Activation barriers for series of exothermic homologous reactions. V. Boron group diatomic species reactions

NASA Astrophysics Data System (ADS)

Blue, Alan S.; Belyung, David P.; Fontijn, Arthur

1997-09-01

Semiempirical configuration interaction (SECI) theory is used to predict activation barriers E, as defined by k(T)=ATn exp(-E/RT). Previously SECI has been applied to homologous series of oxidation reactions of s1, s2, and s2p1 metal atoms. Here it is extended to oxidation reactions of diatomic molecules containing one s2p1 atom. E values are calculated for the reactions of BH, BF, BCl, AlF, AlCl, AlBr, GaF, GaI, InCl, InBr, InI, TlF, TlCl, TlBr, and TlI with O2, CO2, SO2, or N2O. These values correlate with the sums of the ionization potentials and Σ-Π promotion energies of the former minus the electron affinities of the latter. In the earlier work n was chosen somewhat arbitrarily, which affected the absolute values of E. Here it is shown that examination of available experimental and theoretical results allows determination of the best values of n. Using this approach yields n=1.9 for the present series. For the seven reactions which have been studied experimentally, the average deviation of the SECI activation barrier prediction from experiment is 4.0 kJ mol-1. Energy barriers are calculated for another 52 reactions.

Perceived barriers to leisure-time physical activity and associated factors in adolescents.

PubMed

Dias, Douglas Fernando; Loch, Mathias Roberto; Ronque, Enio Ricardo Vaz

2015-11-01

The objective of this study was to identify the prevalence of perceived barriers to leisure-time physical activity in teenagers and to examine the possible association of these barriers with leisure-time physical inactivity. This cross-sectional study was conducted in 2011 and a representative sample of 1,409 high school students from public schools in the city of Londrina/Paraná was selected through multistage sampling. For data collection, the adolescents completed a questionnaire. The relationship between leisure-time physical inactivity (<300 minutes/week) and perceived barriers was analyzed by calculating the prevalence ratio (PR) in Poisson regression models. "Lack of friends company" was the most prevalent barrier for both girls (75.8%) and boys (58.7%). "Feel lazy" for girls (PR: 1.21; CI 95%: 1.08 to 1.36) and "prefer to do other things" for the boys (PR: 1.48; CI 95%: 1.01 to 2.15) were the barriers most strongly associated with leisure-time physical inactivity. For both genders, a strong dose-response relationship was observed between the number of perceived barriers and leisure-time physical inactivity. The perception of barriers was associated with a higher prevalence of leisure-time physical inactivity in adolescents and should therefore be considered in actions for promoting physical activity in this population.The objective of this study was to identify the prevalence of perceived barriers to leisure-time physical activity in teenagers and to examine the possible association of these barriers with leisure-time physical inactivity. This cross-sectional study was conducted in 2011 and a representative sample of 1,409 high school students from public schools in the city of Londrina/Paraná was selected through multistage sampling. For data collection, the adolescents completed a questionnaire. The relationship between leisure-time physical inactivity (<300 minutes/week) and perceived barriers was analyzed by calculating the prevalence ratio (PR) in Poisson regression models. "Lack of friends company" was the most prevalent barrier for both girls (75.8%) and boys (58.7%). "Feel lazy" for girls (PR: 1.21; CI 95%: 1.08 to 1.36) and "prefer to do other things" for the boys (PR: 1.48; CI 95%: 1.01 to 2.15) were the barriers most strongly associated with leisure-time physical inactivity. For both genders, a strong dose-response relationship was observed between the number of perceived barriers and leisure-time physical inactivity. The perception of barriers was associated with a higher prevalence of leisure-time physical inactivity in adolescents and should therefore be considered in actions for promoting physical activity in this population.

Negative influence of pKa on activation energy barrier: A case study for double proton transfer reaction in inorganic acid dimers.

PubMed

Parida, Rakesh; Giri, Santanab

2018-06-15

Strength of acid can be determined by means of pK a value. Attempts have been made to find a relationship between pK a and activation energy barrier for a double proton transfer (DPT) reaction in inorganic acid dimers. Negative influence of pK a is observed on activation energy (E a ) which is contrary to the general convention of pK a . Four different levels of theories with two different basis sets have been used to calculate the activation energy barrier of the DPT reaction in inorganic acid dimers. A model based on first and second order polynomial has been created to find the relationship between activation energy for DPT reaction. © 2018 Wiley Periodicals, Inc. © 2018 Wiley Periodicals, Inc.

Hybrid functional studies of stability and diffusion of hydrogen in Mg-doped GaN

NASA Astrophysics Data System (ADS)

Park, Ji-Sang; Chang, K. J.

2012-02-01

Nitride semiconductors are known to suffer from low p-type doping efficiency due to the high activation energy of Mg acceptors and the compensation of hole carriers. To enhance hole carrier concentration, the hydrogen co-doping method is widely used, in which hydrogen is intentionally doped with Mg dopants and removed by subsequent thermal annealing. In this work, we perform first-principles density functional calculations to study the stability and diffusion of hydrogen in Mg-doped GaN. For the exchange-correlation potential, we employ both the generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof and the hybrid density functional of Heyd, Scuseria, and Ernzerhof. We examine the diffusion pathways and dissociation barriers of H from the Mg-H complex using the nudged elastic band and dimer methods. We compare the results of the GGA and hybrid density functional calculations for the stability of various H interstitial configurations and the migration barriers for H diffusion. Finally, using the calculated migration barriers as inputs, we perform kinetic Monte Carlo simulations for the dissociation of the Mg-H complex and find that the Mg acceptors are activated by thermal annealing up to 700-800 ^oC, in good agreement with experiments.

Removing the barrier to the calculation of activation energies: Diffusion coefficients and reorientation times in liquid water.

PubMed

Piskulich, Zeke A; Mesele, Oluwaseun O; Thompson, Ward H

2017-10-07

General approaches for directly calculating the temperature dependence of dynamical quantities from simulations at a single temperature are presented. The method is demonstrated for self-diffusion and OH reorientation in liquid water. For quantities which possess an activation energy, e.g., the diffusion coefficient and the reorientation time, the results from the direct calculation are in excellent agreement with those obtained from an Arrhenius plot. However, additional information is obtained, including the decomposition of the contributions to the activation energy. These results are discussed along with prospects for additional applications of the direct approach.

Mutagenicity, anticancer activity and blood brain barrier: similarity and dissimilarity of molecular alerts.

PubMed

Toropov, Andrey A; Toropova, Alla P; Benfenati, Emilio; Salmona, Mario

2018-06-01

The aim of the present work is an attempt to define computable measure of similarity between different endpoints. The similarity of structural alerts of different biochemical endpoints can be used to solve tasks of medicinal chemistry. Optimal descriptors are a tool to build up models for different endpoints. The optimal descriptor is calculated with simplified molecular input-line entry system (SMILES). A group of elements (single symbol or pair of symbols) can represent any SMILES. Each element of SMILES can be represented by so-called correlation weight i.e. coefficient that should be used to calculate descriptor. Numerical data on the correlation weights are calculated by the Monte Carlo method, i.e. by optimization procedure, which gives maximal correlation coefficient between the optimal descriptor and endpoint for the training set. Statistically stable correlation weights observed in several runs of the optimization can be examined as structural alerts, which are promoters of the increase or the decrease of a biochemical activity of a substance. Having data on several runs of the optimization correlation weights, one can extract list of promoters of increase and list of promoters of decrease for an endpoint. The study of similarity and dissimilarity of the above lists has been carried out for the following pairs of endpoints: (i) mutagenicity and anticancer activity; (ii) mutagenicity and blood brain barrier; and (iii) blood brain barrier and anticancer activity. The computational experiment confirms that similarity and dissimilarity for pairs of endpoints can be measured.

A New Potential Energy Surface for N+O2: Is There an NOO Minimum?

NASA Technical Reports Server (NTRS)

Walch, Stephen P.

1995-01-01

We report a new calculation of the N+02 potential energy surface using complete active space self-consistent field internally contracted configuration interaction with the Dunning correlation consistent basis sets. The peroxy isomer of N02 is found to be a very shallow minimum separated from NO+O by a barrier of only 0.3 kcal/mol (excluding zero-point effects). The entrance channel barrier height is estimated to be 8.6 kcal/mol for ICCI+Q calculations correlating all but the Ols and N1s electrons with a cc-p VQZ basis set.

Investigation of the CH3Cl + CN(-) reaction in water: Multilevel quantum mechanics/molecular mechanics study.

PubMed

Xu, Yulong; Zhang, Jingxue; Wang, Dunyou

2015-06-28

The CH3Cl + CN(-) reaction in water was studied using a multilevel quantum mechanics/molecular mechanics (MM) method with the multilevels, electrostatic potential, density functional theory (DFT) and coupled-cluster single double triple (CCSD(T)), for the solute region. The detailed, back-side attack SN2 reaction mechanism was mapped along the reaction pathway. The potentials of mean force were calculated under both the DFT and CCSD(T) levels for the reaction region. The CCSD(T)/MM level of theory presents a free energy activation barrier height at 20.3 kcal/mol, which agrees very well with the experiment value at 21.6 kcal/mol. The results show that the aqueous solution has a dominant role in shaping the potential of mean force. The solvation effect and the polarization effect together increase the activation barrier height by ∼11.4 kcal/mol: the solvation effect plays a major role by providing about 75% of the contribution, while polarization effect only contributes 25% to the activation barrier height. Our calculated potential of mean force under the CCSD(T)/MM also has a good agreement with the one estimated using data from previous gas-phase studies.

Self-learning kinetic Monte Carlo simulations of Al diffusion in Mg

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nandipati, Giridhar; Govind, Niranjan; Andersen, Amity

2016-03-16

Atomistic on-lattice self-learning kinetic Monte Carlo (SLKMC) method was used to examine the vacancy-mediated diffusion of an Al atom in pure hcp Mg. Local atomic environment dependent activation barriers for vacancy-atom exchange processes were calculated on-the-fly using climbing image nudged-elastic band method (CI-NEB) and using a Mg-Al binary modified embedded-atom method (MEAM) interatomic potential. Diffusivities of vacancy and Al atom in pure Mg were obtained from SLKMC simulations and are compared with values available in the literature that are obtained from experiments and first-principle calculations. Al Diffusivities obtained from SLKMC simulations are lower, due to larger activation barriers and lowermore » diffusivity prefactors, than those available in the literature but have same order of magnitude. We present all vacancy-Mg and vacancy-Al atom exchange processes and their activation barriers that were identified in SLKMC simulations. We will describe a simple mapping scheme to map a hcp lattice on to a simple cubic lattice that would enable hcp lattices to be simulated in an on-lattice KMC framework. We also present the pattern recognition scheme used in SLKMC simulations.« less

Investigation of the CH3Cl + CN- reaction in water: Multilevel quantum mechanics/molecular mechanics study

NASA Astrophysics Data System (ADS)

Xu, Yulong; Zhang, Jingxue; Wang, Dunyou

2015-06-01

The CH3Cl + CN- reaction in water was studied using a multilevel quantum mechanics/molecular mechanics (MM) method with the multilevels, electrostatic potential, density functional theory (DFT) and coupled-cluster single double triple (CCSD(T)), for the solute region. The detailed, back-side attack SN2 reaction mechanism was mapped along the reaction pathway. The potentials of mean force were calculated under both the DFT and CCSD(T) levels for the reaction region. The CCSD(T)/MM level of theory presents a free energy activation barrier height at 20.3 kcal/mol, which agrees very well with the experiment value at 21.6 kcal/mol. The results show that the aqueous solution has a dominant role in shaping the potential of mean force. The solvation effect and the polarization effect together increase the activation barrier height by ˜11.4 kcal/mol: the solvation effect plays a major role by providing about 75% of the contribution, while polarization effect only contributes 25% to the activation barrier height. Our calculated potential of mean force under the CCSD(T)/MM also has a good agreement with the one estimated using data from previous gas-phase studies.

Revealing the role of the product metal in DNA polymerase β catalysis

PubMed Central

Freudenthal, Bret D.; Beard, William A.; Pedersen, Lee G.; Wilson, Samuel H.

2017-01-01

Abstract DNA polymerases catalyze a metal-dependent nucleotidyl transferase reaction during extension of a DNA strand using the complementary strand as a template. The reaction has long been considered to require two magnesium ions. Recently, a third active site magnesium ion was identified in some DNA polymerase product crystallographic structures, but its role is not known. Using quantum mechanical/ molecular mechanical calculations of polymerase β, we find that a third magnesium ion positioned near the newly identified product metal site does not alter the activation barrier for the chemical reaction indicating that it does not have a role in the forward reaction. This is consistent with time-lapse crystallographic structures following insertion of Sp-dCTPαS. Although sulfur substitution deters product metal binding, this has only a minimal effect on the rate of the forward reaction. Surprisingly, monovalent sodium or ammonium ions, positioned in the product metal site, lowered the activation barrier. These calculations highlight the impact that an active site water network can have on the energetics of the forward reaction and how metals or enzyme side chains may interact with the network to modulate the reaction barrier. These results also are discussed in the context of earlier findings indicating that magnesium at the product metal position blocks the reverse pyrophosphorolysis reaction. PMID:28108654

Successful application of the DBLOC method to the hydroxylation of camphor by cytochrome p450

PubMed Central

Jerome, Steven V.; Hughes, Thomas F.

2015-01-01

Abstract The activation barrier for the hydroxylation of camphor by cytochrome P450 was computed using a mixed quantum mechanics/molecular mechanics (QM/MM) model of the full protein‐ligand system and a fully QM calculation using a cluster model of the active site at the B3LYP/LACVP*/LACV3P** level of theory, which consisted of B3LYP/LACV3P** single point energies computed at B3LYP/LACVP* optimized geometries. From the QM/MM calculation, a barrier height of 17.5 kcal/mol was obtained, while the experimental value was known to be less than or equal to 10 kcal/mol. This process was repeated using the D3 correction for hybrid DFT in order to investigate whether the inadequate treatment of dispersion interaction was responsible for the overestimation of the barrier. While the D3 correction does reduce the computed barrier to 13.3 kcal/mol, it was still in disagreement with experiment. After application of a series of transition metal optimized localized orbital corrections (DBLOC) and without any refitting of parameters, the barrier was further reduced to 10.0 kcal/mol, which was consistent with the experimental results. The DBLOC method to C—H bond activation in methane monooxygenase (MMO) was also applied, as a second, independent test. The barrier in MMO was known, by experiment, to be 15.4 kcal/mol.1 After application of the DBLOC corrections to the MMO barrier compute by B3LYP, in a previous study, and accounting for dispersion with Grimme's D3 method, the unsigned deviation from experiment was improved from 3.2 to 2.3 kcal/mol. These results suggested that the combination of dispersion plus localized orbital corrections could yield significant quantitative improvements in modeling the catalytic chemistry of transition‐metal containing enzymes, within the limitations of the statistical errors of the model, which appear to be on the order of approximately 2 kcal/mole. PMID:26441133

Theoretical study on keto-enol tautomerisation of glutarimide for exploration of the isomerisation reaction pathway of glutamic acid in proteins using density functional theory

NASA Astrophysics Data System (ADS)

Fukuyoshi, Shuichi; Nakayoshi, Tomoki; Takahashi, Ohgi; Oda, Akifumi

2017-03-01

In order to elucidate the reason why glutamic acid residues have lesser racemisation reactivity than asparaginic acid, we investigated the racemisation energy barrier of piperidinedione, which is the presumed intermediate of the isomerisation reaction of L-Glu to D-Glu, by density functional theory calculations. In two-water-molecule-assisted racemisation, the activation barrier for keto-enol isomerisation was 28.1 kcal/mol. The result showed that the activation barrier for the racemisation of glutamic acid residues was not different from that for the racemisation of aspartic acid residues. Thus, glutamic acid residues can possibly cause the racemisation reaction if the cyclic intermediate stably exists.

Effect of an InxGa1-xAs-GaAs blocking heterocathode metal contact on the GaAs TED operation

NASA Astrophysics Data System (ADS)

Arkusha, Yu. V.; Prokhorov, E. D.; Storozhenko, I. P.

2004-09-01

The frequency dependence of the generation efficiency of an mm- -nn:In:InxGaGa1-1-xAs- As-nn:GaAs-:GaAs-nn++:GaAs TED with the 2.5-mm long active region is calculated. The optimum values - which yield the diode maximum generation efficiency - for the :GaAs TED with the 2.5-mm long active region is calculated. The optimum values - which yield the diode maximum generation efficiency - for the nn:In:InxGaGa1-1-xAs cathode length, the cathode concentration of ionized impurities, and the height of the potential barrier on metal contact are determined.As cathode length, the cathode concentration of ionized impurities, and the height of the potential barrier on metal contact are determined.

Chemical Insights into the Design and Development of Face-Centered Cubic Ruthenium Catalysts for Fischer-Tropsch Synthesis.

PubMed

Li, Wei-Zhen; Liu, Jin-Xun; Gu, Jun; Zhou, Wu; Yao, Si-Yu; Si, Rui; Guo, Yu; Su, Hai-Yan; Yan, Chun-Hua; Li, Wei-Xue; Zhang, Ya-Wen; Ma, Ding

2017-02-15

Ruthenium is a promising low-temperature catalyst for Fischer-Tropsch synthesis (FTS). However, its scarcity and modest specific activity limit its widespread industrialization. We demonstrate here a strategy for tuning the crystal phase of catalysts to expose denser and active sites for a higher mass-specific activity. Density functional theory calculations show that upon CO dissociation there are a number of open facets with modest barrier available on the face-centered cubic (fcc) Ru but only a few step edges with a lower barrier on conventional hexagonal-closest packed (hcp) Ru. Guided by theoretical calculations, water-dispersible fcc Ru catalysts containing abundant open facets were synthesized and showed an unprecedented mass-specific activity in the aqueous-phase FTS, 37.8 mol CO ·mol Ru -1 ·h -1 at 433 K. The mass-specific activity of the fcc Ru catalysts with an average size of 6.8 nm is about three times larger than the previous best hcp catalyst with a smaller size of 1.9 nm and a higher specific surface area. The origin of the higher mass-specific activity of the fcc Ru catalysts is identified experimentally from the 2 orders of magnitude higher density of the active sites, despite its slightly higher apparent barrier. Experimental results are in excellent agreement with prediction of theory. The great influence of the crystal phases on site distribution and their intrinsic activities revealed here provides a rationale design of catalysts for higher mass-specific activity without decrease of the particle size.

Effects of a Single Water Molecule on the Reaction Barrier of Interstellar CO2 Formation Reaction.

PubMed

Tachikawa, Hiroto; Kawabata, Hiroshi

2016-08-25

The mechanism by which CO2 is formed in the interstellar space remains a mystery. The most likely reaction is collision between CO and OH; however, previous theoretical works have shown that the activation barrier for CO2 formation is high enough to prevent the reaction at the low thermal conditions of space (∼10 K). The effects of single water molecule on the reaction barrier of CO2 formation from reaction between CO and OH have been investigated here by means of ab initio calculation. The barrier height along the lowest-energy pathway in the reaction between CO and OH in the absence of the H2O molecule was calculated to be 2.3 kcal/mol when CCSD(T) energy corrections are combined with the MP2 basis set limit. In the case of the hydrated (H2O-CO-OH) system, the inclusion of a single H2O molecule into the system significantly decreased the barrier height to 0.2 kcal/mol. This suggests that CO2 can be formed when CO and OH react in the presence of H2O, even under thermal conditions as low as 10 K.

Platinum-catalyzed hydrolysis etching of SiC in water: A density functional theory study

NASA Astrophysics Data System (ADS)

Van Bui, Pho; Toh, Daisetsu; Isohashi, Ai; Matsuyama, Satoshi; Inagaki, Kouji; Sano, Yasuhisa; Yamauchi, Kazuto; Morikawa, Yoshitada

2018-05-01

A comprehensive study of the physicochemical interactions and the reaction mechanism of SiC etching with water by Pt catalysts can reveal key details about the surface treatment and catalytic phenomena at interfaces. Therefore, density functional theory simulations were performed to study the kinetics of Pt-assisted water dissociation and breaking of a Si–C bond compared to the HF-assisted mechanism. These calculations carefully considered the elastic and chemical interaction energies at the Pt–SiC interface, activation barriers of Si–C bond dissociation, and the catalytic role of Pt. It was found that the Pt-catalyzed etching of SiC in water is initiated via hydrolysis reactions that break the topmost Si–C bonds. The activation barrier strongly depends on the elastic and chemical interactions. However, chemical interactions are a dominant factor and mainly contribute to the lowering of the activation barrier, resulting in an increased rate of reaction.

Quantum Mechanics/Molecular Mechanics Modeling of Drug Metabolism: Mexiletine N-Hydroxylation by Cytochrome P450 1A2.

PubMed

Lonsdale, Richard; Fort, Rachel M; Rydberg, Patrik; Harvey, Jeremy N; Mulholland, Adrian J

2016-06-20

The mechanism of cytochrome P450(CYP)-catalyzed hydroxylation of primary amines is currently unclear and is relevant to drug metabolism; previous small model calculations have suggested two possible mechanisms: direct N-oxidation and H-abstraction/rebound. We have modeled the N-hydroxylation of (R)-mexiletine in CYP1A2 with hybrid quantum mechanics/molecular mechanics (QM/MM) methods, providing a more detailed and realistic model. Multiple reaction barriers have been calculated at the QM(B3LYP-D)/MM(CHARMM27) level for the direct N-oxidation and H-abstraction/rebound mechanisms. Our calculated barriers indicate that the direct N-oxidation mechanism is preferred and proceeds via the doublet spin state of Compound I. Molecular dynamics simulations indicate that the presence of an ordered water molecule in the active site assists in the binding of mexiletine in the active site, but this is not a prerequisite for reaction via either mechanism. Several active site residues play a role in the binding of mexiletine in the active site, including Thr124 and Phe226. This work reveals key details of the N-hydroxylation of mexiletine and further demonstrates that mechanistic studies using QM/MM methods are useful for understanding drug metabolism.

Mechanism of H adatoms improving the O2 reduction reaction on the Zn-modified anatase TiO2 (101) surface studied by first principles calculation.

PubMed

Liu, Liangliang; Li, Chongyang; Jiang, Man; Li, Xiaodong; Huang, Xiaowei; Wang, Zhu; Jia, Yu

2018-06-05

First principles calculations were performed to cast insight into the mechanism of the improvement of O2 reduction reaction (ORR) activity by Zn and H interstitials on the anatase TiO2 (101) surface. For the Zn-modified anatase TiO2 (101) surface, both surface and subsurface Zn interstitials could contribute to O2 adsorption and dissociation, but the dissociation barriers of O2 molecules are still too high, which limits the ORR activity. After a H adatom is introduced onto the Zn-modified anatase TiO2 (101) surface, the highest energy barriers are greatly reduced compared with those of the Zn-modified surface. Meanwhile, it is observed that the dissociation barriers decrease almost linearly with the increase of the charge difference of adsorption O2 between initial and transition state configurations. Specifically, subsurface Zn and surface H interstitials facilitate O2 dissociation and subsequent oxidation reactions, and further frequency analysis shows that these dissociation processes are frequent even at the room temperature of 300 K. In a word, this work provides a theoretical support to design a high ORR activity catalyst of the TiO2 nanocrystal comparable to precious Pt catalysts.

Investigation of the CH{sub 3}Cl + CN{sup −} reaction in water: Multilevel quantum mechanics/molecular mechanics study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Yulong; College of Physics and Electronics, Shandong Normal University, Jinan 250014; Zhang, Jingxue

2015-06-28

The CH{sub 3}Cl + CN{sup −} reaction in water was studied using a multilevel quantum mechanics/molecular mechanics (MM) method with the multilevels, electrostatic potential, density functional theory (DFT) and coupled-cluster single double triple (CCSD(T)), for the solute region. The detailed, back-side attack S{sub N}2 reaction mechanism was mapped along the reaction pathway. The potentials of mean force were calculated under both the DFT and CCSD(T) levels for the reaction region. The CCSD(T)/MM level of theory presents a free energy activation barrier height at 20.3 kcal/mol, which agrees very well with the experiment value at 21.6 kcal/mol. The results show thatmore » the aqueous solution has a dominant role in shaping the potential of mean force. The solvation effect and the polarization effect together increase the activation barrier height by ∼11.4 kcal/mol: the solvation effect plays a major role by providing about 75% of the contribution, while polarization effect only contributes 25% to the activation barrier height. Our calculated potential of mean force under the CCSD(T)/MM also has a good agreement with the one estimated using data from previous gas-phase studies.« less

On the validity of the Arrhenius equation for electron attachment rate coefficients.

PubMed

Fabrikant, Ilya I; Hotop, Hartmut

2008-03-28

The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.

Light controllable catalytic activity of Au clusters decorated with photochromic molecules.

PubMed

Guo, Na; Yam, Kah Meng; Zhang, Chun

2018-06-15

By ab initio calculations, we show that when decorated with a photochromic molecule, the catalytic activity of an Au nanocluster can be reversibly controlled by light. The combination of a photochromic thiol-pentacarbonyl azobenzene (TPA) molecule and an Au 8 cluster is chosen as a model catalyst. The TPA molecule has two configurations (trans and cis) that can be reversibly converted to each other upon photo-excitation. Our calculations show that when the TPA takes the trans configuration, the combined system (trans-Au 8 ) is an excellent catalyst for CO oxidation. The reaction barrier of the catalyzed CO oxidation is less than 0.4 eV. While, the reaction barrier of CO oxidation catalyzed by cis-Au 8 is very high (>2.7 eV), indicating that the catalyst is inactive. These results pave the way for a new class of light controllable nanoscale catalysts.

Light controllable catalytic activity of Au clusters decorated with photochromic molecules

NASA Astrophysics Data System (ADS)

Guo, Na; Meng Yam, Kah; Zhang, Chun

2018-06-01

By ab initio calculations, we show that when decorated with a photochromic molecule, the catalytic activity of an Au nanocluster can be reversibly controlled by light. The combination of a photochromic thiol-pentacarbonyl azobenzene (TPA) molecule and an Au8 cluster is chosen as a model catalyst. The TPA molecule has two configurations (trans and cis) that can be reversibly converted to each other upon photo-excitation. Our calculations show that when the TPA takes the trans configuration, the combined system (trans-Au8) is an excellent catalyst for CO oxidation. The reaction barrier of the catalyzed CO oxidation is less than 0.4 eV. While, the reaction barrier of CO oxidation catalyzed by cis-Au8 is very high (>2.7 eV), indicating that the catalyst is inactive. These results pave the way for a new class of light controllable nanoscale catalysts.

Carcinogenesis of urethane: simulation versus experiment.

PubMed

Lajovic, Andrej; Nagy, Leslie D; Guengerich, F Peter; Bren, Urban

2015-04-20

The carcinogenesis of urethane (ethyl carbamate), a byproduct of fermentation that is consistently found in various food products, was investigated with a combination of kinetic experiments and quantum chemical calculations. The main objective of the study was to find ΔG(⧧), the activation free energy for the rate-limiting step of the SN2 reaction among the ultimate carcinogen of urethane, vinyl carbamate epoxide (VCE), and different nucleobases of the DNA. In the experimental part, the second-order reaction rate constants for the formation of the main 7-(2-oxoethyl)guanine adduct in aqueous solutions of deoxyguanosine and in DNA were determined. A series of ab initio, density functional theory (DFT), and semiempirical molecular orbital (MO) calculations was then performed to determine the activation barriers for the reaction between VCE and nucleobases methylguanine, methyladenine, and methylcytosine. Effects of hydration were incorporated with the use of the solvent reaction field method of Tomasi and co-workers and the Langevine dipoles model of Florian and Warshel. The computational results for the main adduct were found to be in good agreement with the experiment, thus presenting strong evidence for the validity of the proposed SN2 mechanism. This allowed us to predict the activation barriers of reactions leading to side products for which kinetic experiments have not yet been performed. Our calculations have shown that the main 7-(2-oxoethyl)deoxyguanosine adduct indeed forms preferentially because the emergence of other adducts either proceeds across a significantly higher activation barrier or the geometry of the reaction requires the Watson-Crick pairs of the DNA to be broken. The computational study also considered the questions of stereoselectivity, the ease of the elimination of the leaving group, and the relative contributions of the two possible reaction paths for the formation of the 1,N(2)-ethenodeoxyguanosine adduct.

Reductive half-reaction of aldehyde oxidoreductase toward acetaldehyde: Ab initio and free energy quantum mechanical/molecular mechanical calculations.

PubMed

Dieterich, Johannes M; Werner, Hans-Joachim; Mata, Ricardo A; Metz, Sebastian; Thiel, Walter

2010-01-21

Energy and free energy barriers for acetaldehyde conversion in aldehyde oxidoreductase are determined for three reaction pathways using quantum mechanical/molecular mechanical (QM/MM) calculations on the solvated enzyme. Ab initio single-point QM/MM energies are obtained at the stationary points optimized at the DFT(B3LYP)/MM level. These ab initio calculations employ local correlation treatments [LMP2 and LCCSD(T0)] in combination with augmented triple- and quadruple-zeta basis sets, and the final coupled cluster results include MP2-based corrections for basis set incompleteness and for the domain approximation. Free energy perturbation (FEP) theory is used to generate free energy profiles at the DFT(B3LYP)/MM level for the most important reaction steps by sampling along the corresponding reaction paths using molecular dynamics. The ab initio and FEP QM/MM results are combined to derive improved estimates of the free energy barriers, which differ from the corresponding DFT(B3LYP)/MM energy barriers by about 3 kcal mol(-1). The present results confirm the qualitative mechanistic conclusions from a previous DFT(B3LYP)/MM study. Most favorable is a three-step Lewis base catalyzed mechanism with an initial proton transfer from the cofactor to the Glu869 residue, a subsequent nucleophilic attack that yields a tetrahedral intermediate (IM2), and a final rate-limiting hydride transfer. The competing metal center activated pathway has the same final step but needs to overcome a higher barrier in the initial step on the route to IM2. The concerted mechanism has the highest free energy barrier and can be ruled out. While confirming the qualitative mechanistic scenario proposed previously on the basis of DFT(B3LYP)/MM energy profiles, the present ab initio and FEP QM/MM calculations provide corrections to the barriers that are important when aiming at high accuracy.

On the pressure field of nonlinear standing water waves

NASA Technical Reports Server (NTRS)

Schwartz, L. W.

1980-01-01

The pressure field produced by two dimensional nonlinear time and space periodic standing waves was calculated as a series expansion in the wave height. The high order series was summed by the use of Pade approximants. Calculations included the pressure variation at great depth, which was considered to be a likely cause of microseismic activity, and the pressure distribution on a vertical barrier or breakwater.

Assessment study for multi-barrier system used in radioactive borate waste isolation based on Monte Carlo simulations.

PubMed

Bayoumi, T A; Reda, S M; Saleh, H M

2012-01-01

Radioactive waste generated from the nuclear applications should be properly isolated by a suitable containment system such as, multi-barrier container. The present study aims to evaluate the isolation capacity of a new multi-barrier container made from cement and clay and including borate waste materials. These wastes were spiked by (137)Cs and (60)Co radionuclides to simulate that waste generated from the primary cooling circuit of pressurized water reactors. Leaching of both radionuclides in ground water was followed and calculated during ten years. Monte Carlo (MCNP5) simulations computed the photon flux distribution of the multi-barrier container, including radioactive borate waste of specific activity 11.22KBq/g and 4.18KBq/g for (137)Cs and (60)Co, respectively, at different periods of 0, 15.1, 30.2 and 302 years. The average total flux for 100cm radius of spherical cell was 0.192photon/cm(2) at initial time and 2.73×10(-4)photon/cm(2) after 302 years. Maximum waste activity keeping the surface radiation dose within the permissible level was calculated and found to be 56KBq/g with attenuation factors of 0.73cm(-1) and 0.6cm(-1) for cement and clay, respectively. The average total flux was 1.37×10(-3)photon/cm(2) after 302 years. Monte Carlo simulations revealed that the proposed multi-barrier container is safe enough during transportation, evacuation or rearrangement in the disposal site for more than 300 years. Copyright © 2011 Elsevier Ltd. All rights reserved.

Fabrication and characterization of tunnel barriers in a multi-walled carbon nanotube formed by argon atom beam irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tomizawa, H.; Department of Applied Physics, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585; Yamaguchi, T., E-mail: tyamag@riken.jp

We have evaluated tunnel barriers formed in multi-walled carbon nanotubes (MWNTs) by an Ar atom beam irradiation method and applied the technique to fabricate coupled double quantum dots. The two-terminal resistance of the individual MWNTs was increased owing to local damage caused by the Ar beam irradiation. The temperature dependence of the current through a single barrier suggested two different contributions to its Arrhenius plot, i.e., formed by direct tunneling through the barrier and by thermal activation over the barrier. The height of the formed barriers was estimated. The fabrication technique was used to produce coupled double quantum dots withmore » serially formed triple barriers on a MWNT. The current measured at 1.5 K as a function of two side-gate voltages resulted in a honeycomb-like charge stability diagram, which confirmed the formation of the double dots. The characteristic parameters of the double quantum dots were calculated, and the feasibility of the technique is discussed.« less

Theoretical insights into the selective oxidation of methane to methanol in copper-exchanged mordenite

DOE PAGES

Zhao, Zhi -Jian; Kulkarni, Ambarish; Vilella, Laia; ...

2016-05-02

Selective oxidation of methane to methanol is one of the most difficult chemical processes to perform. A potential group of catalysts to achieve CH 4 partial oxidation are Cu-exchanged zeolites mimicking the active structure of the enzyme methane monooxygenase. However, the details of this conversion, including the structure of the active site, are still under debate. In this contribution, periodic density functional theory (DFT) methods were employed to explore the molecular features of the selective oxidation of methane to methanol catalyzed by Cu-exchanged mordenite (Cu-MOR). We focused on two types of previously suggested active species, CuOCu and CuOOCu. Our calculationsmore » indicate that the formation of CuOCu is more feasible than that of CuOOCu. In addition, a much lower C–H dissociation barrier is located on the former active site, indicating that C–H bond activation is easily achieved with CuOCu. We calculated the energy barriers of all elementary steps for the entire process, including catalyst activation, CH 4 activation, and CH 3OH desorption. Finally, our calculations are in agreement with experimental observations and present the first theoretical study examining the entire process of selective oxidation of methane to methanol.« less

Dry (CO2) reforming of methane over Pt catalysts studied by DFT and kinetic modeling

NASA Astrophysics Data System (ADS)

Niu, Juntian; Du, Xuesen; Ran, Jingyu; Wang, Ruirui

2016-07-01

Dry reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. In order to design catalysts that minimize the deactivation and improve the selectivity and activity for a high H2/CO yield, it is necessary to understand the elementary reaction steps involved in activation and conversion of CO2 and CH4. In our present work, a microkinetic model based on density functional theory (DFT) calculations is applied to explore the reaction mechanism for methane dry reforming on Pt catalysts. The adsorption energies of the reactants, intermediates and products, and the activation barriers for the elementary reactions involved in the DRM process are calculated over the Pt(1 1 1) surface. In the process of CH4 direct dissociation, the kinetic results show that CH dissociative adsorption on Pt(1 1 1) surface is the rate-determining step. CH appears to be the most abundant species on the Pt(1 1 1) surface, suggesting that carbon deposition is not easy to form in CH4 dehydrogenation on Pt(1 1 1) surface. In the process of CO2 activation, three possible reaction pathways are considered to contribute to the CO2 decomposition: (I) CO2* + * → CO* + O*; (II) CO2* + H* → COOH* + * → CO* + OH*; (III) CO2* + H* → mono-HCOO* + * → bi-HCOO* + * [CO2* + H* → bi-HCOO* + *] → CHO* + O*. Path I requires process to overcome the activation barrier of 1.809 eV and the forward reaction is calculated to be strongly endothermic by 1.430 eV. In addition, the kinetic results also indicate this process is not easy to proceed on Pt(1 1 1) surface. While the CO2 activation by H adsorbed over the catalyst surface to form COOH intermediate (Path II) is much easier to be carried out with the lower activation barrier of 0.746 eV. The Csbnd O bond scission is the rate-determining step along this pathway and the process needs to overcome the activation barrier of 1.522 eV. Path III reveals the CO2 activation through H adsorbed over the catalyst surface to form HCOO intermediate firstly. This reaction requires a quite high activation barrier and is a strongly endothermic process leading to a very low forward rate constant. In conclusion, Path II is the dominant reaction pathway in CO2 activation. Additionally, there are two pathways of CH oxidation by O: (A) CH* + O* → CHO* + * → CO* + H*; (B) CH* + O* → COH* + * → CO* + H*. Both the activation barriers and kinetic results demonstrate that Path A is the prior reaction pathway. Furthermore, in the two pathways of CH oxidation by OH: (C) CH* + OH* → CHOH* + * → CHO* + H*; (D) CH* + OH* → CHOH* + * → COH* + H*. Path C is easier to proceed. In conclusion, the main reaction pathway in CH oxidation according to the mechanism: CH* + OH* → CHOH* + * → CHO* + H* → CO* + 2H*. These results could provide some useful information for the operation of DRM over Pt catalysts, and are helpful to understand the mechanisms of DRM from the atomic scale.

Interference phenomena in the refraction of a surface polariton by vertical dielectric barriers

NASA Technical Reports Server (NTRS)

Shen, T. P.; Wallis, R. F.; Maradudin, A. A.; Stegeman, G. I.

1984-01-01

A normal mode analysis is used to calculate the transmission and reflection coefficients for a surface polariton propagating along the interface between a surface active medium and a dielectric and incident normally on a vertical dielectric barrier of finite thickness or a thin dielectric film of finite length. The efficiencies of conversion of the surface polariton into transmitted and reflected bulk waves are also determined. The radiation patterns associated with the latter waves are presented.

Physical and social environmental characteristics of physical activity for Mexican-origin children: examining differences between school year and summer perceptions.

PubMed

Umstattd Meyer, M Renée; Walsh, Shana M; Sharkey, Joseph R; Morgan, Grant B; Nalty, Courtney C

2014-09-16

Colonias are substandard residential areas along the U.S.-Mexico border. Families of Mexican-origin living in colonias face health burdens characterized by environmental and socioeconomic hardships. Mexican Americans and low-income families, including colonias children, do not frequently participate in physical activity despite the known link to disease risk reduction. For colonias children, schools are the most commonly reported location for physical activity. School closures and extreme temperatures during summer months create a need to explore seasonal differences in environmental supports and barriers in this population. The purpose of this study was to examine the effect of seasonality on perceived environmental barriers, opportunities, and social support for physical activity among colonias children. As a secondary aim, mother-child discordance for each factor was analyzed. Promotora-researchers recruited mother-child dyads (n=101 dyads, n=202 participants) from colonias in Hidalgo County, Texas. Mothers and children were separately administered surveys at two time points to capture perceived barriers, opportunities, and social support for physical activity (school-year: February-May; summertime: July-August). Summative scores for each outcome were calculated and three multilevel longitudinal models for continuous outcomes were examined; children were nested within households. Mother-child discordance was measured using Cohen's Kappa statistic. Physical activity barriers and environmental opportunities (household and neighborhood) increased from school-year to summer by 1.16 and 2.83 points respectively (p≤0.01), after adjusting for covariates. Significant predictors of increased barriers included household income of >$900/month and having more household members. Children of mothers with significant others who were employed part-time or full-time saw significant decreases in barriers. Mother-child agreement of barriers, environmental opportunities, and social support across seasons was slight to fair (range: median κ=0.047 to κ=0.262). These results suggest a complex relationship between dimensions of economic hardship (employment status, household income, etc…) and perceived opportunities and barriers of children's physical activity engagement during the school-year and summer. In this study, both barriers and opportunities increased from school-year to summer, further demonstrating that interactions among these characteristics need to be better understood and addressed when considering physical activity initiatives for colonias and other Mexican-American children, specifically during summer when school-based physical activity resources are unavailable.

Efficient sampling over rough energy landscapes with high barriers: A combination of metadynamics with integrated tempering sampling.

PubMed

Yang, Y Isaac; Zhang, Jun; Che, Xing; Yang, Lijiang; Gao, Yi Qin

2016-03-07

In order to efficiently overcome high free energy barriers embedded in a complex energy landscape and calculate overall thermodynamics properties using molecular dynamics simulations, we developed and implemented a sampling strategy by combining the metadynamics with (selective) integrated tempering sampling (ITS/SITS) method. The dominant local minima on the potential energy surface (PES) are partially exalted by accumulating history-dependent potentials as in metadynamics, and the sampling over the entire PES is further enhanced by ITS/SITS. With this hybrid method, the simulated system can be rapidly driven across the dominant barrier along selected collective coordinates. Then, ITS/SITS ensures a fast convergence of the sampling over the entire PES and an efficient calculation of the overall thermodynamic properties of the simulation system. To test the accuracy and efficiency of this method, we first benchmarked this method in the calculation of ϕ - ψ distribution of alanine dipeptide in explicit solvent. We further applied it to examine the design of template molecules for aromatic meta-C-H activation in solutions and investigate solution conformations of the nonapeptide Bradykinin involving slow cis-trans isomerizations of three proline residues.

Efficient sampling over rough energy landscapes with high barriers: A combination of metadynamics with integrated tempering sampling

NASA Astrophysics Data System (ADS)

Yang, Y. Isaac; Zhang, Jun; Che, Xing; Yang, Lijiang; Gao, Yi Qin

2016-03-01

In order to efficiently overcome high free energy barriers embedded in a complex energy landscape and calculate overall thermodynamics properties using molecular dynamics simulations, we developed and implemented a sampling strategy by combining the metadynamics with (selective) integrated tempering sampling (ITS/SITS) method. The dominant local minima on the potential energy surface (PES) are partially exalted by accumulating history-dependent potentials as in metadynamics, and the sampling over the entire PES is further enhanced by ITS/SITS. With this hybrid method, the simulated system can be rapidly driven across the dominant barrier along selected collective coordinates. Then, ITS/SITS ensures a fast convergence of the sampling over the entire PES and an efficient calculation of the overall thermodynamic properties of the simulation system. To test the accuracy and efficiency of this method, we first benchmarked this method in the calculation of ϕ - ψ distribution of alanine dipeptide in explicit solvent. We further applied it to examine the design of template molecules for aromatic meta-C—H activation in solutions and investigate solution conformations of the nonapeptide Bradykinin involving slow cis-trans isomerizations of three proline residues.

Efficient sampling over rough energy landscapes with high barriers: A combination of metadynamics with integrated tempering sampling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Y. Isaac; Zhang, Jun; Che, Xing

2016-03-07

In order to efficiently overcome high free energy barriers embedded in a complex energy landscape and calculate overall thermodynamics properties using molecular dynamics simulations, we developed and implemented a sampling strategy by combining the metadynamics with (selective) integrated tempering sampling (ITS/SITS) method. The dominant local minima on the potential energy surface (PES) are partially exalted by accumulating history-dependent potentials as in metadynamics, and the sampling over the entire PES is further enhanced by ITS/SITS. With this hybrid method, the simulated system can be rapidly driven across the dominant barrier along selected collective coordinates. Then, ITS/SITS ensures a fast convergence ofmore » the sampling over the entire PES and an efficient calculation of the overall thermodynamic properties of the simulation system. To test the accuracy and efficiency of this method, we first benchmarked this method in the calculation of ϕ − ψ distribution of alanine dipeptide in explicit solvent. We further applied it to examine the design of template molecules for aromatic meta-C—H activation in solutions and investigate solution conformations of the nonapeptide Bradykinin involving slow cis-trans isomerizations of three proline residues.« less

The main types of electron energy distribution determined by model fitting to optical emissions during HF wave ionospheric modification experiments

NASA Astrophysics Data System (ADS)

Vlasov, M. N.; Kelley, M. C.; Hysell, D. L.

2013-06-01

Enhanced optical emissions observed during HF pumping are induced by electrons accelerated by high-power electromagnetic waves. Using measured emission intensities, the energy distribution of accelerated electrons can be inferred. Energy loss from the excitation of molecular nitrogen vibrational levels (the vibrational barrier) strongly influences the electron energy distribution (EED). In airglow calculations, compensation for electron depletion within the 2-3 eV energy range, induced by the vibrational barrier, can be achieved via electrons with an EED similar to a Gaussian distribution and energies higher than 3 eV. This EED has a peak within the 5-10 eV energy range. We show that the main EED features depend strongly on altitude and solar activity. An EED similar to a power law distribution can occur above 270-300 km altitude. Below 270 km altitude, a Gaussian distribution for energies between 3 eV and 10 eV, together with a power law distribution for energies higher than 10 eV, is indicated. A Gaussian distribution combined with an exponential function is needed below 230 km altitude. The transition altitude from Gaussian to power law distribution depends strongly on solar activity, increasing for high solar activity. Electrons accelerated during the initial collisionless stage can inhibit the depletion of fast electrons within the vibrational barrier range, an effect that strongly depends on altitude and solar activity. The approach, based on the effective root square electric field, enables EED calculation, providing the observed red-line intensities for low and high solar activities.

Penetration Barrier of Water through Graphynes' Pores: First-Principles Predictions and Force Field Optimization.

PubMed

Bartolomei, Massimiliano; Carmona-Novillo, Estela; Hernández, Marta I; Campos-Martínez, José; Pirani, Fernando; Giorgi, Giacomo; Yamashita, Koichi

2014-02-20

Graphynes are novel two-dimensional carbon-based materials that have been proposed as molecular filters, especially for water purification technologies. We carry out first-principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne, and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. For graphdiyne, with a pore size almost matching that of water, a low barrier is found that in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is considered. Thus, in contrast with previous determinations, our results do not exclude graphdiyne as a promising membrane for water filtration. In fact, present calculations lead to water permeation probabilities that are 2 orders of magnitude larger than estimations based on common force fields. A new pair potential for the water-carbon noncovalent component of the interaction is proposed for molecular dynamics simulations involving graphdiyne and water.

Edible bioactive fatty acid-cellulosic derivative composites used in food-packaging applications.

PubMed

Sebti, Issam; Ham-Pichavant, Frédérique; Coma, Véronique

2002-07-17

To develop biodegradable packaging that both acts as a moisture barrier and as antimicrobial activity, nisin and stearic acid were incorporated into a hydroxy propyl methyl cellulose (HPMC) based film. Fifteen percent (w/w HPMC) of stearic acid improved film moisture barrier. However, film mechanical resistance and film antimicrobial activity on Listeria monocytogenes and Staphylococcus aureus pathogenic strains were both reduced. This lower film inhibitory activity was due to interactions between nisin and stearic acid. The molecular interaction was modeled, and an equation was developed to calculate the nisin concentration needed to be incorporated into the film matrix to obtain a desired residual antimicrobial activity. Because the molecular interactions were pH dependent, the impact of the pH of the film-forming solution on film inhibitory activity was investigated. Adjusting the pH to 3 totally avoided stearic acid and nisin interaction, inducing a high film inhibitory activity.

MO-D-213-07: RadShield: Semi- Automated Calculation of Air Kerma Rate and Barrier Thickness

DOE Office of Scientific and Technical Information (OSTI.GOV)

DeLorenzo, M; Wu, D; Rutel, I

2015-06-15

Purpose: To develop the first Java-based semi-automated calculation program intended to aid professional radiation shielding design. Air-kerma rate and barrier thickness calculations are performed by implementing NCRP Report 147 formalism into a Graphical User Interface (GUI). The ultimate aim of this newly created software package is to reduce errors and improve radiographic and fluoroscopic room designs over manual approaches. Methods: Floor plans are first imported as images into the RadShield software program. These plans serve as templates for drawing barriers, occupied regions and x-ray tube locations. We have implemented sub-GUIs that allow the specification in regions and equipment for occupancymore » factors, design goals, number of patients, primary beam directions, source-to-patient distances and workload distributions. Once the user enters the above parameters, the program automatically calculates air-kerma rate at sampled points beyond all barriers. For each sample point, a corresponding minimum barrier thickness is calculated to meet the design goal. RadShield allows control over preshielding, sample point location and material types. Results: A functional GUI package was developed and tested. Examination of sample walls and source distributions yields a maximum percent difference of less than 0.1% between hand-calculated air-kerma rates and RadShield. Conclusion: The initial results demonstrated that RadShield calculates air-kerma rates and required barrier thicknesses with reliable accuracy and can be used to make radiation shielding design more efficient and accurate. This newly developed approach differs from conventional calculation methods in that it finds air-kerma rates and thickness requirements for many points outside the barriers, stores the information and selects the largest value needed to comply with NCRP Report 147 design goals. Floor plans, parameters, designs and reports can be saved and accessed later for modification and recalculation. We have confirmed that this software accurately calculates air-kerma rates and required barrier thicknesses for diagnostic radiography and fluoroscopic rooms.« less

Mechanistic differences between methanol and dimethyl ether carbonylation in side pockets and large channels of mordenite.

PubMed

Boronat, Mercedes; Martínez, Cristina; Corma, Avelino

2011-02-21

The activity and selectivity towards carbonylation presented by Brønsted acid sites located inside the 8MR pockets or in the main 12MR channels of mordenite is studied by means of quantum-chemical calculations, and the mechanistic differences between methanol and DME carbonylation are investigated. The selectivity towards carbonylation is higher inside the 8MR pockets, where the competitive formation of DME and hydrocarbons that finally leads to catalyst deactivation is sterically impeded. Moreover, inclusion of dispersion interactions in the calculations leads to agreement between the calculated activation barriers for the rate determining step and the experimentally observed higher reactivity of methoxy groups located inside the 8MR channels.

Nitrogen-doped fullerene as a potential catalyst for hydrogen fuel cells.

PubMed

Gao, Feng; Zhao, Guang-Lin; Yang, Shizhong; Spivey, James J

2013-03-06

We examine the possibility of nitrogen-doped C60 fullerene (N-C60) as a cathode catalyst for hydrogen fuel cells. We use first-principles spin-polarized density functional theory calculations to simulate the electrocatalytic reactions on N-C60. The first-principles results show that an O2 molecule can be adsorbed and partially reduced on the N-C complex sites (Pauling sites) of N-C60 without any activation barrier. Through a direct pathway, the partially reduced O2 can further react with H(+) and additional electrons and complete the water formation reaction (WFR) with no activation energy barrier. In the indirect pathway, reduced O2 reacts with H(+) and additional electrons to form H2O molecules through a transition state (TS) with a small activation barrier (0.22-0.37 eV). From an intermediate state to a TS, H(+) can obtain a kinetic energy of ∼0.95-3.68 eV, due to the Coulomb electric interaction, and easily overcome the activation energy barrier during the WFR. The full catalytic reaction cycles can be completed energetically, and N-C60 fullerene recovers to its original structure for the next catalytic reaction cycle. N-C60 fullerene is a potential cathode catalyst for hydrogen fuel cells.

Comparison of the active-site design of molybdenum oxo-transfer enzymes by quantum mechanical calculations.

PubMed

Li, Jilai; Ryde, Ulf

2014-11-17

There are three families of mononuclear molybdenum enzymes that catalyze oxygen atom transfer (OAT) reactions, named after a typical example from each family, viz., dimethyl sulfoxide reductase (DMSOR), sulfite oxidase (SO), and xanthine oxidase (XO). These families differ in the construction of their active sites, with two molybdopterin groups in the DMSOR family, two oxy groups in the SO family, and a sulfido group in the XO family. We have employed density functional theory calculations on cluster models of the active sites to understand the selection of molybdenum ligands in the three enzyme families. Our calculations show that the DMSOR active site has a much stronger oxidative power than the other two sites, owing to the extra molybdopterin ligand. However, the active sites do not seem to have been constructed to make the OAT reaction as exergonic as possible, but instead to keep the reaction free energy close to zero (to avoid excessive loss of energy), thereby making the reoxidation (SO and XO) or rereduction of the active sites (DMSOR) after the OAT reaction facile. We also show that active-site models of the three enzyme families can all catalyze the reduction of DMSO and that the DMSOR model does not give the lowest activation barrier. Likewise, all three models can catalyze the oxidation of sulfite, provided that the Coulombic repulsion between the substrate and the enzyme model can be overcome, but for this harder reaction, the SO model gives the lowest activation barrier, although the differences are not large. However, only the XO model can catalyze the oxidation of xanthine, owing to its sulfido ligand.

Compound Event Barrier Coverage in Wireless Sensor Networks under Multi-Constraint Conditions.

PubMed

Zhuang, Yaoming; Wu, Chengdong; Zhang, Yunzhou; Jia, Zixi

2016-12-24

It is important to monitor compound event by barrier coverage issues in wireless sensor networks (WSNs). Compound event barrier coverage (CEBC) is a novel coverage problem. Unlike traditional ones, the data of compound event barrier coverage comes from different types of sensors. It will be subject to multiple constraints under complex conditions in real-world applications. The main objective of this paper is to design an efficient algorithm for complex conditions that can combine the compound event confidence. Moreover, a multiplier method based on an active-set strategy (ASMP) is proposed to optimize the multiple constraints in compound event barrier coverage. The algorithm can calculate the coverage ratio efficiently and allocate the sensor resources reasonably in compound event barrier coverage. The proposed algorithm can simplify complex problems to reduce the computational load of the network and improve the network efficiency. The simulation results demonstrate that the proposed algorithm is more effective and efficient than existing methods, especially in the allocation of sensor resources.

Compound Event Barrier Coverage in Wireless Sensor Networks under Multi-Constraint Conditions

PubMed Central

Zhuang, Yaoming; Wu, Chengdong; Zhang, Yunzhou; Jia, Zixi

2016-01-01

It is important to monitor compound event by barrier coverage issues in wireless sensor networks (WSNs). Compound event barrier coverage (CEBC) is a novel coverage problem. Unlike traditional ones, the data of compound event barrier coverage comes from different types of sensors. It will be subject to multiple constraints under complex conditions in real-world applications. The main objective of this paper is to design an efficient algorithm for complex conditions that can combine the compound event confidence. Moreover, a multiplier method based on an active-set strategy (ASMP) is proposed to optimize the multiple constraints in compound event barrier coverage. The algorithm can calculate the coverage ratio efficiently and allocate the sensor resources reasonably in compound event barrier coverage. The proposed algorithm can simplify complex problems to reduce the computational load of the network and improve the network efficiency. The simulation results demonstrate that the proposed algorithm is more effective and efficient than existing methods, especially in the allocation of sensor resources. PMID:28029118

Breakup and n -transfer effects on the fusion reactions Li,76+Sn,119120 around the Coulomb barrier

NASA Astrophysics Data System (ADS)

Fisichella, M.; Shotter, A. C.; Figuera, P.; Lubian, J.; Di Pietro, A.; Fernandez-Garcia, J. P.; Ferreira, J. L.; Lattuada, M.; Lotti, P.; Musumarra, A.; Pellegriti, M. G.; Ruiz, C.; Scuderi, V.; Strano, E.; Torresi, D.; Zadro, M.

2017-03-01

This paper presents values of complete fusion cross sections deduced from activation measurements for the reactions 6Li+120Sn and 7Li+119Sn , and for a projectile energy range from 17.5 to 28 MeV in the center-of-mass system. A new deconvolution analysis technique is used to link the basic activation data to the actual fusion excitation function. The complete fusion cross sections above the barrier are suppressed by about 70 % and 85 % with respect to the universal fusion function, used as a standard reference, in the 6Li and 7Li induced reactions, respectively. From a comparison of the excitation functions of the two systems at energies below the barrier, no significant differences can be observed, despite the two systems have different n -transfer Q values. This observation is supported by the results of coupled reaction channels (CRC) calculations.

Computational studies on non-succinimide-mediated stereoinversion mechanism of aspartic acid residues assisted by phosphate

NASA Astrophysics Data System (ADS)

Nakayoshi, Tomoki; Fukuyoshi, Shuichi; Takahashi, Ohgi; Oda, Akifumi

2018-03-01

Although nearly all of the amino acids that constitute proteins are l-amino acids, d-amino acid residues in human proteins have been recently reported. d-amino acid residues cause a change in the three-dimensional structure of proteins, and d-aspartic acid (Asp) residues are considered to be one of the causes of age-related diseases. The stereoinversion of Asp residues in peptides and proteins is thought to proceed via a succinimide intermediate; however, it has been reported that stereoinversion can occur even under conditions where a succinimide intermediate cannot be formed. In order to elucidate the non-succinimide-mediated stereoinversion pathway, we investigated the stereoinversion of l-Asp to d-Asp catalysed by phosphate and estimated the activation barrier using B3LYP/6-31+G(d,p) density functional theory (DFT) calculations. For the DFT calculations, a model compound in which the Asp residue is capped with acetyl and methyl-amino groups on the N- and C-termini, respectively, was used. The calculated activation barrier was not excessively high for the stereoinversion to occur in vivo. Therefore, this stereoinversion mechanism may compete with the succinimide-mediated mechanism.

Theoretical derivation for reaction rate constants of H abstraction from thiophenol by the H/O radical pool

PubMed Central

Batiha, Marwan; Altarawneh, Mohammednoor; Al-Harahsheh, Mohammad; Altarawneh, Ibrahem; Rawadieh, Saleh

2011-01-01

Reaction and activation energy barriers are calculated for the H abstraction reactions (C6H5SH + X• → C6H5S + XH, X = H, OH and HO2) at the BB1K/GTLarge level of theory. The corresponding reactions with H2S and CH3SH are also investigated using the G3B3 and CBS-QB3 methods in order to demonstrate the accuracy of BB1K functional in finding activation barriers for hydrogen atom transfer reactions. Arrhenius parameters for the title reactions are fitted in the temperature range of 300 K–2000 K. The calculated reaction enthalpies are in good agreement with their corresponding experimental reaction enthalpies. It is found that H abstraction by OH radicals from the thiophenol molecule proceed in a much slower rate in reference to the analogous phenol molecule. ΔfH298o of thiophenoxy radical is calculated to be 63.3 kcal/mol. Kinetic parameters presented herein should be useful in describing the decomposition rate of thiophenol; i.e., one of the major aromatic sulfur carriers, at high temperatures. PMID:22485200

Perspective on Diabatic Models of Chemical Reactivity as Illustrated by the Gas-Phase SN2 Reaction of Acetate Ion with 1,2-Dichloroethane

PubMed Central

Valero, Rosendo; Song, Lingchun; Gao, Jiali; Truhlar, Donald G.

2009-01-01

Diabatic models are widely employed for studying chemical reactivity in condensed phases and enzymes, but there has been little discussion of the pros and cons of various diabatic representations for this purpose. Here we discuss and contrast six different schemes for computing diabatic potentials for a charge rearrangement reaction. They include (i) the variational diabatic configurations (VDC) constructed by variationally optimizing individual valence bond structures and (ii) the consistent diabatic configurations (CDC) obtained by variationally optimizing the ground-state adiabatic energy, both in the nonorthogonal molecular orbital valence bond (MOVB) method, along with the orthogonalized (iii) VDC-MOVB and (iv) CDC-MOVB models. In addition, we consider (v) the fourfold way (based on diabatic molecular orbitals and configuration uniformity), and (vi) empirical valence bond (EVB) theory. To make the considerations concrete, we calculate diabatic electronic states and diabatic potential energies along the reaction path that connects the reactant and the product ion-molecule complexes of the gas-phase bimolecular nucleophilic substitution (SN2) reaction of 1,2-dichloethane (DCE) with acetate ion, which is a model reaction corresponding to the reaction catalyzed by haloalkane dehalogenase. We utilize ab initio block-localized molecular orbital theory to construct the MOVB diabatic states and ab initio multi-configuration quasidegenerate perturbation theory to construct the fourfold-way diabatic states; the latter are calculated at reaction path geometries obtained with the M06-2X density functional. The EVB diabatic states are computed with parameters taken from the literature. The MOVB and fourfold-way adiabatic and diabatic potential energy profiles along the reaction path are in qualitative but not quantitative agreement with each other. In order to validate that these wave-function-based diabatic states are qualitatively correct, we show that the reaction energy and barrier for the adiabatic ground state, obtained with these methods, agree reasonably well with the results of high-level calculations using the composite G3SX and G3SX(MP3) methods and the BMC-CCSD multi-coefficient correlation method. However, a comparison of the EVB gas-phase adiabatic ground-state reaction path with those obtained from MOVB and with the fourfold way reveals that the EVB reaction path geometries show a systematic shift towards the products region, and that the EVB lowest-energy path has a much lower barrier. The free energies of solvation and activation energy in water reported from dynamical calculations based on EVB also imply a low activation barrier in the gas phase. In addition, calculations of the free energy of solvation using the recently proposed SM8 continuum solvation model with CM4M partial atomic charges lead to an activation barrier in reasonable agreement with experiment only when the geometries and the gas-phase barrier are those obtained from electronic structure calculations, i.e., methods i–v. These comparisons show the danger of basing the diabatic states on molecular mechanics without the explicit calculation of electronic wave functions. Furthermore, comparison of schemes i–v with one another shows that significantly different quantitative results can be obtained by using different methods for extracting diabatic states from wave function calculations, and it is important for each user to justify the choice of diabatization method in the context of its intended use. PMID:20047005

Quantum dynamics of hydrogen atoms on graphene. II. Sticking.

PubMed

Bonfanti, Matteo; Jackson, Bret; Hughes, Keith H; Burghardt, Irene; Martinazzo, Rocco

2015-09-28

Following our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface [Bonfanti et al., J. Chem. Phys. 143, 124703 (2015)], we present the results of converged quantum scattering calculations on the activated sticking dynamics. The focus of this study is the collinear scattering on a surface at zero temperature, which is treated with high-dimensional wavepacket propagations with the multi-configuration time-dependent Hartree method. At low collision energies, barrier-crossing dominates the sticking and any projectile that overcomes the barrier gets trapped in the chemisorption well. However, at high collision energies, energy transfer to the surface is a limiting factor, and fast H atoms hardly dissipate their excess energy and stick on the surface. As a consequence, the sticking coefficient is maximum (∼0.65) at an energy which is about one and half larger than the barrier height. Comparison of the results with classical and quasi-classical calculations shows that quantum fluctuations of the lattice play a primary role in the dynamics. A simple impulsive model describing the collision of a classical projectile with a quantum surface is developed which reproduces the quantum results remarkably well for all but the lowest energies, thereby capturing the essential physics of the activated sticking dynamics investigated.

Quantum dynamics of hydrogen atoms on graphene. II. Sticking

NASA Astrophysics Data System (ADS)

Bonfanti, Matteo; Jackson, Bret; Hughes, Keith H.; Burghardt, Irene; Martinazzo, Rocco

2015-09-01

Following our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface [Bonfanti et al., J. Chem. Phys. 143, 124703 (2015)], we present the results of converged quantum scattering calculations on the activated sticking dynamics. The focus of this study is the collinear scattering on a surface at zero temperature, which is treated with high-dimensional wavepacket propagations with the multi-configuration time-dependent Hartree method. At low collision energies, barrier-crossing dominates the sticking and any projectile that overcomes the barrier gets trapped in the chemisorption well. However, at high collision energies, energy transfer to the surface is a limiting factor, and fast H atoms hardly dissipate their excess energy and stick on the surface. As a consequence, the sticking coefficient is maximum (˜0.65) at an energy which is about one and half larger than the barrier height. Comparison of the results with classical and quasi-classical calculations shows that quantum fluctuations of the lattice play a primary role in the dynamics. A simple impulsive model describing the collision of a classical projectile with a quantum surface is developed which reproduces the quantum results remarkably well for all but the lowest energies, thereby capturing the essential physics of the activated sticking dynamics investigated.

Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides

NASA Astrophysics Data System (ADS)

Vassilev, Nikolay G.; Dimitrov, Valentin S.

2003-06-01

The free energies of activation for rotation about the thiocarbonyl C-N bond in X-C(S)N(CH 3) 2 (X=H, F, Cl, CH 3, CF 3) were calculated at the MP2(fc)/6-31+G*//6-31G* and MP2(fc)/6-311++G**//6-311++G** levels and compared with literature NMR gas-phase data. The results of calculations indicate that the nonbonded interactions in ground state (GS) are mainly responsible for the differences in the rotational barriers. For X=H, CH 3 and CF 3, the anti transition state (TS) is more stable; for the case X=Cl, the syn TS is more stable, while for the X=F, the two TS are energetically almost equivalent.

The Stress-Dependent Activation Parameters for Dislocation Nucleation in Molybdenum Nanoparticles.

PubMed

Chachamovitz, Doron; Mordehai, Dan

2018-03-02

Many specimens at the nanoscale are pristine of dislocations, line defects which are the main carriers of plasticity. As a result, they exhibit extremely high strengths which are dislocation-nucleation controlled. Since nucleation is a thermally activated process, it is essential to quantify the stress-dependent activation parameters for dislocation nucleation in order to study the strength of specimens at the nanoscale and its distribution. In this work, we calculate the strength of Mo nanoparticles in molecular dynamics simulations and we propose a method to extract the activation free-energy barrier for dislocation nucleation from the distribution of the results. We show that by deforming the nanoparticles at a constant strain rate, their strength distribution can be approximated by a normal distribution, from which the activation volumes at different stresses and temperatures are calculated directly. We found that the activation energy dependency on the stress near spontaneous nucleation conditions obeys a power-law with a critical exponent of approximately 3/2, which is in accordance with critical exponents found in other thermally activated processes but never for dislocation nucleation. Additionally, significant activation entropies were calculated. Finally, we generalize the approach to calculate the activation parameters for other driving-force dependent thermally activated processes.

Thermal activation of dislocations in large scale obstacle bypass

NASA Astrophysics Data System (ADS)

Sobie, Cameron; Capolungo, Laurent; McDowell, David L.; Martinez, Enrique

2017-08-01

Dislocation dynamics simulations have been used extensively to predict hardening caused by dislocation-obstacle interactions, including irradiation defect hardening in the athermal case. Incorporating the role of thermal energy on these interactions is possible with a framework provided by harmonic transition state theory (HTST) enabling direct access to thermally activated reaction rates using the Arrhenius equation, including rates of dislocation-obstacle bypass processes. Moving beyond unit dislocation-defect reactions to a representative environment containing a large number of defects requires coarse-graining the activation energy barriers of a population of obstacles into an effective energy barrier that accurately represents the large scale collective process. The work presented here investigates the relationship between unit dislocation-defect bypass processes and the distribution of activation energy barriers calculated for ensemble bypass processes. A significant difference between these cases is observed, which is attributed to the inherent cooperative nature of dislocation bypass processes. In addition to the dislocation-defect interaction, the morphology of the dislocation segments pinned to the defects play an important role on the activation energies for bypass. A phenomenological model for activation energy stress dependence is shown to describe well the effect of a distribution of activation energies, and a probabilistic activation energy model incorporating the stress distribution in a material is presented.

Calculation of the Schottky barrier and current–voltage characteristics of metal–alloy structures based on silicon carbide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Altuhov, V. I., E-mail: altukhovv@mail.ru; Kasyanenko, I. S.; Sankin, A. V.

2016-09-15

A simple but nonlinear model of the defect density at a metal–semiconductor interface, when a Schottky barrier is formed by surface defects states localized at the interface, is developed. It is shown that taking the nonlinear dependence of the Fermi level on the defect density into account leads to a Schottky barrier increase by 15–25%. The calculated barrier heights are used to analyze the current–voltage characteristics of n-M/p-(SiC){sub 1–x}(AlN){sub x} structures. The results of calculations are compared to experimental data.

Experimental evaluation of optimization method for developing ultraviolet barrier coatings

NASA Astrophysics Data System (ADS)

Gonome, Hiroki; Okajima, Junnosuke; Komiya, Atsuki; Maruyama, Shigenao

2014-01-01

Ultraviolet (UV) barrier coatings can be used to protect many industrial products from UV attack. This study introduces a method of optimizing UV barrier coatings using pigment particles. The radiative properties of the pigment particles were evaluated theoretically, and the optimum particle size was decided from the absorption efficiency and the back-scattering efficiency. UV barrier coatings were prepared with zinc oxide (ZnO) and titanium dioxide (TiO2). The transmittance of the UV barrier coating was calculated theoretically. The radiative transfer in the UV barrier coating was modeled using the radiation element method by ray emission model (REM2). In order to validate the calculated results, the transmittances of these coatings were measured by a spectrophotometer. A UV barrier coating with a low UV transmittance and high VIS transmittance could be achieved. The calculated transmittance showed a similar spectral tendency with the measured one. The use of appropriate particles with optimum size, coating thickness and volume fraction will result in effective UV barrier coatings. UV barrier coatings can be achieved by the application of optical engineering.

Child Goal Setting of Dietary and Physical Activity in a Serious Videogame.

PubMed

Simons, Monique; Baranowski, Janice; Thompson, Debbe; Buday, Richard; Abdelsamad, Dina; Baranowski, Tom

2013-06-01

To inform child obesity prevention programs, the current article identified what children thought were the most important goals, values, and perceived barriers related to healthy eating and physical activity (PA) within a serious videogame for health, "Escape from Diab" (Archimage Inc., Houston, TX). One hundred three children, 10-12 years of age, played "Escape from Diab." During game play the children were presented with a menu of goals, values, and barriers from which they selected the ones most important to them. The children's selections were transmitted to a central server and stored in a database. Frequencies were calculated and reported. The most important diet-related values and reasons for children were getting good grades and being healthy and fit. The most often reported barrier for fruit intake was that it does not fill you up, and for vegetable intake it was that availability at home was limited. Also, limited availability of bottled water at home was an often chosen barrier. PA-related important values and reasons were not missing school and having energy to do homework. Children preferred to limit sedentary activities for only 30 minutes rather than for 60 minutes. The most frequently mentioned barrier for reducing inactivity was "feeling too tired to do anything else." These findings provide important input for future obesity prevention videogames attempting to motivate children to set healthy diet and PA goals.

Direct mapping of the angle-dependent barrier to reaction for Cl + CHD3 using polarized scattering data

NASA Astrophysics Data System (ADS)

Pan, Huilin; Wang, Fengyan; Czakó, Gábor; Liu, Kopin

2017-12-01

The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD3(v1 = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products—the imprints of small impact-parameter collisions. The deduced bend potential at the transition state agrees with ab initio calculations. We expect that the method should be applicable to many other direct reactions with a collinear barrier.

A Computational Methodology to Screen Activities of Enzyme Variants

PubMed Central

Hediger, Martin R.; De Vico, Luca; Svendsen, Allan; Besenmatter, Werner; Jensen, Jan H.

2012-01-01

We present a fast computational method to efficiently screen enzyme activity. In the presented method, the effect of mutations on the barrier height of an enzyme-catalysed reaction can be computed within 24 hours on roughly 10 processors. The methodology is based on the PM6 and MOZYME methods as implemented in MOPAC2009, and is tested on the first step of the amide hydrolysis reaction catalyzed by the Candida Antarctica lipase B (CalB) enzyme. The barrier heights are estimated using adiabatic mapping and shown to give barrier heights to within 3 kcal/mol of B3LYP/6-31G(d)//RHF/3-21G results for a small model system. Relatively strict convergence criteria (0.5 kcal/(molÅ)), long NDDO cutoff distances within the MOZYME method (15 Å) and single point evaluations using conventional PM6 are needed for reliable results. The generation of mutant structures and subsequent setup of the semiempirical calculations are automated so that the effect on barrier heights can be estimated for hundreds of mutants in a matter of weeks using high performance computing. PMID:23284627

A density functional theory study on the carbon chain growth of ethanol formation on Cu-Co (111) and (211) surfaces

NASA Astrophysics Data System (ADS)

Ren, Bohua; Dong, Xiuqin; Yu, Yingzhe; Wen, Guobin; Zhang, Minhua

2017-08-01

Calculations based on the first-principle density functional theory were carried out to study the most controversial reactions in ethanol formation from syngas on Cu-Co surfaces: CO dissociation mechanism and the key reactions of carbon chain growth of ethanol formation (HCO insertion reactions) on four model surfaces (Cu-Co (111) and (211) with Cu-rich or Co-rich surfaces) to investigate the synergy of the Cu and Co components since the complete reaction network of ethanol formation from syngas is a huge computational burden to calculate on four Cu-Co surface models. We investigated adsorption of important species involved in these reactions, activation barrier and reaction energy of H-assisted dissociation mechanism, directly dissociation of CO, and HCO insertion reactions (CHx + HCO → CHxCHO (x = 1-3)) on four Cu-Co surface models. It was found that reactions on Cu-rich (111) and (211) surfaces all have lower activation barrier in H-assisted dissociation and HCO insertion reactions, especially CH + HCO → CHCHO reaction. The PDOS of 4d orbitals of surface Cu and Co atoms of all surfaces were studied. Analysis of d-band center of Cu and Co atoms and the activation barrier data suggested the correlation between electronic property and catalytic performance. Cu-Co bimetallic with Cu-rich surface allows Co to have higher catalytic activity through the interaction of Cu and Co atom. Then it will improve the adsorption of CO and catalytic activity of Co. Thus it is more favorable to the carbon chain growth in ethanol formation. Our study revealed the factors influencing the carbon chain growth in ethanol production and explained the internal mechanism from electronic property aspect.

Determining collective barrier operation skew in a parallel computer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Faraj, Daniel A.

2015-11-24

Determining collective barrier operation skew in a parallel computer that includes a number of compute nodes organized into an operational group includes: for each of the nodes until each node has been selected as a delayed node: selecting one of the nodes as a delayed node; entering, by each node other than the delayed node, a collective barrier operation; entering, after a delay by the delayed node, the collective barrier operation; receiving an exit signal from a root of the collective barrier operation; and measuring, for the delayed node, a barrier completion time. The barrier operation skew is calculated by:more » identifying, from the compute nodes' barrier completion times, a maximum barrier completion time and a minimum barrier completion time and calculating the barrier operation skew as the difference of the maximum and the minimum barrier completion time.« less

Determining collective barrier operation skew in a parallel computer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Faraj, Daniel A.

Determining collective barrier operation skew in a parallel computer that includes a number of compute nodes organized into an operational group includes: for each of the nodes until each node has been selected as a delayed node: selecting one of the nodes as a delayed node; entering, by each node other than the delayed node, a collective barrier operation; entering, after a delay by the delayed node, the collective barrier operation; receiving an exit signal from a root of the collective barrier operation; and measuring, for the delayed node, a barrier completion time. The barrier operation skew is calculated by:more » identifying, from the compute nodes' barrier completion times, a maximum barrier completion time and a minimum barrier completion time and calculating the barrier operation skew as the difference of the maximum and the minimum barrier completion time.« less

76 FR 38355 - Agency Information Collection Activities: Proposed Collection; Comment Request; Generic Clearance...

Federal Register 2010, 2011, 2012, 2013, 2014

2011-06-30

... calculations that justify the proposed sample size, the expected response rate, methods for assessing potential... of Qualitative Feedback on Agency Service Delivery AGENCY: Architectural and Transportation Barriers...: ``Generic Clearance for the Collection of Qualitative Feedback on Agency Service Delivery'' to the Office of...

Theoretical characterization of the potential energy surface for NH + NO

NASA Technical Reports Server (NTRS)

Walch, Stephen P.

1992-01-01

The potential energy surface (PES) for NH + NO was characterized using complete active space self-consistent field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration interaction (CCI) calculations to refine the energetics. The present results are in qualitative accord with the BAC-MP4 calculations, but there are differences as large as 8 kcal/mol in the detailed energetics. Addition of NH to NO on a (2)A' surface, which correlated with N2 + OH or H + N2O products, involves barriers of 3.2 kcal/mol (trans) and 6.3 kcal/mol (cis). Experimental evidence for these barriers is found in earlier works. The (2)A' surface has no barrier to addition, but does not correlate with products. Surface crossings between the barrierless (2)A' surface and the (2)A' surface may be important. Production of N2 + OH products is predicted to occur via a planar saddle point of (2)A' symmetry. This is in accord with the preferential formation of II(A') lambda doublet levels of OH in earlier experiments. Addition of NH (1)delta to NO is found to occur on an excited state surface and is predicted to lead to N2O product as observed in earlier works.

Quantum Chemical Calculations of Amine-Catalyzed Polymerization of Silanol

NASA Astrophysics Data System (ADS)

Gu, Hongyu; Xu, Wenbin; Zhang, Jinlin; Qi, Zhenyi; Zhang, Tao; Song, Lixin

2018-03-01

Because of the technical importance of organosilicon materials, insight into the related synthetic processes is significantly essential. In this paper, the amine-catalyzed polymerization of silanol has been investigated by the density functional theory (DFT) method. Our data have shown that amines can catalytically promote the hydrogen transfer process by substantially reducing the energy barrier. The activation barrier via hydrogen transfer with catalysis is 38.32 kJ/mol, much lower than that of catalysis-free process (120.88 kJ/mol). The lower energy barrier is in agreement with the much more intense polymerization of silanols with amine catalysts. Based on the above results, amines and other catalysts capable of assisting hydrogen transfer are expected to be used as catalysts for silanol polymerization.

Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a Rh(II) metalloradical: a combined experimental and theoretical study.

PubMed

Chan, Kin Shing; Li, Xin Zhu; Dzik, Wojciech I; de Bruin, Bas

2008-02-13

Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [Rh(II)(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh(III)(tmp)Me] (3) and [Rh(III)(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh(II)(tmp)] is regenerated from [Rh(III)(tmp)H] with formation of 2,2,6,6-tetramethyl-piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [Rh(III)(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the Rh(II) radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [Rh(II)(por)] producing the [TEMPO]+ [RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh(I) with formation of [Rh(III)(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3,4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh(I)(por)]- at one of the C(Me)-C(ring) sigma* orbitals of [TEMPO]+. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (SN2) pathway have barriers which are low enough to explain the experimental kinetic data.

AC electrical conductivity and dielectric relaxation studies on n-type organic thin films of N, N‧-Dimethyl-3,4,9,10-perylenedicarboximide (DMPDC)

NASA Astrophysics Data System (ADS)

Qashou, Saleem I.; Darwish, A. A. A.; Rashad, M.; Khattari, Z.

2017-11-01

Both Alternating current (AC) conductivity and dielectric behavior of n-type organic thin films of N, N‧-Dimethyl-3,4,9,10-perylenedicarboximide (DMPDC) have been investigated. Fourier transformation infrared (FTIR) spectroscopy is used for identifying both powder and film bonds which confirm that there are no observed changes in the bonds between the DMPDC powder and evaporated films. The dependence of AC conductivity on the temperature for DMPDC evaporated films was explained by the correlated barrier hopping (CBH) model. The calculated barrier height using CBH model shows a decreasing behavior with increasing temperature. The mechanism of dielectric relaxation was interpreted on the basis of the modulus of the complex dielectric. The calculated activation energy of the relaxation process was found to be 0.055 eV.

Direct carbon-carbon coupling of furanics with acetic acid over Brønsted zeolites

PubMed Central

Gumidyala, Abhishek; Wang, Bin; Crossley, Steven

2016-01-01

Effective carbon-carbon coupling of acetic acid to form larger products while minimizing CO2 emissions is critical to achieving a step change in efficiency for the production of transportation fuels from sustainable biomass. We report the direct acylation of methylfuran with acetic acid in the presence of water, all of which can be readily produced from biomass. This direct coupling limits unwanted polymerization of furanics while producing acetyl methylfuran. Reaction kinetics and density functional theory calculations illustrate that the calculated apparent barrier for the dehydration of the acid to form surface acyl species is similar to the experimentally measured barrier, implying that this step plays a significant role in determining the net reaction rate. Water inhibits the overall rate, but selectivity to acylated products is not affected. We show that furanic species effectively stabilize the charge of the transition state, therefore lowering the overall activation barrier. These results demonstrate a promising new route to C–C bond–forming reactions for the production of higher-value products from biomass. PMID:27652345

Direct carbon-carbon coupling of furanics with acetic acid over Bronsted zeolites

DOE PAGES

Gumidyala, Abhishek; Wang, Bin; Crossley, Steven

2016-09-16

Effective carbon-carbon coupling of acetic acid to form larger products while minimizing CO 2 emissions is critical to achieving a step change in efficiency for the production of transportation fuels from sustainable biomass. Here, we report the direct acylation of methylfuran with acetic acid in the presence ofwater, all ofwhich can be readily produced from biomass. This direct coupling limits unwanted polymerization of furanics while producing acetyl methylfuran. Reaction kinetics and density functional theory calculations illustrate that the calculated apparent barrier for the dehydration of the acid to form surface acyl species is similar to the experimentally measured barrier, implyingmore » that this step plays a significant role in determining the net reaction rate. Water inhibits the overall rate, but selectivity to acylated products is not affected.We show that furanic species effectively stabilize the charge of the transition state, therefore lowering the overall activation barrier. These results demonstrate a promising new route to C–C bond–forming reactions for the production of higher-value products from biomass.« less

Poster - 11: Radiation barrier thickness calculations for the GammaPod

DOE Office of Scientific and Technical Information (OSTI.GOV)

La Russa, Daniel; Vandervoort, Eric; Wilkins, Davi

A consortium of radiotherapy centers in North America is in the process of evaluating a novel new {sup 60}Co teletherapy device, called the GammaPod™ (Xcision Medical Systems, Columbia Maryland), designed specifically for breast SBRT. The GammaPod consists of 36 collimated {sup 60}Co sources with a total activity of 4320 Ci. The sources are housed in a hemispherical source carrier that rotates during treatment to produce a cylindrically symmetric cone of primary beam spanning 16° – 54° degrees from the horizontal. This unique beam geometry presents challenges when designing or evaluating room shielding for the purposes of meeting regulatory requirements, andmore » for ensuring the safety of staff and the public in surrounding areas. Conventional methods for calculating radiation barrier thicknesses have been adapted so that barrier transmission factors for the GammaPod can be determined from a few relevant distances and characteristics of the primary beam. Simple formalisms have been determined for estimating shielding requirements for primary radiation (with a rotating and non-rotating source carrier), patient-scattered radiation, and leakage radiation. When making worst case assumptions, it was found that conventional barrier thicknesses associated with linac treatment suites are sufficient for shielding all sources of radiation from the GammaPod.« less

Doubly charged coronene clusters—Much smaller than previously observed

NASA Astrophysics Data System (ADS)

Mahmoodi-Darian, Masoomeh; Raggl, Stefan; Renzler, Michael; Goulart, Marcelo; Huber, Stefan E.; Mauracher, Andreas; Scheier, Paul; Echt, Olof

2018-05-01

The smallest doubly charged coronene cluster ions reported so far, Cor152+, were produced by charge exchange between bare coronene clusters and He2+ [H. A. B. Johansson et al., Phys. Rev. A 84, 043201 (2011)]. These dications are at least five times larger than the estimated Rayleigh limit, i.e., the size at which the activation barrier for charge separation vanishes. Such a large discrepancy is unheard of for doubly charged atomic or molecular clusters. Here we report the mass spectrometric observation of doubly charged coronene trimers, produced by electron ionization of helium nanodroplets doped with coronene. The observation implies that Cor32+ features a non-zero fission barrier too large to overcome under the present experimental conditions. The height of the barriers for the dimer and trimer has been estimated by means of density functional theory calculations. A sizeable barrier for the trimer has been revealed in agreement with the experimental findings.

Calculating Transition Energy Barriers and Characterizing Activation States for Steps of Fusion

PubMed Central

Ryham, Rolf J.; Klotz, Thomas S.; Yao, Lihan; Cohen, Fredric S.

2016-01-01

We use continuum mechanics to calculate an entire least energy pathway of membrane fusion, from stalk formation, to pore creation, and through fusion pore enlargement. The model assumes that each structure in the pathway is axially symmetric. The static continuum stalk structure agrees quantitatively with experimental stalk architecture. Calculations show that in a stalk, the distal monolayer is stretched and the stored stretching energy is significantly less than the tilt energy of an unstretched distal monolayer. The string method is used to determine the energy of the transition barriers that separate intermediate states and the dynamics of two bilayers as they pass through them. Hemifusion requires a small amount of energy independently of lipid composition, while direct transition from a stalk to a fusion pore without a hemifusion intermediate is highly improbable. Hemifusion diaphragm expansion is spontaneous for distal monolayers containing at least two lipid components, given sufficiently negative diaphragm spontaneous curvature. Conversely, diaphragms formed from single-component distal monolayers do not expand without the continual injection of energy. We identify a diaphragm radius, below which central pore expansion is spontaneous. For larger diaphragms, prior studies have shown that pore expansion is not axisymmetric, and here our calculations supply an upper bound for the energy of the barrier against pore formation. The major energy-requiring deformations in the steps of fusion are: widening of a hydrophobic fissure in bilayers for stalk formation, splay within the expanding hemifusion diaphragm, and fissure widening initiating pore formation in a hemifusion diaphragm. PMID:26958888

Kinetics of nitric oxide and oxygen gases on porous Y-stabilized ZrO2-based sensors.

PubMed

Killa, Sajin; Cui, Ling; Murray, Erica P; Mainardi, Daniela S

2013-08-16

Using impedance spectroscopy the electrical response of sensors with various porous Y-stabilized ZrO2 (YSZ) microstructures was measured for gas concentrations containing 0-100 ppm NO with 10.5%O2 at temperatures ranging from 600-700 °C. The impedance response increased substantially as the sensor porosity increased from 46%-50%. Activation energies calculated based on data from the impedance measurements increased in magnitude (97.4-104.9 kJ/mol for 100 ppm NO) with respect to increasing YSZ porosity. Analysis of the oxygen partial pressure dependence of the sensors suggested that dissociative adsorption was the dominant rate limiting. The PWC/DNP theory level was used to investigate the gas-phase energy barrier of the 2NO+O2 → 2NO2 reaction on a 56-atom YSZ/Au model cluster using Density Functional Theory and Linear Synchronous Transit/Quadratic Synchronous Transit calculations. The reaction path shows oxygen surface reactions that begin with NO association with adsorbed O2 on a Zr surface site, followed by O2 dissociative adsorption, atomic oxygen diffusion, and further NO2 formation. The free energy barrier was calculated to be 181.7 kJ/mol at PWC/DNP. A qualitative comparison with the extrapolated data at 62% ± 2% porosity representing the YSZ model cluster indicates that the calculated barriers are in reasonable agreement with experiments, especially when the RPBE functional is used.

Mechanistic insights into heterogeneous methane activation

DOE PAGES

Latimer, Allegra A.; Aljama, Hassan; Kakekhani, Arvin; ...

2017-01-11

While natural gas is an abundant chemical fuel, its low volumetric energy density has prompted a search for catalysts able to transform methane into more useful chemicals. This search has often been aided through the use of transition state (TS) scaling relationships, which estimate methane activation TS energies as a linear function of a more easily calculated descriptor, such as final state energy, thus avoiding tedious TS energy calculations. It has been shown that methane can be activated via a radical or surface-stabilized pathway, both of which possess a unique TS scaling relationship. Herein, we present a simple model tomore » aid in the prediction of methane activation barriers on heterogeneous catalysts. Analogous to the universal radical TS scaling relationship introduced in a previous publication, we show that a universal TS scaling relationship that transcends catalysts classes also seems to exist for surface-stabilized methane activation if the relevant final state energy is used. We demonstrate that this scaling relationship holds for several reducible and irreducible oxides, promoted metals, and sulfides. By combining the universal scaling relationships for both radical and surface-stabilized methane activation pathways, we show that catalyst reactivity must be considered in addition to catalyst geometry to obtain an accurate estimation for the TS energy. Here, this model can yield fast and accurate predictions of methane activation barriers on a wide range of catalysts, thus accelerating the discovery of more active catalysts for methane conversion.« less

Mechanistic insights into heterogeneous methane activation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Latimer, Allegra A.; Aljama, Hassan; Kakekhani, Arvin

While natural gas is an abundant chemical fuel, its low volumetric energy density has prompted a search for catalysts able to transform methane into more useful chemicals. This search has often been aided through the use of transition state (TS) scaling relationships, which estimate methane activation TS energies as a linear function of a more easily calculated descriptor, such as final state energy, thus avoiding tedious TS energy calculations. It has been shown that methane can be activated via a radical or surface-stabilized pathway, both of which possess a unique TS scaling relationship. Herein, we present a simple model tomore » aid in the prediction of methane activation barriers on heterogeneous catalysts. Analogous to the universal radical TS scaling relationship introduced in a previous publication, we show that a universal TS scaling relationship that transcends catalysts classes also seems to exist for surface-stabilized methane activation if the relevant final state energy is used. We demonstrate that this scaling relationship holds for several reducible and irreducible oxides, promoted metals, and sulfides. By combining the universal scaling relationships for both radical and surface-stabilized methane activation pathways, we show that catalyst reactivity must be considered in addition to catalyst geometry to obtain an accurate estimation for the TS energy. Here, this model can yield fast and accurate predictions of methane activation barriers on a wide range of catalysts, thus accelerating the discovery of more active catalysts for methane conversion.« less

The barrier for proton transport in aquaporins as a challenge for electrostatic models: the role of protein relaxation in mutational calculations.

PubMed

Kato, Mitsunori; Pisliakov, Andrei V; Warshel, Arieh

2006-09-01

The origin of the barrier for proton transport through the aquaporin channel is a problem of general interest. It is becoming increasingly clear that this barrier is not attributable to the orientation of the water molecules across the channel but rather to the electrostatic penalty for moving the proton charge to the center of the channel. However, the reason for the high electrostatic barrier is still rather controversial. It has been argued by some workers that the barrier is due to the so-called NPA motif and/or to the helix macrodipole or to other specific elements. However, our works indicated that the main reason for the high barrier is the loss of the generalized solvation upon moving the proton charge from the bulk to the center of the channel and that this does not reflect a specific repulsive electrostatic interaction but the absence of sufficient electrostatic stabilization. At this stage it seems that the elucidation and clarification of the origin of the electrostatic barrier can serve as an instructive test case for electrostatic models. Thus, we reexamine the free-energy surface for proton transport in aquaporins using the microscopic free-energy perturbation/umbrella sampling (FEP/US) and the empirical valence bond/umbrella sampling (EVB/US) methods as well as the semimacroscopic protein dipole Langevin dipole model in its linear response approximation version (the PDLD/S-LRA). These extensive studies help to clarify the nature of the barrier and to establish the "reduced solvation effect" as the primary source of this barrier. That is, it is found that the barrier is associated with the loss of the generalized solvation energy (which includes of course all electrostatic effects) upon moving the proton charge from the bulk solvent to the center of the channel. It is also demonstrated that the residues in the NPA region and the helix dipole cannot be considered as the main reasons for the electrostatic barrier. Furthermore, our microscopic and semimacroscopic studies clarify the problems with incomplete alternative calculations, illustrating that the effects of various electrostatic elements are drastically overestimated by macroscopic calculations that use a low dielectric constant and do not consider the protein reorganization. Similarly, it is pointed out that microscopic potential of mean force calculations that do not evaluate the electrostatic barrier relative to the bulk water cannot be used to establish the origin of the electrostatic barrier. The relationship between the present study and calculations of pK(a)s in protein interiors is clarified, pointing out that approaches that are applied to study the aquaporin barrier should be validated by pK(a)s calculations. Such calculations also help to clarify the crucial role of solvation energies in establishing the barrier in aquaporins. (c) 2006 Wiley-Liss, Inc.

High-Efficiency and High-Power Mid-Wave Infrared Cascade Lasers

DTIC Science & Technology

2009-08-01

marked “*”, indicates the first barrier for the design sequence detailed in the main text. The calculation is for an applied electric field of 82 kV...injector regions, consisting of four quantum wells between each set of active regions. The calculation takes the free carrier density into account through a...28: Effective transit time as a function of the dimensionless coupling parameter ( uc ττ⊥Ω4 ) for the structure shown in Fig. 25. 41 Fig. 29: (a

Multidimensional structure of a questionnaire to assess barriers to and motivators of physical activity in recipients of solid organ transplantation.

PubMed

van Adrichem, Edwin J; Krijnen, Wim P; Dekker, Rienk; Ranchor, Adelita V; Dijkstra, Pieter U; van der Schans, Cees P

2017-11-01

To explore the underlying dimensions of the Barriers and Motivators Questionnaire that is used to assess barriers to and motivators of physical activity experienced by recipients of solid organ transplantation and thereby improve the application in research and clinical settings. A cross-sectional study was performed in recipients of solid organ transplantation (n = 591; median (IQR) age = 59 (49; 66); 56% male). The multidimensional structure of the questionnaire was analyzed by exploratory principal component analysis. Cronbach's α was calculated to determine internal consistency of the entire questionnaire and individual components. The barriers scale had a Cronbach's α of 0.86 and was subdivided into four components; α of the corresponding subscales varied between 0.80 and 0.66. The motivator scale had an α of 0.91 and was subdivided into four components with an α between 0.88 to 0.70. Nine of the original barrier items and two motivator items were not included in the component structure. A four-dimensional structure for both the barriers and motivators scale of the questionnaire is supported. The use of the indicated subscales increases the usability in research and clinical settings compared to the overall scores and provide opportunities to identify modifiable constructs to be targeted in interventions. Implications for rehabilitation Organ transplant recipients are less active than the general population despite established health benefits of physical activity. A multidimensional structure is shown in the Barriers and Motivators Questionnaire, the use of the identified subscales increases applicability in research and clinical settings. The use of the questionnaire with its component structure in the clinical practice of a rehabilitation physician could result in a faster assessment of problem areas in daily practice and result in a higher degree of clarity as opposed to the use of the individual items of the questionnaire.

On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients.

PubMed

Hotop, H; Ruf, M-W; Kopyra, J; Miller, T M; Fabrikant, I I

2011-02-14

Rate coefficients k(T) for dissociative electron attachment (DEA) to molecules in many cases exhibit a more or less strong rise with increasing temperature T (the electron temperature T(e) and the molecular temperature T(G) are assumed to be in thermal equilibrium, i.e., T = T(e) = T(G)). This rise is frequently modeled by the Arrhenius equation k(T) = k(A) exp[-E(a)∕(k(B)T)], and an activation energy E(a) is deduced from fits to the experimental data k(T). This behavior reflects the presence of an energy barrier for the anion on its path to the dissociated products. In a recent paper [J. Kopyra, J. Wnorowska, M. Foryś, and I. Szamrej, Int. J. Mass Spectrom. 268, 60 (2007)] it was suggested that the size of the rate coefficients for DEA reactions at room temperature exhibits an exponential dependence on the activation energy, i.e., k(E(a); T ≈ 300 K) = k(1) exp[-E(a)∕E(0)]. More recent experimental data for molecules with high barriers [T. M. Miller, J. F. Friedman, L. C. Schaffer, and A. A. Viggiano, J. Chem. Phys. 131, 084302 (2009)] are compatible with such a correlation. We investigate the validity and the possible origin of this dependence by analyzing the results of R-matrix calculations for temperature-dependent rate coefficients of exothermic DEA processes with intermediate barrier toward dissociation. These include results for model systems with systematically varied barrier height as well as results of molecule-specific calculations for CH(3)Cl, CH(3)Br, CF(3)Cl, and CH(2)Cl(2) (activation energies above 0.2 eV) involving appropriate molecular parameters. A comparison of the experimental and theoretical results for the considered class of molecules (halogenated alkanes) supports the idea that the exponential dependence of k(T = 300 K) on the activation energy reflects a general phenomenon associated with Franck-Condon factors for getting from the initial neutral vibrational levels to the dissociating final anion state in a direct DEA process. Cases are discussed for which the proposed relation does not apply.

Direct vs. indirect pathway for nitrobenzene reduction reaction on a Ni catalyst surface: a density functional study.

PubMed

Mahata, Arup; Rai, Rohit K; Choudhuri, Indrani; Singh, Sanjay K; Pathak, Biswarup

2014-12-21

Density functional theory (DFT) calculations are performed to understand and address the previous experimental results that showed the reduction of nitrobenzene to aniline prefers direct over indirect reaction pathways irrespective of the catalyst surface. Nitrobenzene to aniline conversion occurs via the hydroxyl amine intermediate (direct pathway) or via the azoxybenzene intermediate (indirect pathway). Through our computational study we calculated the spin polarized and dispersion corrected reaction energies and activation barriers corresponding to various reaction pathways for the reduction of nitrobenzene to aniline over a Ni catalyst surface. The adsorption behaviour of the substrate, nitrobenzene, on the catalyst surface was also considered and the energetically most preferable structural orientation was elucidated. Our study indicates that the parallel adsorption behaviour of the molecules over a catalyst surface is preferable over vertical adsorption behaviour. Based on the reaction energies and activation barrier of the various elementary steps involved in direct or indirect reaction pathways, we find that the direct reduction pathway of nitrobenzene over the Ni(111) catalyst surface is more favourable than the indirect reaction pathway.

Glucose transformation to 5-hydroxymethylfurfural in acidic ionic liquid: A quantum mechanical study.

PubMed

Arifin; Puripat, Maneeporn; Yokogawa, Daisuke; Parasuk, Vudhichai; Irle, Stephan

2016-01-30

Isomerization and transformation of glucose and fructose to 5-hydroxymethylfurfural (HMF) in both ionic liquids (ILs) and water has been studied by the reference interaction site model self-consistent field spatial electron density distribution (RISM-SCF-SEDD) method coupled with ab initio electronic structure theory, namely coupled cluster single, double, and perturbative triple excitation (CCSD(T)). Glucose isomerization to fructose has been investigated via cyclic and open chain mechanisms. In water, the calculations support the cyclic mechanism of glucose isomerization; with the predicted activation free energy is 23.8 kcal mol(-1) at experimental condition. Conversely, open ring mechanism is more favorable in ILs with the energy barrier is 32.4 kcal mol(-1) . Moreover, the transformation of fructose into HMF via cyclic mechanism is reasonable; the calculated activation barriers are 16.0 and 21.5 kcal mol(-1) in aqueous and ILs solutions, respectively. The solvent effects of ILs could be explained by the decomposition of free energies and radial distribution functions of solute-solvent that are produced by RISM-SCF-SEDD. © 2015 Wiley Periodicals, Inc.

Ethylene decomposition over Pt(100): A mechanism study from first principle calculation

NASA Astrophysics Data System (ADS)

Wang, Yuchun; Dong, Xiuqin; Yu, Yingzhe; Zhang, Minhua

2016-12-01

First principle based density functional theory was used to calculate the complete step-by-step decomposition network of ethylene (C2H4) over Pt(100) as a model for understanding the carbon deposition of olefin hydrocarbon over transition metal surface. We discussed the structural and energetic properties of all the Csbnd H and Csbnd C bond cleavage reactions in order to fully understand the formation pathway of carbon monomer. It is easier for Csbnd H bond cleavage reactions to take place, as the activation barrier of these reactions is relatively lower than that of Csbnd C bond cleavage as a whole. However, vinyl (CH2CH) is likely to be the precursor of Csbnd C bond scission, as the activation barrier of Csbnd C bond cleavage reaction of CH2CH is much lower than that of CH2CH dehydrogenation and the reaction is exothermic by 0.15 eV. CC was another form of depositional carbon on Pt(100), as it is easy to form but difficult to decompose. Finally we proposed six possible routes of carbon monomer formation.

Electron-phonon interaction in three-barrier nanosystems as active elements of quantum cascade detectors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tkach, N. V., E-mail: ktf@chnu.edu.ua; Seti, Ju. A.; Grynyshyn, Yu. B.

2015-04-15

The theory of electron tunneling through an open nanostructure as an active element of a quantum cascade detector is developed, which takes into account the interaction of electrons with confined and interface phonons. Using the method of finite-temperature Green’s functions and the electron-phonon Hamiltonian in the representation of second quantization over all system variables, the temperature shifts and electron-level widths are calculated and the contributions of different electron-phonon-interaction mechanisms to renormalization of the spectral parameters are analyzed depending on the geometrical configuration of the nanosystem. Due to weak electron-phonon coupling in a GaAs/Al{sub 0.34}Ga{sub 0.66}As-based resonant tunneling nanostructure, the temperaturemore » shift and rf field absorption peak width are not very sensitive to the electron-phonon interaction and result from a decrease in potential barrier heights caused by a difference in the temperature dependences of the well and barrier band gaps.« less

Diffusion of One-Dimensional Crystals in Channels of Single-Walled Carbon Nanotubes

NASA Astrophysics Data System (ADS)

Zhigalina, V. G.; Kumskov, A. S.; Falaleev, N. S.; Vasiliev, A. L.; Kiselev, N. A.

2018-05-01

The transport of one-dimensional CuI crystals in channels of single-walled carbon nanotubes (SWCNTs) has been studied by high resolution electron microscopy. The diffusion kinetics has been investigated by counting the number of CuI atoms escaping from the nanotube channel. The diffusivity is calculated to be 6.8 × 10-21 m2/s, which corresponds to an activation-barrier height of 1 eV/atom. A comparison with the theoretically estimated height of the energy barrier for molecular transport through a graphene layer is indicative of mass transfer through vacancy defects in graphene.

Energetics using the single point IMOMO (integrated molecular orbital+molecular orbital) calculations: Choices of computational levels and model system

NASA Astrophysics Data System (ADS)

Svensson, Mats; Humbel, Stéphane; Morokuma, Keiji

1996-09-01

The integrated MO+MO (IMOMO) method, recently proposed for geometry optimization, is tested for accurate single point calculations. The principle idea of the IMOMO method is to reproduce results of a high level MO calculation for a large ``real'' system by dividing it into a small ``model'' system and the rest and applying different levels of MO theory for the two parts. Test examples are the activation barrier of the SN2 reaction of Cl-+alkyl chlorides, the C=C double bond dissociation of olefins and the energy of reaction for epoxidation of benzene. The effects of basis set and method in the lower level calculation as well as the effects of the choice of model system are investigated in detail. The IMOMO method gives an approximation to the high level MO energetics on the real system, in most cases with very small errors, with a small additional cost over the low level calculation. For instance, when the MP2 (Møller-Plesset second-order perturbation) method is used as the lower level method, the IMOMO method reproduces the results of very high level MO method within 2 kcal/mol, with less than 50% of additional computer time, for the first two test examples. When the HF (Hartree-Fock) method is used as the lower level method, it is less accurate and depends more on the choice of model system, though the improvement over the HF energy is still very significant. Thus the IMOMO single point calculation provides a method for obtaining reliable local energetics such as bond energies and activation barriers for a large molecular system.

Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films

NASA Astrophysics Data System (ADS)

Singh, Tejinder; Valipa, Mayur S.; Mountziaris, T. J.; Maroudas, Dimitrios

2007-11-01

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2×1) surface and molecular-dynamics (MD) simulations of adsorbed SiH3 radical precursor dissociation on surfaces of MD-grown a-Si :H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si :H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50eV. The results are consistent with experimental measurements of a-Si :H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si :H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH(s ), becomes increasingly more dominant as the temperature is increased.

DFT study of CO2 conversion on InZr3(110) surface.

PubMed

Zhang, Minhua; Dou, Maobin; Yu, Yingzhe

2017-11-01

Methanol and methane synthesis from CO 2 hydrogenation on a InZr 3 (110) surface has been studied using density functional theory calculations. The CO 2 can be chemically adsorbed via a polydentated configuration and the H 2 molecule can dissociate to H atoms spontaneously. The methanol is primarily formed via the HCOO route instead of the RWGS route, due to its higher activation barrier of 1.35 eV for HCO hydrogenation. In the HCOO route, the adsorbed CO 2 consecutively hydrogenates to form HCOO, H 2 COO and the H 3 CO species. The H 3 COH is produced via the reaction of H 3 CO with a surface OH group. Furthermore, the C-O bonds of CO, CHO, CH 2 O and CH 3 O species prefer to dissociate to C, CH, CH 2 CH 3 and surface O species. Methane is formed via the hydrogenation of CH x (x = 0-3) monomers with the highest activation barrier of 1.19 eV for CH 3 hydrogenation, which is higher than that of the hydrogenation of H 2 COO in methanol synthesis via the HCOO route. The surface O species formed during CO 2 hydrogenation reacts with the adsorbed H 2 molecule to produce an OH group which reacts with a surface H atom to form H 2 O with an activation barrier of 1.13 eV, which then desorbs to the gas phase. Our calculated results indicate that the InZr 3 alloy is a potential candidate catalyst for CO 2 utilization and conversion.

Barrier distributions and signatures of transfer channels in the Ca40+Ni58,64 fusion reactions at energies around and below the Coulomb barrier

NASA Astrophysics Data System (ADS)

Bourgin, D.; Courtin, S.; Haas, F.; Stefanini, A. M.; Montagnoli, G.; Goasduff, A.; Montanari, D.; Corradi, L.; Fioretto, E.; Huiming, J.; Scarlassara, F.; Rowley, N.; Szilner, S.; Mijatović, T.

2014-10-01

Background: The nuclear structure of colliding nuclei is known to influence the fusion process. Couplings of the relative motion to nuclear shape deformations and vibrations lead to an enhancement of the sub-barrier fusion cross section in comparison with the predictions of one-dimensional barrier penetration models. This enhancement is explained by coupled-channels calculations including these couplings. The sub-barrier fusion cross section is also affected by nucleon transfer channels between the colliding nuclei. Purpose: The aim of the present experiment is to investigate the influence of the projectile and target nuclear structures on the fusion cross sections in the Ca40+Ni58 and Ca40+Ni64 systems. Methods: The experimental and theoretical fusion excitation functions as well as the barrier distributions were compared for these two systems. Coupled-channels calculations were performed using the ccfull code. Results: Good agreement was found between the measured and calculated fusion cross sections for the Ca40+Ni58 system. The situation is different for the Ca40+Ni64 system where the coupled-channels calculations with no nucleon transfer clearly underestimate the fusion cross sections below the Coulomb barrier. The fusion excitation function was, however, well reproduced at low and high energies by including the coupling to the neutron pair-transfer channel in the calculations. Conclusions: The nuclear structure of the colliding nuclei influences the fusion cross sections below the Coulomb barrier for both Ca40+Ni58,64 systems. Moreover, we highlighted the effect of the neutron pair-transfer channel on the fusion cross sections in Ca40+Ni64.

Insight into proton transfer in phosphotungstic acid functionalized mesoporous silica-based proton exchange membrane fuel cells.

PubMed

Zhou, Yuhua; Yang, Jing; Su, Haibin; Zeng, Jie; Jiang, San Ping; Goddard, William A

2014-04-02

We have developed for fuel cells a novel proton exchange membrane (PEM) using inorganic phosphotungstic acid (HPW) as proton carrier and mesoporous silica as matrix (HPW-meso-silica) . The proton conductivity measured by electrochemical impedance spectroscopy is 0.11 S cm(-1) at 90 °C and 100% relative humidity (RH) with a low activation energy of ∼14 kJ mol(-1). In order to determine the energetics associated with proton migration within the HPW-meso-silica PEM and to determine the mechanism of proton hopping, we report density functional theory (DFT) calculations using the generalized gradient approximation (GGA). These DFT calculations revealed that the proton transfer process involves both intramolecular and intermolecular proton transfer pathways. When the adjacent HPWs are close (less than 17.0 Å apart), the calculated activation energy for intramolecular proton transfer within a HPW molecule is higher (29.1-18.8 kJ/mol) than the barrier for intermolecular proton transfer along the hydrogen bond. We find that the overall barrier for proton movement within the HPW-meso-silica membranes is determined by the intramolecular proton transfer pathway, which explains why the proton conductivity remains unchanged when the weight percentage of HPW on meso-silica is above 67 wt %. In contrast, the activation energy of proton transfer on a clean SiO2 (111) surface is computed to be as high as ∼40 kJ mol(-1), confirming the very low proton conductivity on clean silica surfaces observed experimentally.

Self-consistent vertical transport calculations in AlxGa1-xN/GaN based resonant tunneling diode

NASA Astrophysics Data System (ADS)

Rached, A.; Bhouri, A.; Sakr, S.; Lazzari, J.-L.; Belmabrouk, H.

2016-03-01

The formation of two-dimensional electron gases (2DEGs) at AlxGa1-xN/GaN hexagonal double-barriers (DB) resonant tunneling diodes (RTD) is investigated by numerical self-consistent (SC) solutions of the coupled Schrödinger and Poisson equations. Spontaneous and piezoelectric effects across the material interfaces are rigorously taken into account. Conduction band profiles, band edges and corresponding envelope functions are calculated in the AlxGa1-xN/GaN structures and likened to those where no polarization effects are included. The combined effect of the polarization-induced bound charge and conduction band offsets between the hexagonal AlGaN and GaN results in the formation of 2DEGs on one side of the DB and a depletion region on the other side. Using the transfer matrix formalism, the vertical transport (J-V characteristics) in AlGaN/GaN RTDs is calculated with a fully SC calculation in the ballistic regime. Compared to standard calculations where the voltage drop along the structure is supposed to be linear, the SC method leads to strong quantitative changes in the J-V characteristics showing that the applied electric field varies significantly in the active region of the structure. The influences of the aluminum composition and the GaN(AlGaN) thickness layers on the evolution of the current characteristics are also self-consistently investigated and discussed. We show that the electrical characteristics are very sensitive to the potential barrier due to the interplay between the potential symmetry and the barrier height and width. More interestingly, we demonstrate that the figures of merit namely the peak-to-valley ratio (PVR) of GaN/AlGaN RTDs can be optimized by increasing the quantum well width.

Motivating factors and barriers towards exercise in severe mental illness: a systematic review and meta-analysis.

PubMed

Firth, J; Rosenbaum, S; Stubbs, B; Gorczynski, P; Yung, A R; Vancampfort, D

2016-10-01

Exercise can improve clinical outcomes in people with severe mental illness (SMI). However, this population typically engages in low levels of physical activity with poor adherence to exercise interventions. Understanding the motivating factors and barriers towards exercise for people with SMI would help to maximize exercise participation. A search of major electronic databases was conducted from inception until May 2016. Quantitative studies providing proportional data on the motivating factors and/or barriers towards exercise among patients with SMI were eligible. Random-effects meta-analyses were undertaken to calculate proportional data and 95% confidence intervals (CI) for motivating factors and barriers toward exercise. From 1468 studies, 12 independent studies of 6431 psychiatric patients were eligible for inclusion. Meta-analyses showed that 91% of people with SMI endorsed 'improving health' as a reason for exercise (N = 6, n = 790, 95% CI 80-94). Among specific aspects of health and well-being, the most common motivations were 'losing weight' (83% of patients), 'improving mood' (81%) and 'reducing stress' (78%). However, low mood and stress were also identified as the most prevalent barriers towards exercise (61% of patients), followed by 'lack of support' (50%). Many of the desirable outcomes of exercise for people with SMI, such as mood improvement, stress reduction and increased energy, are inversely related to the barriers of depression, stress and fatigue which frequently restrict their participation in exercise. Providing patients with professional support to identify and achieve their exercise goals may enable them to overcome psychological barriers, and maintain motivation towards regular physical activity.

Accelerating wavefunction in density-functional-theory embedding by truncating the active basis set

NASA Astrophysics Data System (ADS)

Bennie, Simon J.; Stella, Martina; Miller, Thomas F.; Manby, Frederick R.

2015-07-01

Methods where an accurate wavefunction is embedded in a density-functional description of the surrounding environment have recently been simplified through the use of a projection operator to ensure orthogonality of orbital subspaces. Projector embedding already offers significant performance gains over conventional post-Hartree-Fock methods by reducing the number of correlated occupied orbitals. However, in our first applications of the method, we used the atomic-orbital basis for the full system, even for the correlated wavefunction calculation in a small, active subsystem. Here, we further develop our method for truncating the atomic-orbital basis to include only functions within or close to the active subsystem. The number of atomic orbitals in a calculation on a fixed active subsystem becomes asymptotically independent of the size of the environment, producing the required O ( N 0 ) scaling of cost of the calculation in the active subsystem, and accuracy is controlled by a single parameter. The applicability of this approach is demonstrated for the embedded many-body expansion of binding energies of water hexamers and calculation of reaction barriers of SN2 substitution of fluorine by chlorine in α-fluoroalkanes.

Conductance and refraction across a Barrier in Phosphorene

NASA Astrophysics Data System (ADS)

Dahal, Dipendra; Gumbs, Godfrey

The transmission coefficient and ballistic conductance for monolayer black phosphorene is calculated when a potential step or square barrier is present. The Landauer-B¨uttiker formalism is employed in our calculations of the conductance. We obtain the refractive index for the step potential barrier when an incident beam of electron travel along different paths so as to observe what role the anisotropy of the energy bands plays. Numerical results are presented for various potential heights and barrier widths and these are compared with those for gapless and gapped graphene.

Modeling the oxidation of ebselen and other organoselenium compounds using explicit solvent networks.

PubMed

Bayse, Craig A; Antony, Sonia

2009-05-14

The oxidation of dimethylselenide, dimethyldiselenide, S-methylselenenyl-methylmercaptan, and truncated and full models of ebselen (N-phenyl-1,2-benzisoselenazol-3(2H)-one) by methyl hydrogen peroxide has been modeled using density functional theory (DFT) and solvent-assisted proton exchange (SAPE), a method of microsolvation that employs explicit solvent networks to facilitate proton transfer reactions. The calculated activation barriers for these systems were substantially lower in energy (DeltaG(double dagger) + DeltaG(solv) = 13 to 26 kcal/mol) than models that neglect the participation of solvent in proton exchange. The comparison of two- and three-water SAPE networks showed a reduction in the strain in the model system but without a substantial reduction in the activation barriers. Truncating the ebselen model to N-methylisoselenazol-3(2H)-one gave a larger activation barrier than ebselen or N-methyl-1,2-benzisoselenazol-3(2H)-one but provided an efficient means of determining an initial guess for larger transition-state models. The similar barriers obtained for ebselen and Me(2)Se(2) (DeltaG(double dagger) + DeltaG(solv) = 20.65 and 20.40 kcal/mol, respectively) were consistent with experimentally determined rate constants. The activation barrier for MeSeSMe (DeltaG(double dagger) + DeltaG(solv) = 21.25 kcal/mol) was similar to that of ebselen and Me(2)Se(2) despite its significantly lower experimental rate for oxidation of an ebselen selenenyl sulfide by hydrogen peroxide relative to ebselen and ebselen diselenide. The disparity is attributed to intramolecular Se-O interactions, which decrease the nucleophilicity of the selenium center of the selenenyl sulfide.

N,N-difluorotris(tert-butyl)silylamine-the first organosilyl difluoroamine. Synthesis and computational studies.

PubMed

Majumder, Utpal; Armantrout, John R; Williams, Richard Vaughan; Shreeve, Jean'ne M

2002-11-29

The synthesis and characterization of the first stable trialkyl(difluoroamino)silane, R3SiNF2, as well as of R3SiNHF and R3SiN(CH3)F in moderate yields are reported. The (difluoroamino)silane has promise as a new synthon for the introduction of the -NF2 group into a variety of electrophilic inorganic and organic substrates. Activation barriers and relative energies were calculated for the unimolecular decompositions of Me3SiCF3 and t-Bu3SiNF2 using density functional theory (B3LYP/6-31G). The calculated activation energies confirm the long-assumed kinetic stability of Me3SiCF3.

Calculating Transition Energy Barriers and Characterizing Activation States for Steps of Fusion.

PubMed

Ryham, Rolf J; Klotz, Thomas S; Yao, Lihan; Cohen, Fredric S

2016-03-08

We use continuum mechanics to calculate an entire least energy pathway of membrane fusion, from stalk formation, to pore creation, and through fusion pore enlargement. The model assumes that each structure in the pathway is axially symmetric. The static continuum stalk structure agrees quantitatively with experimental stalk architecture. Calculations show that in a stalk, the distal monolayer is stretched and the stored stretching energy is significantly less than the tilt energy of an unstretched distal monolayer. The string method is used to determine the energy of the transition barriers that separate intermediate states and the dynamics of two bilayers as they pass through them. Hemifusion requires a small amount of energy independently of lipid composition, while direct transition from a stalk to a fusion pore without a hemifusion intermediate is highly improbable. Hemifusion diaphragm expansion is spontaneous for distal monolayers containing at least two lipid components, given sufficiently negative diaphragm spontaneous curvature. Conversely, diaphragms formed from single-component distal monolayers do not expand without the continual injection of energy. We identify a diaphragm radius, below which central pore expansion is spontaneous. For larger diaphragms, prior studies have shown that pore expansion is not axisymmetric, and here our calculations supply an upper bound for the energy of the barrier against pore formation. The major energy-requiring deformations in the steps of fusion are: widening of a hydrophobic fissure in bilayers for stalk formation, splay within the expanding hemifusion diaphragm, and fissure widening initiating pore formation in a hemifusion diaphragm. Copyright © 2016 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Cardiorespiratory Fitness in Internal Medicine Residents: Are Future Physicians Becoming Deconditioned?

PubMed

Daneshvar, Farshid; Weinreich, Michael; Daneshvar, Danial; Sperling, Michael; Salmane, Chadi; Yacoub, Harout; Gabriels, James; McGinn, Thomas; Smith, Marianne C

2017-02-01

Previous studies have shown a falloff in physicians' physical activity from medical school to residency. Poor fitness may result in stress, increase resident burnout, and contribute to mortality from cardiovascular disease and other causes. Physicians with poor exercise habits are also less likely to counsel patients about exercise. Prior studies have reported resident physical activity but not cardiorespiratory fitness age. The study was conducted in 2 residency programs (3 hospitals) to assess internal medicine residents' exercise habits as well as their cardiorespiratory fitness age. Data regarding physical fitness levels and exercise habits were collected in an anonymous cross-sectional survey. Cardiopulmonary fitness age was determined using fitness calculator based on the Nord-Trøndelag Health Study (HUNT). Of 199 eligible physicians, 125 (63%) responded to the survey. Of respondents, 11 (9%) reported never having exercised prior to residency and 45 (36%) reported not exercising during residency ( P < .001). In addition, 42 (34%) reported exercising every day prior to residency, while only 5 (4%) reported exercising daily during residency ( P < .001), with 99 (79%) participants indicating residency obligations as their main barrier to exercise. We found residents' calculated mean fitness age to be 5.6 years higher than their mean chronological age ( P < .001). Internal medicine residents reported significant decreases in physical activity and fitness. Residents attributed time constraints due to training as a key barrier to physical activity.

First-Principles Study of the Band Diagrams and Schottky-Type Barrier Heights of Aqueous Ta3N5 Interfaces.

PubMed

Watanabe, Eriko; Ushiyama, Hiroshi; Yamashita, Koichi

2017-03-22

The photo(electro)chemical production of hydrogen by water splitting is an efficient and sustainable method for the utilization of solar energy. To improve photo(electro)catalytic activity, a Schottky-type barrier is typically useful to separate excited charge carriers in semiconductor electrodes. Here, we focused on studying the band diagrams and the Schottky-type barrier heights of Ta 3 N 5 , which is one of the most promising materials as a photoanode for water splitting. The band alignments of the undoped and n-type Ta 3 N 5 with adsorbents in a vacuum were examined to determine how impurities and adsorbents affect the band positions and Fermi energies. The band edge positions as well as the density of surface states clearly depended on the density of O N impurities in the bulk and surface regions. Finally, the band diagrams of the n-type Ta 3 N 5 /water interfaces were calculated with an improved interfacial model to include the effect of electrode potential with explicit water molecules. We observed partial Fermi level pinning in our calculations at the Ta 3 N 5 /water interface, which affects the driving force for charge separation.

Theoretical study on stabilization mechanisms of nitrate esters using aromatic amines as stabilizers.

PubMed

Sun, Zhi-Dan; Fu, Xiao-Long; Yu, Hong-Jian; Fan, Xue-Zhong; Ju, Xue-Hai

2017-10-05

The propellants of nitrate esters can be stabilized by some aromatic amines practically. To probe the mechanism of this phenomenon, we performed DFT calculations on: (1) The decompositions of nitrate esters (with and without the catalysis of NO 2 ) and (2) the reaction between the stabilizers and the nitro dioxide (NO 2 is released during the storage of nitrate esters). The structures on the reaction paths (reactants, intermediates and products) were optimized at the (U)B3LYP/6-31G** level. It was shown that NO 2 lowers the activation energy barrier in the decomposition of nitrate ester by 11.82-17.86kJ/mol and efficiently catalyzes the rupture of ONO 2 bond. However, the aromatic amines, typical stabilizers for nitrate esters, can easily eliminate NO 2 with activation barriers as low as 27-113kJ/mol (with one exception of 128kJ/mol). These values are, for most cases, lower or much lower than the activation energy barriers for reactions between nitrate esters and NO 2 (127-137kJ/mol). Consequently, the stabilizers can block the NO 2 catalysis for the decompositions of nitrate esters. Copyright © 2017 Elsevier B.V. All rights reserved.

Fission barriers at the end of the chart of the nuclides

NASA Astrophysics Data System (ADS)

Möller, Peter; Sierk, Arnold J.; Ichikawa, Takatoshi; Iwamoto, Akira; Mumpower, Matthew

2015-02-01

We present calculated fission-barrier heights for 5239 nuclides for all nuclei between the proton and neutron drip lines with 171 ≤A ≤330 . The barriers are calculated in the macroscopic-microscopic finite-range liquid-drop model with a 2002 set of macroscopic-model parameters. The saddle-point energies are determined from potential-energy surfaces based on more than 5 000 000 different shapes, defined by five deformation parameters in the three-quadratic-surface shape parametrization: elongation, neck diameter, left-fragment spheroidal deformation, right-fragment spheroidal deformation, and nascent-fragment mass asymmetry. The energy of the ground state is determined by calculating the lowest-energy configuration in both the Nilsson perturbed-spheroid (ɛ ) and the spherical-harmonic (β ) parametrizations, including axially asymmetric deformations. The lower of the two results (correcting for zero-point motion) is defined as the ground-state energy. The effect of axial asymmetry on the inner barrier peak is calculated in the (ɛ ,γ ) parametrization. We have earlier benchmarked our calculated barrier heights to experimentally extracted barrier parameters and found average agreement to about 1 MeV for known data across the nuclear chart. Here we do additional benchmarks and investigate the qualitative and, when possible, quantitative agreement and/or consistency with data on β -delayed fission, isotope generation along prompt-neutron-capture chains in nuclear-weapons tests, and superheavy-element stability. These studies all indicate that the model is realistic at considerable distances in Z and N from the region of nuclei where its parameters were determined.

Temperature dependence of electrical characteristics of Pt/GaN Schottky diode fabricated by UHV e-beam evaporation.

PubMed

Kumar, Ashish; Arafin, Shamsul; Amann, Markus Christian; Singh, Rajendra

2013-11-15

Temperature-dependent electrical characterization of Pt/n-GaN Schottky barrier diodes prepared by ultra high vacuum evaporation has been done. Analysis has been made to determine the origin of the anomalous temperature dependence of the Schottky barrier height, the ideality factor, and the Richardson constant calculated from the I-V-T characteristics. Variable-temperature Hall effect measurements have been carried out to understand charge transport at low temperature. The modified activation energy plot from the barrier inhomogeneity model has given the value of 32.2 A/(cm2 K2) for the Richardson constant A** in the temperature range 200 to 380 K which is close to the known value of 26.4A/(cm2 K2) for n-type GaN.

Chloroform Hydrodechlorination over Palladium–Gold Catalysts: A First-Principles DFT Study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Lang; Yao, Xiaoqian; Khan, Ahmad

2016-04-20

Hydrodechlorination is a promising method for treating toxic chlorocarbon compounds. Pd is among the most effective catalysts for chloroform hydrodechlorination, and experiments have shown that the Pd–Au alloy catalyst yields superior catalytic performance over pure Pd. In this paper, we examine the chloroform hydrodechlorination mechanism over Pd(1 1 1) and Pd ML/Au(1 1 1) surfaces using periodic, self-consistent density functional theory calculations (DFT, GGA–PW91) and maximum rate analysis. We suggest that the reaction occurs on both surfaces through complete dechlorination of chloroform followed by hydrogenation of CH* to methane, and that the initial dechlorination step is likely the rate-limiting step.more » Finally, on Pd(1 1 1), the chloroform dechlorination barrier is 0.24 eV higher than the desorption barrier, whereas on Pd ML/Au(1 1 1), the chloroform dechlorination barrier is 0.07 eV lower than the desorption barrier, which can explain the higher hydrodechlorination activity of the Pd–Au alloy catalyst.« less

A study on the stability of O{sub 2} on oxometalloporphyrins by the first principles calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kubota, Yoshiyuki; Escano, Mary Clare Sison; Dy, Eben Sy

2007-05-21

The authors investigated the interaction of oxometalloporphyrins (MO(por))--specifically, MoO(por), WO(por), TiO(por), VO(por), and CrO(por)--with O{sub 2} by using first principles calculations. MoO(por) and WO(por) undergo reactions with O{sub 2}; on the other hand, TiO(por), VO(por), and CrO(por) do not. Next, they compared the interaction of MoO(por) and WO(por) with O{sub 2}. Activation barriers for the reactions of MoO(por) and WO(por) with a side-on O{sub 2} are small. For MoO(por)(O{sub 2}), the activation barrier for the reverse reaction that liberates O{sub 2} is also small; however, that for WO(por)(O{sub 2}) is large. The experimental results that photoirradiation with visible light ormore » heating of Mo {sup VI}O(tmp)(O{sub 2}) regenerates Mo {sup VI}O(tmp) by liberating O{sub 2} while W {sup VI}O(tmp)(O{sub 2}) does not [J. Tachibana, T. Imamura, and Y. Sasaki, Bull. Chem. Soc. Jpn. 71, 363 (1998)] are explained by the difference in activation barriers of the reverse reactions. This means that bonds formed between the W atom and O{sub 2} are stronger than those between the Mo atom and O{sub 2}. The bond strengths can be explained by differences in the energy levels between the highest occupied molecular orbital of MoO(por) and WO(por), which are mainly formed from the a orbitals of the central metal atom and {pi}{sup *} orbitals of O{sub 2}.« less

Performing a local barrier operation

DOEpatents

Archer, Charles J; Blocksome, Michael A; Ratterman, Joseph D; Smith, Brian E

2014-03-04

Performing a local barrier operation with parallel tasks executing on a compute node including, for each task: retrieving a present value of a counter; calculating, in dependence upon the present value of the counter and a total number of tasks performing the local barrier operation, a base value, the base value representing the counter's value prior to any task joining the local barrier; calculating, in dependence upon the base value and the total number of tasks performing the local barrier operation, a target value of the counter, the target value representing the counter's value when all tasks have joined the local barrier; joining the local barrier, including atomically incrementing the value of the counter; and repetitively, until the present value of the counter is no less than the target value of the counter: retrieving the present value of the counter and determining whether the present value equals the target value.

Performing a local barrier operation

DOEpatents

Archer, Charles J; Blocksome, Michael A; Ratterman, Joseph D; Smith, Brian E

2014-03-04

Performing a local barrier operation with parallel tasks executing on a compute node including, for each task: retrieving a present value of a counter; calculating, in dependence upon the present value of the counter and a total number of tasks performing the local barrier operation, a base value of the counter, the base value representing the counter's value prior to any task joining the local barrier; calculating, in dependence upon the base value and the total number of tasks performing the local barrier operation, a target value, the target value representing the counter's value when all tasks have joined the local barrier; joining the local barrier, including atomically incrementing the value of the counter; and repetitively, until the present value of the counter is no less than the target value of the counter: retrieving the present value of the counter and determining whether the present value equals the target value.

Communication: An accurate calculation of the S{sub 1} C{sub 2}H{sub 2} cis-trans isomerization barrier height

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baraban, Joshua H.; Matthews, Devin A.; Stanton, John F.

2016-03-21

A high level ab initio calculation of the cis-trans isomerization barrier height in the first excited singlet electronic state of acetylene is found to agree very well with a recent experimental determination.

Communication: An accurate calculation of the S 1 C 2H 2 cis-trans isomerization barrier height

DOE PAGES

Baraban, Joshua H.; Matthews, Devin A.; Stanton, John F.

2016-03-16

In this study, a high level ab initio calculation of the cis-trans isomerization barrier height in the first excited singlet electronic state of acetylene is found to agree very well with a recent experimental determination.

Another Look at the Mechanisms of Hydride Transfer Enzymes with Quantum and Classical Transition Path Sampling.

PubMed

Dzierlenga, Michael W; Antoniou, Dimitri; Schwartz, Steven D

2015-04-02

The mechanisms involved in enzymatic hydride transfer have been studied for years, but questions remain due, in part, to the difficulty of probing the effects of protein motion and hydrogen tunneling. In this study, we use transition path sampling (TPS) with normal mode centroid molecular dynamics (CMD) to calculate the barrier to hydride transfer in yeast alcohol dehydrogenase (YADH) and human heart lactate dehydrogenase (LDH). Calculation of the work applied to the hydride allowed for observation of the change in barrier height upon inclusion of quantum dynamics. Similar calculations were performed using deuterium as the transferring particle in order to approximate kinetic isotope effects (KIEs). The change in barrier height in YADH is indicative of a zero-point energy (ZPE) contribution and is evidence that catalysis occurs via a protein compression that mediates a near-barrierless hydride transfer. Calculation of the KIE using the difference in barrier height between the hydride and deuteride agreed well with experimental results.

When ruthenia met titania: Achieving extraordinary catalytic activity at low temperature by nanostructuring of oxides

DOE PAGES

Graciani, J.; Stacchiola, D.; Yang, F.; ...

2015-09-09

Nanostructured RuO x/TiO 2(110) catalysts have a remarkable catalytic activity for CO oxidation at temperatures in the range of 350–375 K. Furthermore, the RuO 2(110) surface has no activity. The state-of-the-art DFT calculations indicate that the main reasons for such an impressive improvement in the catalytic activity are: (i) a decrease of the diffusion barrier of adsorbed O atoms by around 40%, from 1.07 eV in RuO 2(110) to 0.66 eV in RuO x/TiO 2(110), which explains the shift of the activity to lower temperatures and (ii) a lowering of the barrier by 20% for the association of adsorbed COmore » and O species to give CO 2 (the main barrier for the CO oxidation reaction) passing from around 0.7 eV in RuO 2(110) to 0.55 eV in RuO x/TiO 2(110). We show that the catalytic properties of ruthenia are strongly modified when supported as nanostructures on titania, attaining higher activity at temperatures 100 K lower than that needed for pure ruthenia. As in other systems consisting of ceria nanostructures supported on titania, nanostructured ruthenia shows strongly modified properties compared to the pure oxide, consolidating the fact that the nanostructuring of oxides is a main way to attain higher catalytic activity at lower temperatures.« less

Effects of a TiC substrate on the catalytic activity of Pt for NO reduction.

PubMed

Chu, Xingli; Fu, Zhaoming; Li, Shasha; Zhang, Xilin; Yang, Zongxian

2016-05-11

Density functional theory calculations are used to elucidate the catalytic properties of a Pt monolayer supported on a TiC(001) substrate (Pt/TiC) toward NO reduction. It is found that the compound system of Pt/TiC has a good stability due to the strong Pt-TiC interaction. The diverse dissociation paths (namely the direct dissociation mechanism and the dimeric mechanism) are investigated. The transition state searching calculations suggest that NO has strong diffusion ability and small activation energy for dissociation on the Pt/TiC. For NO reduction on the Pt/TiC surface, we have found that the direct dissociation mechanisms (NO + N + O → NO2 + N and NO + N + O → N2 + O + O) are easier with a smaller dissociation barrier than those on the Pt(111) surface; and the dimeric process (NO + NO → (NO)2 → N2O + O → N2 + O + O) is considered to be dominant or significant with even a lower energy barrier than that of the direct dissociation. The results show that Pt/TiC can serve as an efficient catalyst for NO reduction.

Effect Of N = 40 Shell Closure On Barrier Distributions In 18O+58,60Ni Reactions

NASA Astrophysics Data System (ADS)

Danu, L. S.; Nayak, B. K.; Saxena, A.; Biswas, D. C.; John, B. V.; Thomas, R. G.; Gupta, Y. K.; Choudhury, R. K.

2009-03-01

The quasi-elastic scattering measurements for 18O+58,62Ni systems have been carried out at Θlab = 150° around Coulomb barrier energies to investigate the effect of nuclear shell closure on the barrier distributions. The 18O+58Ni system leads to N = 40 neutron shell closure and 18O+62Ni system is having N = 44 in the compound system. It is observed that target 2+ and 3-, projectile 2+ inelastic and 2n-transfer couplings are required in coupled-channels fusion model (CCFULL) calculations to get good comparison with the experimental barrier distribution of 18O+62Ni system, whereas projectile 2+ inelastic state coupling is not required for 18O+58Ni system. However, the low energy structure observed in the barrier distribution of 18O+58Ni system is not reproduced by coupled-channels calculations. This suggests, a possible additional effect due to N = 40 shell closure in the compound system not accounted for in coupled-channels calculations.

A seven-degree-of-freedom, time-dependent quantum dynamics study on the energy efficiency in surmounting the central energy barrier of the OH + CH{sub 3} → O + CH{sub 4} reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Pengxiu; Wang, Yuping; Li, Yida

2015-04-28

A time-dependent, quantum reaction dynamics calculation with seven degrees of freedom was carried out to study the energy efficiency in surmounting the approximate center energy barrier of OH + CH{sub 3}. The calculation shows the OH vibration excitations greatly enhance the reactivity, whereas the vibrational excitations of CH{sub 3} and the rotational excitations hinder the reactivity. On the basis of equal amount of total energy, although this reaction has a slight early barrier, it is the OH vibrational energy that is the dominate force in promoting the reactivity, not the translational energy. The studies on both the forward O +more » CH{sub 4} and reverse OH + CH{sub 3} reactions demonstrate, for these central barrier reactions, a small change of the barrier location can significantly change the energy efficacy roles on the reactivity. The calculated rate constants agree with the experimental data.« less

Scale transition using dislocation dynamics and the nudged elastic band method

DOE PAGES

Sobie, Cameron; Capolungo, Laurent; McDowell, David L.; ...

2017-08-01

Microstructural features such as precipitates or irradiation-induced defects impede dislocation motion and directly influence macroscopic mechanical properties such as yield point and ductility. In dislocation-defect interactions both atomic scale and long range elastic interactions are involved. Thermally assisted dislocation bypass of obstacles occurs when thermal fluctuations and driving stresses contribute sufficient energy to overcome the energy barrier. The Nudged Elastic Band (NEB) method is typically used in the context of atomistic simulations to quantify the activation barriers for a given reaction. In this work, the NEB method is generalized to coarse-grain continuum representations of evolving microstructure states beyond the discretemore » particle descriptions of first principles and atomistics. The method we employed enables the calculation of activation energies for a View the MathML source glide dislocation bypassing a [001] self-interstitial atom loop of size in the range of 4-10 nm with a spacing larger than 150nm in α-iron for a range of applied stresses and interaction geometries. This study is complemented by a comparison between atomistic and continuum based prediction of barriers.« less

Fission barriers at the end of the chart of the nuclides

DOE PAGES

Möller, Peter; Sierk, Arnold J.; Ichikawa, Takatoshi; ...

2015-02-12

We present calculated fission-barrier heights for 5239 nuclides for all nuclei between the proton and neutron drip lines with 171 ≤ A ≤ 330. The barriers are calculated in the macroscopic-microscopic finite-range liquid-drop (FRLDM) with a 2002 set of macroscopic-model parameters. The saddle-point energies are determined from potential-energy surfaces based on more than five million different shapes, defined by five deformation parameters in the three-quadratic-surface shape parametrization: elongation, neck diameter, left-fragment spheroidal deformation, right-fragment spheroidal deformation, and nascent-fragment mass asymmetry. The energy of the ground state is determined by calculating the lowest-energy configuration in both the Nilsson perturbed-spheroid (ϵ) andmore » the spherical-harmonic (β) parametrizations, including axially asymmetric deformations. The lower of the two results (correcting for zero-point motion) is defined as the ground-state energy. The effect of axial asymmetry on the inner barrier peak is calculated in the (ϵ,γ) parametrization. We have earlier benchmarked our calculated barrier heights to experimentally extracted barrier parameters and found average agreement to about one MeV for known data across the nuclear chart. Here we do additional benchmarks and investigate the qualitative and, when possible, quantitative agreement and/or consistency with data on β-delayed fission, isotope generation along prompt-neutron-capture chains in nuclear-weapons tests, and superheavy-element stability. In addition these studies all indicate that the model is realistic at considerable distances in Z and N from the region of nuclei where its parameters were determined.« less

Surface reaction of silicon chlorides during atomic layer deposition of silicon nitride

NASA Astrophysics Data System (ADS)

Yusup, Luchana L.; Park, Jae-Min; Mayangsari, Tirta R.; Kwon, Young-Kyun; Lee, Won-Jun

2018-02-01

The reaction of precursor with surface active site is the critical step in atomic layer deposition (ALD) process. We performed the density functional theory calculation with DFT-D correction to study the surface reaction of different silicon chloride precursors during the first half cycle of ALD process. SiCl4, SiH2Cl2, Si2Cl6 and Si3Cl8 were considered as the silicon precursors, and an NH/SiNH2*-terminated silicon nitride surface was constructed to model the thermal ALD processes using NH3 as well as the PEALD processes using NH3 plasma. The total energies of the system were calculated for the geometry-optimized structures of physisorption, chemisorption, and transition state. The order of silicon precursors in energy barrier, from lowest to highest, is Si3Cl8 (0.92 eV), Si2Cl6 (3.22 eV), SiH2Cl2 (3.93 eV) and SiCl4 (4.49 eV). Silicon precursor with lower energy barrier in DFT calculation showed lower saturation dose in literature for both thermal and plasma-enhanced ALD of silicon nitride. Therefore, DFT calculation is a promising tool in predicting the reactivity of precursor during ALD process.

Calculation of multidimensional potential energy surfaces for even-even transuranium nuclei: systematic investigation of the triaxiality effect on the fission barrier

NASA Astrophysics Data System (ADS)

Chai, Qing-Zhen; Zhao, Wei-Juan; Liu, Min-Liang; Wang, Hua-Lei

2018-05-01

Static fission barriers for 95 even-even transuranium nuclei with charge number Z = 94–118 have been systematically investigated by means of pairing self-consistent Woods-Saxon-Strutinsky calculations using the potential energy surface approach in multidimensional (β 2, γ, β 4) deformation space. Taking the heavier 252Cf nucleus (with the available fission barrier from experiment) as an example, the formation of the fission barrier and the influence of macroscopic, shell and pairing correction energies on it are analyzed. The results of the present calculated β 2 values and barrier heights are compared with previous calculations and available experiments. The role of triaxiality in the region of the first saddle is discussed. It is found that the second fission barrier is also considerably affected by the triaxial deformation degree of freedom in some nuclei (e.g., the Z=112–118 isotopes). Based on the potential energy curves, general trends of the evolution of the fission barrier heights and widths as a function of the nucleon numbers are investigated. In addition, the effects of Woods-Saxon potential parameter modifications (e.g., the strength of the spin-orbit coupling and the nuclear surface diffuseness) on the fission barrier are briefly discussed. Supported by National Natural Science Foundation of China (11675148, 11505157), the Project of Youth Backbone Teachers of Colleges and Universities of Henan Province (2017GGJS008), the Foundation and Advanced Technology Research Program of Henan Province (162300410222), the Outstanding Young Talent Research Fund of Zhengzhou University (1521317002) and the Physics Research and Development Program of Zhengzhou University (32410017)

Modelisation de la diffusion sur les surfaces metalliques: De l'adatome aux processus de croissance

NASA Astrophysics Data System (ADS)

Boisvert, Ghyslain

Cette these est consacree a l'etude des processus de diffusion en surface dans le but ultime de comprendre, et de modeliser, la croissance d'une couche mince. L'importance de bien mai triser la croissance est primordiale compte tenu de son role dans la miniaturisation des circuits electroniques. Nous etudions ici les surface des metaux nobles et de ceux de la fin de la serie de transition. Dans un premier temps, nous nous interessons a la diffusion d'un simple adatome sur une surface metallique. Nous avons, entre autres, mis en evidence l'apparition d'une correlation entre evenements successifs lorsque la temperature est comparable a la barriere de diffusion, i.e., la diffusion ne peut pas etre associee a une marche aleatoire. Nous proposons un modele phenomenologique simple qui reproduit bien les resultats des simulations. Ces calculs nous ont aussi permis de montrer que la diffusion obeit a la loi de Meyer-Neldel. Cette loi stipule que, pour un processus active, le prefacteur augmente exponentiellement avec la barriere. En plus, ce travail permet de clarifier l'origine physique de cette loi. En comparant les resultats dynamiques aux resultats statiques, on se rend compte que la barriere extraite des calculs dynamiques est essentiellement la meme que celle obtenue par une approche statique, beaucoup plus simple. On peut donc obtenir cette barriere a l'aide de methodes plus precises, i.e., ab initio, comme la theorie de la fonctionnelle de la densite, qui sont aussi malheureusement beaucoup plus lourdes. C'est ce que nous avons fait pour plusieurs systemes metalliques. Nos resultats avec cette derniere approche se comparent tres bien aux resultats experimentaux. Nous nous sommes attardes plus longuement a la surface (111) du platine. Cette surface regorge de particularites interessantes, comme la forme d'equilibre non-hexagonale des i lots et deux sites d'adsorption differents pour l'adatome. De plus, des calculs ab initio precedents n'ont pas reussi a confirmer la forme d'equilibre et surestiment grandement la barriere. Nos calculs, plus complets et dans un formalisme mieux adapte a ce genre de probleme, predisent correctement la forme d'equilibre, qui est en fait due a un relachement different du stress de surface aux deux types de marches qui forment les cotes des i lots. Notre valeur pour la barriere est aussi fortement diminuee lorsqu'on relaxe les forces sur les atomes de la surface, amenant le resultat theorique beaucoup plus pres de la valeur experimentale. Nos calculs pour le cuivre demontre en effet que la diffusion de petits i lots pendant la croissance ne peut pas etre negligee dans ce cas, mettant en doute la valeur des interpretations des mesures experimentales. (Abstract shortened by UMI.)

Structure and energetics of carbon, hexagonal boron nitride, and carbon/hexagonal boron nitride single-layer and bilayer nanoscrolls

NASA Astrophysics Data System (ADS)

Siahlo, Andrei I.; Poklonski, Nikolai A.; Lebedev, Alexander V.; Lebedeva, Irina V.; Popov, Andrey M.; Vyrko, Sergey A.; Knizhnik, Andrey A.; Lozovik, Yurii E.

2018-03-01

Single-layer and bilayer carbon and hexagonal boron nitride nanoscrolls as well as nanoscrolls made of bilayer graphene/hexagonal boron nitride heterostructure are considered. Structures of stable states of the corresponding nanoscrolls prepared by rolling single-layer and bilayer rectangular nanoribbons are obtained based on the analytical model and numerical calculations. The lengths of nanoribbons for which stable and energetically favorable nanoscrolls are possible are determined. Barriers to rolling of single-layer and bilayer nanoribbons into nanoscrolls and barriers to nanoscroll unrolling are calculated. Based on the calculated barriers nanoscroll lifetimes in the stable state are estimated. Elastic constants for bending of graphene and hexagonal boron nitride layers used in the model are found by density functional theory calculations.

First principles investigation of the initial stage of H-induced missing-row reconstruction of Pd(110) surface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Padama, Allan Abraham B.; Kasai, Hideaki, E-mail: kasai@dyn.ap.eng.osaka-u.ac.jp; Center for Atomic and Molecular Technologies, Osaka University, Suita, Osaka 565-0871

2014-06-28

The pathway of H diffusion that will induce the migration of Pd atom is investigated by employing first principles calculations based on density functional theory to explain the origin of missing-row reconstruction of Pd(110).The calculated activation barrier and the H-induced reconstruction energy reveal that the long bridge-to-tetrahedral configuration is the energetically favored process for the initial stage of reconstruction phenomenon. While the H diffusion triggers the migration of Pd atom, it is the latter process that significantly contributes to the activated missing-row reconstruction of Pd(110). Nonetheless, the strong interaction between the diffusing H and the Pd atoms dictates the occurrencemore » of reconstructed surface.« less

High-level QM/MM calculations support the concerted mechanism for Michael addition and covalent complex formation in thymidylate synthase.

PubMed

Kaiyawet, Nopporn; Lonsdale, Richard; Rungrotmongkol, Thanyada; Mulholland, Adrian J; Hannongbua, Supot

2015-02-10

Thymidylate synthase (TS) is a promising cancer target, due to its crucial function in thymine synthesis. It performs the reductive methylation of 2'-deoxyuridine-5'-phosphate (dUMP) to thymidine-5'-phosphate (dTMP), using N-5,10-methylene-5,6,7,8-tetrahydrofolate (mTHF) as a cofactor. After the formation of the dUMP/mTHF/TS noncovalent complex, and subsequent conformational activation, this complex has been proposed to react via nucleophilic attack (Michael addition) by Cys146, followed by methylene-bridge formation to generate the ternary covalent intermediate. Herein, QM/MM (B3LYP-D/6-31+G(d)-CHARMM27) methods are used to model the formation of the ternary covalent intermediate. A two-dimensional potential energy surface reveals that the methylene-bridged intermediate is formed via a concerted mechanism, as indicated by a single transition state on the minimum energy pathway and the absence of a stable enolate intermediate. A range of different QM methods (B3LYP, MP2 and SCS-MP2, and different basis sets) are tested for the calculation of the activation energy barrier for the formation of the methylene-bridged intermediate. We test convergence of the QM/MM results with respect to size of the QM region. Inclusion of Arg166, which interacts with the nucleophilic thiolate, in the QM region is important for reliable results; the MM model apparently does not reproduce energies for distortion of the guanidinium side chain correctly. The spin component scaled-Møller-Plessett perturbation theory (SCS-MP2) approach was shown to be in best agreement (within 1.1 kcal/mol) while the results obtained with MP2 and B3LYP also yielded acceptable values (deviating by less than 3 kcal/mol) compared with the barrier derived from experiment. Our results indicate that using a dispersion-corrected DFT method, or a QM method with an accurate treatment of electron correlation, increases the agreement between the calculated and experimental activation energy barriers, compared with the semiempirical AM1 method. These calculations provide important insight into the reaction mechanism of TS and may be useful in the design of new TS inhibitors.

Resonances for Symmetric Two-Barrier Potentials

ERIC Educational Resources Information Center

Fernandez, Francisco M.

2011-01-01

We describe a method for the accurate calculation of bound-state and resonance energies for one-dimensional potentials. We calculate the shape resonances for symmetric two-barrier potentials and compare them with those coming from the Siegert approximation, the complex scaling method and the box-stabilization method. A comparison of the…

Quantum finance Hamiltonian for coupon bond European and barrier options.

PubMed

Baaquie, Belal E

2008-03-01

Coupon bond European and barrier options are financial derivatives that can be analyzed in the Hamiltonian formulation of quantum finance. Forward interest rates are modeled as a two-dimensional quantum field theory and its Hamiltonian and state space is defined. European and barrier options are realized as transition amplitudes of the time integrated Hamiltonian operator. The double barrier option for a financial instrument is "knocked out" (terminated with zero value) if the price of the underlying instrument exceeds or falls below preset limits; the barrier option is realized by imposing boundary conditions on the eigenfunctions of the forward interest rates' Hamiltonian. The price of the European coupon bond option and the zero coupon bond barrier option are calculated. It is shown that, is general, the constraint function for a coupon bond barrier option can -- to a good approximation -- be linearized. A calculation using an overcomplete set of eigenfunctions yields an approximate price for the coupon bond barrier option, which is given in the form of an integral of a factor that results from the barrier condition times another factor that arises from the payoff function.

Relevance of non-equilibrium defect generation processes to resistive switching in TiO{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abdelouahed, Samir; McKenna, Keith P., E-mail: keith.mckenna@york.ac.uk

First principles calculations are employed to identify atomistic pathways for the generation of vacancy-interstitial pair defects in TiO{sub 2}. We find that the formation of both oxygen and titanium defects induces a net dipole moment indicating that their formation can be assisted by an electric field. We also show that the activation barrier to formation of an oxygen vacancy defect can be reduced by trapping of holes which may be injected by the electrode. The calculated activation energies suggest that generation of titanium defects is more favorable than generation oxygen defects although activation energies in both cases are relatively highmore » (>3.3 eV). These results provide much needed insight into an issue that has been widely debated but for which little definitive experimental information is available.« less

Maximum dose angle for oblique incidence on primary beam protective barriers in the design of medical radiation therapy facilities.

PubMed

Fondevila, Damián; Arbiser, Silvio; Sansogne, Rosana; Brunetto, Mónica; Dosoretz, Bernardo

2008-05-01

Primary barrier determinations for the shielding of medical radiation therapy facilities are generally made assuming normal beam incidence on the barrier, since this is geometrically the most unfavorable condition for that shielding barrier whenever the occupation line is allowed to run along the barrier. However, when the occupation line (for example, the wall of an adjacent building) runs perpendicular to the barrier (especially roof barrier), then two opposing factors come in to play: increasing obliquity angle with respect to the barrier increases the attenuation, while the distance to the calculation point decreases, hence, increasing the dose. As a result, there exists an angle (alpha(max)) for which the equivalent dose results in a maximum, constituting the most unfavorable geometric condition for that shielding barrier. Based on the usual NCRP Report No. 151 model, this article presents a simple formula for obtaining alpha(max), which is a function of the thickness of the barrier (t(E)) and the equilibrium tenth-value layer (TVL(e)) of the shielding material for the nominal energy of the beam. It can be seen that alpha(max) increases for increasing TVL(e) (hence, beam energy) and decreases for increasing t(E), with a range of variation that goes from 13 to 40 deg for concrete barriers thicknesses in the range of 50-300 cm and most commercially available teletherapy machines. This parameter has not been calculated in the existing literature for radiotherapy facilities design and has practical applications, as in calculating the required unoccupied roof shielding for the protection of a nearby building located in the plane of the primary beam rotation.

Solvent effects and potential of mean force study of the SN2 reaction of CH3+CN‑ in water

NASA Astrophysics Data System (ADS)

Li, Chen; Liu, Peng; Li, Yongfang; Wang, Dunyou

2018-03-01

We used a combined quantum mechanics and molecular mechanics (QM/MM) method to investigate the solvent effects and potential of mean force of the CH3F+CN‑ reaction in water. Comparing to gas phase, the water solution substantially affects the structures of the stationary points along the reaction path. We quantitatively obtained the solvent effects’ contributions to the reaction: 1.7 kcal/mol to the activation barrier and ‑26.0 kcal/mol to the reaction free energy. The potential mean of force calculated with the density functional theory/MM theory has a barrier height at 19.7 kcal/mol, consistent with the experimental result at 23.0 kcal/mol; the calculated reaction free energy at ‑43.5 kcal/mol is also consistent with the one estimated based on the gas-phase data at ‑39.7 kcal/mol. Project supported by the National Natural Science Foundation of China (Grant No. 11774206) and Taishan Scholarship Fund from Shandong Province, China.

Investigation of ionic transport in sodium scandium phosphate (NSP) and related compounds

NASA Astrophysics Data System (ADS)

Bhat, Kaustubh; Blügel, Stefan; Lustfeld, Hans

Sodium ionic conductors offer significant advantages for application in large scale energy storage systems. In this study, we investigate the different pathways available for sodium ion conduction in NSP and calculate energy barriers for ionic transport using Density Functional Theory (DFT) and the Nudged Elastic Band Method. We identify the structural parameters that reduce the energy barrier, by calculating the influence of positive and negative external pressure on the energy barrier. Lattice strain can be introduced by cation or anion substitution within the NASICON structure. We substitute the scandium atom with other trivalent atoms such as aluminium and yttrium, and calculate the resulting energy barriers. Sodium thiophosphate (Na3PS4) has previously shown about two orders of magnitude higher ionic conductivity than sodium phosphate (Na3PO4). We investigate the effect of substituting oxygen with sulphur in NSP. We acknowledge discussions with our experimental colleagues F. Tietz and M. Guin toward this work

Revealing the Origin of Activity in Nitrogen-Doped Nanocarbons towards Electrocatalytic Reduction of Carbon Dioxide.

PubMed

Xu, Junyuan; Kan, Yuhe; Huang, Rui; Zhang, Bingsen; Wang, Bolun; Wu, Kuang-Hsu; Lin, Yangming; Sun, Xiaoyan; Li, Qingfeng; Centi, Gabriele; Su, Dangsheng

2016-05-23

Carbon nanotubes (CNTs) are functionalized with nitrogen atoms for reduction of carbon dioxide (CO2 ). The investigation explores the origin of the catalyst's activity and the role of nitrogen chemical states therein. The catalysts show excellent performances, with about 90 % current efficiency for CO formation and stability over 60 hours. The Tafel analyses and density functional theory calculations suggest that the reduction of CO2 proceeds through an initial rate-determining transfer of one electron to CO2 , which leads to the formation of carbon dioxide radical anion (CO2 (.-) ). The initial reduction barrier is too high on pristine CNTs, resulting in a very high overpotentials at which the hydrogen evolution reaction dominates over CO2 reduction. The doped nitrogen atoms stabilize the radical anion, thereby lowering the initial reduction barrier and improving the intrinsic activity. The most efficient nitrogen chemical state for this reaction is quaternary nitrogen, followed by pyridinic and pyrrolic nitrogen. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Truncation-based energy weighting string method for efficiently resolving small energy barriers

NASA Astrophysics Data System (ADS)

Carilli, Michael F.; Delaney, Kris T.; Fredrickson, Glenn H.

2015-08-01

The string method is a useful numerical technique for resolving minimum energy paths in rare-event barrier-crossing problems. However, when applied to systems with relatively small energy barriers, the string method becomes inconvenient since many images trace out physically uninteresting regions where the barrier has already been crossed and recrossing is unlikely. Energy weighting alleviates this difficulty to an extent, but typical implementations still require the string's endpoints to evolve to stable states that may be far from the barrier, and deciding upon a suitable energy weighting scheme can be an iterative process dependent on both the application and the number of images used. A second difficulty arises when treating nucleation problems: for later images along the string, the nucleus grows to fill the computational domain. These later images are unphysical due to confinement effects and must be discarded. In both cases, computational resources associated with unphysical or uninteresting images are wasted. We present a new energy weighting scheme that eliminates all of the above difficulties by actively truncating the string as it evolves and forcing all images, including the endpoints, to remain within and cover uniformly a desired barrier region. The calculation can proceed in one step without iterating on strategy, requiring only an estimate of an energy value below which images become uninteresting.

First principles investigations of small bimetallic PdGa clusters as catalysts for hydrogen dissociation

NASA Astrophysics Data System (ADS)

Kaul, Indu; Ghosh, Prasenjit

2017-04-01

Using first principles density functional theory based calculations, we have studied hydrogen dissociation on sub nanometer bimetallic clusters formed from d-block (Pd) and p-block (Ga) elements in gas phase to explore the feasibility of using them as cheap catalysts for hydrogen dissociation. Our calculations show that the dimers, trimers and tetramers of these clusters are thermodynamically more stable than the pure ones for all Ga concentrations. For a given cluster size, we find that the clusters containing equal amount of Pd and Ga are the most stable ones. In contrast to bulk PdGa, the contribution of Pd-d states to the highest occupied molecular orbitals of the bimetallic clusters are either very small or absent. Study of adsorption of hydrogen molecule on these clusters show that hydrogen binds in an activated form only on the Pd rich clusters. From the calculations of hydrogen dissociation barriers on tetramers of pure Pd, 25% Ga (Pd3Ga) and 50% Ga (Pd2Ga2) we find that Pd3Ga is the most efficient catalyst for hydrogen dissociation with barriers even lower than that on the PdGa surfaces.

Electronic states of aryl radical functionalized graphenes: Density functional theory study

NASA Astrophysics Data System (ADS)

Tachikawa, Hiroto; Kawabata, Hiroshi

2016-06-01

Functionalized graphenes are known as a high-performance molecular device. In the present study, the structures and electronic states of the aryl radical functionalized graphene have been investigated by the density functional theory (DFT) method to elucidate the effects of functionalization on the electronic states of graphene (GR). Also, the mechanism of aryl radical reaction with GR was investigated. The benzene, biphenyl, p-terphenyl, and p-quaterphenyl radicals [denoted by (Bz) n (n = 1-4), where n means numbers of benzene rings in aryl radical] were examined as aryl radicals. The DFT calculation of GR-(Bz) n (n = 1-4) showed that the aryl radical binds to the carbon atom of GR, and a C-C single bond was formed. The binding energies of aryl radicals to GR were calculated to be ca. 6.0 kcal mol-1 at the CAM-B3LYP/6-311G(d,p) level. It was found that the activation barrier exists in the aryl radical addition: the barrier heights were calculated to be 10.0 kcal mol-1. The electronic states of GR-(Bz) n were examined on the basis of theoretical results.

200-BP-1 Prototype Hanford Barrier -- 15 Years of Performance Monitoring

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ward, Anderson L.; Draper, Kathryn E.; Link, Steven O.

2011-09-30

Monitoring is an essential component of engineered barrier system design and operation. A composite capacitive cover, including a capillary break and an evapotranspiration (ET) barrier at the Hanford Site, is generating data that can be used to help resolve these issues. The prototype Hanford barrier was constructed over the 216-B-57 Crib in 1994 to evaluate surface-barrier constructability, construction costs, and physical and hydrologic performance at the field scale. The barrier has been routinely monitored between November 1994 and September 1998 as part of a Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) treatability test of barrier performance formore » the 200 BP 1 Operable Unit. Since FY 1998, monitoring has focused on a more limited set of key water balance, stability, and biotic parameters. In FY 2009, data collection was focused on: (1) water-balance monitoring, consisting of precipitation, runoff, soil moisture storage, and drainage measurements with evapotranspiration calculated by difference; (2) stability monitoring, consisting of asphalt-layer-settlement, basalt-side-slope-stability, and surface-elevation measurements; (3) vegetation dynamics; and (4) animal use. September 2009 marked 15 years since the start of monitoring and the collection of performance data. This report describes the results of monitoring activities during the period October 1, 2008, through September 30, 2009, and summarizes the 15 years of performance data collected from September 1994 through September 2009.« less

Temperature dependent current-voltage characteristics of Au/n-Si Schottky barrier diodes and the effect of transition metal oxides as an interface layer

NASA Astrophysics Data System (ADS)

Mahato, Somnath; Puigdollers, Joaquim

2018-02-01

Temperature dependent current-voltage (I‒V) characteristics of Au/n-type silicon (n-Si) Schottky barrier diodes have been investigated. Three transition metal oxides (TMO) are used as an interface layer between gold and silicon. The basic Schottky diode parameters such as ideality factor (n), barrier height (ϕb 0) and series resistance (Rs) are calculated and successfully explained by the thermionic emission (TE) theory. It has been found that ideality factor decreased and barrier height increased with increased of temperature. The conventional Richardson plot of ln(I0/T2) vs. 1000/T is determined the activation energy (Ea) and Richardson constant (A*). Whereas value of 'A*' is much smaller than the known theoretical value of n-type Si. The temperature dependent I-V characteristics obtained the mean value of barrier height (ϕb 0 bar) and standard deviation (σs) from the linear plot of ϕap vs. 1000/T. From the modified Richardson plot of ln(I0/T2) ˗ (qσ)2/2(kT)2 vs. 1000/T gives Richardson constant and homogeneous barrier height of Schottky diodes. Main observation in this present work is the barrier height and ideality factor shows a considerable change but the series resistance value exhibits negligible change due to TMO as an interface layer.

Theoretical evidence of PtSn alloy efficiency for CO oxidation.

PubMed

Dupont, Céline; Jugnet, Yvette; Loffreda, David

2006-07-19

The efficiency of PtSn alloy surfaces toward CO oxidation is demonstrated from first-principles theory. Oxidation kinetics based on atomistic density-functional theory calculations shows that the Pt3Sn surface alloy exhibits a promising catalytic activity for fuel cells. At room temperature, the corresponding rate outstrips the activity of Pt(111) by several orders of magnitude. According to the oxidation pathways, the activation barriers are actually lower on Pt3Sn(111) and Pt3Sn/Pt(111) surfaces than on Pt(111). A generalization of Hammer's model is proposed to elucidate the key role of tin on the lowering of the barriers. Among the energy contributions, a correlation is evidenced between the decrease of the barrier and the strengthening of the attractive interaction energy between CO and O moieties. The presence of tin modifies also the symmetry of the transition states which are composed of a CO adsorbate on a Pt near-top position and an atomic O adsorption on an asymmetric mixed PtSn bridge site. Along the reaction pathways, a CO2 chemisorbed surface intermediate is obtained on all the surfaces. These results are supported by a thorough vibrational analysis including the coupling with the surface phonons which reveals the existence of a stretching frequency between the metal substrate and the CO2 molecule.

Relative role of different radii in the dynamics of 8B+58Ni reaction

NASA Astrophysics Data System (ADS)

Kaur, Amandeep; Sandhu, Kirandeep; Sharma, Manoj K.

2018-05-01

In the present work, we intend to analyze the significance of three different radius terms in the framework of dynamical cluster-decay model (DCM) based calculations. In the majority of DCM based calculations the impact of mass- dependent radius R(A) is extensively analyzed. The other two factors on which the radius term may depend are, the neutron- proton asymmetry and the charge of the decaying fragments. Hence, the asymmetry dependent radius term R(I) and charge dependent radius term R(Z) are incorporated in DCM based calculations to investigate their effect on the reaction dynamics involved. Here, we present an extension of an earlier work based on the decay of 66As* compound nucleus by including R(I) and R(Z) radii in addition to the R(A) term. The effect of replacement of R(A) with R(I) and R(Z) is analyzed via fragmentation structure, tunneling probabilities (P) and other barrier characteristics like barrier height (VB), barrier position (RB), barrier turning point Ra etc. The role of temperature, deformations and angular momentum is duly incorporated in the present calculations.

Resonant activation in a colored multiplicative thermal noise driven closed system.

PubMed

Ray, Somrita; Mondal, Debasish; Bag, Bidhan Chandra

2014-05-28

In this paper, we have demonstrated that resonant activation (RA) is possible even in a thermodynamically closed system where the particle experiences a random force and a spatio-temporal frictional coefficient from the thermal bath. For this stochastic process, we have observed a hallmark of RA phenomena in terms of a turnover behavior of the barrier-crossing rate as a function of noise correlation time at a fixed noise variance. Variance can be fixed either by changing temperature or damping strength as a function of noise correlation time. Our another observation is that the barrier crossing rate passes through a maximum with increase in coupling strength of the multiplicative noise. If the damping strength is appreciably large, then the maximum may disappear. Finally, we compare simulation results with the analytical calculation. It shows that there is a good agreement between analytical and numerical results.

Localized saddle-point search and application to temperature-accelerated dynamics

NASA Astrophysics Data System (ADS)

Shim, Yunsic; Callahan, Nathan B.; Amar, Jacques G.

2013-03-01

We present a method for speeding up temperature-accelerated dynamics (TAD) simulations by carrying out a localized saddle-point (LSAD) search. In this method, instead of using the entire system to determine the energy barriers of activated processes, the calculation is localized by only including a small chunk of atoms around the atoms directly involved in the transition. Using this method, we have obtained N-independent scaling for the computational cost of the saddle-point search as a function of system size N. The error arising from localization is analyzed using a variety of model systems, including a variety of activated processes on Ag(100) and Cu(100) surfaces, as well as multiatom moves in Cu radiation damage and metal heteroepitaxial growth. Our results show significantly improved performance of TAD with the LSAD method, for the case of Ag/Ag(100) annealing and Cu/Cu(100) growth, while maintaining a negligibly small error in energy barriers.

Exchange coupling and magnetic anisotropy in a family of bipyrimidyl radical-bridged dilanthanide complexes: density functional theory and ab initio calculations.

PubMed

Zhang, Yi-Quan; Luo, Cheng-Lin; Zhang, Qiang

2014-05-05

The origin of the magnetic anisotropy energy barriers in a series of bpym(-) (bpym = 2,2'-bipyrimidine) radical-bridged dilanthanide complexes [(Cp*2Ln)2(μ-bpym)](+) [Cp* = pentamethylcyclopentadienyl; Ln = Gd(III) (1), Tb(III) (2), Dy(III) (3), Ho(III) (4), Er(III) (5)] has been explored using density functional theory (DFT) and ab initio methods. DFT calculations show that the exchange coupling between the two lanthanide ions for each complex is very weak, but the antiferromagnetic Ln-bpym(-) couplings are strong. Ab initio calculations show that the effective energy barrier of 2 or 3 mainly comes from the contribution of a single Tb(III) or Dy(III) fragment, which is only about one third of a single Ln energy barrier. For 4 or 5, however, both of the two Ho(III) or Er(III) fragments contribute to the total energy barrier. Thus, it is insufficient to only increase the magnetic anisotropy energy barrier of a single Ln ion, while enhancing the Ln-bpym(-) couplings is also very important. Copyright © 2014 Wiley Periodicals, Inc.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Möller, Peter; Sierk, Arnold J.; Ichikawa, Takatoshi

We present calculated fission-barrier heights for 5239 nuclides for all nuclei between the proton and neutron drip lines with 171 ≤ A ≤ 330. The barriers are calculated in the macroscopic-microscopic finite-range liquid-drop (FRLDM) with a 2002 set of macroscopic-model parameters. The saddle-point energies are determined from potential-energy surfaces based on more than five million different shapes, defined by five deformation parameters in the three-quadratic-surface shape parametrization: elongation, neck diameter, left-fragment spheroidal deformation, right-fragment spheroidal deformation, and nascent-fragment mass asymmetry. The energy of the ground state is determined by calculating the lowest-energy configuration in both the Nilsson perturbed-spheroid (ϵ) andmore » the spherical-harmonic (β) parametrizations, including axially asymmetric deformations. The lower of the two results (correcting for zero-point motion) is defined as the ground-state energy. The effect of axial asymmetry on the inner barrier peak is calculated in the (ϵ,γ) parametrization. We have earlier benchmarked our calculated barrier heights to experimentally extracted barrier parameters and found average agreement to about one MeV for known data across the nuclear chart. Here we do additional benchmarks and investigate the qualitative and, when possible, quantitative agreement and/or consistency with data on β-delayed fission, isotope generation along prompt-neutron-capture chains in nuclear-weapons tests, and superheavy-element stability. In addition these studies all indicate that the model is realistic at considerable distances in Z and N from the region of nuclei where its parameters were determined.« less

A molecular dynamics study of intramolecular proton transfer reaction of malonaldehyde in solution based upon a mixed quantum-classical approximation. II. Proton transfer reaction in non-polar solvent

NASA Astrophysics Data System (ADS)

Kojima, H.; Yamada, A.; Okazaki, S.

2015-05-01

The intramolecular proton transfer reaction of malonaldehyde in neon solvent has been investigated by mixed quantum-classical molecular dynamics (QCMD) calculations and fully classical molecular dynamics (FCMD) calculations. Comparing these calculated results with those for malonaldehyde in water reported in Part I [A. Yamada, H. Kojima, and S. Okazaki, J. Chem. Phys. 141, 084509 (2014)], the solvent dependence of the reaction rate, the reaction mechanism involved, and the quantum effect therein have been investigated. With FCMD, the reaction rate in weakly interacting neon is lower than that in strongly interacting water. However, with QCMD, the order of the reaction rates is reversed. To investigate the mechanisms in detail, the reactions were categorized into three mechanisms: tunneling, thermal activation, and barrier vanishing. Then, the quantum and solvent effects were analyzed from the viewpoint of the reaction mechanism focusing on the shape of potential energy curve and its fluctuations. The higher reaction rate that was found for neon in QCMD compared with that found for water solvent arises from the tunneling reactions because of the nearly symmetric double-well shape of the potential curve in neon. The thermal activation and barrier vanishing reactions were also accelerated by the zero-point energy. The number of reactions based on these two mechanisms in water was greater than that in neon in both QCMD and FCMD because these reactions are dominated by the strength of solute-solvent interactions.

Confined Li ion migration in the silicon-graphene complex system: An ab initio investigation

NASA Astrophysics Data System (ADS)

Wang, Guoqing; Xu, Bo; Shi, Jing; Lei, Xueling; Ouyang, Chuying

2018-04-01

Silicon-Carbon complex systems play an important role in enhancing the performance of Si-based anode materials for Li ion batteries. In this work, the Li migration property of the Silicon-Graphene (Si-Gr) complex systems are investigated by using first-principles calculations. Especially, the effects of graphene coating on the migration of Li ions are discussed in detail. The distance between Si surface and graphene in the Si-Gr system significantly affects the lateral migration of Li ions. With the decrease of the distance from 4.715 to 3.844 Å, the energy barrier of Li ion migration also decreases from 0.115 to 0.067 eV, which are all lower than that of the case without graphene d(0.135 eV). However, smaller distance (3.586 Å) brings the high energy barrier (0.237 eV). Through AIMD calculations, it is found that the graphene coating in the Si-Gr complex system would result in the larger intercalation depths, more uniform distributions, and higher migration coefficients of Li ions. Further calculations of migration coefficients of Li ions at different temperature are used to obtained the activation energy for Li ions migration in the Si-Gr system, which is as low as 0.028 eV. This low activation energy shows that it is easy for Li ions migrating in the Si-Gr system. Our study provided the basically information to understand the migration mechanism of Li ions in Si-C system.

New Global Calculation of Nuclear Masses and Fission Barriers for Astrophysical Applications

NASA Astrophysics Data System (ADS)

Möller, P.; Sierk, A. J.; Bengtsson, R.; Ichikawa, T.; Iwamoto, A.

2008-05-01

The FRDM(1992) mass model [1] has an accuracy of 0.669 MeV in the region where its parameters were determined. For the 529 masses that have been measured since, its accuracy is 0.46 MeV, which is encouraging for applications far from stability in astrophysics. We are developing an improved mass model, the FRDM(2008). The improvements in the calculations with respect to the FRDM(1992) are in two main areas. (1) The macroscopic model parameters are better optimized. By simulation (adjusting to a limited set of now known nuclei) we can show that this actually makes the results more reliable in new regions of nuclei. (2) The ground-state deformation parameters are more accurately calculated. We minimize the energy in a four-dimensional deformation space (ɛ2, V3, V4, V6,) using a grid interval of 0.01 in all 4 deformation variables. The (non-finalized) FRDM (2008-a) has an accuracy of 0.596 MeV with respect to the 2003 Audi mass evaluation before triaxial shape degrees of freedom are included (in progress). When triaxiality effects are incorporated preliminary results indicate that the model accuracy will improve further, to about 0.586 MeV. We also discuss very large-scale fission-barrier calculations in the related FRLDM (2002) model, which has been shown to reproduce very satisfactorily known fission properties, for example barrier heights from 70Se to the heaviest elements, multiple fission modes in the Ra region, asymmetry of mass division in fission and the triple-humped structure found in light actinides. In the superheavy region we find barriers consistent with the observed half-lives. We have completed production calculations and obtain barrier heights for 5254 nuclei heavier than A = 170 for all nuclei between the proton and neutron drip lines. The energy is calculated for 5009325 different shapes for each nucleus and the optimum barrier between ground state and separated fragments is determined by use of an ``immersion'' technique.

SU-G-206-17: RadShield: Semi-Automated Shielding Design for CT Using NCRP 147 and Isodose Curves

DOE Office of Scientific and Technical Information (OSTI.GOV)

DeLorenzo, M; Rutel, I; Yang, K

2016-06-15

Purpose: Computed tomography (CT) exam rooms are shielded more quickly and accurately compared to manual calculations using RadShield, a semi-automated diagnostic shielding software package. Last year, we presented RadShield’s approach to shielding radiographic and fluoroscopic rooms calculating air kerma rate and barrier thickness at many points on the floor plan and reporting the maximum values for each barrier. RadShield has now been expanded to include CT shielding design using not only NCRP 147 methodology but also by overlaying vendor provided isodose curves onto the floor plan. Methods: The floor plan image is imported onto the RadShield workspace to serve asmore » a template for drawing barriers, occupied regions and CT locations. SubGUIs are used to set design goals, occupancy factors, workload, and overlay isodose curve files. CTDI and DLP methods are solved following NCRP 147. RadShield’s isodose curve method employs radial scanning to extract data point sets to fit kerma to a generalized power law equation of the form K(r) = ar^b. RadShield’s semiautomated shielding recommendations were compared against a board certified medical physicist’s design using dose length product (DLP) and isodose curves. Results: The percentage error found between the physicist’s manual calculation and RadShield’s semi-automated calculation of lead barrier thickness was 3.42% and 21.17% for the DLP and isodose curve methods, respectively. The medical physicist’s selection of calculation points for recommending lead thickness was roughly the same as those found by RadShield for the DLP method but differed greatly using the isodose method. Conclusion: RadShield improves accuracy in calculating air-kerma rate and barrier thickness over manual calculations using isodose curves. Isodose curves were less intuitive and more prone to error for the physicist than inverse square methods. RadShield can now perform shielding design calculations for general scattering bodies for which isodose curves are provided.« less

SU-F-P-53: RadShield: Semi-Automated Shielding Design for CT Using NCRP 147 and Isodose Curves

DOE Office of Scientific and Technical Information (OSTI.GOV)

DeLorenzo, M; Rutel, I; Wu, D

Purpose: Computed tomography (CT) exam rooms are shielded more quickly and accurately compared to manual calculations using RadShield, a semi-automated diagnostic shielding software package. Last year, we presented RadShield’s approach to shielding radiographic and fluoroscopic rooms calculating air kerma rate and barrier thickness at many points on the floor plan and reporting the maximum values for each barrier. RadShield has now been expanded to include CT shielding design using not only NCRP 147 methodology but also by overlaying vendor provided isodose curves onto the floor plan. Methods: The floor plan image is imported onto the RadShield workspace to serve asmore » a template for drawing barriers, occupied regions and CT locations. SubGUIs are used to set design goals, occupancy factors, workload, and overlay isodose curve files. CTDI and DLP methods are solved following NCRP 147. RadShield’s isodose curve method employs radial scanning to extract data point sets to fit kerma to a generalized power law equation of the form K(r) = ar^b. RadShield’s semi-automated shielding recommendations were compared against a board certified medical physicist’s design using dose length product (DLP) and isodose curves. Results: The percentage error found between the physicist’s manual calculation and RadShield’s semi-automated calculation of lead barrier thickness was 3.42% and 21.17% for the DLP and isodose curve methods, respectively. The medical physicist’s selection of calculation points for recommending lead thickness was roughly the same as those found by RadShield for the DLP method but differed greatly using the isodose method. Conclusion: RadShield improves accuracy in calculating air-kerma rate and barrier thickness over manual calculations using isodose curves. Isodose curves were less intuitive and more prone to error for the physicist than inverse square methods. RadShield can now perform shielding design calculations for general scattering bodies for which isodose curves are provided.« less

Mechanistic Insights on the Hydrogenation of α,β-Unsaturated Ketones and Aldehydes to Unsaturated Alcohols over Metal Catalysts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ide, Matthew S.; Hao, Bing; Neurock, Matthew

The selective hydrogenation of unsaturated ketones (methyl vinyl ketone and benzalacetone) and unsaturated aldehydes (crotonaldehyde and cinnamaldehyde) was carried out with H₂ at 2 bar absolute over Pd/C, Pt/C, Ru/C, Au/C, Au/TiO₂, or Au/Fe₂O₃ catalysts in ethanol or water solvent at 333 K. Comparison of the turnover frequencies revealed Pd/C to be the most active hydrogenation catalyst, but the catalyst failed to produce unsaturated alcohols, indicating hydrogenation of the C=C bond was highly preferred over the C=O bond on Pd. The Pt and Ru catalysts were able to produce unsaturated alcohols from unsaturated aldehydes, but not from unsaturated ketones. Althoughmore » Au/ Fe₂O₃ was able to partially hydrogenate unsaturated ketones to unsaturated alcohols, the overall hydrogenation rate over gold was the lowest of all of the metals examined. First-principles density functional theory calculations were therefore used to explore the reactivity trends of methyl vinyl ketone (MVK) and benzalacetone (BA) hydrogenation over model Pt(111) and Ru(0001) surfaces. The observed selectivity over these metals is likely controlled by the significantly higher activation barriers to hydrogenate the C=O bond compared with those required to hydrogenate the C=C bond. Both the unsaturated alcohol and the saturated ketone, which are the primary reaction products, are strongly bound to Ru and can react further to the saturated alcohol. The lower calculated barriers for the hydrogenation steps over Pt compared with Ru account for the higher observed turnover frequencies for the hydrogenation of MVK and BA over Pt. The presence of a phenyl substituent α to the C=C bond in BA increased the barrier for C=C hydrogenation over those associated with the C=C bond in MVK; however, the increase in barriers with phenyl substitution was not adequate to reverse the selectivity trend.« less

Oxygen self-diffusion mechanisms in monoclinic Zr O2 revealed and quantified by density functional theory, random walk analysis, and kinetic Monte Carlo calculations

NASA Astrophysics Data System (ADS)

Yang, Jing; Youssef, Mostafa; Yildiz, Bilge

2018-01-01

In this work, we quantify oxygen self-diffusion in monoclinic-phase zirconium oxide as a function of temperature and oxygen partial pressure. A migration barrier of each type of oxygen defect was obtained by first-principles calculations. Random walk theory was used to quantify the diffusivities of oxygen interstitials by using the calculated migration barriers. Kinetic Monte Carlo simulations were used to calculate diffusivities of oxygen vacancies by distinguishing the threefold- and fourfold-coordinated lattice oxygen. By combining the equilibrium defect concentrations obtained in our previous work together with the herein calculated diffusivity of each defect species, we present the resulting oxygen self-diffusion coefficients and the corresponding atomistically resolved transport mechanisms. The predicted effective migration barriers and diffusion prefactors are in reasonable agreement with the experimentally reported values. This work provides insights into oxygen diffusion engineering in Zr O2 -related devices and parametrization for continuum transport modeling.

Computer-assisted study on the reaction between pyruvate and ylide in the pathway leading to lactyl-ThDP.

PubMed

Alvarado, Omar; Jaña, Gonzalo; Delgado, Eduardo J

2012-08-01

In this study the formation of the lactyl-thiamin diphosphate intermediate (L-ThDP) is addressed using density functional theory calculations at X3LYP/6-31++G(d,p) level of theory. The study includes potential energy surface scans, transition state search, and intrinsic reaction coordinate calculations. Reactivity is analyzed in terms of Fukui functions. The results allow to conclude that the reaction leading to the formation of L-ThDP occurs via a concerted mechanism, and during the nucleophilic attack on the pyruvate molecule, the ylide is in its AP form. The calculated activation barrier for the reaction is 19.2 kcal/mol, in agreement with the experimental reported value.

Reaction pathways and free energy profiles for cholinesterase-catalyzed hydrolysis of 6-monoacetylmorphine

PubMed Central

Qiao, Yan; Han, Keli; Zhan, Chang-Guo

2014-01-01

As the most active metabolite of heroin, 6-monoacetylmorphine (6-MAM) can penetrate into the brain for the rapid onset of heroin effects. The primary enzymes responsible for the metabolism of 6-MAM to the less potent morphine in humans are acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The detailed reaction pathways for AChE- and BChE-catalyzed hydrolysis of 6-MAM to morphine have been explored, for the first time, in the present study by performing first-principles quantum mechanical/molecular mechanical free energy calculations. It has been demonstrated that the two enzymatic reaction processes follow the similar catalytic reaction mechanism, and the whole catalytic reaction pathway for each enzyme consists of four reaction steps. According to the calculated results, the second reaction step associated with the transition state TS2a/TS2b should be rate-determining for the AChE/BChE-catalyzed hydrolysis, and the free energy barrier calculated for the AChE-catalyzed hydrolysis (18.3 kcal/mol) is 2.5 kcal/mol lower than that for the BChE-catalyzed hydrolysis (20.8 kcal/mol). The free energy barriers calculated for the AChE- and BChE-catalyzed reactions are in good agreement with the experimentally derived activation free energies (17.5 and 20.7 kcal/mol for the AChE- and BChE-catalyzed reactions, respectively). Further structural analysis reveals that the aromatic residues Phe295 and Phe297 in the acyl pocket of AChE (corresponding to Leu286 and Val288 in BChE) contribute to the lower energy of TS2a relative to TS2b. The obtained structural and mechanistic insights could be valuable for use in future rational design of a novel therapeutic treatment of heroin abuse. PMID:24595354

Fundamental Reaction Pathway for Peptide Metabolism by Proteasome: Insights from First-principles Quantum Mechanical/Molecular Mechanical Free Energy Calculations

PubMed Central

Wei, Donghui; Fang, Lei; Tang, Mingsheng; Zhan, Chang-Guo

2013-01-01

Proteasome is the major component of the crucial nonlysosomal protein degradation pathway in the cells, but the detailed reaction pathway is unclear. In this study, first-principles quantum mechanical/molecular mechanical free energy calculations have been performed to explore, for the first time, possible reaction pathways for proteasomal proteolysis/hydrolysis of a representative peptide, succinyl-leucyl-leucyl-valyl-tyrosyl-7-amino-4-methylcoumarin (Suc-LLVY-AMC). The computational results reveal that the most favorable reaction pathway consists of six steps. The first is a water-assisted proton transfer within proteasome, activating Thr1-Oγ. The second is a nucleophilic attack on the carbonyl carbon of a Tyr residue of substrate by the negatively charged Thr1-Oγ, followed by the dissociation of the amine AMC (third step). The fourth step is a nucleophilic attack on the carbonyl carbon of the Tyr residue of substrate by a water molecule, accompanied by a proton transfer from the water molecule to Thr1-Nz. Then, Suc-LLVY is dissociated (fifth step), and Thr1 is regenerated via a direct proton transfer from Thr1-Nz to Thr1-Oγ. According to the calculated energetic results, the overall reaction energy barrier of the proteasomal hydrolysis is associated with the transition state (TS3b) for the third step involving a water-assisted proton transfer. The determined most favorable reaction pathway and the rate-determining step have provided a reasonable interpretation of the reported experimental observations concerning the substituent and isotopic effects on the kinetics. The calculated overall free energy barrier of 18.2 kcal/mol is close to the experimentally-derived activation free energy of ~18.3–19.4 kcal/mol, suggesting that the computational results are reasonable. PMID:24111489

Transport of the moving barrier driven by chiral active particles

NASA Astrophysics Data System (ADS)

Liao, Jing-jing; Huang, Xiao-qun; Ai, Bao-quan

2018-03-01

Transport of a moving V-shaped barrier exposed to a bath of chiral active particles is investigated in a two-dimensional channel. Due to the chirality of active particles and the transversal asymmetry of the barrier position, active particles can power and steer the directed transport of the barrier in the longitudinal direction. The transport of the barrier is determined by the chirality of active particles. The moving barrier and active particles move in the opposite directions. The average velocity of the barrier is much larger than that of active particles. There exist optimal parameters (the chirality, the self-propulsion speed, the packing fraction, and the channel width) at which the average velocity of the barrier takes its maximal value. In particular, tailoring the geometry of the barrier and the active concentration provides novel strategies to control the transport properties of micro-objects or cargoes in an active medium.

Thermochemistry of Rare Earth Silicates for Environmental Barrier Coatings

NASA Technical Reports Server (NTRS)

Costa, Gustavo; Jacobson, Nathan

2015-01-01

Rare earth silicates are promising candidates as environmental protective coatings (EBCs) for silica-forming ceramics and composites in combustion environments since they are predicted to have lower reactivity with the water vapor combustion products. The reactivity of rare earth silicates is assessed by the thermodynamic activity of the silica component which is best measured by Knudsen effusion mass spectrometry (KEMS). Here, we discuss a novel method based on a reducing agent to increase the partial pressure of SiO(g) which is then used to calculate thermodynamic activity of silica in Y2O3-SiO2 and Yb2O3-SiO2 systems. After the KEMS measurements, samples were probed by X-ray diffraction and their phase content was calculated from Rietveld refinement.

Theoretical investigation on the dimerization of the deprotonated aquo ion of Al(III) in water.

PubMed

Qian, Zhaosheng; Feng, Hui; Zhang, Zhenjiang; Yang, Wenjing; Jin, Jing; Miao, Qiang; He, Lina; Bi, Shuping

2009-01-21

Reaction pathways, solvent effects and energy barriers have been investigated for the dimerization of the deprotonated aquo ion of Al(III) in aqueous solution by performing supramolecule density functional theory calculations. Two competing reaction pathways were investigated, sharing a common first step and third step, i.e. the formation of the aggregate II of two aluminium monomers and the doubly bridged dimer. One pathway involves a nucleophilic attack to undercoordinated metal center in the first step and then the loss of a coordinated water molecule. Another pathway involves a water exchange reaction in the first step and then the formation of the hydroxo bridge. The calculated results indicate that both pathways I and II are possible in aqueous solution. The direct participation of the solvent water molecule facilitates the dimerization, but the extremely large solvent shifts of the energy barriers for each reaction are attributed mainly to the bulk effect. The computed activation energies for the water exchange reactions are in good agreement with the available experimental values, namely, the calculated value 37.5 kJ mol(-1) compared to the experimental value 36.4 (+/-5) kJ mol(-1). In agreement with experimental observations in aqueous solution, the calculated results favor the transformation of singly-bridged to doubly-bridged aluminium ion, which is helpful to understand the complicated hydrolytic polymerizaiton of Al(III).

Theory and simulation of ion conduction in the pentameric GLIC channel.

PubMed

Zhu, Fangqiang; Hummer, Gerhard

2012-10-09

GLIC is a bacterial member of the large family of pentameric ligand-gated ion channels. To study ion conduction through GLIC and other membrane channels, we combine the one-dimensional potential of mean force for ion passage with a Smoluchowski diffusion model, making it possible to calculate single-channel conductance in the regime of low ion concentrations from all-atom molecular dynamics (MD) simulations. We then perform MD simulations to examine sodium ion conduction through the GLIC transmembrane pore in two systems with different bulk ion concentrations. The ion potentials of mean force, calculated from umbrella sampling simulations with Hamiltonian replica exchange, reveal a major barrier at the hydrophobic constriction of the pore. The relevance of this barrier for ion transport is confirmed by a committor function that rises sharply in the barrier region. From the free evolution of Na(+) ions starting at the barrier top, we estimate the effective diffusion coefficient in the barrier region, and subsequently calculate the conductance of the pore. The resulting diffusivity compares well with the position-dependent ion diffusion coefficient obtained from restrained simulations. The ion conductance obtained from the diffusion model agrees with the value determined via a reactive-flux rate calculation. Our results show that the conformation in the GLIC crystal structure, with an estimated conductance of ~1 picosiemens at 140 mM ion concentration, is consistent with a physiologically open state of the channel.

Mechanism of tungsten-dependent acetylene hydratase from quantum chemical calculations.

PubMed

Liao, Rong-Zhen; Yu, Jian-Guo; Himo, Fahmi

2010-12-28

Acetylene hydratase is a tungsten-dependent enzyme that catalyzes the nonredox hydration of acetylene to acetaldehyde. Density functional theory calculations are used to elucidate the reaction mechanism of this enzyme with a large model of the active site devised on the basis of the native X-ray crystal structure. Based on the calculations, we propose a new mechanism in which the acetylene substrate first displaces the W-coordinated water molecule, and then undergoes a nucleophilic attack by the water molecule assisted by an ionized Asp13 residue at the active site. This is followed by proton transfer from Asp13 to the newly formed vinyl anion intermediate. In the subsequent isomerization, Asp13 shuttles a proton from the hydroxyl group of the vinyl alcohol to the α-carbon. Asp13 is thus a key player in the mechanism, but also W is directly involved in the reaction by binding and activating acetylene and providing electrostatic stabilization to the transition states and intermediates. Several other mechanisms are also considered but the energetic barriers are found to be very high, ruling out these possibilities.

Aryl nitrene rearrangements: spectroscopic observation of a benzazirine and its ring expansion to a ketenimine by heavy-atom tunneling.

PubMed

Inui, Hiroshi; Sawada, Kazuhiro; Oishi, Shigero; Ushida, Kiminori; McMahon, Robert J

2013-07-17

In the photodecompositions of 4-methoxyphenyl azide (1) and 4-methylthiophenyl azide (5) in argon matrixes at cryogenic temperatures, benzazirine intermediates were identified on the basis of IR spectra. As expected, the benzazirines photochemically rearranged to the corresponding ketenimines and triplet nitrenes. Interestingly, with the methylthio substituent, the rearrangement of benzazirine 8 to ketenimine 7 occurred at 1.49 × 10(-5) s(-1) even in the dark at 10 K, despite a computed activation barrier of 3.4 kcal mol(-1). Because this rate is 10(57) times higher than that calculated for passing over the barrier and because it shows no temperature dependence, the rearrangement mechanism is interpreted in terms of heavy-atom tunneling.

Characterization of the Minimum Energy Paths and Energetics for the Reaction of Vinylidene with Acetylene

NASA Technical Reports Server (NTRS)

Walch, Stephen P.; Taylor, Peter R.

1995-01-01

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinyl-acetylene and for a number of isomers of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations (CCSD(T)) to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinyl-acetylene.

Characterization of the Minimum Energy Paths and Energetics for the reaction of Vinylidene with Acetylene

NASA Technical Reports Server (NTRS)

Walch, Stephen P.; Taylor, Peter R.

1995-01-01

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinylacetylene and for a number of isomers Of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations (CCSD(T)) to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinylacetylene.

Investigation of complete and incomplete fusion in the 7Li+124Sn reaction near Coulomb barrier energies

NASA Astrophysics Data System (ADS)

Parkar, V. V.; Sharma, Sushil K.; Palit, R.; Upadhyaya, S.; Shrivastava, A.; Pandit, S. K.; Mahata, K.; Jha, V.; Santra, S.; Ramachandran, K.; Nag, T. N.; Rath, P. K.; Kanagalekar, Bhushan; Trivedi, T.

2018-01-01

The complete and incomplete fusion cross sections for the 7Li+124Sn reaction were measured using online and offline characteristic γ -ray detection techniques. The complete fusion (CF) cross sections at energies above the Coulomb barrier were found to be suppressed by ˜26 % compared to the coupled channel calculations. This suppression observed in complete fusion cross sections is found to be commensurate with the measured total incomplete fusion (ICF) cross sections. There is a distinct feature observed in the ICF cross sections, i.e., t capture is found to be dominant compared to α capture at all the measured energies. A simultaneous explanation of complete, incomplete, and total fusion (TF) data was also obtained from the calculations based on the continuum discretized coupled channel method with short range imaginary potentials. The cross section ratios of CF/TF and ICF/TF obtained from the data as well as the calculations showed the dominance of ICF at below-barrier energies and CF at above-barrier energies.

Self-learning kinetic Monte Carlo simulations of diffusion in ferromagnetic α -Fe–Si alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nandipati, Giridhar; Jiang, Xiujuan; Vemuri, Rama S.

Diffusion in α-Fe-Si alloys is studied using AKSOME, an on-lattice self-learning KMC code, in the ferromagnetic state. Si diffusivity in the α-Fe matrix were obtained with and without the magnetic disorder in various temperature ranges. In addition we studied vacancy diffusivity in ferromagnetic α-Fe at various Si concentrations up to 12.5at.% in the temperature range of 350–550 K. The results were compared with available experimental and theoretical values in the literature. Local Si-atom dependent activation energies for vacancy hops were calculated using a broken-model and were stored in a database. The migration barrier and prefactors for Si-diffusivity were found tomore » be in reasonable agreement with available modeling results in the literature. Magnetic disorder has a larger effect on the prefactor than on the migration barrier. Prefactor was approximately an order of magnitude and the migration barrier a tenth of an electron-volt higher with magnetic disorder when compared to a fully ferromagnetic ordered state. In addition, the correlation between various have a larger effect on the Si-diffusivity extracted in various temperature range than the magnetic disorder. In the case of vacancy diffusivity, the migration barrier more or less remained constant while the prefactor decreased with increasing Si concentration in the disordered or A2-phase of Fe-Si alloy. Important vacancy-Si/Fe atom exchange processes and their activation barriers were also identified and discuss the effect of energetics on the formation of ordered phases in Fe-Si alloys.« less

Temperature dependent barrier height and ideality factor of electrodeposited n-CdSe/Cu Schottky barrier diode

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mahato, S., E-mail: som.phy.ism@gmail.com; Shiwakoti, N.; Kar, A. K.

2015-06-24

This article reports the measurement of temperature-dependent barrier height and ideality factor of n-CdSe/Cu Schottky barrier diode. The Cadmium Selenide (CdSe) thin films have been deposited by simple electrodeposition technique. The XRD measurements ravels the deposited single phase CdSe films are highly oriented on (002) plane and the average particle size has been calculated to be ~18 nm. From SEM characterization, it is clear that the surface of CdSe thin films are continuous, homogeneous and the film is well adhered to the substrate and consists of fine grains which are irregular in shape and size. Current-Voltage characteristics have been measured atmore » different temperatures in the range (298 K – 353 K). The barrier height and ideality factor are found to be strongly temperature dependent. The inhomogenious barrier height increases and ideality factor decreases with increase in temperature. The expectation value has been calculated and its value is 0.30 eV.« less

Tactical maneuvering and calculated risks: independent child migrants and the complex terrain of flight.

PubMed

Denov, Myriam; Bryan, Catherine

2012-01-01

Similar to refugees in general, independent child migrants are frequently constructed in academic and popular discourse as passive and powerless or as untrustworthy and potentially threatening. Such portrayals fail to capture how these youth actively navigate the complex experiences of forced migration. Drawing on interviews with independent child migrants who arrived in Canada and on the conceptual framework of social navigation, we argue that contrary to being powerless, and despite significant structural barriers, these youth deliberately and thoughtfully navigate flight by making strategic decisions and taking calculated risks thereby ensuring their survival and well-being. Copyright © 2012 Wiley Periodicals, Inc., A Wiley Company.

High-throughput ab-initio dilute solute diffusion database.

PubMed

Wu, Henry; Mayeshiba, Tam; Morgan, Dane

2016-07-19

We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world.

Perceived somatic and affective barriers for self-efficacy and physical activity.

PubMed

Warner, Lisa M; Wolff, Julia K; Spuling, Svenja M; Wurm, Susanne

2017-05-01

According to Bandura's social-cognitive theory, perceptions of somatic and affective barriers are sources of self-efficacy. This longitudinal study compares general indicators of health barriers with measures of perceived somatic and affective barriers to predict self-efficacy and accelerometer-assessed physical activity in a subsample of n = 153 (selected at random from N = 310) community-dwelling German older adults. Perceived somatic and affective barriers longitudinally predicted physical activity mediated by self-efficacy, whereas general health barriers did not. Perceived health barriers to physical activity might be more important than more objective health barriers for older adults' physical activity levels.

Pharmacokinetics and In Vitro Blood-Brain Barrier Screening of the Plant-Derived Alkaloid Tryptanthrin.

PubMed

Jähne, Evelyn A; Eigenmann, Daniela E; Sampath, Chethan; Butterweck, Veronika; Culot, Maxime; Cecchelli, Roméo; Gosselet, Fabien; Walter, Fruzsina R; Deli, Mária A; Smieško, Martin; Hamburger, Matthias; Oufir, Mouhssin

2016-07-01

The indolo[2,1-b]quinazoline alkaloid tryptanthrin was previously identified as a potent anti-inflammatory compound with a unique pharmacological profile. It is a potent inhibitor of cyclooxygenase-2, 5-lipooxygenase-catalyzed leukotriene synthesis, and nitric oxide production catalyzed by the inducible nitric oxide synthase. To characterize the pharmacokinetic properties of tryptanthrin, we performed a pilot in vivo study in male Sprague-Dawley rats (2 mg/kg bw i. v.). Moreover, the ability of tryptanthrin to cross the blood-brain barrier was evaluated in three in vitro human and animal blood-brain barrier models. Bioanalytical UPLC-MS/MS methods used were validated according to current international guidelines. A half-life of 40.63 ± 6.66 min and a clearance of 1.00 ± 0.36 L/h/kg were found in the in vivo pharmacokinetic study. In vitro data obtained with the two primary animal blood-brain barrier models showed a good correlation with an immortalized human monoculture blood-brain barrier model (hBMEC cell line), and were indicative of a high blood-brain barrier permeation potential of tryptanthrin. These findings were corroborated by the in silico prediction of blood-brain barrier penetration. P-glycoprotein interaction of tryptanthrin was assessed by calculation of the efflux ratio in bidirectional permeability assays. An efflux ratio below 2 indicated that tryptanthrin is not subjected to active efflux. Georg Thieme Verlag KG Stuttgart · New York.

Barriers to Leisure-Time Physical Activities in Individuals with Spinal Cord Injury.

PubMed

Hwang, Eric J; Groves, Mary D; Sanchez, Jacqueline N; Hudson, Cassandra E; Jao, Rachel G; Kroll, Meghan E

2016-07-01

This study investigated the personal, environmental, and activity barriers to leisure-time physical activities (LTPAs) among individuals with spinal cord injury (SCI). A survey instrument was administered to 85 participants with SCI. Personal barriers to LTPAs included issues involving motivation, pain, scheduling, and financial resources. Environmental barriers marked the issues regarding availability and accessibility to specialized programs, activities, and professional services. Activity barriers included limitations in equipment, training, and personal skills required by the selected activities. Significant negative correlations were found between these barriers and the levels of physical activity and satisfaction with physical activity. While working with clients with SCI, occupational therapists should identify those LTPA barriers and possible solutions in order to establish individualized action plans for enhancing participation in LTPAs.

Numerical modeling of the destruction of steel plates with a gradient substrate

NASA Astrophysics Data System (ADS)

Orlov, M. Yu.; Glazyrin, V. P.; Orlov, Yu. N.

2017-10-01

The paper presents the results of numerical simulation of the shock loading process of steel barriers with a gradient substrate. In an elastic plastic axisymmetric statement, a shock is simulated along the normal in the range of initial velocities up to 300 m / s. A range of initial velocities was revealed, in which the presence of a substrate "saved" the obstacle from spallation. New tasks were announced to deepen scientific knowledge about the behavior of unidirectional gradient barriers at impact. The results of calculations are obtained in the form of graphs, calculated configurations of the "impact - barrier" and tables.

Determinants of reactivity and selectivity in soluble epoxide hydrolase from quantum mechanics/molecular mechanics modeling.

PubMed

Lonsdale, Richard; Hoyle, Simon; Grey, Daniel T; Ridder, Lars; Mulholland, Adrian J

2012-02-28

Soluble epoxide hydrolase (sEH) is an enzyme involved in drug metabolism that catalyzes the hydrolysis of epoxides to form their corresponding diols. sEH has a broad substrate range and shows high regio- and enantioselectivity for nucleophilic ring opening by Asp333. Epoxide hydrolases therefore have potential synthetic applications. We have used combined quantum mechanics/molecular mechanics (QM/MM) umbrella sampling molecular dynamics (MD) simulations (at the AM1/CHARMM22 level) and high-level ab initio (SCS-MP2) QM/MM calculations to analyze the reactions, and determinants of selectivity, for two substrates: trans-stilbene oxide (t-SO) and trans-diphenylpropene oxide (t-DPPO). The calculated free energy barriers from the QM/MM (AM1/CHARMM22) umbrella sampling MD simulations show a lower barrier for phenyl attack in t-DPPO, compared with that for benzylic attack, in agreement with experiment. Activation barriers in agreement with experimental rate constants are obtained only with the highest level of QM theory (SCS-MP2) used. Our results show that the selectivity of the ring-opening reaction is influenced by several factors, including proximity to the nucleophile, electronic stabilization of the transition state, and hydrogen bonding to two active site tyrosine residues. The protonation state of His523 during nucleophilic attack has also been investigated, and our results show that the protonated form is most consistent with experimental findings. The work presented here illustrates how determinants of selectivity can be identified from QM/MM simulations. These insights may also provide useful information for the design of novel catalysts for use in the synthesis of enantiopure compounds.

Quantum Mechanics and Molecular Mechanics Study of the Catalytic Mechanism of Human AMSH-LP Domain Deubiquitinating Enzymes.

PubMed

Zhu, Wenyou; Liu, Yongjun; Ling, Baoping

2015-08-25

Deubiquitinating enzymes (DUBs) catalyze the cleavage of the isopeptide bond in polyubiquitin chains to control and regulate the deubiquitination process in all known eukaryotic cells. The human AMSH-LP DUB domain specifically cleaves the isopeptide bonds in the Lys63-linked polyubiquitin chains. In this article, the catalytic mechanism of AMSH-LP has been studied using a combined quantum mechanics and molecular mechanics method. Two possible hydrolysis processes (Path 1 and Path 2) have been considered. Our calculation results reveal that the activation of Zn(2+)-coordinated water molecule is the essential step for the hydrolysis of isopeptide bond. In Path 1, the generated hydroxyl first attacks the carbonyl group of Gly76, and then the amino group of Lys63 is protonated, which is calculated to be the rate limiting step with an energy barrier of 13.1 kcal/mol. The energy barrier of the rate limiting step and the structures of intermediate and product are in agreement with the experimental results. In Path 2, the protonation of amino group of Lys63 is prior to the nucleophilic attack of activated hydroxyl. The two proton transfer processes in Path 2 correspond to comparable overall barriers (33.4 and 36.1 kcal/mol), which are very high for an enzymatic reaction. Thus, Path 2 can be ruled out. During the reaction, Glu292 acts as a proton transfer mediator, and Ser357 mainly plays a role in stabilizing the negative charge of Gly76. Besides acting as a Lewis acid, Zn(2+) also influences the reaction by coordinating to the reaction substrates (W1 and Gly76).

Perceived benefits and barriers to leisure-time physical activity during pregnancy in previously inactive and active women.

PubMed

Da Costa, Deborah; Ireland, Kierla

2013-01-01

This study compared perceived benefits and barriers to leisure-time physical activity during pregnancy among women who were insufficiently active or inactive before pregnancy. Eighty-two pregnant women completed questionnaires assessing leisure-time physical activity benefits/barriers, exercise self-efficacy, social support, depressed mood, pre-pregnancy and current physical activity and fatigue. Multivariable regression analyses identified factors associated with exercise benefits/barriers for the two pre-pregnancy leisure-time physical activity groups. Both pre-pregnancy leisure-time physical activity groups reported more benefits than barriers to exercise during pregnancy. Previously inactive women reported fewer perceived benefits and greater perceived barriers to leisure-time physical activity during pregnancy. Higher self-efficacy for exercise during pregnancy was significantly associated with greater benefits of leisure-time physical activity during pregnancy for both groups. Less family support for exercise and lower self-efficacy for exercise were significantly related to greater leisure-time physical activity barriers during pregnancy for previously inactive women. Lower self-efficacy for exercise, higher depressed mood scores, and younger age were associated with greater leisure-time physical activity barriers for active women. Findings suggest that the intensities of perceived leisure-time physical activity benefits and barriers during pregnancy differ for women, depending on their pre-pregnancy leisure-time physical activity status. Consideration of pre-pregnancy leisure-time physical activity status may thus be important when tailoring strategies to overcome barriers to promote initiation and maintenance of physical activity during pregnancy.

Theoretical study of the promotional effect of ZrO2 on In2O3 catalyzed methanol synthesis from CO2 hydrogenation

NASA Astrophysics Data System (ADS)

Zhang, Minhua; Dou, Maobin; Yu, Yingzhe

2018-03-01

Methanol synthesis from CO2 hydrogenation on the ZrO2 doped In2O3(110) surface (Zr-In2O3(110)) with oxygen vacancy has been studied using the density functional theory calculations. The calculated results show that the doped ZrO2 species prohibits the excessive formation of oxygen vacancies and dissociation of H2 on In2O3 surface slightly, but enhances the adsorption of CO2 on both perfect and defective Zr-In2O3(110) surface. Methanol is formed via the HCOO route. The hydrogenation of CO2 to HCOO is both energetically and kinetically facile. The HCOO hydrogenates to polydentate H2CO (p-H2CO) species with an activation barrier of 0.75 eV. H3CO is produced from the hydrogenation of monodentate H2CO (mono-H2CO), transformation from p-H2CO with 0.82 eV reaction energy, with no barrier whether there is hydroxyl group between the mono-H2CO and the neighboring hydride or not. Methanol is the product of H3CO protonation with 0.75 eV barrier. The dissociation and protonation of CO2 are both energetically and kinetically prohibited on Zr-In2O3(110) surface. The doped ZrO2 species can further enhance the adsorption of all the intermediates involved in CO2 hydrogenation to methanol, activate the adsorbed CO2 and H2CO, and stabilize the HCOO, H2CO and H3CO, especially prohibit the dissociation of H2CO or the reaction of H2CO with neighboring hydride to form HCOO and gas phase H2. All these effects make the ZrO2 supported In2O3 catalyst exhibit higher activity and selectivity on methanol synthesis from CO2 hydrogenation.

Reaction mechanisms at 4H-SiC/SiO2 interface during wet SiC oxidation

NASA Astrophysics Data System (ADS)

Akiyama, Toru; Hori, Shinsuke; Nakamura, Kohji; Ito, Tomonori; Kageshima, Hiroyuki; Uematsu, Masashi; Shiraishi, Kenji

2018-04-01

The reaction processes at the interface between SiC with 4H structure (4H-SiC) and SiO2 during wet oxidation are investigated by electronic structure calculations within the density functional theory. Our calculations for 4H-SiC/SiO2 interfaces with various orientations demonstrate characteristic features of the reaction depending on the crystal orientation of SiC: On the Si-face, the H2O molecule is stable in SiO2 and hardly reacts with the SiC substrate, while the O atom of H2O can form Si-O bonds at the C-face interface. Two OH groups are found to be at least necessary for forming new Si-O bonds at the Si-face interface, indicating that the oxidation rate on the Si-face is very low compared with that on the C-face. On the other hand, both the H2O molecule and the OH group are incorporated into the C-face interface, and the energy barrier for OH is similar to that for H2O. By comparing the calculated energy barriers for these reactants with the activation energies of oxide growth rate, we suggest the orientation-dependent rate-limiting processes during wet SiC oxidation.

A QM/MM Metadynamics Study of the Direct Decarboxylation Mechanism for Orotidine-5'-monophosphate Decarboxylase using Two Different QM Regions: Acceleration too Small to Explain Rate of Enzyme Catalysis

PubMed Central

Stanton, Courtney; Kuo, I-Feng W.; Mundy, Christopher J.; Laino, Teodoro; Houk, K. N.

2011-01-01

Despite decades of study, the mechanism by which orotidine-5'-monophosphate decarboxylase (ODCase) catalyzes the decarboxylation of orotidine monophosphate remains unresolved. A computational investigation of the direct decarboxylation mechanism has been performed using mixed quantum mechanical/molecular mechanical (QM/MM) dynamics simulations. The study was performed with the program CP2K that integrates classical dynamics and ab initio dynamics based on the Born-Oppenheimer approach. Two different QM regions were explored. The free energy barriers for decarboxylation of orotidine-5'-monophosphate (OMP) in solution and in the enzyme (using the larger QM region) were determined with the metadynamics method to be 40 kcal/mol and 33 kcal/mol, respectively. The calculated change in activation free energy (ΔΔG±) on going from solution to the enzyme is therefore −7 kcal/mol, far less than the experimental change of −23 kcal/mol (for kcat/kuncat Radzicka, A.; Wolfenden, R., Science. 1995, 267, 90–92). These results do not support the direct decarboxylation mechanism that has been proposed for the enzyme. However, in the context of QM/MM calculations, it was found that the size of the QM region has a dramatic effect on the calculated reaction barrier. PMID:17927240

Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters.

PubMed

Ishikawa, Akio; Neurock, Matthew; Iglesia, Enrique

2007-10-31

The identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steady-state vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.

Theoretical studies of the transition state structures and free energy barriers for base-catalyzed hydrolysis of amides

PubMed Central

Xiong, Ying; Zhan, Chang-Guo

2010-01-01

The transition state structures and free energy barriers for the rate-determining step (i.e. the formation of a tetrahedral intermediate) of base-catalyzed hydrolysis of a series of amides in aqueous solution have been studied by performing first-principle electronic structure calculations using a hybrid supermolecule-polarizable continuum approach. The calculated results and a revisit of recently reported experimental proton inventory data reveal that the favorable transition state structure optimized for the tetrahedral intermediate formation of hydroxide ion-catalyzed hydrolysis of formamide may have three solvating water molecules remaining on the attacking hydroxide oxygen and two additional water molecules attached to the carbonyl oxygen of formamide. The calculated results have also demonstrated interesting substituent effects on the optimized transition state geometries, on the transition-state stabilization, and on the calculated free energy barriers for the base-catalyzed hydrolysis of amides. When some or all of the hydrogen atoms of formamide are replaced by methyl groups, the total number of water molecules hydrogen-bonding with the attacking hydroxide in the transition state decreases from three for formamide to two for N-methylacetamide, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). The larger substituents of the amide hinder the solvent water molecules approaching the attacking hydroxide oxygen in the transition state and, therefore, destabilize the transition state structure and increase the free energy barrier. By using the optimized most favorable transition state structures, the calculated free energy barriers, i.e. 21.6 (or 21.7), 22.7, 23.1, and 26.0 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively, are in good agreement with the available experimental free energy barriers, i.e. 21.2, 21.5, 22.6, and 24.1 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively. PMID:17107116

Theoretical studies of the potential surface for the F - H2 greater than HF + H reaction

NASA Technical Reports Server (NTRS)

Bauschlicher, Charles W., Jr.; Walch, Stephen, P.; Langhoff, Stephen R.; Taylor, Peter R.; Jaffe, Richard L.

1987-01-01

The F + H2 yields HF + H potential energy hypersurface was studied in the saddle point and entrance channel regions. Using a large (5s 5p 3d 2f 1g/4s 3p 2d) atomic natural orbital basis set, a classical barrier height of 1.86 kcal/mole was obtained at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mole. The location of the bottleneck on the lowest vibrationally adiabatic potential curve was also computed and the translational energy threshold determined from a one-dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0 to 1.5 kcal/mole. Combining the results of the direct estimates of the classical barrier height with the empirical values obtained from the approximation calculations of the dynamical threshold, it is predicted that the true classical barrier height is 1.4 + or - 0.4 kcal/mole. Arguments are presented in favor of including the relatively large +Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level.

DFT investigations of phosphotriesters hydrolysis in aqueous solution: a model for DNA single strand scission induced by N-nitrosoureas.

PubMed

Liu, Tingting; Zhao, Lijiao; Zhong, Rugang

2013-02-01

DNA phosphotriester adducts are common alkylation products of DNA phosphodiester moiety induced by N-nitrosoureas. The 2-hydroxyethyl phosphotriester was reported to hydrolyze more rapidly than other alkyl phosphotriesters both in neutral and in alkaline conditions, which can cause DNA single strand scission. In this work, DFT calculations have been employed to map out the four lowest activation free-energy profiles for neutral and alkaline hydrolysis of triethyl phosphate (TEP) and diethyl 2-hydroxyethyl phosphate (DEHEP). All the hydrolysis pathways were illuminated to be stepwise involving an acyclic or cyclic phosphorane intermediate for TEP or DEHEP, respectively. The rate-limiting step for all the hydrolysis reactions was found to be the formation of phosphorane intermediate, with the exception of DEHEP hydrolysis in alkaline conditions that the decomposition process turned out to be the rate-limiting step, owing to the extraordinary low formation barrier of cyclic phosphorane intermediate catalyzed by hydroxide. The rate-limiting barriers obtained for the four reactions are all consistent with the available experimental information concerning the corresponding hydrolysis reactions of phosphotriesters. Our calculations performed on the phosphate triesters hydrolysis predict that the lower formation barriers of cyclic phosphorane intermediates compared to its acyclic counter-part should be the dominant factor governing the hydrolysis rate enhancement of DEHEP relative to TEP both in neutral and in alkaline conditions.

Computing the Viscosity of Supercooled Liquids: Markov Network Model

PubMed Central

Li, Ju; Kushima, Akihiro; Eapen, Jacob; Lin, Xi; Qian, Xiaofeng; Mauro, John C.; Diep, Phong; Yip, Sidney

2011-01-01

The microscopic origin of glass transition, when liquid viscosity changes continuously by more than ten orders of magnitude, is challenging to explain from first principles. Here we describe the detailed derivation and implementation of a Markovian Network model to calculate the shear viscosity of deeply supercooled liquids based on numerical sampling of an atomistic energy landscape, which sheds some light on this transition. Shear stress relaxation is calculated from a master-equation description in which the system follows a transition-state pathway trajectory of hopping among local energy minima separated by activation barriers, which is in turn sampled by a metadynamics-based algorithm. Quantitative connection is established between the temperature variation of the calculated viscosity and the underlying potential energy and inherent stress landscape, showing a different landscape topography or “terrain” is needed for low-temperature viscosity (of order 107 Pa·s) from that associated with high-temperature viscosity (10−5 Pa·s). Within this range our results clearly indicate the crossover from an essentially Arrhenius scaling behavior at high temperatures to a low-temperature behavior that is clearly super-Arrhenius (fragile) for a Kob-Andersen model of binary liquid. Experimentally the manifestation of this crossover in atomic dynamics continues to raise questions concerning its fundamental origin. In this context this work explicitly demonstrates that a temperature-dependent “terrain” characterizing different parts of the same potential energy surface is sufficient to explain the signature behavior of vitrification, at the same time the notion of a temperature-dependent effective activation barrier is quantified. PMID:21464988

A molecular dynamics study of intramolecular proton transfer reaction of malonaldehyde in solution based upon a mixed quantum–classical approximation. II. Proton transfer reaction in non-polar solvent

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kojima, H.; Yamada, A.; Okazaki, S., E-mail: okazaki@apchem.nagoya-u.ac.jp

2015-05-07

The intramolecular proton transfer reaction of malonaldehyde in neon solvent has been investigated by mixed quantum–classical molecular dynamics (QCMD) calculations and fully classical molecular dynamics (FCMD) calculations. Comparing these calculated results with those for malonaldehyde in water reported in Part I [A. Yamada, H. Kojima, and S. Okazaki, J. Chem. Phys. 141, 084509 (2014)], the solvent dependence of the reaction rate, the reaction mechanism involved, and the quantum effect therein have been investigated. With FCMD, the reaction rate in weakly interacting neon is lower than that in strongly interacting water. However, with QCMD, the order of the reaction rates ismore » reversed. To investigate the mechanisms in detail, the reactions were categorized into three mechanisms: tunneling, thermal activation, and barrier vanishing. Then, the quantum and solvent effects were analyzed from the viewpoint of the reaction mechanism focusing on the shape of potential energy curve and its fluctuations. The higher reaction rate that was found for neon in QCMD compared with that found for water solvent arises from the tunneling reactions because of the nearly symmetric double-well shape of the potential curve in neon. The thermal activation and barrier vanishing reactions were also accelerated by the zero-point energy. The number of reactions based on these two mechanisms in water was greater than that in neon in both QCMD and FCMD because these reactions are dominated by the strength of solute–solvent interactions.« less

Quantal Study of the Exchange Reaction for N + N2 using an ab initio Potential Energy Surface

NASA Technical Reports Server (NTRS)

Wang, Dunyou; Stallcop, James R.; Huo, Winifred M.; Dateo, Christopher E.; Schwenke, David W.; Partridge, Harry; Kwak, Dochan (Technical Monitor)

2002-01-01

The N + N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground A" state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N2(J=O). The J - K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasi-classical calculation using a LEPS PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate 3D quantal dynamics study for N + N2 reaction system and the ab initio PES reported here is the first such surface for N3.

New Global Calculation of Nuclear Masses and Fission Barriers for Astrophysical Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moeller, P.; Sierk, A. J.; Bengtsson, R.

The FRDM(1992) mass model [1] has an accuracy of 0.669 MeV in the region where its parameters were determined. For the 529 masses that have been measured since, its accuracy is 0.46 MeV, which is encouraging for applications far from stability in astrophysics. We are developing an improved mass model, the FRDM(2008). The improvements in the calculations with respect to the FRDM(1992) are in two main areas. (1) The macroscopic model parameters are better optimized. By simulation (adjusting to a limited set of now known nuclei) we can show that this actually makes the results more reliable in new regionsmore » of nuclei. (2) The ground-state deformation parameters are more accurately calculated. We minimize the energy in a four-dimensional deformation space ({epsilon}{sub 2}, {epsilon}{sub 3}, {epsilon}{sub 4}, {epsilon}{sub 6},) using a grid interval of 0.01 in all 4 deformation variables. The (non-finalized) FRDM (2008-a) has an accuracy of 0.596 MeV with respect to the 2003 Audi mass evaluation before triaxial shape degrees of freedom are included (in progress). When triaxiality effects are incorporated preliminary results indicate that the model accuracy will improve further, to about 0.586 MeV.We also discuss very large-scale fission-barrier calculations in the related FRLDM (2002) model, which has been shown to reproduce very satisfactorily known fission properties, for example barrier heights from {sup 70}Se to the heaviest elements, multiple fission modes in the Ra region, asymmetry of mass division in fission and the triple-humped structure found in light actinides. In the superheavy region we find barriers consistent with the observed half-lives. We have completed production calculations and obtain barrier heights for 5254 nuclei heavier than A = 170 for all nuclei between the proton and neutron drip lines. The energy is calculated for 5009325 different shapes for each nucleus and the optimum barrier between ground state and separated fragments is determined by use of an ''immersion'' technique.« less

RadShield: semiautomated shielding design using a floor plan driven graphical user interface

PubMed Central

Wu, Dee H.; Yang, Kai; Rutel, Isaac B.

2016-01-01

The purpose of this study was to introduce and describe the development of RadShield, a Java‐based graphical user interface (GUI), which provides a base design that uniquely performs thorough, spatially distributed calculations at many points and reports the maximum air‐kerma rate and barrier thickness for each barrier pursuant to NCRP Report 147 methodology. Semiautomated shielding design calculations are validated by two approaches: a geometry‐based approach and a manual approach. A series of geometry‐based equations were derived giving the maximum air‐kerma rate magnitude and location through a first derivative root finding approach. The second approach consisted of comparing RadShield results with those found by manual shielding design by an American Board of Radiology (ABR)‐certified medical physicist for two clinical room situations: two adjacent catheterization labs, and a radiographic and fluoroscopic (R&F) exam room. RadShield's efficacy in finding the maximum air‐kerma rate was compared against the geometry‐based approach and the overall shielding recommendations by RadShield were compared against the medical physicist's shielding results. Percentage errors between the geometry‐based approach and RadShield's approach in finding the magnitude and location of the maximum air‐kerma rate was within 0.00124% and 14 mm. RadShield's barrier thickness calculations were found to be within 0.156 mm lead (Pb) and 0.150 mm lead (Pb) for the adjacent catheterization labs and R&F room examples, respectively. However, within the R&F room example, differences in locating the most sensitive calculation point on the floor plan for one of the barriers was not considered in the medical physicist's calculation and was revealed by the RadShield calculations. RadShield is shown to accurately find the maximum values of air‐kerma rate and barrier thickness using NCRP Report 147 methodology. Visual inspection alone of the 2D X‐ray exam distribution by a medical physicist may not be sufficient to accurately select the point of maximum air‐kerma rate or barrier thickness. PACS number(s): 87.55.N, 87.52.‐g, 87.59.Bh, 87.57.‐s PMID:27685128

RadShield: semiautomated shielding design using a floor plan driven graphical user interface.

PubMed

DeLorenzo, Matthew C; Wu, Dee H; Yang, Kai; Rutel, Isaac B

2016-09-08

The purpose of this study was to introduce and describe the development of RadShield, a Java-based graphical user interface (GUI), which provides a base design that uniquely performs thorough, spatially distributed calculations at many points and reports the maximum air-kerma rate and barrier thickness for each barrier pursuant to NCRP Report 147 methodology. Semiautomated shielding design calculations are validated by two approaches: a geometry-based approach and a manual approach. A series of geometry-based equations were derived giv-ing the maximum air-kerma rate magnitude and location through a first derivative root finding approach. The second approach consisted of comparing RadShield results with those found by manual shielding design by an American Board of Radiology (ABR)-certified medical physicist for two clinical room situations: two adjacent catheterization labs, and a radiographic and fluoroscopic (R&F) exam room. RadShield's efficacy in finding the maximum air-kerma rate was compared against the geometry-based approach and the overall shielding recommendations by RadShield were compared against the medical physicist's shielding results. Percentage errors between the geometry-based approach and RadShield's approach in finding the magnitude and location of the maximum air-kerma rate was within 0.00124% and 14 mm. RadShield's barrier thickness calculations were found to be within 0.156 mm lead (Pb) and 0.150 mm lead (Pb) for the adjacent catheteriza-tion labs and R&F room examples, respectively. However, within the R&F room example, differences in locating the most sensitive calculation point on the floor plan for one of the barriers was not considered in the medical physicist's calculation and was revealed by the RadShield calculations. RadShield is shown to accurately find the maximum values of air-kerma rate and barrier thickness using NCRP Report 147 methodology. Visual inspection alone of the 2D X-ray exam distribution by a medical physicist may not be sufficient to accurately select the point of maximum air-kerma rate or barrier thickness. © 2016 The Authors.

Perceived barriers to walking for physical activity.

PubMed

Dunton, Genevieve F; Schneider, Margaret

2006-10-01

Although the health benefits of walking for physical activity have received increasing research attention, barriers specific to walking are not well understood. In this study, questions to measure barriers to walking for physical activity were developed and tested among college students. The factor structure, test-retest and internal consistency reliability, and discriminant and criterion validity of the perceived barriers were evaluated. A total of 305 undergraduate students participated. Participants had a mean age (+/- SD) of 20.6 (+/- 3.02) years, and 70.3% were female. Participants responded to a questionnaire assessing barriers specific to walking for physical activity. Perceived barriers to vigorous exercise, walking for transportation and recreation, and participation in lifestyle activities (such as taking the stairs instead of the elevator) were also assessed. Subsamples completed the walking barriers instrument a second time after 5 days in order to determine test-retest reliability (n = 104) and wore an accelerometer to measure moderate-intensity physical activity (n = 85). Factor analyses confirmed the existence of three factors underlying the perceived barriers to walking questions: appearance (four items), footwear (three items), and situation (three items). Appearance and situational barriers demonstrated acceptable reliability, discriminant validity, and relations with physical activity criteria. After we controlled for barriers to vigorous exercise, appearance and situational barriers to walking explained additional variation in objectively-measured moderate physical activity. The prediction of walking for physical activity, especially walking that is unstructured and spontaneous, may be improved by considering appearance and situational barriers. Assessing barriers specific to walking may have important implications for interventions targeting walking as means for engaging in physical activity.

Conformational dependence of a protein kinase phosphate transfer reaction.

PubMed

Henkelman, Graeme; LaBute, Montiago X; Tung, Chang-Shung; Fenimore, P W; McMahon, Benjamin H

2005-10-25

Atomic motions and energetics for a phosphate transfer reaction catalyzed by the cAMP-dependent protein kinase are calculated by plane-wave density functional theory, starting from structures of proteins crystallized in both the reactant conformation (RC) and the transition-state conformation (TC). In TC, we calculate that the reactants and products are nearly isoenergetic with a 20-kJ/mol barrier, whereas phosphate transfer is unfavorable by 120 kJ/mol in the RC, with an even higher barrier. With the protein in TC, the motions involved in reaction are small, with only P(gamma) and the catalytic proton moving >0.5 A. Examination of the structures reveals that in the RC the active site cleft is not completely closed and there is insufficient space for the phosphorylated serine residue in the product state. Together, these observations imply that the phosphate transfer reaction occurs rapidly and reversibly in a particular conformation of the protein, and that the reaction can be gated by changes of a few tenths of an angstrom in the catalytic site.

Energetics of Glutamate Binding to an Ionotropic Glutamate Receptor.

PubMed

Yu, Alvin; Lau, Albert Y

2017-11-22

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that are responsible for the majority of excitatory transmission at the synaptic cleft. Mechanically speaking, agonist binding to the ligand binding domain (LBD) activates the receptor by triggering a conformational change that is transmitted to the transmembrane region, opening the ion channel pore. We use fully atomistic molecular dynamics simulations to investigate the binding process in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, an iGluR subtype. The string method with swarms of trajectories was applied to calculate the possible pathways glutamate traverses during ligand binding. Residues peripheral to the binding cleft are found to metastably bind the ligand prior to ligand entry into the binding pocket. Umbrella sampling simulations were performed to compute the free energy barriers along the binding pathways. The calculated free energy profiles demonstrate that metastable interactions contribute substantially to the energetics of ligand binding and form local minima in the overall free energy landscape. Protein-ligand interactions at sites outside of the orthosteric agonist-binding site may serve to lower the transition barriers of the binding process.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, James B.

We report the third in a series of ’exact’ quantum Monte Carlo calculations for the potential energy of the saddle point of the barrier for the reaction H + H{sub 2} → H{sub 2} + H. The barrier heights determined are 9.61 ± 0.01 in 1992/94, 9.608 ± 0.001 in 2003, and 9.6089 ± 0.0001 in 2016 (this work), all in kcal/mole and successively a factor of ten more accurate. The new value is below the lowest value from explicitly correlated Gaussian calculations and within the estimated limits of extrapolated multireference configuration calculations.

Interaction of tetraethoxysilane with OH-terminated SiO2 (0 0 1) surface: A first principles study

NASA Astrophysics Data System (ADS)

Deng, Xiaodi; Song, Yixu; Li, Jinchun; Pu, Yikang

2014-06-01

First principles calculates have been performed to investigate the surface reaction mechanism of tetraethoxysilane (TEOS) with fully hydroxylated SiO2(0 0 1) substrate. In semiconductor industry, this is the key step to understand and control the SiO2 film growth in chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. During the calculation, we proposed a model which breaks the surface dissociative chemisorption into two steps and we calculated the activation barriers and thermochemical energies for each step. Our calculation result for step one shows that the first half reaction is thermodynamically favorable. For the second half reaction, we systematically studied the two potential reaction pathways. The comparing result indicates that the pathway which is more energetically favorable will lead to formation of crystalline SiO2 films while the other will lead to formation of disordered SiO2 films.

On the Traversal Time of Barriers

NASA Astrophysics Data System (ADS)

Aichmann, Horst; Nimtz, Günter

2014-06-01

Fifty years ago Hartman studied the barrier transmission time of wave packets (J Appl Phys 33:3427-3433, 1962). He was inspired by the tunneling experiments across thin insulating layers at that time. For opaque barriers he calculated faster than light propagation and a transmission time independent of barrier length, which is called the Hartman effect. A faster than light (FTL or superluminal) wave packet velocity was deduced in analog tunneling experiments with microwaves and with infrared light thirty years later. Recently, the conjectured zero time of electron tunneling was claimed to have been observed in ionizing helium inside the barrier. The calculated and measured short tunneling time arises at the barrier front. This tunneling time was found to be universal for elastic fields as well as for electromagnetic fields. Remarkable is that the delay time is the same for the reflected and the transmitted waves in the case of symmetric barriers. Several theoretical physicists predicted this strange nature of the tunneling process. However, even with this background many members of the physics community do not accept a FTL signal velocity interpretation of the experimental tunneling results. Instead a luminal front velocity was calculated to explain the FTL experimental results frequently. However, Brillouin stated in his book on wave propagation and group velocity that the front velocity is given by the group velocity of wave packets in the case of physical signals, which have only finite frequency bandwidths. Some studies assumed barriers to be cavities and the observed tunneling time does represent the cavity lifetime. We are going to discus these continuing misleading interpretations, which are found in journals and in textbooks till today.

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in murine epidermis. Modulation of enzyme content and activation state by barrier requirements.

PubMed Central

Proksch, E; Elias, P M; Feingold, K R

1990-01-01

Epidermal cholesterol biosynthesis is regulated by barrier function. We quantitated the amount and activation state (phosphorylation-dephosphorylation) of the rate-limiting enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, in epidermis before and after barrier disruption. In murine epidermis we found high enzyme activity (1.75 +/- 0.02 nmol/min per mg protein). After acute barrier disruption, enzyme activity began to increase after 1.5 h, reaching a maximum increase by 2.5 h, and returned to normal by 15 h. Chronic barrier disruption increased total enzyme activity by 83%. In normal epidermis, measurement of HMG CoA reductase activity in microsomes isolated in NaF- vs. NaCl-containing buffers demonstrated that 46 +/- 2% of the enzyme was in the active form. After acute or chronic barrier disruption, a marked increase in the percentage of HMG CoA reductase in the active form was observed. Acute disruption increased enzyme activation state as early as 15 min, reaching a maximum after 2.5 h, with an increase still present at 15 h, indicating that changes in activation state had a close temporal relationship with barrier function. Increases in total HMG CoA reductase activity occurred only after profound barrier disruption, whereas changes in activation state occur with lesser degrees of barrier disruption. Artificial correction of barrier function prevented the increase in total HMG CoA reductase activity, and partially prevented the increase in enzyme activation. These results show that barrier requirements regulate epidermal cholesterol synthesis by modulating both the HMG CoA reductase amount and activation state. Images PMID:2312730

Spin-resolved conductance of Dirac electrons through multibarrier arrays

NASA Astrophysics Data System (ADS)

Dahal, Dipendra; Gumbs, Godfrey; Iurov, Andrii

We use a transfer matrix method to calculate the transmission coefficient of Dirac electrons through an arbitrary number of square potential barrier in gapped monolayer graphene(MLG) and bilayer graphene (BLG). The widths of barriers may not be chosen equal. The shift in the angle of incidence and the width of the barrier required for resonance are investigated numerically for both MLG and BLG. We compare the effects due to energy gap on these two transmission coefficient for each of these two structures (MLG and BLG). We present our results as functions of barrier width, height as well as incoming electron energy as well as band gap and examine the conditions for which perfect reflection or transmission occurs. Our transmission data are further used to calculate conductivity.

Computational characterization of lightweight multilayer MXene Li-ion battery anodes

NASA Astrophysics Data System (ADS)

Ashton, Michael; Hennig, Richard G.; Sinnott, Susan B.

2016-01-01

MXenes, a class of two-dimensional transition metal carbides and nitrides, have shown promise experimentally and computationally for use in energy storage applications. In particular, the most lightweight members of the monolayer MXene family (M = Sc, Ti, V, or Cr) are predicted to have gravimetric capacities above 400 mAh/g, higher than graphite. Additionally, intercalation of ions into multilayer MXenes can be accomplished at low voltages, and low diffusion barriers exist for Li diffusing across monolayer MXenes. However, large discrepancies have been observed between the calculated and experimental reversible capacities of MXenes. Here, dispersion-corrected density functional theory calculations are employed to predict reversible capacities and other battery-related properties for six of the most promising members of the MXene family (O-functionalized Ti- and V-based carbide MXenes) as bilayer structures. The calculated reversible capacities of the V2CO2 and Ti2CO2 bilayers agree more closely with experiment than do previous calculations for monolayers. Additionally, the minimum energy paths and corresponding energy barriers along the in-plane [1000] and [0100] directions for Li travelling between neighboring MXene layers are determined. V4C3O2 exhibits the lowest diffusion barrier of the compositions considered, at 0.42 eV, but its reversible capacity (148 mAh/g) is dragged down by its heavy formula unit. Conversely, the V2CO2 MXene shows good reversible capacity (276 mAh/g), but a high diffusion barrier (0.82 eV). We show that the diffusion barriers of all bilayer structures are significantly higher than those calculated for the corresponding monolayers, advocating the use of dispersed monolayer MXenes instead of multilayers in high performance anodes.

Coupled-channel calculation for cross section of fusion and barrier distribution of {}^{16,17,18}O + {}^{16}O reactions

NASA Astrophysics Data System (ADS)

Fereidonnejad, R.; Sadeghi, H.; Ghambari, M.

2018-03-01

In this work, the effect of multi-phonon excitation on heavy-ion fusion reactions has been studied and fusion barrier distributions of energy intervals near and below the Coulomb barrier have been studied for 16,17,18O + 16O reactions. The structure and deformation of nuclear projectiles have been studied. Given the adaptation of computations to experimental data, our calculations predict the behavior of reactions in intervals of energy in which experimental measurements are not available. In addition the S-factor for these reactions has been calculated. The results showed that the structure and deformation of a nuclear projectile are important factors. The S-factor, obtained in the coupled-channel calculations for the {}^{16}O + {}^{16}O, {}^{17}O +{}^{16}O and {}^{18}O +{}^{16}O reactions, showed good agreement with the experimental data and had a maximum value at an energy near 5, 4.5 and 4 MeV, respectively.

Diffusion of Zr, Ru, Ce, Y, La, Sr and Ba fission products in UO 2

DOE PAGES

Perriot, R.; Liu, X. -Y.; Stanek, C. R.; ...

2015-01-08

The diffusivity of the solid fission products (FP) Zr (Zr 4+), Ru (Ru 4+, Ru 3+), Ce (Ce 4+), Y (Y 3+), La (La 3+), Sr (Sr 2+) and Ba (Ba 2+) by a vacancy mechanism has been calculated, using a combination of density functional theory (DFT) and empirical potential (EP) calculations. The activation energies for the solid fission products are compared to the activation energy for Xe fission gas atoms calculated previously. Apart from Ru, the solid fission products all exhibit higher activation energy than Xe. Furthermore, for all solid FPs except Y 3+, the migration of the FPmore » has lower barrier than the migration of a neighboring U atom, making the latter the rate limiting step for direct migration. An indirect mechanism, consisting of two successive migrations around the FP, is also investigated. The calculated diffusivities show that most solid fission products diffuse with rates similar to U self-diffusion. But, Ru, Ba and Sr exhibit faster diffusion than the other solid FPs, with Ru 3+ and Ru 4+ diffusing even faster than Xe for T < 1200 K. The diffusivities correlate with the observed fission product solubility in UO 2, and the tendency to form metallic and oxide second phase inclusions.« less

The mechanism for water exchange in [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-), as studied by quantum chemical methods.

PubMed

Vallet, V; Wahlgren, U; Schimmelpfennig, B; Szabó, Z; Grenthe, I

2001-12-05

The mechanisms for the exchange of water between [UO(2)(H(2)O)(5)](2+), [UO(2)(oxalate)(2)(H(2)O)](2)(-)(,) and water solvent along dissociative (D), associative (A) and interchange (I) pathways have been investigated with quantum chemical methods. The choice of exchange mechanism is based on the computed activation energy and the geometry of the identified transition states and intermediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and predictable difference between the activation energy of the gas phase and solvent models: the energy barrier for the D-mechanism increases in the solvent as compared to the gas phase, while it decreases for the A- and I-mechanisms. The calculated activation energy, Delta U(++), for the water exchange in [UO(2)(H(2)O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-mechanisms in the solvent, as compared to the experimental value Delta H(++) = 26 +/- 1 kJ/mol. This indicates that the D-mechanism for this system can be ruled out. The energy barrier between the intermediates and the transition states is small, indicating a lifetime for the intermediate approximately 10(-10) s, making it very difficult to distinguish between the A- and I-mechanisms experimentally. There is no direct experimental information on the rate and mechanism of water exchange in [UO(2)(oxalate)(2)(H(2)O)](2-) containing two bidentate oxalate ions. The activation energy and the geometry of transition states and intermediates along the D-, A-, and I-pathways were calculated both in the gas phase and in a water solvent model, using a single-point MP2 calculation with the gas phase geometry. The activation energy, Delta U(++), in the solvent for the D-, A-, and I-mechanisms is 56, 12, and 53 kJ/mol, respectively. This indicates that the water exchange follows an associative reaction mechanism. The geometry of the A- and I-transition states for both [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-) indicates that the entering/leaving water molecules are located outside the plane formed by the spectator ligands.

Catalytic mechanism of human N-acetylserotonin methyltransferase: a theoretical investigation

NASA Astrophysics Data System (ADS)

Wang, Li; Zhang, Ting; Li, Jieqiong; He, Chaozheng; He, Hongqing; Zhang, Jinglai

2015-11-01

The methyl-transfer mechanism of human N-acetylserotonin methyltransferase and the roles of several residues around the active sites are investigated by density function theory method. This enzyme will catalyse the conversion of N-acetylserotonin and S-adenosyl-L-methionine (SAM) into melatonin and S-asenosylhomocysteine, which is the terminal step in the melatonin (N-acetyl-5-methoxytryptamine) biosynthesis. The calculated results confirm that the methyl transfer and proton transfer will take place via a SN2 step with a concerted mechanism, which is different from the experimental estimation via a water bridge. The residues H255, D256, E311, and R252 play an important role in reducing the barrier height and inducing methyl transfer. In addition, a full SAM molecule is considered in this work, which is never explored in previous reports. We find that some residues around the SAM in the centre of active site are essential factors to influence the mechanism and barrier height. So a truncated SAM model may not be suitable for all reactions.

The doping effect on the catalytic activity of graphene for oxygen evolution reaction in a lithium-air battery: a first-principles study.

PubMed

Ren, Xiaodong; Wang, Beizhou; Zhu, Jinzhen; Liu, Jianjun; Zhang, Wenqing; Wen, Zhaoyin

2015-06-14

A lithium-air battery as an energy storage technology can be used in electric vehicles due to its large energy density. However, its poor rate capability, low power density and large overpotential problems limit its practical usage. In this paper, the first-principles thermodynamic calculations were performed to study the catalytic activity of X-doped graphene (X = B, N, Al, Si, and P) materials as potential cathodes to enhance charge reactions in a lithium-air battery. Among these materials, P-doped graphene exhibits the highest catalytic activity in reducing the charge voltage by 0.25 V, while B-doped graphene has the highest catalytic activity in decreasing the oxygen evolution barrier by 0.12 eV. By combining these two catalytic effects, B,P-codoped graphene was demonstrated to have an enhanced catalytic activity in reducing the O2 evolution barrier by 0.70 eV and the charge voltage by 0.13 V. B-doped graphene interacts with Li2O2 by Li-sited adsorption in which the electron-withdrawing center can enhance charge transfer from Li2O2 to the substrate, facilitating reduction of O2 evolution barrier. In contrast, X-doped graphene (X = N, Al, Si, and P) prefers O-sited adsorption toward Li2O2, forming a X-O2(2-)···Li(+) interface structure between X-O2(2-) and the rich Li(+) layer. The active structure of X-O2(2-) can weaken the surrounding Li-O2 bonds and significantly reduce Li(+) desorption energy at the interface. Our investigation is helpful in developing a novel catalyst to enhance oxygen evolution reaction (OER) in Li-air batteries.

Examining empirical evidence of the effect of superfluidity on the fusion barrier

NASA Astrophysics Data System (ADS)

Scamps, Guillaume

2018-04-01

Background: Recent time-dependent Hartree-Fock-Bogoliubov (TDHFB) calculations predict that superfluidity enhances fluctuations of the fusion barrier. This effect is not fully understood and not yet experimentally revealed. Purpose: The goal of this study is to empirically investigate the effect of superfluidity on the distribution width of the fusion barrier. Method: Two new methods are proposed in the present study. First, the local regression method is introduced and used to determine the barrier distribution. The second method, which requires only the calculation of an integral of the cross section, is developed to determine accurately the fluctuations of the barrier. This integral method, showing the best performance, is systematically applied to 115 fusion reactions. Results: Fluctuations of the barrier for open-shell systems are, on average, larger than those for magic or semimagic nuclei. This is due to the deformation and the superfluidity. To disentangle these two effects, a comparison is made between the experimental width and the width estimated from a model that takes into account the tunneling, the deformation, and the vibration effect. This study reveals that superfluidity enhances the fusion barrier width. Conclusions: This analysis shows that the predicted effect of superfluidity on the width of the barrier is real and is of the order of 1 MeV.

Simulation of solute transport across low-permeability barrier walls

USGS Publications Warehouse

Harte, P.T.; Konikow, Leonard F.; Hornberger, G.Z.

2006-01-01

Low-permeability, non-reactive barrier walls are often used to contain contaminants in an aquifer. Rates of solute transport through such barriers are typically many orders of magnitude slower than rates through the aquifer. Nevertheless, the success of remedial actions may be sensitive to these low rates of transport. Two numerical simulation methods for representing low-permeability barriers in a finite-difference groundwater-flow and transport model were tested. In the first method, the hydraulic properties of the barrier were represented directly on grid cells and in the second method, the intercell hydraulic-conductance values were adjusted to approximate the reduction in horizontal flow, allowing use of a coarser and computationally efficient grid. The alternative methods were tested and evaluated on the basis of hypothetical test problems and a field case involving tetrachloroethylene (PCE) contamination at a Superfund site in New Hampshire. For all cases, advective transport across the barrier was negligible, but preexisting numerical approaches to calculate dispersion yielded dispersive fluxes that were greater than expected. A transport model (MODFLOW-GWT) was modified to (1) allow different dispersive and diffusive properties to be assigned to the barrier than the adjacent aquifer and (2) more accurately calculate dispersion from concentration gradients and solute fluxes near barriers. The new approach yields reasonable and accurate concentrations for the test cases. ?? 2006.

Direct hydride shift mechanism and stereoselectivity of P450nor confirmed by QM/MM calculations.

PubMed

Krámos, Balázs; Menyhárd, Dóra K; Oláh, Julianna

2012-01-19

Nitric oxide reductase (P450(nor)) found in Fusarium oxysporum catalyzes the reduction of nitric oxide to N(2)O in a multistep process. The reducing agent, NADH, is bound in the distal pocket of the enzyme, and direct hydride transfer occurs from NADH to the nitric oxide bound heme enzyme, forming intermediate I. Here we studied the possibility of hydride transfer from NADH to both the nitrogen and oxygen of the heme-bound nitric oxide, using quantum chemical and combined quantum mechanics/molecular mechanics (QM/MM) calculations, on two different protein models, representing both possible stereochemistries, a syn- and an anti-NADH arrangement. All calculations clearly favor hydride transfer to the nitrogen of nitric oxide, and the QM-only barrier and kinetic isotope effects are good agreement with the experimental values of intermediate I formation. We obtained higher barriers in the QM/MM calculations for both pathways, but hydride transfer to the nitrogen of nitric oxide is still clearly favored. The barriers obtained for the syn, Pro-R conformation of NADH are lower and show significantly less variation than the barriers obtained in the case of anti conformation. The effect of basis set and wide range of functionals on the obtained results are also discussed.

Sub-barrier fusion and transfers in the 40Ca + 58,64Ni systems

NASA Astrophysics Data System (ADS)

Bourgin, D.; Courtin, S.; Haas, F.; Goasduff, A.; Stefanini, A. M.; Montagnoli, G.; Montanari, D.; Corradi, L.; Huiming, J.; Scarlassara, F.; Fioretto, E.; Simenel, C.; Rowley, N.; Szilner, S.; Mijatović, T.

2016-05-01

Fusion cross sections have been measured in the 40Ca + 58Ni and 40Ca + 64Ni systems at energies around and below the Coulomb barrier. The 40Ca beam was delivered by the XTU Tandem accelerator of the Laboratori Nazionali di Legnaro and evaporation residues were measured at very forward angles with the LNL electrostatic beam deflector. Coupled-channels calculations were performed which highlight possible strong effects of neutron transfers on the fusion below the barrier in the 40Ca + 64Ni system. Microscopic time-dependent Hartree-Fock calculations have also been performed for both systems. Preliminary results are shown.

Observation of internal transport barrier in ELMy H-mode plasmas on the EAST tokamak

NASA Astrophysics Data System (ADS)

Yang, Y.; Gao, X.; Liu, H. Q.; Li, G. Q.; Zhang, T.; Zeng, L.; Liu, Y. K.; Wu, M. Q.; Kong, D. F.; Ming, T. F.; Han, X.; Wang, Y. M.; Zang, Q.; Lyu, B.; Li, Y. Y.; Duan, Y. M.; Zhong, F. B.; Li, K.; Xu, L. Q.; Gong, X. Z.; Sun, Y. W.; Qian, J. P.; Ding, B. J.; Liu, Z. X.; Liu, F. K.; Hu, C. D.; Xiang, N.; Liang, Y. F.; Zhang, X. D.; Wan, B. N.; Li, J. G.; Wan, Y. X.; EAST Team

2017-08-01

The internal transport barrier (ITB) has been obtained in ELMy H-mode plasmas by neutron beam injection and lower hybrid wave heating on the Experimental Advanced Superconducting Tokamak (EAST). The ITB structure has been observed in profiles of ion temperature, electron temperature, and electron density within ρ < 0.5. It was also observed that the ITB formation is stepwise. Due to the ITB formation, the confinement quality H 98y2 increases from 1 to 1.1 and the normalized beta, β N, increases from 1.5 to near 2. The fishbone activity observed during the ITB phase suggests the central safety factor q(0) ˜ 1. Transport coefficients are calculated by particle balance and power balance analysis, showing an obvious reduction after the ITB formation.

A network-analysis-based comparative study of the throughput behavior of polymer melts in barrier screw geometries

NASA Astrophysics Data System (ADS)

Aigner, M.; Köpplmayr, T.; Kneidinger, C.; Miethlinger, J.

2014-05-01

Barrier screws are widely used in the plastics industry. Due to the extreme diversity of their geometries, describing the flow behavior is difficult and rarely done in practice. We present a systematic approach based on networks that uses tensor algebra and numerical methods to model and calculate selected barrier screw geometries in terms of pressure, mass flow, and residence time. In addition, we report the results of three-dimensional simulations using the commercially available ANSYS Polyflow software. The major drawbacks of three-dimensional finite-element-method (FEM) simulations are that they require vast computational power and, large quantities of memory, and consume considerable time to create a geometric model created by computer-aided design (CAD) and complete a flow calculation. Consequently, a modified 2.5-dimensional finite volume method, termed network analysis is preferable. The results obtained by network analysis and FEM simulations correlated well. Network analysis provides an efficient alternative to complex FEM software in terms of computing power and memory consumption. Furthermore, typical barrier screw geometries can be parameterized and used for flow calculations without timeconsuming CAD-constructions.

Two Equivalent Methyl Internal Rotations in 2,5-DIMETHYLTHIOPHENE Investigated by Microwave Spectroscopy

NASA Astrophysics Data System (ADS)

Van, Vinh; Stahl, Wolfgang; Nguyen, Ha Vinh Lam

2016-06-01

The microwave spectrum of 2,5-dimethylthiophene, a sulfur-containing five-membered heterocyclic molecule with two conjugated double bonds, was recorded using two molecular beam Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Highly accurate molecular parameters were determined. The rotational constants obtained by geometry optimizations at different levels of theory are in good agreement with the experimental values. A C2v equilibrium structure was calculated, where one hydrogen atom of each methyl group is antiperiplanar to the sulfur atom, and the two methyl groups are thus equivalent. Transition states were optimized at different levels of theory using the Berny algorithm to calculate the barrier height of the two equivalent methyl rotors. The fitted experimental torsional barrier of 247.95594(30) wn is in reasonable agreement with the calculated barriers. Similar barriers to internal rotation were found for the monomethyl derivatives 2-methylthiophene (194.1 wn) and 3-methylthiophene (258.8 wn). A labeling scheme for the group G36 written as the semi-direct product (C3I x C3I) (x C2v was introduced.

Monte Carlo simulations on atropisomerism of thienotriazolodiazepines applicable to slow transition phenomena using potential energy surfaces by ab initio molecular orbital calculations.

PubMed

Morikami, Kenji; Itezono, Yoshiko; Nishimoto, Masahiro; Ohta, Masateru

2014-01-01

Compounds with a medium-sized flexible ring often show atropisomerism that is caused by the high-energy barriers between long-lived conformers that can be isolated and often have different biological properties to each other. In this study, the frequency of the transition between the two stable conformers, aS and aR, of thienotriazolodiazepine compounds with flexible 7-membered rings was estimated computationally by Monte Carlo (MC) simulations and validated experimentally by NMR experiments. To estimate the energy barriers for transitions as precisely as possible, the potential energy (PE) surfaces used in the MC simulations were calculated by molecular orbital (MO) methods. To accomplish the MC simulations with the MO-based PE surfaces in a practical central processing unit (CPU) time, the MO-based PE of each conformer was pre-calculated and stored before the MC simulations, and then only referred to during the MC simulations. The activation energies for transitions calculated by the MC simulations agreed well with the experimental ΔG determined by the NMR experiments. The analysis of the transition trajectories of the MC simulations revealed that the transition occurred not only through the transition states, but also through many different transition paths. Our computational methods gave us quantitative estimates of atropisomerism of the thienotriazolodiazepine compounds in a practical period of time, and the method could be applicable for other slow-dynamics phenomena that cannot be investigated by other atomistic simulations.

Barriers to activity and participation for stroke survivors in rural China.

PubMed

Zhang, Lifang; Yan, Tiebin; You, Liming; Li, Kun

2015-07-01

To investigate environmental barriers reported by stroke survivors in the rural areas of China and to determine the impact of environmental barriers on activity and participation relative to demographic characteristics and body functioning. Cross-sectional survey. Structured interviews in the participants' homes. Community-dwelling stroke survivors in the rural areas of China (N=639). Not applicable. Activity and participation (Chinese version of the World Health Organization Disability Assessment Schedule 2.0), environmental barriers (Craig Hospital Inventory of Environmental Factors), neurological function (Canadian Neurological Scale), cognitive function (Abbreviated Mental Test), and depression (6-item Hamilton Rating Scale for Depression). Physical/structural barriers are the major impediment to activity and participation for these participants (odds ratio, 1.86 and 1.99 for activity and participation, respectively; P<.01). Services/assistance barriers primarily impede participation rather than activity (odds ratio, 1.58 in participation; P<.05). Physical/structural and services/assistance barriers were considered the dominant barriers to activity and participation for stroke survivors in the rural areas of China. Attitudinal/support and policy barriers did not emerge as serious concerns. To generate an enabling environment, physical/structural and services/assistance barriers are the environmental barriers to be decreased and eliminated first. Copyright © 2015 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.

Thermal E/ Z Isomerization in First Generation Molecular Motors.

PubMed

Kuwahara, Shunsuke; Suzuki, Yuri; Sugita, Naoya; Ikeda, Mari; Nagatsugi, Fumi; Harada, Nobuyuki; Habata, Yoichi

2018-04-20

Determination of a thermal E/ Z isomerization barrier of first generation molecular motors is reported. Stable ( E)-1a directly converts to stable ( Z)-1c without photochemical E/ Z isomerization. The activation Gibbs energy of the isomerization was determined to be 123 kJ mol -1 by circular dichroism spectral changes. Density functional theory calculations show that ( Z)-1c is ∼11.4 kJ mol -1 more stable than ( E)-1a.

Reverse cope elimination of hydroxylamines and alkenes or alkynes: theoretical investigation of tether length and substituent effects.

PubMed

Krenske, Elizabeth H; Davison, Edwin C; Forbes, Ian T; Warner, Jacqueline A; Smith, Adrian L; Holmes, Andrew B; Houk, K N

2012-02-01

Quantum mechanical calculations have been used to study the intramolecular additions of hydroxylamines to alkenes and alkynes ("reverse Cope eliminations"). In intermolecular reverse Cope eliminations, alkynes are more reactive than alkenes. However, competition experiments have shown that tethering the hydroxylamine to the alkene or alkyne can reverse the reactivity order from that normally observed. The exact outcome depends on the length of the tether. In agreement with experiment, a range of density functional theory methods and CBS-QB3 calculations predict that the activation energies for intramolecular reverse Cope eliminations follow the order 6-exo-dig < 5-exo-trig < 5-exo-dig ≈ 7-exo-dig. The order of the barriers for the 5-, 6-, and 7-exo-dig reactions of alkynes arises mainly from differences in tether strain in the transition states (TSs), but is also influenced by the TS interaction between the hydroxylamine and alkyne. Cyclization onto an alkene in the 5-exo-trig fashion incurs slightly less tether strain than a 6-exo-dig alkyne cyclization, but its activation energy is higher because the hydroxylamine fragment must distort more before the TS is reached. If the alkene terminus is substituted with two methyl groups, the barrier becomes so much higher that it is also disfavored compared to the 5- and 7-exo-dig cyclizations. © 2012 American Chemical Society

The Reverse Cope Elimination of Hydroxylamines and Alkenes or Alkynes: Theoretical Investigation of Tether Length and Substituent Effects

PubMed Central

Krenske, Elizabeth H.; Davison, Edwin C.; Forbes, Ian T.; Warner, Jacqueline A.; Smith, Adrian L.; Holmes, Andrew B.; Houk, K. N.

2012-01-01

Quantum mechanical calculations have been used to study the intramolecular additions of hydroxylamines to alkenes and alkynes (“reverse Cope eliminations”). In intermolecular reverse Cope eliminations, alkynes are more reactive than alkenes. However, competition experiments have shown that tethering the hydroxylamine to the alkene or alkyne can reverse the reactivity order from that normally observed. The exact outcome depends on the length of the tether. In agreement with experiment, a range of density functional theory methods and CBS-QB3 calculations predict that the activation energies for intramolecular reverse Cope eliminations follow the order 6-exo-dig < 5-exo-trig < 5-exo-dig ≈ 7-exo-dig. The order of the barriers for the 5-, 6-, and 7-exo-dig reactions of alkynes arises mainly from differences in tether strain in the transition states, but is also influenced by the transition-state interaction between the hydroxylamine and alkyne. Cyclization onto an alkene in the 5-exo-trig fashion incurs slightly less tether strain than a 6-exo-dig alkyne cyclization, but its activation energy is higher because the hydroxylamine fragment must distort more before the TS is reached. If the alkene terminus is substituted with two methyl groups, the barrier becomes so much higher that it is also disfavored compared to the 5- and 7-exo-dig cyclizations. PMID:22280245

High-throughput ab-initio dilute solute diffusion database

PubMed Central

Wu, Henry; Mayeshiba, Tam; Morgan, Dane

2016-01-01

We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world. PMID:27434308

Extended optical model for fission

DOE PAGES

Sin, M.; Capote, R.; Herman, M. W.; ...

2016-03-07

A comprehensive formalism to calculate fission cross sections based on the extension of the optical model for fission is presented. It can be used for description of nuclear reactions on actinides featuring multi-humped fission barriers with partial absorption in the wells and direct transmission through discrete and continuum fission channels. The formalism describes the gross fluctuations observed in the fission probability due to vibrational resonances, and can be easily implemented in existing statistical reaction model codes. The extended optical model for fission is applied for neutron induced fission cross-section calculations on 234,235,238U and 239Pu targets. A triple-humped fission barrier ismore » used for 234,235U(n,f), while a double-humped fission barrier is used for 238U(n,f) and 239Pu(n,f) reactions as predicted by theoretical barrier calculations. The impact of partial damping of class-II/III states, and of direct transmission through discrete and continuum fission channels, is shown to be critical for a proper description of the measured fission cross sections for 234,235,238U(n,f) reactions. The 239Pu(n,f) reaction can be calculated in the complete damping approximation. Calculated cross sections for 235,238U(n,f) and 239Pu(n,f) reactions agree within 3% with the corresponding cross sections derived within the Neutron Standards least-squares fit of available experimental data. Lastly, the extended optical model for fission can be used for both theoretical fission studies and nuclear data evaluation.« less

The triaxiality and Coriolis effects on the fission barrier in isovolumic nuclei with mass number A = 256 based on multidimensional total Routhian surface calculations

NASA Astrophysics Data System (ADS)

Chai, Qing-Zhen; Zhao, Wei-Juan; Wang, Hua-Lei; Liu, Min-Liang; Xu, Fu-Rong

2018-05-01

The triaxiality and Coriolis effects on the first fission barrier in even-even nuclei with A=256 have been studied in terms of the approach of multidimensional total Routhian surface calculations. The present results are compared with available data and other theories, showing a good agreement. Based on the deformation energy or Routhian curves, the first fission barriers are analyzed, focusing on their shapes, heights, and evolution with rotation. It is found that, relative to the effect on the ground-state minimum, the saddle point, at least the first one, can be strongly affected by the triaxial deformation degree of freedom and Coriolis force. The evolution trends of the macroscopic and microscopic (shell and pairing) contributions as well as the triaxial fission barriers are briefly discussed.

Hydroxyl Radical (OH•) Reaction with Guanine in an Aqueous Environment: A DFT Study

PubMed Central

Kumar, Anil; Pottiboyina, Venkata; Sevilla, Michael D.

2011-01-01

The reaction of hydroxyl radical (OH•) with DNA accounts for about half of radiation-induced DNA damage in living systems. Previous literature reports point out that the reaction of OH• with DNA proceeds mainly through the addition of OH• to the C=C bond of the DNA bases. However, recently it has been reported that the principal reaction of OH• with dGuo (deoxyguanosine) is the direct hydrogen atom abstraction from its exocyclic amine group rather than addition of OH• to the C=C bond. In the present work, these two reaction pathways of OH• attack on guanine (G) in the presence of water molecules (aqueous environment) are investigated using the density functional theory (DFT) B3LYP method with 6-31G* and 6-31++G** basis sets. The calculations show that the initial addition of the OH• at C4=C5 double bond of guanine is barrier free and the adduct radical (G-OH•) has only a small activation barrier of ca. 1 – 6 kcal/mol leading to the formation of a metastable ion-pair intermediate (G•+---OH−). The formation of ion-pair is a result of the highly oxidizing nature of the OH• in aqueous media. The resulting ion-pair (G•+---OH−) deprotonates to form H2O and neutral G radicals favoring G(N1-H)• with an activation barrier of ca. 5 kcal/mol. The overall process from the G(C4)-OH• (adduct) to G(N1-H)• and water is found to be exothermic in nature by more than 13 kcal/mol. (G-OH•), (G•+---OH−), and G(N1-H)• were further characterized by the CAM-B3LYP calculations of their UV-visible spectra and good agreement between theory and experiment is achieved. Our calculations for the direct hydrogen abstraction pathway from N1 and N2 sites of guanine by the OH• show that this is also a competitive route to produce G(N2-H)•, G(N1-H)• and H2O. PMID:22050033

Migration mechanisms and diffusion barriers of vacancies in Ga2O3

NASA Astrophysics Data System (ADS)

Kyrtsos, Alexandros; Matsubara, Masahiko; Bellotti, Enrico

2017-06-01

We employ the nudged elastic band and the dimer methods within the standard density functional theory (DFT) formalism to study the migration of the oxygen and gallium vacancies in the monoclinic structure of β -Ga2O3 . We identify all the first nearest neighbor paths and calculate the migration barriers for the diffusion of the oxygen and gallium vacancies. We also identify the metastable sites of the gallium vacancies which are critical for the diffusion of the gallium atoms. The migration barriers for the diffusion of the gallium vacancies are lower than the migration barriers for oxygen vacancies by 1 eV on average, suggesting that the gallium vacancies are mobile at lower temperatures. Using the calculated migration barriers we estimate the annealing temperature of these defects within the harmonic transition state theory formalism, finding excellent agreement with the observed experimental annealing temperatures. Finally, we suggest the existence of percolation paths which enable the migration of the species without utilizing all the migration paths of the crystal.

Perceived barriers, benefits, and motives for physical activity: two primary-care physical activity prescription programs.

PubMed

Patel, Asmita; Schofield, Grant M; Kolt, Gregory S; Keogh J, W L

2013-01-01

This study examined whether perceived barriers, benefits, and motives for physical activity differed based on allocation to 2 different types of primary-care activity-prescription programs (pedometer-based vs. time-based Green Prescription). Eighty participants from the Healthy Steps study completed a questionnaire that assessed their perceived barriers, benefits, and motives for physical activity. Factor analysis was carried out to identify common themes of barriers, benefits, and motives for physical activity. Factor scores were then used to explore between-groups differences for perceived barriers, benefits, and motives based on group allocation and demographic variables. No significant differences were found in factor scores based on allocation. Demographic variables relating to the existence of chronic health conditions, weight status, and older age were found to significantly influence perceived barriers, benefits, and motives for physical activity. Findings suggest that the addition of a pedometer to the standard Green Prescription does not appear to increase perceived motives or benefits or decrease perceived barriers for physical activity in low-active older adults.

Performance evaluation of intermediate cover soil barrier for removal of heavy metals in landfill leachate.

PubMed

Suzuki, Kazuyuki; Anegawa, Aya; Endo, Kazuto; Yamada, Masato; Ono, Yusaku; Ono, Yoshiro

2008-11-01

This pilot-scale study evaluated the use of intermediate cover soil barriers for removing heavy metals in leachate generated from test cells for co-disposed fly ash from municipal solid waste incinerators, ash melting plants, and shredder residue. Cover soil barriers were mixtures of Andisol (volcanic ash soil), waste iron powder, (grinder dust waste from iron foundries), and slag fragments. The cover soil barriers were installed in the test cells' bottom layer. Sorption/desorption is an important process in cover soil bottom barrier for removal of heavy metals in landfill leachate. Salt concentrations such as those of Na, K, and Ca in leachate were extremely high (often greater than 30 gL(-1)) because of high salt content in fly ash from ash melting plants. Concentrations of all heavy metals (nickel, manganese, copper, zinc, lead, and cadmium) in test cell leachates with a cover soil barrier were lower than those of the test cell without a cover soil barrier and were mostly below the discharge limit, probably because of dilution caused by the amount of leachate and heavy metal removal by the cover soil barrier. The cover soil barriers' heavy metal removal efficiency was calculated. About 50% of copper, nickel, and manganese were removed. About 20% of the zinc and boron were removed, but lead and cadmium were removed only slightly. Based on results of calculation of the Langelier saturation index and analyses of core samples, the reactivity of the cover soil barrier apparently decreases because of calcium carbonate precipitation on the cover soil barriers' surfaces.

Conformational state of β-hydroxynaphthylamides: Barriers for the rotation of the amide group around CN bond and dynamics of the morpholine ring

NASA Astrophysics Data System (ADS)

Kozlecki, Tomasz; Tolstoy, Peter M.; Kwocz, Agnieszka; Vovk, Mikhail A.; Kochel, Andrzej; Polowczyk, Izabela; Tretyakov, Peter Yu.; Filarowski, Aleksander

2015-10-01

Three β-hydroxynaphthylamides (morpholine, pyrrolidine and dimethylamine derivatives) have been synthesized and their conformational state was analyzed by NMR, X-ray and DFT calculations. In aprotic solution the molecules contain intramolecular OHO hydrogen bonds, which change into intermolecular ones in solid state. The energy barriers for the amide group rotation around the CN bond were estimated from the line shape analysis of 1H and 13C NMR signals. A tentative correlation between the barrier height and the strength of OHO bond was proposed. Calculations of the potential energy profiles for the rotations around CC and CN bonds were done. In case of morpholine derivative experimental indications of additional dynamics: chair-chair 'ring flip' in combination with the twisting around CC bond were obtained and confirmed by quantum chemistry calculations.

Electric field control of spin transfer torque in multiferroic tunnel junctions

NASA Astrophysics Data System (ADS)

Useinov, Artur; Kalitsov, Alan; Velev, Julian; Kioussis, Nicholas

2014-03-01

Based on model calculations we predict that the spin transfer torque (STT) in magnetic tunnel junctions with ferroelectric barriers can be strongly influenced by the saturated polarization of the barrier. The STT in such multiferroic tunnel junctions is calculated within the non-equilibrium Keldysh formalism generalized for non-collinear transport and implemented in the framework of a single-band tight-binding (TB) model. We calculate the bias dependence of both the in-plane (T∥) and out-of-plane (T⊥) components of STT as a function of the ferroelectric polarization (P) in the barrier. We find that the components of STT strongly depend on both the magnitude and the direction of the polarization. In particular switching of the polarization direction can dramatically alter the value of the STT and can even lead to a change of sign of T∥ and the voltage-induced part of T⊥. The effect is proportional to the magnitude of the polarization.

Autonomous motivation and quality of life as predictors of physical activity in patients with schizophrenia.

PubMed

Costa, Raquel; Bastos, Tânia; Probst, Michel; Seabra, André; Vilhena, Estela; Corredeira, Rui

2018-02-08

Being physically active is a complex behaviour in patients with schizophrenia. Several factors were identified as barriers to achieving active behaviours in this population. Therefore, the purpose of this study was to investigate among a number of barriers what predicts the most on physical activity (PA) in patients with schizophrenia. A total of 114 patients (28♀) with schizophrenia were included. Body mass index (BMI) was calculated. Autonomous and controlled motivation (Behavioural Regulation in Exercise Questionnaire - 3), self-esteem (Rosenberg Self-esteem scale), quality of life (World Health Organization Quality of Life Scale - Brief version) and functional exercise capacity (6-minute walk test - 6MWT) were evaluated. Multiple Regression Analysis was applied to assess the effect of these variables on Total PA per week (International Physical Activity Questionnaire - short version). Autonomous motivation and domains of quality of life were positively correlated with Total PA per week. Stepwise multiple regression analyses showed that of all the candidate factors to predict PA, autonomous motivation and global domain of quality of life were found as significant predictors. Our findings help to understand the importance of autonomous motivation and quality of life for PA in patients with schizophrenia. Knowledge about these predictors may provide guidance to improve PA behaviour in this population.

Progress on single barrier varactors for submillimeter wave power generation

NASA Technical Reports Server (NTRS)

Nilsen, Svein M.; Groenqvist, Hans; Hjelmgren, Hans; Rydberg, Anders; Kollberg, Erik L.

1992-01-01

Theoretical work on Single Barrier Varactor (SBV) diodes, indicate that the efficiency for a multiplier has a maximum for a considerably smaller capacitance variation than previously thought. The theoretical calculations are performed, both with a simple theoretical model and a complete computer simulation using the method of harmonic balance. Modeling of the SBV is carried out in two steps. First, the semiconductor transport equations are solved simultaneously using a finite difference scheme in one dimension. Secondly, the calculated I-V, and C-V characteristics are input to a multiplier simulator which calculates the optimum impedances, and output powers at the frequencies of interest. Multiple barrier varactors can also be modeled in this way. Several examples on how to design the semiconductor layers to obtain certain characteristics are given. The calculated conversion efficiencies of the modeled structures, in a multiplier circuit, are also presented. Computer simulations for a case study of a 750 GHz multiplier show that InAs diodes perform favorably compared to GaAs diodes. InAs and InGaAs SBV diodes have been fabricated and their current vs. voltage characteristics are presented. In the InAs diode, was the large bandgap semiconductor AlSb used as barrier. The InGaAs diode was grown lattice matched to an InP substrate with InAlAs as a barrier material. The current density is greatly reduced for these two material combinations, compared to that of GaAs/AlGaAs SBV diodes. GaAs based diodes can be biased to higher voltages than InAs diodes.

Are barriers to physical activity similar for adults with and without abnormal glucose metabolism?

PubMed

Hume, Clare; Dunstan, David; Salmon, Jo; Healy, Genevieve; Andrianopoulos, Nick; Owen, Neville

2010-01-01

The purpose of this study was to examine perceived barriers to physical activity among adults with and without abnormal glucose metabolism (AGM), and whether barriers varied according to physical activity status. The 1999 to 2000 Australian Diabetes, Obesity, and Lifestyle Study (AusDiab) was a population-based cross-sectional study among adults aged > or =25 years. AGM was identified through an oral glucose tolerance test. The previous week's physical activity and individual, social, and environmental barriers to physical activity were self-reported. Logistic regression analyses examined differences in barriers to physical activity between those with and without AGM, and for those with and without AGM who did and did not meet the minimum recommendation of 150 minutes/week of moderate-to-vigorous intensity physical activity. Of the 7088 participants (47.5 +/- 12.7 years; 46% male), 18.5% had AGM. Approximately 47.5% of those with AGM met the physical activity recommendation, compared to 54.7% of those without AGM (P < .001). Key barriers to physical activity included lack of time, other priorities, and being tired. Following adjustment for sociodemographic and behavioral factors, there were few differences in barriers to physical activity between those with and without AGM, even after stratifying according to physical activity. Adults with AGM report similar barriers to physical activity, as do those without AGM. Programs for those with AGM can therefore focus on the known generic adult-reported barriers to physical activity.

Diffusion quantum Monte Carlo calculations of SrFeO 3 and LaFeO 3

DOE PAGES

Santana, Juan A.; Krogel, Jaron T.; Kent, Paul R. C.; ...

2017-07-18

The equations of state, formation energy, and migration energy barrier of the oxygen vacancy in SrFeO 3 and LaFeO 3 were calculated in this paper with the diffusion quantum Monte Carlo (DMC) method. Calculations were also performed with various Density Functional Theory (DFT) approximations for comparison. DMC reproduces the measured cohesive energies of these materials with errors below 0.23(5) eV and the structural properties within 1% of the experimental values. The DMC formation energies of the oxygen vacancy in SrFeO 3 and LaFeO 3 under oxygen-rich conditions are 1.3(1) and 6.24(7) eV, respectively. Similar calculations with semi-local DFT approximations formore » LaFeO 3 yielded vacancy formation energies 1.5 eV lower. Comparison of charge density evaluated with DMC and DFT approximations shows that DFT tends to overdelocalize the electrons in defected SrFeO 3 and LaFeO 3. Finally, calculations with DMC and local density approximation yield similar vacancy migration energy barriers, indicating that steric/electrostatic effects mainly determine migration barriers in these materials.« less

Carbon-hydrogen vs. carbon-halogen oxidative addition of chlorobenzene by a neutral iridium complex explored by DFT.

PubMed

Wu, Hong; Hall, Michael B

2009-08-14

Density functional theory (DFT) is used to explore the competitive C-H and C-Cl oxidative additions (OA) of chlorobenzene to the neutral Ir(i) complex: (PNP)Ir(I) [PNP = bis(Z-2-(dimethylphosphino)vinyl)amino]. Consistent with experimental results, our calculation shows that C-H OA is kinetically favored with an activation free-energy barrier of DeltaG(double dagger) = 17.2 kcal mol(-1) that is significantly lower than that for the C-Cl activation at DeltaG(double dagger) = 24.2 kcal mol(-1). However, C-Cl OA is thermodynamically preferred and the C-Cl OA product is 22.6 kcal mol(-1) more stable than the most stable C-H OA product. The calculations also show that the lowest energy path for the conversion of the C-H OA product to the more stable C-Cl OA product is intramolecular through a "benzyne"-type intermediate.

Simulations of chemical catalysis

NASA Astrophysics Data System (ADS)

Smith, Gregory K.

This dissertation contains simulations of chemical catalysis in both biological and heterogeneous contexts. A mixture of classical, quantum, and hybrid techniques are applied to explore the energy profiles and compare possible chemical mechanisms both within the context of human and bacterial enzymes, as well as exploring surface reactions on a metal catalyst. A brief summary of each project follows. Project 1 - Bacterial Enzyme SpvC The newly discovered SpvC effector protein from Salmonella typhimurium interferes with the host immune response by dephosphorylating mitogen-activated protein kinases (MAPKs) with a beta-elimination mechanism. The dynamics of the enzyme substrate complex of the SpvC effector is investigated with a 3.2 ns molecular dynamics simulation, which reveals that the phosphorylated peptide substrate is tightly held in the active site by a hydrogen bond network and the lysine general base is positioned for the abstraction of the alpha hydrogen. The catalysis is further modeled with density functional theory (DFT) in a truncated active-site model at the B3LYP/6-31 G(d,p) level of theory. The truncated model suggested the reaction proceeds via a single transition state. After including the enzyme environment in ab initio QM/MM studies, it was found to proceed via an E1cB-like pathway, in which the carbanion intermediate is stabilized by an enzyme oxyanion hole provided by Lys104 and Tyr158 of SpvC. Project 2 - Human Enzyme CDK2 Phosphorylation reactions catalyzed by kinases and phosphatases play an indispensable role in cellular signaling, and their malfunctioning is implicated in many diseases. Ab initio quantum mechanical/molecular mechanical studies are reported for the phosphoryl transfer reaction catalyzed by a cyclin-dependent kinase, CDK2. Our results suggest that an active-site Asp residue, rather than ATP as previously proposed, serves as the general base to activate the Ser nucleophile. The corresponding transition state features a dissociative, metaphosphate-like structure, stabilized by the Mg(II) ion and several hydrogen bonds. The calculated free-energy barrier is consistent with experimental values. Project 3 - Bacterial Enzyme Anthrax Lethal Factor In this dissertation, we report a hybrid quantum mechanical and molecular mechanical study of the catalysis of anthrax lethal factor, an important first step in designing inhibitors to help treat this powerful bacterial toxin. The calculations suggest that the zinc peptidase uses the same general base-general acid mechanism as in thermolysin and carboxypeptidase A, in which a zinc-bound water is activated by Glu687 to nucleophilically attack the scissile carbonyl carbon in the substrate. The catalysis is aided by an oxyanion hole formed by the zinc ion and the side chain of Tyr728, which provide stabilization for the fractionally charged carbonyl oxygen. Project 4 - Methanol Steam Reforming on PdZn alloy Recent experiments suggested that PdZn alloy on ZnO support is a very active and selective catalyst for methanol steam reforming (MSR). Plane-wave density functional theory calculations were carried out on the initial steps of MSR on both PdZn and ZnO surfaces. Our calculations indicate that the dissociation of both methanol and water is highly activated on flat surfaces of PdZn such as (111) and (100), while the dissociation barriers can be lowered significantly by surface defects, represented here by the (221), (110), and (321) faces of PdZn. The corresponding processes on the polar Zn-terminated ZnO(0001) surfaces are found to have low or null barriers. Implications of these results for both MSR and low temperature mechanisms are discussed.

Validation and divergence of the activation energy barrier crossing transition at the AOT/lecithin reverse micellar interface.

PubMed

Narayanan, S Shankara; Sinha, Sudarson Sekhar; Sarkar, Rupa; Pal, Samir Kumar

2008-03-13

In this report, the validity and divergence of the activation energy barrier crossing model for the bound to free type water transition at the interface of the AOT/lecithin mixed reverse micelle (RM) has been investigated for the first time in a wide range of temperatures by time-resolved solvation of fluorophores. Here, picosecond-resolved solvation dynamics of two fluorescent probes, ANS (1-anilino-8-naphthalenesulfonic acid, ammonium salt) and Coumarin 500 (C-500), in the mixed RM have been carefully examined at 293, 313, 328, and 343 K. Using the dynamic light scattering (DLS) technique, the size of the mixed RMs at different temperatures was found to have an insignificant change. The solvation process at the reverse micellar interface has been found to be the activation energy barrier crossing type, in which interface-bound type water molecules get converted into free type water molecules. The activation energies, Ea, calculated for ANS and C-500 are 7.4 and 3.9 kcal mol(-1), respectively, which are in good agreement with that obtained by molecular dynamics simulation studies. However, deviation from the regular Arrhenius type behavior was observed for ANS around 343 K, which has been attributed to the spatial heterogeneity of the probe environments. Time-resolved fluorescence anisotropy decay of the probes has indicated the existence of the dyes in a range of locations in RM. With the increase in temperature, the overall anisotropy decay becomes faster revealing the lability of the microenvironment at elevated temperatures.

DFT study on dry reforming of methane over Ni2Fe overlayer of Ni(1 1 1) surface

NASA Astrophysics Data System (ADS)

Xu, Li-li; Wen, Hong; Jin, Xin; Bing, Qi-ming; Liu, Jing-yao

2018-06-01

We reported the complete catalytic cycle of dry reforming of methane (DRM) on Ni2Fe overlayer of Ni(1 1 1) surface by periodic density functional theory (DFT) calculations. The pathways for dehydrogenation of CH4 and CO2 activation were located. Our results demonstrate that compared with pure Ni(1 1 1) surface, the introduction Fe into Ni increases the energy barrier of CH dissociation to carbon and hydrogen atoms, thereby suppressing coke deposition on the surface, while it promotes the H-induced CO2 activation pathway to form OH radical, and thus not only the surface oxygen but also OH are responsible for the oxidation of CHx (x = 0,1) on the Ni2Fe overlayer. The most favorable pathway of CH/C oxidation is found to be CH∗ + OH∗ → CHOH∗ → CHO∗ + H∗ → CO∗ + 2H∗, with the rate-limiting energy barrier of 1.12 eV. Furthermore, since Fe is oxidized partially to FeO leading to a partial dealloying under DRM conditions, we also studied the surface-carbon removal and the activity for the reforming of methane on the FeO ribbon supported Ni(1 1 1) (FeO/Ni) interface by DFT+U method. The surface C reacts with lattice oxygen of FeO to produce CO via a Mars-van Krevelen (MvK) mechanism, with a very lower energy barrier of 0.16 eV. The present results show that the introduction of Fe into Ni has a positive effect on the activity toward DRM and has an improved coke resistance.

Barriers to front propagation in laminar, three-dimensional fluid flows

NASA Astrophysics Data System (ADS)

Doan, Minh; Simons, J. J.; Lilienthal, Katherine; Solomon, Tom; Mitchell, Kevin A.

2018-03-01

We present experiments on one-way barriers that block reaction fronts in a fully three-dimensional (3D) fluid flow. Fluorescent Belousov-Zhabotinsky reaction fronts are imaged with laser-scanning in a laminar, overlapping vortex flow. The barriers are analyzed with a 3D extension to burning invariant manifold (BIM) theory that was previously applied to two-dimensional advection-reaction-diffusion processes. We discover tube and sheet barriers that guide the front evolution. The experimentally determined barriers are explained by BIMs calculated from a model of the flow.

[Prevalence of barriers for physical activity in adolescents].

PubMed

Santos, Mariana Silva; Hino, Adriano Akira Ferreira; Reis, Rodrigo Siqueira; Rodriguez-Añez, Ciro Romélio

2010-03-01

The aim of this study was to analyze the prevalence and association of barriers to physical activity among adolescents. This cross-sectional study evaluated a representative sample of public high school students in Curitiba-PR, Brazil. A total of 1,609 school adolescents (59.7% male) between 14 and 18 years of age answered a questionnaire on physical activity status and barriers to physical activity. Logistic regressions were conducted for each barrier investigated to verify the association between the prevalence of barriers and physical activity, adjusting for confounding variables (age and socioeconomic status). Analyses were done separately for boys and girls. Only 22% of boys and 9% of girls achieved the current physical activity recommendation. Among the 12 barriers investigated, only "there is nobody to take" did not differ between boys and girls. The perception of barriers was higher for girls than boys (p < 0.05) for all other barriers. "Lack of friends company" and "feel lazy" were the barriers most often reported by boys (30.4%) and girls (51.8%) respectively; however, the barrier most strongly associated with prevalence of physical inactivity was "prefer to do other things" for both boys (OR = 5.02 (2.69 - 9.37); p < 0.05) and girls (OR = 7.10 (3.71 - 13.60); p < 0.05). Perceived barriers for the practice of physical activity were more prevalent in girls and differed as to the extent of importance between genders.

Thermoelectric properties of nano-granular indium-tin-oxide within modified electron filtering model with chemisorption-type potential barriers

NASA Astrophysics Data System (ADS)

Brinzari, V.; Nika, D. L.; Damaskin, I.; Cho, B. K.; Korotcenkov, G.

2016-07-01

In this work, an approach to the numerical study of the thermoelectric parameters of nanoscale indium tin oxide (ITO, Sn content<10 at%) based on an electron filtering model (EFM) was developed. Potential barriers at grain boundaries were assumed to be responsible for a filtering effect. In the case of the dominant inelastic scattering of electrons, the maximal distance between potential barriers was limited in this modified model. The algorithm for such characteristic length calculation was proposed, and its value was evaluated for ITO. In addition, the contributions of different scattering mechanisms (SMs) in electron transport were examined. It was confirmed that in bulk ITO, the scattering on polar optical phonons (POPs) and ionized impurities dominates, limiting electron transport. In the framework of the filtering model, the basic thermoelectric parameters (i.e., electrical conductivity, mobility, Seebeck coefficient, and power factor (PF)) were calculated for ITO in the temperature range of 100-500 °C as a function of potential barrier height. The results demonstrated a sufficient rise of the Seebeck coefficient with an increase in barrier height and specific behavior of PF. It was found that PF is very sensitive to barrier height, and at its optimal value for granular ITO, it may exceed the PF for bulk ITO by 3-5 times. The PF maximum was achieved by band bending, slightly exceeding Fermi energy. The nature of surface potential barriers in nano-granular ITO with specific grains is due to the oxygen chemisorption effect, and this can be observed despite of the degeneracy of the conduction band (CB). This hypothesis and the corresponding calculations are in good agreement with recent experimental studies [Brinzari et al. Thin Solid Films 552 (2014) 225].

Barriers to outdoor physical activity in wintertime among Somali youth.

PubMed

Rothe, Elizabeth; Holt, Christina; Kuhn, Celine; McAteer, Timothy; Askari, Isabella; O'Meara, Mary; Sharif, Abdimajid; Dexter, William

2010-10-01

To identify barriers to outdoor physical activity in winter among Somali youth in Maine. Despite the many proven health benefits of physical activity among children, such as cardiovascular fitness and health status as an adult, there has been a decrease in physical activity among children in recent years. Specifically, children who are of low socio-economic status or are from communities where many immigrants are at increased risk for developing obesity. Immigrants are also less likely to be physically active. There are many potential barriers to wintertime physical activity among Somali youth in Maine, such as lack of financial resources, transportation, proper winter clothing, and appropriate knowledge of winter safety, and language and cultural barriers. For females, different attire required for outdoor activity may be a barrier. Somali parents and children were recruited from Portland, Maine to participate in focus groups led by a trained facilitator with a Somali translator and cultural broker. Transcripts were coded using NVIVO software to identify barriers to physical activity among Somali youth outside in winter. Eight focus groups were conducted. Sixty-one Somali community members were recruited. Participants felt outdoor physical activity is important, but note that it is decreased in winter. Barriers to outdoor activity in winter cited by focus group participants were lack of resources, health concerns, gender barriers for females, and knowledge barriers. Concern over lack of supervision while children play outside was also cited. This study revealed many of the underlying beliefs, barriers and cultural issues that impact Somali families' intention to be active and ability to be active outdoors in winter. These findings can be used to generate research hypotheses and public health interventions regarding outdoor physical activity among Somali youth.

Nitride based quantum well light-emitting devices having improved current injection efficiency

DOEpatents

Tansu, Nelson; Zhao, Hongping; Liu, Guangyu; Arif, Ronald

2014-12-09

A III-nitride based device provides improved current injection efficiency by reducing thermionic carrier escape at high current density. The device includes a quantum well active layer and a pair of multi-layer barrier layers arranged symmetrically about the active layer. Each multi-layer barrier layer includes an inner layer abutting the active layer; and an outer layer abutting the inner layer. The inner barrier layer has a bandgap greater than that of the outer barrier layer. Both the inner and the outer barrier layer have bandgaps greater than that of the active layer. InGaN may be employed in the active layer, AlInN, AlInGaN or AlGaN may be employed in the inner barrier layer, and GaN may be employed in the outer barrier layer. Preferably, the inner layer is thin relative to the other layers. In one embodiment the inner barrier and active layers are 15 .ANG. and 24 .ANG. thick, respectively.

Multidimensionally constrained relativistic mean-field study of triple-humped barriers in actinides

NASA Astrophysics Data System (ADS)

Zhao, Jie; Lu, Bing-Nan; Vretenar, Dario; Zhao, En-Guang; Zhou, Shan-Gui

2015-01-01

Background: Potential energy surfaces (PES's) of actinide nuclei are characterized by a two-humped barrier structure. At large deformations beyond the second barrier, the occurrence of a third barrier was predicted by macroscopic-microscopic model calculations in the 1970s, but contradictory results were later reported by a number of studies that used different methods. Purpose: Triple-humped barriers in actinide nuclei are investigated in the framework of covariant density functional theory (CDFT). Methods: Calculations are performed using the multidimensionally constrained relativistic mean field (MDC-RMF) model, with the nonlinear point-coupling functional PC-PK1 and the density-dependent meson exchange functional DD-ME2 in the particle-hole channel. Pairing correlations are treated in the BCS approximation with a separable pairing force of finite range. Results: Two-dimensional PES's of 226,228,230,232Th and 232,235,236,238U are mapped and the third minima on these surfaces are located. Then one-dimensional potential energy curves along the fission path are analyzed in detail and the energies of the second barrier, the third minimum, and the third barrier are determined. The functional DD-ME2 predicts the occurrence of a third barrier in all Th nuclei and 238U . The third minima in 230 ,232Th are very shallow, whereas those in 226 ,228Th and 238U are quite prominent. With the functional PC-PK1 a third barrier is found only in 226 ,228 ,230Th . Single-nucleon levels around the Fermi surface are analyzed in 226Th, and it is found that the formation of the third minimum is mainly due to the Z =90 proton energy gap at β20≈1.5 and β30≈0.7 . Conclusions: The possible occurrence of a third barrier on the PES's of actinide nuclei depends on the effective interaction used in multidimensional CDFT calculations. More pronounced minima are predicted by the DD-ME2 functional, as compared to the functional PC-PK1. The depth of the third well in Th isotopes decreases with increasing neutron number. The origin of the third minimum is due to the proton Z =90 shell gap at relevant deformations.

Conformational stability, structural parameters and vibrational assignment from variable temperature infrared spectra of krypton solutions and ab initio calculations of ethylisothiocyanate.

PubMed

Durig, James R; Zheng, Chao

2007-11-01

Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.

Enhancing the Photocatalytic Hydrogen Evolution Performance of a Metal/Semiconductor Catalyst through Modulation of the Schottky Barrier Height by Controlling the Orientation of the Interface.

PubMed

Liu, Yang; Gu, Xin; Qi, Wen; Zhu, Hong; Shan, Hao; Chen, Wenlong; Tao, Peng; Song, Chengyi; Shang, Wen; Deng, Tao; Wu, Jianbo

2017-04-12

Construction of a metal-semiconductor heterojunction is a promising method to improve heterogeneous photocatalysis for various reactions. Although the structure and photocatalytic performance of such a catalyst system have been extensively studied, few reports have demonstrated the effect of interface orientation at the metal-semiconductor junction on junction-barrier bending and the electronic transport properties. Here, we construct a Pt/PbS heterojunction, in which Pt nanoparticles are used as highly active catalysts and PbS nanocrystals (NCs) with well-controlled shapes are used as light-harvesting supports. Experimental results show that the photoelectrocatalytic activities of the Pt/PbS catalyst are strongly dependent on the contacting facets of PbS at the junction. Pt/octahedral PbS NCs with exposed PbS(111) facets show the highest photoinduced enhancement of hydrogen evolution reaction activity, which is ∼14.38 times higher than that of the ones with only PbS(100) facets (Pt/cubic PbS NCs). This enhancement can further be rationalized by the different energy barriers of the Pt/PbS Schottky junction due to the specific band structure and electron affinity, which is also confirmed by the calculations based on density functional theory. Therefore, controlling the contacting interfaces of a metal/semiconductor material may offer an effective approach to form the desired heterojunction for optimization of the catalytic performance.

Hydrogenation of Carbon Dioxide to Methanol Catalyzed by Iron, Cobalt, and Manganese Cyclopentadienone Complexes: Mechanistic Insights and Computational Design.

PubMed

Ge, Hongyu; Chen, Xiangyang; Yang, Xinzheng

2017-07-03

Density functional theory study of the hydrogenation of carbon dioxide to methanol catalyzed by iron, cobalt, and manganese cyclopentadienone complexes reveals a self-promoted mechanism, which features a methanol- or water-molecule-assisted proton transfer for the cleavage of H 2 . The total free energy barrier of the formation of methanol from CO 2 and H 2 catalyzed by Knölker's iron cyclopentadienone complex, [2,5-(SiMe 3 ) 2 -3,4-(CH 2 ) 4 (η 5 -C 4 COH)]Fe(CO) 2 H, is 26.0 kcal mol -1 in the methanol solvent. We also evaluated the catalytic activities of 8 other experimentally reported iron cyclopentadienone complexes and 37 iron, cobalt, and manganese cyclopentadienone complexes proposed in this study. In general, iron and manganese complexes have relatively higher catalytic activities. Among all calculated complexes, [2,5-(SiMe 3 ) 2 -3,4-CH 3 CHSCH 2 (η 5 -C 4 COH)]Fe(CO) 2 H (1 Fe-Casey-S-CH3 ) is the most active one with a total free energy barrier of 25.1 kcal mol -1 in the methanol solvent. Such a low barrier indicates that 1 Fe-Casey-S-CH3 is a very promising low-cost and high efficiency catalyst for the conversion of CO 2 and H 2 to methanol under mild conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Perceived barriers to physical activity among Nigerian stroke survivors.

PubMed

Idowu, Opeyemi Ayodiipo; Adeniyi, Ade Fatai; Ogwumike, Omoyemi Olubunmi; Fawole, Henrietta Oluwafunmilola; Akinrolie, Olayinka

2015-01-01

Benefits of physical activity in the prevention and management of stroke are well documented in the literature. There is increasing evidence that stroke survivors in South-West Nigeria are physically inactive. Data on barriers to the achievement of the recommended physical activity levels including its differences along socio-demographic characteristics among stroke survivors in South-West Nigeria are needed. The Exercise Benefits and Barrier Scale and the International Physical Activity Questionnaire were administered on 121 stroke survivors to determine their perceived barriers to physical activity and physical activity levels respectively. Information on socio-demographic data and clinical variables were also collected. The sample included 70.2% males, with majority of the participants reporting low physical activity levels (80.2%) and high perceived barriers (Mean = 48.13, SD = 7.88). The four most reported common barriers among stroke survivors were access to exercise facilities (95.0%), being embarrassed to exercise (94.2%), economic cost demands of exercise (94.2%) and notion that people in exercise clothes look funny (94.2%) respectively. There were no significant differences found in barriers to physical activity between gender (U = 1471.00, P = 0.74) and across each of: occupational status (H = 4.37, P = 0.22), age group (H = 0.82, P = 0.84) and educational levels (H = 4.56, P = 0.33). Significant difference however existed in perceived barriers across marital status categories (H = 12.87, P = 0.05). Stroke survivors indicated high perceived barriers to physical activity and these barriers were associated with marital status.

An exploratory study of physical activity and perceived barriers to exercise in ambulant people with neuromuscular disease compared with unaffected controls.

PubMed

Phillips, Margaret; Flemming, Nicola; Tsintzas, Kostas

2009-08-01

To determine activity patterns and perceived barriers to exercise in ambulant people with neuromuscular disease compared with ambulatory controls. Prospective controlled parallel group design. Outpatient clinic and community. Thirteen ambulatory people with neuromuscular disease and 18 ambulatory controls. Heart rates were recorded during sedentary activity and treadmill walking at various speeds to indicate activity threshold (flex heart rate), followed by ambulatory heart rate monitoring over two weekdays and one weekend day. The EPIC-Norfolk Physical Activity Questionnaire-2 and Barriers to Physical Activity and Disability Survey were completed. Participants with neuromuscular disease were less active than controls as estimated by both the EPIC-Norfolk Physical Activity Questionnaire-2, P<0.004, and the flex heart rate method, P<0.05. The number of perceived barriers was greater in the neuromuscular group, a mean of 7 (SD 4.2) barriers, compared with mean 3 (SD 2.1) barriers for controls, P<0.05. Specific barriers differed, with the barriers of 'pain', 'lack of energy' and 'exercise is too difficult' showing the greatest discrepancy and being higher in the neuromuscular disease group. Physical activity, as determined by two different methods, was less and barriers to exercise greater in people with neuromuscular disease compared with healthy controls. Specific barriers were different in the two groups. This information could assist in the design of achievable and effective exercise programmes for people with neuromuscular disease.

2D barrier in a superconducting niobium square

DOE Office of Scientific and Technical Information (OSTI.GOV)

Joya, Miryam R., E-mail: mrinconj@unal.edu.co; Barba-ortega, J., E-mail: jjbarbao@unal.edu.co; Sardella, Edson, E-mail: edsonsdl@gmail.com

The presence of barriers changes the vortex structure in superconducting Nb square in presence of a uniform applied magnetic field. The Cooper pair configurations in a mesoscopics superconducting square of Nb with a barrier are calculated within the nonlinear Ginzburg Landau equations. We predict the nucleation of multi-vortex states into the sample and a soft entry of the magnetic field inside and around into the barrier. A novel and non-conventional vortex configurations occurs at determined magnetic field.

Perceived barriers to healthy eating and physical activity among adolescents in seven Arab countries: a cross-cultural study.

PubMed

Musaiger, Abdulrahman O; Al-Mannai, Mariam; Tayyem, Reema; Al-Lalla, Osama; Ali, Essa Y A; Kalam, Faiza; Benhamed, Mofida M; Saghir, Sabri; Halahleh, Ismail; Djoudi, Zahra; Chirane, Manel

2013-01-01

To highlight the perceived personal, social, and environmental barriers to healthy eating and physical activity among Arab adolescents. A multistage stratified sampling method was used to select 4698 students aged 15-18 years (2240 males and 2458 females) from public schools. Seven Arab counties were included in the study, namely, Algeria, Jordan, Kuwait, Libya, Palestine, Syria, and the United Arab Emirates. Self-reported questionnaire was used to list the barriers to healthy eating and physical activity facing these adolescents. It was found that lack of information on healthy eating, lack of motivation to eat a healthy diet, and not having time to prepare or eat healthy food were the main barriers to healthy eating among both genders. For physical activity, the main barriers selected were lack of motivation to do physical activity, less support from teachers, and lack of time to do physical activity. In general, females faced more barriers to physical activity than males in all countries included. There were significant differences between males and females within each country and among countries for most barriers. Intervention programmes to combat obesity and other chronic noncommunicable diseases in the Arab world should include solutions to overcome the barriers to weight maintenance, particularly the sociocultural barriers to practising physical activity.

A comparative theoretical study of the catalytic activities of Au2(-) and AuAg(-) dimers for CO oxidation.

PubMed

Liu, Peng; Song, Ke; Zhang, Dongju; Liu, Chengbu

2012-05-01

The detailed mechanisms of catalytic CO oxidation over Au(2)(-) and AuAg(-) dimers, which represent the simplest models for monometal Au and bimetallic Au-Ag nanoparticles, have been studied by performing density functional theory calculations. It is found that both Au(2)(-) and AuAg(-) dimers catalyze the reaction according to the similar mono-center Eley-Rideal mechanism. The catalytic reaction is of the multi-channel and multi-step characteristic, which can proceed along four possible pathways via two or three elementary steps. In AuAg(-), the Au site is more active than the Ag site, and the calculated energy barrier values for the rate-determining step of the Au-site catalytic reaction are remarkably smaller than those for both the Ag-site catalytic reaction and the Au(2)(-) catalytic reaction. The better catalytic activity of bimetallic AuAg(-) dimer is attributed to the synergistic effect between Au and Ag atom. The present results provide valuable information for understanding the higher catalytic activity of Au-Ag nanoparticles and nanoalloys for low-temperature CO oxidation than either pure metallic catalyst.

Neutron-induced 63Ni activity and microscopic observation of copper samples exposed to the Hiroshima atomic bomb

NASA Astrophysics Data System (ADS)

Shizuma, Kiyoshi; Endo, Satoru; Shinozaki, Kenji; Fukushima, Hiroshi

2013-05-01

Fast neutron activation data for 63Ni in copper samples exposed to the Hiroshima atomic bomb are important in evaluating neutron doses to the survivors. Up to until now, accelerator mass spectrometry and liquid scintillation counting methods have been applied in 63Ni measurements and data were accumulated within 1500 m from the hypocenter. The slope of the activation curve versus distance shows reasonable agreement with the calculation result, however, data near the hypocenter are scarce. In the present work, two copper samples obtained from the Atomic bomb dome (155 m from the hypocenter) and the Bank of Japan building (392 m) were utilized in 63Ni beta-ray measurement with a Si surface barrier detector. Additionally, microscopic observation of the metal surfaces was performed for the first time. Only upper limit of 63Ni production was obtained for copper sample of the Atomic bomb dome. The result of the 63Ni measurement for Bank of Japan building show reasonable agreement with the AMS measurement and to fast neutron activation calculations based on the Dosimetry System 2002 (DS02) neutrons.

Chemical instability leads to unusual chemical-potential-independent defect formation and diffusion in perovskite solar cell material CH 3 NH 3 PbI 3

DOE PAGES

Ming, Wenmei; Chen, Shiyou; East China Normal Univ.; ...

2016-10-13

Methylammonium (MA) lead triiodide (MAPbI 3) has recently emerged as a promising solar cell material. But, MAPbI3 is known to have chemical instability, i.e., MAPbI3 is prone to decomposition into MAI and PbI 2 even at moderate temperatures (e.g. 330 K). Here, we show that the chemical instability, as reflected by the calculated negligible enthalpy of formation of MAPbI 3 (with respect to MAI and PbI 2), has an unusual and important consequence for defect properties, i.e., defect formation energies in low-carrier-density MAPbI 3 are nearly independent of the chemical potentials of constituent elements and thus can be uniquely determined. This allows straightforward calculations of defect concentrations and the activation energy of ionic conductivity (the sum of the formation energy and the diffusion barrier of the charged mobile defect) in MAPbI 3. Furthermore, the calculated activation energy for ionic conductivity due to Vmore » $$+\\atop{1}$$ diffusion is in excellent agreement with the experimental values, which demonstrates unambiguously that V$$+\\atop{1}$$ is the dominant diffusing defect and is responsible for the observed ion migration and device polarization in MAPbI3 solar cells. The calculated low formation energy of a Frenkel pair (V$$+\\atop{1}$$ -I$$-\\atop{i}$$ and low diffusion barriers of V$$+\\atop{1}$$ and Image I$$-\\atop{i}$$ suggest that the iodine ion migration and the resulting device polarization may occur even in single-crystal devices and grain-boundary-passivated polycrystalline thin film devices (which were previously suggested to be free from ion-migration-induced device polarization), leading to device degradation. Moreover, the device polarization due to the Frenkel pair (which has a relatively low concentration) may take a long time to develop and thus may avoid the appearance of the current–voltage hysteresis at typical scan rates.« less

Hydrogen atom migration in the oxidation of aldehydes - O(3P) + H2CO

NASA Technical Reports Server (NTRS)

Dupuis, M.; Lester, W. A., Jr.

1984-01-01

An ab initio study of hydrogen atom migration in methylenebis(oxy)H2CO2(3B2) to form triplet formic acid HCOOH (3A1) is reported. From HF, MCHF, and CI calculated energy barriers, the activation energy is estimated to be no less than 30 kcal/mol. It is concluded that the hydrogen migration channel is not accessible in recent room temperature experiments on the O(3P) + H2CO reaction.

Free energy barrier for molecular motions in bistable [2]rotaxane molecular electronic devices.

PubMed

Kim, Hyungjun; Goddard, William A; Jang, Seung Soon; Dichtel, William R; Heath, James R; Stoddart, J Fraser

2009-03-12

Donor-acceptor binding of the pi-electron-poor cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) with the pi-electron-rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) stations provides the basis for electrochemically switchable, bistable [2]rotaxanes, which have been incorporated and operated within solid-state devices to form ultradense memory circuits (ChemPhysChem 2002, 3, 519-525; Nature 2007, 445, 414-417) and nanoelectromechanical systems. The rate of CBPQT(4+) shuttling at each oxidation state of the [2]rotaxane dictates critical write-and-retention time parameters within the devices, which can be tuned through chemical synthesis. To validate how well computational chemistry methods can estimate these rates for use in designing new devices, we used molecular dynamics simulations to calculate the free energy barrier for the shuttling of the CBPQT(4+) ring between the TTF and the DNP. The approach used here was to calculate the potential of mean force along the switching pathway, from which we calculated free energy barriers. These calculations find a turn-on time after the rotaxane is doubly oxidized of approximately 10(-7) s (suggesting that the much longer experimental turn-on time is determined by the time scale of oxidization). The return barrier from the DNP to the TTF leads to a predicted lifetime of 2.1 s, which is compatible with experiments.

Breaking the BBC (Buoyancy Barriers to Cryovolcanism)

NASA Astrophysics Data System (ADS)

McGovern, P. J.; White, O. L.

2018-06-01

Like Australian table wines, Cryovolcanism has been poo-poohed, because of a perceived negative buoyancy problem. Here we point out that several basaltic planets have overcome far worse barriers, and calculate a scenario for cyrovolcanism on Pluto.

First-principles study of transition-metal nitrides as diffusion barriers against Al

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mei, Zhi-Gang; Yacout, Abdellatif M.; Kim, Yeon Soo

2016-04-01

Using density-functional theory based first-principles calculations we provided a comparative study of the diffusion barrier properties of TiN, ZrN, and HfN against Al for U-Mo dispersion fuel applications. We firstly examined the thermodynamic stability of these transition-metal nitrides with Al. The calculated heats of reaction show that both TiN and ZrN are thermodynamically unstable diffusion barrier materials, which might be decomposed by Al at relatively high temperatures. As a comparison, HfN is a stable diffusion barrier material for Al. To evaluate the kinetic stability of these nitride systems against Al diffusion, we investigated the diffusion mechanisms of Al in TiN,more » ZrN and HfN using atomic scale simulations. The effect of non-stoichiometry on the defect formation and Al migration was systematically studied. (C) 2015 ELSEVIER B.V. All rights reserved« less

Effect of the scheme of plasmachemical processes on the calculated characteristics of a barrier discharge in xenon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Avtaeva, S. V.; Kulumbaev, E. B.

2008-06-15

The dynamics of a repetitive barrier discharge in xenon at a pressure of 400 Torr is simulated using a one-dimensional drift-diffusion model. The thicknesses of identical barriers with a dielectric constant of 4 are 2 mm, and the gap length is 4 mm. The discharge is fed with an 8-kV ac voltage at a frequency of 25 or 50 kHz. The development of the ionization wave and the breakdown and afterglow phases of a barrier discharge are analyzed using two different kinetic schemes of elementary processes in a xenon plasma. It is shown that the calculated waveforms of the dischargemore » voltage and current, the instant of breakdown, and the number of breakdowns per voltage half-period depend substantially on the properties of the kinetic scheme of plasmachemical processes.« less

Barriers for recess physical activity: a gender specific qualitative focus group exploration.

PubMed

Pawlowski, Charlotte Skau; Tjørnhøj-Thomsen, Tine; Schipperijn, Jasper; Troelsen, Jens

2014-06-23

Many children, in particular girls, do not reach the recommended amount of daily physical activity. School recess provides an opportunity for both boys and girls to be physically active, but barriers to recess physical activity are not well understood. This study explores gender differences in children's perceptions of barriers to recess physical activity. Based on the socio-ecological model four types of environmental barriers were distinguished: natural, social, physical and organizational environment. Data were collected through 17 focus groups (at 17 different schools) with in total 111 children (53 boys) from fourth grade, with a mean age of 10.4 years. The focus groups included an open group discussion, go-along group interviews, and a gender segregated post-it note activity. A content analysis of the post-it notes was used to rank the children's perceived barriers. This was verified by a thematic analysis of transcripts from the open discussions and go-along interviews. The most frequently identified barriers for both boys and girls were weather, conflicts, lack of space, lack of play facilities and a newly-found barrier, use of electronic devices. While boys and girls identified the same barriers, there were both inter- and intra-gender differences in the perception of these barriers. Weather was a barrier for all children, apart from the most active boys. Conflicts were perceived as a barrier particularly by those boys who played ballgames. Girls said they would like to have more secluded areas added to the school playground, even in large schoolyards where lack of space was not a barrier. This aligned with girls' requests for more "hanging-out" facilities, whereas boys primarily wanted activity promoting facilities. Based on the results from this study, we recommend promoting recess physical activity through a combination of actions, addressing barriers within the natural, social, physical and organizational environment.

Quantum chemical modeling of zeolite-catalyzed methylation reactions: toward chemical accuracy for barriers.

PubMed

Svelle, Stian; Tuma, Christian; Rozanska, Xavier; Kerber, Torsten; Sauer, Joachim

2009-01-21

The methylation of ethene, propene, and t-2-butene by methanol over the acidic microporous H-ZSM-5 catalyst has been investigated by a range of computational methods. Density functional theory (DFT) with periodic boundary conditions (PBE functional) fails to describe the experimentally determined decrease of apparent energy barriers with the alkene size due to inadequate description of dispersion forces. Adding a damped dispersion term expressed as a parametrized sum over atom pair C(6) contributions leads to uniformly underestimated barriers due to self-interaction errors. A hybrid MP2:DFT scheme is presented that combines MP2 energy calculations on a series of cluster models of increasing size with periodic DFT calculations, which allows extrapolation to the periodic MP2 limit. Additionally, errors caused by the use of finite basis sets, contributions of higher order correlation effects, zero-point vibrational energy, and thermal contributions to the enthalpy were evaluated and added to the "periodic" MP2 estimate. This multistep approach leads to enthalpy barriers at 623 K of 104, 77, and 48 kJ/mol for ethene, propene, and t-2-butene, respectively, which deviate from the experimentally measured values by 0, +13, and +8 kJ/mol. Hence, enthalpy barriers can be calculated with near chemical accuracy, which constitutes significant progress in the quantum chemical modeling of reactions in heterogeneous catalysis in general and microporous zeolites in particular.

Faculty Perceptions about Barriers to Active Learning

ERIC Educational Resources Information Center

Michael, Joel

2007-01-01

Faculty may perceive many barriers to active learning in their classrooms. Four groups of participants in a faculty development workshop were asked to list their perceived barriers to active learning. Many of the problems identified were present on more than one list. The barriers fall into three categories: student characteristics, issues…

Barriers to physical activity among working mothers.

PubMed

Dombrowski, Jill J

2011-04-01

Working mothers experience several barriers to physical activity. If these barriers can be identified by occupational health nurses and they can partner with working mothers to reduce these perceived barriers, the health of these workers can be improved and chronic disease risk prevented. The purpose of this study was to measure the effect of self-regulatory efficacy on physical activity among working mothers and to describe specific barriers to physical activity. The Barriers Specific Self-Efficacy Scale (BARSE) and the Kaiser Physical Activity Survey (KPAS) were used to measure the variables. Self-regulatory efficacy was found to be a strong predictor of physical activity in a diverse sample of working mothers who did not meet current recommendations for physical activity. Occupational health nurses can use these findings to design programs for groups and for counseling individuals. Copyright 2011, SLACK Incorporated.

78 FR 4983 - Proposed Information Collection; Women Veterans Healthcare Barriers Survey Activity: Comment Request

Federal Register 2010, 2011, 2012, 2013, 2014

2013-01-23

... Veterans Healthcare Barriers Survey Activity: Comment Request AGENCY: Veterans Health Administration... Healthcare Barriers Survey'' in any correspondence. During the comment period, comments may be viewed online... Veterans Healthcare Barriers Survey . OMB Control Number: 2900-New (Women Veterans Healthcare Barriers...

Semiclassical treatment of fusion and breakup processes of ^{6,8}He halo nuclei

NASA Astrophysics Data System (ADS)

Majeed, Fouad A.; Abdul-Hussien, Yousif A.

2016-06-01

A semiclassical approach has been used to study the effect of channel coupling on the calculations of the total fusion reaction cross section σ _{fus}, and the fusion barrier distribution D_{fus} for the systems 6He +^{238}U and 8He +^{197}Au. Since these systems invloves light exotic nuclei, breakup states channel play an important role that should be considered in the calculations. In semiclassical treatment, the relative motion between the projectile and target nuclei is approximated by a classical trajectory while the intrinsic dynamics is handled by time-dependent quantum mechanics. The calculations of the total fusion cross section σ _{fus}, and the fusion barrier distribution D_{fus} are compared with the full quantum mechanical calculations using the coupled-channels calculations with all order coupling using the computer code and with the available experimental data.

Design and Application of a High-Temperature Linear Ion Trap Reactor

NASA Astrophysics Data System (ADS)

Jiang, Li-Xue; Liu, Qing-Yu; Li, Xiao-Na; He, Sheng-Gui

2018-01-01

A high-temperature linear ion trap reactor with hexapole design was homemade to study ion-molecule reactions at variable temperatures. The highest temperature for the trapped ions is up to 773 K, which is much higher than those in available reports. The reaction between V2O6 - cluster anions and CO at different temperatures was investigated to evaluate the performance of this reactor. The apparent activation energy was determined to be 0.10 ± 0.02 eV, which is consistent with the barrier of 0.12 eV calculated by density functional theory. This indicates that the current experimental apparatus is prospective to study ion-molecule reactions at variable temperatures, and more kinetic details can be obtained to have a better understanding of chemical reactions that have overall barriers. [Figure not available: see fulltext.

Theoretical study of water-gas shift reaction on the silver nanocluster

NASA Astrophysics Data System (ADS)

Arab, Ali; Sharafie, Darioush; Fazli, Mostafa

2017-10-01

The kinetics of water gas shift reaction (WGSR) on the silver nanocluster was investigated using density functional theory according to the carboxyl associative mechanism. The hybrid B3PW91 functional along with the 6-31+G* and LANL2DZ basis sets were used throughout the calculations. It was observed that CO and H2O molecules adsorb physically on the Ag5 cluster without energy barrier as the initial steps of WGSR. The next three steps including H2Oads dissociation, carboxyl (OCOHads) formation, and CO2(ads) formation were accompanied by activation barrier. Transition states, as well as energy profiles of these three steps, were determined and analyzed. Our results revealed that the carboxyl and CO2(ads) formation were fast steps whereas H2Oads dissociation was the slowest step of WGSR.

Un-catalyzed peptide bond formation between two monomers of glycine, alanine, serine, threonine, and aspartic acid in gas phase: a density functional theory study

NASA Astrophysics Data System (ADS)

Bhunia, Snehasis; Singh, Ajeet; Ojha, Animesh K.

2016-05-01

In the present report, un-catalyzed peptide bond formation between two monomers of glycine (Gly), alanine (Ala), serine (Ser), threonine (Thr), and aspartic acid (Asp) has been investigated in gas phase via two steps reaction mechanism and concerted mechanism at B3LYP/6-31G(d,p) and M062X/6-31G(d,p) level of theories. The peptide bond is formed through a nucleophilic reaction via transition states, TS1 and TS2 in stepwise mechanism. The TS1 reveals formation of a new C-N bond while TS2 illustrate the formation of C=O bond. In case of concerted mechanism, C-N bond is formed by a single four-centre transition state (TS3). The energy barrier is used to explain the involvement of energy at each step of the reaction. The energy barrier (20-48 kcal/mol) is required for the transformation of reactant state R1 to TS1 state and intermediate state I1 to TS2 state. The large value of energy barrier is explained in terms of distortion and interaction energies for stepwise mechanism. The energy barrier of TS3 in concerted mechanism is very close to the energy barrier of the first transition state (TS1) of the stepwise mechanism for the formation of Gly-Gly and Ala-Ala di- peptide. However, in case of Ser-Ser, Thr-Thr and Asp-Asp di-peptide, the energy barrier of TS3 is relatively high than that of the energy barrier of TS1 calculated at B3LYP/6-31G(d,p) and M062X/6-31G(d,p) level of theories. In both the mechanisms, the value of energy barrier calculated at B3LYP/6-31G(d,p) level of theory is greater than that of the value calculated at M062X/6-31G(d,p) level of theory.

Structural Shielding Design of a 6 MV Flattening Filter Free Linear Accelerator: Indian Scenario.

PubMed

Mishra, Bibekananda; Selvam, T Palani; Sharma, P K Dash

2017-01-01

Detailed structural shielding of primary and secondary barriers for a 6 MV medical linear accelerator (LINAC) operated with flattening filter (FF) and flattening filter free (FFF) modes are calculated. The calculations have been carried out by two methods, one using the approach given in National Council on Radiation Protection (NCRP) Report No. 151 and the other based on the monitor units (MUs) delivered in clinical practice. Radiation survey of the installations was also carried out. NCRP approach suggests that the primary and secondary barrier thicknesses are higher by 24% and 26%. respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes with an assumption that only 20% of the workload is shared in FFF mode. Primary and secondary barrier thicknesses calculated from MUs delivered on clinical practice method also show the same trend and are higher by 20% and 19%, respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes. Overall, the barrier thickness for a LINAC operated in FF mode is higher about 20% to that of a LINAC operated in both FF and FFF modes.

Structural Shielding Design of a 6 MV Flattening Filter Free Linear Accelerator: Indian Scenario

PubMed Central

Mishra, Bibekananda; Selvam, T. Palani; Sharma, P. K. Dash

2017-01-01

Detailed structural shielding of primary and secondary barriers for a 6 MV medical linear accelerator (LINAC) operated with flattening filter (FF) and flattening filter free (FFF) modes are calculated. The calculations have been carried out by two methods, one using the approach given in National Council on Radiation Protection (NCRP) Report No. 151 and the other based on the monitor units (MUs) delivered in clinical practice. Radiation survey of the installations was also carried out. NCRP approach suggests that the primary and secondary barrier thicknesses are higher by 24% and 26%. respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes with an assumption that only 20% of the workload is shared in FFF mode. Primary and secondary barrier thicknesses calculated from MUs delivered on clinical practice method also show the same trend and are higher by 20% and 19%, respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes. Overall, the barrier thickness for a LINAC operated in FF mode is higher about 20% to that of a LINAC operated in both FF and FFF modes. PMID:28405104

First-principles study of the effects of Silicon doping on the Schottky barrier of TiSi2/Si interfaces

NASA Astrophysics Data System (ADS)

Wang, Han; Silva, Eduardo; West, Damien; Sun, Yiyang; Restrepo, Oscar; Zhang, Shengbai; Kota, Murali

As scaling of semiconductor devices is pursued in order to improve power efficiency, quantum effects due to the reduced dimensions on devices have become dominant factors in power, performance, and area scaling. In particular, source/drain contact resistance has become a limiting factor in the overall device power efficiency and performance. As a consequence, techniques such as heavy doping of source and drain have been explored to reduce the contact resistance, thereby shrinking the width of depletion region and lowering the Schottky barrier height. In this work, we study the relation between doping in Silicon and the Schottky barrier of a TiSi2/Si interface with first-principles calculation. Virtual Crystal Approximation (VCA) is used to calculate the average potential of the interface with varying doping concentration, while the I-V curve for the corresponding interface is calculated with a generalized one-dimensional transfer matrix method. The relation between substitutional and interstitial Boron and Phosphorus dopant near the interface, and their effect on tuning the Schottky barrier is studied. These studies provide insight to the type of doping and the effect of dopant segregation to optimize metal-semiconductor interface resistance.

I-V characteristics of graphene nanoribbon/h-BN heterojunctions and resonant tunneling.

PubMed

Wakai, Taiga; Sakamoto, Shoichi; Tomiya, Mitsuyoshi

2018-07-04

We present the first principle calculations of the electrical properties of graphene sheet/h-BN heterojunction (GS/h-BN) and 11-armchair graphene nanoribbon/h-BN heterojunction (11-AGNR/h-BN), which are carried out using the density functional theory (DFT) method and the non-equilibrium Green's function (NEGF) technique. Since 11-AGNR belongs to the conductive (3n-1)-family of AGNR, both are metallic nanomaterials with two transverse arrays of h-BN, which is a wide-gap semi-conductor. The two h-BN arrays act as double barriers. The transmission functions (TF) and I-[Formula: see text] characteristics of GS/h-BN and 11-AGNR/h-BN are calculated by DFT and NEGF, and they show that quantum double barrier tunneling occurs. The TF becomes very spiky in both materials, and it leads to step-wise I-[Formula: see text] characteristics rather than negative resistance, which is the typical behavior of double barriers in semiconductors. The results of our first principle calculations are also compared with 1D Dirac equation model for the double barrier system. The model explains most of the peaks of the transmission functions nearby the Fermi energy quite well. They are due to quantum tunneling.

Design of catalysts by different substituent groups to the ;cut g-C3N4; single layer

NASA Astrophysics Data System (ADS)

Xu, Weiwei; Tang, Chao; Chen, Chongyang; Li, Youyong; Xu, Lai

2017-09-01

Graphitic carbon nitride has been wildly studied as a kind of promising photocatalysts for hydrogen evolution. However, it has a low intrinsic activity. Herein, we designed new periodic structures "cut g-C3N4", and adding the new substituent groups. We employed density functional theory to calculate the charge distribution and catalytic properties of hydrogen evolution on the structures. We got a theoretical view that introducing conjugate substituents can enhance the catalytic performance for hydrogen evolution. Furthermore, it provided a theoretical guidance for the reasonable design of two dimensional non-metallic photocatalysts, with lower activation barrier of the catalytic reaction.

Oxygen-Promoted Methane Activation on Copper

DOE PAGES

Niu, Tianchao; Jiang, Zhao; Zhu, Yaguang; ...

2017-11-01

The role of oxygen in the activation of C–H bonds in methane on clean and oxygen-precovered Cu(111) and Cu 2O(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and “moderate pressures” was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C–H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole–dipole attraction of O–H and Cu–CH 3 species. Furthermore, the C–H bond activation barriersmore » on Cu 2O(111) surfaces are large due to the weak stabilization of H and CH 3 fragments.« less

Oxygen-Promoted Methane Activation on Copper

DOE Office of Scientific and Technical Information (OSTI.GOV)

Niu, Tianchao; Jiang, Zhao; Zhu, Yaguang

The role of oxygen in the activation of C–H bonds in methane on clean and oxygen-precovered Cu(111) and Cu 2O(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and “moderate pressures” was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C–H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole–dipole attraction of O–H and Cu–CH 3 species. Furthermore, the C–H bond activation barriersmore » on Cu 2O(111) surfaces are large due to the weak stabilization of H and CH 3 fragments.« less

Aromatic hydroxylation by cytochrome P450: model calculations of mechanism and substituent effects.

PubMed

Bathelt, Christine M; Ridder, Lars; Mulholland, Adrian J; Harvey, Jeremy N

2003-12-10

The mechanism and selectivity of aromatic hydroxylation by cytochrome P450 enzymes is explored using new B3LYP density functional theory computations. The calculations, using a realistic porphyrin model system, show that rate-determining addition of compound I to an aromatic carbon atom proceeds via a transition state with partial radical and cationic character. Reactivity is shown to depend strongly on ring substituents, with both electron-withdrawing and -donating groups strongly decreasing the addition barrier in the para position, and it is shown that the calculated barrier heights can be reproduced by a new dual-parameter equation based on radical and cationic Hammett sigma parameters.

Attempting to be active: Self-efficacy and barrier limitation differentiate activity levels of working mothers.

PubMed

Gierc, Madelaine; Locke, Sean; Jung, Mary; Brawley, Lawrence

2016-07-01

Working mothers are less physically active than working women without children and mothers who do not work. The purpose of this study was to examine concurrent self-regulatory efficacy and barriers to physical activity in a sample of working mothers. Women completed a mixed-methods survey which included measures of physical activity, concurrent self-regulatory efficacy, and barriers. Sufficiently active women experienced significantly greater concurrent self-regulatory efficacy and significantly less barrier limitation and frequency. No significant group differences were found for age, domestic duties performed, and children's extracurricular activities. Thematic analysis of barriers revealed six themes of common and unique factors, including limited time and family activities. © The Author(s) 2014.

Self-learning kinetic Monte Carlo simulations of diffusion in ferromagnetic α-Fe-Si alloys

NASA Astrophysics Data System (ADS)

Nandipati, Giridhar; Jiang, Xiujuan; Vemuri, Rama S.; Mathaudhu, Suveen; Rohatgi, Aashish

2018-01-01

Diffusion of Si atom and vacancy in the A2-phase of α-Fe-Si alloys in the ferromagnetic state, with and without magnetic order and in various temperature ranges, are studied using AKSOME, an on-lattice self-learning KMC code. Diffusion of the Si atom and the vacancy are studied in the dilute limit and up to 12 at.% Si, respectively, in the temperature range 350-700 K. Local Si neighborhood dependent activation energies for vacancy hops were calculated on-the-fly using a broken-bond model based on pairwise interaction. The migration barrier and prefactor for the Si diffusion in the dilute limit were obtained and found to agree with published data within the limits of uncertainty. Simulations results show that the prefactor and the migration barrier for the Si diffusion are approximately an order of magnitude higher, and a tenth of an electron-volt higher, respectively, in the magnetic disordered state than in the fully ordered state. However, the net result is that magnetic disorder does not have a significant effect on Si diffusivity within the range of parameters studied in this work. Nevertheless, with increasing temperature, the magnetic disorder increases and its effect on the Si diffusivity also increases. In the case of vacancy diffusion, with increasing Si concentration, its diffusion prefactor decreases while the migration barrier more or less remained constant and the effect of magnetic disorder increases with Si concentration. Important vacancy-Si/Fe atom exchange processes and their activation barriers were identified, and the effect of energetics on ordered phase formation in Fe-Si alloys are discussed.

Physical activity behavior change in middle-aged and older women: the role of barriers and of environmental characteristics.

PubMed

Kowal, John; Fortier, Michelle Sheila

2007-06-01

The majority of North American women are insufficiently active. Using an ecological approach to examine physical activity behavior in a sample of middle-aged and older women, this study aimed to (1) describe barriers to physical activity behavior change as well as environmental characteristics present in their neighborhoods, (2) examine relationships between barriers and physical activity behavior change, and (3) investigate environmental characteristics that may contribute to physical activity behavior change. Participants were 149 women ranging in age between 39 and 68. At Time 1, self-reported physical activity was assessed. Six months later (Time 2), barriers and environmental characteristics were measured, and physical activity was re-assessed. The most prevalent barriers were daily activities and fatigue. Over time, inactive women reported higher levels of barriers (e.g. fatigue, lack of interest in physical activity) than women who remained active or increased their physical activity level. Certain environmental characteristics (e.g. enjoyable scenery, seeing others exercising in their neighborhood) are suggested as potential contributors to physical activity behavior change.

MEMS Micropropulsion Activities at JPL

NASA Technical Reports Server (NTRS)

Mueller, Juergen; Chakraborty, Indrani; Vargo, Stephen; Bame, David; Marrese, Colleen; Tang, William C.

1999-01-01

A status of MEMS-based micropropulsion activities conducted at JPL will be given. These activities include work conducted on the so called Vaporizing Liquid Micro-Thruster (VLM) which recently underwent proof-of-concept testing, demonstrating the ability to vaporize water propellant at 2 W and 2 V. Micro-ion engine technologies, such m field emitter arrays and micro-grids are being studied. Focus in the field emitter area is on arrays able to survive in thruster plumes and micro-ion engine plasmas to serve as neutralizers aW engine cathodes. Integrated, batch-fabricated Ion repeller grid structures are being studied as well as different emitter tip materials are being investigated to meet these goals. A micro-isolation valve is being studied to isolate microspacecraft feed system during long interplanetary cruises, avoiding leakage and prolonging lifetime and reliability of such systems. This concept relies on the melting of a thin silicon barrier. Burst pressure values as high as 2,900 psig were obtained for these valves and power requirements to melt barriers ranging between 10 - 50 microns in thickness, as determined through thermal finite element calculations, varied between 10 - 30 W to be applied over a duration of merely 0.5 ms.

Theoretical study of the H2 reaction with a Pt4 (111) cluster

NASA Astrophysics Data System (ADS)

Cruz, A.; Bertin, V.; Poulain, E.; Benitez, J. I.; Castillo, S.

2004-04-01

The Cs symmetry reaction of the H2 molecule on a Pt4 (111) clusters, has been studied using ab initio multiconfiguration self-consistent field plus extensive multireference configuration interaction variational and perturbative calculations. The H2 interaction by the vertex and by the base of a tetrahedral Pt4 cluster were studied in ground and excited triplet and singlet states (closed and open shells), where the reaction curves are obtained through many avoided crossings. The Pt4 cluster captures and activates the hydrogen molecule; it shows a similar behavior compared with other Ptn (n=1,2,3) systems. The Pt4 cluster in their lowest five open and closed shell electronic states: 3B2, 1B2, 1A1 3A1, 1A1, respectively, may capture and dissociate the H2 molecule without activation barriers for the hydrogen molecule vertex approach. For the threefolded site reaction, i.e., by the base, the situation is different, the hydrogen adsorption presents some barriers. The potential energy minima occur outside and inside the cluster, with strong activation of the H-H bond. In all cases studied, the Pt4 cluster does not absorb the hydrogen molecule.

Conductivity equations of protons transporting through 2D crystals obtained with the rate process theory and free volume concept

NASA Astrophysics Data System (ADS)

Hao, Tian; Xu, Yuanze; Hao, Ting

2018-04-01

The Eyring's rate process theory and free volume concept are employed to treat protons (or other particles) transporting through a 2D (two dimensional) crystal like graphene and hexagonal boron nitride. The protons are assumed to be activated first in order to participate conduction and the conduction rate is dependent on how much free volume available in the system. The obtained proton conductivity equations show that only the number of conduction protons, proton size and packing structure, and the energy barrier associated with 2D crystals are critical; the quantization conductance is unexpectedly predicted with a simple Arrhenius type temperature dependence. The predictions agree well with experimental observations and clear out many puzzles like much smaller energy barrier determined from experiments than from the density function calculations and isotope separation rate independent of the energy barrier of 2D crystals, etc. Our work may deepen our understandings on how protons transport through a membrane and has direct implications on hydrogen related technology and proton involved bioprocesses.

Observations of barrier recombination in GaAs-AlGaAs quantum well structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blood, P.; Tsui, E.S.; Fletcher, E.D.

1989-05-29

Using laser structures with a window in the contact stripe, we have observed recombination from the wells and barrier regions of GaAs-AlGaAs quantum well lasers. The magnitude of the ratio of emission intensities from the barrier and the well, and the dependence of this ratio upon injection current, are in good agreement with a calculation in which the carrier populations in well and barrier are in thermal equilibrium at the lattice temperature (300 K).

Excited state free energy calculations of Cy3 in different environments

NASA Astrophysics Data System (ADS)

Sawangsang, Pilailuk; Buranachai, Chittanon; Punwong, Chutintorn

2015-05-01

Cy3, a cyanine dye, is one of the most widely used dyes in investigating the structure and dynamics of biomolecules by means of fluorescence methods. However, Cy3 fluorescence emission is strongly competed by trans-cis isomerization, whose efficiency is dictated by the isomerization energy barrier and the environment of Cy3. The fluorescence quantum yield of Cy3 is very low when the dye is free in homogeneous solution but it is considerably enhanced in an environment that rigidifies the structure, e.g. when it is attached to a DNA strand. In this work, the barriers for isomerization on the excited state of free Cy3, and Cy3 attached to single- and double-stranded DNA in methanol, are presented. The free energy and subsequently the isomerization barrier calculations are performed using the umbrella sampling technique with the weighted histogram analysis method. The hybrid quantum mechanics/molecular mechanics (QM/MM) approach is employed to provide the potential energy surfaces for the excited state dynamics simulations in umbrella sampling. The semiempirical floating occupation molecular orbital configuration interaction method is used for electronic excited state calculations of the QM region (Cy3). From the free energy calculations, the barrier of Cy3 attached to the single-stranded DNA is highest, in agreement with previously reported experimental results. This is likely due to the stacking interaction between Cy3 and DNA. Such a stacking interaction is likely associated with steric hindrance that prevents the rotation around the conjugated bonds of Cy3. If Cy3 experiences high steric hindrance, it has a higher isomerization barrier and thus the efficiency of fluorescence emission increases.

How thin barrier metal can be used to prevent Co diffusion in the modern integrated circuits?

NASA Astrophysics Data System (ADS)

Dixit, Hemant; Konar, Aniruddha; Pandey, Rajan; Ethirajan, Tamilmani

2017-11-01

In modern integrated circuits (ICs), billions of transistors are connected to each other via thin metal layers (e.g. copper, cobalt, etc) known as interconnects. At elevated process temperatures, inter-diffusion of atomic species can occur among these metal layers, causing sub-optimal performance of interconnects, which may lead to the failure of an IC. Thus, typically a thin barrier metal layer is used to prevent the inter-diffusion of atomic species within interconnects. For ICs with sub-10 nm transistors (10 nm technology node), the design rule (thickness scaling) demands the thinnest possible barrier layer. Therefore, here we investigate the critical thickness of a titanium-nitride (TiN) barrier that can prevent the cobalt diffusion using multi-scale modeling and simulations. First, we compute the Co diffusion barrier in crystalline and amorphous TiN with the nudged elastic band method within first-principles density functional theory simulations. Later, using the calculated activation energy barriers, we quantify the Co diffusion length in the TiN metal layer with the help of kinetic Monte Carlo simulations. Such a multi-scale modelling approach yields an exact critical thickness of the metal layer sufficient to prevent the Co diffusion in IC interconnects. We obtain a diffusion length of a maximum of 2 nm for a typical process of thermal annealing at 400 °C for 30 min. Our study thus provides useful physical insights for the Co diffusion in the TiN layer and further quantifies the critical thickness (~2 nm) to which the metal barrier layer can be thinned down for sub-10 nm ICs.

Perceived Barriers to Healthy Eating and Physical Activity among Adolescents in Seven Arab Countries: A Cross-Cultural Study

PubMed Central

Musaiger, Abdulrahman O.; Tayyem, Reema; Al-Lalla, Osama; Ali, Essa Y. A.; Kalam, Faiza; Benhamed, Mofida M.; Saghir, Sabri; Halahleh, Ismail; Djoudi, Zahra; Chirane, Manel

2013-01-01

Objective. To highlight the perceived personal, social, and environmental barriers to healthy eating and physical activity among Arab adolescents. Method. A multistage stratified sampling method was used to select 4698 students aged 15–18 years (2240 males and 2458 females) from public schools. Seven Arab counties were included in the study, namely, Algeria, Jordan, Kuwait, Libya, Palestine, Syria, and the United Arab Emirates. Self-reported questionnaire was used to list the barriers to healthy eating and physical activity facing these adolescents. Results. It was found that lack of information on healthy eating, lack of motivation to eat a healthy diet, and not having time to prepare or eat healthy food were the main barriers to healthy eating among both genders. For physical activity, the main barriers selected were lack of motivation to do physical activity, less support from teachers, and lack of time to do physical activity. In general, females faced more barriers to physical activity than males in all countries included. There were significant differences between males and females within each country and among countries for most barriers. Conclusion. Intervention programmes to combat obesity and other chronic noncommunicable diseases in the Arab world should include solutions to overcome the barriers to weight maintenance, particularly the sociocultural barriers to practising physical activity. PMID:24348144

Atropisomerism about Aryl-C(sp(3)) Bonds: Conformational Behavior of Substituted Phenylcyclohexanes in Solution.

PubMed

Flos, Manon; Lameiras, Pedro; Denhez, Clément; Mirand, Catherine; Berber, Hatice

2016-03-18

A catalytic hydrogenation of cannabidiol derivatives known as phenylcyclohexenes was used to prepare epimeric (1R,1S) and/or rotameric (M,P) phenylcyclohexanes. The reaction is diastereoselective, in favor of the 1S epimer, when large groups are attached to the phenyl ring. For each epimer, variable-temperature NMR experiments, including EXSY spectroscopy and DFT calculations, were used to determine the activation energies of the conformational exchange arising from the restricted rotation about the aryl-C(sp(3)) bond that led to two unequally populated rotamers. The conformational preference arises essentially from steric interactions between substituents vicinal to the pivot bond. The conformers of epimers (1S)-2e,f show high rotational barriers of up to 92 kJ mol(-1), unlike those of (1R)-2e,f and with much lower barriers of ∼72 kJ mol(-1). The height of the barriers not only depends on the substituents at the axis of chirality but also is influenced by the position of a methyl group on the monoterpene ring. The feature most favorable to high rotational barriers is when the methyl at C1 lies equatorially. This additional substituent effect, highlighted for the first time, seems fundamental to allowing atropisomerism in hindered ortho-substituted phenylcyclohexanes.

Room temperature current-voltage (I-V) characteristics of Ag/InGaN/n-Si Schottky barrier diode

NASA Astrophysics Data System (ADS)

Erdoğan, Erman; Kundakçı, Mutlu

2017-02-01

Metal-semiconductors (MSs) or Schottky barrier diodes (SBDs) have a significant potential in the integrated device technology. In the present paper, electrical characterization of Ag/InGaN/n-Si Schottky diode have been systematically carried out by simple Thermionic method (TE) and Norde function based on the I-V characteristics. Ag ohmic and schottky contacts are deposited on InGaN/n-Si film by thermal evaporation technique under a vacuum pressure of 1×10-5 mbar. Ideality factor, barrier height and series resistance values of this diode are determined from I-V curve. These parameters are calculated by TE and Norde methods and findings are given in a comparetive manner. The results show the consistency for both method and also good agreement with other results obtained in the literature. The value of ideality factor and barrier height have been determined to be 2.84 and 0.78 eV at room temperature using simple TE method. The value of barrier height obtained with Norde method is calculated as 0.79 eV.

Mapping intermolecular interactions and active site conformations: from human MMP-1 crystal structure to molecular dynamics free energy calculations.

PubMed

Nash, Anthony; Birch, Helen L; de Leeuw, Nora H

2017-02-01

The zinc-dependent Matrix Metalloproteinases (MMPs) found within the extracellular matrix (ECM) of vertebrates are linked to pathological processes such as arthritis, skin ulceration and cancer. Although a general backbone proteolytic mechanism is understood, crystallographic data continue to suggest an active site that is too narrow to encompass the respective substrate. We present a fully parameterised Molecular Dynamics (MD) study of the structural properties of an MMP-1-collagen crystallographic structure (Protein Data Bank - 4AUO), followed by an exploration of the free energy surface of a collagen polypeptide chain entering the active site, using a combined meta-dynamics and umbrella sampling (MDUS) approach. We conclude that the interactions between MMP-1 and the collagen substrate are in good agreement with a number of experimental studies. As such, our unrestrained MD simulations and our MDUS results, which indicate an energetic barrier for a local uncoiling and insertion event, can inform future investigations of the collagen-peptide non-bonded association steps with the active site prior to proteolytic mechanisms. The elucidation of such free energy barriers provides a better understanding of the role of the enzyme in the ECM and is important in the design of future MMP inhibitors.

Explore the reaction mechanism of the Maillard reaction: a density functional theory study.

PubMed

Ren, Ge-Rui; Zhao, Li-Jiang; Sun, Qiang; Xie, Hu-Jun; Lei, Qun-Fang; Fang, Wen-Jun

2015-05-01

The mechanism of Maillard reaction has been investigated by means of density functional theory calculations in the gaseous phase and aqueous solution. The Maillard reaction is a cascade of consecutive and parallel reaction. In the present model system study, glucose and glycine were taken as the initial reactants. On the basis of previous experimental results, the mechanisms of Maillard reaction have been proposed, and the possibility for the formation of different compounds have been evaluated through calculating the relative energy changes for different steps of reaction under different pH conditions. Our calculations reveal that the TS3 in Amadori rearrangement reaction is the rate-determining step of Maillard reaction with the activation barriers of about 66.7 and 68.8 kcal mol(-1) in the gaseous phase and aqueous solution, respectively. The calculation results are in good agreement with previous studies and could provide insights into the reaction mechanism of Maillard reaction, since experimental evaluation of the role of intermediates in the Maillard reaction is quite complicated.

First principles calculations of ceramics surfaces and interfaces: Examples from beta-silicon nitride and alpha-alumina

NASA Astrophysics Data System (ADS)

Dunn, Jennifer Synowczynski

The goal of this thesis was to use first principles calculations to provide a fundamental understanding at the atomistic level of the mechanisms (e.g. structural relaxations of ceramic surfaces/interfaces, charge transfer reactions, adsorption and dissociation phenomena, localized debonding) behind macroscopic behavior in ceramics (e.g. fracture toughness, corrosion, catalysis). This thesis includes the results from three independent Density Functional Theory (DFT) studies of beta-Si3N4 and alpha-Al2O 3. Due to the computational complexity of first principles calculations, the models in this thesis do not consider temperature or pressure effects and are limited to describing the behavior of systems containing less than 200 atoms. In future studies, these calculations can be used to train a reactive molecular dynamics force field (REAXFF) so that larger scale phenomena including temperature effects can be explicitly simulated. In the first study, the effect of over 30 dopants on the stability of the interface between beta-Si3N4 grains and the intergranular glassy SiON film (IGF) was investigated. The dopants chosen not only represented commonly known glass modifiers and sintering aides but also enabled us to search for dependencies based on atomic size and electronic orbital configuration. To ensure that the approximations used in our model captured the key physical phenomena occurring on the beta-Si3N4 (100) surface and at the Si3N4/ IGF interface, we compared to experimental data (i.e. High Angle Annual Dark Field-Scanning Transmission Electron Microscopy atomic positions and fracture toughness values (Mikijelj B., 2009)). We identified a computational metric (the interfacial stability factor S) which correlates with experimentally measured fracture toughness values. The interfacial stability factor S is defined as the binding energy of the doped system minus the binding energy of the undoped system, where the binding energy is the total energy of the system minus the sum of the energies of the constituent atoms. In the second study, we performed constrained geometry barrier calculations of the interaction of CO with the (001) beta-Si3N4 surface to answer the following questions: (1) Does the CO combustion product interact with the Si3N4 surface and if so, what is the mechanism? (2) Once adsorbed, can CO further dissociate into isolated surface active C and O species? (3) Is it more energetically favorable for C to diffuse into the bulk beta-Si3N4 or along its surface? and (4) What is the barrier to C diffusing into an amorphous SiO2 intergranular film? Our calculations indicated that CO spontaneously adsorbs to the (001) beta-Si 3N4 surface. However, at ambient temperatures, further dissociation into isolated surface adsorbed C and O species was not thermodynamically or kinetically feasible. The barrier to C diffusing interstitially 1A and 5A into the bulk crystalline lattice is 2.12 and 4.42 eV respectively for a defect free, clean surface. However, the barrier for C surface diffusion is much smaller, ˜ 0.87 eV. Therefore, we concluded that surface is rich in C which can diffuse to the Si3N4/SiO2 interface and contribute to chemical erosion near the grain boundary interface. In the final study, we created a DFT model to investigate the 'inverse spillover effect' that occurs during hydrogen combustion on catalytically active Pt clusters supported by alpha-Al2O3. Our results indicated that the dissociation of O2 was not thermodynamically favored on the alpha-Al2O3 surface. However, both H2 and H2O dissociated, forming hydroxyls with oxygen atoms in the second atomic layer. Once dissociated, the oxygen species could diffuse locally but encountered a large barrier to long-range surface diffusion in the absence of defects or other species. In contrast, the barrier to the long-range surface diffusion of hydrogen was modest under ideal conditions. We also identified several adsorption and dissociation products for Pt, Pt-O [ads] Pt3, O, H, O2, H2, and H 2O on the alpha-Al2O3 (0001) surface and described how these structures changed the surface reconstruction. Specifically, we concluded that the adsorption of molecular H2O, atomic Pt, and Pt trimers changed the termination for the alpha-Al2O3 (0001) surface from aluminum to oxygen terminated in the vicinity of the adsorption products. This should have a dramatic affect on catalytic activity and surface diffusion. We confirmed this for O surface diffusion near surface Al where the presence of atomic Pt decreased the diffusion barrier from 1.17 to 0.22 eV.

Smart Houses

NASA Technical Reports Server (NTRS)

1987-01-01

GWS takes plans for a new home and subjects them to intensive computerized analysis that does 10,000 calculations relative to expected heat loss and heat gain, then provides specifications designed specifically for each structure as to heating, cooling, ventilation and insulation. As construction progresses, GWS inspects the work of the electrical, plumbing and insulation contractors and installs its own Smart House Radiant Barrier. On completion of the home, GWS technicians use a machine that creates a vacuum in the house and enables computer calculation of the air exchanged, a measure of energy efficiency. Key factor is the radiant barrier, borrowed from the Apollo program. This is an adaptation of a highly effective aluminized heat shield as a radiation barrier holding in or keeping out heat, cold air and water vapor.

Preferences for and Barriers to Formal and Informal Athletic Training Continuing Education Activities

PubMed Central

Armstrong, Kirk J.; Weidner, Thomas G.

2011-01-01

Context: Our previous research determined the frequency of participation and perceived effect of formal and informal continuing education (CE) activities. However, actual preferences for and barriers to CE must be characterized. Objective: To determine the types of formal and informal CE activities preferred by athletic trainers (ATs) and barriers to their participation in these activities. Design: Cross-sectional study. Setting: Athletic training practice settings. Patients or Other Participants: Of a geographically stratified random sample of 1000 ATs, 427 ATs (42.7%) completed the survey. Main Outcome Measure(s): As part of a larger study, the Survey of Formal and Informal Athletic Training Continuing Education Activities (FIATCEA) was developed and administered electronically. The FIATCEA consists of demographic characteristics and Likert scale items (1 = strongly disagree, 5 = strongly agree) about preferred CE activities and barriers to these activities. Internal consistency of survey items, as determined by Cronbach α, was 0.638 for preferred CE activities and 0.860 for barriers to these activities. Descriptive statistics were computed for all items. Differences between respondent demographic characteristics and preferred CE activities and barriers to these activities were determined via analysis of variance and dependent t tests. The α level was set at .05. Results: Hands-on clinical workshops and professional networking were the preferred formal and informal CE activities, respectively. The most frequently reported barriers to formal CE were the cost of attending and travel distance, whereas the most frequently reported barriers to informal CE were personal and job-specific factors. Differences were noted between both the cost of CE and travel distance to CE and all other barriers to CE participation (F1,411 = 233.54, P < .001). Conclusions: Overall, ATs preferred formal CE activities. The same barriers (eg, cost, travel distance) to formal CE appeared to be universal to all ATs. Informal CE was highly valued by ATs because it could be individualized. PMID:22488195

Parental perceptions of barriers to active commuting to school in Spanish children and adolescents.

PubMed

Huertas-Delgado, Francisco Javier; Herrador-Colmenero, Manuel; Villa-González, Emilio; Aranda-Balboa, María Jesús; Cáceres, María Victoria; Mandic, Sandra; Chillón, Palma

2017-06-01

: Understanding parental barriers is crucial to promote active commuting to school since the parental perceptions influence how young people commute. This study examined parental barriers to active commuting to school among Spanish children and adolescents, and their association with their gender and the usual mode of commuting. Parents of children ( n = 628) and parents of adolescents ( n = 151) from Granada (Spain) completed a paper-based questionnaire about perceived parental barriers to active commuting to school and mode of commuting. Data were analyzed using the Chi-square test. Among Spanish parents, the most common barriers reported by parents of children were traffic volume and dangerous intersections, whereas the most frequent barriers reported by parents of adolescents were distance to school and dangerous intersections. Compared to parents of children, a greater proportion of parents of adolescents reported distance to school and crime and smaller proportion reported traffic volume as barriers to active commuting to school. Among parents of children, crime was a more commonly reported as a barrier by parents of girls. Although some barriers reported by parents of passive commuters were similar for children and adolescents (such as distance to school and absence of a policeman at crosswalks), other barriers were specific to parents of children. The main parental barriers to active commuting in children were traffic volume and dangerous intersections whereas for adolescents were distance and dangerous intersections. Among Spanish parents, parental barriers to active commuting are influenced by children's age, gender and mode of commuting to school. © The Author 2017. Published by Oxford University Press on behalf of the European Public Health Association. All rights reserved.

Stability and mobility of Cu-vacancy clusters in Fe-Cu alloys: A computational study based on the use of artificial neural networks for energy barrier calculations

NASA Astrophysics Data System (ADS)

Pascuet, M. I.; Castin, N.; Becquart, C. S.; Malerba, L.

2011-05-01

An atomistic kinetic Monte Carlo (AKMC) method has been applied to study the stability and mobility of copper-vacancy clusters in Fe. This information, which cannot be obtained directly from experimental measurements, is needed to parameterise models describing the nanostructure evolution under irradiation of Fe alloys (e.g. model alloys for reactor pressure vessel steels). The physical reliability of the AKMC method has been improved by employing artificial intelligence techniques for the regression of the activation energies required by the model as input. These energies are calculated allowing for the effects of local chemistry and relaxation, using an interatomic potential fitted to reproduce them as accurately as possible and the nudged-elastic-band method. The model validation was based on comparison with available ab initio calculations for verification of the used cohesive model, as well as with other models and theories.

Unsupervised Calculation of Free Energy Barriers in Large Crystalline Systems

NASA Astrophysics Data System (ADS)

Swinburne, Thomas D.; Marinica, Mihai-Cosmin

2018-03-01

The calculation of free energy differences for thermally activated mechanisms in the solid state are routinely hindered by the inability to define a set of collective variable functions that accurately describe the mechanism under study. Even when possible, the requirement of descriptors for each mechanism under study prevents implementation of free energy calculations in the growing range of automated material simulation schemes. We provide a solution, deriving a path-based, exact expression for free energy differences in the solid state which does not require a converged reaction pathway, collective variable functions, Gram matrix evaluations, or probability flux-based estimators. The generality and efficiency of our method is demonstrated on a complex transformation of C 15 interstitial defects in iron and double kink nucleation on a screw dislocation in tungsten, the latter system consisting of more than 120 000 atoms. Both cases exhibit significant anharmonicity under experimentally relevant temperatures.

El problema de la barrera linguistica en el desarrollo cientifico y tecnologico (The Problem of the Language Barrier in Scientific and Technological Development).

ERIC Educational Resources Information Center

Zierer, Ernesto

This monograph discusses the problem of the language barrier in scientific and technological development in terms of several parameters describing the flow of scientific information from one language to another. The numerical values of the language barrier parameters of the model are calculated in the field of information on second language…

Global Account of Barriers and Facilitators of Physical Activity Among Patients with Diabetes Mellitus: A Narrative Review of the Literature.

PubMed

Adeniyi, Ade F; Anjana, Ranjit M; Weber, Mary B

2016-01-01

With diabetes rates escalating globally, there is the need for a better integration of all aspects of diabetes care for improved population outcomes. An understanding, not only of regional but global literature on physical activity barriers and its facilitators is important if healthcare providers and policy makers are to create programs tailored to their populations. Herein, we report the results of a narrative review of the global barriers and facilitators of physical activity for patients with diabetes mellitus. An in-depth literature search was conducted to identify English-language studies that examined physical activity barriers and associated facilitators among patients with diabetes mellitus. Major electronic literature databases that were searched included Google Scholar, PubMed, Hub-Med, and Highwire. Studies were available from Africa, Asia, Australia, Europe, and, predominantly North America. A total of 34 predominantly internal barriers emerged globally. The most commonly reported were time constrains, fear of provoking additional disorders, exercise venue and weather related barriers. Facilitators of physical activity were reported for most of the internal barriers (e.g. time constraints, lack of knowledge etc) while the external barriers (e.g. weather, environmental pollution etc) received only a minimal attention. Globally, patients with diabetes are confronted with an enormous number of physical activity barriers. Unlike the robust solutions proffered for the internal barriers, the literature is largely silent about solutions to the external barriers, which though fewer, may be highly influential. Additional data is needed to better understand physical activity behaviors in populations outside of North America.

"What I Wish You Knew": Social Barriers toward Physical Activity in Youth with Congenital Heart Disease (CHD)

ERIC Educational Resources Information Center

Moola, Fiona; Fusco, Caroline; Kirsh, Joel A.

2011-01-01

Despite the benefits of physical activity for youth with congenital heart disease (CHD), most patients are inactive. Although literature has addressed medical and psychological barriers to participation, little is known about the social barriers that youth encounter. This qualitative study explored sociocultural barriers to physical activity from…

Circularly polarized antennas for active holographic imaging through barriers

DOEpatents

McMakin, Douglas L [Richland, WA; Severtsen, Ronald H [Richland, WA; Lechelt, Wayne M [West Richland, WA; Prince, James M [Kennewick, WA

2011-07-26

Circularly-polarized antennas and their methods of use for active holographic imaging through barriers. The antennas are dielectrically loaded to optimally match the dielectric constant of the barrier through which images are to be produced. The dielectric loading helps to remove barrier-front surface reflections and to couple electromagnetic energy into the barrier.

Horizontal hydraulic conductivity estimates for intact coal barriers between closed underground mines

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mccoy, K.J.; Donovan, J.J.; Leavitt, B.R.

2006-08-15

Unmined blocks of coal, called barriers, separate and restrict horizontal leakage between adjacent bituminous coal mines. Understanding the leakage rate across such barriers is important in planning mine closure and strongly affects recharge calculations for postmining flooding. This study presents upper-limit estimates for hydraulic conductivity (K) of intact barriers in two closed mines at moderate depth (75-300 m) in the Pittsburgh coal basin. The estimates are based on pumping rates from these mines for the years ranging from 1992 to 2000. The two mines do not approach the outcrop and are sufficiently deep that vertical infiltration is thought to bemore » negligible. Similarly, there are no saturated zones on the pumped mines' side of shared barriers with other mines, and therefore pumping is the only outflow. Virtually all of the pumping is attributed to leakage across or over the top of barriers shared with upgradient flooded mines. The length of shared barriers totals 24 km for the two mines, and the barriers range in thickness from 15 to 50 m. K values calculated independently for each of the 9 years of the pumping record ranged from 0.037 m/d to 0.18 m/d using an isotropic model of barrier flow. Using an anisotropic model for differential K in the face cleat (K{sub f}) and butt cleat (K{sub b}) directions, results range from 0.074 to 0.34 m/d for K{sub f} and from 0.022 to 0.099 m/d for K{sub b}.« less

Temperature dependent current transport of Pd/ZnO nanowire Schottky diodes

NASA Astrophysics Data System (ADS)

Gayen, R. N.; Bhattacharyya, S. R.; Jana, P.

2014-09-01

Zinc oxide (ZnO) nanowire based Schottky barrier diodes are fabricated by depositing Pd metal contact on top of vertically well-aligned ZnO nanowire arrays. A vertical array of ZnO nanowires on indium tin oxide (ITO) coated glass substrates is synthesized by hybrid wet chemical route. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurement confirm the formation of stoichiometric well-aligned hexagonal (h-ZnO) nanowire arrays with wurtzite structure. Temperature dependent current-voltage (I-V) measurements on palladium-ZnO (Pd/ZnO) nanowire Schottky junctions in the temperature range 303-383 K exhibit excellent rectifying character. From these nonlinear I-V plots, different electrical parameters of diode-like reverse saturation current, barrier height and ideality factor are determined as a function of temperature assuming pure thermionic emission model. The ideality factor is found to decrease while the barrier height increases with the increase in temperature. The series resistance values calculated from Cheung’s functions also show temperature dependency. Such behavior can be attributed to the presence of defects that traps carriers, and barrier height inhomogeneity at the interface of the barrier junction. After barrier height inhomogeneity correction, considering a Gaussian distributed barrier height fluctuation across the Pd/ZnO interface, the estimated values of mean barrier height and modified Richardson constant are more closely matched to the theoretically predicted value for Pd/ZnO Schottky barrier diodes. The variation of density of interface states as a function of interface state energy is also calculated.

Design and simulation of GaN based Schottky betavoltaic nuclear micro-battery.

PubMed

San, Haisheng; Yao, Shulin; Wang, Xiang; Cheng, Zaijun; Chen, Xuyuan

2013-10-01

The current paper presents a theoretical analysis of Ni-63 nuclear micro-battery based on a wide-band gap semiconductor GaN thin-film covered with thin Ni/Au films to form Schottky barrier for carrier separation. The total energy deposition in GaN was calculated using Monte Carlo methods by taking into account the full beta spectral energy, which provided an optimal design on Schottky barrier width. The calculated results show that an 8 μm thick Schottky barrier can collect about 95% of the incident beta particle energy. Considering the actual limitations of current GaN growth technique, a Fe-doped compensation technique by MOCVD method can be used to realize the n-type GaN with a carrier concentration of 1×10(15) cm(-3), by which a GaN based Schottky betavoltaic micro-battery can achieve an energy conversion efficiency of 2.25% based on the theoretical calculations of semiconductor device physics. Copyright © 2013 Elsevier Ltd. All rights reserved.

Perceived Barriers, Facilitators and Benefits for Regular Physical Activity and Exercise in Patients with Rheumatoid Arthritis: A Review of the Literature.

PubMed

Veldhuijzen van Zanten, Jet J C S; Rouse, Peter C; Hale, Elizabeth D; Ntoumanis, Nikos; Metsios, George S; Duda, Joan L; Kitas, George D

2015-10-01

Rheumatoid arthritis (RA) is an autoimmune disease, which not only affects the joints but can also impact on general well-being and risk for cardiovascular disease. Regular physical activity and exercise in patients with RA have numerous health benefits. Nevertheless, the majority of patients with RA are physically inactive. This indicates that people with RA might experience additional or more severe barriers to physical activity or exercise than the general population. This narrative review provides an overview of perceived barriers, benefits and facilitators of physical activity and exercise in RA. Databases were searched for articles published until September 2014 using the terms 'rheumatoid arthritis', 'physical activity', 'exercise', 'barriers', 'facilitators', 'benefits', 'motivation', 'motivators' and 'enablers'. Similarities were found between disease-specific barriers and benefits of physical activity and exercise, e.g. pain and fatigue are frequently mentioned as barriers, but reductions in pain and fatigue are perceived benefits of physical activity and exercise. Even though exercise does not influence the existence of barriers, physically active patients appear to be more capable of overcoming them. Therefore, exercise programmes should enhance self-efficacy for exercise in order to achieve long-term physical activity and exercise behaviour. Encouragement from health professionals and friends/family are facilitators for physical activity and exercise. There is a need for interventions that support RA patients in overcoming barriers to physical activity and exercise and help sustain this important health behaviour.

Gender-Associated Perceptions of Barriers and Motivators to Physical Activity Participation in South Asian Punjabis Living in Western Canada.

PubMed

Caperchione, Cristina M; Chau, Shirley; Walker, Gordon J; Mummery, W Kerry; Jennings, Cally

2015-05-01

Gender is a sociocultural factor known to impact the physical activity (PA) behaviors of South Asians. The purpose of this research was to examine gender-associated perceptions of barriers and motivators for PA in a South Asian population living Canada. A random sample (N = 204) of South Asian Punjabi adults (18yrs+) completed a computer assisted telephone interview concerning their perceptions to PA participation. Content analysis was used to identify relevant main themes and chi-square analysis was used to calculate gender differences. Results indicated that women more often reported a lack of time due to work and family (χ2 = 7.284, df = 1, P = .007) and a lack of motivation (χ2 = 4.982, df = 1, P = .026), yet men more often reported climate (χ2 = 7.045, df = 1, P = .008) as a barrier. Regarding motivators, men more often reported prevention and reduction of disease (χ2 = 4.451, df = 1, P = .034) and watching others perform (χ2 = 10.827, df = 1, P = .001); however, reducing weight gain (χ2 = 4.806, df = 1, P = .028) and looking like others (χ2 = 4.730, df = 1, P = .029) were reported more often by women. Gender-associated differences concerning PA are present in this population and must be considered in the design and implementation of effective interventions.

X-ray crystallography and QM/MM investigation on the oligosaccharide synthesis mechanism of rice BGlu1 glycosynthases.

PubMed

Wang, Jinhu; Pengthaisong, Salila; Cairns, James R Ketudat; Liu, Yongjun

2013-02-01

Nucleophile mutants of retaining β-glycosidase can act as glycosynthases to efficiently catalyze the synthesis of oligosaccharides. Previous studies proved that rice BGlu1 mutants E386G, E386S and E386A catalyze the oligosaccharide synthesis with different rates. The E386G mutant gave the fastest transglucosylation rate, which was approximately 3- and 19-fold faster than those of E386S and E386A. To account for the differences of their activities, in this paper, the X-ray crystal structures of BGlu1 mutants E386S and E386A were solved and compared with that of E386G mutant. However, they show quite similar active sites, which implies that their activities cannot be elucidated from the crystal structures alone. Therefore, a combined quantum mechanical/molecular mechanical (QM/MM) calculations were further performed. Our calculations reveal that the catalytic reaction follows a single-step mechanism, i.e., the extraction of proton by the acid/base, E176, and the formation of glycosidic bond are concerted. The energy barriers are calculated to be 19.9, 21.5 and 21.9kcal/mol for the mutants of E386G, E386S and E386A, respectively, which is consistent with the order of their experimental relative activities. But based on the calculated activation energies, 1.1kcal/mol energy difference may translate to nearly 100 fold rate difference. Although the rate limiting step in these mutants has not been established, considering the size of the product and the nature of the active site, it is likely that the product release, rather than chemistry, is rate limiting in these oligosaccharides synthesis catalyzed by BGlu1 mutants. Copyright © 2012 Elsevier B.V. All rights reserved.

A comparative DFT study on the dehydrogenation of methanol on Rh(100) and Rh(110)

NASA Astrophysics Data System (ADS)

Zhang, Minhua; Wu, Xingyu; Yu, Yingzhe

2018-04-01

Numerous density functional theory calculations have been performed to investigate the complete mechanisms of methanol dehydrogenation on Rh(100) and Rh(110) surfaces. The adsorption properties of relevant species were discussed in details. In addition, a comprehensive reaction network including four reaction pathways was built and analyzed. It is found that the initial Osbnd H bond scission of CH3OH seems to be more favorable than Csbnd H bond cleavage on both Rh(100) and Rh(110) surfaces from the perspective of activation barriers. It is also concluded that path1 (CH3OH → CH3O → CH2O → CHO → CO) is the predominant pathway on both Rh(100) and Rh (110) surfaces. On the whole, in most of the dehydrogenation reactions investigated, the energy barriers on Rh(100) are lower than those on Rh (110). Remarkable differences in the activity and predominant reaction pathway on Rh(100), Rh(110) and Rh(111) indicate that the dehydrogenation of methanol might be structure-sensitive.

Computing the Free Energy Barriers for Less by Sampling with a Coarse Reference Potential while Retaining Accuracy of the Target Fine Model.

PubMed

Plotnikov, Nikolay V

2014-08-12

Proposed in this contribution is a protocol for calculating fine-physics (e.g., ab initio QM/MM) free-energy surfaces at a high level of accuracy locally (e.g., only at reactants and at the transition state for computing the activation barrier) from targeted fine-physics sampling and extensive exploratory coarse-physics sampling. The full free-energy surface is still computed but at a lower level of accuracy from coarse-physics sampling. The method is analytically derived in terms of the umbrella sampling and the free-energy perturbation methods which are combined with the thermodynamic cycle and the targeted sampling strategy of the paradynamics approach. The algorithm starts by computing low-accuracy fine-physics free-energy surfaces from the coarse-physics sampling in order to identify the reaction path and to select regions for targeted sampling. Thus, the algorithm does not rely on the coarse-physics minimum free-energy reaction path. Next, segments of high-accuracy free-energy surface are computed locally at selected regions from the targeted fine-physics sampling and are positioned relative to the coarse-physics free-energy shifts. The positioning is done by averaging the free-energy perturbations computed with multistep linear response approximation method. This method is analytically shown to provide results of the thermodynamic integration and the free-energy interpolation methods, while being extremely simple in implementation. Incorporating the metadynamics sampling to the algorithm is also briefly outlined. The application is demonstrated by calculating the B3LYP//6-31G*/MM free-energy barrier for an enzymatic reaction using a semiempirical PM6/MM reference potential. These modifications allow computing the activation free energies at a significantly reduced computational cost but at the same level of accuracy compared to computing full potential of mean force.

Computing the Free Energy Barriers for Less by Sampling with a Coarse Reference Potential while Retaining Accuracy of the Target Fine Model

PubMed Central

2015-01-01

Proposed in this contribution is a protocol for calculating fine-physics (e.g., ab initio QM/MM) free-energy surfaces at a high level of accuracy locally (e.g., only at reactants and at the transition state for computing the activation barrier) from targeted fine-physics sampling and extensive exploratory coarse-physics sampling. The full free-energy surface is still computed but at a lower level of accuracy from coarse-physics sampling. The method is analytically derived in terms of the umbrella sampling and the free-energy perturbation methods which are combined with the thermodynamic cycle and the targeted sampling strategy of the paradynamics approach. The algorithm starts by computing low-accuracy fine-physics free-energy surfaces from the coarse-physics sampling in order to identify the reaction path and to select regions for targeted sampling. Thus, the algorithm does not rely on the coarse-physics minimum free-energy reaction path. Next, segments of high-accuracy free-energy surface are computed locally at selected regions from the targeted fine-physics sampling and are positioned relative to the coarse-physics free-energy shifts. The positioning is done by averaging the free-energy perturbations computed with multistep linear response approximation method. This method is analytically shown to provide results of the thermodynamic integration and the free-energy interpolation methods, while being extremely simple in implementation. Incorporating the metadynamics sampling to the algorithm is also briefly outlined. The application is demonstrated by calculating the B3LYP//6-31G*/MM free-energy barrier for an enzymatic reaction using a semiempirical PM6/MM reference potential. These modifications allow computing the activation free energies at a significantly reduced computational cost but at the same level of accuracy compared to computing full potential of mean force. PMID:25136268

A systematic review of perceived barriers and motivators to physical activity after stroke.

PubMed

Nicholson, Sarah; Sniehotta, Falko F; van Wijck, Frederike; Greig, Carolyn A; Johnston, Marie; McMurdo, Marion E T; Dennis, Martin; Mead, Gillian E

2013-07-01

Physical fitness is impaired after stroke, may contribute to disability, yet is amenable to improvement through regular physical activity. To facilitate uptake and maintenance of physical activity, it is essential to understand stroke survivors' perceived barriers and motivators. Therefore, we undertook a systematic review of perceived barriers and motivators to physical activity after stroke. Electronic searches of EMBASE, Medline, CINAHL, and PsychInfo were performed. We included peer-reviewed journal articles, in English, between 1 January 1966 and 30 August 2010 reporting stroke survivors' perceived barriers and motivators to physical activity. Searches identified 73,807 citations of which 57 full articles were retrieved. Six articles were included, providing data on 174 stroke survivors (range 10 to 83 per article). Two reported barriers and motivators, two reported only motivators, and two reported only barriers. Five were qualitative articles and one was quantitative. The most commonly reported barriers were lack of motivation, environmental factors (e.g. transport), health concerns, and stroke impairments. The most commonly reported motivators were social support and the need to be able to perform daily tasks. This review has furthered our understanding of the perceived barriers and motivators to physical activity after a stroke. This review will enable the development of tailored interventions to target barriers, while building upon perceived motivators to increase and maintain stroke survivors' physical activity. © 2012 The Authors. International Journal of Stroke © 2012 World Stroke Organization.

Calculated rate constants for the reaction ClO + O yields Cl + O2 between 220 and 1000 deg K. [molecular trajectories and stratospheric ozone

NASA Technical Reports Server (NTRS)

Jaffee, R. L.

1978-01-01

Classical trajectory calculations are presented for the reaction ClO + O yields Cl + O2, a reaction which is an important step in the chlorine-catalyzed destruction of ozone which is thought to occur in the 220 and 1000 K. The calculated rate constant is 4.36 x 10 to the minus 11th power exp (-191/T)cu cm molecule (-1)s(-1) and its value at 300 K is 2.3 plus or minus 10 to the 11th power cu cm molecule (-1)s(-1), about a factor of 2 lower than recent experimental data. The empirical potential energy surface used in the calculations was constructed to fit experimental data for ClO, O2 and ClOO molecules. Other important features of this potential surface, such as the barrier to reaction, were varied systematically and calculations were performed for a range of conditions to determine the best theoretical rate constants. Results demonstrate the utility of classical trajectory methods for determining activation energies and other kinetic data for important atmospheric reactions.

Battery Second-Use Repurposing Cost Calculator | Transportation Research |

Science.gov Websites

NREL Second-Use Repurposing Cost Calculator Battery Second-Use Repurposing Cost Calculator For Cost Calculator Download tool B2U strategies involve repurposing one single battery: first in an both the automotive and electricity industries, helping to overcome lithium-ion battery cost barriers

Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)-Oxo Sites in Magnesium-Diluted Fe2(dobdc).

PubMed

Verma, Pragya; Vogiatzis, Konstantinos D; Planas, Nora; Borycz, Joshua; Xiao, Dianne J; Long, Jeffrey R; Gagliardi, Laura; Truhlar, Donald G

2015-05-06

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.

Base Mechanism to the Hydrolysis of Phosphate Triester Promoted by the Cd2+/Cd2+ Active site of Phosphotriesterase: A Computational Study.

PubMed

Chagas, Marcelo A; Pereira, Eufrásia S; Godinho, Marina P B; Da Silva, Júlio Cosme S; Rocha, Willian R

2018-05-21

In the present work, density functional theory (DFT) calculations at the B3LYP/6-31+G(d) and including dispersion effects were used to investigate the hydrolysis of paraoxon, using a cluster model of the active site of Cd 2+ /Cd 2+ -phosphotriesterase (PTE) from Pseudomonas diminuta. The mechanism proposed here consist of (i) Exchange of the coordinated water molecule and coordination of the substrate to the more solvent exposed Cd β center in monodentate fashion, (ii) protonation of the μ-hydroxo bridge by the uncoordinated water molecule and in situ formation of the nucleophile, (iii) formation of a pentacoordinate intermediate with significant bond breaking to the leaving group and bond formation to the nucleophile, and (iv) protonation of the Asp301 residue and restoration of the active site through the coordination of another water molecule of the medium. The water molecules initially coordinated to the active site play a crucial role in stabilizing the transition states and the pentacoordinate intermediate. The reaction takes place in a two-step (A N + D N ) mechanism, with energy barriers of 12.9 and 1.9 kcal/mol for the first and second steps, respectively, computed at the B3LYP-D3/6-311++G(2d,2p) level of theory, in excellent agreement with the experimental findings. Dispersion effects alone contribute to diminish the energy barriers as much as 26%. The base mechanism for the Cd 2+ /Cd 2+ -PTE proposed here, in conjunction with the agreement found with the experimental energetic value for the energy barrier, makes it a consistent and kinetically viable mechanistic proposal for the hydrolysis of phosphate triesters promoted by the Cd 2+ substituted PTE enzyme.

A qualitative theory guided analysis of stroke survivors' perceived barriers and facilitators to physical activity.

PubMed

Nicholson, Sarah L; Donaghy, Marie; Johnston, Marie; Sniehotta, Falko F; van Wijck, Frederike; Johnston, Derek; Greig, Carolyn; McMurdo, Marion E T; Mead, Gillian

2014-01-01

After stroke, physical activity and physical fitness levels are low, impacting on health, activity and participation. It is unclear how best to support stroke survivors to increase physical activity. Little is known about the barriers and facilitators to physical activity after stroke. Thus, our aim was to explore stroke survivors' perceived barriers and facilitators to physical activity. Semi-structured interviews with 13 ambulatory stroke survivors exploring perceived barriers and facilitators to physical activity post stroke were conducted in participants' homes, audio-recorded and transcribed verbatim. The Theoretical Domains Framework (TDF) informed content analysis of the interview transcripts. Data saturation was reached after interviews with 13 participants (median age of 76 years (inter-quartile range (IQR) = 69-83 years). The median time since stroke was 345 d (IQR = 316-366 d). The most commonly reported TDF domains were "beliefs about capabilities", "environmental context and resources" and "social influence". The most commonly reported perceived motivators were: social interaction, beliefs of benefits of exercise, high self-efficacy and the necessity of routine behaviours. The most commonly reported perceived barriers were: lack of professional support on discharge from hospital and follow-up, transport issues to structured classes/interventions, lack of control and negative affect. Stroke survivors perceive several different barriers and facilitators to physical activity. Stroke services need to address barriers to physical activity and to build on facilitators to promote physical activity after stroke. Physical activity post stroke can improve physical fitness and function, yet physical activity remains low among stroke survivors. Understanding stroke survivors' perceived barriers and facilitators to physical activity is essential to develop targeted interventions to increase physical activity. Beliefs about capabilities, environmental context and resources and social influences were the mostly commonly report influences on stroke survivors' perceived barriers and facilitators to physical activity.

Barriers to Physical Activity on University Student

NASA Astrophysics Data System (ADS)

Jajat; Sultoni, K.; Suherman, A.

2017-03-01

The purpose of the research is to analyze the factors that become barriers to physical activity in university students based on physical activity level. An internet-based survey was conducted. The participants were 158 University students from Universitas Pendidikan Indonesia. Barriers to Physical Activity Quiz (BPAQ) were used to assessed the factors that become barriers to physical activity in university students. IPAQ (short form) were used to assessed physical activity level. The results show there was no differences BPAQ based on IPAQ level. But when analyzed further based on seven factors barriers there are differences in factors “social influence and lack of willpower” based IPAQ level. Based on this it was concluded that the “influence from other and lack of willpower” an inhibiting factor on students to perform physical activity.

Multicomponent kinetic analysis and theoretical studies on the phenolic intermediates in the oxidation of eugenol and isoeugenol catalyzed by laccase.

PubMed

Qi, Yan-Bing; Wang, Xiao-Lei; Shi, Ting; Liu, Shuchang; Xu, Zhen-Hao; Li, Xiqing; Shi, Xuling; Xu, Ping; Zhao, Yi-Lei

2015-11-28

Laccase catalyzes the oxidation of natural phenols and thereby is believed to initialize reactions in lignification and delignification. Numerous phenolic mediators have also been applied in laccase-mediator systems. However, reaction details after the primary O-H rupture of phenols remain obscure. In this work two types of isomeric phenols, EUG (eugenol) and ISO (trans-/cis-isoeugenol), were used as chemical probes to explore the enzymatic reaction pathways, with the combined methods of time-resolved UV-Vis absorption spectra, MCR-ALS, HPLC-MS, and quantum mechanical (QM) calculations. It has been found that the EUG-consuming rate is linear to its concentration, while the ISO not. Besides, an o-methoxy quinone methide intermediate, (E/Z)-4-allylidene-2-methoxycyclohexa-2,5-dienone, was evidenced in the case of EUG with the UV-Vis measurement, mass spectra and TD-DFT calculations; in contrast, an ISO-generating phenoxyl radical, a (E/Z)-2-methoxy-4-(prop-1-en-1-yl) phenoxyl radical, was identified in the case of ISO. Furthermore, QM calculations indicated that the EUG-generating phenoxyl radical (an O-centered radical) can easily transform into an allylic radical (a C-centered radical) by hydrogen atom transfer (HAT) with a calculated activation enthalpy of 5.3 kcal mol(-1) and then be fast oxidized to the observed eugenol quinone methide, rather than an O-radical alkene addition with barriers above 12.8 kcal mol(-1). In contrast, the ISO-generating phenoxyl radical directly undergoes a radical coupling (RC) process, with a barrier of 4.8 kcal mol(-1), while the HAT isomerization between O- and C-centered radicals has a higher reaction barrier of 8.0 kcal mol(-1). The electronic conjugation of the benzyl-type radical and the aromatic allylic radical leads to differentiation of the two pathways. These results imply that competitive reaction pathways exist for the nascent reactive intermediates generated in the laccase-catalyzed oxidation of natural phenols, which is important for understanding the lignin polymerization and may shed some light on the development of efficient laccase-mediator systems.

"I don't have the heart": a qualitative study of barriers to and facilitators of physical activity for people with coronary heart disease and depressive symptoms.

PubMed

Rogerson, Michelle C; Murphy, Barbara M; Bird, Stephen; Morris, Tony

2012-11-30

Physical activity has been shown to reduce depression in people with coronary heart disease (CHD), however many people with CHD do not engage in sufficient levels of physical activity to reap its positive effects. People with depression and CHD are at particular risk of non-adherence to physical activity. Little is known about the barriers to and facilitators of physical activity for people with CHD and depressive symptoms. Using qualitative interviews, the aim of this study was to explore the barriers to and facilitators of physical activity for cardiac patients with depressive symptoms. Fifteen participants with CHD and depressive symptoms (assessed using the Cardiac Depression Scale) participated in in-depth semi-structured interviews. The interviews were focussed on investigating participants' experiences of physical activity since their cardiac event. Interviews were content analysed to determine major themes. Participants identified a number of barriers to and facilitators of physical activity. Barriers included having negative perceptions towards health and life changes as a result of the cardiac event, having low mood and low motivation to exercise, feeling physically restricted towards or fearful of exercise, lacking knowledge regarding exercise and perceiving external barriers. Facilitators included having a reason for exercising, being able to identify the psychological benefits of exercise, having positive social support and using psychological strategies. 'Inactive' participants reported more barriers and fewer facilitators than did 'active' participants. The barriers reported in this study were highly salient for a number of participants. Health professionals and researchers can use this information to assist people with CHD and depressive symptoms to identify and possibly overcome barriers to physical activity. Relevant barriers and facilitators could be taken into account to increase their effectiveness when designing interventions to encourage physical activity maintenance in this population.

Structure, vibrational spectrum, and ring puckering barrier of cyclobutane.

PubMed

Blake, Thomas A; Xantheas, Sotiris S

2006-09-07

We present the results of high level ab initio calculations for the structure, harmonic and anharmonic spectroscopic constants, and ring puckering barrier of cyclobutane (C4H8) in an effort to establish the minimum theoretical requirements needed for their accurate description. We have found that accurate estimates for the barrier between the minimum (D(2d)) and transition state (D(4h)) configurations require both higher levels of electron correlation [MP4, CCSD(T)] and orbital basis sets of quadruple-zeta quality or larger. By performing CCSD(T) calculations with basis sets as large as cc-pV5Z, we were able to obtain, for the first time, a value for the puckering barrier that lies within 10 cm(-1) (or 2%) from experiment, whereas the best previously calculated values were in errors exceeding 40% of experiment. Our best estimate of 498 cm(-1) for the puckering barrier is within 10 cm(-1) of the experimental value proposed originally, but it lies approximately 50 cm(-1) higher than the revisited value, which was obtained more recently using different assumptions regarding the coupling between the various modes. It is therefore suggested that revisiting the analysis of the experimental data might be warranted. Our best computed values (at the CCSD(T)/aug-cc-pVTZ level of theory) for the equilibrium structural parameters of C4H8 are r(C-C) = 1.554 A, r(C-H(alpha)) = 1.093 A, r(C-H(beta)) = 1.091 A, phi(C-C-C) = 88.1 degrees , alpha(H(alpha)-C-H(beta)) = 109.15 degrees , and theta = 29.68 degrees for the puckering angle. We have found that the puckering angle theta is more sensitive to the level of electron correlation than to the size of the basis set for a given method. We furthermore present anharmonic calculations that are based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structure. We have found that the anharmonic calculations predict the experimentally measured fundamental band origins within 1% (< or =30 cm(-1)) for most vibrations. The results of the current study can serve as a guide for future calculations on the substituted four-member ring hydrocarbon compounds. To this end we present a method for estimating the puckering barrier height at higher levels of electron correlation [MP4, CCSD(T)] from the MP2 results that can be used in chemically similar compounds.

Promoting Physical Activity: Addressing Barriers and Moving Forward

ERIC Educational Resources Information Center

Beighle, Aaron; Morrow, James R.

2014-01-01

The barriers that keep individuals from adopting and maintaining active lifestyles are very complex. Strategies for overcoming these barriers and to incentivize and assist inactive individuals to benefit from physical activity are necessary. In addition, it is important to examine the impact of public policy on active living. As youth physical…

An internet survey of the characteristics and physical activity of community-dwelling Australian adults with acquired brain injury: Exploring interest in an internet-delivered self-management program focused on physical activity.

PubMed

Jones, Taryn M; Dean, Catherine M; Dear, Blake F; Hush, Julia M; Titov, Nickolai

2016-01-01

Individuals with acquired brain injury (ABI) are more likely to be physically inactive and experience barriers to accessing services to address inactivity. This study was designed to guide the development of an internet-delivered self-management program to increase physical activity after ABI. The aims of this study were to examine the current physical activity status of community-dwelling Australian adults with ABI, the barriers to physical activity they experience and to explore interest an internet-delivered self-management program aimed at increasing physical activity. An online survey of Australian adults with ABI was used to collect information about demographic characteristics; general health; emotional well-being; mobility and physical activity status, and satisfaction; barriers to physical activity; confidence in overcoming barriers, and; interest in an internet self-management program. Data were analyzed descriptively and correlational analyses examined relationships between variables. Data were analyzed from 59 respondents. Over half were not satisfied with their current physical activity status. The most frequently reported barriers were pain/discomfort, fatigue and fear, and confidence to overcome these barriers was very low. Interest in an internet-delivered self-management program was high (74%) and not related to the amount of physical activity, satisfaction with physical activity and mobility status or total number of barriers. Australian adults with ABI are not satisfied with their activity levels and experience barriers in maintaining their physical activity levels. Participants were interested in accessing an internet-delivered self-management program aimed at improving physical activity levels. Therefore such a program warrants development and evaluation. Copyright © 2016 Elsevier Inc. All rights reserved.

Mechanistic insight into the hydrazine decomposition on Rh(111): effect of reaction intermediate on catalytic activity.

PubMed

Deng, Zhigang; Lu, Xiaoqing; Wen, Zengqiang; Wei, Shuxian; Liu, Yunjie; Fu, Dianling; Zhao, Lianming; Guo, Wenyue

2013-10-14

Periodic density functional theory (DFT) calculations have been performed to systematically investigate the effect of reaction intermediate on catalytic activity for hydrazine (N2H4) decomposition on Rh(111). Reaction mechanisms via intramolecular and NH2-assisted N2H4 decompositions are comparatively analyzed, including adsorption configuration, reaction energy and barrier of elementary step, and reaction network. Our results show that the most favorable N2H4 decomposition pathway starts with the initial N-N bond scission to the NH2 intermediate, followed by stepwise H stripping from adsorbed N2Hx (x = 1-4) species, and finally forms the N2 and NH3 products. Comparatively, the stepwise intramolecular dehydrogenation via N2H4→ N2H3→ N2H2→ N2H → N2, and N2H4→ NH2→ NH → N with or without NH2 promotion effect, are unfavorable due to higher energy barriers encountered. Energy barrier analysis, reaction rate constants, and electronic structures are used to identify the crucial competitive route. The promotion effect of the NH2 intermediate is structurally reflected in the weakening of the N-H bond and strengthening of the N-N bond in N2Hx in the coadsorption system; it results intrinsically from the less structural deformation of the adsorbate, and weakening of the interaction between dehydrogenated fragment and departing H in transition state. Our results highlight the crucial effect of reaction intermediate on catalytic activity and provide a theoretical approach to analyze the effect.

Density functional study on the mechanism for the highly active palladium monolayer supported on titanium carbide for the oxygen reduction reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mao, Jianjun; Zhang, Yanxing; Chu, Xingli

2016-05-28

The adsorption, diffusion, and dissociation of O{sub 2} on the palladium monolayer supported on TiC(001) surface, MLPd/TiC(001), are investigated using ab initio density functional theory calculations. Strong adhesion of palladium monolayer to the TiC(001) support, accompanied by a modification of electronic structure of the supported palladium, is evidenced. Compared with Pt(111) surface, the MLPd/TiC(001) can enhance the adsorption of O{sub 2}, leading to comparable dissociation barrier and a smaller diffusion barrier of O{sub 2}. Whilst the adsorption strength of atomic O (the dissociation product of O{sub 2}) on MLPd/TiC(001) is similar to that on the Pt(111) surface, possessing high mobility,more » our theoretical results indicate that MLPd/TiC(001) may serve as a good catalyst for the oxygen reduction reaction.« less

A charge-based model of Junction Barrier Schottky rectifiers

NASA Astrophysics Data System (ADS)

Latorre-Rey, Alvaro D.; Mudholkar, Mihir; Quddus, Mohammed T.; Salih, Ali

2018-06-01

A new charge-based model of the electric field distribution for Junction Barrier Schottky (JBS) diodes is presented, based on the description of the charge-sharing effect between the vertical Schottky junction and the lateral pn-junctions that constitute the active cell of the device. In our model, the inherently 2-D problem is transformed into a simple but accurate 1-D problem which has a closed analytical solution that captures the reshaping and reduction of the electric field profile responsible for the improved electrical performance of these devices, while preserving physically meaningful expressions that depend on relevant device parameters. The validation of the model is performed by comparing calculated electric field profiles with drift-diffusion simulations of a JBS device showing good agreement. Even though other fully 2-D models already available provide higher accuracy, they lack physical insight making the proposed model an useful tool for device design.

A DIM model for sodium cluster-ions interacting with a charged conducting sphere

NASA Astrophysics Data System (ADS)

Kuntz, P. J.

A diatomics-in-molecules (DIM) model for the energy, shape and charge distribution of metal cluster ions in the presence of a charged insulated conducting sphere is presented. The electrostatic interaction between the sphere and the cluster-ion is introduced in a self-consistent manner which allows the sphere to be polarized by the ion and the ion by the sphere. This interaction appears in the diagonal elements of the model Hamiltonian matrix in such a way that the lowest eigenvalue includes the correct electrostatic energy for the charge distribution in the ground state. The model is applied to the calculation of fusion barriers for Na+2 and Na+3 ions. When both the charge distribution and the geometric configuration of the cluster-ion are allowed to relax freely, the energy as a function of distance from the sphere is nearly the same as that calculated from the electrostatic energy alone, which implies that details of the molecular structure of the cluster-ion can be neglected in calculating fusion barriers from charge polarization alone. That the fusion barriers lie sufficiently far away from the sphere so that the molecule does not dissociate under the influence of the Coulomb interaction confirms that it is meaningful to speak of two separate entities at the barrier position.

Interaction of healthcare staff's attitude with barriers to physical activity in hemodialysis patients: A quantitative assessment.

PubMed

Regolisti, Giuseppe; Maggiore, Umberto; Sabatino, Alice; Gandolfini, Ilaria; Pioli, Sarah; Torino, Claudia; Aucella, Filippo; Cupisti, Adamasco; Pistolesi, Valentina; Capitanini, Alessandro; Caloro, Giorgia; Gregorini, Mariacristina; Battaglia, Yuri; Mandreoli, Marcora; Dani, Lucia; Mosconi, Giovanni; Bellizzi, Vincenzo; Di Iorio, Biagio Raffaele; Conti, Paolo; Fiaccadori, Enrico

2018-01-01

In hemodialysis patients, sedentarism is a potentially modifiable mortality risk factor. We explored whether healthcare staff's attitude towards exercise interacts with patient-perceived barriers in modifying the level of physical activity in this population. In this prospective, cross-sectional, multicenter study we recruited 608 adult patients and 330 members of the healthcare staff in 16 hemodialysis units in Italy. We assessed patient-perceived barriers to, and healthcare staff's attitude towards, exercise by specific questionnaires. We fitted multilevel linear models to analyze the relationships of either barriers or staff's attitude, and their interaction, with a measure of patient self-reported physical activity (the Human Activity Profile-Adjusted Activity Score [HAP-AAS]), adjusting for multiple confounders. We also employed latent class analysis to dichotomize patients into those endorsing or not endorsing barriers. Most barriers were negatively associated with the HAP-AAS (adjusted change attributable to a given barrier ranging between -5.1 ["Feeling too old", 95% Confidence Interval: -9.4 to -0.8] and -15.6 ["Ulcers on legs and feet", 95%CI: -24.8 to -6.5]. We found a significant interaction between staff's attitude and barriers (adjusted P values ranging between 0.03 ["I do not believe that it is physician's or nurse's role providing advice on exercise to patients on dialysis"] and 0.001 ["I do not often ask patients about exercise"]). A beneficial effect of a proactive staff's attitude was evident only in patients not endorsing barriers. Barriers and non-proactive staff's attitude reduce physical activity in hemodialysis patients. Patients not endorsing barriers benefit the most from a proactive staff's attitude.

Systematic study of fission barriers of excited superheavy nuclei

NASA Astrophysics Data System (ADS)

Sheikh, J. A.; Nazarewicz, W.; Pei, J. C.

2009-07-01

A systematic study of fission-barrier dependence on excitation energy has been performed using the self-consistent finite-temperature Hartree-Fock + BCS (FT-HF + BCS) formalism with the SkM* Skyrme energy density functional. The calculations have been carried out for even-even superheavy nuclei with Z ranging between 110 and 124. For an accurate description of fission pathways, the effects of triaxial and reflection-asymmetric degrees of freedom have been fully incorporated. Our survey demonstrates that the dependence of isentropic fission barriers on excitation energy changes rapidly with particle number, pointing to the importance of shell effects even at large excitation energies characteristic of compound nuclei. The fastest decrease of fission barriers with excitation energy is predicted for deformed nuclei around N=164 and spherical nuclei around N=184 that are strongly stabilized by ground-state shell effects. For the nuclei Pu240 and Fm256, which exhibit asymmetric spontaneous fission, our calculations predict a transition to symmetric fission at high excitation energies owing to the thermal quenching of static reflection asymmetric deformations.

Charge transport through DNA based electronic barriers

NASA Astrophysics Data System (ADS)

Patil, Sunil R.; Chawda, Vivek; Qi, Jianqing; Anantram, M. P.; Sinha, Niraj

2018-05-01

We report charge transport in electronic 'barriers' constructed by sequence engineering in DNA. Considering the ionization potentials of Thymine-Adenine (AT) and Guanine-Cytosine (GC) base pairs, we treat AT as 'barriers'. The effect of DNA conformation (A and B form) on charge transport is also investigated. Particularly, the effect of width of 'barriers' on hole transport is investigated. Density functional theory (DFT) calculations are performed on energy minimized DNA structures to obtain the electronic Hamiltonian. The quantum transport calculations are performed using the Landauer-Buttiker framework. Our main findings are contrary to previous studies. We find that a longer A-DNA with more AT base pairs can conduct better than shorter A-DNA with a smaller number of AT base pairs. We also find that some sequences of A-DNA can conduct better than a corresponding B-DNA with the same sequence. The counterions mediated charge transport and long range interactions are speculated to be responsible for counter-intuitive length and AT content dependence of conductance of A-DNA.

An integral equation method for calculating sound field diffracted by a rigid barrier on an impedance ground.

PubMed

Zhao, Sipei; Qiu, Xiaojun; Cheng, Jianchun

2015-09-01

This paper proposes a different method for calculating a sound field diffracted by a rigid barrier based on the integral equation method, where a virtual boundary is assumed above the rigid barrier to divide the whole space into two subspaces. Based on the Kirchhoff-Helmholtz equation, the sound field in each subspace is determined with the source inside and the boundary conditions on the surface, and then the diffracted sound field is obtained by using the continuation conditions on the virtual boundary. Simulations are carried out to verify the feasibility of the proposed method. Compared to the MacDonald method and other existing methods, the proposed method is a rigorous solution for whole space and is also much easier to understand.

Identifying barriers to remaining physically active after rehabilitation: differences in perception between physical therapists and older adult patients.

PubMed

Zalewski, Kathryn; Alt, Carlynn; Arvinen-Barrow, Monna

2014-06-01

Cross-sectional study. To describe readiness for change and barriers to physical activity in older adults and to contrast perceptions of physical therapists and patients using the Barriers to Being Active Quiz. Regular physical activity is vital to recovery after discharge from physical therapy. Physical therapists are positioned to support change in physical activity habits for those transitioning to home care. Understanding of readiness for change and barriers to physical activity could optimize recovery. Thirteen physical therapists enrolled in the study and invited patients who met the inclusion criteria to enroll (79 patients enrolled). The physical therapists provided the ICD-9 code, the physical therapist diagnosis, and completed the Barriers to Being Active Quiz as they perceived their patients would. The enrolled patients provided demographics and filled out the Satisfaction With Life Scale, the stages-of-change scale for physical activity, and the Barriers to Being Active Quiz. Patients were predominantly in the early stages of readiness for change. Both patients and physical therapists identified lack of willpower as the primary barrier to physical activity. Patients identified lack of willpower and social influence as critical barriers more often than physical therapists, whereas physical therapists identified fear of injury and lack of time more often than their patients did. Differences between physical therapists and their patients were noted for fear of injury (z = 2.66, P = .008) and lack of time (z = 3.46, P = .001). The stage of change for physical activity impacted perception of social influence (χ2 = 9.64, P<.05), lack of willpower (χ2 = 21.91, P<.01), and lack of skill (χ2 = 12.46, P<.05). Women ranked fear of injury higher than men did (χ2 = 6.76, P<.01). Understanding readiness for change in and barriers to physical activity may allow physical therapists to better tailor intervention strategies to impact physical activity behavior change.

Polymer in a pore: Effect of confinement on the free energy barrier

NASA Astrophysics Data System (ADS)

Kumar, Sanjiv; Kumar, Sanjay

2018-06-01

We investigate the transfer of a polymer chain from cis- side to trans- side through two types of pores: cone-shaped channel and flat-channel. Using the exact enumeration technique, we obtain the free energy landscapes of a polymer chain for such systems. We have also calculated the free-energy barrier of a polymer chain attached to the edge of the pore. The model system allows us to calculate the force required to pull polymer from the pore and stall-force to confine polymer within the pore.

Diffusion and Stability of Hydrogen in Mg-Doped GaN: A Density Functional Study

NASA Astrophysics Data System (ADS)

Park, Ji-Sang; Chang, Kee Joo

2012-06-01

Using hybrid functional calculations, we study the diffusion and thermal stability of hydrogen in Mg-doped GaN. Compared with the generalized gradient approximation, we obtain a higher activation barrier for dissociating a Mg-H complex, which is attributed to the increase in the binding energy of Mg-H. Kinetic Monte Carlo simulations yield the annealing temperature of around 800 °C for activating Mg acceptors, close to the measured values. The results provide an insight to understanding the annealing effect such that the annealing temperature generally increases with the Mg-H concentration, and the retrapping of H is partly responsible for the low doping efficiencies at high Mg concentrations.

A model SN2 reaction ‘on water’ does not show rate enhancement

NASA Astrophysics Data System (ADS)

Nelson, Katherine V.; Benjamin, Ilan

2011-05-01

Molecular dynamics calculations of the benchmark nucleophilic substitution reaction (SN2) Cl- + CH3Cl are carried out at the water liquid/vapor interface. The reaction free energy profile and the activation free energy are determined as a function of the reactants' location normal to the surface. The activation free energy remains almost constant relative to that in bulk water, despite the fact that the barrier is expected to significantly decrease as the reaction is carried out near the vapor phase. We show that this is due to the combined effects of a clustering of water molecules around the nucleophile and a relatively weak hydration of the transition state.

Perceived barriers to weight maintenance among university students in Kuwait: the role of gender and obesity.

PubMed

Musaiger, Abdulrahman O; Al-Kandari, Fawzia I; Al-Mannai, Mariam; Al-Faraj, Alaa M; Bouriki, Fajer A; Shehab, Fatima S; Al-Dabous, Lulwa A; Al-Qalaf, Wassin B

2014-05-01

To investigate the barriers to weight maintenance among university students in Kuwait by gender and obesity. A sample of 530 students was selected at convenience from four universities in Kuwait (2 public and 2 private). The age of students ranged from 19 to 26 years. A self-reported pretested questionnaire was used to obtain the barriers, which were divided into barriers to healthy eating and barriers to physical activity. Weight and height were based on self-reporting, and the students were grouped into non-obese and obese according to the WHO classification. The response options to barriers were: very important, somewhat important and not important. The main barriers to healthy eating for both genders were: "Do not have skills to plan, shop for, prepare or cook healthy foods" and "Not having time to prepare or eat healthy food". In general, there were no significant differences between men and women in barriers to healthy eating. There were highly significant differences between men and women regarding barriers to physical activity (P values ranged from <0.001 to <0.016). "Not having time to be physically active" and "The climate is not suitable for practising exercise" were the main barriers reported. Obese men were more likely to face barriers to healthy eating than non-obese men. There were no significant differences between obese and non-obese women regarding barriers to healthy eating and physical activity. The findings of this study can be utilized in intervention activities to promote a healthy lifestyle and to combat obesity in Kuwait, and maybe in other Arab countries.

The substitution reaction of (CNC)Fe-2N₂ with CO.

PubMed

Liu, Hongyan; Liu, Shuangshuang; Zhang, Xiang

2013-06-01

The substitution mechanism of two N₂ ligands in (CNC)Fe-2N₂ replaced by CO was studied theoretically at the B3LYP/LACVP* level. Both SN1 and SN₂ mechanisms were considered. The calculated results for the gas phase suggested that: 1) in SN1 mechanism, N₂ elimination, which involves S₀-T₁ PESs crossing, is the rate control step for both substitution stages. The barrier heights are 9.7 kcal mol(-1) and 13.05 kcal mol(-1), respectively. 2) In SN2 mechanism, the calculated barrier heights on LS PES are respectively 13.7 and 19.83 kcal mol(-1) for the two substitution steps, but S₀-T₁ PESs crossing lowers the two barriers to 10.7 and 15.7 kcal mol(-1), respectively. 3) Inclusion of solvation effect of THF by PCM model, the relative energies of all the key species (including minima, transition states and S₀-T₁ crossing points) do not have great difference from their gas phase relative energies. Considering that for each substitution step, SN1 barrier heights is slightly smaller than SN2 barrier, SN1 mechanism seems to be slightly preferable to SN2 mechanism.

Dose Rate Calculation of TRU Metal Ingot in Pyroprocessing - 12202

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Yoon Hee; Lee, Kunjai

Spent fuel management has been a main problem to be solved for continuous utilization of nuclear energy. Spent fuel management policy of Korea is 'Wait and See'. It is focused on Pyro-process and SFR (Sodium-cooled Fast Reactor) for closed-fuel cycle research and development in Korea. For peaceful use of nuclear facilities, the proliferation resistance has to be proved. Proliferation resistance is one of key constraints in the deployment of advanced nuclear energy systems. Non-proliferation and safeguard issues have been strengthening internationally. Barriers to proliferation are that reduces desirability or attractiveness as an explosive and makes it difficult to gain accessmore » to the materials, or makes it difficult to misuse facilities and/or technologies for weapons applications. Barriers to proliferation are classified into intrinsic and extrinsic barriers. Intrinsic barrier is inherent quality of reactor materials or the fuel cycle that is built into the reactor design and operation such as material and technical barriers. As one of the intrinsic measures, the radiation from the material is considered significantly. Therefore the radiation of TRU metal ingot from the pyro-process was calculated using ORIGEN and MCNP code. (authors)« less

Free energy barrier for melittin reorientation from a membrane-bound state to a transmembrane state.

PubMed

Irudayam, Sheeba J; Pobandt, Tobias; Berkowitz, Max L

2013-10-31

An important step in a phospholipid membrane pore formation by melittin antimicrobial peptide is a reorientation of the peptide from a surface into a transmembrane conformation. Experiments measure the fraction of peptides in the surface state and the transmembrane state, but no computational study exists that quantifies the free energy curve for the reorientation. In this work we perform umbrella sampling simulations to calculate the potential of mean force (PMF) for the reorientation of melittin from a surface-bound state to a transmembrane state and provide a molecular level insight in understanding the peptide-lipid properties that influence the existence of the free energy barrier. The PMFs were calculated for a peptide to lipid (P/L) ratio of 1/128 and 4/128. We observe that the free energy barrier is reduced when the P/L ratio increases. In addition, we study the cooperative effect; specifically we investigate if the reorientation barrier is smaller for a second melittin, given that another neighboring melittin was already in the transmembrane orientation. We observe that indeed the barrier of the PMF curve is reduced in this case, thus confirming the presence of a cooperative effect.

Analysis of high reverse currents of 4H-SiC Schottky-barrier diodes

NASA Astrophysics Data System (ADS)

Okino, Hiroyuki; Kameshiro, Norifumi; Konishi, Kumiko; Shima, Akio; Yamada, Ren-ichi

2017-12-01

Nickel (Ni), titanium (Ti), and molybdenum (Mo) 4H-silicon carbide Schottky-barrier diodes (SiC SBDs) were fabricated and used to investigate the relation between forward and reverse currents. Temperature dependence of reverse current follows a theory that includes tunneling in regard to thermionic emission, namely, temperature dependence is weak at low temperature but strong at high temperatures. On the other hand, the reverse currents of the Ni and Mo SBDs are higher than their respective currents calculated from their Schottky barrier heights (SBHs), whereas the reverse current of the Ti SBD agrees well with that calculated from its SBH. The cause of the high reverse currents was investigated from the viewpoints of low barrier patch, Gaussian distribution of barrier height (GD), thin surface barrier, and electron effective mass. The high reverse current of the Ni and Mo SBDs can be explained not in terms of a low-barrier patch, GD, or thin surface barrier but in terms of small effective masses. Investigation of crystal structures at the Schottky interface revealed a large lattice mismatch between the metals (Ni, Ti, or Mo) and SiC for the Ni and Mo SBDs. The small effective mass is possibly attributed to the large lattice mismatch, which might generate transition layers at the Schottky interface. It is concluded from these results that the lattice constant as well as the work function is an important factor in selecting the metal species as the Schottky metal for wide band-gap SBDs, for which tunneling current dominates reverse current.

Multilevel Quantum Mechanics Theories and Molecular Mechanics Calculations of the Cl- + CH3I Reaction in Water.

PubMed

Liu, Peng; Li, Chen; Wang, Dunyou

2017-10-19

The Cl - + CH 3 I → CH 3 Cl + I - reaction in water was studied using combined multilevel quantum mechanism theories and molecular mechanics with an explicit water solvent model. The study shows a significant influence of aqueous solution on the structures of the stationary points along the reaction pathway. A detailed, atomic-level evolution of the reaction mechanism shows a concerted one-bond-broken and one-bond-formed mechanism, as well as a synchronized charge-transfer process. The potentials of mean force calculated with the CCSD(T) and DFT treatments of the solute produce a free activation barrier at 24.5 and 19.0 kcal/mol, respectively, which agrees with the experimental one at 22.0 kcal/mol. The solvent effects have also been quantitatively analyzed: in total, the solvent effects raise the activation energy by 20.2 kcal/mol, which shows a significant impact on this reaction in water.

Coupled-channel analyses on 16O + 147,148,150,152,154Sm heavy-ion fusion reactions

NASA Astrophysics Data System (ADS)

Erol, Burcu; Yılmaz, Ahmet Hakan

2018-02-01

Heavy-ion collisons are typically characterized by the presence of many open reaction channels. In the energies around the Coulomb barrier, the main processes are elastic scattering, inelastic excitations of low-lying modes and fusion operations of one or two nuclei. The fusion process is generally defined as the effect of one-dimensional barrier penetration model, taking scattering potential as the sum of Coulomb and proximity potential. We have performed heay-ion fusion reactions with coupled-channel (CC) calculations. Coupled-channel formalism is carried out under barrier energy in heavy-ion fusion reactions. In this work fusion cross sections have been calculated and analyzed in detail for the five systems 16O + 147,148,150,152,154sm in the framework of coupled-channel approach (using the codes CCFUS and CCDEF) and Wong Formula. Calculated results are compared with experimental data, CC calculations using code CCFULL and with the cross section datas taken from `nrv'. CCDEF, CCFULL and Wong Formula explains the fusion reactions of heavy-ions very well, while using the scattering potential as WOODS-SAXON volume potential with Akyuz-Winther parameters. It was observed that AW potential parameters are able to reproduce the experimentally observed fusion cross sections reasonably well for these systems. There is a good agreement between the calculated results with the experimental and nrv[8] results.

Ion Transport through Membrane-Spanning Nanopores Studied by Molecular Dynamics Simulations and Continuum Electrostatics Calculations

PubMed Central

Peter, Christine; Hummer, Gerhard

2005-01-01

Narrow hydrophobic regions are a common feature of biological channels, with possible roles in ion-channel gating. We study the principles that govern ion transport through narrow hydrophobic membrane pores by molecular dynamics simulation of model membranes formed of hexagonally packed carbon nanotubes. We focus on the factors that determine the energetics of ion translocation through such nonpolar nanopores and compare the resulting free-energy barriers for pores with different diameters corresponding to the gating regions in closed and open forms of potassium channels. Our model system also allows us to compare the results from molecular dynamics simulations directly to continuum electrostatics calculations. Both simulations and continuum calculations show that subnanometer wide pores pose a huge free-energy barrier for ions, but a small increase in the pore diameter to ∼1 nm nearly eliminates that barrier. We also find that in those wider channels the ion mobility is comparable to that in the bulk phase. By calculating local electrostatic potentials, we show that the long range Coulomb interactions of ions are strongly screened in the wide water-filled channels. Whereas continuum calculations capture the overall energetics reasonably well, the local water structure, which is not accounted for in this model, leads to interesting effects such as the preference of hydrated ions to move along the pore wall rather than through the center of the pore. PMID:16006629

Barriers to physical activity and restorative care for residents in long-term care: a review of the literature.

PubMed

Benjamin, Kathleen; Edwards, Nancy; Ploeg, Jenny; Legault, Frances

2014-01-01

Despite the benefits of physical activity, residents living in long-term care (LTC) are relatively sedentary. Designing successful physical activity and restorative care programs requires a good understanding of implementation barriers. A database search (2002-2013) yielded seven studies (nine articles) that met our inclusion criteria. We also reviewed 31 randomized controlled trials (RCTs) to determine if the authors explicitly discussed the barriers encountered while implementing their interventions. Eleven RCTs (13 articles) included a discussion of the barriers. Hence, a total of 18 studies (22 articles) were included in this review. Barriers occurred at resident (e.g., health status), environmental (e.g., lack of space for physical activity), and organizational (e.g., staffing and funding constraints) levels. These barriers intersect to adversely affect the physical activity of older people living in LTC. Future studies targeting physical activity interventions for residents living in LTC are needed to address these multiple levels of influence.

Pivotal role of water in terminating enzymatic function: a density functional theory study of the mechanism-based inactivation of cytochromes P450.

PubMed

Hirao, Hajime; Cheong, Zhi Hao; Wang, Xiaoqing

2012-07-12

The importance of the mechanism-based inactivation (MBI) of enzymes, which has a variety of physiological effects and therapeutic implications, has been garnering appreciation. Density functional theory calculations were undertaken to gain a clear understanding of the MBI of a cytochrome P450 enzyme (CYP2B4) by tert-butylphenylacetylene (tBPA). The results of calculations suggest that, in accordance with previous proposals, the reaction proceeds via a ketene-type metabolic intermediate. Once an oxoiron(IV) porphyryn π-cation radical intermediate (compound I) of P450 is generated at the heme reaction site, ketene formation is facile, as the terminal acetylene of tBPA can form a C-O bond with the oxo unit of compound I with a relatively low reaction barrier (14.1 kcal/mol). Unexpectedly, it was found that the ketene-type intermediate was not very reactive. Its reaction with the hydroxyl group of a threonine (Thr302) to form an ester bond required a substantial barrier (38.2 kcal/mol). The high barrier disfavored the mechanism by which these species react directly. However, the introduction of a water molecule in the reaction center led to its active participation in the reaction. The water was capable of donating its proton to the tBPA molecule, while accepting the proton of threonine. This water-mediated mechanism lowered the reaction barrier for the formation of an ester bond by about 20 kcal/mol. Therefore, our study suggests that a water molecule, which can easily gain access to the threonine residue through the proton-relay channel, plays a critical role in enhancing the covalent modification of threonine by terminal acetylene compounds. Another type of MBI by acetylenes, N-alkylation of the heme prosthetic group, was less favorable than the threonine modification pathway.

Cross-slip in face-centered cubic metals: a general Escaig stress-dependent activation energy line tension model

NASA Astrophysics Data System (ADS)

Malka-Markovitz, Alon; Mordehai, Dan

2018-02-01

Cross-slip is a dislocation mechanism by which screw dislocations can change their glide plane. This thermally activated mechanism is an important mechanism in plasticity and understanding the energy barrier for cross-slip is essential to construct reliable cross-slip rules in dislocation models. In this work, we employ a line tension model for cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier under Escaig stresses. The analysis shows that the activation energy is proportional to the stacking fault energy, the unstressed dissociation width and a typical length for cross-slip along the dislocation line. Linearisation of the interaction forces between the partial dislocations yields that this typical length is related to the dislocation length that bows towards constriction during cross-slip. We show that the application of Escaig stresses on both the primary and the cross-slip planes varies the typical length for cross-slip and we propose a stress-dependent closed form expression for the activation energy for cross-slip in a large range of stresses. This analysis results in a stress-dependent activation volume, corresponding to the typical volume surrounding the stressed dislocation at constriction. The expression proposed here is shown to be in agreement with previous models, and to capture qualitatively the essentials found in atomistic simulations. The activation energy function can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality.

Investigation of the mechanism of the cell wall DD-carboxypeptidase reaction of penicillin-binding protein 5 of Escherichia coli by quantum mechanics/molecular mechanics calculations.

PubMed

Shi, Qicun; Meroueh, Samy O; Fisher, Jed F; Mobashery, Shahriar

2008-07-23

Penicillin-binding protein 5 (PBP 5) of Escherichia coli hydrolyzes the terminal D-Ala-D-Ala peptide bond of the stem peptides of the cell wall peptidoglycan. The mechanism of PBP 5 catalysis of amide bond hydrolysis is initial acylation of an active site serine by the peptide substrate, followed by hydrolytic deacylation of this acyl-enzyme intermediate to complete the turnover. The microscopic events of both the acylation and deacylation half-reactions have not been studied. This absence is addressed here by the use of explicit-solvent molecular dynamics simulations and ONIOM quantum mechanics/molecular mechanics (QM/MM) calculations. The potential-energy surface for the acylation reaction, based on MP2/6-31+G(d) calculations, reveals that Lys47 acts as the general base for proton abstraction from Ser44 in the serine acylation step. A discrete potential-energy minimum for the tetrahedral species is not found. The absence of such a minimum implies a conformational change in the transition state, concomitant with serine addition to the amide carbonyl, so as to enable the nitrogen atom of the scissile bond to accept the proton that is necessary for progression to the acyl-enzyme intermediate. Molecular dynamics simulations indicate that transiently protonated Lys47 is the proton donor in tetrahedral intermediate collapse to the acyl-enzyme species. Two pathways for this proton transfer are observed. One is the direct migration of a proton from Lys47. The second pathway is proton transfer via an intermediary water molecule. Although the energy barriers for the two pathways are similar, more conformers sample the latter pathway. The same water molecule that mediates the Lys47 proton transfer to the nitrogen of the departing D-Ala is well positioned, with respect to the Lys47 amine, to act as the hydrolytic water in the deacylation step. Deacylation occurs with the formation of a tetrahedral intermediate over a 24 kcal x mol(-1) barrier. This barrier is approximately 2 kcal x mol(-1) greater than the barrier (22 kcal x mol(-1)) for the formation of the tetrahedral species in acylation. The potential-energy surface for the collapse of the deacylation tetrahedral species gives a 24 kcal x mol(-1) higher energy species for the product, signifying that the complex would readily reorganize and pave the way for the expulsion of the product of the reaction from the active site and the regeneration of the catalyst. These computational data dovetail with the knowledge on the reaction from experimental approaches.

Perceived barriers to leisure-time physical activity during pregnancy: A literature review of quantitative and qualitative evidence.

PubMed

Coll, Carolina V N; Domingues, Marlos R; Gonçalves, Helen; Bertoldi, Andréa D

2017-01-01

Identify perceived barriers to leisure-time physical activity during pregnancy to inform future interventions aimed at improving physical activity levels in pregnancy. PubMed/Medline and Web of Science databases were systematically searched using a reference period between 1986 and January/2016. A comprehensive search strategy was developed combining the following keywords: (barriers OR constraints OR perceptions OR attitudes) AND (physical activity OR exercise OR motor activity) AND (pregnancy OR pregnant women OR antenatal OR prenatal). Thematic synthesis was conducted to analyze the data. A socioecological model was used to categorize the reported barriers. Twelve quantitative studies and 14 qualitative studies were included. Barriers belonging to the intrapersonal level of the socioecological model were the most reported in the studies and were categorized in five themes as follows: (1) Pregnancy-related symptoms and limitations; (2) Time constraints; (3) Perceptions of already being active, (4) Lack of motivation and (5) Mother-child safety concerns. At the interpersonal level, barriers were coded into two descriptive themes: (1) Lack of advice and information and (2) Lack of social support. Two other themes were used to summarize Environmental, Organizational and Policy barriers: (1) Adverse weather and (2) Lack of resources. A range of relevant barriers to leisure-time physical-activity engagement during pregnancy were identified in this literature review. Pregnancy-related symptoms and limitations barriers were the most reported in studies, regardless of study design. Mother-child safety concerns, lack of advice/information and lack of social support were also important emphasized pregnancy-related barriers to be targeted in future interventions. Copyright © 2016 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.

Multiconfiguration pair-density functional theory: barrier heights and main group and transition metal energetics.

PubMed

Carlson, Rebecca K; Li Manni, Giovanni; Sonnenberger, Andrew L; Truhlar, Donald G; Gagliardi, Laura

2015-01-13

Kohn-Sham density functional theory, resting on the representation of the electronic density and kinetic energy by a single Slater determinant, has revolutionized chemistry, but for open-shell systems, the Kohn-Sham Slater determinant has the wrong symmetry properties as compared to an accurate wave function. We have recently proposed a theory, called multiconfiguration pair-density functional theory (MC-PDFT), in which the electronic kinetic energy and classical Coulomb energy are calculated from a multiconfiguration wave function with the correct symmetry properties, and the rest of the energy is calculated from a density functional, called the on-top density functional, that depends on the density and the on-top pair density calculated from this wave function. We also proposed a simple way to approximate the on-top density functional by translation of Kohn-Sham exchange-correlation functionals. The method is much less expensive than other post-SCF methods for calculating the dynamical correlation energy starting with a multiconfiguration self-consistent-field wave function as the reference wave function, and initial tests of the theory were quite encouraging. Here, we provide a broader test of the theory by applying it to bond energies of main-group molecules and transition metal complexes, barrier heights and reaction energies for diverse chemical reactions, proton affinities, and the water dimerization energy. Averaged over 56 data points, the mean unsigned error is 3.2 kcal/mol for MC-PDFT, as compared to 6.9 kcal/mol for Kohn-Sham theory with a comparable density functional. MC-PDFT is more accurate on average than complete active space second-order perturbation theory (CASPT2) for main-group small-molecule bond energies, alkyl bond dissociation energies, transition-metal-ligand bond energies, proton affinities, and the water dimerization energy.

Deep tunneling in the unimolecular decay of CH 3CHOO Criegee intermediates to OH radical products

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fang, Yi; Liu, Fang; Barber, Victoria P.

Unimolecular decay of Criegee intermediates produced in alkene ozonolysis is known to be a significant source of OH radicals in the troposphere. In this work, unimolecular decay of the methyl-substituted Criegee intermediate, syn-CH 3CHOO, to OH products is shown to occur at energies significantly below the transition state barrier for a 1,4 hydrogen transfer that leads to these products [Y. Fang et al., J. Chem. Phys. 144, 061102 (2016)]. The rate of appearance of OH products arising from tunneling through the barrier is obtained through direct time-domain measurements following the vibrational activation of syn-CH 3CHOO. IR excitation of syn-CH 3CHOOmore » at energies nearly 2000 cm -1 below the barrier is achieved through combination bands involving CH stretch and another lower frequency mode, and the resultant OH products are detected by UV laser-induced fluorescence. The observed syn-CH 3CHOO combination bands in the 4100–4350 cm -1 region are identified by comparison with the computed IR absorption spectrum. The experimental decay rates are found to be ca. 106 s -1 in this deep tunneling regime, which is approximately 100-times slower than that in the vicinity of the barrier.The experimental results are consistent with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the microcanonical decay rates with tunneling through the barrier, and notable deviations may originate from the sparsity in the density of states for syn-CH 3CHOO at lower energies. Thermal unimolecular decay of syn-CH 3CHOO is predicted to have significant contribution from microcanonical rates at energies that are much below the barrier.« less

Work-related barriers, facilitators, and strategies of breast cancer survivors working during curative treatment.

PubMed

Sun, Wenjun; Chen, Karen; Terhaar, Abigail; Wiegmann, Douglas A; Heidrich, Susan M; Tevaarwerk, Amye J; Sesto, Mary E

2016-01-01

Research has identified barriers and facilitators affecting cancer survivors' return to work (RTW) following the end of active treatment (surgery, chemotherapy and/or radiation therapy). However, few studies have focused on barriers and facilitators that cancer survivors experience while working during active treatment. Strategies used by cancer survivors to solve work-related problems during active treatment are underexplored. The aim of this study was to describe factors that impact, either positively or negatively, breast cancer survivors' work activities during active treatment. Semi-structured, recorded interviews were conducted with 35 breast cancer survivors who worked during active treatment. Transcripts of interviews were analyzed using inductive content analysis to identify themes regarding work-related barriers, facilitators and strategies. Barriers identified included symptoms, emotional distress, appearance change, time constraints, work characteristics, unsupportive supervisors and coworkers, family issues and other illness. Facilitators included positive aspects of work, support outside of work, and coworker and supervisor support. Strategies included activities to improve health-related issues and changes to working conditions and tasks. Breast cancer survivors encounter various barriers during active treatment. Several facilitators and strategies can help survivors maintain productive work activities.

Work-related barriers, facilitators, and strategies of breast cancer survivors working during curative treatment

PubMed Central

Sun, Wenjun; Chen, Karen; Terhaar, Abigail; Wiegmann, Douglas A.; Heidrich, Susan M.; Tevaarwerk, Amye J.; Sesto, Mary E.

2017-01-01

BACKGROUND Research has identified barriers and facilitators affecting cancer survivors’ return to work (RTW) following the end of active treatment (surgery, chemotherapy and/or radiation therapy). However, few studies have focused on barriers and facilitators that cancer survivors experience while working during active treatment. Strategies used by cancer survivors to solve work-related problems during active treatment are underexplored. OBJECTIVE The aim of this study was to describe factors that impact, either positively or negatively, breast cancer survivors’ work activities during active treatment. METHODS Semi-structured, recorded interviews were conducted with 35 breast cancer survivors who worked during active treatment. Transcripts of interviews were analyzed using inductive content analysis to identify themes regarding work-related barriers, facilitators and strategies. RESULTS Barriers identified included symptoms, emotional distress, appearance change, time constraints, work characteristics, unsupportive supervisors and coworkers, family issues and other illness. Facilitators included positive aspects of work, support outside of work, and coworker and supervisor support. Strategies included activities to improve health-related issues and changes to working conditions and tasks. CONCLUSIONS Breast cancer survivors encounter various barriers during active treatment. Several facilitators and strategies can help survivors maintain productive work activities. PMID:28059814

Synthesis, Chemical and Physical Characterization of TKX-50

NASA Astrophysics Data System (ADS)

Klapoetke, Thomas

2015-06-01

TKX-50 (bis(hydroxylammonium) 5,5'-bis(tetrazolate-1 N-oxide)) is one of the most promising ionic salts as a possible replacement for RDX. The thermal behavior of TKX-50 (bis(hydroxylammonium) 5,5'-(tetrazolate-1 N-oxide)) and the kinetics of its thermal decomposition were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The calculated results of the detonation parameters and equations of state for the detonation products (EOS DP) of explosive materials TKX-50 and MAD-X1 and several of their derivatives were obtained using the computer program EXPLO5 V.6.01. These values were also calculated for standard explosive materials which are commonly used such as TNT, PETN, RDX, HMX as well as for the more powerful explosive material CL-20 to allow comparisons to be made. The determination of the detonation parameters and EOS DP was conducted both for explosive materials having the maximum crystalline density and for porous right up to 50% in volume materials. The influence of the content of plastic binder polyisobutylene used (up to 20% in volume) on all of the investigated properties was also examined. Calculated results on shock wave loading of different inert barriers in a wide range of their dynamic properties under explosion on their surfaces of concrete size charges of different explosive materials in various initial states were obtained with the use of the one-dimensional computer hydrocode EP. Barriers due to materials such as polystyrene, textolite, magnesium, aluminum, zinc, copper, tantalum or tungsten were examined (Fig. 1). Initial values of pressure and other parameters of loading on the interface explosive-barrier were determined in the process of conducted calculations. Phenomena of propagation and attenuation of shock waves in barrier materials were considered too for all possible situations. From these calculations, an essentially complete overview of the explosion properties and characteristics of shock wave action onto barriers was obtained for several new and also for several standard explosive materials as a comparison. Work done in collaboration with Golubev/Fischer/Stierstorfer/Bohanek/Dobrilovic.

Perceived barriers to and facilitators of physical activity in young adults with childhood-onset physical disabilities.

PubMed

Buffart, Laurien M; Westendorp, Tessa; van den Berg-Emons, Rita J; Stam, Henk J; Roebroeck, Marij E

2009-11-01

To explore the main barriers to and facilitators of physical activity in young adults with childhood-onset physical disabilities. Qualitative study using focus groups. Sixteen persons (12 men and 4 women) aged 22.4 (standard deviation 3.4) years, of whom 50% were wheelchair-dependent, participated in the study. Eight were diagnosed with myelomeningocele, 4 with cerebral palsy, 2 with acquired brain injury and 2 with rheumatoid arthritis. Three focus group sessions of 1.5 h were conducted using a semi-structured question route to assess perceived barriers to and facilitators of physical activity. Tape recordings were transcribed verbatim and content analysed. According to the Physical Activity for People with a Physical Disability model, barriers and facilitators were subdivided into personal factors and environmental factors. Participants reported several barriers related to attitude and motivation. In addition, lack of energy, existing injury or fear of developing injuries or complications, limited physical activity facilities, and lack of information and knowledge, appeared to be barriers to physical activity. Fun and social contacts were mentioned as facilitators of engaging in physical activity, as well as improved health and fitness. Young adults with childhood-onset physical disabilities perceived various personal and environmental factors as barriers to or facilitators of physical activity. These should be taken into account when developing interventions to promote physical activity in this population.

Iron catalyst chemistry in modeling a high-pressure carbon monoxide nanotube reactor

NASA Technical Reports Server (NTRS)

Scott, Carl D.; Povitsky, Alexander; Dateo, Christopher; Gokcen, Tahir; Willis, Peter A.; Smalley, Richard E.

2003-01-01

The high-pressure carbon monoxide (HiPco) technique for producing single-wall carbon nanotubes (SWNTs) is analyzed with the use of a chemical reaction model coupled with flow properties calculated along streamlines, calculated by the FLUENT code for pure carbon monoxide. Cold iron pentacarbonyl, diluted in CO at about 30 atmospheres, is injected into a conical mixing zone, where hot CO is also introduced via three jets at 30 degrees with respect to the axis. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Then iron nucleates and forms clusters that catalyze the formation of SWNTs by a disproportionation reaction (Boudouard) of CO on Fe-containing clusters. Alternative nucleation rates are estimated from the theory of hard sphere collision dynamics with an activation energy barrier. The rate coefficient for carbon nanotube growth is estimated from activation energies in the literature. The calculated growth was found be about an order of magnitude greater than measured, regardless of the nucleation rate. A study of cluster formation in an incubation zone prior to injection into the reactor shows that direct dimer formation from Fe atoms is not as important as formation via an exchange reaction of Fe with CO in FeCO.

Iron catalyst chemistry in modeling a high-pressure carbon monoxide nanotube reactor.

PubMed

Scott, Carl D; Povitsky, Alexander; Dateo, Christopher; Gökçen, Tahir; Willis, Peter A; Smalley, Richard E

2003-01-01

The high-pressure carbon monoxide (HiPco) technique for producing single-wall carbon nanotubes (SWNTs) is analyzed with the use of a chemical reaction model coupled with flow properties calculated along streamlines, calculated by the FLUENT code for pure carbon monoxide. Cold iron pentacarbonyl, diluted in CO at about 30 atmospheres, is injected into a conical mixing zone, where hot CO is also introduced via three jets at 30 degrees with respect to the axis. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Then iron nucleates and forms clusters that catalyze the formation of SWNTs by a disproportionation reaction (Boudouard) of CO on Fe-containing clusters. Alternative nucleation rates are estimated from the theory of hard sphere collision dynamics with an activation energy barrier. The rate coefficient for carbon nanotube growth is estimated from activation energies in the literature. The calculated growth was found be about an order of magnitude greater than measured, regardless of the nucleation rate. A study of cluster formation in an incubation zone prior to injection into the reactor shows that direct dimer formation from Fe atoms is not as important as formation via an exchange reaction of Fe with CO in FeCO.

Nonequilibrium study of the intrinsic free-energy profile across a liquid-vapour interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Braga, Carlos, E-mail: ccorreia@imperial.ac.uk; Muscatello, Jordan, E-mail: jordan.muscatello@imperial.ac.uk; Lau, Gabriel, E-mail: gabriel.lau07@imperial.ac.uk

2016-01-28

We calculate an atomistically detailed free-energy profile across a heterogeneous system using a nonequilibrium approach. The path-integral formulation of Crooks fluctuation theorem is used in conjunction with the intrinsic sampling method to calculate the free-energy profile for the liquid-vapour interface of the Lennard-Jones fluid. Free-energy barriers are found corresponding to the atomic layering in the liquid phase as well as a barrier associated with the presence of an adsorbed layer as revealed by the intrinsic density profile. Our findings are in agreement with profiles calculated using Widom’s potential distribution theorem applied to both the average and the intrinsic profiles asmore » well as the literature values for the excess chemical potential.« less

Schottky barrier at graphene/metal oxide interfaces: insight from first-principles calculations

NASA Astrophysics Data System (ADS)

Cheng, Kai; Han, Nannan; Su, Yan; Zhang, Junfeng; Zhao, Jijun

2017-02-01

Anode materials play an important role in determining the performance of lithium ion batteries. In experiment, graphene (GR)/metal oxide (MO) composites possess excellent electrochemical properties and are promising anode materials. Here we perform density functional theory calculations to explore the interfacial interaction between GR and MO. Our result reveals generally weak physical interactions between GR and several MOs (including Cu2O, NiO). The Schottky barrier height (SBH) in these metal/semiconductor heterostructures are computed using the macroscopically averaged electrostatic potential method, and the role of interfacial dipole is discussed. The calculated SBHs below 1 eV suggest low contact resistance; thus these GR/MO composites are favorable anode materials for better lithium ion batteries.

Schottky barrier at graphene/metal oxide interfaces: insight from first-principles calculations.

PubMed

Cheng, Kai; Han, Nannan; Su, Yan; Zhang, Junfeng; Zhao, Jijun

2017-02-06

Anode materials play an important role in determining the performance of lithium ion batteries. In experiment, graphene (GR)/metal oxide (MO) composites possess excellent electrochemical properties and are promising anode materials. Here we perform density functional theory calculations to explore the interfacial interaction between GR and MO. Our result reveals generally weak physical interactions between GR and several MOs (including Cu2O, NiO). The Schottky barrier height (SBH) in these metal/semiconductor heterostructures are computed using the macroscopically averaged electrostatic potential method, and the role of interfacial dipole is discussed. The calculated SBHs below 1 eV suggest low contact resistance; thus these GR/MO composites are favorable anode materials for better lithium ion batteries.

Effect of hydrogen adsorption on the formation and annealing of Stone-Wales defects in graphene

NASA Astrophysics Data System (ADS)

Podlivaev, A. I.; Openov, L. A.

2015-12-01

The heights of energy barriers preventing the formation and annealing of Stone-Wales defects in graphene with a hydrogen atom adsorbed on the defect or in its immediate vicinity have been calculated using the atomistic computer simulation. It has been shown that, in the presence of hydrogen, both barriers are significantly lower than those in the absence of hydrogen. Based on the analysis of the potential energy surface, the frequency factors have been calculated for two different paths of the Stone-Wales transformation, and the temperature dependences of the corresponding annealing times of the defects have been found. The results obtained have been compared with the first-principles calculations and molecular dynamics data.

Barriers to involvement in physical activities of persons with mental illness.

PubMed

Shor, Ron; Shalev, Anat

2016-03-01

Participating in physical activities could be essential for reducing the multiple risk factors for health problems that persons with severe mental illness (SMI) may suffer. However, people with SMI are significantly less active than the general population. To develop knowledge about factors related to the perceived barriers hindering this population's participation in physical activities and the benefits this participation would have, a study was conducted in Israel with 86 people with mental illness living in community mental health facilities prior to their participation in a health promotion program. A mixed method was implemented and included: a scale designed to measure participants' perceptions of the barriers to and benefits of involvement in physical activities; instruments focusing on bio-psycho-social factors that may affect the level of barriers experienced; and personal interviews. The findings revealed high ranking for accessibility barriers hindering the participation in physical activities. Bio-psycho-social factors stemming from the participants' mental health, such as level of depression, were correlated with higher ranking of accessibility barriers. Bio-psycho-social factors reflecting positive mental health and health, such as positive appraisal of body weight, were correlated with lower ranking of accessibility barriers. Other barriers may include organizational and broader systemic barriers in the mental health facilities where the participants reside. These findings illuminate the need to consider the unique challenges that persons with mental illness may face in any attempt to advance their involvement in physical activity. © The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Quantum chemical ab initio prediction of proton exchange barriers between CH{sub 4} and different H-zeolites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tuma, Christian; Sauer, Joachim, E-mail: js@chemie.hu-berlin.de

2015-09-14

A hybrid MP2:DFT (second-order Møller–Plesset perturbation theory–density functional theory) method that combines MP2 calculations for cluster models with DFT calculations for the full periodic structure is used to localize minima and transition structures for proton jumps at different Brønsted sites in different frameworks (chabazite, faujasite, ferrierite, and ZSM-5) and at different crystallographic positions of a given framework. The MP2 limit for the periodic structures is obtained by extrapolating the results of a series of cluster models of increasing size. A coupled-cluster (CCSD(T)) correction to MP2 energies is calculated for cluster models consisting of three tetrahedra. For the adsorption energies, thismore » difference is small, between 0.1 and 0.9 kJ/mol, but for the intrinsic proton exchange barriers, this difference makes a significant (10.85 ± 0.25 kJ/mol) and almost constant contribution across different systems. The total values of the adsorption energies vary between 22 and 34 kJ/mol, whereas the total proton exchange energy barriers fall in the narrow range of 152–156 kJ/mol. After adding nuclear motion contributions (harmonic approximation, 298 K), intrinsic enthalpy barriers between 134 and 141 kJ/mol and apparent energy barriers between 105 and 118 kJ/mol are predicted for the different sites examined for the different frameworks. These predictions are consistent with experimental results available for faujasite, ferrierite, and ZSM-5.« less

Best Practices and Barriers to Obesity Prevention in Head Start: Differences Between Director and Teacher Perceptions.

PubMed

Byrd-Williams, Courtney; Dooley, Erin E; Sharma, Shreela V; Chuang, Ru-Jye; Butte, Nancy; Hoelscher, Deanna M

2017-12-21

Practices and barriers to promoting healthy eating and physical activity at Head Start centers may influence children's energy balance behaviors. We examined differences between directors' and teachers' perspectives on best practices and barriers to promoting healthy eating and physical activity in Head Start centers. We conducted a cross-sectional study of directors (n = 23) and teachers (n = 113) at 23 Head Start centers participating in the baseline assessment of the Texas Childhood Obesity Research Demonstration study. Participants completed surveys about practices and barriers to promoting healthy eating and physical activity. Multilevel regression models examined differences between director and teacher responses. More than half of directors and teachers reported meeting most best practices related to nutrition and physical activity; few directors or teachers (<25%) reported conducting physical activity for more than 60 minutes a day, and less than 40% of teachers helped children attend to satiety cues. Significantly more directors than teachers reported meeting 2 nutrition-related best practices: "Teachers rarely eat less healthy foods (especially sweets, salty snacks, and sugary drinks) in front of children" and "Teachers talk to children about trying/enjoying new foods" (P < .05). No barrier to healthy eating or physical activity was reported by more than 25% of directors or teachers. Significantly more teachers than directors reported barriers to healthy eating, citing lack of food service staff support, limited time, and insufficient funds (P < .05). More barriers to healthy eating were reported than were barriers to physical activity indicating that more support may be needed for healthy eating. Differences between responses of directors and teachers may have implications for future assessments of implementation of best practices and barriers to implementation related to nutrition and physical activity in early care and education centers.

Molecular oxygen adsorption and dissociation on Au12M clusters with M = Cu, Ag or Ir

NASA Astrophysics Data System (ADS)

Jiménez-Díaz, Laura M.; Pérez, Luis A.

2018-03-01

In this work, we present a density functional theory study of the structural and electronic properties of isolated neutral clusters of the type Au12M, with M = Cu, Ag, or Ir. On the other hand, there is experimental evidence that gold-silver, gold-copper and gold-iridium nanoparticles have an enhanced catalytic activity for the CO oxidation reaction. In order to address these phenomena, we also performed density functional calculations of the adsorption and dissociation of O2 on these nanoparticles. Moreover, to understand the effects of Cu, Ag, and Ir impurity atoms on the dissociation of O2, we also analyze this reaction in the corresponding pure gold cluster. The results indicate that the substitution of one gold atom in a Au13 cluster by Ag, Cu or Ir diminishes the activation energy barrier for the O2 dissociation by nearly 1 eV. This energy barrier is similar for Au12Ag and Au12Cu, whereas for Au12Ir is even lower. These results suggest that the addition of other transition metal atoms to gold nanoclusters can enhance their catalytic activity towards the CO oxidation reaction, independently of the effect that the substrate could have on supported nanoclusters.

Associative desorption of hydrogen isotopologues from copper surfaces: Characterization of two reaction mechanisms

NASA Astrophysics Data System (ADS)

Kaufmann, Sven; Shuai, Quan; Auerbach, Daniel J.; Schwarzer, Dirk; Wodtke, Alec M.

2018-05-01

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H2, HD, and D2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a "fast" channel and a "slow" channel on both facets. The "fast channel" is promoted by both hydrogen incidence translational and vibrational energy, while the "slow channel" is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the "fast channel" on Cu(111) are in agreement with theory within "chemical accuracy" (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle - quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the "slow channel," which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments.

Reaction mechanism of guanidinoacetate methyltransferase, concerted or step-wise.

PubMed

Zhang, Xiaodong; Bruice, Thomas C

2006-10-31

We describe a quantum mechanics/molecular mechanics investigation of the guanidinoacetate methyltransferase catalyzed reaction, which shows that proton transfer from guanidinoacetate (GAA) to Asp-134 and methyl transfer from S-adenosyl-L-methionine (AdoMet) to GAA are concerted. By self-consistent-charge density functional tight binding/molecular mechanics, the bond lengths in the concerted mechanism's transition state are 1.26 A for both the OD1 (Asp-134)-H(E) (GAA) and H(E) (GAA)-N(E) (GAA) bonds, and 2.47 and 2.03 A for the S8 (AdoMet)-C9 (AdoMet) and C9 (AdoMet)-N(E) (GAA) bonds, respectively. The potential-energy barrier (DeltaE++) determined by single-point B3LYP/6-31+G*//MM is 18.9 kcal/mol. The contributions of the entropy (-TDeltaS++) and zero-point energy corrections Delta(ZPE)++ by normal mode analysis are 2.3 kcal/mol and -1.7 kcal/mol, respectively. Thus, the activation enthalpy of this concerted mechanism is predicted to be DeltaH++ = DeltaE++ plus Delta(ZPE)++ = 17.2 kcal/mol. The calculated free-energy barrier for the concerted mechanism is DeltaG++ = 19.5 kcal/mol, which is in excellent agreement with the value of 19.0 kcal/mol calculated from the experimental rate constant (3.8 +/- 0.2.min(-1)).

Plasmachemical and heterogeneous processes in ozonizers with oxygen activation by a dielectric barrier discharge

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mankelevich, Yu. A., E-mail: ymankelevich@mics.msu.su; Voronina, E. N.; Poroykov, A. Yu.

Plasmachemical and heterogeneous processes of generation and loss of ozone in the atmosphericpressure dielectric barrier discharge in oxygen are studied theoretically. Plasmachemical and electronic kinetics in the stage of development and decay of a single plasma filament (microdischarge) are calculated numerically with and without allowance for the effects of ozone vibrational excitation and high initial ozone concentration. The developed analytical approach is applied to determine the output ozone concentration taking into account ozone heterogeneous losses on the Al{sub 2}O{sub 3} dielectric surface. Using the results of quantummechanical calculations by the method of density functional theory, a multistage catalytic mechanism ofmore » heterogeneous ozone loss based on the initial passivation of a pure Al{sub 2}O{sub 3} surface by ozone and the subsequent interaction of O{sub 3} molecules with the passivated surface is proposed. It is shown that the conversion reaction 2O{sub 3} → 3O{sub 2} of a gas-phase ozone molecule with a physically adsorbed ozone molecule can result in the saturation of the maximum achievable ozone concentration at high specific energy depositions, the nonstationarity of the output ozone concentration, and its dependence on the prehistory of ozonizer operation.« less

Barriers Affecting Physical Activity in Rural Communities: Perceptions of Parents and Children

ERIC Educational Resources Information Center

McWhinney, Sharon; McDonald, Andrea; Dawkins-Moultin, Lenna; Outley, Corliss; McKyer, E. Lisako; Thomas, Audrene

2011-01-01

A comprehensive understanding of the barriers inhibiting physical activity among children is critical in the fight against childhood obesity. This qualitative interview study examined parents' and children's perceptions of the barriers to physical activity in rural communities of low socioeconomic status. Parents and children concurred that the…

Barriers and Facilitators to Being Physically Active on a Rural U.S. Northern Plains American Indian Reservation

PubMed Central

Jahns, Lisa; McDonald, Leander R.; Wadsworth, Ann; Morin, Charles; Liu, Yan

2014-01-01

The objective of the present study was to identify barriers to and facilitators of physical activity among American Indian adults living on a rural, U.S. Northern Plains reservation using the nominal group technique (NGT). NGT is a method of data generation and interpretation that combines aspects of qualitative (free generation of responses) and quantitative (systematic ranking of responses) methodologies. Adults participated in one of two NGT sessions asking about either barriers to (n = 6), or facilitators of (n = 5), being physically active. Participants nominated and ranked 21 barriers and 18 facilitators. Barriers indicated lack of knowledge of how to fit physical activity into a daily schedule, work, caring for family members, and prioritizing sedentary pursuits. Other responses included environmental barriers such as lack of access and transportation to a gym, unsafe walking conditions, and inclement weather. Facilitators to following recommendations included knowledge of health benefits of physical activity and the perception of physical activity as enjoyable, including feeling good when working out. Environmental facilitators included being outdoors walking and biking as well as parks and exercise facilities. Responses provided direction for locally designed community-based programs to promote facilitators and decrease barriers to individual’s engagement in physical activity. PMID:25421064

Barriers and facilitators to being physically active on a rural U.S. Northern Plains American Indian reservation.

PubMed

Jahns, Lisa; McDonald, Leander R; Wadsworth, Ann; Morin, Charles; Liu, Yan

2014-11-21

The objective of the present study was to identify barriers to and facilitators of physical activity among American Indian adults living on a rural, U.S. Northern Plains reservation using the nominal group technique (NGT). NGT is a method of data generation and interpretation that combines aspects of qualitative (free generation of responses) and quantitative (systematic ranking of responses) methodologies. Adults participated in one of two NGT sessions asking about either barriers to (n = 6), or facilitators of (n = 5), being physically active. Participants nominated and ranked 21 barriers and 18 facilitators. Barriers indicated lack of knowledge of how to fit physical activity into a daily schedule, work, caring for family members, and prioritizing sedentary pursuits. Other responses included environmental barriers such as lack of access and transportation to a gym, unsafe walking conditions, and inclement weather. Facilitators to following recommendations included knowledge of health benefits of physical activity and the perception of physical activity as enjoyable, including feeling good when working out. Environmental facilitators included being outdoors walking and biking as well as parks and exercise facilities. Responses provided direction for locally designed community-based programs to promote facilitators and decrease barriers to individual's engagement in physical activity.

Experimental and theoretical research of the interaction between high-strength supercavitation impactors and monolithic barriers in water

NASA Astrophysics Data System (ADS)

Ishchenko, A. N.; Afanas'eva, S. A.; Burkin, V. V.; Diachkovskii, A. S.; Zykova, A. I.; Khabibullin, M. V.; Chupashev, A. V.; Yugov, N. T.

2017-09-01

The article describes experimental and theoretical research of the interaction between supercavitating impactors and underwater aluminum alloy and steel barriers. Strong alloys are used for making impactors. An experimental research technique based on a high-velocity hydro-ballistic complex was developed. Mathematical simulation of the collision the impactor and barrier is based on the continuum mechanics inclusive of the deformation and destruction of interacting bodies. Calculated and experimental data on the ultimate penetration thickness of barriers made of aluminum alloy D16T and steel for the developed supercavitating impactor are obtained.

Stability and migration of large oxygen clusters in UO(2+x): density functional theory calculations.

PubMed

Andersson, D A; Espinosa-Faller, F J; Uberuaga, B P; Conradson, S D

2012-06-21

Using ab initio molecular dynamics simulations and nudged elastic band calculations we examine the finite temperature stability, transition pathways, and migration mechanisms of large oxygen clusters in UO(2+x). Here we specifically consider the recently proposed split quad-interstitial and cuboctahedral oxygen clusters. It is shown that isolated cuboctahedral clusters may transform into more stable configurations that are closely linked to the split quad-interstitial. The split quad-interstitial is stable with respect to single interstitials occupying the empty octahedral holes of the UO(2) lattice. In order to better understand discrepancies between theory and experiments, the simulated atomic pair distribution functions for the split quad-interstitial structures are analyzed with respect to the distribution function for U(4)O(9) previously obtained from neutron diffraction data. Our nudged elastic band calculations suggest that the split quad-interstitial may migrate by translating one of its constituent di-interstitial clusters via a barrier that is lower than the corresponding barrier for individual interstitials, but higher than the barrier for the most stable di-interstitial cluster.

Evolutionary combinatorial chemistry, a novel tool for SAR studies on peptide transport across the blood-brain barrier. Part 2. Design, synthesis and evaluation of a first generation of peptides.

PubMed

Teixidó, Meritxell; Belda, Ignasi; Zurita, Esther; Llorà, Xavier; Fabre, Myriam; Vilaró, Senén; Albericio, Fernando; Giralt, Ernest

2005-12-01

The use of high-throughput methods in drug discovery allows the generation and testing of a large number of compounds, but at the price of providing redundant information. Evolutionary combinatorial chemistry combines the selection and synthesis of biologically active compounds with artificial intelligence optimization methods, such as genetic algorithms (GA). Drug candidates for the treatment of central nervous system (CNS) disorders must overcome the blood-brain barrier (BBB). This paper reports a new genetic algorithm that searches for the optimal physicochemical properties for peptide transport across the blood-brain barrier. A first generation of peptides has been generated and synthesized. Due to the high content of N-methyl amino acids present in most of these peptides, their syntheses were especially challenging due to over-incorporations, deletions and DKP formations. Distinct fragmentation patterns during peptide cleavage have been identified. The first generation of peptides has been studied by evaluation techniques such as immobilized artificial membrane chromatography (IAMC), a cell-based assay, log Poctanol/water calculations, etc. Finally, a second generation has been proposed. (c) 2005 European Peptide Society and John Wiley & Sons, Ltd.

Understanding contextual barriers, supports, and opportunities for physical activity among Mexican-origin children in Texas border colonias: a descriptive study.

PubMed

Umstattd Meyer, M Renée; Sharkey, Joseph R; Patterson, Megan S; Dean, Wesley R

2013-01-08

The increasing numbers of colonias along the U.S.-Mexico border are characterized by disproportionately poor families of Mexican-origin, limited access to resources and health services, and heightened risk for obesity and diabetes. Despite consistent evidence supporting physical activity (PA) in prevention of chronic diseases, many individuals of Mexican-origin, including children, fail to meet PA recommendations. Environmental influences on PA, founded in ecological and social cognitive perspectives, have not been examined among children living in colonias. The purpose of this study was to identify and better understand (1) household and neighborhood environmental PA resources/supports, (2) perceived barriers to engaging in PA, and (3) PA offerings, locations, and transportation characteristics for Mexican-origin children living in colonias. Data for this study were collected by promotora-researchers (indigenous community health workers trained in research methods) using face-to-face interviews conducted in Spanish. The sample consists of 94 mother-child dyads from Texas border colonias in Hidalgo County. Interviews included questionnaire items addressing PA barriers, household and neighborhood environmental support assessments conducted with each dyad, and open-ended questions that were coded to identify availability and locations of PA opportunities and transportation options. Descriptive statistics were calculated and differences between genders, birth countries, and BMI categories of children were determined using chi-square tests. All children were of Mexican-origin. The most frequently reported barriers were unleashed dogs in the street, heat, bad weather, traffic, no streetlights, and no place like a park to exercise. Prominent locations for current PA included schools, home, and parks. Common PA options for children were exercise equipment, running, playing, and sports. Environmental assessments identified exercise equipment (bicycles/tricycles, balls, etc.…), paved/good streets, yard/patio space, and social norms as the most frequent household or neighborhood resources within these colonias. Differences in PA barriers, options, and environmental resources for genders, birth countries, and BMI categories were detected. This study suggests that PA environmental resources, barriers, and opportunities for colonias children are similar to previous studies and distinctively unique. As expected, built resources in these communities are limited and barriers exist; however, knowledge of PA opportunities and available PA resources within colonias households and neighborhoods offers insight to help guide future research, policy, and PA initiatives.

Understanding contextual barriers, supports, and opportunities for physical activity among Mexican-origin children in Texas border colonias: A descriptive study

PubMed Central

2013-01-01

Background The increasing numbers of colonias along the U.S.-Mexico border are characterized by disproportionately poor families of Mexican-origin, limited access to resources and health services, and heightened risk for obesity and diabetes. Despite consistent evidence supporting physical activity (PA) in prevention of chronic diseases, many individuals of Mexican-origin, including children, fail to meet PA recommendations. Environmental influences on PA, founded in ecological and social cognitive perspectives, have not been examined among children living in colonias. The purpose of this study was to identify and better understand (1) household and neighborhood environmental PA resources/supports, (2) perceived barriers to engaging in PA, and (3) PA offerings, locations, and transportation characteristics for Mexican-origin children living in colonias. Methods Data for this study were collected by promotora-researchers (indigenous community health workers trained in research methods) using face-to-face interviews conducted in Spanish. The sample consists of 94 mother-child dyads from Texas border colonias in Hidalgo County. Interviews included questionnaire items addressing PA barriers, household and neighborhood environmental support assessments conducted with each dyad, and open-ended questions that were coded to identify availability and locations of PA opportunities and transportation options. Descriptive statistics were calculated and differences between genders, birth countries, and BMI categories of children were determined using chi-square tests. Results All children were of Mexican-origin. The most frequently reported barriers were unleashed dogs in the street, heat, bad weather, traffic, no streetlights, and no place like a park to exercise. Prominent locations for current PA included schools, home, and parks. Common PA options for children were exercise equipment, running, playing, and sports. Environmental assessments identified exercise equipment (bicycles/tricycles, balls, etc.…), paved/good streets, yard/patio space, and social norms as the most frequent household or neighborhood resources within these colonias. Differences in PA barriers, options, and environmental resources for genders, birth countries, and BMI categories were detected. Conclusions This study suggests that PA environmental resources, barriers, and opportunities for colonias children are similar to previous studies and distinctively unique. As expected, built resources in these communities are limited and barriers exist; however, knowledge of PA opportunities and available PA resources within colonias households and neighborhoods offers insight to help guide future research, policy, and PA initiatives. PMID:23297793

Effect of sulfation on the surface activity of CaO for N2O decomposition

NASA Astrophysics Data System (ADS)

Wu, Lingnan; Hu, Xiaoying; Qin, Wu; Dong, Changqing; Yang, Yongping

2015-12-01

Limestone addition to circulating fluidized bed boilers for sulfur removal affects nitrous oxide (N2O) emission at the same time, but mechanism of how sulfation process influences the surface activity of CaO for N2O decomposition remains unclear. In this paper, we investigated the effect of sulfation on the surface properties and catalytic activity of CaO for N2O decomposition using density functional theory calculations. Sulfation of CaO (1 0 0) surface by the adsorption of a single gaseous SO2 or SO3 molecule forms stable local CaSO3 or CaSO4 on the CaO (1 0 0) surface with strong hybridization between the S atom of SOx and the surface O anion. The formed local CaSO3 increases the barrier energy of N2O decomposition from 0.989 eV (on the CaO (1 0 0) surface) to 1.340 eV, and further sulfation into local CaSO4 remarkably increases the barrier energy to 2.967 eV. Sulfation from CaSO3 into CaSO4 is therefore the crucial step for deactivating the surface activity for N2O decomposition. Completely sulfated CaSO4 (0 0 1) and (0 1 0) surfaces further validate the negligible catalytic ability of CaSO4 for N2O decomposition.

1,2-Fluorine Radical Rearrangements: Isomerization Events in Perfluorinated Radicals.

PubMed

Van Hoomissen, Daniel J; Vyas, Shubham

2017-11-16

Devising effective degradation technologies for perfluoroalkyl substances (PFASs) is an active area of research, where the molecular mechanisms involving both oxidative and reductive pathways are still elusive. One commonly neglected pathway in PFAS degradation is fluorine atom migration in perfluoroalkyl radicals, which was largely assumed to be implausible because of the high C-F bond strength. Using density functional theory calculations, it was demonstrated that 1,2-F atom migrations are thermodynamically favored when the fluorine atom migrated from a less branched carbon center to a more branched carbon center. Activation barriers for these rearrangements were within 19-29 kcal/mol, which are possible to easily overcome at elevated temperatures or in photochemically activated species in the gas or aqueous phase. It was also found that the activation barriers for the 1,2-F atom migration are lowered as much as by 10 kcal/mol when common oxidative degradation products such as HF assisted the rearrangements or if the resulting radical center was stabilized by vicinal π-bonds. Natural bond orbital analyses showed that fluorine moves as a radical in a noncharge-separated state. These findings add an important reaction to the existing knowledge of mechanisms for PFAS degradation and highlights the fact that 1,2-F atom shifts may be a small channel for isomerization of these compounds, but upon availability of mineralization products, this isomerization process could become more prominent.

A Comparative Study of AlGaN and InGaN Back-Barriers in Ultrathin-Barrier AlN/GaN Heterostructures

NASA Astrophysics Data System (ADS)

All Abbas, J. M.; Atmaca, G.; Narin, P.; Kutlu, E.; Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.

2017-08-01

Investigations of the effects of back-barrier introduction on the two-dimensional electron gas (2DEG) of ultrathin-barrier AlN/GaN heterostructures with AlGaN and InGaN back-barriers are carried out using self-consistent solutions of 1-dimensional Schrödinger-Poisson equations. Inserted AlGaN and InGaN back-barriers are used to provide a good 2DEG confinement thanks to raising the conduction band edge of GaN buffer with respect to GaN channel layer. Therefore, in this paper the influence of these back-barrier layers on sheet carrier density, 2DEG confinement, and mobility are systematically and comparatively investigated. As a result of calculations, although sheet carrier density is found to decrease with InGaN back-barrier layer, it is not changed with AlGaN back-barrier layer for suggested optimise heterostructures. Obtained results can give some insights for further experimental studies.

Biological and Sociocultural Differences in Perceived Barriers to Physical Activity among 5th–7th Grade Urban Girls

PubMed Central

Vermeesch, Amber L.; Ling, Jiying; Voskuil, Vicki R.; Bakhoya, Marion; Wesolek, Stacey M.; Bourne, Kelly A.; Pfeiffer, Karin A.; Robbins, Lorraine B.

2015-01-01

Background Inadequate physical activity (PA) contributes to the high prevalence of overweight and obesity among U.S. adolescent girls. Barriers preventing adolescent girls from meeting PA guidelines have not been thoroughly examined. Objectives The threefold purpose of this study was to: (a) determine pubertal stage, racial/ethnic, and socioeconomic status (SES) differences in ratings of interference of barriers to PA; (b) examine relationships between perceived barriers and age, body mass index (BMI), recreational screen time, sedentary activity, and PA; and (c) identify girls’ top-rated perceived barriers to PA. Methods Girls (N = 509) from eight Midwestern U.S. schools participated. Demographic, pubertal stage, perceived barriers, and recreational screen time data were collected via surveys. Height and weight were measured. Accelerometers measured sedentary activity, moderate-to-vigorous physical activity (MVPA), and light plus MVPA. Results Girls of low SES reported greater interference of perceived barriers to PA than those who were not of low SES (1.16 vs. 0.97, p = .01). Girls in early/middle puberty had lower perceived barriers than those in late puberty (1.03 vs. 1.24, p < .001). Girls’ perceived barriers were negatively related to MVPA (r = −.10, p = .03) and light plus MVPA (r = −.11, p = .02). Girls’ top five perceived barriers included lack of skills, hating to sweat, difficulty finding programs, being tired, and having pain. Discussion Innovative interventions, particularly focusing on skill development, are needed to assist girls in overcoming their perceived barriers to PA. PMID:26325276

Lateral tunneling through voltage-controlled barriers

NASA Technical Reports Server (NTRS)

Manion, S. J.; Bell, L. D.; Kaiser, W. J.; Maker, P. D.; Muller, R. E.

1991-01-01

The paper reports on a detailed experimental investigation of lateral tunneling between electrodes of a two-dimensional electron gas separated by the voltage-controlled barrier of a nanometer Schottky gate. The experimental data are modeled using the WKB method to calculate the tunneling probability of electrons through a barrier whose shape is determined from a solution of the two-dimensional Poisson equation. This model is in excellent agreement with the experimental data over a two order of magnitude range of current.

Magnetic barriers and their q95 dependence at DIII-D

NASA Astrophysics Data System (ADS)

Volpe, F. A.; Kessler, J.; Ali, H.; Evans, T. E.; Punjabi, A.

2012-05-01

It is well known that externally generated resonant magnetic perturbations (RMPs) can form islands in the plasma edge. In turn, large overlapping islands generate stochastic fields, which are believed to play a role in the avoidance and suppression of edge localized modes (ELMs) at DIII-D. However, large coalescing islands can also generate, in the middle of these stochastic regions, KAM surfaces effectively acting as ‘barriers’ against field-line dispersion and, indirectly, particle diffusion. It was predicted in Ali and Punjabi (2007 Plasma Phys. Control. Fusion 49 1565-82) that such magnetic barriers can form in piecewise analytic DIII-D plasma equilibria. In this work, the formation of magnetic barriers at DIII-D is corroborated by field-line tracing calculations using experimentally constrained EFIT (Lao et al 1985 Nucl. Fusion 25 1611) DIII-D equilibria perturbed to include the vacuum field from the internal coils utilized in the experiments. According to these calculations, the occurrence and location of magnetic barriers depend on the edge safety factor q95. It was thus suggested that magnetic barriers might contribute to narrowing the edge stochastic layer and play an indirect role in the RMPs failing to control ELMs for certain values of q95. The analysis of DIII-D discharges where q95 was varied, however, does not show anti-correlation between barrier formation and ELM suppression.

The dynamical conductance of graphene tunnelling structures.

PubMed

Zhang, Huan; Chan, K S; Lin, Zijing

2011-12-16

The dynamical conductances of graphene tunnelling structures were numerically calculated using the scattering matrix method with the interaction effect included in a phenomenological approach. The overall single-barrier dynamical conductance is capacitative. Transmission resonances in the single-barrier structure lead to dips in the capacitative imaginary part of the response. This is different from the ac responses of typical semiconductor nanostructures, where transmission resonances usually lead to inductive peaks. The features of the dips depend on the Fermi energy. When the Fermi energy is below half of the barrier height, the dips are sharper. When the Fermi energy is higher than half of the barrier height, the dips are broader. Inductive behaviours can be observed in a double-barrier structure due to the resonances formed by reflection between the two barriers.

Computed potential energy surfaces for chemical reactions

NASA Technical Reports Server (NTRS)

Walch, Stephen P.

1988-01-01

The minimum energy path for the addition of a hydrogen atom to N2 is characterized in CASSCF/CCI calculations using the (4s3p2d1f/3s2p1d) basis set, with additional single point calculations at the stationary points of the potential energy surface using the (5s4p3d2f/4s3p2d) basis set. These calculations represent the most extensive set of ab initio calculations completed to date, yielding a zero point corrected barrier for HN2 dissociation of approx. 8.5 kcal mol/1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional Transition State Theory and a method which utilizes an Eckart barrier to compute one dimensional quantum mechanical tunneling effects. It is concluded that the lifetime of the HN2 species is very short, greatly limiting its role in both termolecular recombination reactions and combustion processes.

Perceived barriers and facilitators to physical activity in men with prostate cancer: possible influence of androgen deprivation therapy.

PubMed

Keogh, J W L; Patel, A; MacLeod, R D; Masters, J

2014-03-01

While physical activity is beneficial for men with prostate cancer, too few perform sufficient activity for such benefit. This study examined perceptions of men with prostate cancer of their barriers and facilitators to physical activity, and how androgen deprivation therapy (ADT) may influence these perceptions. Two focus groups were conducted, involving six ADT and eight non-ADT patients respectively. Data were transcribed verbatim and themes developed using a general inductive thematic approach. Facilitators to physical activity common to both groups of cancer survivors included clinician and spousal involvement, with pre-existing co-morbidities and increased age cited as barriers by both groups. The ADT subgroup cited personal involvement as a facilitator to physical activity, with fatigue, reduced motivation and a relative lack of specific advice from their clinician as additional barriers. The non-ADT subgroup had no additional facilitators to physical activity but cited time constraints as a barrier. These results highlight the important role that cancer clinicians and spouses play in promoting physical activity for men with prostate cancer and how ADT may influence their other facilitators and barriers. As physical activity is beneficial for prostate cancer survivors, especially those on ADT, cancer clinicians should regularly discuss physical activity with their patients. © 2013 John Wiley & Sons Ltd.

Studies of aircraft differential maneuvering. Report 75-27: Calculating of differential-turning barrier surfaces. Report 75-26: A user's guide to the aircraft energy-turn and tandem-motion computer programs. Report 75-7: A user's guide to the aircraft energy-turn hodograph program. [numerical analysis of tactics and aircraft maneuvers of supersonic attack aircraft

NASA Technical Reports Server (NTRS)

Kelley, H. J.; Lefton, L.

1976-01-01

The numerical analysis of composite differential-turn trajectory pairs was studied for 'fast-evader' and 'neutral-evader' attitude dynamics idealization for attack aircraft. Transversality and generalized corner conditions are examined and the joining of trajectory segments discussed. A criterion is given for the screening of 'tandem-motion' trajectory segments. Main focus is upon the computation of barrier surfaces. Fortunately, from a computational viewpoint, the trajectory pairs defining these surfaces need not be calculated completely, the final subarc of multiple-subarc pairs not being required. Some calculations for pairs of example aircraft are presented. A computer program used to perform the calculations is included.

Study of Fission Barrier Heights of Uranium Isotopes by the Macroscopic-Microscopic Method

NASA Astrophysics Data System (ADS)

Zhong, Chun-Lai; Fan, Tie-Shuan

2014-09-01

Potential energy surfaces of uranium nuclei in the range of mass numbers 229 through 244 are investigated in the framework of the macroscopic-microscopic model and the heights of static fission barriers are obtained in terms of a double-humped structure. The macroscopic part of the nuclear energy is calculated according to Lublin—Strasbourg-drop (LSD) model. Shell and pairing corrections as the microscopic part are calculated with a folded-Yukawa single-particle potential. The calculation is carried out in a five-dimensional parameter space of the generalized Lawrence shapes. In order to extract saddle points on the potential energy surface, a new algorithm which can effectively find an optimal fission path leading from the ground state to the scission point is developed. The comparison of our results with available experimental data and others' theoretical results confirms the reliability of our calculations.

Density functional calculations on the effect of sulfur substitution for 2'-hydroxypropyl-p-nitrophenyl phosphate: C-O vs. P-O bond cleavage.

PubMed

Xia, Futing; Zhu, Hua

2012-02-01

Density functional theory calculations have been used to investigate the intra-molecular attack of 2'-hydroxypropyl-p-nitrophenyl phosphate (HPpNP) and its analogous compound 2-thiouridyl-p-nitrophenyl phosphate (s-2'pNP). Bulk solvent effect has been tested at the geometry optimization level with the polarized continuum model. It is found that the P-path involving the intra-molecular attack at the phosphorus atom and C-path involving the attack at the beta carbon atom proceed through the S(N)2-type mechanism for HPpNP and s-2'pNP. The calculated results indicate that the P-path with the free energy barrier of about 11 kcal/mol is more accessible than the C-path for the intra-molecular attack of HPpNP, which favors the formation of the five-membered phosphate diester. While for s-2'pNP, the C-path with the free energy barrier of about 21 kcal/mol proceeds more favorably than the P-path. The calculated energy barriers of the favorable pathways for HPpNP and s-2'pNP are both in agreement with the experimental results. Crown Copyright © 2011. Published by Elsevier Inc. All rights reserved.

Why bistetracenes are much less reactive than pentacenes in Diels-Alder reactions with fullerenes.

PubMed

Cao, Yang; Liang, Yong; Zhang, Lei; Osuna, Sílvia; Hoyt, Andra-Lisa M; Briseno, Alejandro L; Houk, K N

2014-07-30

The Diels-Alder (DA) reactions of pentacene (PT), 6,13-bis(2-trimethylsilylethynyl)pentacene (TMS-PT), bistetracene (BT), and 8,17-bis(2-trimethylsilylethynyl)bistetracene (TMS-BT) with the [6,6] double bond of [60]fullerene have been investigated by density functional theory calculations. Reaction barriers and free energies have been obtained to assess the effects of frameworks and substituent groups on the DA reactivity and product stability. Calculations indicate that TMS-BT is about 5 orders of magnitude less reactive than TMS-PT in the reactions with [60]fullerene. This accounts for the observed much higher stability of TIPS-BT than TIPS-PT when mixed with PCBM. Surprisingly, calculations predict that the bulky silylethynyl substituents of TMS-PT and TMS-BT have only a small influence on reaction barriers. However, the silylethynyl substituents significantly destabilize the corresponding products due to steric repulsions in the adducts. This is confirmed by experimental results here. Architectures of the polycyclic aromatic hydrocarbons (PAHs) play a crucial role in determining both the DA barrier and the adduct stability. The reactivities of different sites in various PAHs are related to the loss of aromaticity, which can be predicted using the simple Hückel molecular orbital localization energy calculations.

Spectral response, dark current, and noise analyses in resonant tunneling quantum dot infrared photodetectors.

PubMed

Jahromi, Hamed Dehdashti; Mahmoodi, Ali; Sheikhi, Mohammad Hossein; Zarifkar, Abbas

2016-10-20

Reduction of dark current at high-temperature operation is a great challenge in conventional quantum dot infrared photodetectors, as the rate of thermal excitations resulting in the dark current increases exponentially with temperature. A resonant tunneling barrier is the best candidate for suppression of dark current, enhancement in signal-to-noise ratio, and selective extraction of different wavelength response. In this paper, we use a physical model developed by the authors recently to design a proper resonant tunneling barrier for quantum infrared photodetectors and to study and analyze the spectral response of these devices. The calculated transmission coefficient of electrons by this model and its dependency on bias voltage are in agreement with experimental results. Furthermore, based on the calculated transmission coefficient, the dark current of a quantum dot infrared photodetector with a resonant tunneling barrier is calculated and compared with the experimental data. The validity of our model is proven through this comparison. Theoretical dark current by our model shows better agreement with the experimental data and is more accurate than the previously developed model. Moreover, noise in the device is calculated. Finally, the effect of different parameters, such as temperature, size of quantum dots, and bias voltage, on the performance of the device is simulated and studied.

Quantitative data analysis of perceived barriers and motivators to physical activity in stroke survivors.

PubMed

Nicholson, S L; Greig, C A; Sniehotta, F; Johnston, M; Lewis, S J; McMurdo, M E; Johnston, D; Scopes, J; Mead, G E

2017-09-01

Levels of physical activity after stroke are low, despite multiple health benefits. We explored stroke survivors' perceived barriers, motivators, self-efficacy and intention to physical activity. Fifty independently mobile stroke survivors were recruited prior to hospital discharge. Participants rated nine possible motivators and four possible barriers based on the Mutrie Scale, as having 'no influence', 'some influence' or 'a major influence' on physical activity. Participants also rated their self-efficacy and intention to increasing walking. The most common motivator was 'physical activity is good for health' [34 (68%)]. The most common barrier was 'feeling too tired' [24 (48%)]. Intention and self-efficacy were high. Self-efficacy was graded as either 4 or 5 (highly confident) on a five-point scale by [34 (68%)] participants, while 42 (84%) 'strongly agreed' or 'agreed' that they intended to increase their walking. Participants felt capable of increasing physical activity but fatigue was often perceived as a barrier to physical activity. This needs to be considered when encouraging stroke survivors to be more active.

Understanding barriers and facilitators to healthy eating and physical activity from patients either before and after knee arthroplasty.

PubMed

Pellegrini, Christine A; Ledford, Gwendolyn; Chang, Rowland W; Cameron, Kenzie A

2017-05-05

We sought to identify patient-reported barriers and facilitators to healthy eating and physical activity among patients before or after knee arthroplasty. Twenty patients with knee osteoarthritis aged 40-79 years who had knee arthroplasty surgery scheduled or completed within 3 months were interviewed. Interview topics included perceived barriers and facilitators to healthy eating and activity before or after surgery. Interviews were coded and analyzed using constant comparative analysis. Interviews were completed with 11 pre-operative (67.1 ± 7.6 years, 45.5% female, BMI 31.2 ± 6.3) and nine post-operative patients (61.7 ± 11.7 years, 44.4% female, BMI 30.2 ± 4.7 kg/m 2 ). The most commonly identified personal barriers to healthy eating identified were desire for high-fat/high-calorie foods, managing overconsumption and mood. Factors related to planning, portion control and motivation to improve health were identified as healthy eating facilitators. Identified personal barriers for activity included pain, physical limitations and lack of motivation, whereas facilitators included having motivation to improve knee symptoms/outcomes, personal commitment to activity and monitoring activity levels. Identifying specific eating and activity barriers and facilitators, such as mood and motivation to improve outcomes, provides critical insight from the patient perspective, which will aid in developing weight management programs during rehabilitation for knee arthroplasty patients. Implications for rehabilitation This study provides insight into the identified barriers and facilitators to healthy eating and physical activity in knee arthroplasty patients, both before and after surgery. Intrapersonal barriers that may hinder engagement in physical activity and rehabilitation include pain, physical limitations and lack of motivation; factors that may help to improve activity and the rehabilitation process include being motivated to improve knee outcomes, having a personal commitment to activity and tracking activity levels. Barriers that may interfere with healthy eating behaviors and knee arthroplasty rehabilitation include the desire for high-fat/high-calorie foods, overeating and mood; whereas planning and portion control may help to facilitate healthy eating. Understanding barriers and facilitators to healthy eating and physical activity can help guide rehabilitation professionals with their discussions on weight management with patients who had or are contemplating knee arthroplasty.

Theoretical analysis of hydrogen spillover mechanism on carbon nanotubes

PubMed Central

Juarez-Mosqueda, Rosalba; Mavrandonakis, Andreas; Kuc, Agnieszka B.; Pettersson, Lars G. M.; Heine, Thomas

2015-01-01

The spillover mechanism of molecular hydrogen on carbon nanotubes in the presence of catalytically active platinum clusters was critically and systematically investigated by using density-functional theory. Our simulation model includes a Pt4 cluster for the catalyst nanoparticle and curved and planar circumcoronene for two exemplary single-walled carbon nanotubes (CNT), the (10,10) CNT and one of large diameter, respectively. Our results show that the H2 molecule dissociates spontaneously on the Pt4 cluster. However, the dissociated H atoms have to overcome a barrier of more than 2 eV to migrate from the catalyst to the CNT, even if the Pt4 cluster is at full saturation with six adsorbed and dissociated hydrogen molecules. Previous investigations have shown that the mobility of hydrogen atoms on the CNT surface is hindered by a barrier. We find that instead the Pt4 catalyst may move along the outer surface of the CNT with activation energy of only 0.16 eV, and that this effect offers the possibility of full hydrogenation of the CNT. Thus, although we have not found a low-energy pathway to spillover onto the CNT, we suggest, based on our calculations and calculated data reported in the literature, that in the hydrogen-spillover process the observed saturation of the CNT at hydrogen background pressure occurs through mobile Pt nanoclusters, which move on the substrate more easily than the substrate-chemisorbed hydrogens, and deposit or reattach hydrogens in the process. Initial hydrogenation of the carbon substrate, however, is thermodynamically unfavoured, suggesting that defects should play a significant role. PMID:25699250

“My hair or my health”: Overcoming Barriers to Physical Activity in African American women with a focus on hairstyle-related factors

PubMed Central

Huebschmann, Amy G.; Campbell, Lucille Johnson; Brown, Candace S.; Dunn, Andrea L.

2016-01-01

Physical activity disparities among African American (AA) women may be related to sociocultural barriers, including difficulties with restyling hair after exercise. We sought to identify physical activity barriers and facilitators in AA women with a focus on sociocultural factors related to hairstyle maintenance. Participants (n=51) were AA women aged 19–73 years who completed valid surveys and participated in structured focus groups, stratified by age and physical activity levels, from 11/2012 to 2/2013. The Constant Comparison method was used to develop qualitative themes for barriers and facilitators. The most frequently reported general physical activity barrier among exercisers was “lack of money” (27%) and among non-exercisers was “lack of self-discipline” (57%). A hairstyle-related barrier of “sweating out my hairstyle,” was reported by 7% of exercisers and 29% of non-exercisers. This hairstyle-related barrier included the need for extra time and money to restyle hair due to perspiration. Hairstyle-related facilitators included: prioritizing health over hairstyle and high self-efficacy to restyle hair after perspiration. Participants were interested in resources to simplify hairstyle maintenance. AA women whose hairstyle is affected by perspiration may avoid physical activity due to time and financial burdens. Increasing self-efficacy to restyle hair after perspiration may help to overcome this barrier. PMID:26495938

Thermal, dielectric characteristics and conduction mechanism of azodyes derived from quinoline and their copper complexes.

PubMed

El-Ghamaz, N A; Diab, M A; El-Bindary, A A; El-Sonbati, A Z; Nozha, S G

2015-05-15

A novel series of (5-(4'-derivatives phenyl azo)-8-hydroxy-7-quinolinecarboxaldehyde) (AQLn) (n=1, p-OCH3; n=2, R=H; and n=3; p-NO2) and their complexes [Cu(AQLn)2]·5H2O are synthesized and investigated. The optimized bond lengths, bond angles and the calculated quantum chemical parameters for AQLn are investigated. HOMO-LUMO energy gap, absolute electronegativities, chemical potentials, and absolute hardness are also calculated. The thermal properties, dielectric properties, alternating current conductivity (σac) and conduction mechanism are investigated in the frequency range 0.1-100kHz and temperature range 293-568K for AQL1-3 and 318-693K for [Cu(AQL1-3)2]·5H2O complexes. The thermal properties are of ligands (AQLn) and their Cu(II) complexes investigated by thermogravimetric analysis (TGA). The temperature and frequency dependence of the real and the imaginary part of the dielectric constant are studied. The values of the thermal activation energy of conduction mechanism for AQLn and their complexes [Cu(AQLn)2]·5H2O under investigation are calculated at different test frequencies. The values of thermal activation energies ΔE1 and ΔE2 for AQLn and [Cu(AQLn)2]·5H2O decrease with increasing the values of frequency. The ac conductivity is found to be depending on the chemical structure of the compounds. Different conduction mechanisms have been proposed to explain the obtained experimental data. The small polaron tunneling (SPT) is the dominant conduction mechanism for AQL1 and its complex [Cu(AQL1)2]·5H2O. The quantum mechanical tunneling (QMT) is the dominant conduction mechanism for AQL2 and its complex [Cu(AQL2)2]·5H2O. The correlated barrier hopping (CBH) is the dominant conduction mechanism for AQL3 and its complex [Cu(AQL3)2]·5H2O, and the values of the maximum barrier height (Wm) are calculated. Copyright © 2015 Elsevier B.V. All rights reserved.

Radial Angular Momentum Transfer and Magnetic Barrier for Short-type Gamma-Ray-burst Central Engine Activity

NASA Astrophysics Data System (ADS)

Liu, Tong; Liang, En-Wei; Gu, Wei-Min; Hou, Shu-Jin; Lei, Wei-Hua; Lin, Lin; Dai, Zi-Gao; Zhang, Shuang-Nan

2012-11-01

Soft extended emission (EE) following initial hard spikes up to 100 s was observed with Swift/BAT for about half of known short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of black-hole-neutron-star merger models, we study the roles of radial angular momentum transfer in the disk and the magnetic barrier around the black hole in the activity of SGRB central engines. We show that radial angular momentum transfer may significantly prolong the lifetime of the accretion process, which may be divided into multiple episodes by the magnetic barrier. Our numerical calculations based on models of neutrino-dominated accretion flows suggest that disk mass is critical for producing the observed EE. In the case of the mass being ~0.8 M ⊙, our model can reproduce the observed timescale and luminosity of both the main and the EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE within about one order of magnitude and the timescale is shorter than 20 s if the disk mass is ~0.2 M ⊙. Swift/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component could be a probe for the merger process and disk formation for compact star mergers.

Iron-embedded C2N monolayer: a promising low-cost and high-activity single-atom catalyst for CO oxidation.

PubMed

He, B L; Shen, J S; Tian, Z X

2016-09-21

An Fe-embedded C2N monolayer as a promising single-atom catalyst for CO oxidation by O2 has been investigated based on first-principles calculations. It is found that the single Fe atom can be strongly trapped in the cavity of the C2N monolayer with a large adsorption energy of 4.55 eV and a high diffusion barrier of at least 3.00 eV to leave the cavity, indicating that Fe should exist in the isolated single-atom form. Due to the localized metal 3d orbitals near the Fermi level, the embedded Fe single-atom catalyst has a high chemical activity for the adsorption of CO and O2 molecules. CO oxidation by O2 on the catalyst would proceed via a two-step mechanism. The first step of the CO oxidation reaction has been studied via the Langmuir-Hinshelwood and Eley-Rideal mechanisms with energy barriers of 0.46 and 0.65 eV, respectively. The second step of the CO oxidation reaction follows the Eley-Rideal mechanism with a much smaller energy barrier of 0.24 eV. For both the steps, the CO2 molecules produced are weakly adsorbed on the substrates, suggesting that the proposed catalyst will not be poisoned by the generated CO2. Our results indicate that the Fe-embedded C2N monolayer is a promising single-atom catalyst for CO oxidation by O2 at low temperatures.

Ab initio and kinetic Monte Carlo study of lithium diffusion in LiSi, Li12Si7, Li13Si5 and Li15Si4

NASA Astrophysics Data System (ADS)

Moon, Janghyuk; Lee, Byeongchan; Cho, Maenghyo; Cho, Kyeongjae

2016-10-01

The kinetics of lithium atoms in various Li-Si binary compounds are investigated using density functional theory calculations and kinetic Monte Carlo calculations. The values of the Li migration energy barriers are identified by NEB calculations with vacancy-mediated, interstitial and exchange migration mechanisms in crystalline LiSi, Li12Si7, Li13Si4, and Li15Si4. A comparison of these NEB results shows that the vacancy-mediated Li migration is identified as the dominant diffusion mechanisms in Li-Si compounds. The diffusion coefficients of Li in Li-Si compounds at room temperature are determined by KMC simulation. From the KMC results, the recalculated migration energy barriers in LiSi, Li12Si7, Li13Si4, and Li15Si4 correspond to 0.306, 0.301, 0.367 and 0.320 eV, respectively. Compared to the Li migration energy barrier of 0.6 eV in crystalline Si, the drastic reduction in the Li migration energy barriers in the lithiated silicon indicates that the initial lithiation of the Si anode is the rate-limiting step. Furthermore, it is also found that Si migration is possible in Li-rich configurations. On the basis of these findings, the underlying mechanisms of kinetics on the atomic scale details are elucidated.

Why do kids eat healthful food? Perceived benefits of and barriers to healthful eating and physical activity among children and adolescents.

PubMed

O'dea, Jennifer A

2003-04-01

The goal was to have children and adolescents identify and rank the major perceived benefits of and barriers to healthful eating and physical activity and to suggest strategies for overcoming barriers. Semistructured, in-depth focus groups were undertaken using standardized questions and prompts. Students in grades 2 through 11(ages 7 through 17; N=213) from 34 randomly selected schools participated in 38 focus groups. Major benefits of healthful eating included improvements to cognitive and physical performance, fitness, endurance, psychological benefits, physical sensation (feeling good physically), and production of energy. Barriers included convenience, taste, and social factors. Benefits of physical activity included social benefits, enhancement of psychological status, physical sensation, and sports performance. Barriers included a preference for indoor activities, lack of energy and motivation, time constraints, and social factors. Suggested strategies for overcoming barriers included support from parents and school staff, better planning, time management, self-motivation, education, restructuring the physical environment, and greater variety of physical activities.

Temperature dependence of current-and capacitance-voltage characteristics of an Au/4H-SiC Schottky diode

NASA Astrophysics Data System (ADS)

Gülnahar, Murat

2014-12-01

In this study, the current-voltage (I-V) and capacitance-voltage (C-V) measurements of an Au/4H-SiC Schottky diode are characterized as a function of the temperature in 50-300 K temperature range. The experimental parameters such as ideality factor and apparent barrier height presents to be strongly temperature dependent, that is, the ideality factor increases and the apparent barrier height decreases with decreasing temperature, whereas the barrier height values increase with the temperature for C-V data. Likewise, the Richardson plot deviates at low temperatures. These anomaly behaviors observed for Au/4H-SiC are attributed to Schottky barrier inhomogeneities. The barrier anomaly which relates to interface of Au/4H-SiC is also confirmed by the C-V measurements versus the frequency measured in 300 K and it is interpreted by both Tung's lateral inhomogeneity model and multi-Gaussian distribution approach. The values of the weighting coefficients, standard deviations and mean barrier height are calculated for each distribution region of Au/4H-SiC using the multi-Gaussian distribution approach. In addition, the total effective area of the patches NAe is obtained at separate temperatures and as a result, it is expressed that the low barrier regions influence meaningfully to the current transport at the junction. The homogeneous barrier height value is calculated from the correlation between the ideality factor and barrier height and it is noted that the values of standard deviation from ideality factor versus q/3kT curve are in close agreement with the values obtained from the barrier height versus q/2kT variation. As a result, it can be concluded that the temperature dependent electrical characteristics of Au/4H-SiC can be successfully commented on the basis of the thermionic emission theory with both models.

The mechanism of transition-metal (Cu or Pd)-catalyzed synthesis of benzimidazoles from amidines: theoretical investigation.

PubMed

Li, Juan; Gu, Honghong; Wu, Caihong; Du, Lijuan

2014-11-28

In this study, the Cu(OAc)2- and [PdCl2(PhCN)2]-catalyzed syntheses of benzimidazoles from amidines were theoretically investigated using density functional theory calculations. For the Cu-catalyzed system, our calculations supported a four-step-pathway involving C-H activation of an arene with Cu(II) via concerted metalation-deprotonation (CMD), followed by oxidation of the Cu(II) intermediate and deprotonation of the imino group by Cu(III), and finally reductive elimination from Cu(III). In our calculations, the barriers for the CMD step and the oxidation step are the same. The results are different from the ones reported by Fu et al. in which the whole reaction mechanism includes three steps and the CMD step is rate determining. On the basis of the calculation results for the [PdCl2(PhCN)2]-catalyzed system, C-H bond breaking by CMD occurs first, followed by the rate-determining C-N bond formation and N-H deprotonation. Pd(III) species is not involved in the [PdCl2(PhCN)2]-catalyzed syntheses of benzimidazoles from amidines.

Thermal decomposition pathways of ethane

NASA Astrophysics Data System (ADS)

Gordon, Mark S.; Truong, Thanh N.; Pople, John A.

1986-10-01

The alternate thermal decomposition pathways for ethane in its ground state have been investigated, using ab initio electronic structure calculations. Single-point energies were obtained at the full MP4/6-311 G ∗∗ level, using 6-31 G ∗ geometries for reactant, products, and transition states. The thermodynamically favored products are ethylene and molecular hydrogen, but a very large barrier (130 kcal/mol) is found for the direct 1,2-elimination of hydrogen. When calculated barriers are taken into account, the lowest-energy process is the homolytic cleavage of the C-C bond to form two methyl radicals.

[Perceived barriers to active commuting to school: reliability and validity of a scale].

PubMed

Molina-García, Javier; Queralt, Ana; Estevan, Isaac; Álvarez, Octavio; Castillo, Isabel

To examine the reliability and validity of a scale to measure perceived barriers to active commuting to school among Spanish young people. The validity of the scale was assessed in a sample of 465 adolescents (14-18 years) using a confirmatory factor analysis and studying its association with self-reported active commuting to school. The reliability of the instrument was evaluated in a sub-sample that completed the scale twice separated by a one-week interval. The results showed that the barriers scale had satisfactory fit indices, and two factors were determined. The first included environment- and safety-related items (α=0.72), while the other concerned planning and psychosocial items (α=0.64). Active commuting to school showed significant correlations with the total score of the barriers scale (rho=-0.27; p <0.001), with the environmental/safety barriers (rho=-0.22; p <0.001), as well as with the planning/psychosocial barriers (rho=-0.29; p <0.001). Test-retest ICCs for the barriers ranged from 0.68 to 0.77. The developed scale has acceptable validity and good reliability to assess barriers to active commuting to school among Spanish young people. Copyright © 2016 SESPAS. Publicado por Elsevier España, S.L.U. All rights reserved.

Gender comparisons of perceived benefits of and barriers to physical activity in middle school youth.

PubMed

Robbins, Lorraine B; Sikorskii, Alla; Hamel, Lauren M; Wu, Tsu-Yin; Wilbur, JoEllen

2009-04-01

Perceived benefits of and barriers to physical activity (PA) reported by 206 middle school boys and girls in a survey were compared. Only "take care of myself, stay in shape, and be healthier" emerged as a greater benefit for girls than boys. Among students not on a sports team, boys reported fewer barriers than girls. Among those selecting an active pursuit, boys perceived more barriers than girls. When controlling for sports team participation and perceived benefits and barriers, boys reported more minutes of vigorous PA than girls. As boys and girls reported relatively similar benefits of and barriers to PA, nurse counseling with both groups can focus on the same information. Effort is particularly needed to increase PA among girls.

Perceived barriers and benefits to physical activity in colorectal cancer patients.

PubMed

Fisher, Abigail; Wardle, J; Beeken, R J; Croker, H; Williams, K; Grimmett, C

2016-02-01

There is emerging evidence for the benefits of physical activity (PA) post-diagnosis for colorectal cancer (CRC) survivors. However, population studies suggest activity levels in these patients are very low. Understanding perceived barriers and benefits to activity is a crucial step in designing effective interventions. Patients who were between 6 months and 5 years post-diagnosis with non-metastasised disease were identified from five London (UK) hospitals. Four hundred and ninety five completed a lifestyle survey that included open-ended questions on their perceived barriers (what things would stop you from doing more physical activity?) and benefits (what do you think you would gain from doing more physical activity?). Patients also recorded their activity levels using the Godin Leisure Time Exercise Questionnaire, along with sociodemographic and treatment variables. The most commonly reported barriers related to cancer and its treatments (e.g. fatigue). Age and mobility-related comorbidities (e.g. impaired mobility) were also frequently cited. Those who reported age and mobility as barriers, or reported any barrier, were significantly less active even after adjustment for multiple confounders. The most frequently reported benefits were physiological (e.g. improving health and fitness). Cancer-related benefits (such as prevention of recurrence) were rarely reported. Those perceiving physiological benefits or perceiving any benefits were more active in unadjusted models, but associations were not significant in adjusted models. We have identified important barriers and facilitators in CRC survivors that will aid in the design of theory-based PA interventions.

Predictors of perceived barriers to physical activity in the general adult population: a cross-sectional study.

PubMed

Herazo-Beltrán, Yaneth; Pinillos, Yisel; Vidarte, José; Crissien, Estela; Suarez, Damaris; García, Rafael

The perception of personal barriers to physical activity varies according to the sociodemographic characteristics of individuals. To determine the predictors of the perception of barriers to physical activity in the adult population. A cross-sectional study with 1066 adult women and 1036 adult men. The sociodemographic variables (age, gender, marital status, socioeconomic level, level of education), the perception of barriers that do not allow performance of physical activity (i.e. lack of time, social support, energy, motivation, skill, resources, and fear of injury during practice); and the level of physical activity through the International Physical Activity Questionnaire in its short-form version were evaluated. Individuals from low socioeconomic level (1 and 2) have higher risks of perceiving barriers such as lack of motivation [OR 1.76 (95% CI (1.4-2.1))] and lack of resources [OR 1.37 (95% CI (1.1-1.6))]; individuals with partners did not perceive the lack of social support [OR 0.29 (95% CI (0.2-0.4))] and lack of motivation [OR 0.54 (95% CI (0.4-0.7))] as barriers to physical activity. Individuals with low schooling perceived lack of social support [OR 3.81 (95% CI (3-4.7))], lack of resources [OR 2.78 (95% CI (2.2-3.3))], and fear of injury [OR 2.70 (95% CI (2.2-3.3))] as barrier to physical activity. Factors such as socioeconomic level, marital status, level of education, and self-perception of health are predictors of barriers to physical activity. Copyright © 2017 Associação Brasileira de Pesquisa e Pós-Graduação em Fisioterapia. Publicado por Elsevier Editora Ltda. All rights reserved.

Barriers to physical activity in chronic hemodialysis patients: a single-center pilot study in an Italian dialysis facility.

PubMed

Fiaccadori, Enrico; Sabatino, Alice; Schito, Franco; Angella, Francesca; Malagoli, Martina; Tucci, Marco; Cupisti, Adamasco; Capitanini, Alessandro; Regolisti, Giuseppe

2014-01-01

In patients on chronic dialysis a sedentary lifestyle is a strong, yet potentially modifiable, predictor of mortality. The present single-center pilot study evaluated social, psychological and clinical barriers that may hinder physical activity in this population. We explored the association between barriers to physical activity and sedentarism in adult patients at a chronic dialysis facility in Parma, Italy. We used different questionnaries exploring participation in physical activity, physical functioning, patient attitudes and preferences, and barriers to physical activity perceived by either patients or dialysis doctors and nurses. We enrolled 104 patients, (67 males, 65%), mean age 69 years (79% of patients older than 60 years); median dialysis vintage 60 months (range 8-440); mean Charlson score 5.55, ADL (Activities of Daily Living) score 5.5. Ninety-two participants (88.5%) reported at least one barrier to physical activity. At multivariable analysis, after adjusting for age and sex, feeling to have too many medical problems (OR 2.99, 95% CI 1.27 to 7.07; P=0.012), chest pain (OR 10.78, 95% CI 1.28 to 90.28; P=0.029) and sadness (OR 2.59, 95% CI 1.10 to 6.09; P=0.030) were independently associated with physical inactivity. Lack of time for exercise counseling and the firm belief about low compliance/interest by the patients toward exercise were the most frequent barriers reported by doctors and nurses. We identified a number of patient-related and health personnel-related barriers to physical activity in patients on chronic dialysis. Solutions for these barriers should be addressed in future studies aimed at increasing the level of physical activity in this population. © 2014 S. Karger AG, Basel.

Determinants of physical activity in primary school students using the health belief model.

PubMed

Ar-Yuwat, Sireewat; Clark, Mary Jo; Hunter, Anita; James, Kathy S

2013-01-01

Thailand is a middle-income country in which two-thirds of children demonstrate an insufficient level of physical activity. Physical inactivity is a major risk factor for obesity and many other health-related consequences in children. Thus, it is important to understand how primary school children perceive things in their daily life as determinants of physical activity. The purpose of this study was to investigate the impact of cues, perceived benefits, and perceived barriers on the level of physical activity among primary school students. A cross-sectional study was conducted in Phitsanulok Province, Thailand, in 2011. Multistage sampling selected a total of 123 primary school students. The Physical Activity Questionnaire for Older Children and the Cues, Perceived Benefits, and Barriers to Physical Activity Questionnaire were used to assess the student levels of physical activity, as well as the perceived benefits, barriers, and cues to action. The association between these factors and the level of physical activity was determined by correlation statistics and confirmed by robust regression. Multivariate analysis of variance compared health belief model determinants: perceived benefits, perceived barriers, and cues to action on physical activity between male and female students. Self-administered questionnaires were validated and tested in a pilot study. The level of activity among primary school children was significantly influenced by perceived barriers, such as fear of strangers when playing outdoors, bad weather, and too much homework. However, activity was not influenced by cues to action or perceived benefits. Perceived benefits, barriers, and cues to physical activity did not differ by gender. A safe environment and fewer barriers, such as amount of homework, could enhance physical activity in primary school children.

Hydrologic Evaluation of a Humid Climate Poplar Phytoremediation Barrier

NASA Astrophysics Data System (ADS)

Swensen, K.; Rabideau, A. J.

2016-12-01

The emplacement of hybrid poplar trees to function as phytoremediation barriers is an appealing and sustainable groundwater management strategy because of low maintenance costs and the potential to extract large amounts of groundwater without pumping. While the effectiveness of poplar barriers has been assessed by groundwater quality monitoring, less attention has been given to physical hydrologic evaluations needed to improve barrier designs. In this research, a five year hydrologic evaluation was conducted at a poplar phytoremediation site in western NY, with the goal of quantifying ETg (evapotranspiration from groundwater) as a measure of the barrier's effectiveness in a humid climate. To consider transpiration from both vadose zone and groundwater, the hydrologic evaluation included four components: physical ET measurements, theoretical ET calculations, analysis of diurnal groundwater table fluctuations, and vadose zone modeling. The direct measurements of ETT (total) were obtained using sap flow meters installed on multiple trees within the barrier. These data were interpreted using a regression model that included theoretical ET calculations and site-specific measurements of weather parameters and poplar trunk area. Application of this model was challenged by the spatial variation in rooting depth as determined by tree excavations. To further quantify the removal of groundwater by the phytobarrier (ETg), the White Method was applied to interpret diurnal groundwater fluctuations from monitoring wells located within the barrier, in conjunction with a variably saturated-saturated flow model configured to confirm water extraction from ETg. Taken together, the results of this five year hydrologic evaluation highlight the complexity in quantifying humid climate groundwater extraction, as a large number of variables were found to influence these rates. Improved understanding of these controls will contribute to improved barrier designs that maximize ETg.

Perceived Barriers to Healthy Eating and Physical Activity Among Participants in a Workplace Obesity Intervention.

PubMed

Stankevitz, Kayla; Dement, John; Schoenfisch, Ashley; Joyner, Julie; Clancy, Shayna M; Stroo, Marissa; Østbye, Truls

2017-08-01

To characterize barriers to healthy eating (BHE) and physical activity (BPA) among participants in a workplace weight management intervention. Steps to health participants completed a questionnaire to ascertain barriers to physical activity and healthy eating faced. Exploratory factor analysis was used to determine the factor structure for BPA and BHE. The relationships of these factors with accelerometer data and dietary behaviors were assessed using linear regression. Barriers to physical activity included time constraints and lack of interest and motivation, and to healthy eating, lack of self-control and convenience, and lack of access to healthy foods. Higher BHE correlated with higher sugary beverage intake but not fruit and vegetable and fat intake. To improve their effectiveness, workplace weight management programs should consider addressing and reducing barriers to healthy eating and physical activity.

Functional role of R462 in the degradation of hyaluronan catalyzed by hyaluronate lyase from Streptococcus pneumoniae.

PubMed

Li, Fengxue; Xu, Dingguo

2015-08-01

Hyaluronan lyase from Streptococcus pneumoniae can degrade hyaluronic acid, which is one of the major components in the extracellular matrix. Hyaluronan can regulate water balance, osmotic pressure, and act as an ion exchange resin. Followed by our recent work on the catalytic reaction mechanism and substrate binding mode, we in this work further investigate the functional role of active site arginine residue, R462, in the degradation of hyaluronan. The site directed mutagenesis simulation of R462A and R462Q were modeled using a combined quantum mechanical and molecular mechanical method. The overall substrate binding features upon mutations do not have significant changes. The energetic profiles for the reaction processes are essentially the same as that in wild type enzyme, but significant activation barrier height changes can be observed. Both mutants were shown to accelerate the overall enzymatic activity, e.g., R462A can reduce the barrier height by about 2.8 kcal mol(-1), while R462Q reduces the activation energy by about 2.9 kcal mol(-1). Consistent with the active site model calculated using density functional theory, our results can support that the positive charge on R462 guanidino side chain group plays a negative role in the catalysis. Finally, the functional role of R462 was proposed to facilitate the formation of initial enzyme-substrate complex, but not in the subsequent catalytic degradation reaction. Graphical Abstract Degradation of hyaluronan catalyzed by hyaluronate lyase from Streptococcus pneumoniae.

Barriers to participation in physical activity and exercise among middle-aged and elderly individuals.

PubMed

Justine, Maria; Azizan, Azliyana; Hassan, Vaharli; Salleh, Zoolfaiz; Manaf, Haidzir

2013-10-01

INTRODUCTION Although the benefits of physical activity and exercise are widely acknowledged, many middle-aged and elderly individuals remain sedentary. This cross-sectional study aimed to identify the external and internal barriers to physical activity and exercise participation among middle-aged and elderly individuals, as well as identify any differences in these barriers between the two groups. METHODS Recruited individuals were categorised into either the middle-aged (age 45-59 years, n = 60) or elderly (age ≥ 60 years, n = 60) group. Data on demographics, anthropometry, as well as external and internal barriers to participation in physical activity and exercise were collected. RESULTS Analysis showed no significant differences in the total scores of all internal barriers between the two groups (p > 0.05). The total scores for most external barriers between the two groups also showed no significant differences (p > 0.05); only 'cost' (p = 0.045) and 'exercise interferes with social/family activities' (p = 0.011) showed significant differences. The most common external barriers among the middle-aged and elderly respondents were 'not enough time' (46.7% vs. 48.4%), 'no one to exercise with' (40.0% vs. 28.3%) and 'lack of facilities' (33.4% vs. 35.0%). The most common internal barriers for middle-aged respondents were 'too tired' (48.3%), 'already active enough' (38.3%), 'do not know how to do it' (36.7%) and 'too lazy' (36.7%), while those for elderly respondents were 'too tired' (51.7%), 'lack of motivation' (38.4%) and 'already active enough' (38.4%). CONCLUSION Middle-aged and elderly respondents presented with similar external and internal barriers to physical activity and exercise participation. These factors should be taken into account when healthcare policies are being designed and when interventions such as the provision of facilities to promote physical activity and exercise among older people are being considered.

The electrical characteristics of the dielectric barrier discharges

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yehia, Ashraf, E-mail: yehia30161@yahoo.com; Department of Physics, Faculty of Science, Assiut University, Assiut 71516

2016-06-15

The electrical characteristics of the dielectric barrier discharges have been studied in this paper under different operating conditions. The dielectric barrier discharges were formed inside two reactors composed of electrodes in the shape of two parallel plates. The dielectric layers inside these reactors were pasted on the surface of one electrode only in the first reactor and on the surfaces of the two electrodes in the second reactor. The reactor under study has been fed by atmospheric air that flowed inside it with a constant rate at the normal temperature and pressure, in parallel with applying a sinusoidal ac voltagemore » between the electrodes of the reactor. The amount of the electric charge that flows from the reactors to the external circuit has been studied experimentally versus the ac peak voltage applied to them. An analytical model has been obtained for calculating the electrical characteristics of the dielectric barrier discharges that were formed inside the reactors during a complete cycle of the ac voltage. The results that were calculated by using this model have agreed well with the experimental results under the different operating conditions.« less

Multiscale Investigations of the Early Stage Oxidation on Cu Surfaces

NASA Astrophysics Data System (ADS)

Zhu, Qing; Xiao, Penghao; Lian, Xin; Yang, Shen-Che; Henkelman, Grame; Saidi, Wissam; Yang, Judith; University of Pittsburgh Team; University of Texas at Austin Team

Previous in situ TEM experiments have shown that the oxidation of the three low index Cu surfaces (100), (110) and (111) exhibit different oxide nucleation rates, and the resulting oxides have 3-dimensional (3D) island shapes or 2D rafts under different conditions. In order to better understand these results, we have investigated the early stages of Cu oxidation using a multiscale computational approach that employs density functional theory (DFT), reactive force field (ReaxFF), and kinetic Mote Carlo (KMC). With DFT calculation, we have compared O2 dissociation barriers on Cu (100), (110) and (111) surfaces at high oxygen coverage to evaluate the kinetic barrier of sublayer oxidization. We found that O2 dissociation barriers on Cu(111) surface are all lower than those on (110) and (100) surfaces. This trend agrees with experimental observations that (111) surface is easier to oxidize. These DFT calculated energy barriers are then incorporated into KMC simulations. The large scale ReaxFF molecular dynamics and KMC simulations detail the oxidation dynamics of the different Cu surfaces, and show the formation of various oxide morphologies that are consistent with experimental observations.

Naval Waste Package Design Sensitivity

DOE Office of Scientific and Technical Information (OSTI.GOV)

T. Schmitt

2006-12-13

The purpose of this calculation is to determine the sensitivity of the structural response of the Naval waste packages to varying inner cavity dimensions when subjected to a comer drop and tip-over from elevated surface. This calculation will also determine the sensitivity of the structural response of the Naval waste packages to the upper bound of the naval canister masses. The scope of this document is limited to reporting the calculation results in terms of through-wall stress intensities in the outer corrosion barrier. This calculation is intended for use in support of the preliminary design activities for the license applicationmore » design of the Naval waste package. It examines the effects of small changes between the naval canister and the inner vessel, and in these dimensions, the Naval Long waste package and Naval Short waste package are similar. Therefore, only the Naval Long waste package is used in this calculation and is based on the proposed potential designs presented by the drawings and sketches in References 2.1.10 to 2.1.17 and 2.1.20. All conclusions are valid for both the Naval Long and Naval Short waste packages.« less

Thermally activated switching at long time scales in exchange-coupled magnetic grains

NASA Astrophysics Data System (ADS)

Almudallal, Ahmad M.; Mercer, J. I.; Whitehead, J. P.; Plumer, M. L.; van Ek, J.; Fal, T. J.

2015-10-01

Rate coefficients of the Arrhenius-Néel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of M H hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modeled as two exchange-coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of noninteracting ECC grains are then integrated numerically in an applied field with constant sweep rate R =-d H /d t and the magnetization calculated as a function of the applied field H . M H hysteresis loops are presented for a range of values I for sweep rates 105Oe /s ≤R ≤1010Oe /s and a figure of merit that quantifies the advantages of ECC media is proposed. M H hysteresis loops are also calculated based on the stochastic Landau-Lifshitz-Gilbert equations for 108Oe /s ≤R ≤1010Oe /s and are shown to be in good agreement with those obtained from the direct integration of rate equations. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer-based formalism and which provide some useful insight into the reversal process and its dependence on the coupling strength and sweep rate. Of particular interest is the clustering of minimum energy states that are separated by relatively low-energy barriers into "metastates." It is shown that while approximating the reversal process in terms of "metastates" results in little loss of accuracy, it can reduce the run time of a kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by 2 -3 orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC-based simulation studies on systems of interacting dual layer ECC media.

Perceived barriers and facilitators to physical activity for children with disability: a systematic review.

PubMed

Shields, Nora; Synnot, Anneliese Jane; Barr, Megan

2012-11-01

The aim of this systematic review was to investigate the perceived barriers and facilitators to physical activity among children with disability. 10 electronic databases were searched from the earliest time available to September 2010 to identify relevant articles. Articles were included if they examined the barriers or facilitators to physical activity for children with disability and were written in English. Articles were excluded if they included children with an acute, transient or chronic medical condition, examined sedentary leisure activities, or societal participation in general. Two reviewers independently assessed the search yields, extracted the data and assessed trial quality. Data were analysed descriptively. 14 articles met the inclusion criteria. Barriers included lack of knowledge and skills, the child's preferences, fear, parental behaviour, negative attitudes to disability, inadequate facilities, lack of transport, programmes and staff capacity, and cost. Facilitators included the child's desire to be active, practising skills, involvement of peers, family support, accessible facilities, proximity of location, better opportunities, skilled staff and information. Personal, social, environmental, and policy and programme-related barriers and facilitators influence the amount of activity children with disability undertake. The barriers to physical activity have been studied more comprehensively than the facilitators.

Comparison of Methodologies of Activation Barrier Measurements for Reactions with Deactivation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Zhenhua; Yan, Binhang; Zhang, Li

In this work, methodologies of activation barrier measurements for reactions with deactivation were theoretically analyzed. Reforming of ethane with CO 2 was introduced as an example for reactions with deactivation to experimentally evaluate these methodologies. Both the theoretical and experimental results showed that due to catalyst deactivation, the conventional method would inevitably lead to a much lower activation barrier, compared to the intrinsic value, even though heat and mass transport limitations were excluded. In this work, an optimal method was identified in order to provide a reliable and efficient activation barrier measurement for reactions with deactivation.

Comparison of Methodologies of Activation Barrier Measurements for Reactions with Deactivation

DOE PAGES

Xie, Zhenhua; Yan, Binhang; Zhang, Li; ...

2017-01-25

In this work, methodologies of activation barrier measurements for reactions with deactivation were theoretically analyzed. Reforming of ethane with CO 2 was introduced as an example for reactions with deactivation to experimentally evaluate these methodologies. Both the theoretical and experimental results showed that due to catalyst deactivation, the conventional method would inevitably lead to a much lower activation barrier, compared to the intrinsic value, even though heat and mass transport limitations were excluded. In this work, an optimal method was identified in order to provide a reliable and efficient activation barrier measurement for reactions with deactivation.

Role of the supersymmetric semiclassical approach in barrier penetration and heavy-ion fusion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sil, T.; Dutt, R.; Varshni, Y.P.

1994-11-01

The problem of heavy-ion fusion reactions in the one-dimensional barrier penetration model (BPM) has been reexamined in light of supersymmetry-inspired WKB (SWKB) method. Motivated by our recent work [Phys. Lett. A 184, 209 (1994)] describing the SWKB method for the computation of the transmission coefficient [ital T]([ital E]), we have performed similar calculations for a potential barrier that mimics the proximity potential obtained by fitting experimentally measured fusion cross section [sigma][sub [ital F

Enhancing the effective energy barrier of a Dy(III) SMM using a bridged diamagnetic Zn(II) ion.

PubMed

Upadhyay, Apoorva; Singh, Saurabh Kumar; Das, Chinmoy; Mondol, Ranajit; Langley, Stuart K; Murray, Keith S; Rajaraman, Gopalan; Shanmugam, Maheswaran

2014-08-18

Field induced single-molecule-magnet behaviour is observed for both a heterodinuclear [ZnDy(L(-))2](3+) complex (1) and a mononuclear [Dy(HL)2](3+) complex (2), with effective energy barriers of 83 cm(-1) and 16 cm(-1), respectively. Insights into the relaxation mechanism(s) and barrier heights are provided via ab initio and DFT calculations. Our findings reveal an interesting observation that the U(eff) of SMMs can be enhanced by incorporating diamagnetic metal ions.

Socio-demographic and behavioral variation in barriers to leisure-time physical activity.

PubMed

Borodulin, Katja; Sipilä, Noora; Rahkonen, Ossi; Leino-Arjas, Päivi; Kestilä, Laura; Jousilahti, Pekka; Prättälä, Ritva

2016-02-01

We examined the socio-demographic and behavioral determinants of perceived barriers to leisure-time physical activity (LTPA) in a population-based sample of working-aged adults. Data comprised the National FINRISK 2002 Study, a population-based health examination study. Analyses were restricted to those aged 25-64 years and who perceived that their amount of LTPA did not reach sufficient levels. They reported barriers to LTPA, defined as a lack of time, motivation and lack of companionship to be active with, as well as high expenses. Age, education, household income, employment status, family type, physical activity, smoking and body mass index (BMI) were included as explanatory variables. Lack of time was the most frequent barrier. Each barrier was explained by a different set of factors that also varied between genders. The strongest and most systematic associations with the barriers were found for age, employment status and family type. Lack of time was less often reported as a barrier among the unemployed, singles without children and older people. Lacking motivation as a barrier was most common among singles without children. High expenses as a barrier was more often reported by the unemployed, and less often reported in the highest income group. When considering actions to promote LTPA, there is not one single solution, because the perceived barriers vary by population subgroups. © 2015 the Nordic Societies of Public Health.

Biological and Sociocultural Differences in Perceived Barriers to Physical Activity Among Fifth- to Seventh-Grade Urban Girls.

PubMed

Vermeesch, Amber L; Ling, Jiying; Voskuil, Vicki R; Bakhoya, Marion; Wesolek, Stacey M; Bourne, Kelly A; Pfeiffer, Karin A; Robbins, Lorraine B

2015-01-01

Inadequate physical activity (PA) contributes to the high prevalence of overweight and obesity among U.S. adolescent girls. Barriers preventing adolescent girls from meeting PA guidelines have not been thoroughly examined. The threefold purpose of this study was to (a) determine pubertal stage, racial/ethnic, and socioeconomic status (SES) differences in ratings of interference of barriers to PA; (b) examine relationships between perceived barriers and age, body mass index, recreational screen time, sedentary activity, and PA; and (c) identify girls' top-rated perceived barriers to PA. Girls (N = 509) from eight Midwestern U.S. schools participated. Demographic, pubertal stage, perceived barriers, and recreational screen time data were collected via surveys. Height and weight were measured. Accelerometers measured sedentary activity, moderate-to-vigorous PA (MVPA), and light plus MVPA. Girls of low SES reported greater interference of perceived barriers to PA than those who were not of low SES (1.16 vs. 0.97, p = .01). Girls in early/middle puberty had lower perceived barriers than those in late puberty (1.03 vs. 1.24, p < .001). Girls' perceived barriers were negatively related to MVPA (r = -.10, p = .03) and light plus MVPA (r = -.11, p = .02). Girls' top five perceived barriers included lack of skills, hating to sweat, difficulty finding programs, being tired, and having pain. Innovative interventions, particularly focusing on skill development, are needed to assist girls in overcoming their perceived barriers to PA.

Mapping barriers and intervention activities to behaviour change theory for Mobilization of Vulnerable Elders in Ontario (MOVE ON), a multi-site implementation intervention in acute care hospitals.

PubMed

Moore, Julia E; Mascarenhas, Alekhya; Marquez, Christine; Almaawiy, Ummukulthum; Chan, Wai-Hin; D'Souza, Jennifer; Liu, Barbara; Straus, Sharon E

2014-10-30

As evidence-informed implementation interventions spread, they need to be tailored to address the unique needs of each setting, and this process should be well documented to facilitate replication. To facilitate the spread of the Mobilization of Vulnerable Elders in Ontario (MOVE ON) intervention, the aim of the current study is to develop a mapping guide that links identified barriers and intervention activities to behaviour change theory. Focus groups were conducted with front line health-care professionals to identify perceived barriers to implementation of an early mobilization intervention targeted to hospitalized older adults. Participating units then used or adapted intervention activities from an existing menu or developed new activities to facilitate early mobilization. A thematic analysis was performed on the focus group data, emphasizing concepts related to barriers to behaviour change. A behaviour change theory, the 'capability, opportunity, motivation-behaviour (COM-B) system', was used as a taxonomy to map the identified barriers to their root causes. We also mapped the behaviour constructs and intervention activities to overcome these. A total of 46 focus groups were conducted across 26 hospital inpatient units in Ontario, Canada, with 261 participants. The barriers were conceptualized at three levels: health-care provider (HCP), patient, and unit. Commonly mentioned barriers were time constraints and workload (HCP), patient clinical acuity and their perceived 'sick role' (patient), and lack of proper equipment and human resources (unit level). Thirty intervention activities to facilitate early mobilization of older adults were implemented across hospitals; examples of unit-developed intervention activities include the 'mobility clock' communication tool and the use of staff champions. A mapping guide was created with barriers and intervention activities matched though the lens of the COM-B system. We used a systematic approach to develop a guide, which maps barriers, intervention activities, and behaviour change constructs in order to tailor an implementation intervention to the local context. This approach allows implementers to identify potential strategies to overcome local-level barriers and to document adaptations.

Stochastic Resonance and First Arrival Time for Excitable Systems

NASA Astrophysics Data System (ADS)

Duki, Solomon Fekade; Taye, Mesfin Asfaw

2018-04-01

We study the noise induced thermally activated barrier crossing of Brownian particles that hop in a piecewise linear potential. Using the exact analytic solutions and via numerical simulations not only we explore the dependence for the first passage time of a single particle but also we calculate the first arrival time for one particle out of N particles. The first arrival time decreases as the number of particles increases as expected. We then explore the thermally activated barrier crossing rate of the system in the presence of time varying signal. The dependence of signal to noise ratio SNR as well as the power amplification (η ) on model parameters is explored. η and SNR depict a pronounced peak at particular noise strength. In the presence of N particles, η is considerably amplified as N steps up showing the weak periodic signal plays a vital role in controlling the noise induced dynamics of the system. Moreover, for the sake of generality, the viscous friction γ is considered to decrease exponentially when the temperature T of the medium increases (γ =Be^{-A T} ) as proposed originally by Reynolds (Philos Trans R Soc Lond 177:157, 1886).

Stochastic Resonance and First Arrival Time for Excitable Systems

NASA Astrophysics Data System (ADS)

Duki, Solomon Fekade; Taye, Mesfin Asfaw

2018-06-01

We study the noise induced thermally activated barrier crossing of Brownian particles that hop in a piecewise linear potential. Using the exact analytic solutions and via numerical simulations not only we explore the dependence for the first passage time of a single particle but also we calculate the first arrival time for one particle out of N particles. The first arrival time decreases as the number of particles increases as expected. We then explore the thermally activated barrier crossing rate of the system in the presence of time varying signal. The dependence of signal to noise ratio SNR as well as the power amplification (η ) on model parameters is explored. η and SNR depict a pronounced peak at particular noise strength. In the presence of N particles, η is considerably amplified as N steps up showing the weak periodic signal plays a vital role in controlling the noise induced dynamics of the system. Moreover, for the sake of generality, the viscous friction γ is considered to decrease exponentially when the temperature T of the medium increases (γ =Be^{-A T}) as proposed originally by Reynolds (Philos Trans R Soc Lond 177:157, 1886).

Fullerene formation and annealing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mintmire, J.W.

1996-04-05

Why does the highly symmetric carbon cluster C{sub 60} form in such profusion under the right conditions? This question was first asked in 1985, when Kroto suggested that the predominance of the C{sub 60} carbon clusters observed in the molecular beam experiments could be explained by the truncated icosahedral (or soccer ball) form. The name given to this cluster, buckminsterfullerene, led to the use of the term fullerenes for the family of hollow-cage carbon clusters made up of even numbers of triply coordinated carbons arranged with 12 pentagonal rings and an almost arbitrary number of hexagonal rings. More than amore » decade later, we still lack a completely satisfying understanding of the fundamental chemistry that takes place during fullerene formation. Most current models for fullerene formation require a facile mechanism for ring rearrangement in the fullerene structure, but the simplest proposed mechanisms are believed to have unrealistically high activation barriers. In recent research calculations have suggested that atomic carbon in the reaction mixture could act as a catalyst and allow substantially lower activation barriers for fullerene annealing. This article discusses the background for this research and other adjunct research. 14 refs.« less

Current-induced changes of migration energy barriers in graphene and carbon nanotubes

NASA Astrophysics Data System (ADS)

Obodo, J. T.; Rungger, I.; Sanvito, S.; Schwingenschlögl, U.

2016-05-01

An electron current can move atoms in a nanoscale device with important consequences for the device operation and breakdown. We perform first principles calculations aimed at evaluating the possibility of changing the energy barriers for atom migration in carbon-based systems. In particular, we consider the migration of adatoms and defects in graphene and carbon nanotubes. Although the current-induced forces are large for both the systems, in graphene the force component along the migration path is small and therefore the barrier height is little affected by the current flow. In contrast, the same barrier is significantly reduced in carbon nanotubes as the current increases. Our work also provides a real-system numerical demonstration that current-induced forces within density functional theory are non-conservative.An electron current can move atoms in a nanoscale device with important consequences for the device operation and breakdown. We perform first principles calculations aimed at evaluating the possibility of changing the energy barriers for atom migration in carbon-based systems. In particular, we consider the migration of adatoms and defects in graphene and carbon nanotubes. Although the current-induced forces are large for both the systems, in graphene the force component along the migration path is small and therefore the barrier height is little affected by the current flow. In contrast, the same barrier is significantly reduced in carbon nanotubes as the current increases. Our work also provides a real-system numerical demonstration that current-induced forces within density functional theory are non-conservative. Electronic supplementary information (ESI) available. See DOI: 10.1039/C6NR00534A

A comprehensive mechanistic picture of the isomerizing alkoxycarbonylation of plant oils.

PubMed

Roesle, Philipp; Caporaso, Lucia; Schnitte, Manuel; Goldbach, Verena; Cavallo, Luigi; Mecking, Stefan

2014-12-03

Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective.

High level QM/MM modeling of the formation of the tetrahedral intermediate in the acylation of wild type and K73A mutant TEM-1 class A beta-lactamase.

PubMed

Hermann, Johannes C; Pradon, Juliette; Harvey, Jeremy N; Mulholland, Adrian J

2009-10-29

The breakdown of beta-lactam antibiotics by beta-lactamases is the most important resistance mechanism of gram negative bacteria against these drugs. The reaction mechanism of class A beta-lactamases, the most widespread family of these enzymes, consists of two main steps: acylation of an active site serine by the antibiotic, followed by deacylation and release of the cleaved compound. We have investigated the first step in acylation (the formation of the tetrahedral intermediate) for the reaction of benzylpenicillin in the TEM-1 enzyme using high level combined quantum mechanics/molecular mechanics (QM/MM) methods. Structures were optimized at the B3LYP/6-31+G(d)/CHARMM27 level, with energies for key points calculated up to the ab initio SCS-MP2/aug-cc-pVTZ/CHARMM27 level. The results support a mechanism in which Glu166 removes a proton (via an intervening water molecule) from Ser70, which in turn attacks the beta-lactam of the antibiotic. Depending on the method used, the calculated barriers range from 3 to 12 kcal mol(-1) for this step, consistent with experimental data. We have also modeled this reaction step in a model of the K73A mutant enzyme. The barrier to reaction in this mutant model is found to be slightly higher: the results indicate that Lys73 stabilizes the transition state, in particular deprotonated Ser70, lowering the barrier by about 1.7 kcal mol(-1). This finding may help to explain the conservation of Lys73, in addition to the role we have previously found for it in the later stages of the reaction (Hermann et al. Org. Biomol. Chem. 2006, 4, 206-210).

High Level QM/MM Modeling of the Formation of the Tetrahedral Intermediate in the Acylation of Wild Type and K73A Mutant TEM-1 Class A β-Lactamase

NASA Astrophysics Data System (ADS)

Hermann, Johannes C.; Pradon, Juliette; Harvey, Jeremy N.; Mulholland, Adrian J.

2009-09-01

The breakdown of β-lactam antibiotics by β-lactamases is the most important resistance mechanism of Gram negative bacteria against these drugs. The reaction mechanism of class A β-lactamases, the most widespread family of these enzymes, consists of two main steps: acylation of an active site serine by the antibiotic, followed by deacylation and release of the cleaved compound. We have investigated the first step in acylation (the formation of the tetrahedral intermediate) for the reaction of benzylpenicillin in the TEM-1 enzyme using high level combined quantum mechanics/molecular mechanics (QM/MM) methods. Structures were optimized at the B3LYP/6-31+G(d)/CHARMM27 level, with energies for key points calculated up to the ab initio SCS-MP2/aug-cc-pVTZ/CHARMM27 level. The results support a mechanism in which Glu166 removes a proton (via an intervening water molecule) from Ser70, which in turn attacks the β-lactam of the antibiotic. Depending on the method used, the calculated barriers range from 3 to 12 kcal mol-1 for this step, consistent with experimental data. We have also modeled this reaction step in a model of the K73A mutant enzyme. The barrier to reaction in this mutant model is found to be slightly higher: the results indicate that Lys73 stabilizes the transition state, in particular deprotonated Ser70, lowering the barrier by about 1.7 kcal mol-1. This finding may help to explain the conservation of Lys73, in addition to the role we have previously found for it in the later stages of the reaction ( Hermann et al. Org. Biomol. Chem. 2006, 4, 206 - 210 ).

Molecular dynamics and brownian dynamics investigation of ion permeation and anesthetic halothane effects on a proton-gated ion channel.

PubMed

Cheng, Mary Hongying; Coalson, Rob D; Tang, Pei

2010-11-24

Bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC) is activated to cation permeation upon lowering the solution pH. Its function can be modulated by anesthetic halothane. In the present work, we integrate molecular dynamics (MD) and Brownian dynamics (BD) simulations to elucidate the ion conduction, charge selectivity, and halothane modulation mechanisms in GLIC, based on recently resolved X-ray crystal structures of the open-channel GLIC. MD calculations of the potential of mean force (PMF) for a Na(+) revealed two energy barriers in the extracellular domain (R109 and K38) and at the hydrophobic gate of transmembrane domain (I233), respectively. An energy well for Na(+) was near the intracellular entrance: the depth of this energy well was modulated strongly by the protonation state of E222. The energy barrier for Cl(-) was found to be 3-4 times higher than that for Na(+). Ion permeation characteristics were determined through BD simulations using a hybrid MD/continuum electrostatics approach to evaluate the energy profiles governing the ion movement. The resultant channel conductance and a near-zero permeability ratio (P(Cl)/P(Na)) were comparable to experimental data. On the basis of these calculations, we suggest that a ring of five E222 residues may act as an electrostatic gate. In addition, the hydrophobic gate region may play a role in charge selectivity due to a higher dehydration energy barrier for Cl(-) ions. The effect of halothane on the Na(+) PMF was also evaluated. Halothane was found to perturb salt bridges in GLIC that may be crucial for channel gating and open-channel stability, but had no significant impact on the single ion PMF profiles.

A multi-level quantum mechanics and molecular mechanics study of SN2 reaction at nitrogen: NH2Cl + OH(-) in aqueous solution.

PubMed

Lv, Jing; Zhang, Jingxue; Wang, Dunyou

2016-02-17

We employed a multi-level quantum mechanics and molecular mechanics approach to study the reaction NH2Cl + OH(-) in aqueous solution. The multi-level quantum method (including the DFT method with both the B3LYP and M06-2X exchange-correlation functionals and the CCSD(T) method, and both methods with the aug-cc-pVDZ basis set) was used to treat the quantum reaction region in different stages of the calculation in order to obtain an accurate potential of mean force. The obtained free energy activation barriers at the DFT/MM level of theory yielded a big difference of 21.8 kcal mol(-1) with the B3LYP functional and 27.4 kcal mol(-1) with the M06-2X functional respectively. Nonetheless, the barrier heights become very close when shifted from DFT to CCSD(T): 22.4 kcal mol(-1) and 22.9 kcal mol(-1) at CCSD(T)(B3LYP)/MM and CCSD(T)(M06-2X)/MM levels of theory, respectively. The free reaction energy obtained using CCSD(T)(M06-2X)/MM shows an excellent agreement with the one calculated using the available gas-phase data. Aqueous solution plays a significant role in shaping the reaction profile. In total, the water solution contributes 13.3 kcal mol(-1) and 14.6 kcal mol(-1) to the free energy barrier heights at CCSD(T)(B3LYP)/MM and CCSD(T)(M06-2X)/MM respectively. The title reaction at nitrogen is a faster reaction than the corresponding reaction at carbon, CH3Cl + OH(-).

Integrated study of first principles calculations and experimental measurements for Li-ionic conductivity in Al-doped solid-state LiGe2(PO4)3 electrolyte

NASA Astrophysics Data System (ADS)

Kang, Joonhee; Chung, Habin; Doh, Chilhoon; Kang, Byoungwoo; Han, Byungchan

2015-10-01

Understanding of the fundamental mechanisms causing significant enhancement of Li-ionic conductivity by Al3+ doping to a solid LiGe2(PO4)3 (LGP) electrolyte is pursued using first principles density functional theory (DFT) calculations combined with experimental measurements. Our results indicate that partial substitution Al3+ for Ge4+ in LiGe2(PO4)3 (LGP) with aliovalent (Li1+xAlxGe2-x(PO4)3, LAGP) improves the Li-ionic conductivity about four-orders of the magnitude. To unveil the atomic origin we calculate plausible diffusion paths of Li in LGP and LAGP materials using DFT calculations and a nudged elastic band method, and discover that LAGP had additional transport paths for Li with activation barriers as low as only 34% of the LGP. Notably, these new atomic channels manifest subtle electrostatic environments facilitating cooperative motions of at least two Li atoms. Ab-initio molecular dynamics predict Li-ionic conductivity for the LAGP system, which is amazingly agreed experimental measurement on in-house made samples. Consequently, we suggest that the excess amounts of Li caused by the aliovalent Al3+ doping to LGP lead to not only enhancing Li concentration but also opening new conducting paths with substantially decreases activation energies and thus high ionic conductivity of LAGP solid-state electrolyte.

Anti-site defected MoS2 sheet for catalytic application

NASA Astrophysics Data System (ADS)

Sharma, Archana; Husain, Mushahid; Khan, Mohd. Shahid

2018-04-01

To prevent harmful and poisonous CO gas molecules, catalysts are needed for converting them into benign substances. Density functional theory (DFT) calculations have been used to investigate CO oxidation on the surface of MoS2 monolayer with Mo atom embedded at S-vacancy site (anti-site defect). The stronger interaction between Mo metal with O2 molecule as compared with CO molecule suggests high catalytic activity. The complete oxidation of CO is studied in a two-step procedure using Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms with a low overall energy barrier of 0.35 eV. Creation of anti-site defect makes the surface of MoS2 nanosheet catalytically active for the CO oxidation to take place.

Atomically Dispersed Pd–O Species on CeO 2(111) as Highly Active Sites for Low-Temperature CO Oxidation

DOE PAGES

Spezzati, Giulia; Su, Yaqiong; Hofmann, Jan P.; ...

2017-09-07

Ceria-supported Pd is a promising heterogeneous catalyst for CO oxidation relevant to environmental cleanup reactions. Pd loaded onto a nanorod form of ceria exposing predominantly (111) facets is already active at 50 °C. Here we report a combination of CO-FTIR spectroscopy and theoretical calculations that allows assigning different forms of Pd on the CeO 2(111) surface during reaction conditions. Single Pd atoms stabilized in the form of PdO and PdO 2 in a CO/O 2 atmosphere participate in a catalytic cycle involving very low activation barriers for CO oxidation. In conclusion, the presence of single Pd atoms on the Pd/CeOmore » 2-nanorod, corroborated by aberration-corrected TEM and CO-FTIR spectroscopy, is considered pivotal to its high CO oxidation activity.« less

Atomically Dispersed Pd–O Species on CeO 2(111) as Highly Active Sites for Low-Temperature CO Oxidation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Spezzati, Giulia; Su, Yaqiong; Hofmann, Jan P.

Ceria-supported Pd is a promising heterogeneous catalyst for CO oxidation relevant to environmental cleanup reactions. Pd loaded onto a nanorod form of ceria exposing predominantly (111) facets is already active at 50 °C. Here we report a combination of CO-FTIR spectroscopy and theoretical calculations that allows assigning different forms of Pd on the CeO 2(111) surface during reaction conditions. Single Pd atoms stabilized in the form of PdO and PdO 2 in a CO/O 2 atmosphere participate in a catalytic cycle involving very low activation barriers for CO oxidation. In conclusion, the presence of single Pd atoms on the Pd/CeOmore » 2-nanorod, corroborated by aberration-corrected TEM and CO-FTIR spectroscopy, is considered pivotal to its high CO oxidation activity.« less

Combined experimental and theoretical study on the reactivity of compounds I and II in horseradish peroxidase biomimetics.

PubMed

Ji, Li; Franke, Alicja; Brindell, Małgorzata; Oszajca, Maria; Zahl, Achim; van Eldik, Rudi

2014-10-27

For the exploration of the intrinsic reactivity of two key active species in the catalytic cycle of horseradish peroxidase (HRP), Compound I (HRP-I) and Compound II (HRP-II), we generated in situ [Fe(IV) O(TMP(+.) )(2-MeIm)](+) and [Fe(IV) O(TMP)(2-MeIm)](0) (TMP=5,10,15,20-tetramesitylporphyrin; 2-MeIm=2-methylimidazole) as biomimetics for HRP-I and HRP-II, respectively. Their catalytic activities in epoxidation, hydrogen abstraction, and heteroatom oxidation reactions were studied in acetonitrile at -15 °C by utilizing rapid-scan UV/Vis spectroscopy. Comparison of the second-order rate constants measured for the direct reactions of the HRP-I and HRP-II mimics with the selected substrates clearly confirmed the outstanding oxidizing capability of the HRP-I mimic, which is significantly higher than that of HRP-II. The experimental study was supported by computational modeling (DFT calculations) of the oxidation mechanism of the selected substrates with the involvement of quartet and doublet HRP-I mimics ((2,4) Cpd I) and the closed-shell triplet spin HRP-II model ((3) Cpd II) as oxidizing species. The significantly lower activation barriers calculated for the oxidation systems involving (2,4) Cpd I than those found for (3) Cpd II are in line with the much higher oxidizing efficiency of the HRP-I mimic proven in the experimental part of the study. In addition, the DFT calculations show that all three reaction types catalyzed by HRP-I occur on the doublet spin surface in an effectively concerted manner, whereas these reactions may proceed in a stepwise mechanism with the HRP-II mimic as oxidant. However, the high desaturation or oxygen rebound barriers during CH bond activation processes by the HRP-II mimic predict a sufficient lifetime for the substrate radical formed through hydrogen abstraction. Thus, the theoretical calculations suggest that the dissociation of the substrate radical may be a more favorable pathway than desaturation or oxygen rebound processes. Importantly, depending on the electronic nature of the oxidizing species, that is, (2,4) Cpd I or (3) Cpd II, an interesting region-selective conversion phenomenon between sulfoxidation and H-atom abstraction was revealed in the course of the oxidation reaction of dimethylsulfide. The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of the HRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

[Effects of mulberry/soybean intercropping on the plant growth and rhizosphere soil microbial number and enzyme activities].

PubMed

Hu, Ju-Wei; Zhu, Wen-Xu; Zhang, Hui-Hui; Xu, Nan; Li, Xin; Yue, Bing-Bing; Sun, Guang-yu

2013-05-01

A root separation experiment was conducted to investigate the plant growth and rhizosphere soil microbes and enzyme activities in a mulberry/soybean intercropping system. As compared with those in plastic barrier and nylon mesh barrier treatments, the plant height, leaf number, root length, root nodule number, and root/shoot ratio of mulberry and soybean in non-barrier treatment were significantly higher, and the soybean's effective nodule number was larger. The available phosphorous content in the rhizosphere soils of mulberry and soybean in no barrier and nylon mesh barrier treatments was increased by 10.3% and 11.1%, and 5.1% and 4.6%, respectively, as compared with that in plastic barrier treatment. The microbial number, microbial diversity, and enzyme activities in the rhizosphere soils of mulberry and soybean were higher in the treatments of no barrier and nylon mesh barrier than in the treatment of plastic barrier. All the results indicated that there was an obvious interspecific synergistic effect between mulberry and soybean in the mulberry/soybean intercropping system.

Physical activity barriers and facilitators among working mothers and fathers.

PubMed

Mailey, Emily L; Huberty, Jennifer; Dinkel, Danae; McAuley, Edward

2014-06-27

The transition to parenthood is consistently associated with declines in physical activity. In particular, working parents are at risk for inactivity, but research exploring physical activity barriers and facilitators in this population has been scarce. The purpose of this study was to qualitatively examine perceptions of physical activity among working parents. Working mothers (n = 13) and fathers (n = 12) were recruited to participate in one of four focus group sessions and discuss physical activity barriers and facilitators. Data were analyzed using immersion/crystallization in NVivo 10. Major themes for barriers included family responsibilities, guilt, lack of support, scheduling constraints, and work. Major themes for facilitators included being active with children or during children's activities, being a role model for children, making time/prioritizing, benefits to health and family, and having support available. Several gender differences emerged within each theme, but overall both mothers and fathers reported their priorities had shifted to focus on family after becoming parents, and those who were fitting in physical activity had developed strategies that allowed them to balance their household and occupational responsibilities. The results of this study suggest working mothers and fathers report similar physical activity barriers and facilitators and would benefit from interventions that teach strategies for overcoming barriers and prioritizing physical activity amidst the demands of parenthood. Future interventions might consider targeting mothers and fathers in tandem to create an optimally supportive environment in the home.

Enhanced spin-torque in double tunnel junctions using a nonmagnetic-metal spacer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, C. H.; Cheng, Y. H.; Ko, C. W.

2015-10-12

This study proposes an enhancement in the spin-transfer torque of a magnetic tunnel junction (MTJ) designed with double-barrier layer structure using a nonmagnetic metal spacer, as a replacement for the ferromagnetic material, which is traditionally used in these double-barrier stacks. Our calculation results show that the spin-transfer torque and charge current density of the proposed double-barrier MTJ can be as much as two orders of magnitude larger than the traditional double-barrier one. In other words, the proposed double-barrier MTJ has a spin-transfer torque that is three orders larger than that of the single-barrier stack. This improvement may be attributed tomore » the quantum-well states that are formed in the nonmagnetic metal spacer and the resonant tunneling mechanism that exists throughout the system.« less

Fusion barrier characteristics of actinides

NASA Astrophysics Data System (ADS)

Manjunatha, H. C.; Sridhar, K. N.

2018-03-01

We have studied fusion barrier characteristics of actinide compound nuclei with atomic number range 89 ≤ Z ≤ 103 for all projectile target combinations. After the calculation of fusion barrier heights and positions, we have searched for their parameterization. We have achieved the empirical formula for fusion barrier heights (VB), positions (RB), curvature of the inverted parabola (ħω) of actinide compound nuclei with atomic number range 89 ≤ Z ≤ 103 for all projectile target combinations (6

Motivational readiness for active commuting by university students: incentives and barriers.

PubMed

Cole, Rachel; Leslie, Eva; Donald, Maria; Cerin, Ester; Neller, Anne; Owen, Neville

2008-12-01

Walking for transport can contribute significantly to health-enhancing physical activity. We examined the associations of stages of motivational readiness for active transport with perceived barriers and incentives to walking to and from university among students. Mail-back surveys were completed by 781 students in a regional university in south-east Queensland. They identified one of eight options on motivational readiness for active commuting, which were then classified as: pre-contemplation; contemplation-preparation; or, action-maintenance. Open-ended questions were used to identify relevant barriers and incentives. Logistic regressions were used to examine the barriers and incentives that distinguished between those at different stages of motivational readiness. Barriers most frequently reported were long travel distances, inconvenience and time constraints. Incentives most frequently reported were shorter travel distance, having more time, supportive infrastructure and better security. Those not considering active commuting (pre-contemplation) were significantly more likely to report shorter travel distance as an incentive compared to those in contemplation-preparation. Those in contemplation-preparation were significantly more likely to report lack of motivation, inadequate infrastructure, shorter travel distance and inconvenience as barriers; and, having more time, supportive infrastructure, social support and incentive programs as encouragement. Different barriers and incentives to walking to or from university exist for students in the different stages of motivational readiness for active commuting. Interventions targeted specifically to stage of motivational readiness may be potentially helpful in increasing activity levels, through active transport.

Physical inactivity is associated with low self efficacy and social support among patients with hypertension in Nigeria.

PubMed

Idowu, O A; Adeniyi, A F; Atijosan, O J; Ogwumike, O O

2013-06-01

BACKGROUND Physical inactivity is a major factor in the development of many chronic illnesses, including hypertension. Evidence highlighting links among physical activity participation and psychosocial constructs such as self efficacy, social support and perceived barriers among hypertensive patients in the Nigerian population is scarce. This study explored the associations between physical activity and each of self efficacy, social support and perceived barriers. METHODS Two hundred and twelve patients receiving treatment in two tertiary health institutions located in Ekiti State, Nigeria were surveyed cross-sectionally. Physical activity level, self efficacy, social support and perceived barriers were measured with the International Physical Activity Questionnaire, Exercise Self-Efficacy Scale, Medical Outcomes Social Support Scale and Exercise Benefits and Barrier Scale respectively. RESULTS Level of physical activity was significantly associated with self efficacy (r(s) = 0.67, p < 0.01, = 0.45) and social support (r(s) = 0.80, p < 0.01, = 0.64), with most participants (56.1%) being physically inactive. However, no association was found between physical activity level and perceived barriers (r(s) = 0.07, p > 0.01, = 0.005). CONCLUSION Most of the hypertensive patients presented with low levels of physical activity. Physical activity was associated with psychosocial constructs including self efficacy and social support but not with perceived barriers.

Fission properties of Po isotopes in different macroscopic-microscopic models

NASA Astrophysics Data System (ADS)

Bartel, J.; Pomorski, K.; Nerlo-Pomorska, B.; Schmitt, Ch

2015-11-01

Fission-barrier heights of nuclei in the Po isotopic chain are investigated in several macroscopic-microscopic models. Using the Yukawa-folded single-particle potential, the Lublin-Strasbourg drop (LSD) model, the Strutinsky shell-correction method to yield the shell corrections and the BCS theory for the pairing contributions, fission-barrier heights are calculated and found in quite good agreement with the experimental data. This turns out, however, to be only the case when the underlying macroscopic, liquid-drop (LD) type, theory is well chosen. Together with the LSD approach, different LD parametrizations proposed by Moretto et al are tested. Four deformation parameters describing respectively elongation, neck-formation, reflectional-asymmetric, and non-axiality of the nuclear shape thus defining the so called modified Funny Hills shape parametrization are used in the calculation. The present study clearly demonstrates that nuclear fission-barrier heights constitute a challenging and selective tool to discern between such different macroscopic approaches.

Towards a converged barrier height for the entrance channel transition state of the N( 2D) + CH 4 reaction and its implication for the chemistry in Titan's atmosphere

NASA Astrophysics Data System (ADS)

Ouk, Chanda-Malis; Zvereva-Loëte, Natalia; Bussery-Honvault, Béatrice

2011-10-01

The N( 2D) + CH 4 reaction appears to be a key reaction for the chemistry of Titan's atmosphere, opening the door to nitrile formation as recently observed by the Cassini-Huygens mission. Faced to the controversy concerning the existence or not of a potential barrier for this reaction, we have carried out accurate ab initio calculations by means of multi-state multi-reference configuration interaction (MS-MR-SDCI) method. These calculations have been partially corrected for the size-consistency errors (SCE) by Davidson, Pople or AQCC corrections. We suggest a barrier height of 3.86 ± 0.84 kJ/mol, including ZPE, for the entrance transition state, in good agreement with the experimental value. Its implication in Titan's atmopsheric chemistry is discussed.

Quantum instanton calculation of rate constant for CH4 + OH → CH3 + H2O reaction: Torsional anharmonicity and kinetic isotope effect

NASA Astrophysics Data System (ADS)

Wang, Wenji; Zhao, Yi

2012-12-01

Thermal rate constants for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The results reveal that the quantum effect is remarkable for the reaction at both low and high temperatures, and the obtained rates are in good agreement with experimental measurements at high temperatures. Compared to the harmonic approximation, the torsional anharmonic effect of the internal rotation has a little influence on the rates at low temperatures, however, it enhances the rate by about 20% at 1000 K. In addition, the free energy barriers for the isotopic reactions and the temperature dependence of kinetic isotope effects are also investigated. Generally speaking, for the title reaction, the replacement of OH with OD will reduce the free energy barrier, while substituting D for H (connected to C) will increase the free energy barrier.

The rate of the reaction between CN and C2H2 at interstellar temperatures.

PubMed

Woon, D E; Herbst, E

1997-03-01

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.

Music therapists' research activity and utilization barriers: a survey of the membership.

PubMed

Waldon, Eric G

2015-01-01

Music therapists have access to a rapidly expanding body of research supporting the use of music-based interventions. What is not known is the extent to which music therapists access these resources and what factors may prevent them from incorporating research findings into clinical work. After constructing the Music Therapists' Research Activity and Utilization Barrier (MTRAUB) database, the purposes of this study involved: assessing the extent to which American Music Therapy Association (AMTA) members engage in certain research-related activities; and identifying respondents' perceived barriers to integrating research into clinical practice. This study employed a quantitative, non-experimental approach using an online survey. Respondents included professional, associate, student/graduate student, retired, inactive, and honorary life members of AMTA. Instrumentation involved a researcher-designed Background Questionnaire as well as the Barriers to Research Utilization Scale (BARRIERS; Funk, Champagne, Wiese, & Tornquist, 1991), a tool designed to assess perceived barriers to incorporating research into practice. Of the 3,194 survey invitations distributed, 974 AMTA members replied (a response rate of 30%). Regarding research-related activities, descriptive findings indicate that journal reading is the most frequently reported research-related activity while conducting research is the least frequently reported activity. Results from the BARRIERS Scale indicated that Organizational and Communication factors are perceived as interfering most prominently with the ability to utilize research in clinical practice. Findings suggest that research-related activity and perceived barriers vary as a function of educational attainment, work setting, and occupational role. The author discusses these differential findings in detail, suggests supportive mechanisms to encourage increased research activity and utilization, and offers recommendations for further analysis of the MTRAUB data. © the American Music Therapy Association 2015. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Motivations associated with physical activity in young breast cancer survivors.

PubMed

Voege, Patricia; Bower, Julienne E; Stanton, Annette L; Ganz, Patricia A

2015-01-01

Physical activity is associated with positive health outcomes in breast cancer survivors. However, factors that promote or discourage physical activity in this population are not fully understood. This cross-sectional study was designed to examine approach and avoidance motivations, barriers for exercise, and their association with physical activity in breast cancer survivors younger than 50 years old at time of diagnosis. Current physical activity levels, approach and avoidance motivations, and barriers to exercise were assessed through self-report questionnaires in young breast cancer survivors (N = 156). Results indicated that barriers to exercise were negatively associated with physical activity (p < .01) while approach motivations were positively associated with physical activity (p < .01) and were most relevant in the context of low perceived barriers (p < .05). Avoidance motivations were not associated with physical activity (p = .91).

A molecular dynamics study of intramolecular proton transfer reaction of malonaldehyde in solutions based upon mixed quantum-classical approximation. I. Proton transfer reaction in water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yamada, Atsushi; Kojima, Hidekazu; Okazaki, Susumu, E-mail: okazaki@apchem.nagoya-u.ac.jp

2014-08-28

In order to investigate proton transfer reaction in solution, mixed quantum-classical molecular dynamics calculations have been carried out based on our previously proposed quantum equation of motion for the reacting system [A. Yamada and S. Okazaki, J. Chem. Phys. 128, 044507 (2008)]. Surface hopping method was applied to describe forces acting on the solvent classical degrees of freedom. In a series of our studies, quantum and solvent effects on the reaction dynamics in solutions have been analysed in detail. Here, we report our mixed quantum-classical molecular dynamics calculations for intramolecular proton transfer of malonaldehyde in water. Thermally activated proton transfermore » process, i.e., vibrational excitation in the reactant state followed by transition to the product state and vibrational relaxation in the product state, as well as tunneling reaction can be described by solving the equation of motion. Zero point energy is, of course, included, too. The quantum simulation in water has been compared with the fully classical one and the wave packet calculation in vacuum. The calculated quantum reaction rate in water was 0.70 ps{sup −1}, which is about 2.5 times faster than that in vacuum, 0.27 ps{sup −1}. This indicates that the solvent water accelerates the reaction. Further, the quantum calculation resulted in the reaction rate about 2 times faster than the fully classical calculation, which indicates that quantum effect enhances the reaction rate, too. Contribution from three reaction mechanisms, i.e., tunneling, thermal activation, and barrier vanishing reactions, is 33:46:21 in the mixed quantum-classical calculations. This clearly shows that the tunneling effect is important in the reaction.« less

LED structure with enhanced mirror reflectivity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bergmann, Michael; Donofrio, Matthew; Heikman, Sten

2014-04-01

Embodiments of the present invention are generally related to LED chips having improved overall emission by reducing the light-absorbing effects of barrier layers adjacent mirror contacts. In one embodiment, a LED chip comprises one or more LEDs, with each LED having an active region, a first contact under the active region having a highly reflective mirror, and a barrier layer adjacent the mirror. The barrier layer is smaller than the mirror such that it does not extend beyond the periphery of the mirror. In another possible embodiment, an insulator is further provided, with the insulator adjacent the barrier layer andmore » adjacent portions of the mirror not contacted by the active region or by the barrier layer. In yet another embodiment, a second contact is provided on the active region. In a further embodiment, the barrier layer is smaller than the mirror such that the periphery of the mirror is at least 40% free of the barrier layer, and the second contact is below the first contact and accessible from the bottom of the chip.« less

Computational Design of Cobalt Catalysts for Hydrogenation of Carbon Dioxide and Dehydrogenation of Formic Acid.

PubMed

Ge, Hongyu; Jing, Yuanyuan; Yang, Xinzheng

2016-12-05

A series of cobalt complexes with acylmethylpyridinol and aliphatic PNP pincer ligands are proposed based on the active site structure of [Fe]-hydrogenase. Density functional theory calculations indicate that the total free energy barriers of the hydrogenation of CO 2 and dehydrogenation of formic acid catalyzed by these Co complexes are as low as 23.1 kcal/mol in water. The acylmethylpyridinol ligand plays a significant role in the cleavage of H 2 by forming a strong Co-H δ- ···H δ+ -O dihydrogen bond in a fashion of frustrated Lewis pairs.

Quantum Mechanics Studies of Fuel Cell Catalysts and Proton Conducting Ceramics with Validation by Experiment

NASA Astrophysics Data System (ADS)

Tsai, Ho-Cheng

We carried out quantum mechanics (QM) studies aimed at improving the performance of hydrogen fuel cells. In part I, The challenge was to find a replacement for the Pt cathode that would lead to improved performance for the Oxygen Reduction Reaction (ORR) while remaining stable under operational conditions and decreasing cost. Our design strategy was to find an alloy with composition Pt3M that would lead to surface segregation such that the top layer would be pure Pt, with the second and subsequent layers richer in M. Under operating conditions we expect the surface to have significant O and/or OH chemisorbed on the surface; we searched for M that would remain segregated under these conditions. Using QM we examined surface segregation for 28 Pt3M alloys, where M is a transition metal. We found that only Pt3Os and Pt3Ir showed significant surface segregation when O and OH are chemisorbed on the catalyst surfaces. This result indicates that Pt3Os and Pt 3Ir favor formation of a Pt-skin surface layer structure that would resist the acidic electrolyte corrosion during fuel cell operation environments. We chose to focus on Os because the phase diagram for Pt-Ir indicated that Pt-Ir could not form a homogeneous alloy at lower temperature. To determine the performance for ORR, we used QM to examine intermediates, reaction pathways, and reaction barriers involved in the processes for which protons from the anode reactions react with O2 to form H2O. These QM calculations used our Poisson-Boltzmann implicit solvation model include the effects of the solvent (water with dielectric constant 78 with pH 7 at 298K). We also carried out similar QM studies followed by experimental validation for the Os/Pt core-shell catalyst fabricated by the underpotential deposition (UPD) method. The QM results indicated that the RDS for ORR is a compromise between the OOH formation step (0.37 eV for Pt, 0.23 eV for Pt2ML/Os core-shell) and H2O formation steps (0.32 eV for Pt, 0.22 eV for Pt2ML /Os core-shell). We found that Pt2ML/Os has the highest activity (compared to pure Pt and to the Pt3Os alloy) because the 0.37 eV barrier decreases to 0.23 eV. To understand what aspects of the core shell structure lead to this improved performance, we considered the effect on ORR of compressing the alloy slab to the dimensions of pure Pt. However this had the same RDS barrier 0.37 eV. Experimental materials characterization proves the core-shell feature of our catalyst. In part II, we used QM calculations to study methane stream reforming on a Ni-alloy catalyst surfaces for solid oxide fuel cell (SOFC) application. SOFC has wide fuel adaptability but the coking and sulfur poisoning will reduce its stability. We carried out QM calculations on surface segregation and found that the most stable configuration for Ni4Fe has a Fe atom distribution of (0%, 50%, 25%, 25%, 0%) starting at the bottom layer. We calculated that the binding of C atoms on the Ni4Fe surface is 142.9 Kcal/mol, which is about 10 Kcal/mol weaker compared to the pure Ni surface. This result confirms the experimental observation. The reaction energy barriers for CH x decomposition and C binding on various alloy surface, Ni4X (X=Fe, Co, Mn, and Mo), showed Ni4Fe, Ni4Co, and Fe4Mn all have better coking resistance than pure Ni, but that only Ni4Fe and Fe4Mn have (slightly) improved activity compared to pure Ni. In part III, we used QM to examine the proton transport in doped perovskite-ceramics. Here we used a 2x2x2 supercell of perovskite with composition Ba8X 7M1(OH)1O23 where X=Ce or Zr and M=Y, Gd, or Dy. Thus in each case a 4+ X is replace by a 3 + M plus a proton on one O. Here we predicted the barriers for proton diffusion allowing both includes intra-octahedron and inter-octahedra proton transfer. Without any restriction, we only observed the inter-octahedra proton transfer with similar energy barrier as previous computational work but 0.2 eV higher than experimental result for Y doped zirconate. For one restriction in our calculations is that the Odonor-Oacceptor atoms were kept at fixed distances, we found that the barrier difference between cerates/zirconates with various dopants are only 0.02~0.03 eV. To fully address performance one would need to examine proton transfer at grain boundaries, which will require larger scale ReaxFF reactive dynamics for systems with millions of atoms. The QM calculations used here will be used to train the ReaxFF force field. (Abstract shortened by UMI.).

Short and long-term lifestyle coaching approaches used to address diverse participant barriers to weight loss and physical activity adherence.

PubMed

Venditti, Elizabeth M; Wylie-Rosett, Judith; Delahanty, Linda M; Mele, Lisa; Hoskin, Mary A; Edelstein, Sharon L

2014-02-12

Individual barriers to weight loss and physical activity goals in the Diabetes Prevention Program, a randomized trial with 3.2 years average treatment duration, have not been previously reported. Evaluating barriers and the lifestyle coaching approaches used to improve adherence in a large, diverse participant cohort can inform dissemination efforts. Lifestyle coaches documented barriers and approaches after each session (mean session attendance = 50.3 ± 21.8). Subjects were 1076 intensive lifestyle participants (mean age = 50.6 years; mean BMI = 33.9 kg/m²; 68% female, 48% non-Caucasian). Barriers and approaches used to improve adherence were ranked by the percentage of the cohort for whom they applied. Barrier groupings were also analyzed in relation to baseline demographic characteristics. Top weight loss barriers reported were problems with self-monitoring (58%); social cues (58%); holidays (54%); low activity (48%); and internal cues (thought/mood) (44%). Top activity barriers were holidays (51%); time management (50%); internal cues (30%); illness (29%), and motivation (26%). The percentage of the cohort having any type of barrier increased over the long-term intervention period. A majority of the weight loss barriers were significantly associated with younger age, greater obesity, and non-Caucasian race/ethnicity (p-values vary). Physical activity barriers, particularly thought and mood cues, social cues and time management, physical injury or illness and access/weather, were most significantly associated with being female and obese (p < 0.001 for all). Lifestyle coaches used problem-solving with most participants (≥75% short-term; > 90% long term) and regularly reviewed self-monitoring skills. More costly approaches were used infrequently during the first 16 sessions (≤10%) but increased over 3.2 years. Behavioral problem solving approaches have short and long term dissemination potential for many kinds of participant barriers. Given minimal resources, increased attention to training lifestyle coaches in the consistent use of these approaches appears warranted.

Free-energy landscape of ion-channel voltage-sensor–domain activation

PubMed Central

Delemotte, Lucie; Kasimova, Marina A.; Klein, Michael L.; Tarek, Mounir; Carnevale, Vincenzo

2015-01-01

Voltage sensor domains (VSDs) are membrane-bound protein modules that confer voltage sensitivity to membrane proteins. VSDs sense changes in the transmembrane voltage and convert the electrical signal into a conformational change called activation. Activation involves a reorganization of the membrane protein charges that is detected experimentally as transient currents. These so-called gating currents have been investigated extensively within the theoretical framework of so-called discrete-state Markov models (DMMs), whereby activation is conceptualized as a series of transitions across a discrete set of states. Historically, the interpretation of DMM transition rates in terms of transition state theory has been instrumental in shaping our view of the activation process, whose free-energy profile is currently envisioned as composed of a few local minima separated by steep barriers. Here we use atomistic level modeling and well-tempered metadynamics to calculate the configurational free energy along a single transition from first principles. We show that this transition is intrinsically multidimensional and described by a rough free-energy landscape. Remarkably, a coarse-grained description of the system, based on the use of the gating charge as reaction coordinate, reveals a smooth profile with a single barrier, consistent with phenomenological models. Our results bridge the gap between microscopic and macroscopic descriptions of activation dynamics and show that choosing the gating charge as reaction coordinate masks the topological complexity of the network of microstates participating in the transition. Importantly, full characterization of the latter is a prerequisite to rationalize modulation of this process by lipids, toxins, drugs, and genetic mutations. PMID:25535341

Free-energy landscape of ion-channel voltage-sensor-domain activation.

PubMed

Delemotte, Lucie; Kasimova, Marina A; Klein, Michael L; Tarek, Mounir; Carnevale, Vincenzo

2015-01-06

Voltage sensor domains (VSDs) are membrane-bound protein modules that confer voltage sensitivity to membrane proteins. VSDs sense changes in the transmembrane voltage and convert the electrical signal into a conformational change called activation. Activation involves a reorganization of the membrane protein charges that is detected experimentally as transient currents. These so-called gating currents have been investigated extensively within the theoretical framework of so-called discrete-state Markov models (DMMs), whereby activation is conceptualized as a series of transitions across a discrete set of states. Historically, the interpretation of DMM transition rates in terms of transition state theory has been instrumental in shaping our view of the activation process, whose free-energy profile is currently envisioned as composed of a few local minima separated by steep barriers. Here we use atomistic level modeling and well-tempered metadynamics to calculate the configurational free energy along a single transition from first principles. We show that this transition is intrinsically multidimensional and described by a rough free-energy landscape. Remarkably, a coarse-grained description of the system, based on the use of the gating charge as reaction coordinate, reveals a smooth profile with a single barrier, consistent with phenomenological models. Our results bridge the gap between microscopic and macroscopic descriptions of activation dynamics and show that choosing the gating charge as reaction coordinate masks the topological complexity of the network of microstates participating in the transition. Importantly, full characterization of the latter is a prerequisite to rationalize modulation of this process by lipids, toxins, drugs, and genetic mutations.

Cleavage of hydrogen by activation at a single non-metal centre - towards new hydrogen storage materials.

PubMed

Grabowski, Sławomir J

2015-05-28

Molecular surfaces of non-metal species are often characterized by both positive and negative regions of electrostatic potential (EP) at a non-metal centre. This centre may activate molecular hydrogen which further leads to the addition reaction. The positive EP regions at the non-metal centres correspond to σ-holes; the latter sites are enhanced by electronegative substituents. This is why the following simple moieties; PFH2, SFH, AsFH2, SeFH, BrF3, PF(CH3)2 and AsF(CH3)2, were chosen here to analyze the H2 activation and its subsequent splitting at the P, As, S, Se and Br centres. Also the reverse H-H bond reforming process is analyzed. MP2/aug-cc-pVTZ calculations were performed for systems corresponding to different stages of these processes. The sulphur centre in the SFH moiety is analyzed in detail since the potential barrier height for the addition reaction for this species is the lowest of the moieties analyzed here. The results of calculations show that the SFH + H2 → SFH3 reaction in the gas phase is endothermic but it is exothermic in polar solvents.

Motivators of and Barriers to Engaging in Physical Activity: Perspectives of Low-Income Culturally Diverse Adolescents and Adults

ERIC Educational Resources Information Center

Bragg, Marie A.; Tucker, Carolyn M.; Kaye, Lily B.; Desmond, Frederic

2009-01-01

Background: Obesity rates are rising in the United States, especially among low-income and racial/ethnic minority individuals. Exploring motivators and barriers relative to engaging in physical activity is imperative. Purpose: The purpose of this study was to identify motivators and barriers relative to engagement in physical activity as reported…

The role of perceived barriers and objectively measured physical activity in adults aged 65-100.

PubMed

Gellert, Paul; Witham, Miles D; Crombie, Iain K; Donnan, Peter T; McMurdo, Marion E T; Sniehotta, Falko F

2015-05-01

to test the predictive utility of perceived barriers to objectively measured physical activity levels in a stratified sample of older adults when accounting for social-cognitive determinants proposed by the Theory of Planned Behaviour (TPB), and economic and demographic factors. data were analysed from the Physical Activity Cohort Scotland survey, a representative and stratified (65-80 and 80+ years; deprived and affluent) sample of 584 community-dwelling older people, resident in Tayside, Scotland. Physical activity was measured objectively by accelerometry. perceived barriers clustered around the areas of poor health, lack of interest, lack of safety and lack of access. Perceived poor health and lack of interest, but not lack of access or concerns about personal safety, predicted physical activity after controlling for demographic, economic and TPB variables. perceived person-related barriers (poor health and lack of interest) seem to be more strongly associated with physical activity levels than perceived environmental barriers (safety and access) in a large sample of older adults. Perceived barriers are modifiable and may be a target for future interventions. © The Author 2015. Published by Oxford University Press on behalf of the British Geriatrics Society. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Barriers to Physical Activity in a Mass Transit Population: A Qualitative Study.

PubMed

Das, Bhibha M; Petruzzello, Steven J

2016-01-01

The physical inactivity epidemic continues be one of the greatest public health challenges in contemporary society in the United States. The transportation industry is at greater risk of physical inactivity, compared with individuals in other sectors of the workforce. The aim of this study was to use the Nominal Group Technique, a focus group technique, to examine mass transit employees' perceptions of the barriers to physical activity at their worksite. Three focus groups (n = 31) were conducted to examine mass transit employees' perceptions of barriers to physical activity at the worksite. Salient barriers included (1) changing work schedules, (2) poor weather conditions, and (3) lack of scheduled and timely breaks. Findings were consistent with previous research demonstrating shift work, poor weather, and lack of breaks can negatively impact mass transit employees' ability to be physically active. Although physical activity barriers for this population have been consistent for the last 20 years, public health practice and policy have not changed to address these barriers. Future studies should include conducing focus groups stratified by job classification (eg, operators, maintenance, and clerical) along with implementing and evaluating worksite-based physical activity interventions and policy changes.

Exercise barriers in Korean colorectal cancer patients.

PubMed

Kang, Dong-Woo; Chung, Jae Youn; Lee, Mi Kyung; Lee, Junga; Park, Ji-Hye; Kim, Dong-Il; Jones, Lee W; Ahn, Joong Bae; Kim, Nam Kyu; Jeon, Justin Y

2014-01-01

To identify barriers to exercise in Korean colorectal cancer patients and survivors, and to analyze differences in exercise barriers by age, gender, treatment status, and physical activity level. A total of 427 colorectal cancer patients and survivors from different stages and medical status completed a self-administered questionnaire that surveyed their barriers to exercise and exercise participation. The greatest perceived exercise barriers for the sampled population as a whole were fatigue, low level of physical fitness, and poor health. Those under 60-years old reported lack of time (p=0.008), whereas those over 60 reported low level of physical fitness (p=0.014) as greater exercise barriers than their counterparts. Women reported fatigue as a greater barrier than men (p<0.001). Those who were receiving treatment rated poor health (p=0.0005) and cancer-related factors as greater exercise barriers compared to those who were not receiving treatment. A multivariate model found that other demographic and medical status were not potential factors that may affect exercise participation. Further, for those who were not participating in physical activity, tendency to be physically inactive (p<0.001) and lack of exercise skill (p<0.001) were highly significant barriers, compared to those who were participating in physical activity. Also, for those who were not meeting ACSM guidelines, cancer-related exercise barriers were additionally reported (p<0.001), compared to those who were. Our study suggests that fatigue, low level of physical fitness, and poor health are most reported exercise barriers for Korean colorectal cancer survivors and there are differences in exercise barriers by age, sex, treatment status, and physical activity level. Therefore, support for cancer patients should be provided considering these variables to increase exercise participation.

Linear ground-water flow, flood-wave response program for programmable calculators

USGS Publications Warehouse

Kernodle, John Michael

1978-01-01

Two programs are documented which solve a discretized analytical equation derived to determine head changes at a point in a one-dimensional ground-water flow system. The programs, written for programmable calculators, are in widely divergent but commonly encountered languages and serve to illustrate the adaptability of the linear model to use in situations where access to true computers is not possible or economical. The analytical method assumes a semi-infinite aquifer which is uniform in thickness and hydrologic characteristics, bounded on one side by an impermeable barrier and on the other parallel side by a fully penetrating stream in complete hydraulic connection with the aquifer. Ground-water heads may be calculated for points along a line which is perpendicular to the impermeable barrie and the fully penetrating stream. Head changes at the observation point are dependent on (1) the distance between that point and the impermeable barrier, (2) the distance between the line of stress (the stream) and the impermeable barrier, (3) aquifer diffusivity, (4) time, and (5) head changes along the line of stress. The primary application of the programs is to determine aquifer diffusivity by the flood-wave response technique. (Woodard-USGS)

Observation of millimeter-wave oscillations from resonant tunneling diodes and some theoretical considerations of ultimate frequency limits

NASA Technical Reports Server (NTRS)

Sollner, T. C. L. G.; Brown, E. R.; Goodhue, W. D.; Le, H. Q.

1987-01-01

Recent observations of oscillation frequencies up to 56 GHz in resonant tunneling structures are discussed in relation to calculations by several authors of the ultimate frequency limits of these devices. It is found that calculations relying on the Wentzel-Kramers-Brillouin (WKB) approximation give limits well below the observed oscillation frequencies. Two other techniques for calculating the upper frequency limit were found to give more reasonable results. One method employs the solution of the time-dependent Schroedinger equation obtained by Kundrotas and Dargys (1986); the other uses the energy width of the transmission function for electrons through the double-barrier structure. This last technique is believed to be the most accurate since it is based on general results for the lifetime of any resonant state. It gives frequency limits on the order of 1 THz for two recently fabricated structures. It appears that the primary limitation of the oscillation frequency for double-barrier resonant-tunneling diodes is imposed by intrinsic device circuit parameters and by the transit time of the depletion layer rather than by time delays encountered in the double-barrier region.

Performance of Density Functional Theory Procedures for the Calculation of Proton-Exchange Barriers: Unusual Behavior of M06-Type Functionals.

PubMed

Chan, Bun; Gilbert, Andrew T B; Gill, Peter M W; Radom, Leo

2014-09-09

We have examined the performance of a variety of density functional theory procedures for the calculation of complexation energies and proton-exchange barriers, with a focus on the Minnesota-class of functionals that are generally highly robust and generally show good accuracy. A curious observation is that M05-type and M06-type methods show an atypical decrease in calculated barriers with increasing proportion of Hartree-Fock exchange. To obtain a clearer picture of the performance of the underlying components of M05-type and M06-type functionals, we have investigated the combination of MPW-type and PBE-type exchange and B95-type and PBE-type correlation procedures. We find that, for the extensive E3 test set, the general performance of the various hybrid-DFT procedures improves in the following order: PBE1-B95 → PBE1-PBE → MPW1-PBE → PW6-B95. As M05-type and M06-type procedures are related to PBE1-B95, it would be of interest to formulate and examine the general performance of an alternative Minnesota DFT method related to PW6-B95.

Charge and energy transports via poly-phenylacetylene based dendrimers

NASA Astrophysics Data System (ADS)

Shin, Yongwoo; Li, Minghai; Lin, Xi

2010-03-01

Poly-Phenylacetylene (PPA) is widely used in photoconductivity, photoluminescence, and light harvesting applications. In this work, we investigate the charge and exciton transport energetics and mechanisms in the PPA-based dendrimers using our recently developed adapted Su-Schrieffer-Heeger (SSH) model Hamiltonians and ab initio Hartree-Fock (HF) calculations. We found both doping and photo-excitation lead to the formation of optical phonon dressed pi electron states, namely the self-localized polarons, in the energy gap. Independent from their origins, these polarons can be self-trapped at multiple lattice locations along the PPA chain, and migrate from one to the next with an activation barrier of ˜0.006 eV, slightly higher than the corresponding barrier found in trans-polyacetylene. The PPA-based dendrimers can be constructed via the meta-positions of phenyl rings. In this case, we found the dendrimer junctions form attractive potential wells for both polarons and excitons, and the width and height of these junction potential wells can be controlled by the geometry of the dendrimers.

A density functional study on adsorption and dissociation of O 2 on Ir(1 0 0) surface

NASA Astrophysics Data System (ADS)

Erikat, I. A.; Hamad, B. A.; Khalifeh, J. M.

2011-06-01

The adsorption and the reaction barrier for the dissociation of O 2 on Ir(1 0 0) surface are studied using periodic self-consistent density functional theory (DFT) calculations. Dissociative adsorption is found to be energetically more favorable compared to molecular adsorption. Parallel approaches Prl1 and Prl2 on a hollow site with the same adsorption energy of -3.93 eV for both of them are found to have the most energetically preferred sites of adsorptions among all the studied cases. Hybridization between p-O 2 and d-metal orbitals is responsible for the dissociative adsorption. The minimum energy path is determined by using the nudge elastic band method (NEB). We found that the dissociation occurs immediately and very early in the dissociation path with a small activation barrier (0.26 eV), which means that molecular adsorption of O 2 on Ir(1 0 0) surface occurs at very low temperatures; this is consistent with previous experimental and theoretical studies on Ir surfaces.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Yongli; Wang, Xianjie; Sui, Yu

Here in this article, we investigated the dielectric properties of (In + Nb) co-doped rutile TiO 2 single crystal and polycrystalline ceramics. Both of them showed colossal, up to 10 4, dielectric permittivity at room temperature. The single crystal sample showed one dielectric relaxation process with a large dielectric loss. The voltage-dependence of dielectric permittivity and the impedance spectrum suggest that the high dielectric permittivity of single crystal originated from the surface barrier layer capacitor (SBLC). The impedance spectroscopy at different temperature confirmed that the (In+Nb) co-doped rutile TiO 2 polycrystalline ceramic had semiconductor grains and insulating grain boundaries, andmore » that the activation energies were calculated to be 0.052 eV and 0.35 eV for grain and grain boundary, respectively. The dielectric behavior and impedance spectrum of the polycrystalline ceramic sample indicated that the internal barrier layer capacitor (IBLC) mode made a major contribution to the high ceramic dielectric permittivity, instead of the electron-pinned defect-dipoles.« less

The reaction mechanism with free energy barriers at constant potentials for the oxygen evolution reaction at the IrO 2 (110) surface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ping, Yuan; Nielsen, Robert J.; Goddard, William A.

How to efficiently oxidize H 2O to O 2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO 2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OERmore » at the IrO 2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. Furthermore, this allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.« less

Numerical investigation of the effect of driving voltage pulse shapes on the characteristics of low-pressure argon dielectric barrier discharge

DOE Office of Scientific and Technical Information (OSTI.GOV)

Eslami, E., E-mail: eeslami@iust.ac.ir; Barjasteh, A.; Morshedian, N.

2015-06-15

In this work, we numerically compare the effect of a sinusoidal, triangular, and rectangular pulsed voltage profile on the calculated particle production, electric current, and gas voltage in a dielectric barrier discharge. The total argon gas pressure of 400 Pa, the distance between dielectrics of 5 mm, the dielectric thickness of 0.7 mm, and the temperature of T = 300 K were considered as input parameters. The different driving voltage pulse shapes (triangular, rectangular, and sinusoidal) are considered as applied voltage with a frequency of 7 kHz and an amplitude of 700 V peak to peak. It is shown thatmore » applying a rectangular voltage, as compared with a sinusoidal or triangle voltage, increases the current peak, while the peak width is decreased. Higher current density is related to high production of charged particles, which leads to the generation of some highly active species, such as Ar* (4s level), and Ar** (4p level) in the gap.« less

Exploring the possibilities of two-dimensional transition metal carbides as anode materials for sodium batteries.

PubMed

Yang, Eunjeong; Ji, Hyunjun; Kim, Jaehoon; Kim, Heejin; Jung, Yousung

2015-02-21

Recently a group of two-dimensional materials called MXenes have been discovered and they have demonstrated their potential in Li rechargeable batteries. Herein, the Na storage and ion migration properties of M2C-type MXenes (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were investigated using density functional theory (DFT) calculations, and were compared to the Li case. Based on the average voltage and migration barrier of surface ions, we suggest that M = Ti, V, Cr, Mn, and Mo are suitable for sodium ion battery (SIB) anodes. These screened M2C materials can provide a theoretical capacity of 190-288 mA h g(-1) by accommodating two alkali ions per formula unit. They also exhibit an activation barrier of 0.1-0.2 eV for ionic motion, suggesting that the M2C materials are promising for high-power applications. The underlying aspects of the voltage differences between M2C materials are also discussed using electrostatic considerations.

The reaction mechanism with free energy barriers at constant potentials for the oxygen evolution reaction at the IrO 2 (110) surface

DOE PAGES

Ping, Yuan; Nielsen, Robert J.; Goddard, William A.

2016-12-09

How to efficiently oxidize H 2O to O 2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO 2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OERmore » at the IrO 2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. Furthermore, this allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.« less

Diffusion of hydrogen into and through γ-iron by density functional theory

NASA Astrophysics Data System (ADS)

Chohan, Urslaan K.; Koehler, Sven P. K.; Jimenez-Melero, Enrique

2018-06-01

This study is concerned with the early stages of hydrogen embrittlement on an atomistic scale. We employed density functional theory to investigate hydrogen diffusion through the (100), (110) and (111) surfaces of γ-Fe. The preferred adsorption sites and respective energies for hydrogen adsorption were established for each plane, as well as a minimum energy pathway for diffusion. The H atoms adsorb on the (100), (110) and (111) surfaces with energies of ∼4.06 eV, ∼3.92 eV and ∼4.05 eV, respectively. The barriers for bulk-like diffusion for the (100), (110) and (111) surfaces are ∼0.6 eV, ∼0.5 eV and ∼0.7 eV, respectively. We compared these calculated barriers with previously obtained experimental data in an Arrhenius plot, which indicates good agreement between experimentally measured and theoretically predicted activation energies. Texturing austenitic steels such that the (111) surfaces of grains are preferentially exposed at the cleavage planes may be a possibility to reduce hydrogen embrittlement.

Physical Activity With and Without TV Viewing: Effects on Enjoyment of Physical Activity and TV, Exercise Self-Efficacy, and Barriers to Being Active in Overweight Adults.

PubMed

Steeves, Jeremy A; Bassett, David R; Fitzhugh, Eugene C; Raynor, Hollie; Cho, Chi; Thompson, Dixie L

2016-04-01

Physical activity (PA) is enjoyable, but there are barriers to participation. TV viewing is highly enjoyable with limited barriers. Exercising while viewing TV may impact enjoyment, exercise self-efficacy, and barriers to PA, compared with exercising without TV. 58 sedentary, overweight adults were randomized to 1 of 2 PA prescriptions: one that increased PA during TV viewing (TV Commercial Stepping), and another that focused solely on PA (Walking). Random effects models tested changes in enjoyment of TV and PA, exercise self-efficacy, and barriers to PA across time (baseline, 3, and 6 months) and PA prescription during a 6-month PA intervention. At baseline, TV was more enjoyable than PA. Over the 6-month intervention, enjoyment of TV viewing did not change, but enjoyment of PA and exercise self-efficacy significantly increased, while barriers to PA significantly decreased for both groups compared with baseline (P < .05). While enjoyment of TV viewing remained constant, PA became more enjoyable, confidence to exercise increased, and barriers to being active were reduced for previously sedentary adults participating in a behavioral PA intervention. These findings highlight the importance of encouraging inactive adults to engage in some form of PA, whether it occurs with or without TV viewing.

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations.

PubMed

Barmak, Katayun; Liu, Jiaxing; Harlan, Liam; Xiao, Penghao; Duncan, Juliana; Henkelman, Graeme

2017-10-21

The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

Three-dimensional biomimetic vascular model reveals a RhoA, Rac1, and N-cadherin balance in mural cell-endothelial cell-regulated barrier function.

PubMed

Alimperti, Stella; Mirabella, Teodelinda; Bajaj, Varnica; Polacheck, William; Pirone, Dana M; Duffield, Jeremy; Eyckmans, Jeroen; Assoian, Richard K; Chen, Christopher S

2017-08-15

The integrity of the endothelial barrier between circulating blood and tissue is important for blood vessel function and, ultimately, for organ homeostasis. Here, we developed a vessel-on-a-chip with perfused endothelialized channels lined with human bone marrow stromal cells, which adopt a mural cell-like phenotype that recapitulates barrier function of the vasculature. In this model, barrier function is compromised upon exposure to inflammatory factors such as LPS, thrombin, and TNFα, as has been observed in vivo. Interestingly, we observed a rapid physical withdrawal of mural cells from the endothelium that was accompanied by an inhibition of endogenous Rac1 activity and increase in RhoA activity in the mural cells themselves upon inflammation. Using a system to chemically induce activity in exogenously expressed Rac1 or RhoA within minutes of stimulation, we demonstrated RhoA activation induced loss of mural cell coverage on the endothelium and reduced endothelial barrier function, and this effect was abrogated when Rac1 was simultaneously activated. We further showed that N -cadherin expression in mural cells plays a key role in barrier function, as CRISPR-mediated knockout of N -cadherin in the mural cells led to loss of barrier function, and overexpression of N -cadherin in CHO cells promoted barrier function. In summary, this bicellular model demonstrates the continuous and rapid modulation of adhesive interactions between endothelial and mural cells and its impact on vascular barrier function and highlights an in vitro platform to study the biology of perivascular-endothelial interactions.

A computational study of the catalytic aerobic epoxidation of propylene over the coordinatively unsaturated metal-organic framework Fe3(btc)2: formation of propylene oxide and competing reactions.

PubMed

Maihom, Thana; Sawangphruk, Montree; Probst, Michael; Limtrakul, Jumras

2018-02-28

The aerobic epoxidation of propylene over the metal-organic framework Fe 3 (btc) 2 (btc = 1,3,5-benzentricarboxylate) as catalyst has been investigated by means of density functional calculations. The mechanisms of the reaction towards propylene oxide, carbonylic products (acetone and propanal) and a pi-allyl radical were investigated to assess the efficiency of Fe 3 (btc) 2 for the selective formation of propylene oxide. Propylene oxide and carbonylic products are formed on Fe 3 (btc) 2 by proceeding via propyleneoxy intermediates in the first step. Subsequently, the intermediates can then either be transformed to propylene oxide by way of ring closure of the intermediate or to the carbonylic compounds of propanal and acetone via 1,2-hydride shift. The results show that the formation of propylene oxide is favored over the formation of carbonylic products mainly due to the activation barriers being 2-3 times smaller. The activation barriers for the formation of the propyleneoxy intermediates on the Fe 3 (btc) 2 catalyst for the first and second reaction cycle are also lower than the barriers obtained for the formation of the pi-allyl radical that acts as the precursor to combustion products. On the basis of these computational results, we therefore expect a high catalytic selectivity of the Fe 3 (btc) 2 catalyst with respect to the formation of propylene oxide. We also compared the catalytic activities of Fe 3 (btc) 2 and Cu 3 (btc) 2 . The activation energy of the rate-determining step is almost 2 times lower for Fe 3 (btc) 2 than that for Cu 3 (btc) 2 , due to a larger charge transfer from the catalytic site to the O 2 molecule in the case of Fe 3 (btc) 2 .

Reaction Mechanism for Direct Propylene Epoxidation by Alumina-Supported Silver Aggregates. The Role of the Particle / Support Interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheng, Lei; Yin, Chunrong; Mehmood, Faisal

2013-11-21

Sub-nanometer Ag aggregates on alumina supports have been found to be active toward direct propylene epoxidation to propylene oxide by molecular oxygen at low temperatures, with a negligible amount of carbon dioxide formation (Science 328, p. 224, 2010). In this work, we computationally and experimentally investigate the origin of the high reactivity of the sub-nanometer Ag aggregates. Computationally, we study O 2 dissociation and propylene epoxidation on unsupported Ag 19 and Ag 20 clusters, as well as alumina-supported Ag 19. The O 2 dissociation and propylene epoxidation apparent barriers at the interface between the Ag aggregate and the alumina supportmore » are calculated to be 0.2 and 0.2~0.4 eV, respectively. These barriers are somewhat lower than those on sites away from the interface. The mechanism at the interface is similar to what was previously found for the silver trimer on alumina and can account for the high activity observed for the direct oxidation of propylene on the Ag aggregates. The barriers for oxygen dissociation on these model systems both at the interface and on the surfaces are small compared to crystalline surfaces, indicating that availability of oxygen will not be a rate limiting step for the aggregates, as in the case of the crystalline surfaces. Experimentally, we investigate Ultrananocrystalline Diamond (UNCD)-supported silver aggregates under reactive conditions of propylene partial oxidation. The UNCD-supported Ag clusters are found to be not measurably active toward propylene oxidation, in contrast to the alumina supported Ag clusters. This suggests that the lack of metal-oxide interfacial sites of the Ag-UNCD catalyst, limits the epoxidation catalytic activity. This combined computational and experimental study shows the importance of the metal-oxide interface as well as the non-crystalline nature of the alumina-supported sub-nanometer Ag aggregate catalysts for propylene epoxidation.« less

Physical activity barriers and facilitators among working mothers and fathers

PubMed Central

2014-01-01

Background The transition to parenthood is consistently associated with declines in physical activity. In particular, working parents are at risk for inactivity, but research exploring physical activity barriers and facilitators in this population has been scarce. The purpose of this study was to qualitatively examine perceptions of physical activity among working parents. Methods Working mothers (n = 13) and fathers (n = 12) were recruited to participate in one of four focus group sessions and discuss physical activity barriers and facilitators. Data were analyzed using immersion/crystallization in NVivo 10. Results Major themes for barriers included family responsibilities, guilt, lack of support, scheduling constraints, and work. Major themes for facilitators included being active with children or during children’s activities, being a role model for children, making time/prioritizing, benefits to health and family, and having support available. Several gender differences emerged within each theme, but overall both mothers and fathers reported their priorities had shifted to focus on family after becoming parents, and those who were fitting in physical activity had developed strategies that allowed them to balance their household and occupational responsibilities. Conclusions The results of this study suggest working mothers and fathers report similar physical activity barriers and facilitators and would benefit from interventions that teach strategies for overcoming barriers and prioritizing physical activity amidst the demands of parenthood. Future interventions might consider targeting mothers and fathers in tandem to create an optimally supportive environment in the home. PMID:24974148

Thermal Analysis of a Carbon Fiber Rope Barrier for Use in the Reusable Solid Rocket Motor Nozzle Joint-2

NASA Technical Reports Server (NTRS)

Clayton, J. Louie

2002-01-01

This study provides development and verification of analysis methods used to assess performance of a carbon fiber rope (CFR) thermal barrier system that is currently being qualified for use in Reusable Solid Rocket Motor (RSRM) nozzle joint-2. Modeled geometry for flow calculations considers the joint to be vented with the porous CFR barriers placed in the 'open' assembly gap. Model development is based on a 1-D volume filling approach where flow resistances (assembly gap and CFRs) are defined by serially connected internal flow and the porous media 'Darcy' relationships. Combustion gas flow rates are computed using the volume filling code by assuming a lumped distribution total joint fill volume on a per linear circumferential inch basis. Gas compressibility, friction and heat transfer are included in the modeling. Gas-to-wall heat transfer is simulated by concurrent solution of the compressible flow equations and a large thermal 2-D finite element (FE) conduction grid. The derived numerical technique loosely couples the FE conduction matrix with the compressible gas flow equations. Free constants that appear in the governing equations are calibrated by parametric model comparison to hot fire subscale test results. The calibrated model is then used to make full-scale motor predictions using RSRM aft dome environments. Model results indicate that CFR thermal barrier systems will provide a thermally benign and controlled pressurization environment for the RSRM nozzle joint-2 primary seal activation.

Thermal Analysis of a Carbon Fiber Rope Barrier for Use in the Reusable Solid Rocket Motor Nozzle Joint-2

NASA Technical Reports Server (NTRS)

Clayton, J. Louie; Phelps, Lisa (Technical Monitor)

2001-01-01

This study provides for development and verification of analysis methods used to assess performance of a carbon fiber rope (CFR) thermal barrier system that is currently being qualified for use in Reusable Solid Rocket Motor (RSRM) nozzle joint-2. Modeled geometry for flow calculations considers the joint to be vented with the porous CFR barriers placed in the "open' assembly gap. Model development is based on a 1-D volume filling approach where flow resistances (assembly gap and CFRs) are defined by serially connected internal flow and the porous media "Darcy" relationships. Combustion gas flow rates are computed using the volume filling code by assuming a lumped distribution total joint fill volume on a per linear circumferential inch basis. Gas compressibility, friction and heat transfer are included in the modeling. Gas-to-wall heat transfer is simulated by concurrent solution of the compressible flow equations and a large thermal 2-D finite element (FE) conduction grid. The derived numerical technique loosely couples the FE conduction matrix with the compressible gas flow equations, Free constants that appear in the governing equations are calibrated by parametric model comparison to hot fire subscale test results. The calibrated model is then used to make full-scale motor predictions using RSRM aft dome environments. Model results indicate that CFR thermal barrier systems will provide a thermally benign and controlled pressurization environment for the RSRM nozzle joint-2 primary seal activation.

Adiabatic and coupled channels calculations for near barrier fusion of 16O +238U using realistic nucleon-nucleon interaction

NASA Astrophysics Data System (ADS)

Ismail, M.; Seif, W. M.; Botros, M. M.

2016-04-01

We investigate the fusion cross-section and the fusion barrier distribution of 16O +238U at near- and sub-barrier energies. We use an interaction potential generated by the semi-microscopic double folding model-based on density dependent (DD) form of the realistic Michigan-three-Yukawa (M3Y) Reid nucleon-nucleon (NN) interaction. We studied the role of both the static and dynamic deformations of the target nucleus on the fusion process. Rotational and vibrational degrees of freedom of 238U-nucleus are considered. We found that the deformation and the octupole vibrations in 238U enhance its sub-barrier fusion cross-section. The signature of the the octupole vibrational modes of 238U appears clearly in its fusion barrier distribution profile.

A calculation model to half-life estimate of two-proton radioactive decay process

NASA Astrophysics Data System (ADS)

Tavares, O. A. P.; Medeiros, E. L.

2018-04-01

Partial half-life of the radioactive decay by the two-proton emission mode has been estimated for proton-rich nuclei of mass number 18 < A < 68 by a model based on the quantum mechanical tunneling mechanism through a potential barrier. The Coulomb, centrifugal and overlapping contributions to the barrier have been considered within the spherical nucleus approximation. The present calculation method has been shown to be adequate in reproducing the existing experimental half-life data for 19Mg, 45Fe, 48Ni, and 54Zn 2p-emitter nuclides within a factor six. For 67Kr parent nucleus the calculated partial 2p-decay half-life has been found to be ten times greater than the recent, unique measured value at RIKEN Nishina Center. Prediction for new, yet unmeasured cases of two-proton radioactivity are also reported.

Parents' Perceived Barriers to Healthful Eating and Physical Activity for Low-Income Adolescents Who Are at Risk for Type 2 Diabetes

ERIC Educational Resources Information Center

Peterson, Sharon L.; Bell, Toya Wilson; Hasin, Afroza

2009-01-01

Healthful eating and regular physical activity are vitally important for low-income adolescents who are at risk for developing type 2 diabetes (T2DM). To design a relevant, community-based intervention for these at risk adolescents, parent perceptions of barriers to healthful eating and physical activity should be assessed. Such barriers have been…

Breakup of 8B on 58Ni at energies around the Coulomb barrier and the astrophysical S17(0) factor revisited

NASA Astrophysics Data System (ADS)

Morales-Rivera, J. C.; Belyaeva, T. L.; Amador-Valenzuela, P.; Aguilera, E. F.; Martinez-Quiroz, E.; Kolata, J. J.

2018-01-01

Calculations of breakup and direct proton transfer for the 8B+58Ni system at energies around the Coulomb barrier (EB,lab=22.95 MeV) were performed by the continuum-discretized coupled channels (CDCC) method and the coupled-reaction-channels (CRC) method, respectively. For the 7Be+58Ni interaction, we used a semimicroscopic optical model potential (OMP) that combines microscopic calculations of the mean-field double folding potential and a phenomenological construction of the dynamical polarization potential (DPP). The 7Be angular distribution at Elab=25.75 MeV from the 8B breakup on 58Ni was calculated and the spectroscopic factor for 8B → 7Be+p vertex, Sexpt = 1.10 ± 0.05, was deduced. The astrophysical S17(0) factor was calculated equal to 20.7 ±1.1 eV•b, being in good agreement with the previously reported values.

Ford Motor Company NDE facility shielding design.

PubMed

Metzger, Robert L; Van Riper, Kenneth A; Jones, Martin H

2005-01-01

Ford Motor Company proposed the construction of a large non-destructive evaluation laboratory for radiography of automotive power train components. The authors were commissioned to design the shielding and to survey the completed facility for compliance with radiation doses for occupationally and non-occupationally exposed personnel. The two X-ray sources are Varian Linatron 3000 accelerators operating at 9-11 MV. One performs computed tomography of automotive transmissions, while the other does real-time radiography of operating engines and transmissions. The shield thickness for the primary barrier and all secondary barriers were determined by point-kernel techniques. Point-kernel techniques did not work well for skyshine calculations and locations where multiple sources (e.g. tube head leakage and various scatter fields) impacted doses. Shielding for these areas was determined using transport calculations. A number of MCNP [Briesmeister, J. F. MCNPCA general Monte Carlo N-particle transport code version 4B. Los Alamos National Laboratory Manual (1997)] calculations focused on skyshine estimates and the office areas. Measurements on the operational facility confirmed the shielding calculations.

On the enhanced sampling over energy barriers in molecular dynamics simulations.

PubMed

Gao, Yi Qin; Yang, Lijiang

2006-09-21

We present here calculations of free energies of multidimensional systems using an efficient sampling method. The method uses a transformed potential energy surface, which allows an efficient sampling of both low and high energy spaces and accelerates transitions over barriers. It allows efficient sampling of the configuration space over and only over the desired energy range(s). It does not require predetermined or selected reaction coordinate(s). We apply this method to study the dynamics of slow barrier crossing processes in a disaccharide and a dipeptide system.

Coupled Heat and Moisture Transport Simulation on the Re-saturation of Engineered Clay Barrier

NASA Astrophysics Data System (ADS)

Huang, W. H.; Chuang, Y. F.

2014-12-01

Engineered clay barrier plays a major role for the isolation of radioactive wastes in a underground repository. This paper investigates the resaturation processes of clay barrier, with emphasis on the coupling effects of heat and moisture during the intrusion of groundwater to the repository. A reference bentonite and a locally available clay were adopted in the laboratory program. Soil suction of clay specimens was measured by psychrometers embedded in clay specimens and by vapor equilibrium technique conducted at varying temperatures so as to determine the soil water characteristic curves of the two clays at different temperatures. And water uptake tests were conducted on clay specimens compacted at various densities to simulate the intrusion of groundwater into the clay barrier. Using the soil water characteristic curve, an integration scheme was introduced to estimate the hydraulic conductivity of unsaturated clay. It was found that soil suction decreases as temperature increases, resulting in a reduction in water retention capability. The finite element method was then employed to carry out the numerical simulation of the saturation process in the near field of a repository. Results of the numerical simulation were validated using the degree of saturation profile obtained from the water uptake tests on the clays. The numerical scheme was then extended to establish a model simulating the resaturation process after the closure of a repository. Finally, the model was then used to evaluate the effect of clay barrier thickness on the time required for groundwater to penetrate the clay barrier and approach saturation. Due to the variation in clay suction and thermal conductivity with temperature of clay barrier material, the calculated temperature field shows a reduction as a result of incorporating the hydro-properties in the calculations.

Effect of a gap opening on the conductance of graphene with magnetic barrier structures

NASA Astrophysics Data System (ADS)

Esmailpour, Mohammad

2018-04-01

In the present study Klein tunneling in a single-layer gapped graphene was investigated by transfer matrix method under normal magnetic field for one and two magnetic barriers. Calculations show that electron transmission through a magnetic barrier is deflected to positive angles and reduces as the magnitude of magnetic field and especially the energy gap increases. This reduction is even more significant in larger fields so that after reaching a specific value of energy gap, an effective confinement for fermions and suppression of Klein tunneling is reached particularly in normal incidence and the conductance becomes zero. Unlike one barrier, the process of tunneling through two magnetic barriers induces symmetric transmission probability versus the incident angle; even, for lower energy gaps, electron transmission probability increases which in turn reduces total conductance via proper changes in the value of the magnetic field and energy gap. In general, it is concluded that confining electrons in asymmetric transmission through one barrier is conducted better than two barriers.

Systematics of capture and fusion dynamics in heavy-ion collisions

NASA Astrophysics Data System (ADS)

Wang, Bing; Wen, Kai; Zhao, Wei-Juan; Zhao, En-Guang; Zhou, Shan-Gui

2017-03-01

We perform a systematic study of capture excitation functions by using an empirical coupled-channel (ECC) model. In this model, a barrier distribution is used to take effectively into account the effects of couplings between the relative motion and intrinsic degrees of freedom. The shape of the barrier distribution is of an asymmetric Gaussian form. The effect of neutron transfer channels is also included in the barrier distribution. Based on the interaction potential between the projectile and the target, empirical formulas are proposed to determine the parameters of the barrier distribution. Theoretical estimates for barrier distributions and calculated capture cross sections together with experimental cross sections of 220 reaction systems with 182 ⩽ZPZT ⩽ 1640 are tabulated. The results show that the ECC model together with the empirical formulas for parameters of the barrier distribution work quite well in the energy region around the Coulomb barrier. This ECC model can provide prediction of capture cross sections for the synthesis of superheavy nuclei as well as valuable information on capture and fusion dynamics.

A prospective examination of exercise and barrier self-efficacy to engage in leisure-time physical activity during pregnancy.

PubMed

Cramp, Anita G; Bray, Steven R

2009-06-01

Pregnant women without medical contraindications should accumulate 30 min of moderate exercise on most days of the week, yet many pregnant women do not exercise at recommended levels. The purpose the study was to examine barriers to leisure-time physical activity (LTPA) and investigate barrier and exercise self-efficacy as predictors of self-reported LTPA during pregnancy. Pregnant women (n = 160) completed questionnaires eliciting barriers to LTPA, measures of exercise and barrier self-efficacy, and 6-week LTPA recall at gestational weeks 18, 24, 30, and 36. A total of 1,168 barriers were content-analyzed, yielding nine major themes including fatigue, time constraints, and physical limitations. Exercise self-efficacy predicted LTPA from gestational weeks 18 to 24 (beta = 0.32, R(2) = 0.26) and weeks 30 to 36 (beta = 0.41, R(2) = 0.37), while barrier self-efficacy predicted LTPA from weeks 24 to 30 (beta = 0.40, R(2) = 0.32). Pregnant women face numerous barriers to LTPA during pregnancy, the nature of which may change substantially over the course of pregnancy. Higher levels of self-efficacy to exercise and to overcome exercise barriers are associated with greater LTPA during pregnancy. Research and interventions to understand and promote LTPA during pregnancy should explore the dynamic nature of exercise barriers and foster women's confidence to overcome physical activity barriers.

Assessment of physical inactivity and perceived barriers to physical activity among health college students, south-western Saudi Arabia.

PubMed

Awadalla, N J; Aboelyazed, A E; Hassanein, M A; Khalil, S N; Aftab, R; Gaballa, I I; Mahfouz, A A

2014-10-20

Physical inactivity is a public health problem in Saudi Arabia. A cross-sectional study was carried out to evaluate the pattern of physical activity, predictors of physical inactivity and perceived barriers to physical activity among health college students in King Khalid University. A total of 1257 students (426 males and 831 females) were recruited. The Arabic short form of the International Physical Activity Questionnaire was used. Overall, 58.0% of the students were physically inactive. Only 13.4% of the students performed vigorous physical activity, 14.8% moderate-intensity physical activity and 29.9% walking activities which met World Health Organization criteria of health-enhancing physical activities. The prevalence of inactive leisure time was 47.5%. The independent predictors of physical inactivity were non-membership of sports clubs and being a medical student. The top reported barrier to physical activity among inactive students was time limitations (51.3%). Overcoming perceived barriers may increase physical activity among students.

Barriers to a healthy lifestyle among patients attending primary care clinics at a university hospital in Riyadh.

PubMed

AlQuaiz, Aljoharah M; Tayel, Salwa A

2009-01-01

The occurrence and progress of chronic non-communicable diseases (NCDs) is associated with unhealthy lifestyles and behaviors. Modification of barriers to healthy lifestyle can produce great benefits. The objective of this study was to identify barriers to physical activity and healthy eating among patients attending primary health care clinics in Riyadh city. A cross-sectional study was conducted at King Khalid University Hospital (KKUH) in Riyadh city. Four hundred and fifty participants attending primary health care clinics (PHCC) from 1 March to 30 April 2007 were randomly selected. A questionnaire about barriers to physical activity and healthy eating was adapted from the CDC web site. The prevalence of physical inactivity among the Saudi population in the study was 82.4% (371/450). Females were more physically inactive (87.6%, 268/306) compared to males (71.5%, 103/144) (P<.001). The most common barrier to physical activity was lack of resources (80.5%, 326/405), which was significantly higher among females than males and among the lower income versus the higher income group. The most common barrier to healthy diet was lack of willpower. More than four-fifths (80.3%, 354/441) of the study group stated that they did not have enough will to stick to a diet. Lack of resources was the most important barrier for physical activity, while lack of willpower and social support were both barriers for adherence to physical activity and a healthy diet.

Reaction mechanism of guanidinoacetate methyltransferase, concerted or step-wise

PubMed Central

Zhang, Xiaodong; Bruice, Thomas C.

2006-01-01

We describe a quantum mechanics/molecular mechanics investigation of the guanidinoacetate methyltransferase catalyzed reaction, which shows that proton transfer from guanidinoacetate (GAA) to Asp-134 and methyl transfer from S-adenosyl-l-methionine (AdoMet) to GAA are concerted. By self-consistent-charge density functional tight binding/molecular mechanics, the bond lengths in the concerted mechanism's transition state are 1.26 Å for both the OD1 (Asp-134)–HE (GAA) and HE (GAA)–NE (GAA) bonds, and 2.47 and 2.03 Å for the S8 (AdoMet)–C9 (AdoMet) and C9 (AdoMet)–NE (GAA) bonds, respectively. The potential-energy barrier (ΔE‡) determined by single-point B3LYP/6–31+G*//MM is 18.9 kcal/mol. The contributions of the entropy (−TΔS‡) and zero-point energy corrections Δ(ZPE)‡ by normal mode analysis are 2.3 kcal/mol and −1.7 kcal/mol, respectively. Thus, the activation enthalpy of this concerted mechanism is predicted to be ΔH‡ = ΔE‡ + Δ(ZPE)‡ = 17.2 kcal/mol. The calculated free-energy barrier for the concerted mechanism is ΔG‡ = 19.5 kcal/mol, which is in excellent agreement with the value of 19.0 kcal/mol calculated from the experimental rate constant (3.8 ± 0.2·min−1). PMID:17053070

FA(I):A(+) and FA(II):Cu(+) laser activity and photographic sensitization at the low coordinated surfaces of AgBr ab initio calculations.

PubMed

Shalabi, A S

2002-08-01

The twofold potentials of F(A)(I):Au(+) and F(A)(II)Cu(+) color centers at the low coordinated surfaces of AgBr thin films in providing tunable laser activity and photographic sensitization were investigated using ab initio methods of molecular electronic structure calculations. Clusters of variable size were embedded in simulated Coulomb fields that closely approximated the Madelung fields of the host surfaces, and the nearest neighbor ions to the F(A) defect site were allowed to relax to equilibrium in each case. Based on the calculated Stokes shifted optical transition bands and horizontal shifts along the configuration coordinate diagrams, both F(A)(I):Au(+) and F(A)(II):Cu(+) color centers were found to be laser active. The laser activity faded quickly as the bromide ion coordination decreased from 5 (flat) to 4 (edge) to 3 (corner) and as the size of the impurity cation increased from Cu(+) to Au(+). The latter relation was explainable in terms of the axial perturbation of the impurity cation. The smallest calculated Stokes-shift at the corner surface suggested that emission had the same oscillator strength as absorption. All relaxed excited states RESs of the defect containing surfaces were deep below the lower edges of the conduction bands of the defect free ground state surfaces, indicating that F(A)(I):Au(+) and F(A)(II):Cu(+) are suitable laser defects. The probability of orientational destruction of the two centers attributed to the assumed RES saddle point ion configurations along the <110> axis was found to be directly proportional to the size of the impurity cation, with activation energy barriers of about 0.655-3.294 eV for Cu(+), and about 1.887-3.404 eV for Au(+). The possibility of exciton (energy) transfer from the sites of higher coordination to those of lower coordination is demonstrated. The more laser active F(A)(II):Cu(+) center was more easily formed than the less laser active F(A)(I):Au(+) center. The Glasner-Tompkins empirical relation was generalized to include F(A) centers at the low coordinated surfaces of silver bromide thin film. As far as color photographic sensitization is concerned, the lowest unoccupied molecular orbitals of the selected dye molecules in the excited states were high enough for electron injection. F(A) defect formation and rotational diffusion of silver clusters reduced the energy gaps between the excited dye molecules and the lower edges of the conduction bands and allowed for hole injection. About 54-60% of the reduction of silver ions at the flat surface of AgBr was attributed to the host anions and F(A) defect formation, leaving about 40-46% for the reduction of photoelectrons as well as the electrons of the developer or dye molecules. The unrelaxed rotational diffusions of the central Ag(4) by 90 degrees decreased the latter percentage, but were severely hindered by activation energy barriers. Copyright 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1104-1120, 2002

Barriers to Physical Activity in East Harlem, New York

PubMed Central

Fox, Ashley M.; Mann, Devin M.; Ramos, Michelle A.; Kleinman, Lawrence C.; Horowitz, Carol R.

2012-01-01

Background. East Harlem is an epicenter of the intertwining epidemics of obesity and diabetes in New York. Physical activity is thought to prevent and control a number of chronic illnesses, including diabetes, both independently and through weight control. Using data from a survey collected on adult (age 18+) residents of East Harlem, this study evaluated whether perceptions of safety and community-identified barriers were associated with lower levels of physical activity in a diverse sample. Methods. We surveyed 300 adults in a 2-census tract area of East Harlem and took measurements of height and weight. Physical activity was measured in two ways: respondents were classified as having met the weekly recommended target of 2.5 hours of moderate physical activity (walking) per week (or not) and reporting having engaged in at least one recreational physical activity (or not). Perceived barriers were assessed through five items developed by a community advisory board and perceptions of neighborhood safety were measured through an adapted 7-item scale. Two multivariate logistic regression models with perceived barriers and concerns about neighborhood safety were modeled separately as predictors of engaging in recommended levels of exercise and recreational physical activity, controlling for respondent weight and sociodemographic characteristics. Results. The most commonly reported perceived barriers to physical activity identified by nearly half of the sample were being too tired or having little energy followed by pain with exertion and lack of time. Multivariate regression found that individuals who endorsed a greater number of perceived barriers were less likely to report having met their weekly recommended levels of physical activity and less likely to engage in recreational physical activity controlling for covariates. Concerns about neighborhood safety, though prevalent, were not associated with physical activity levels. Conclusions. Although safety concerns were prevalent in this low-income, minority community, it was individual barriers that correlated with lower physical activity levels. PMID:22848797

Large-scale field testing on flexible shallow landslide barriers

NASA Astrophysics Data System (ADS)

Bugnion, Louis; Volkwein, Axel; Wendeler, Corinna; Roth, Andrea

2010-05-01

Open shallow landslides occur regularly in a wide range of natural terrains. Generally, they are difficult to predict and result in damages to properties and disruption of transportation systems. In order to improve the knowledge about the physical process itself and to develop new protection measures, large-scale field experiments were conducted in Veltheim, Switzerland. Material was released down a 30° inclined test slope into a flexible barrier. The flow as well as the impact into the barrier was monitored using various measurement techniques. Laser devices recording flow heights, a special force plate measuring normal and shear basal forces as well as load cells for impact pressures were installed along the test slope. In addition, load cells were built in the support and retaining cables of the barrier to provide data for detailed back-calculation of load distribution during impact. For the last test series an additional guiding wall in flow direction on both sides of the barrier was installed to achieve higher impact pressures in the middle of the barrier. With these guiding walls the flow is not able to spread out before hitting the barrier. A special constructed release mechanism simulating the sudden failure of the slope was designed such that about 50 m3 of mixed earth and gravel saturated with water can be released in an instant. Analysis of cable forces combined with impact pressures and velocity measurements during a test series allow us now to develop a load model for the barrier design. First numerical simulations with the software tool FARO, originally developed for rockfall barriers and afterwards calibrated for debris flow impacts, lead already to structural improvements on barrier design. Decisive for the barrier design is the first dynamic impact pressure depending on the flow velocity and afterwards the hydrostatic pressure of the complete retained material behind the barrier. Therefore volume estimation of open shallow landslides by assessing the thickness of the failure layer and the width of the possible failure are essential for the required barrier design parameter height and width. First results of the calculated drag coefficients of dynamic impact pressure measurements showed that the dynamic coefficient cw is much lower than 1.0 which is contradictory to most of existing dimensioning property protection guidelines. It appears to us that special adaptation to the system like smaller mesh sizes and special ground-barrier interface compared to normal rock-fall barriers and channelised debris flow barriers are necessary to improve the retention behavior of shallow landslide barriers. Detailed analysis of the friction coefficient in relationship with pore water pressure measurements gives interesting insights into the dynamic of fluid-solid mixed flows. Impact pressures dependencies on flow features are analyzed and discussed with respect to existing models and guidelines for shallow landslides.

Barriers and facilitators to participation in workplace health promotion (WHP) activities: results from a cross-sectional survey of public-sector employees in Tasmania, Australia.

PubMed

Kilpatrick, Michelle; Blizzard, Leigh; Sanderson, Kristy; Teale, Brook; Jose, Kim; Venn, Alison

2017-12-01

Issue addressed Workplaces are promising settings for health promotion, yet employee participation in workplace health promotion (WHP) activities is often low or variable. This study explored facilitating factors and barriers associated with participation in WHP activities that formed part of a comprehensive WHP initiative run within the Tasmanian State Service (TSS) between 2009 and 2013. Methods TSS employee (n=3228) completed surveys in 2013. Data included sociodemographic characteristics, employee-perceived availability of WHP activities, employee-reported participation in WHP activities, and facilitators and barriers to participation. Ordinal log-link regression was used in cross-sectional analyses. Results Significant associations were found for all facilitating factors and participation. Respondents who felt their organisation placed a high priority on WHP, who believed that management supported participation or that the activities could improve their health were more likely to participate. Time- and health-related barriers were associated with participation in fewer activities. All associations were independent of age, sex, work schedule and employee-perceived availability of programs. Part-time and shift-work patterns, and location of activities were additionally identified barriers. Conclusion Facilitating factors relating to implementation, peer and environmental support, were associated with participation in more types of activities, time- and health-related barriers were associated with less participation. So what? Large and diverse organisations should ensure WHP efforts have manager support and adopt flexible approaches to maximise employee engagement.