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1

The rate constant for the reaction of O/3P/ atoms with ClONO2  

NASA Technical Reports Server (NTRS)

Experiments were conducted in a conventional fast flow apparatus to study the rate constant and the activation energy for the reaction O(3P) + ClONO2. The air-afterglow technique was used for the detection of oxygen atoms under pseudo first order conditions. A value of (3.4 plus or minus 0.6) x 10 to the -12th exp/(-840 plus or minus 60)T/cu cm/molecule/s was obtained for the rate constant. The observed rate is sufficiently slow so that oxygen atom attack is of minor importance in the stratosphere compared to photolysis.

Molina, L. T.; Spencer, J. E.; Molina, M. J.

1977-01-01

2

Absolute rate constant of the reaction between chlorine /2P/ atoms and hydrogen peroxide from 298 to 424 K  

NASA Technical Reports Server (NTRS)

The absolute rate constant of the reaction between chlorine (2P) atoms and hydrogen peroxide was determined from 298 to 424 K, using the discharge flow resonance fluorescence technique. Pseudo-first-order conditions were used with hydrogen peroxide in large excess. A fast flow-sampling procedure limited hydrogen peroxide decomposition to less than 5% over the temperature range studied. At 298 K, the rate constant is (4.1 plus or minus 0.2) x 10 to the minus 13th cu cm/molecule-sec.

Keyser, L. F.

1980-01-01

3

Reaction of limonene with F2: rate coefficient and products.  

PubMed

The kinetics of the reaction of limonene (C10H16) with F2 has been studied using a low pressure (P = 1 Torr) and a high pressure turbulent (P = 100 Torr) flow reactor coupled with an electron impact ionization and chemical ionization mass spectrometers, respectively: F2 + Limonene ? products (1). The rate constant of the title reaction was determined under pseudo-first-order conditions by monitoring either limonene or F2 decay in excess of F2 or C10H16, respectively. The reaction rate constant, k1 = (1.15 ± 0.25) × 10(-12) exp(160 ± 70)/T) was determined over the temperature range 278-360 K, independent of pressure between 1 (He) and 100 (N2) Torr. F atom and HF were found to be formed in reaction 1 , with the yields of 0.60 ± 0.13 and 0.39 ± 0.09, respectively, independent of temperature in the range 296-355 K. PMID:25327409

Bedjanian, Yuri; Romanias, Manolis N; Morin, Julien

2014-11-01

4

Temperature dependence of the rate constants for reactions of O/1D/ atoms with a number of halocarbons  

NASA Technical Reports Server (NTRS)

Results are reported for an experimental study of the reactions of O(1D) with CCl4, CFCl3, CF2Cl2, CHFCl2, CHF2Cl, and CF2ClCFCl2 over the temperature range from 173 to 343 K, based on the time-resolved emission of O(1D) at 630 nm. The experiments involved photolysis of O3 by Nd-YAG laser pulses and measurements of 630-nm emission intensity in mixtures with He and the various halocarbons. Pseudo-first-order rate constants are derived from plots of the logarithms of the 630-nm emission intensities as a function of time for particular He/O3/halocarbon mixtures, and second-order rate constants are then obtained from plots of these pseudo-first-order rate constants as a function of halocarbon concentration at constant He and O3 concentrations. No temperature dependence of the second-order rate constants is observed over the range studied, within the precision of the measurements. It is found that the rate constants decrease as H is substituted for Cl in a halocarbon molecule and as F is substituted for H or Cl.

Davidson, J. A.; Schiff, H. I.; Brown, T. J.; Howard, C. J.

1978-01-01

5

Reaction rate constants of H-abstraction by OH from large ketones: measurements and site-specific rate rules.  

PubMed

Reaction rate constants of the reaction of four large ketones with hydroxyl (OH) are investigated behind reflected shock waves using OH laser absorption. The studied ketones are isomers of hexanone and include 2-hexanone, 3-hexanone, 3-methyl-2-pentanone, and 4-methl-2-pentanone. Rate constants are measured under pseudo-first-order kinetics at temperatures ranging from 866 K to 1375 K and pressures near 1.5 atm. The reported high-temperature rate constant measurements are the first direct measurements for these ketones under combustion-relevant conditions. The effects of the position of the carbonyl group (C=O) and methyl (CH3) branching on the overall rate constant with OH are examined. Using previously published data, rate constant expressions covering, low-to-high temperatures, are developed for acetone, 2-butanone, 3-pentanone, and the hexanone isomers studied here. These Arrhenius expressions are used to devise rate rules for H-abstraction from various sites. Specifically, the current scheme is applied with good success to H-abstraction by OH from a series of n-ketones. Finally, general expressions for primary and secondary site-specific H-abstraction by OH from ketones are proposed as follows (the subscript numbers indicate the number of carbon atoms bonded to the next-nearest-neighbor carbon atom, the subscript CO indicates that the abstraction is from a site next to the carbonyl group (C=O), and the prime is used to differentiate different neighboring environments of a methylene group): PMID:24817270

Badra, Jihad; Elwardany, Ahmed E; Farooq, Aamir

2014-06-28

6

Nucleophilic substitution at centers other than carbon. 2. Reaction at the chlorine of N-chloroacetanilide with substituted phenoxides  

SciTech Connect

All reactions were carried out in buffered 10% aqueous acetonitrile brought to an ionic strength of 1.350 M with NaClO/sub 4/ at 39.36/sup 0/C. A series of meta- and para-substituted phenols (H, p-CH/sub 3/, m-Cl, m-NO/sub 2/, and p-NO/sub 2/) were allowed to react with N-chloroacetanilide under pseudo-first-order conditions with the phenol in an 11- to 16-fold excess. The pseudo-first-order rate constants, k/sub obsd/, were obtained by monitoring the disappearance of the N-chloroacetanilide as a function of time.

Dietze, P.E.; Underwood, G.R.

1984-06-29

7

The Rate Constant for the Reaction H + C2H5 at T = 295 - 150K  

NASA Technical Reports Server (NTRS)

The reaction between the hydrogen atom and the ethyl (C2H3) radical is predicted by photochemical modeling to be the most important loss process for C2H5 radicals in the atmospheres of Jupiter and Saturn. This reaction is also one of the major sources for the methyl radicals in these atmospheres. These two simplest hydrocarbon radicals are the initial species for the synthesis of larger hydrocarbons. Previous measurements of the rate constant for the H + C2H5 reaction varied by a factor of five at room temperature, and some studies showed a dependence upon temperature while others showed no such dependence. In addition, the previous studies were at higher temperatures and generally higher pressures than that needed for use in planetary atmospheric models. The rate constant for the reaction H + C2H5 has been measured directly at T = 150, 202 and 295 K and at P = 1.0 Torr He for all temperatures and additionally at P = 0.5 and 2.0 Torr He at T = 202 K. The measurements were performed in a discharge - fast flow system. The decay of the C2H5 radical in the presence of excess hydrogen was monitored by low-energy electron impact mass spectrometry under pseudo-first order conditions. H atoms and C2H5 radicals were generated rapidly and simultaneously by the reaction of fluorine atoms with H2 and C2H6, respectively. The total rate constant was found to be temperature and pressure independent. The measured total rate constant at each temperature are: k(sub 1)(295K) = (1.02+/-0.24)x10(exp -10), k(sub 1)(202K) = (1.02+/-0.22)x10(exp -10) and k(sub 1)(150K) = (0.93+/-0.21)x10(exp -10), all in units of cu cm/molecule/s. The total rate constant derived from all the combined measurements is k(sub 1) = (l.03+/-0.17)x10(exp -10) cu cm/molecule/s. At room temperature our results are about a factor of two higher than the recommended rate constant and a factor of three lower than the most recently published study.

Pimentel, Andre S.; Payne, Walter A.; Nesbitt, Fred L.; Cody, Regina J.; Stief, Louis J.

2004-01-01

8

The gas-phase reactions of O( 3P) with perhalogenated butenes: Absolute rate constants and temperature dependences  

NASA Astrophysics Data System (ADS)

The temperature dependences of the gas-phase reactions of O( 3P) atoms with CF 3CF dbnd CFCF 3, CF 3CF dbnd CClCF 3 and CF 3CCl dbnd CClCF 3 were studied under pseudo-first-order conditions in the range 298-359 K, using the discharge flow-chemiluminescence technique. The Arrhenius expressions obtained (in units of cm 3 molecule -1 s -1) were: k1 = (0.32 ± 0.05) × 10 -12 exp[-(8.1 ± 0.5) × 10 3/ RT], k2 = (1.50 ± 0.74) × 10 -11 exp[-(17.7 ± 1.4) × 10 3/ RT] and k3 = (2.58 ± 1.10) × 10 -11 exp[-(18.0 ± 1.2) × 10 3/ RT], with Ea in J mol -1. The positive temperature dependences point to the importance of the irreversibility of the addition channel. Complete substitution by halogen atoms in the alkenes results in dramatic reduction of the rate constants. The reactivity trend with the ionization potential is also analyzed.

Cometto, Pablo M.; Taccone, Raúl A.; Lane, Silvia I.

2007-07-01

9

Reaction Rates, Catalysis, and Pasteurization  

NSDL National Science Digital Library

In this activity, by the Concord Consortium's Molecular Literacy project, students are introduced to "key ideas of chemistry, including reaction rates, bond strength, activation energy, catalysis, and equilibrium." Upon completion of this activity students should be able to describe and manipulate models to discover how reaction rates, energy, bond strength, catalysis, and equilibrium are related; manipulate reaction rates by changing temperature and concentration; demonstrate their understanding by creating an endothermic reaction that releases a lot of heat energy, one that requires a catalyst, and one that reaches equilibrium between reactants and products. The activity itself is a java-based interactive resource built upon the free, open source Molecular Workbench software. In the activity, students are allowed to explore at their own pace in a digital environment full of demonstrations, illustrations, and models they can manipulate. In addition to the activity, visitors will find an overview of the activity, a test and rubric, central concepts, and their correlation to AAAS standards.

2008-10-23

10

CF3CF=CH2 and CF3CF=CHF: Temperature Dependent OH Rate Coefficients and Global Warming Potentials  

Microsoft Academic Search

Rate coefficients, k(T), over the temperature range 206-380 K are reported for the gas phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF3CF=CH2) and 1,2,3,3,3-pentafluoropropene (CF3CF=CHF), which are major components in proposed substitutes for HFC134a (CF3CFH2) in mobile air conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH and laser induced fluorescence

J. B. Burkholder; V. C. Papadimitriou; R. K. Talukdar; R. W. Portmann; A. R. Ravishankara

2007-01-01

11

Method of controlling fusion reaction rates  

DOEpatents

A method of controlling the reaction rates of the fuel atoms in a fusion reactor comprises the step of polarizing the nuclei of the fuel atoms in a particular direction relative to the plasma confining magnetic field. Fusion reaction rates can be increased or decreased, and the direction of emission of the reaction products can be controlled, depending on the choice of polarization direction.

Kulsrud, Russell M. (Princeton, NJ); Furth, Harold P. (Princeton, NJ); Valeo, Ernest J. (Princeton Junction, NJ); Goldhaber, Maurice (Bayport, NY)

1988-01-01

12

Factors influencing the mechanism of surfactant catalyzed reaction of vitamin C-ferric chloride hexahydrate system  

NASA Astrophysics Data System (ADS)

The kinetics of vitamin C by ferric chloride hexahydrate has been investigated in the aqueous ethanol solution of basic surfactant viz. octadecylamine (ODA) under pseudo-first order conditions. The critical micelle concentration (CMC) of surfactant was determined by surface tension measurement. The effect of pH (2.5-4.5) and temperature (15-35°C) in the presence and absence of surfactant were investigated. Activation parameters, ? E a, ? H #, ? S #, ? G ?, for the reaction were calculated by using Arrhenius and Eyring plot. Surface excess concentration (?max), minimum area per surfactant molecule ( A min), average area occupied by each molecule of surfactant ( a), surface pressure at the CMC (?max), Gibb's energy of micellization (? G M°), Gibb's energy of adsorption (? G ad°), were calculated. It was found that the reaction in the presence of surfactant showed faster oxidation rate than the aqueous ethanol solution. Reaction mechanism has been deduced in the presence and absence of surfactant.

Farrukh, Muhammad Akhyar; Kauser, Robina; Adnan, Rohana

2013-09-01

13

PhET Simulation: Reactions & Rates  

NSDL National Science Digital Library

This applet is designed for study of the kinetics of chemical reactions. It uses a model gas of three atomic species to illustrate the formation and disassociation of the different possible diatomic molecules. The user can run experiments with either one of each atom type or a gas of many atoms, controlling the density of the various species, the temperature, and energetics of the reaction. Data collection tools are available to calculate rate coefficients. Tips for teachers, ideas and activities are included. The direct link to the simulation is given as a mirror URL. This is part of a larger collection developed by the Physics Education Technology project (PhET).

2008-11-03

14

Kinetic study of the reaction of the acetyl radical, CH 3CO, with O 3 using cavity ring-down spectroscopy  

NASA Astrophysics Data System (ADS)

Rate coefficients for the gas-phase reaction CH 3CO + O 3 ? products (1) were measured between 248 and 403 K at 60-200 Torr (N 2). Rate coefficients were measured under pseudo-first-order conditions in CH 3CO with CH 3CO produced by 248 nm pulsed laser photolysis of acetone and monitored using cavity ring-down spectroscopy at 532 nm. Rate coefficients were found to be pressure independent with a weak negative temperature dependence k1(T)=1.50-0.30+0.25×10-11exp[(300±50)/T]cmmolecules and k1(296 K) = (4.4 ± 0.5) × 10 -11 cm 3 molecule -1 s -1 where the quoted uncertainties are 2 ? and include estimated systematic errors.

Gierczak, Tomasz; Rajakumar, B.; Flad, Jonathan E.; Burkholder, James B.

2010-01-01

15

Nuclear reaction rates and the nova outburst  

SciTech Connect

In this paper we examined the consequences of improving the nuclear reaction library on our simulations of TNRs on 1.25M, WD and 1.35M, WDS. We have found that the changes in the rates have affected the nucleosynthesis predictions of our calculations but not, to any great extent, the gross features. In addition, we have used a lower mass accretion rate than in our previous studies in order to accrete (and eject) more material. This has, as expected, caused the peak values of some important parameters to increase over our previous studies at the same WD mass. However, because some important reaction rates have declined in the new compilation this has not increased the abundances for nuclei above aluminum and, in fact, they have declined while the abundances of both {sup 26}Al and {sup 27}Al have increased at both WD masses. In contrast, the abundance of {sup 22}Na has declined at both WD masses over the values predicted in our earlier work. This has important implications with respect to predictions of the observability of novae with INTEGRAL.

Starrfield, S.G.; Iliadis, C. [and others

2000-10-01

16

Fusion Reaction Rate in an Inhomogeneous Plasma  

SciTech Connect

The local fusion rate, obtained from the assumption that the distribution is a local Maxwellian, is inaccurate if mean-free-paths of fusing particles are not sufficiently small compared with the inhomogeneity length of the plasma. We calculate the first order correction of P0 in terms of the small spatial gradient and obtain a non-local modification of P(sub)0 in a shock region when the gradient is not small. Use is made of the fact that the fusion reaction cross section has a relatively sharp peak as a function of energy.

S. Son; N.J. Fisch

2004-09-03

17

Conjugate mass transfer to a spherical drop accompanied by a second-order chemical reaction inside the drop  

Microsoft Academic Search

Conjugate mass transfer between a drop and a surrounding fluid flow with second-order (inclusive the particular case – pseudo-first-order), irreversible chemical reaction in the dispersed phase has been analyzed. The dispersed phase reactant is insoluble in the continuous phase and its complete depletion is allowed. Two sphere models were considered: the rigid sphere and the fluid sphere with internal circulation.

Gh Juncu

2002-01-01

18

OH + (E)- and (Z)-1-Chloro-3,3,3-trifluoropropene-1 (CF3CH?CHCl) Reaction Rate Coefficients: Stereoisomer-Dependent Reactivity.  

PubMed

Rate coefficients for the gas-phase reaction of the OH radical with (E)- and (Z)-CF3CH?CHCl (1-chloro-3,3,3-trifluoropropene-1, HFO-1233zd) (k1(T) and k2(T), respectively) were measured under pseudo-first-order conditions in OH over the temperature range 213-376 K. OH was produced by pulsed laser photolysis, and its temporal profile was measured using laser-induced fluorescence. The obtained rate coefficients were independent of pressure between 25 and 100 Torr (He, N2) with k1(296 K) = (3.76 ± 0.35) × 10(-13) cm(3) molecule(-1) s(-1) and k2(296 K) = (9.46 ± 0.85) × 10(-13) cm(3) molecule(-1) s(-1) (quoted uncertainties are 2? and include estimated systematic errors). k2(T) showed a weak non-Arrhenius behavior over this temperature range. The (E)- and (Z)- stereoisomer rate coefficients were found to have opposite temperature dependencies that are well represented by k1(T) = (1.14 ± 0.15) × 10(-12) exp[(-330 ± 10)/T] cm(3) molecule(-1) s(-1) and k2(T) = (7.22 ± 0.65) × 10(-19) × T(2) × exp[(800 ± 20)/T] cm(3) molecule(-1) s(-1). The present results are compared with a previous room temperature relative rate coefficient study of k1, and an explanation for the discrepancy is presented. CF3CHO, HC(O)Cl, and CF3CClO, were observed as stable end-products following the OH radical initiated degradation of (E)- and (Z)-CF3CH?CHCl in the presence of O2. In addition, chemically activated isomerization was also observed. Atmospheric local lifetimes of (E)- and (Z)-CF3CH?CHCl, due to OH reactive loss, were estimated to be ?34 and ?11 days, respectively. Infrared absorption spectra measured in this work were used to estimate radiative efficiencies and well-mixed global warming potentials of ?10 and ?3 for (E)- and (Z)-CF3CH?CHCl, respectively, on the 100-year time horizon. PMID:25333607

Gierczak, Tomasz; Baasandorj, M; Burkholder, James B

2014-11-20

19

Representing Rate Equations for Enzyme-Catalyzed Reactions  

ERIC Educational Resources Information Center

Rate equations for enzyme-catalyzed reactions are derived and presented in a way that makes it easier for the nonspecialist to see how the rate of an enzyme-catalyzed reaction depends upon kinetic constants and concentrations. This is done with distribution equations that show how the rate of the reaction depends upon the relative quantities of…

Ault, Addison

2011-01-01

20

INITIAL TEST OF THE BENCHMARK CHEMICAL APPROACH FOR PREDICTING MICROBIAL TRANSFORMATION RATES IN AQUATIC ENVIRONMENTS  

EPA Science Inventory

Using 2,4-dichlorophenoxyacetic acid methyl ester (2,4-DME) as a benchmark chemical, we determined relative pseudo-first-order rate coefficients for butoxyethyl ester of 2,4-dichlorophenoxyacetic acid (2,4-DBE), methyl parathion, and methyl-3-chlorobenzoate in a diversity of micr...

21

Temporal Scaling of Biogeochemical Reaction Rates  

NASA Astrophysics Data System (ADS)

In at least two disparate areas of organic and inorganic geochemistry---the microbial degradation of detritus and the dissolution of minerals in sediments and soils---apparent rate constants k have been observed to diminish with the "age" t of the substrate like k(t) ~eq a t-b, where a ~eq 0.2 and b ~eq 1.0. Published reports display up to ten orders of magnitude in time [1,2]. Because the accuracy of biogeochemical models typically depends crucially on the specification of such rates, an understanding of this scaling law has important implications for predicting the evolution of biogeochemical cycles, especially the cycles of carbon and oxygen. The power-law decay of rates likely derives from a combination of chemical and physical heterogeneity. In a purely chemical scenario, an intrinsically heterogeneous substrate (e.g., a mixture of organic matter ranging from "labile" to "recalcitrant") is assumed to produce the observed slowdown of k(t). In contrast, a physical model assumes a homogeneous substrate in which rates nevertheless vary microscopically due to spatially varying physical constraints. Here we consider the extreme case of a purely physical origin and test its consistency with observations [3]. We first show how a diffusion-limited reaction-diffusion system leads to a logarithmic decay of the substrate. We then show how the power-law for k(t) derives from this logarithmic decay. We obtain not only the correct exponent b=1 but also a good approximation of the prefactor a. By constructing an extensive database of previously published measurements, we show that observations compare well to predictions. The particular way in which diffusion-limitation manifests itself varies from problem to problem. In the case of detrital decay in sediments, we suggest that rates are limited by contact of substrate with extracellular enzymes [3]. Mechanisms in soils are likely similar. For mineral dissolution is sediments, we suggest that rates are limited by diffusion of reactants from the seafloor. [1] J.~J.~Middelburg, Geochim.~Cosmochim.~Acta 53, 1577 (1989). [2] K.~Maher, D.~J.~DePaolo, J.~C.-F.~Lin, Geochim.~Cosmochim.~Acta 68, 4629. [3] D.~H.~Rothman and D.~C.~Forney, Science 316, 1325 (2004).

Rothman, D. H.; Forney, D. C.

2007-12-01

22

Gas-phase rate coefficients for the OH + n-, i-, s-, and t-butanol reactions measured between 220 and 380 K: non-Arrhenius behavior and site-specific reactivity.  

PubMed

Butanol (C4H9OH) is a potential biofuel alternative in fossil fuel gasoline and diesel formulations. The usage of butanol would necessarily lead to direct emissions into the atmosphere; thus, an understanding of its atmospheric processing and environmental impact is desired. Reaction with the OH radical is expected to be the predominant atmospheric removal process for the four aliphatic isomers of butanol. In this work, rate coefficients, k, for the gas-phase reaction of the n-, i-, s-, and t-butanol isomers with the OH radical were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to monitor its temporal profile. Rate coefficients were measured over the temperature range 221-381 K at total pressures between 50 and 200 Torr (He). The reactions exhibited non-Arrhenius behavior over this temperature range and no dependence on total pressure with k(296 K) values of (9.68 ± 0.75), (9.72 ± 0.72), (8.88 ± 0.69), and (1.04 ± 0.08) (in units of 10(-12) cm(3) molecule(-1) s(-1)) for n-, i-, s-, and t-butanol, respectively. The quoted uncertainties are at the 2? level and include estimated systematic errors. The observed non-Arrhenius behavior is interpreted here to result from a competition between the available H-atom abstraction reactive sites, which have different activation energies and pre-exponential factors. The present results are compared with results from previous kinetic studies, structure-activity relationships (SARs), and theoretical calculations and the discrepancies are discussed. Results from this work were combined with available high temperature (1200-1800 K) rate coefficient data and room temperature reaction end-product yields, where available, to derive a self-consistent site-specific set of reaction rate coefficients of the form AT(n) exp(-E/RT) for use in atmospheric and combustion chemistry modeling. PMID:23627621

McGillen, Max R; Baasandorj, Munkhbayar; Burkholder, James B

2013-06-01

23

Kinetic studies of inverse electron demand Diels–Alder reactions (iEDDA) of norbornenes and 3,6-dipyridin-2-yl-1,2,4,5-tetrazine  

PubMed Central

Inverse electron demand Diels–Alder additions (iEDDA) between 1,2,4,5-tetrazines and olefins have recently found widespread application as a novel ‘click chemistry’ scheme and as a mild technique for the modification of materials. Norbornenes are, due to their straightforward synthetic availability, especially interesting in the latter context. Therefore, the reactivity of different norbornene-based compounds was compared with unsubstituted norbornene and other alkenes using UV-vis measurements for the determination of reaction rates under pseudo first order conditions. Thereby, exo,exo-5-norbornene-2,3-dimethanol was found to be almost as reactive as unsubstituted norbornene whereas (±)-endo,exo-dimethyl-5-norbornene-2,3-dicarboxylate reacted only insignificanty faster than unstrained alkenes. PMID:25152544

Knall, Astrid-Caroline; Hollauf, Manuel; Slugovc, Christian

2014-01-01

24

Kinetic study of Nafion degradation by Fenton reaction  

NASA Astrophysics Data System (ADS)

To predict the durability of polymer electrolyte membranes in fuel cells, the degradation reactions of Nafion 117 films were studied as oxidation reactions with hydroxyl radicals as oxidation accelerators. The radical species were generated by the Fenton reaction between hydrogen peroxide (H2O2) and iron ions (Fe2+). The Nafion degradation kinetics were estimated by fluorine ion (F-) generation. The H2O2 and Nafion degradation reactions fit a pseudo-first-order rate constant. The values of the activation energy and frequency factor are 85 kJ mol-1 and 3.97 × 108 s-1 for H2O2 decomposition in the presence of a Nafion film and 97 kJ mol-1 and 9.88 × 108 s-1 for F- generation. The Nafion surface morphology became rough after reaction for 12 h; small cracks, approximately 100 ?m in length, were observed at temperatures below 60 °C. These cracks connected to make larger gaps of approximately 1 mm at temperatures above 70 °C. We also found a linear relationship between H2O2 consumption and F- generation. The rate constant is temperature dependent and expressed as ln(d[F-]/d[decomposed H2O2]) = -19.5 × 103 K-1 + 42.8. F- generated and H2O2 consumed along with the Nafion degradation conditions can be predicted using this relation.

Sugawara, Tomoko; Kawashima, Norimichi; Murakami, Takurou N.

2011-03-01

25

Let the microbes explicitly carry out the reactions: a new paradigm to the modeling of soil carbon-nutrient dynamics (Invited)  

NASA Astrophysics Data System (ADS)

Soil carbon-nutrient dynamics are carried out by a broad spectrum of microbial functional guilds through a network of different reactions, all impacted by abiotic factors, including temperature, moisture, pH, redox, and mineralogy. Few existing soil biogeochemical models explicitly represent microbes and the related biotic and abiotic chemical reactions. Rather, these models usually assume microbes are perfectly adaptive such that their ecological impacts on carbon and nutrient dynamics can be sufficiently described by pseudo-first order decay parameters. These decay parameters are empirically formulated as functions of organic matter composition (e.g., lignocellulos index, CN ratio), soil temperature, and soil moisture that are obtained by ad hoc regression fitting to experimental and observational measurements. These pseudo-first order decay models claim predictability at large spatial and long temporal scales; however, their lack of explicit microbial ecology and microbe relevant biogeochemical and biogeophysical interactions have limited these models' capability to mechanistically explain some fundamental patterns, such as the divergent temperature response of decomposition, episodic emissions in drying-rewetting cycles, and predator-prey effects on organic matter decomposition. We here propose a new modeling paradigm that explicitly considers microbial ecology and reactions in modeling soil carbon-nutrient dynamics. We show this approach requires a new formulation of substrate kinetics to properly account for the intrinsic network structure of the biogeochemical processes and that the classical Michaelis-Menten kinetics is of limited use for this purpose. With a simple model based on this new paradigm, we mechanistically explained lignin dynamics in litter decomposition experiments without the ad hoc use of a lignin shielding effect. We further show, using existing experimental results, that decomposition dynamics are co-shaped by litter chemistry and associated microbial ecology. We also show that interactions between soil mineralogy and organic substrates can significantly reduce the microbial decomposition rate. Finally, we will discuss a few open questions and the data necessary to make such a model structure feasible for practical applications.

Tang, J.; Riley, W. J.; Bouskill, N.; Brodie, E.

2013-12-01

26

Typewriting rate as a function of reaction time.  

PubMed

This study was designed to determine the relationship between reaction time and typewriting rate. Subjects were 24 typists ranging in age from 19 to 39 yr. Reaction times (.001 sec) to a light were recorded for each finger and to each alphabetic character and three punctuation marks. Analysis of variance yielded significant differences in reaction time among subjects and fingers. Correlation between typewriting rate and average reaction time to the alphabetic characters and three punctuation marks was --.75. Correlation between typewriting rate and the difference between the reaction time of the hands was --.42. Factors influencing typewriting rate may include reaction time of the fingers, difference between the reaction time of the hands, and reaction time to individual keys on the typewriter. Implications exist for instructional methodology and further research. PMID:604897

Hayes, V; Wilson, G D; Schafer, R L

1977-12-01

27

Dynamic Phase Transition, Enhanced Reaction Rate, and  

E-print Network

reactions, biological populations, growth- deposition, economics, etc. Catalytic processes have many-reaction model that describes kinetics aspects of the gas- phase catalytic oxidation of carbon monoxide arriving to the surface is CO. Proportional to partial pressure of CO. #12;Two kinetic phase transitions

Rikvold, Per Arne

28

Reaction of Tris(2-chloroethyl)phosphate with Reduced Sulfur Species  

PubMed Central

Tris(2-chloroethyl)phosphates (TCEP) is a widely used flame retardant in the U.S. It has recently been identified as one of the most frequently detected contaminants in U.S. streams. This contaminant is of toxicological concern in sensitive coastal ecosystems such as estuaries and salt marshes. It is likely that reactions with reduced sulfur species such as polysulfides (Sn2?), bisulfide (HS?), and thiophenolate (PhS?) present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such a contaminant. The kinetics of reaction of reduced sulfur species with tris(2-chloroethyl)phosphate have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain the second-order rate constants from the observed pseudo-first order rate constants. The determined second-order rate constant for the reaction of TCEP with polysulfide at 25°C is 5.0 (± 1.4) × 10?4 M?1 s?1, with thiophenolate at 50 °C is 34 (± 2) × 10?4 M?1 s?1 and with bisulfide at 50 °C is 0.9 × 10?4 M?1 s?1, respectively. In addition, the degradation products of hydrolysis and the reactions with polysulfides, thiophenolate, and bisulfide with TCEP were studied with GC-FID and LC-MS-MS and were quantified. PMID:21419471

Hilaire, Dickens Saint; Ismail, Kamal Z.; Jans, Urs

2014-01-01

29

NACRE: A European Compilation of Reaction rates for Astrophysics  

SciTech Connect

We report on the program and results of the NACRE network (Nuclear Astrophysics Compilation of REaction rates). We have compiled low-energy cross section data for 86 charged-particle induced reactions involving light (1{<=}Z{<=}14) nuclei. The corresponding Maxwellian-averaged thermonuclear reactions rates are calculated in the temperature range from 10{sup 6} K to 10{sup 10} K. The web site http://pntpm.ulb.ac.be/nacre.htm, including the cross section data base and the reaction rates, allows users to browse electronically all the information on the reactions studied in this compilation.

Angulo, Carmen [Institut de Physique Nucleaire, Universite catholique de Louvain, Chemin du cyclotron, 2, B-1348 Louvain-la-Neuve (Belgium)

1999-11-16

30

Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations  

E-print Network

Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and gamma-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples ...

Iliadis, Christian; Coc, Alain; Timmes, F X; Champagne, Art E

2014-01-01

31

DSMC predictions of non-equilibrium reaction rates.  

SciTech Connect

A set of Direct Simulation Monte Carlo (DSMC) chemical-reaction models recently proposed by Bird and based solely on the collision energy and the vibrational energy levels of the species involved is applied to calculate nonequilibrium chemical-reaction rates for atmospheric reactions in hypersonic flows. The DSMC non-equilibrium model predictions are in good agreement with theoretical models and experimental measurements. The observed agreement provides strong evidence that modeling chemical reactions using only the collision energy and the vibrational energy levels provides an accurate method for predicting non-equilibrium chemical-reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2010-04-01

32

Imaginary-time theory for triple-alpha reaction rate  

E-print Network

Using imaginary-time theory, it is shown that the triple-alpha reaction rate can be reliably calculated without the need to solve scattering problems involving three charged particles. The calculated reaction rate is found to agree well with the empirical NACRE rate, which is widely adopted in stellar evolution calculations. The reason for this is explained using $R$-matrix theory. Extremely slow convergence is found to occur when a coupled-channel expansion is introduced, which helps to explain the very different reaction rates obtained using different theoretical approaches.

T. Akahori; Y. Funaki; K. Yabana

2014-01-17

33

Multidimensional reaction rate theory with anisotropic diffusion.  

PubMed

An analytical expression is derived for the rate constant that describes diffusive transitions between two deep wells of a multidimensional potential. The expression, in contrast to the Kramers-Langer formula for the rate constant, is valid even when the diffusion is highly anisotropic. Our approach is based on a variational principle for the reactive flux and uses a trial function for the splitting probability or commitor. The theoretical result is validated by Brownian dynamics simulations. PMID:25429932

Berezhkovskii, Alexander M; Szabo, Attila; Greives, Nicholas; Zhou, Huan-Xiang

2014-11-28

34

Multidimensional reaction rate theory with anisotropic diffusion  

NASA Astrophysics Data System (ADS)

An analytical expression is derived for the rate constant that describes diffusive transitions between two deep wells of a multidimensional potential. The expression, in contrast to the Kramers-Langer formula for the rate constant, is valid even when the diffusion is highly anisotropic. Our approach is based on a variational principle for the reactive flux and uses a trial function for the splitting probability or commitor. The theoretical result is validated by Brownian dynamics simulations.

Berezhkovskii, Alexander M.; Szabo, Attila; Greives, Nicholas; Zhou, Huan-Xiang

2014-11-01

35

Reactions of Three Halogenated Organophosphorus Flame Retardants with Reduced Sulfur Species  

PubMed Central

Tris(haloalkyl)phosphates (THAPs) are among the most widely used flame retardants in the U.S. They have been identified as one of the most frequently detected contaminants in U.S. streams. These contaminants are of toxicological concern in sensitive coastal ecosystems such as estuaries and salt marshes. It is likely that reactions with reduced sulfur species such as polysulfides (Sn2?) and bisulfide (HS?), present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such contaminants, especially since no significant degradation reactions in the environment (e.g., hydrolysis, biological degradation) is reported for these compounds. The kinetics of the reaction of reduced sulfur species with three structurally related THAPs have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain second-order rate constants from the observed pseudo-first order rate constants. The degradation products were studied with GC-FID and LC-MS. The reactivity of Sn2?, thiophenolate, and HS? were compared and steric, as well as electronic factors are used to explain the relative reactivity of the three THAPs with these three sulfur species. PMID:23948611

Saint-Hilaire, Dickens; Jans, Urs

2014-01-01

36

Measured ion-molecule reaction rates for modelling Titan's atmosphere  

NASA Technical Reports Server (NTRS)

Rate coefficients for several two- and three-body ion-molecule reactions involving hydrocarbons have been determined at thermal energies and above using drift tube-mass spectrometer techniques. The measured rates for clustering and breakup reactions involving CH5(+) and C2H5(+) ions in methane are found to be strongly temperature dependent in the range from 80 to 240 K. The equilibrium constants determined for these reactions differ somewhat from those of Hiraoka and Kebarle (1975). Rate coefficients for two-body reactions of CH5(+), C2H5(+), N(+), H(+), and D(+) ions with methane and/or ethane have been measured. The results indicate that the product yields of several of the fast ion-molecule reactions depend strongly on ion energy (temperature), and therefore previous room-temperature results may be of limited value for model calculations of Titan's atmosphere.

Dheandhanoo, S.; Johnsen, R.; Biondi, M. A.

1984-01-01

37

Urea degradation by electrochemically generated reactive chlorine species: products and reaction pathways.  

PubMed

This study investigated the transformation of urea by electrochemically generated reactive chlorine species (RCS). Solutions of urea with chloride ions were electrolyzed using a bismuth doped TiO2 (BiOx/TiO2) anode coupled with a stainless steel cathode at applied anodic potentials (Ea) of either +2.2 V or +3.0 V versus the normal hydrogen electrode. In NaCl solution, the current efficiency of RCS generation was near 30% at both potentials. In divided cell experiments, the pseudo-first-order rate of total nitrogen decay was an order of magnitude higher at Ea of +3.0 V than at +2.2 V, presumably because dichlorine radical (Cl2(-)·) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N2 and CO2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N2 (91%) and NO3(-) (9%). The higher reaction selectivity with respect to N2 production can be ascribed to a low operational ratio of free available chlorine to N. The mass-balance analysis recovered urea-C as CO2 at 77%, while CO generation most likely accounts for the residual carbon. In light of these results, we propose a reaction mechanism involving chloramines and chloramides as reaction intermediates, where the initial chlorination is the rate-determining step in the overall sequence of reactions. PMID:25219459

Cho, Kangwoo; Hoffmann, Michael R

2014-10-01

38

The kinetics and mechanisms of the reactions of iron(III) with quercetin and morin.  

PubMed

The kinetics and mechanisms of the reactions of a pseudo-first order excess of iron(III) with the flavonoids quercetin and morin have been investigated in aqueous solution at 25 degrees C and an ionic strength of 0.5M. Mechanisms have been proposed which account satisfactorily for the kinetic data. The data are consistent with a mechanism in which the metal:ligand complex formed initially on reaction of iron(III) with the ligand subsequently decomposes through an electron transfer step. Morin forms a 1:1 metal:ligand complex while quercetin forms a 2:1 metal:ligand complex. Both ligands showed evidence for the involvement of the iron hydroxo dimer Fe2(OH)2(4+) in the complex formation reaction at the hydroxy-carbonyl moiety. The iron(III) assisted decomposition of the initial iron(III) complex formed was also investigated and the rate constants evaluated. Both the complex formation and subsequent electron transfer reactions of iron(III) with these ligands were monitored using UV-visible spectrophotometry. All of the suggested mechanisms and calculated rate constants are supported by calculations carried out using global analysis of time dependant spectra. PMID:17868888

Ryan, Paul; Hynes, Michael J

2008-01-01

39

Workshop on Thermonuclear Reaction Rates for Astrophysics Applications  

E-print Network

rates key reactions: experiment and theory s-, r- and p-process nucleosynthesis nuclear experiments. Research activities carried out at INP include nuclear structure, nuclear reactions and astrophysics of Excellence in Low-energy Ion-Beam Research and Applications (LIBRA). Location NCSR "Demokritos" is located

40

A compilation of charged-particle induced thermonuclear reaction rates  

Microsoft Academic Search

Low-energy cross section data for 86 charged-particle induced reactions involving light (1 ? Z ? 14), mostly stable, nuclei are compiled. The corresponding Maxwellian-averaged thermonuclear reaction rates of relevance in astrophysical plasmas at temperatures in the range from 106 K to 1010 K are calculated. These evaluations assume either that the target nuclei are in their ground state, or that

C. Angulo; M. Arnould; M. Rayet; P. Descouvemont; D. Baye; C. Leclercq-Willain; A. Coc; S. Barhoumi; P. Aguer; C. Rolfs; R. Kunz; J. W. Hammer; A. Mayer; T. Paradellis; S. Kossionides; C. Chronidou; K. Spyrou; S. Degl'Innocenti; G. Fiorentini; B. Ricci; S. Zavatarelli; C. Providencia; H. Wolters; J. Soares; C. Grama; J. Rahighi; A. Shotter; M. Lamehi Rachti

1999-01-01

41

Thermonuclear reaction rate of 17O(p,?)18F  

NASA Astrophysics Data System (ADS)

The 17O(p,?)18F and 17O(p,?)14N reactions have a profound influence on hydrogen-burning nucleosynthesis in a number of stellar sites, including red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. Previously evaluated thermonuclear rates for both reactions carry large uncertainties. We investigated the proton-capture reaction on 17O in the bombarding energy range of Elabp=180-540keV. We observed a previously undiscovered resonance at ElabR=193.2±0.9keV. The resonance strength amounts to (??)p?=(1.2±0.2)×10-6eV. With this value, the uncertainties of the 17O(p,?)18F reaction rates are reduced by orders of magnitude in the peak temperature range of classical novae (T=0.1-0.4GK). We also report on a reevaluation of the 17O(p,?)18F reaction rates at lower temperatures that are pertinent to red giants, AGB stars, or massive stars. The present work establishes the 17O(p,?)18F reaction rates over a temperature range of T=0.01-1.5GK with statistical uncertainties of 10 50%. The new recommended reaction rates deviate from the previously accepted values by an order of magnitude around T?0.2GK and by factors of 2 3 at T<0.1GK.

Fox, C.; Iliadis, C.; Champagne, A. E.; Fitzgerald, R. P.; Longland, R.; Newton, J.; Pollanen, J.; Runkle, R.

2005-05-01

42

Non-resonant triple alpha reaction rate at low temperature  

SciTech Connect

Our experimental goal is to study the non-resonant triple alpha reaction rate at low temperture (T < 10{sup 8} K). The {sup 13}C(p,d) reaction at 66 MeV has been used to probe the alpha-unbound continuum state in {sup 12}C just below the 2{sup nd} 0{sup +} state at 7.65 MeV. The transition strength to the continuum state is predicted to be sensitive to the non-resonant triple alpha reaction rate. The experiment has been performed at iThemba LABS. We report the present status of the experiment.

Itoh, T.; Tamii, A.; Aoi, N.; Fujita, H.; Hashimoto, T.; Miki, K.; Ogata, K. [Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047 (Japan); Carter, J.; Donaldson, L.; Sideras-Haddad, E. [Schools of Physics, University of Witwatersrand, Johannesburg 2050 (South Africa); Furuno, T.; Kawabata, T. [Departments of Physics, Kyoto University, Sakyo, Kyoto, 606-8502 (Japan); Kamimura, M. [RIKEN Nishina Center, Wako, Saitama, 351-0198 (Japan); Nemulodi, F.; Neveling, R.; Smit, F. D.; Swarts, C. [iThemba Laboratory for Accelerator Based Sciences Somerset, West, 7129 (South Africa)

2014-05-02

43

Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations  

E-print Network

Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and gamma-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples are given for applications to s-process neutron sources, core-collapse supernovae, classical novae, and big bang nucleosynthesis.

Christian Iliadis; Richard Longland; Alain Coc; F. X. Timmes; Art E. Champagne

2014-09-19

44

Reaction rate constant for uranium in water and water vapor  

SciTech Connect

The literature on uranium oxidation in water and oxygen free water vapor was reviewed. Arrhenius rate equations were developed from the review data. These data and equations will be used as a baseline from which to compare reaction rates measured for K Basin fuel.

TRIMBLE, D.J.

1998-11-09

45

Extension of a Kinetic-Theory Approach for Computing Chemical-Reaction Rates to Reactions with Charged Particles  

NASA Technical Reports Server (NTRS)

Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties (i.e., no macroscopic reaction rate information) are extended to include reactions involving charged particles and electronic energy levels. The proposed extensions include ionization reactions, exothermic associative ionization reactions, endothermic and exothermic charge exchange reactions, and other exchange reactions involving ionized species. The extensions are shown to agree favorably with the measured Arrhenius rates for near-equilibrium conditions.

Liechty, Derek S.; Lewis, Mark J.

2010-01-01

46

Extension of a Kinetic-Theory Approach for Computing Chemical-Reaction Rates to Reactions with Charged Particles  

NASA Astrophysics Data System (ADS)

Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties (i.e., no macroscopic reaction rate information) are extended to include reactions involving charged particles and electronic energy levels. The proposed extensions include ionization reactions, exothermic associative ionization reactions, endothermic and exothermic charge exchange reactions, and other exchange reactions involving ionized species. The extensions are shown to agree favorably with the measured Arrhenius rates for near-equilibrium conditions.

Liechty, Derek S.; Lewis, Mark J.

2011-05-01

47

Rate coefficients, binding probabilities, and related quantities for area reactivity models.  

PubMed

We further develop the general theory of the area reactivity model that describes the diffusion-influenced reaction of an isolated receptor-ligand pair in terms of a generalized Feynman-Kac equation and that provides an alternative to the classical contact reactivity model. Analyzing both the irreversible and reversible reaction, we derive the equation of motion of the survival probability as well as several relationships between single pair quantities and the reactive flux at the encounter distance. Building on these relationships, we derive the equation of motion of the many-particle survival probability for irreversible pseudo-first-order reactions. Moreover, we show that the usual definition of the rate coefficient as the reactive flux is deficient in the area reactivity model. Numerical tests for our findings are provided through Brownian Dynamics simulations. We calculate exact and approximate expressions for the irreversible rate coefficient and show that this quantity behaves differently from its classical counterpart. Furthermore, we derive approximate expressions for the binding probability as well as the average lifetime of the bound state and discuss on- and off-rates in this context. Throughout our approach, we point out similarities and differences between the area reactivity model and its classical counterpart, the contact reactivity model. The presented analysis and obtained results provide a theoretical framework that will facilitate the comparison of experiment and model predictions. PMID:25416882

Prüstel, Thorsten; Meier-Schellersheim, Martin

2014-11-21

48

Rate coefficients, binding probabilities, and related quantities for area reactivity models  

NASA Astrophysics Data System (ADS)

We further develop the general theory of the area reactivity model that describes the diffusion-influenced reaction of an isolated receptor-ligand pair in terms of a generalized Feynman-Kac equation and that provides an alternative to the classical contact reactivity model. Analyzing both the irreversible and reversible reaction, we derive the equation of motion of the survival probability as well as several relationships between single pair quantities and the reactive flux at the encounter distance. Building on these relationships, we derive the equation of motion of the many-particle survival probability for irreversible pseudo-first-order reactions. Moreover, we show that the usual definition of the rate coefficient as the reactive flux is deficient in the area reactivity model. Numerical tests for our findings are provided through Brownian Dynamics simulations. We calculate exact and approximate expressions for the irreversible rate coefficient and show that this quantity behaves differently from its classical counterpart. Furthermore, we derive approximate expressions for the binding probability as well as the average lifetime of the bound state and discuss on- and off-rates in this context. Throughout our approach, we point out similarities and differences between the area reactivity model and its classical counterpart, the contact reactivity model. The presented analysis and obtained results provide a theoretical framework that will facilitate the comparison of experiment and model predictions.

Prüstel, Thorsten; Meier-Schellersheim, Martin

2014-11-01

49

On reaction rate constants IO+NO, IO+CO  

NASA Astrophysics Data System (ADS)

A method, allowing to define, whether an investigated chemical reaction in a gas phase or on a surface of a reactor for those cases when dependence of the reaction rate on concentration of reagents for heterogeneous and for homogeneous reaction are identical, has been suggested. The method has been used for investigation of reactions of IO with nitric oxide and carbon monoxide. Experiments have been run in the flow reactor with using a method of resonance fluorescence to record atoms of iodine. It has been proved that the reaction of radical IO with nitric oxide proceeds in the gas phase, whereas the reaction of radical IO with carbon monoxide proceeds on the surface of the reactor. It has been obtained that reaction rate constant of IO with nitrogen monoxide in the temperature range 298-353 K is kNO(T)=(1,01±0,2)•10^-11•exp((210±80)/T) cm3*molecule*s-1 and the same for reaction of IO with carbon monoxide is kCO(T)=(1,14±0,64)•10^-14•exp((612±41)/T) cm3*molecule*s-1.

Larin, Igor; Spasskiy, Alexandr; Trofimova, Elen

2013-04-01

50

General properties of astrophysical reaction rates in explosive nucleosynthesis  

E-print Network

Fundamental differences in the prediction of reaction rates with intermediate and heavy target nuclei compared to the ones with light nuclei are discussed, with special emphasis on stellar modifications of the rates. Ground and excited state contributions to the stellar rates are quantified, deriving a linear weighting of excited state contributions despite of a Boltzmann population of the nuclear states. A Coulomb suppression effect of the excited state contributions is identified, acting against the usual Q-value rule in some reactions. The proper inclusion of experimental data in revised stellar rates is shown, containing revised uncertainties. An application to the s-process shows that the actual uncertainties in the neutron capture rates are larger than would be expected from the experimental errors alone. Sensitivities of reaction rates and cross sections are defined and their application in reaction studies is discussed. The conclusion provides a guide to experiment as well as theory on how to best improve the rates used in astrophysical simulations and how to assess their uncertainties.

T. Rauscher

2012-09-25

51

Glow Fast, Glow Slow: Alter the Rate of a Reaction!  

NSDL National Science Digital Library

Learners investigate one factor affecting reaction rates: temperature. In a darkened room, two identical lightsticks are placed in water -- one in hot water and one in cold water. The lightstick in the hot water glows brighter than the lightstick in the cold water since the light-producing reaction is faster in hot water. This activity, located on page 2 of the PDF, is one of three activities in the Take Home guide for the Chemical Reactions unit in OMSI's Chemistry Lab. Also included on the PDF are two additional activities: Cloudy Globs and Gas Production.

Oregon Museum of Science and Industry

1997-01-01

52

Imaginary-time method for radiative capture reaction rate  

E-print Network

We propose a new computational method for astrophysical reaction rate of radiative capture process. In the method, an evolution of a wave function is calculated along the imaginary-time axis which is identified as the inverse temperature. It enables direct evaluation of reaction rate as a function of temperature without solving any scattering problem. The method is tested for two-body radiative capture reaction, ${^{16}{\\rm O}}(\\alpha,\\gamma){^{20}{\\rm Ne}}$, showing that it gives identical results to that calculated by the ordinary procedure. The new method will be suited for calculation of triple-alpha radiative capture rate for which an explicit construction of the scattering solution is difficult.

Kazuhiro Yabana; Yasuro Funaki

2012-02-15

53

Dissipation Scale Fluctuations and Chemical Reaction Rates in Turbulent Flows  

E-print Network

Small separation between reactants, not exceeding $10^{-8}-10^{-7}cm$, is the necessary condition for various chemical reactions. It is shown that random advection and stretching by turbulence leads to formation of scalar-enriched sheets of {\\it strongly fluctuating thickness} $\\eta_{c}$. The molecular-level mixing is achieved by diffusion across these sheets (interfaces) separating the reactants. Since diffusion time scale is $\\tau_{d}\\propto \\eta_{c}^{2}$, the knowledge of probability density $Q(\\eta_{c},Re)$ is crucial for evaluation of chemical reaction rates. In this paper we derive the probability density $Q(\\eta_{c},Re,Sc)$ and predict a transition in the reaction rate behavior from ${\\cal R}\\propto \\sqrt{Re}$ ($Re\\leq 10^{4}$) to the high-Re asymptotics ${\\cal R}\\propto Re^{0}$. The theory leads to an approximate universality of transitional Reynolds number $Re_{tr}\\approx 10^{4}$. It is also shown that if chemical reaction involves short-lived reactants, very strong anomalous fluctuations of the length-scale $\\eta_{c}$ may lead to non-negligibly small reaction rates.

Victor Yakhot

2007-06-29

54

Direct measurement and theoretical calculation of the rate coefficient for Cl + CH3 from T = 202 - 298 K.  

SciTech Connect

The rate coefficient has been measured under pseudo-first-order conditions for the Cl + CH{sub 3} association reaction at T = 202, 250, and 298 K and P = 0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH{sub 3} were generated rapidly and simultaneously by reaction of F with HCl and CH{sub 4}, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F{sub 2} in He. The decay of CH{sub 3} was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH{sub 3} concentration ([Cl]{sub 0}/[CH{sub 3}]{sub 0} = 9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T = 202 K, the rate coefficient increases from 8.4 x 10{sup -12} at P = 0.30 Torr He to 1.8 x 10{sup -11} at P = 2.00 Torr He, both in units of cm{sup 3} molecule{sup -1} s{sup -1}. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k{sub 2} = 6.0 x 10{sup -11} cm{sup 3} molecule{sup -1} s{sup -1}, independent of temperature in the range from 200 to 300 K.

Payne, Walter A. (NASA/Goddard Space Flight Center, Greenbelt, MD); Harding, Lawrence B. (Argonne National Laboratory, Argonne, IL); Stief, Louis J. (NASA/Goddard Space Flight Center, Greenbelt, MD); Parker, James F. (James Fletcher), 1925-; Klippenstein, Stephen J.; Nesbitt, Fred L. (NASA/Goddard Space Flight Center, Greenbelt, MD); Cody, Regina J. (NASA/Goddard Space Flight Center, Greenbelt, MD)

2004-10-01

55

Reaction rate uncertainties and the {nu}p-process  

SciTech Connect

Current hydrodynamical simulations of core collapse supernovae find proton-rich early ejecta. At the same time, the models fail to eject neutron-rich matter, thus leaving the origin of the main r-process elements unsolved. However, the proton-rich neutrino-driven winds from supernovae have been identified as a possible production site for light n-capture elements beyond iron (such as Ge, Sr, Y, Zr) through the {nu}p-process. The detailed nucleosynthesis patterns of the {nu}p-process depend on the hydrodynamic conditions and the nuclear reaction rates of key reactions. We investigate the impact of reaction rate uncertainties on the {nu}p-process nucleosynthesis.

Froehlich, C.; Rauscher, T. [Department of Physics, North Carolina State University, Raleigh, NC 27695 (United States); Dept. of Physics, University of Basel, 4056 Basel (Switzerland)

2012-11-12

56

Reaction rates of graphite with ozone measured by etch decoration  

NASA Technical Reports Server (NTRS)

Etch-decoration technique of detecting vacancies in graphite has been used to determine the reaction rates of graphite with ozone in the directions parallel and perpendicular to the layer planes. It consists essentially of peeling single atom layers off graphite crystals without affecting the remainder of the crystal.

Hennig, G. R.; Montet, G. L.

1968-01-01

57

EXPERIMENTAL PROTOCOL FOR DETERMINING OZONE REACTION RATE CONSTANTS  

EPA Science Inventory

An experimental protocol for the determination of room temperature rate constants for the reactions of ozone with chemicals in the gas phase has been developed and is described in detail. This protocol will thus provide a basis for evaluating the relative importance of one atmosp...

58

Nuclear fusion in dense matter: Reaction rate and carbon burning  

E-print Network

In this paper we analyze the nuclear fusion rate between equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S-factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate which covers all cases. The parameters of this formula can be varied, taking into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the ^{12}C+^{12}C fusion reaction. This reaction is very important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovae, and in accreting neutron stars. The S-factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S-factor using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities rho > 10^9 g/cc.

L. R. Gasques; A. V. Afanasjev; E. F. Aguilera; M. Beard; L. C. Chamon; P. Ring; M. Wiescher; D. G. Yakovlev

2005-06-16

59

Kinetics of heterogeneous reaction of ozone with linoleic acid and its dependence on temperature, physical state, RH, and ozone concentration.  

PubMed

Heterogeneous reaction between ozone and linoleic acid (LA) thin film was investigated by a flow reactor coupled to attenuated total reflection infrared spectroscopy (FR-ATR-IR) over wide ranges of temperature, relative humidity (RH), and ozone concentration under atmospheric pressure condition. Pseudo-first-order rate constants kapp and overall reactive uptake coefficients ? were acquired on the basis of changes in absorbance from peaks located near 1743, 1710, 1172, and 1110 cm(-1), which can be assigned to C?O in ester, C?O in acid, and C-C and C-O stretching modes, respectively. Results showed that the kapp and ? increased nearly by a factor of 6 with increasing temperatures from 258 to 314 K. It was noted the temperature effect on the reaction kinetics was much more pronounced at lower temperatures. Such behavior can be explained by a change in the physical state of LA at lower temperatures. In addition, kapp and ? were enhanced by 2-fold as the RH increased from 0 to 80%. Moreover, the effect of ozone concentration on the reaction kinetics was reported for the first time. kapp was found to display a Langmuir-Hinshelwood dependence on ozone concentration with KO3 = (1.146 ± 0.017) × 10(-15) molecules cm(-3) and k[S] = 0.0522 ± 0.0004 s(-1), where KO3 is a parameter that describes the partitioning of ozone to the thin film surface, and k[S] is the maximum pseudo-first-order coefficient at high ozone concentration. Furthermore, yields and hygroscopic properties of reaction products were also investigated by FTIR spectroscopy. The intensity ratio of two C?O stretching bands, A1743/A1710, which was utilized as an indicator of the product yields, increased sharply with increasing temperatures in the lower temperature region (258-284 K), and then remained nearly constant in the higher temperature region (284-314 K). The product yields showed no significant variation with RH, for the intensity ratio of A1743/A1710 barely changed in the wide RH range 0-80%. Water uptake studies showed that the LA thin film absorbed water with an increasing RH, and the hygroscopicity of the thin film was enhanced after ozone exposure. PMID:23347186

Zeng, Guang; Holladay, Sara; Langlois, Danielle; Zhang, Yunhong; Liu, Yong

2013-03-01

60

Suitable combination of promoter and micellar catalyst for kilo fold rate acceleration on benzaldehyde to benzoic acid conversion in aqueous media at room temperature: a kinetic approach.  

PubMed

The kinetics of oxidation of benzaldehyde by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, alkyl phenyl polyethylene glycol, Triton X-100 and N-cetylpyridinium chloride, CPC) media have been investigated in the presence of promoter at 303 K. The pseudo-first-order rate constants (kobs) were determined from a logarithmic plot of absorbance as a function time. The rate constants were found to increase with introduction of heteroaromatic nitrogen base promoters such as Picolinic acid (PA), 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen). The product benzoic acid has been characterized by conventional melting point experiment, NMR, HRMS and FTIR spectral analysis. The mechanism of both unpromoted and promoted reaction path has been proposed for the reaction. In presence of the anionic surfactant SDS, cationic surfactant CPC and neutral surfactant TX-100 the reaction can undergo simultaneously in both aqueous and micellar phase with an enhanced rate of oxidation in the micellar phase. Both SDS and TX-100 produce normal micellar effect whereas CPC produce reverse micellar effect in the presence of benzaldehyde. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. SDS and bipy combination is the suitable one for benzaldehyde oxidation. PMID:23501718

Ghosh, Aniruddha; Saha, Rumpa; Ghosh, Sumanta K; Mukherjee, Kakali; Saha, Bidyut

2013-05-15

61

Suitable combination of promoter and micellar catalyst for kilo fold rate acceleration on benzaldehyde to benzoic acid conversion in aqueous media at room temperature: A kinetic approach  

NASA Astrophysics Data System (ADS)

The kinetics of oxidation of benzaldehyde by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, alkyl phenyl polyethylene glycol, Triton X-100 and N-cetylpyridinium chloride, CPC) media have been investigated in the presence of promoter at 303 K. The pseudo-first-order rate constants (kobs) were determined from a logarithmic plot of absorbance as a function time. The rate constants were found to increase with introduction of heteroaromatic nitrogen base promoters such as Picolinic acid (PA), 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen). The product benzoic acid has been characterized by conventional melting point experiment, NMR, HRMS and FTIR spectral analysis. The mechanism of both unpromoted and promoted reaction path has been proposed for the reaction. In presence of the anionic surfactant SDS, cationic surfactant CPC and neutral surfactant TX-100 the reaction can undergo simultaneously in both aqueous and micellar phase with an enhanced rate of oxidation in the micellar phase. Both SDS and TX-100 produce normal micellar effect whereas CPC produce reverse micellar effect in the presence of benzaldehyde. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. SDS and bipy combination is the suitable one for benzaldehyde oxidation.

Ghosh, Aniruddha; Saha, Rumpa; Ghosh, Sumanta K.; Mukherjee, Kakali; Saha, Bidyut

2013-05-01

62

Photochemical Generation of Strong One-Electron Reductants via Light-Induced Electron Transfer with Reversible Donors Followed by Cross Reaction with Sacrificial Donors.  

PubMed

This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru(LL)3](2+) (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E(0)(2+/1+) values between -1.0 and -1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru(LL)3](2+) complex, reacts irreversibly with various sacrificial electron donating amines that are kinetically unable to directly react with the photoexcited complex. The electron transfer between the TTA(+) and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA(+). A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA(+). This electron-donating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production. PMID:24882233

Shan, Bing; Schmehl, Russell

2014-11-13

63

Kinetic studies of the reactions of HSO with NO/sub 2/, NO, and O/sub 2/  

SciTech Connect

A far-infrared rotational transition in HSO has been observed by using an optically pumped laser magnetic resonance (LMR) spectrometer. LMR detection of HSO was utilized to study the reactions of HSO with NO/sub 2/, NO, and O/sub 2/. Rate coefficients were measured under pseudo-first-order conditions in a discharge flow system coupled to the spectrometer. The results are k(HSO + NO/sub 2/) = (9.6 +/- 2.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/; k(HSO + NO) less than or equal to 1 x 10/sup -15/ cm/sup 3/ molecule/sup -1/ s/sup -1/; and k(HSO + O/sub 2/) less than or equal to 2 x 10/sup -17/ cm/sup 3/ molecule/sup -1/ s/sup -1/. All measurements were made at low pressure (1-5 Torr) and room temperature (296 +/- 3 K). Experiments were also conducted to determine the products of the HSO + NO/sub 2/ reaction. The results show that a species is produced which reacts with O/sub 2/ to form HO/sub 2/ with a rate coefficient of (3 +/- 2) x 10/sup -13/ cm/sup 3/ molecule/sup -1/ s/sup -1/ which was determined from the rate of HO/sub 2/ formation. They propose that the unidentified intermediate is HSO/sub 2/.

Lovejoy, E.R.; Wang, N.S.; Howard, C.J.

1987-10-22

64

Temperature Dependence of the Rate Constants of Charge Recombination Reactions in Bacterial Reaction Centers  

NASA Astrophysics Data System (ADS)

The bacterial reaction center couples light-induced electron transfer via a tightly bound ubiquinone (QA) to a mobile ubiquinone (QB). Based on the electron transfer theory by Marcus, we have investigated the rate of charge recombination reactions from Rhodopseudomonas viridis and Rhodobacter sphaeroides, by mean of finding an approximation formula. The results obtained are verified for not only at high and low temperature as the previous works but also at the medium temperature range.

Thuy, T. T.; Yen, V. T. H.; Thao, T. T.; Viet, Nguyen Ai

65

Reaction rates from pressure-gauge measurements in reacting explosives  

SciTech Connect

The proper hydrodynamic data and an equation of state are sufficient to describe quantitatively the reaction rates of explosives during the shock-to-detonation transition. Manganin pressure gauges embedded in the reacting explosive have provided these data for the explosives PETN, PBX 9404, TATB, and TNT. Once a pressure-field history has been assembled from individual pressure histories at different depths in the explosive, the conservation equations can be applied in a Lagrangian analysis of the data. The combination of a reactant-product equation of state with this analysis then allows the calculation of the extent of reaction and reaction rate. Successful correlation of the calculated reaction rate values with other thermodynamic variables, such as pressure or temperature, allows formulation of a rate law and the prediction of initiation behavior under circumstances quite different from the experiments that led to the rate law. The best dynamic piezoresistive pressure gauge for most applications would have a substantial output voltage and present negligible disturbance to the flow. In explosives, however, requirements for survival in the extreme temperature and pressure environment encountered by the gauge dictate compromise. Low electrical resistance (approx. 20 m..cap omega..) helps to minimize shunt conductivity failures, but this drastically reduces output and demands that much attention be given to reducingnoise. Although relatively thick insulation perturbs the flow to some extent, survivability requirements dictate its use. Pressure measurements in reactive flow can now be made routinely with gauges that successfully produce data leading to a description of the flow and a powerful predictive capability.

Ginsberg, M.J.; Anderson, A.B.; Wackerle, J.

1981-01-01

66

Suppression of excited-state contributions to stellar reaction rates  

NASA Astrophysics Data System (ADS)

It has been shown in previous work [Kiss , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.101.191101 101, 191101 (2008); Rauscher , Phys. Rev. C10.1103/PhysRevC.80.035801 80, 035801 (2009)] that a suppression of the stellar enhancement factor (SEF) occurs in some endothermic reactions at and far from stability. This effect is re-evaluated using the ground-state contributions to the stellar reaction rates, which were shown to be better suited to judging the importance of excited-state contributions than the previously applied SEFs. An update of the tables shown in the latter work is given. The new evaluation finds 2350 cases (out of a full set of 57 513 reactions) for which the ground-state contribution is larger in the reaction direction with a negative reaction Q value than in the exothermic direction, thus providing exceptions to the commonly applied Q value rule. The results confirm the Coulomb suppression effect but lead to a larger number of exceptions than previously found. This is due to the fact that often a large variation in the g.s. contribution does not lead to a sizable change in the SEF. On the other hand, several previously identified cases do not appear anymore because it is found that their g.s. contribution is smaller than inferred from the SEF.

Rauscher, T.

2013-09-01

67

Suppression of Excited-State Contributions to Stellar Reaction Rates  

E-print Network

It has been shown in previous work [Phys. Rev. Lett. 101, 191101 (2008); Phys. Rev. C 80, 035801 (2009)] that a suppression of the stellar enhancement factor (SEF) occurs in some endothermic reactions at and far from stability. This effect is re-evaluated using the ground-state contributions to the stellar reaction rates, which were shown to be better suited to judge the importance of excited state contributions than the previously applied SEFs. An update of the tables shown in Phys. Rev. C 80, 035801 (2009) is given. The new evalution finds 2350 cases (out of a full set of 57513 reactions) for which the ground-state contribution is larger in the reaction direction with negative reaction Q-value than in the exothermic direction, thus providing exceptions to the commonly applied Q-value rule. The results confirm the Coulomb suppression effect but lead to a larger number of exceptions than previously found. This is due to the fact that often a large variation in the g.s. contribution does not lead to a sizeable...

Rauscher, T

2013-01-01

68

Updating the nuclear reaction rate library REACLIB. (I. Experimental reaction rates of the proton-proton chain)  

E-print Network

REACLIB is one of the most comprehensive and popular astrophysical reaction rate libraries. However, its experimentally obtained rates for light isotopes still rely mainly on the Caughlan & Fowler (1988) compilation and have never been updated despite the progress in many relevant nuclear astrophysics experiments. Moreover, due to fitting errors REACLIB is not reliable at temperatures lower than 10^{7}K. In this work we establish the formalism for updating the obsolete Caughlan-Fowler experimental rates of REACLIB. Then we use the NACRE compilation and results from the LUNA experiments to update some important charged-particle induced rates of REACLIB focusing on the proton-proton chain. The updated rates (available also in digital form) can now be used in the low temperature regime (below 10^{7}K) which was forbidden to the old version of REACLIB.

Theodore E. Liolios

2005-02-03

69

Application of semiclassical methods to reaction rate theory  

SciTech Connect

This work is concerned with the development of approximate methods to describe relatively large chemical systems. This effort has been divided into two primary directions: First, we have extended and applied a semiclassical transition state theory (SCTST) originally proposed by Miller to obtain microcanonical and canonical (thermal) rates for chemical reactions described by a nonseparable Hamiltonian, i.e. most reactions. Second, we have developed a method to describe the fluctuations of decay rates of individual energy states from the average RRKM rate in systems where the direct calculation of individual rates would be impossible. Combined with the semiclassical theory this latter effort has provided a direct comparison to the experimental results of Moore and coworkers. In SCTST, the Hamiltonian is expanded about the barrier and the ``good`` action-angle variables are obtained perturbatively; a WKB analysis of the effectively one-dimensional reactive direction then provides the transmission probabilities. The advantages of this local approximate treatment are that it includes tunneling effects and anharmonicity, and it systematically provides a multi-dimensional dividing surface in phase space. The SCTST thermal rate expression has been reformulated providing increased numerical efficiency (as compared to a naive Boltzmann average), an appealing link to conventional transition state theory (involving a ``prereactive`` partition function depending on the action of the reactive mode), and the ability to go beyond the perturbative approximation.

Hernandez, R.

1993-11-01

70

A Study of the ^30S(,)^33Cl Reaction Rate  

NASA Astrophysics Data System (ADS)

The ^30S(,)^33Cl reaction rate has major implications for x-ray bursts (XRBs). No experimental information exists for this reaction rate, though XRB models have shown that it affects final isotopic abundances and the total energy output.footnotetextA. Parikh et al., ApJ SS 178, 110 (2008). This rate may also influence XRB observables such as the structure of double-peaked luminosity curvesfootnotetextJ.L. Fisker et al., ApJ 608, L61 (2004). and the composition of the neutron star crust.footnotetextH. Schatz and K.E. Rehm, NPA 777, 601 (2006). We have studied the time-inverse reaction p(^33Cl,^30S)? at ATLAS using a radioactive ^33Cl beam. The residual ^30S nuclei were detected at the focal plane of the split-pole spectrograph, which was used in gas-filled mode, in coincidence with the ? particles, which were detected in a double-sided Si detector. The experimental results and conclusions about the impact on XRB nucleosynthesis will be discussed.

Deibel, C. M.; Jiang, C. L.; Kay, B. P.; Lee, H. Y.; Pardo, R. C.; Rehm, K. E.; Ugalde, C.; Woodard, A.; Figueira, J. M.; Marley, S. T.; Patel, N. R.; Paul, M.; Wuosmaa, A.

2009-10-01

71

Kinetics of Imidazole Catalyzed Ester Hydrolysis: Use of Buffer Dilutions to Determine Spontaneous Rate, Catalyzed Rate, and Reaction Order.  

ERIC Educational Resources Information Center

Described is an advanced undergraduate kinetics experiment using buffer dilutions to determine spontaneous rate, catalyzed rate, and reaction order. The reaction utilized is hydrolysis of p-nitro-phenyl acetate in presence of imidazole, which has been shown to enhance rate of the reaction. (Author/JN)

Lombardo, Anthony

1982-01-01

72

r-PROCESS Reaction Rates for the Actinides and Beyond  

NASA Astrophysics Data System (ADS)

We discuss the importance of different fission rates for the formation of heavy and superheavy nuclei in the astrophysical r-process. Neutron-induced reaction rates, including fission and neutron capture, are calculated in the temperature range 108 ? T(K) ? 1010 within the framework of the statistical model for targets with the atomic number 84 ? Z ? 118 (from Po to Uuo) from the neutron to the proton drip-line for different mass and fission barrier predictions based on Thomas-Fermi (TF), Extended Thomas-Fermi plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and Hartree-Fock-Bogolyubov (HFB) approaches. The contribution of spontaneous fission as well as beta-delayed fission to the recycling r-process is discussed. We also discuss the possibility of rate tests, based on mini r-processed yields in nuclear explosions.

Panov, I. V.; Korneev, I. Yu.; Rauscher, T.; Thielemann, F.-K.

2011-10-01

73

Thermonuclear reaction rate of 18O(p,?)19F  

NASA Astrophysics Data System (ADS)

For stars with 0.8 M? ? M ? 8.0 M?, nucleosynthesis enters its final phase during the asymptotic giant branch (AGB) stage. During this evolutionary period, grain condensation occurs in the stellar atmosphere, and the star experiences significant mass loss. The production of presolar grains can often be attributed to this unique stellar environment. A subset of presolar oxide grains features dramatic 18O depletion that cannot be explained by the standard AGB star burning stages and dredge-up models. An extra mixing process, referred to as cool bottom processing (CBP), was proposed for low-mass AGB stars. The 18O depletion observed within certain stellar environments and within presolar grain samples may result from the 18O+p processes during CBP. We report here on a study of the 18O(p,?)19F reaction at low energies. Based on our new results, we found that the resonance at ERlab=95 keV has a negligible affect on the reaction rate at the temperatures associated with CBP. We also determined that the direct capture S factor is almost a factor of 2 lower than the previously recommended value at low energies. An improved thermonuclear reaction rate for 18O(p,?)19F is presented.

Buckner, M. Q.; Iliadis, C.; Cesaratto, J. M.; Howard, C.; Clegg, T. B.; Champagne, A. E.; Daigle, S.

2012-12-01

74

Neutron detectors for fusion reaction-rate measurements  

SciTech Connect

Fusion reactions in an inertial-confinement fusion (ICF) target filled with deuterium or a deuterium/tritium fuel release nearly monoenergetic neutrons. Because most the neutrons leave the compressed target without collision, they preserve reaction-rate information as they travel radially outward from their point of origin. Three fast, neutron detector techniques, each capable of measuring the fusion reaction-rate of ICF targets, have been demonstrated. The most advanced detector is based on the fast rise-time of a commercial plastic scintillator material (BC-422) which acts as a neutron-to-light converter. Signals, which are recorded with a fast optical streak camera, have a resolution of 25 ps. Good signals can be recorded for targets producing only 5 x 10{sup 7} DT neutrons. Two other detectors use knock-on collisions between neutrons and protons in a thin polyethylene (CH{sub 2}) converter. In one, the converter is placed in front of the photocathode of an x-ray streak camera. Recoil protons pass through the photocathode and knock out electrons which are accelerated and deflected to produce a signal. Resolutions < 25 ps are possible. In the other, the converter is placed in front of a microchannel plate (MCP) with a gated microstrip. Recoil protons eject electrons from the gold layer forming the microstrip. If a gate pulse is present, the signal is amplified. Present gate times are about 80 ps.

Lerche, R.A.; Phillion, D.W.; Landen, O.L.; Murphy, T.J. [Lawrence Livermore National Lab., CA (United States); Jaanimagi, P.A. [Univ. of Rochester, NY (United States). Laboratory for Laser Energetics

1994-02-10

75

Kinetics and Mechanism of Oxidation Reactions of Porphyrin-Iron(IV)-Oxo Intermediates  

PubMed Central

The kinetics of reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1–2 × 10?2 M?1 s?1 for reactions with styrene and 3 × 10?2 M?1 s?1 for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett ?+ substituent constants and with a dual parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states. PMID:17630728

Pan, Zhengzheng; Newcomb, Martin

2010-01-01

76

RPMDRATE: Bimolecular chemical reaction rates from ring polymer molecular dynamics  

NASA Astrophysics Data System (ADS)

We present RPMDRATE, a computer program for the calculation of gas phase bimolecular reaction rate coefficients using the ring polymer molecular dynamics (RPMD) method. The RPMD rate coefficient is calculated using the Bennett-Chandler method as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (ring polymer transmission coefficient) factor. The computational procedure is general and can be used to treat bimolecular polyatomic reactions of any complexity in their full dimensionality. The program has been tested for the H+H2, H+CH4, OH+CH4 and H+C2H6 reactions. Catalogue identifier: AENW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AENW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: MIT license No. of lines in distributed program, including test data, etc.: 94512 No. of bytes in distributed program, including test data, etc.: 1395674 Distribution format: tar.gz Programming language: Fortran 90/95, Python (version 2.6.x or later, including any version of Python 3, is recommended). Computer: Not computer specific. Operating system: Any for which Python, Fortran 90/95 compiler and the required external routines are available. Has the code been vectorized or parallelized?: The program can efficiently utilize 4096+ processors, depending on problem and available computer. At low temperatures, 110 processors are reasonable for a typical umbrella integration run with an analytic potential energy function and gradients on the latest x86-64 machines.

Suleimanov, Yu. V.; Allen, J. W.; Green, W. H.

2013-03-01

77

Rate-Controlled Constrained-Equilibrium Theory of Chemical Reactions  

NASA Astrophysics Data System (ADS)

The Rate-Controlled Constrained-Equilibrium (RCCE) method for simplifying the treatment of reactions in complex systems is summarized and the selection of constraints for both close-to and far-from equilibrium systems is discussed. Illustrative examples of RCCE calculations of carbon monoxide concentrations in the exhaust products of an internal combustion engine and ignition delays for methane-oxygen mixtures in a constant volume adiabatic chamber are given and compared with "detailed" calculations. The advantages of RCCE calculations over "detailed" calculations are discussed.

Keck, James C.

2008-08-01

78

Novel technique for Constraining r-Process (n, ?) Reaction Rates.  

PubMed

A novel technique has been developed, which will open exciting new opportunities for studying the very neutron-rich nuclei involved in the r process. As a proof of principle, the ? spectra from the ? decay of ^{76}Ga have been measured with the SuN detector at the National Superconducting Cyclotron Laboratory. The nuclear level density and ?-ray strength function are extracted and used as input to Hauser-Feshbach calculations. The present technique is shown to strongly constrain the ^{75}Ge(n,?)^{76}Ge cross section and reaction rate. PMID:25526121

Spyrou, A; Liddick, S N; Larsen, A C; Guttormsen, M; Cooper, K; Dombos, A C; Morrissey, D J; Naqvi, F; Perdikakis, G; Quinn, S J; Renstrøm, T; Rodriguez, J A; Simon, A; Sumithrarachchi, C S; Zegers, R G T

2014-12-01

79

Novel technique for constraining r-process (n,$?$) reaction rates  

E-print Network

A novel technique has been developed, which will open exciting new opportunities for studying the very neutron-rich nuclei involved in the r-process. As a proof-of-principle, the $\\gamma$-spectra from the $\\beta$-decay of $^{76}$Ga have been measured with the SuN detector at the National Superconducting Cyclotron Laboratory. The nuclear level density and $\\gamma$-ray strength function are extracted and used as input to Hauser-Feshbach calculations. The present technique is shown to strongly constrain the $^{75}$Ge($n,\\gamma$)$^{76}$Ge cross section and reaction rate.

A. Spyrou; S. N. Liddick; A. C. Larsen; M. Guttormsen; K. Cooper; A. C. Dombos; D. J. Morrissey; F. Naqvi; G. Perdikakis; S. J. Quinn; T. Renstrøm; J. A. Rodriguez; A. Simon; C. S. Sumithrarachchi; R. G. T. Zegers

2014-08-27

80

Constraining kinetic rates of mineral reactions using reactive transport models  

NASA Astrophysics Data System (ADS)

We use a reactive transport model to better understand results of experiments to obtain kinetic rates of mineral reactions in closed systems. Closed system experiments pose special challenges in that secondary minerals may form that modify the fluid composition evolution and may grow on the dissolving minerals thus armoring the surface. Even so, such closed system experiments provide critical data for what minerals would actually form in field applications and how coupled dissolution and precipitation mineral reactions are strongly linked. Comparing to experimental observations can test the reactive transport model, and the experimental observations can be better understood by comparing the results to the modeling. We apply a 0D end member of the model to understand the dissolution of single crystals of forsterite in a variety of settings (low pH, high pH, or NaHCO3 initial fluids, at 100 C and 1 bar, or 200 C and 150 bar). Depending on the initial conditions, we observe the precipitation of talc, brucite, amorphous silica, chrysotile, or magnesite, in various combinations. We compare simulation results to fluid compositions and the presence of secondary minerals experimentally sampled at various times. Insight from the simulations helped create an inverse model to extract the rates of forsterite dissolution and to create a simple forward model useful for exploring the influence of system size, secondary mineral surface areas, etc. Our reactive transport model allows secondary minerals to armor the forsterite surface, which can strongly decrease the dissolution rate as the system evolves. Tuning our model with experimentally derived rates and assuring relevant processes are included so as to reproduce experimental observations is necessary before upscaling to heterogeneous field conditions. The reactive transport model will be used for field-scale sequestration simulations and coupled with a geomechanical model that includes the influence of deformation.

Bolton, E. W.; Wang, Z.; Ague, J.; Bercovici, D.; Cai, Z.; Karato, S.; Oristaglio, M. L.; Qiu, L.

2012-12-01

81

The Rate of the Proton-Proton Reaction  

E-print Network

We re-evaluate the matrix element for the proton-proton reaction which is important for stellar-evolution calculations and for the solar-neutrino problem. We self-consistently determine the effect of vacuum polarization on the matrix element by first correcting the low-energy scattering data to account for vacuum polarization. We then calculate the proton-proton wave function by integrating the Schrodinger equation with vacuum polarization included. We use improved data for proton-proton scattering and for the deuteron wave function. We evaluate the uncertainties that are due to experimental error and estimate those that are due to theoretical inadequacies. We estimate the theoretical uncertainty by using six different deuteron potentials and five different proton-proton potentials. Vacuum polarization decreases the calculated value by $0.6_{-0.4}^{+0.1}$\\%. The complete result is $\\Lambda^2=6.92\\times(1\\pm0.002^{+0.014}_{-0.009})$ where the first uncertainty is due to experimental errors and the second uncertainty is due to theoretical uncertainties. Our value of $\\Lambda^2$ is 2\\% smaller than the value obtained in 1969 by Bahcall and May. The improved calculations of the rate of the $pp$ reaction described here increase slightly the predicted event rates for the chlorine and the Kamiokande solar-neutrino experiments.

Marc Kamionkowski; John N. Bahcall

1993-06-24

82

Spatial rock-paper-scissors models with inhomogeneous reaction rates.  

PubMed

We study several variants of the stochastic four-state rock-paper-scissors game or, equivalently, cyclic three-species predator-prey models with conserved total particle density, by means of Monte Carlo simulations on one- and two-dimensional lattices. Specifically, we investigate the influence of spatial variability of the reaction rates and site occupancy restrictions on the transient oscillations of the species densities and on spatial correlation functions in the quasistationary coexistence state. For small systems, we also numerically determine the dependence of typical extinction times on the number of lattice sites. In stark contrast with two-species stochastic Lotka-Volterra systems, we find that for our three-species models with cyclic competition quenched disorder in the reaction rates has very little effect on the dynamics and the long-time properties of the coexistence state. Similarly, we observe that site restriction only has a minor influence on the system's dynamical properties. Our results therefore demonstrate that the features of the spatial rock-paper-scissors system are remarkably robust with respect to model variations, and stochastic fluctuations as well as spatial correlations play a comparatively minor role. PMID:21230502

He, Qian; Mobilia, Mauro; Täuber, Uwe C

2010-11-01

83

New insights into atrazine degradation by cobalt catalyzed peroxymonosulfate oxidation: Kinetics, reaction products and transformation mechanisms.  

PubMed

The widespread occurrence of atrazine in waters poses potential risk to ecosystem and human health. In this study, we investigated the underlying mechanisms and transformation pathways of atrazine degradation by cobalt catalyzed peroxymonosulfate (Co(II)/PMS). Co(II)/PMS was found to be more efficient for ATZ elimination in aqueous solution than Fe(II)/PMS process. ATZ oxidation by Co(II)/PMS followed pseudo-first-order kinetics, and the reaction rate constant (kobs) increased appreciably with increasing Co(II) concentration. Increasing initial PMS concentration favored the decomposition of ATZ, however, no linear relationship between kobs and PMS concentration was observed. Higher efficiency of ATZ oxidation was observed around neutral pH, implying the possibility of applying Co(II)/PMS process under environmental realistic conditions. Natural organic matter (NOM), chloride (Cl(-)) and bicarbonate (HCO3(-)) showed detrimental effects on ATZ degradation, particularly at higher concentrations. Eleven products were identified by applying solid phase extraction-liquid chromatography-mass spectrometry (SPE-LC/MS) techniques. Major transformation pathways of ATZ included dealkylation, dechlorination-hydroxylation, and alkyl chain oxidation. Detailed mechanisms responsible for these transformation pathways were discussed. Our results reveal that Co(II)/PMS process might be an efficient technique for remediation of groundwater contaminated by ATZ and structurally related s-triazine herbicides. PMID:25544494

Ji, Yuefei; Dong, Changxun; Kong, Deyang; Lu, Junhe

2015-03-21

84

Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles  

E-print Network

under illumination; this method pro- vides only the average reaction rate of all the various redox reacWater-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles of the gas-evolving reaction rate. Optical microscopy was used to record the bubble growth on single

Atwater, Harry

85

Reaction rate calculation with time-dependent invariant manifolds Thomas Bartsch,1  

E-print Network

Reaction rate calculation with time-dependent invariant manifolds Thomas Bartsch,1 F. Revuelta,2 R mechanism, and provides at the same time a simple approximation to the reaction rate. More specifically, TST 2012) The identification of trajectories that contribute to the reaction rate is the crucial dynamical

86

Atmospheric Environment 38 (2004) 21772185 Proton transfer reaction rate constants between hydronium ion  

E-print Network

Atmospheric Environment 38 (2004) 2177­2185 Proton transfer reaction rate constants between in revised form 3 December 2003; accepted 14 January 2004 Abstract We report proton transfer reaction rate. The newly calculated proton transfer rate constants facilitate the use of the proton transfer reaction mass

87

Neural network - based estimation of reaction rates with partly unknown states and completely known kinetics coefficients  

Microsoft Academic Search

This work is focused on developing a more efficient computational scheme for estimation of process reaction rates based on NN models. In contrast to the traditional way of process reaction rates estimation by exhaustive and expensive search for the most appropriate parameterized structure, a neural network (NN) based procedure is proposed here to identify the reaction rates in the framework

Petia Georgieva; Sebastião Feyo de Azevedo

2008-01-01

88

Kinetics of tetrabromobisphenol A (TBBPA) reactions with H?SO?, HNO? and HCl: implication for hydrometallurgy of electronic wastes.  

PubMed

Hydrometallurgy is an acid leaching based process widely used for recovering precious metals from electronic wastes (e-wastes). The effects of acid leaching on the fate of brominated flame retardants (BFRs) in typical hydrometallurgical processes remain largely unknown. This study was aimed at evaluating the fate of tetrabromobisphenol A (TBBPA), a commonly used BFR, in three acid leaching reagents (i.e. H2SO4, HNO3, and HCl) commonly used in hydrometallurgy. It was found that the reactions of TBBPA with concentrated H2SO4 followed a pseudo-zero-order rate and the reaction rates declined rapidly as the concentrations of H2SO4 decreased. In contrast, TBBPA could be easily transformed in less concentrated HNO3 solutions (<21.7 wt%) and the reactions followed a pseudo-first-order rate. The reaction products identified by GC-MS indicated different transformation pathways of TBBPA in H2SO4 and HNO3. HCl or HCl/H2SO4 mixtures (3:1, v/v) did not appear to react with TBBPA, while aqua regia (3:1 HCl/HNO3, v/v) reacted violently with TBBPA and led to almost complete disappearance of TBBPA within a minute. It suggested that HNO3 significantly affected the fate of TBBPA and the use of HNO3 as leaching reagents in hydrometallurgy of e-wastes should be carefully evaluated. Collectively, our findings of distinct fate of TBBPA in different acid leaching reagents provided fundamental information for design of hydrometallurgical treatment of e-wastes to minimize acid reactions with BFRs within plastics matrix and to maximize acid leaching efficiency for metals recycling processes. PMID:24594840

Zhong, Yin; Li, Dan; Mao, Zhe; Huang, Weilin; Peng, Ping'an; Chen, Pei; Mei, Jun

2014-04-15

89

The influence of steric hindrance on kinetics and isotope effects in the reaction of 2,2-bis(4-dimethylaminophenyl)-1-nitro-1-(4-nitrophenyl)ethane with DBU base in acetonitrile  

NASA Astrophysics Data System (ADS)

The pKa value for 2,2-bis(4-dimethylaminophenyl)-1-nitro-1-(4-nitrophenyl)ethane, (dmap)2 (pKa = 25.11) has been measured spectrophotometrically using buffer solutions of a few strong amine bases: 1,8-diazabicyclo[5.4.0]undec-7-ene, (DBU); 1,1,3,3-tetramethylguanidine, (TMG); 1,5,7-triazabicyclo[4.4.0]dec-5-ene, (TBD); 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, (MTBD) and their salts. The low energy conformers of nitrophenyl nitroalkanes have been determined using the semiempirical PM6 methods, (B3-LYP) density functional theory (DFT) together with the 6-31G(d,p) basis set. The participation of the low energy conformer in the proton transfer reaction to DBU base has been discussed. The kinetic data for proton transfer reactions between (dmap)2 and DBU in acetonitrile (MeCN) at pseudo-first order conditions have been presented. The influence of steric hindrance brought by reacting C-acid and organic base on the stability of the transition state has been discussed. The rates of second-order rate constants for series of nitrophenyl nitroalkanes, NO2PhCHRNO2 (R = Me; Et; iPr; dimethylaminophenyl = (dmap)2) are presented and discussed.

Nowak, Iwona; Jarczewski, Arnold

2014-11-01

90

Reaction-rate, mixture-fraction, and temperature imaging in turbulent methane\\/air jet flames  

Microsoft Academic Search

Instantaneous two-dimensional measurements of reaction rate, mixture fraction, and temperature are demonstrated in turbulent partially premixed methane\\/air jet flames. The forward reaction rate of the reaction CO OH ? CO2 H is measured by simultaneous OH laser-induced fluorescence (LIF) and two-photon CO LIF. The product of the two LIF signals is shown to be proportional to the reaction rate. Temperature

Jonathan H. Frank; Sebastian A. Kaiser; Marshall B. Long

2002-01-01

91

Enhanced reaction rates in NDP analysis with neutron scattering  

SciTech Connect

Neutron depth profiling (NDP) makes accessible quantitative information on a few isotopic concentration profiles ranging from the surface into the sample a few micrometers. Because the candidate analytes for NDP are few, there is little interference encountered. Furthermore, neutrons have no charge so mixed chemical states in the sample are of no direct concern. There are a few nuclides that exhibit large probabilities for neutron scattering. The effect of neutron scattering on NDP measurements has not previously been evaluated as a basis for either enhancing the reaction rates or as a source of measurement error. Hydrogen is a common element exhibiting large neutron scattering probability found in or around sample volumes being analyzed by NDP. A systematic study was conducted to determine the degree of signal change when neutron scattering occurs during analysis. The relative signal perturbation was evaluated for materials of varied neutron scattering probability, concentration, total mass, and geometry. Signal enhancements up to 50% are observed when the hydrogen density is high and in close proximity to the region of analysis with neutron beams of sub thermal energies. Greater signal enhancements for the same neutron number density are reported for thermal neutron beams. Even adhesive tape used to position the sample produces a measureable signal enhancement. Because of the shallow volume, negligible distortion of the NDP measured profile shape is encountered from neutron scattering.

Downing, R. Gregory, E-mail: gregory.downing@nist.gov [National Institute of Standards and Technology, Chemical Sciences Division, Gaithersburg, Maryland 20899 (United States)

2014-04-15

92

Enhanced reaction rates in NDP analysis with neutron scattering.  

PubMed

Neutron depth profiling (NDP) makes accessible quantitative information on a few isotopic concentration profiles ranging from the surface into the sample a few micrometers. Because the candidate analytes for NDP are few, there is little interference encountered. Furthermore, neutrons have no charge so mixed chemical states in the sample are of no direct concern. There are a few nuclides that exhibit large probabilities for neutron scattering. The effect of neutron scattering on NDP measurements has not previously been evaluated as a basis for either enhancing the reaction rates or as a source of measurement error. Hydrogen is a common element exhibiting large neutron scattering probability found in or around sample volumes being analyzed by NDP. A systematic study was conducted to determine the degree of signal change when neutron scattering occurs during analysis. The relative signal perturbation was evaluated for materials of varied neutron scattering probability, concentration, total mass, and geometry. Signal enhancements up to 50% are observed when the hydrogen density is high and in close proximity to the region of analysis with neutron beams of sub thermal energies. Greater signal enhancements for the same neutron number density are reported for thermal neutron beams. Even adhesive tape used to position the sample produces a measureable signal enhancement. Because of the shallow volume, negligible distortion of the NDP measured profile shape is encountered from neutron scattering. PMID:24784661

Downing, R Gregory

2014-04-01

93

Influence of Chemical Kinetics on Postcolumn Reaction in a Capillary Taylor Reactor with Catechol Analytes and Photoluminescence Following Electron Transfer  

PubMed Central

Postcolumn derivatization reactions can enhance detector sensitivity and selectivity, but their successful combination with capillary liquid chromatography has been limited because of the small peak volumes in capillary chromatography. A capillary Taylor reactor (CTR), developed in our laboratory, provides simple and effective mixing and reaction in a 25-?m-radius postcolumn capillary. Homogenization of reactant streams occurs by radial diffusion, and a chemical reaction follows. Three characteristic times for a given reaction process can be predicted using simple physical and chemical parameters. Two of these times are the homogenization time, which governs how long it takes the molecules in the analyte and reagent streams to mix, and the reaction time, which governs how long the molecules in a homogeneous solution take to react. The third characteristic time is an adjustment to the reaction time called the start time, which represents an estimate of the average time the analyte stream spends without exposure to reagent. In this study, laser-induced fluorescence monitored the extent of the postcolumn reaction (reduction of Os(bpy)33+ by analyte to the photoluminescent Os(bpy)32+) in a CTR. The reaction time depends on the reaction rates. Analysis of product versus time data yielded second-order reaction rate constants between the PFET reagent, tris(2,2?-bipyridine)osmium, and standards ((ferrocenylmethyl)trimethylammonium cation and p-hydroquinone) or catechols (dopamine, epinephrine, norepinephrine, 3, 4-dihydroxyphenylacetic acid. The extent of the reactions in a CTR were then predicted from initial reaction conditions and compared to experimental results. Both the theory and experimental results suggested the reactions of catechols were generally kinetically controlled, while those of the standards were controlled by mixing time (1–2 s). Thus, the extent of homogenization can be monitored in a CTR using the relatively fast reaction of the reagent and p-hydroquinone. Kinetically controlled reactions of catechols, however, could be also completed in a reasonable time at increased reagent concentration. A satisfactory reactor, operating at 1.7 cm/s (2 ?L/min) velocity with solutes having diffusion coefficients in the 5 × 10?6 cm2/s range, can be constructed from 8.0 cm of 25-?m-radius capillary. Slower reactions require longer reaction times, but theoretical calculations expect that a CTR does not broaden a chromatographic peak (N = 14 000) from a 100-?m-capillary chromatography column by 10% if the pseudo-first-order rate constant is larger than 0.1 s?1. PMID:15858975

Jung, Moon Chul; Weber, Stephen G.

2006-01-01

94

Kinetics study of heterogeneous reactions of ozone with erucic acid using an ATR-IR flow reactor.  

PubMed

The ozone initiated heterogeneous oxidation of erucic acid (EA) thin film was investigated using a flow system combined with attenuated total reflection infrared spectroscopy (ATR-IR) over wide ranges of ozone concentrations (0.25-60 ppm), thin film thickness (0.1-1.0 ?m), temperatures (263-298 K), and relative humidities (0-80% RH) for the first time. Pseudo-first-order rate constants, kapp, and overall reactive uptake coefficients, ?, were obtained through changes in the absorbance of C[double bond, length as m-dash]O stretching bands at 1695 cm(-1), which is assigned to the carbonyl group in carboxylic acid. Results showed that the reaction followed the Langmuir-Hinshelwood mechanism and kapp was largely dominated by surface reaction over bulk phase reaction. In addition, both the kapp and the ? values showed very strong temperature dependences (?two orders of magnitude) over the temperature range; in contrast, they only slightly increased with increasing RH values from 0-80%. According to the kapp values as a function of temperature, the activation energy for the heterogeneous reaction was estimated to be 80.6 kJ mol(-1). Our results have suggested that heterogeneous reactions between ozone and unsaturated solid surfaces likely have a substantially greater temperature dependence than liquid ones. Moreover, the hygroscopic properties of EA thin films before and after exposure to ozone were also studied by measurement of water uptake. Based on the hygroscopicity data, the insignificant RH effect on reaction kinetics was probably due to the relatively weak water uptake by the unreacted and reacted EA thin films. PMID:24457621

Leng, Chunbo; Hiltner, Joseph; Pham, Hai; Kelley, Judas; Mach, Mindy; Zhang, Yunhong; Liu, Yong

2014-03-01

95

Cross sections and reaction rates of relevance to aeronomy  

SciTech Connect

Experimental and theoretical data relevant to models and measurements of the chemical and thermal structures and luminosity of the thermospheres of the earth and planets published during the last four years are surveyed. Among chemical processes, attention is given to ion-molecule reactions, dissociative recombination of molecular ions, and reactions between neutral species. Both reactions between ground state species and species in excited states are considered, including energy transfer and quenching. Measured and calculated cross sections for interactions of solar radiation with atmospheric species, such as photoabsorption, photoionization, and photodissociation and related processes are surveyed.

Fox, J.L. (USAF, Geophysics Laboratory, Hanscom AFB, MA (United States))

1991-01-01

96

A study of rates of reaction of some substituted diphenyldiazomethanes and benzoic acid  

E-print Network

A STUDY OF RATES OF REACTION OF SOME SUBSTITUTED DIPHENYLDIAZ(METHANES AM) BENZOIC ACID A Dissertation By Ralph Fredrick Gilby, Jr. Approved as to style and content by; May 195^ A STOUT 'OF1 WffBS CF KEAC9XOI O f SOB ffliHfiiyi'f ilfl.... . . . . . . . . 11 II. Reaction Rate Constants (k) and Reaction Rate of if,-Dimethoxydiphenyldiazomethane and Benzoic Acid................................. ... . %8 IV...

Gilby, Ralph Fredrick

1954-01-01

97

Reaction rate oscillations during catalytic CO oxidation: A brief overview  

NASA Technical Reports Server (NTRS)

It is not the intent here to present a comprehensive review of the dynamic behavior of the catalytic oxidation of CO. This reaction is one of the most widely studied in the field of catalysis. A review paper by Engel and Ertl has examined the basic kinetic and mechanistic aspects, and a comprehensive paper by Razon and Schmitz was recently devoted to its dynamic behavior. Those interested in further study of the subject should consult these reviews and a number of general review papers on catalytic reaction dynamics. The goal is to present a brief overview of certain interesting aspects of the dynamic behavior of this reaction and to discuss a few questions and issues, which are still the subject of study and debate.

Tsotsis, T. T.; Sane, R. C.

1987-01-01

98

Rate Constants for the Reactions of Hydroxyl Radical with Several Alkanes, Cycloalkanes, and Dimethyl Ether  

NASA Technical Reports Server (NTRS)

Relative rate experiements were used to measure rate constants and temperature denpendencies of the reactions of OH with propane, n-butane, n-pentane, n-hexane, cyclopropane, cyclobutane, cyclopentane, and dimethyl ether.

DeMore, W.; Bayes, K.

1998-01-01

99

Derivation of rate expressions for nonadiabatic proton-coupled electron transfer reactions in solution  

E-print Network

Derivation of rate expressions for nonadiabatic proton-coupled electron transfer reactions, and the transferring proton s are treated quantum mechanically. In this formulation, a PCET reaction is described. For PCET reactions involving the transfer of one electron and one proton, these mixed electronic/proton

Hammes-Schiffer, Sharon

100

Estimation of the Mutation Rate during Error-prone Polymerase Chain Reaction  

E-print Network

Estimation of the Mutation Rate during Error-prone Polymerase Chain Reaction Dai Wang1 , Cheng-prone polymerase chain reaction (PCR) is widely used to introduce point mutations during in vitro evolution step of in vitro evolution is mutagenesis. Error-prone polymerase chain reaction (PCR) (Leung et al

Sun, Fengzhu - Sun, Fengzhu

101

Interlaboratory reaction rate program. 12th progress report, November 1976-October 1979  

SciTech Connect

The Interlaboratory Reaction Rate UILRR) program is establishing the capability to accurately measure neutron-induced reactions and reaction rates for reactor fuels and materials development programs. The goal for the principal fission reactions, /sup 235/U, /sup 238/U and /sup 239/Pu, is an accuracy to within +- 5% at the 95% confidence level. Accurate measurement of other fission and nonfission reactions is also required, but to a lesser accuracy, between +- 5% and 10% at the 95% confidence level. A secondary program objective is improvement in knowledge of the nuclear parameters involved in the standarization of fuels and materials dosimetry measurements of neutron flux, spectra, fluence and burnup.

Lippincott, E.P.; McElroy, W.N.; Preston, C.C. (comps.)

1980-09-01

102

RATE CONSTANT FOR THE REACTION OF OH RADICALS WITH DIACETYLENE AT 297 + OR - 2K  

EPA Science Inventory

Using a relative rate technique, the rate constant for the gas phase reaction of OH radicals with diacetylene, a reaction considered to be of importance in fuel-rich acetylene oxidation, has been determined at 297 + or - 2 K and atmospheric pressure....

103

Acceleration of Key Reactions as a Strategy to Elucidate the Rate-Limiting Chemistry Underlying  

E-print Network

Acceleration of Key Reactions as a Strategy to Elucidate the Rate-Limiting Chemistry Underlying of accelerating each step separately, while monitoring the rate of formation of the end product of the series of phototransduction turn-off. These reactions were accelerated separately and to- gether by adding hydroxylamine and

Wensel, Theodore G.

104

Overall Rate Constant Measurements of the Reaction of Chloroalkylperoxy Radicals with Nitric Oxide  

E-print Network

Overall Rate Constant Measurements of the Reaction of Chloroalkylperoxy Radicals with Nitric Oxide of the NO reaction with chloroalkylperoxy radicals derived from the Cl-initiated oxidation of several atmospherically the OH-initiated oxidation of alkenes) with NO yielded identical rate constants for all of the alkenes

Elrod, Matthew J.

105

The Effect of the Triple-? Reaction Rate on Stellar Evolution at Low-Metallicity  

NASA Astrophysics Data System (ADS)

We investigate the effect of the triple-? reaction rates on the evolution of low-mass stars and massive stars. The former is compared with the observations of metal-poor stars known to date. For the latter, we discuss the impact of recent calculation of triple-? reaction rate by Ogata et al. (2009, PTP, 122, 1055) on the evolution until carbon burning.

Suda, Takuma; Hirschi, Raphael; Fujimoto, Masayuki Y.

2010-06-01

106

Estimation of rate constants in lysozyme-catalyzed reaction of chitooligosaccharides.  

PubMed

The rate constants of the cleavage of glycoside linkage, hydration (hydrolysis) and transglycosylation in a lysozyme-catalyzed reaction of substrate chitooligosaccharides were evaluated by computer analysis of the experimentally obtained reaction time-courses. In the computer analysis, the rate equation was numerically solved by use of the known binding constants for each subsite. Because of the complexity of the lysozyme-catalyzed reaction, optimal values of rate constants were determined by checking the sensitivity of each rate constant to the computed time-courses. It was not possible to estimate uniquely the rate constants for transglycosylation and hydration, owing to the nature of the enzymatic reaction, but it was possible to estimate accurately their ratio. The estimated values were 0.94 s-1 for the rate constant for the cleavage of glycosidic linkage and 133 for the ratio of rate constants of transglycosylation and hydration. PMID:7309713

Masaki, A; Fukamizo, T; Otakara, A; Torikata, T; Hayashi, K; Imoto, T

1981-10-01

107

Reaction between plutonium and deuterium. Part II. Rate measurements by weight changes  

Microsoft Academic Search

The effects of pressure and temperature upon the rate and mechanism of the reaction between plutonium and deuterium have been studied for temperatures to 400°C at pressures from 1 to 15 kPa. The rate was determined by measuring the weight changes of plutonium disks as the reaction proceeded. Below 200°C the reaction product spalled to expose fresh metal. The equation

David F. Bowersox

1977-01-01

108

Reaction rate constants of HO2 + O3 in the temperature range 233-400 K  

NASA Technical Reports Server (NTRS)

The reaction rate constants of HO2 + O3 were measured in the temperature range 233-400 K using a discharge flow system with photofragment emission detection. In the range 233-253 K, the constants are approximately a constant value, and then increase with increasing temperature. This result suggests that the reaction may have two different channels. An expression representing the reaction rate constants is presented.

Wang, Xiuyan; Suto, Masako; Lee, L. C.

1988-01-01

109

Absolute rate parameters for the reaction of ground state atomic oxygen with carbonyl sulfide  

NASA Technical Reports Server (NTRS)

The rate parameters for the reaction of O(3P) with carbonyl sulfide, O(3P) + OCS yields CO + SO, have been determined directly by monitoring O(3P) using the flash photolysis-resonance fluorescence technique. The value for reaction rate was measured over a temperature range of 263-502 K and the data were fitted to an Arrhenius expression with good linearity. A comparison of the present results with those from previous studies of this reaction is also presented.

Klemm, R. B.; Stief, L. J.

1974-01-01

110

Rate of recombination of oxygen atoms and CO at temperatures below ambient  

NASA Technical Reports Server (NTRS)

The measurements reported were conducted with 30.0 torr CO, 40.0 torr argon, and varying amounts of carbon dioxide from about 20 to 230 torr. Measurements were made at 277, 272, 263, and 257 K. The pseudo-first-order decay rate as a function of carbon dioxide pressure is shown in a graph. An Arrhenius plot for the rate constants obtained from the measurements is also presented. A value for the activation energy was determined on the basis of a linear least-squares fit to the data.

Inn, E. C. Y.

1974-01-01

111

X-ray burster and nuclear reaction rates.  

NASA Astrophysics Data System (ADS)

The X-ray burst duration is sufficient to complete certain proton capturing nuclear reaction cycle, at high stellar temperature. Ne-Na cycle are found to be completed within 10 to 100 secs for a variety of temperatures and density. Considering the cycle to be in equilibrium the authors have calculated the number density ratios of Ne20, Ne21, Ne22 and they are compared with meteoritic samples.

Deka, I. R.; Duorah, K.; Duorah, H. L.

1996-12-01

112

Error Rate Comparison during Polymerase Chain Reaction by DNA Polymerase  

PubMed Central

As larger-scale cloning projects become more prevalent, there is an increasing need for comparisons among high fidelity DNA polymerases used for PCR amplification. All polymerases marketed for PCR applications are tested for fidelity properties (i.e., error rate determination) by vendors, and numerous literature reports have addressed PCR enzyme fidelity. Nonetheless, it is often difficult to make direct comparisons among different enzymes due to numerous methodological and analytical differences from study to study. We have measured the error rates for 6 DNA polymerases commonly used in PCR applications, including 3 polymerases typically used for cloning applications requiring high fidelity. Error rate measurement values reported here were obtained by direct sequencing of cloned PCR products. The strategy employed here allows interrogation of error rate across a very large DNA sequence space, since 94 unique DNA targets were used as templates for PCR cloning. The six enzymes included in the study, Taq polymerase, AccuPrime-Taq High Fidelity, KOD Hot Start, cloned Pfu polymerase, Phusion Hot Start, and Pwo polymerase, we find the lowest error rates with Pfu, Phusion, and Pwo polymerases. Error rates are comparable for these 3 enzymes and are >10x lower than the error rate observed with Taq polymerase. Mutation spectra are reported, with the 3 high fidelity enzymes displaying broadly similar types of mutations. For these enzymes, transition mutations predominate, with little bias observed for type of transition. PMID:25197572

McInerney, Peter; Adams, Paul; Hadi, Masood Z.

2014-01-01

113

Hydrolysis of 1-(X-substituted-benzoyl)-4-aminopyridinium ions: effect of substituent X on reactivity and reaction mechanism.  

PubMed

A kinetic study is reported for hydrolysis of 1-(X-substituted-benzoyl)-4-aminopyridinium ions 2a-i, which were generated in situ from the nucleophilic substitution reaction of 2,4-dinitrophenyl X-substituted-benzoates 1a-i with 4-aminopyridine in 80 mol% H(2)O/20 mol% DMSO at 25.0 ± 0.1 °C. The plots of pseudo-first-order rate constants k(obsd) vs. pyridine concentration are linear with a large positive intercept, indicating that the hydrolysis of 2a-i proceeds through pyridine-catalyzed and uncatalyzed pathways with the rate constant k(cat) and k(o), respectively. The Hammett plots for k(cat) and k(o) consist of two intersecting straight lines, which might be taken as evidence for a change in the rate-determining step (RDS). However, it has been proposed that the nonlinear Hammett plots are not due to a change in the RDS but are caused by stabilization of 2a-i in the ground state through a resonance interaction between the ?-electron-donor substituent X and the carbonyl functionality. This is because the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with ?(X) = 1.45 and r = 0.76 for k(cat) while ?(X) = 1.39 and r = 0.72 for k(o). A possibility that the hydrolysis of 2a-i proceeds through a concerted mechanism has been ruled out on the basis of the large ?(X) values. Thus, the reaction has been concluded to proceed through a stepwise mechanism in which the leaving group departs after the RDS since OH(-) is more basic and a poorer nucleofuge than 4-aminopyridine. PMID:22024890

Um, Ik-Hwan; Kim, Eun-Hee; Kang, Ji-Sun

2011-12-01

114

The Gaseous Explosive Reaction : the Effect of Pressure on the Rate of Propagation of the Reaction Zone and upon the Rate of Molecular Transformation  

NASA Technical Reports Server (NTRS)

This study of gaseous explosive reaction has brought out a number of important fundamental characteristics of the explosive reaction indicating that the basal processes of the transformation are much simpler and corresponds more closely to the general laws and principles of ordinary transformations than is usually supposed. The report calls attention to the point that the rate of molecular transformation within the zone was found in all cases to be proportional to pressure, that the transformation within the zone is the result of binary impacts. This result is of unusual interest in the case of the reaction of heavy hydrocarbon fuels and the reaction mechanism proposed by the recent kinetic theory of chain reactions.

Stevens, F W

1932-01-01

115

Upper atmosphere research: Reaction rate and optical measurements  

NASA Technical Reports Server (NTRS)

The objective is to provide photochemical, kinetic, and spectroscopic information necessary for photochemical models of the Earth's upper atmosphere and to examine reactions or reactants not presently in the models to either confirm the correctness of their exclusion or provide evidence to justify future inclusion in the models. New initiatives are being taken in technique development (many of them laser based) and in the application of established techniques to address gaps in the photochemical/kinetic data base, as well as to provide increasingly reliable information.

Stief, L. J.; Allen, J. E., Jr.; Nava, D. F.; Payne, W. A., Jr.

1990-01-01

116

Chiral polymerization in open systems from chiral-selective reaction rates.  

PubMed

We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates. PMID:22610131

Gleiser, Marcelo; Nelson, Bradley J; Walker, Sara Imari

2012-08-01

117

Chiral Polymerization in Open Systems From Chiral-Selective Reaction Rates  

NASA Astrophysics Data System (ADS)

We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates.

Gleiser, Marcelo; Nelson, Bradley J.; Walker, Sara Imari

2012-08-01

118

Rate constant for the reaction between OH and CS2 at 298 and 520 K  

NASA Technical Reports Server (NTRS)

In an attempt to resolve discrepancies between published values of the rate constant for the reaction between the hydroxyl radical and carbon disulfide, the reaction has been studied in a discharge flow system by using resonance fluorescence for kinetic measurements and mass spectrometry for product analysis. On the basis of the measured rate constant for disappearance of OH, and measurements of the amount of carbonyl sulfide formed, it was estimated that for the reaction HO + CS2 yields HS + OCS, rate constant values are not greater than 3 x 10 to the -15th/cu cm per sec at 520 K and not greater than 7 x 10 to the -15th/cu cm per sec at 298 K, upper limits are specified because of the inability to isolate exclusively this reaction channel, and because of possible involvement of wall reactions. These results confirm the low values found for this rate constant in two very recent studies.

Leu, M.-T.; Smith, R. H.

1982-01-01

119

The rate of the He-3(p,e+ nu) - He-4 reaction  

NASA Astrophysics Data System (ADS)

The reaction rate of He-3 with a proton to produce He-4 + e(+) nu is estimated. The matrix elements of the He-3 + p reaction and of thermal neutron capture are calculated rigorously, and the assumptions used by Werntz and Brennan (1967) in their calculation are shown to be incorrect. Meson exchange contributions are estimated at 20 percent, and the reaction rate at solar temperatures is calculated using experimental cross sections of the (n, gamma) reaction: the cross section factor S(Ep = 0) is between 4 and 25 x 10 to the -20th keV barns, or about 6 times larger than predicted by Werntz and Brennan. However, the neutrinos from the reaction are found to contribute less than 3 percent to the predicted Cl-37 capture rate and 0.03 percent to the Ga-71 capture rate.

Tegner, P. E.; Bargholtz, Chr.

1983-09-01

120

Rate constants for the atmospheric reactions of alkoxy radicals: An updated estimation method  

NASA Astrophysics Data System (ADS)

Alkoxy radicals are key intermediates in the atmospheric degradations of volatile organic compounds, and can typically undergo reaction with O 2, unimolecular decomposition or unimolecular isomerization. Previous structure-reactivity relationships for the estimation of rate constants for these processes for alkoxy radicals [Atkinson, R., 1997. Atmospheric reactions of alkoxy and ?-hydroxyalkoxy radicals. International Journal of Chemical Kinetics, 29, 99-111; Aschmann, S.M., Atkinson, R., 1999. Products of the gas-phase reactions of the OH radical with n-butyl methyl ether and 2-isopropoxyethanol: reactions of ROC(O rad )< radicals. International Journal of Chemical Kinetics, 31, 501-513] have been updated to incorporate recent kinetic data from absolute and relative rate studies. Temperature-dependent rate expressions are derived allowing rate constants for all three of these alkoxy radical reaction pathways to be calculated at atmospherically relevant temperatures.

Atkinson, Roger

121

Introduction Intracellular reaction rates are dependent both on the catalytic  

E-print Network

in some fish white muscles, which may devote mitochondria, whereas extremely high metabolic rates are observed in insect flight muscle, where dense clusters of mitochondria may exceed 40% of the fiber volume. Thus, the diversity of muscle structure and function found in nature encompasses

Kinsey, Stephen

122

Sensitivity study of explosive nucleosynthesis in Type Ia supernovae: I. Modification of individual thermonuclear reaction rates  

E-print Network

We explore the sensitivity of the nucleosynthesis due to type Ia supernovae with respect to uncertainties in nuclear reaction rates. We have adopted a standard one-dimensional delayed detonation model of the explosion of a Chandrasekhar-mass white dwarf, and have post-processed the thermodynamic trajectories of every mass-shell with a nucleosynthetic code, with increases (decreases) by a factor of ten on the rates of 1196 nuclear reactions. We have computed as well hydrodynamic models for different rates of the fusion reactions of 12C and of 16O. For selected reactions, we have recomputed the nucleosynthesis with alternative prescriptions for their rates taken from the JINA REACLIB database, and have analyzed the temperature ranges where modifications of their rates have the strongest effect on nucleosynthesis. The nucleosynthesis resulting from the Type Ia supernova models is quite robust with respect to variations of nuclear reaction rates, with the exception of the reaction of fusion of 12C nuclei. The energy of the explosion changes by less than \\sim4%. The changes in the nucleosynthesis due to the modification of the rates of fusion reactions are as well quite modest, for instance no species with a mass fraction larger than 0.02 experiences a variation of its yield larger than a factor of two. We provide the sensitivity of the yields of the most abundant species with respect to the rates of the most intense reactions with protons, neutrons, and alphas. In general, the yields of Fe-group nuclei are more robust than the yields of intermediate-mass elements. Among the charged particle reactions, the most influential on supernova nucleosynthesis are 30Si + p \\rightleftarrows 31P + {\\gamma}, 20Ne + {\\alpha} \\rightleftarrows 24Mg + {\\gamma}, and 24Mg + {\\alpha} \\rightleftarrows 27Al + p. The temperatures at which a modification of their rate has a larger impact are in the range 2 < T < 4 GK. (abridged)

Eduardo Bravo; Gabriel Martínez-Pinedo

2012-04-09

123

Estimation of the prevalence and rate of acute transfusion reactions occurring in Windhoek, Namibia  

PubMed Central

Background Acute transfusion reactions are probably common in sub-Saharan Africa, but transfusion reaction surveillance systems have not been widely established. In 2008, the Blood Transfusion Service of Namibia implemented a national acute transfusion reaction surveillance system, but substantial under-reporting was suspected. We estimated the actual prevalence and rate of acute transfusion reactions occurring in Windhoek, Namibia. Methods The percentage of transfusion events resulting in a reported acute transfusion reaction was calculated. Actual percentage and rates of acute transfusion reactions per 1,000 transfused units were estimated by reviewing patients’ records from six hospitals, which transfuse >99% of all blood in Windhoek. Patients’ records for 1,162 transfusion events occurring between 1st January – 31st December 2011 were randomly selected. Clinical and demographic information were abstracted and Centers for Disease Control and Prevention National Healthcare Safety Network criteria were applied to categorize acute transfusion reactions1. Results From January 1 – December 31, 2011, there were 3,697 transfusion events (involving 10,338 blood units) in the selected hospitals. Eight (0.2%) acute transfusion reactions were reported to the surveillance system. Of the 1,162 transfusion events selected, medical records for 785 transfusion events were analysed, and 28 acute transfusion reactions were detected, of which only one had also been reported to the surveillance system. An estimated 3.4% (95% confidence interval [CI]: 2.3–4.4) of transfusion events in Windhoek resulted in an acute transfusion reaction, with an estimated rate of 11.5 (95% CI: 7.6–14.5) acute transfusion reactions per 1,000 transfused units. Conclusion The estimated actual rate of acute transfusion reactions is higher than the rate reported to the national haemovigilance system. Improved surveillance and interventions to reduce transfusion-related morbidity and mortality are required in Namibia. PMID:24333079

Meza, Benjamin P.L.; Lohrke, Britta; Wilkinson, Robert; Pitman, John P.; Shiraishi, Ray W.; Bock, Naomi; Lowrance, David W.; Kuehnert, Matthew J.; Mataranyika, Mary; Basavaraju, Sridhar V.

2014-01-01

124

Thermonuclear reaction rate of 18Ne(? ,p ) 21Na from Monte Carlo calculations  

NASA Astrophysics Data System (ADS)

The 18Ne(? ,p ) 21Na reaction impacts the break-out from the hot CNO cycles to the r p process in type-I x-ray bursts. We present a revised thermonuclear reaction rate, which is based on the latest experimental data. The new rate is derived from Monte Carlo calculations, taking into account the uncertainties of all nuclear physics input quantities. In addition, we present the reaction rate uncertainty and probability density versus temperature. Our results are also consistent with estimates obtained using different indirect approaches.

Mohr, P.; Longland, R.; Iliadis, C.

2014-12-01

125

Time-Resolved O3 Chemical Chain Reaction Kinetics Via High-Resolution IR Laser Absorption Methods  

NASA Technical Reports Server (NTRS)

Excimer laser photolysis in combination with time-resolved IR laser absorption detection of OH radicals has been used to study O3/OH(v = 0)/HO2 chain reaction kinetics at 298 K, (i.e.,(k(sub 1) is OH + 03 yields H02 + 02 and (k(sub 2) is H02 + 03 yields OH + 202). From time-resolved detection of OH radicals with high-resolution near IR laser absorption methods, the chain induction kinetics have been measured at up to an order of magnitude higher ozone concentrations ([03] less than or equal to 10(exp 17) molecules/cu cm) than accessible in previous studies. This greater dynamic range permits the full evolution of the chain induction, propagation, and termination process to be temporally isolated and measured in real time. An exact solution for time-dependent OH evolution under pseudo- first-order chain reaction conditions is presented, which correctly predicts new kinetic signatures not included in previous OH + 03 kinetic analyses. Specifically, the solutions predict an initial exponential loss (chain "induction") of the OH radical to a steady-state level ([OH](sub ss)), with this fast initial decay determined by the sum of both chain rate constants, k(sub ind) = k(sub 1) + k(sub 2). By monitoring the chain induction feature, this sum of the rate constants is determined to be k(sub ind) = 8.4(8) x 10(exp -14) cu cm/molecule/s for room temperature reagents. This is significantly higher than the values currently recommended for use in atmospheric models, but in excellent agreement with previous results from Ravishankara et al.

Kulcke, Axel; Blackmon, Brad; Chapman, William B.; Kim, In Koo; Nesbitt, David J.

1998-01-01

126

A mixed-phase relative rates technique for measuring aerosol reaction kinetics  

NASA Astrophysics Data System (ADS)

A mixed-phase relative rates approach for measuring rates of reaction in aerosols is presented. Using this method the rate of reaction of methyl oleate (MO) particles, normalized to the gas-particle collision rate, was measured to be ?MO = 1.12 (+/-0.36) × 10-3 with 2-methyl-2-butene as the gas-phase reference. This value compares favorably with our previously published value of 1.23 × 10-3 measured using an absolute technique. Reaction of bis(2-ethylhexyl) sebacate (BES) particles with Cl and OH radicals was also studied using acetone and hexanal, respectively, as the gas-phase references. The rates of reaction of BES, normalized to the gas-particle collision rate, were measured to be ?BES = 1.8 (-0.3+0.8) and ?BES = 2.0 (-0.1+0.6) with Cl and OH, respectively. These fast rates of reaction (?BES > 1) imply that secondary reactions, perhaps involving radical chain mechanisms, could impact the rate at which organic particles are oxidized in the atmosphere.

Hearn, John D.; Smith, Geoffrey D.

2006-09-01

127

Rate constants measured for hydrated electron reactions with peptides and proteins  

NASA Technical Reports Server (NTRS)

Effects of ionizing radiation on the amino acids of proteins and the reactivity of the protonated amino group depends upon the pK subscript a of the group. Estimates of the rate constants for reactions involving the amino acid side chains are presented. These rate constants gave an approximate rate constant for three different protein molecules.

Braams, R.

1968-01-01

128

Enhancing reaction rate of transesterification of glycerol monostearate and methanol by CO(2).  

PubMed

The effect of CO2 as a green additive on the reaction rate of transesterification of glycerol monostearate with methanol was studied at 333.15 and 343.15 K up to 10.5 MPa. It was demonstrated that addition of CO2 in the reaction system could increase the reaction rate significantly. The phase behavior of the CO2+methanol+ glycerol monostearate ternary system was also determined at 333.15 and 343.15 K. It is shown that addition of CO2 can enhance the miscibility of the reactant. The main reasons for the reaction rate enhancement are that CO2 can enhance the miscibility of the reactants, reduce the viscosity of the reaction mixture, and increase the diffusion coefficients of the reactants. PMID:17975900

Song, Jiyuan; Hou, Minqiang; Jiang, Tao; Han, Buxing; Li, Xiaoyong; Liu, Gang; Yang, Guanyin

2007-11-29

129

Fluctuation Spectroscopy: The Determination of Chemical Reaction Rates Based on the Optical Density Fluctuation  

Microsoft Academic Search

The analysis of the half-width of the frequency spectrum due to the concentration fluctuation around the equilibrium value should give the rate constant of a chemical reaction under the appropriate condition. Specially, in a chemical reaction among the molecular species which have respectively different light absorption coefficients, the concentration fluctuations of each component molecule induce the corresponding fluctuation of the

Akiyoshi Watanabe; Yoshi-ichi Sato; Yoshito Amako

1978-01-01

130

Up-Scaling Geochemical Reaction Rates Accompanying Acidic CO2-Saturated Brine Flow in Sandstone Aquifers  

E-print Network

1 Up-Scaling Geochemical Reaction Rates Accompanying Acidic CO2-Saturated Brine Flow in Sandstone in the pore networks corresponding to three different sandstones. The simulations were used to study up

New York at Stoney Brook, State University of

131

Diffusion-controlled reaction rates for two active sites on a sphere  

PubMed Central

Background The diffusion-limited reaction rate of a uniform spherical reactant is generalized to anisotropic reactivity. Previous work has shown that the protein model of a uniform sphere is unsatisfactory in many cases. Competition of ligands binding to two active sites, on a spherical enzyme or cell is studied analytically. Results The reaction rate constant is given for two sites at opposite ends of the species of interest. This is compared with twice the reaction rate for a single site. It is found that the competition between sites lowers the reaction rate over what is expected for two sites individually. Competition between sites does not show up, until the site half angle is greater than 30 degrees. Conclusions Competition between sites is negligible until the site size becomes large. The competitive effect grows as theta becomes large. The maximum effect is given for theta?=?pi/2. PMID:24982756

2014-01-01

132

Proton transfer reaction rate constants between hydronium ion (H 3O +) and volatile organic compounds  

Microsoft Academic Search

We report proton transfer reaction rate constants between the hydronium ion (H3O+) and selected atmospherically important volatile organic compounds (VOCs). The quantum chemical method was used to determine the structures of the organic species employing the density function theory-B3LYP. The ion–molecule reaction rates were determined using the average-dipole-orientation theory, along with the permanent dipole moment and polarizability of the organic

Jun Zhao; Renyi Zhang

2004-01-01

133

Measurement of positional isotope exchange rates in enzyme catalyzed reactions by fast atom bombardment mass spectrometry  

E-print Network

MEASUREMENT OF POSITIONAL ISOTOPE EXCHANGE RATES IN ENZYME CATALYZED REACTIONS BY FAST ATOM BOMBARDMENT MASS SPECTROMETRY A Thesis by LARRY NAYNE HILSCHER Submitted to the Gradu te College of Texas A&M University in partial fulfil" mert... of the requiremer ts for the degree of MASTEP. QF SCIENCE August 1985 Major Subject: Chemistry MEASUREMENT OF POSITIONAL ISOTOPE EXCHANGE RATES IN ENZYME CATALYZED REACTIONS BY FAST ATOM BOMBARDMENT MASS SPECTROMETRY A Thesis LARRY NAYNE HILSCHER Approved...

Hilscher, Larry Wayne

1985-01-01

134

Rated and measured impulsivity in children is associated with diminished cardiac reactions to acute psychological stress.  

PubMed

This study examined the association between impulsivity and heart rate reactions to a brief psychological stress in pre-adolescent children. Impulsivity was assessed by two response inhibition tasks and maternal self-report. Heart rate was measured at rest and in response to a mental arithmetic challenge. Children high in impulsivity showed blunted cardiac stress reactions. This result resonates with previous findings that blunted stress reactivity is characteristic of a range of problematic behaviours often associated with impulsivity. PMID:25038303

Bennett, Carmel; Blissett, Jackie; Carroll, Douglas; Ginty, Annie T

2014-10-01

135

Up-Scaling Geochemical Reaction Rates for Carbon Dioxide (CO2) in Deep Saline Aquifers  

SciTech Connect

The overall goal of the project was to bridge the gap between our knowledge of small-scale geochemical reaction rates and reaction rates meaningful for modeling transport at core scales. The working hypothesis was that reaction rates, determined from laboratory measurements based upon reactions typically conducted in well mixed batch reactors using pulverized reactive media may be significantly changed in in situ porous media flow due to rock microstructure heterogeneity. Specifically we hypothesized that, generally, reactive mineral surfaces are not uniformly accessible to reactive fluids due to the random deposition of mineral grains and to the variation in flow rates within a pore network. Expected bulk reaction rates would therefore have to be correctly up-scaled to reflect such heterogeneity. The specific objective was to develop a computational tool that integrates existing measurement capabilities with pore-scale network models of fluid flow and reactive transport. The existing measurement capabilities to be integrated consisted of (a) pore space morphology, (b) rock mineralogy, and (c) geochemical reaction rates. The objective was accomplished by: (1) characterizing sedimentary sandstone rock morphology using X-ray computed microtomography, (2) mapping rock mineralogy using back-scattered electron microscopy (BSE), X-ray dispersive spectroscopy (EDX) and CMT, (3) characterizing pore-accessible reactive mineral surface area, and (4) creating network models to model acidic CO{sub 2} saturated brine injection into the sandstone rock samples.

Lindquist, W Brent

2009-03-03

136

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reaction flows  

NASA Technical Reports Server (NTRS)

The use of silane (SiH4) as an effective ignitor and flame stabilizing pilot fuel is well documented. A reliable chemical kinetic mechanism for prediction of its behavior at the conditions encountered in the combustor of a SCRAMJET engine was calculated. The effects of hydrogen addition on hydrocarbon ignition and flame stabilization as a means for reduction of lengthy ignition delays and reaction times were studied. The ranges of applicability of chemical kinetic models of hydrogen-air combustors were also investigated. The CHARNAL computer code was applied to the turbulent reaction rate modeling.

Magnotti, F.; Diskin, G.; Matulaitis, J.; Chinitz, W.

1984-01-01

137

Astrophysical S-Factors and Reaction Rates of Threshold (p, n)-Reactions on {sup 99-102}Ru  

SciTech Connect

Astrophysical S-factors of (p, n) reactions on {sup 99}Ru, {sup 100}Ru, {sup 101}Ru, and {sup 102}Ru were derived from the sum of experimental isomeric and ground states cross sections measured in the incident proton energy range of 5-9 MeV. They were compared with Hauser-Feshbach statistical model predictions of the NON-SMOKER code. Good agreement was found in the majority of cases. Reaction rates were derived up to 8.7 GK stellar temperature by combining experiment and theory.

Skakun, Ye. [Kharkiv Institute of Physics and Technology, Academicheskaja str., 1, 61108 Kharkiv (Ukraine); Rauscher, T. [Department of Physics, University of Basel, Klingelbergstr, 82, CH-4056 Basel (Switzerland)

2010-08-12

138

La(3+)-Catalyzed methanolysis of hydroxypropyl-p-nitrophenyl phosphate as a model for the RNA transesterification reaction.  

PubMed

The methanolysis of hydroxypropyl-p-nitrophenyl phosphate (HPNPP, 1) promoted by La(OTf)(3) under buffered conditions was studied in methanol as a function of pH at 25 degrees C. (31)P NMR studies at -90 degrees C indicate that there are at least three La/1 complexes formed at pH approximately 5.3 of 1:1, 2:2, and 1:2 stoichiometry. Kinetic studies of the observed pseudo-first-order rate constants for the methanolysis of 1 as a function of [La(3+)] at 4.5 < pH < 10.5 indicate there are two general pH regimes. In the low pH regime between 4.5 and 7.6, the plots of k(obs) versus [La(3+)] exhibit saturation behavior with very strong 1:1 binding, with a plateau rate constant that depends on [OCH(3)(-)]. The catalytically productive species is shown to be a 2:2 complex of La(3+) and 1, where the phosphate is proposed to be doubly activated, thereby promoting the methoxide reaction by some 4.6 x 10(10)-fold. In the high pH regime from 7.9 to 10.5, 1:1, 2:2, and 2:1 La(3+)/1 complexes are formed with the La(3+) coordinated in the form of [La(3+)(OCH(3)(-))](1,2). Throughout this pH regime at high [La(3+)], a saturation complex, (La(3+)OCH(3)(-))(2)/1, is formed that spontaneously decomposes with a rate constant of (5-10) x 10(-)(3) s(-)(1), leading to an acceleration of 10(9)-fold at pH 8.0. PMID:12568616

Tsang, Josephine S W; Neverov, Alexei A; Brown, R S

2003-02-12

139

Proton-capture Nucleosynthesis In Low Mass Stars: Effects of New Reaction Rates  

SciTech Connect

We present computations of nucleosynthesis in low-mass asymptotic-giant-branch stars of solar metallicity experiencing deep mixing. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass nuclei. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that especially the new reaction rate for the {sup 14}N(p,{gamma}){sup 15}O reaction implies considerable modifications in the composition of low mass red giant stars.

Palmerini, S.; Busso, M. [Dipartimento di Fisica, Universita degli Studi di Perugia and INFN Sezione di Perugia (Italy); La Cognata, M. [DMFCI - Universita di Catania and Laboratori Nazionali del Sud - INFN, Catania (Italy); Cristallo, S. [Departamento de Fisica Teorica y del Cosmos, Universidad de Granada (Spain); INAF Osservatorio Astronomico di Collurania, Teramo (Italy)

2011-10-28

140

STARLIB: A NEXT-GENERATION REACTION-RATE LIBRARY FOR NUCLEAR ASTROPHYSICS  

SciTech Connect

STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For the first time, this library provides the rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the rate and its uncertainty but also its distribution. Because a majority of rates are lognormally distributed, this allows the construction of rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate reaction rates, where uncertainties are rigorously defined. In STARLIB, experimental rates are supplemented with: (1) theoretical TALYS rates for reactions for which no experimental input is available, and (2) laboratory and theoretical weak rates. STARLIB includes all types of reactions of astrophysical interest to Z = 83, such as (p, {gamma}), (p, {alpha}), ({alpha}, n), and corresponding reverse rates. Strong rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based rates.

Sallaska, A. L. [National Institute of Standards and Technology, Gaithersburg, MD 20899-8462 (United States); Iliadis, C.; Champange, A. E. [University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 (United States); Goriely, S. [Institut d'Astronomie et d'Astrophysique, Universite Libre de Bruxelles, C.P. 226, B-1050 Brussels (Belgium); Starrfield, S.; Timmes, F. X., E-mail: anne.sallaska@nist.gov [Arizona State University, Tempe, AZ 85287-1504 (United States)

2013-07-15

141

The nuclear fusion reaction rate based on relativistic equilibrium velocity distribution  

E-print Network

The Coulomb barrier is in general much higher than thermal energy. Nuclear fusion reactions occur only among few protons and nuclei with higher relative energies than Coulomb barrier. It is the equilibrium velocity distribution of these high-energy protons and nuclei that participates in determining the rate of nuclear fusion reactions. In the circumstance it is inappropriate to use the Maxwellian velocity distribution for calculating the nuclear fusion reaction rate. We use the relativistic equilibrium velocity distribution for this purpose. The rate based on the relativistic equilibrium velocity distribution has a reduction factor with respect to that based on the Maxwellian distribution, which factor depends on the temperature, reduced mass and atomic numbers of the studied nuclear fusion reactions. This signifies much to the solar neutrino problem.

Jian-Miin Liu

2002-10-20

142

Determination of shift in electrodic reaction rates due to the presence of stress  

NASA Astrophysics Data System (ADS)

An extension of Butler-Volmer formulation is proposed to determine the stress-induced changes in electrodic reaction rates. Gibbs-Duhem equation is used to determine the stress-dependent chemical potential and the corresponding change in the reaction rate. The scope of possible amplification or reduction in the reaction rates due to tensile and compressive stress fields is explored numerically. Though quantitative experimental validation remains to be pursued, behavioral agreement of the extended Butler-Volmer model with some observations made in the field of corrosive dissolution is established. Our numerical results also indicate that in addition to altering the speed of a reaction, a stress field can modify the shape of an anodic dissolution front. The effect of stress-generated surface patterns is also considered. It is well-established that a stress field can create surface patterns due to surface wrinkling or surface diffusion. We determine the possible significance of such patterns on the reaction rate, and identify the factors that may enhance their contribution to electrodic reaction rates.

Sarkar, Swarnavo; Aquino, Wilkins

2013-03-01

143

Method and apparatus for obtaining enhanced production rate of thermal chemical reactions  

DOEpatents

Reactors and processes are disclosed that can utilize high heat fluxes to obtain fast, steady-state reaction rates. Porous catalysts used in conjunction with microchannel reactors to obtain high rates of heat transfer are also disclosed. Reactors and processes that utilize short contact times, high heat flux and low pressure drop are described. Improved methods of steam reforming are also provided.

Tonkovich, Anna Lee Y. (Pasco, WA); Wang, Yong (Richland, WA); Wegeng, Robert S. (Richland, WA); Gao, Yufei (Kennewick, WA)

2006-05-16

144

The Effect of Conceptual Change Pedagogy on Students' Conceptions of Rate of Reaction  

ERIC Educational Resources Information Center

This paper reports on an investigation of the effect of conceptual change pedagogy on students' conceptions of "rate of reaction" concepts. The study used a pre-test/post-test non-equivalent comparison group design approach and the sample consisted of 72 Turkish grade-11 students (aged 16-18 years) selected from two intact classrooms. The "Rate of…

Calik, Muammer; Kolomuc, Ali; Karagolge, Zafer

2010-01-01

145

Venus volcanism: Rate estimates from laboratory studies of sulfur gas-solid reactions  

NASA Technical Reports Server (NTRS)

Thermochemical reactions between sulfur-bearing gases in the atmosphere of Venus and calcium-, iron-, magnesium-, and sulfur-bearing minerals on the surface of Venus are an integral part of a hypothesized cycle of thermochemical and photochemical reactions responsible for the maintenance of the global sulfuric acid cloud cover on Venus. SO2 is continually removed from the Venus atmosphere by reaction with calcium bearing minerals on the planet's surface. The rate of volcanism required to balance SO2 depletion by reactions with calcium bearing minerals on the Venus surface can therefore be deduced from a knowledge of the relevant gas-solid reaction rates combined with reasonable assumptions about the sulfur content of the erupted material (gas + magma). A laboratory program was carried out to measure the rates of reaction between SO2 and possible crustal minerals on Venus. The reaction of CaCO3(calcite) + SO2 yields CaSO4 (anhydrite) + CO was studied. Brief results are given.

Ehlers, K.; Fegley, B., Jr.; Prinn, R. G.

1989-01-01

146

Rates of grain boundary diffusion through enstatite and forsterite reaction rims  

Microsoft Academic Search

The growth rates of enstatite rims produced by reaction of Fo92 and SiO2 were determined at 250-1500 MPa and 900-1100°C for a wide range of water contents. Growth rates were also determined for forsterite rims between MgO and Mg2Si2O6 and between MgO and SiO2. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of

Richard A. Yund

1997-01-01

147

NUCLEAR PHYSICS: Determination of the stellar reaction rate for 12C(?, ?)16O: using a new expression with the reaction mechanism  

NASA Astrophysics Data System (ADS)

The astrophysical reaction rate of 12C(?, ?)16O plays a key role in massive star evolution. However, this reaction rate and its uncertainties have not been well determined yet, especially at T9 = 0.2. The existing results even disagree with each other to a certain extent. In this paper, the E1, E2 and total (E1+E2) 12C(?, ?)16O reaction rates are calculated in the temperature range from T9 = 0.3 to 2 according to all the available cross section data. A new analytic expression of the 12C(?, ?)16O reaction rate is brought forward based on the reaction mechanism. In this expression, each part embodies the underlying physics of the reaction. Unlike previous works, some physical parameters are chosen from experimental results directly, instead of all the parameters obtained from fitting. These parameters in the new expression, with their 3? fit errors, are obtained from fit to our calculated reaction rate from T9 = 0.3 to 2. Using the fit results, the analytic expression of 12C(?, ?)16O reaction rate is extrapolated down to T9 = 0.05 based on the underlying physics. The 12C(?, ?)16O reaction rate at T9 = 0.2 is (8.78 ± 1.52) × 1015 cm's-1 mol-1. Some comparisons and discussions about our new 12C(?, ?)16O reaction rate are presented, and the contributions of the reaction rate correspond to the different part of reaction mechanism are given. The agreements of the reaction rate below T9 = 2 between our results and previous works indicate that our results are reliable, and they could be included in the astrophysical reaction rate network. Furthermore, we believe our method to investigate the 12C(?, ?)16O reaction rate is reasonable, and this method can also be employed to study the reaction rate of other astrophysical reactions. Finally, a new constraint of the supernovae production factor of some isotopes are illustrated according to our 12C(?, ?)16O reaction rates.

Xu, Yi; Xu, Wang; Ma, Yu-Gang; Cai, Xiang-Zhou; Chen, Jin-Gen; Fan, Gong-Tao; Fan, Guang-Wei; Guo, Wei; Luo, Wen; Pan, Qiang-Yan; Shen, Wen-Qing; Yang, Li-Feng

2009-04-01

148

Scale-dependent rates of uranyl surface complexation reaction in sediments  

NASA Astrophysics Data System (ADS)

Scale-dependency of uranyl[U(VI)] surface complexation rates was investigated in stirred flow-cell and column systems using a U(VI)-contaminated sediment from the US Department of Energy, Hanford site, WA. The experimental results were used to estimate the apparent rate of U(VI) surface complexation at the grain-scale and in porous media. Numerical simulations using molecular, pore-scale, and continuum models were performed to provide insights into and to estimate the rate constants of U(VI) surface complexation at the different scales. The results show that the grain-scale rate constant of U(VI) surface complexation was over 3-10 orders of magnitude smaller than the rate constant calculated using the molecular simulations. The largest rate constant at the grain-scale decreased additional 2 orders of magnitude when the rate was scaled to the porous media in the column. The scaling effect from the grain-scale to the porous media became less important for the slower sorption sites. Pore-scale simulations revealed the importance of coupled mass transport and reactions in both intragranular and inter-granular domains, which caused both spatial and temporal dependence of U(VI) surface complexation rates in the sediment. Pore-scale simulations also revealed that the rate of coupled diffusion and molecular surface complexation reaction in the intragranular porous domains was slower than either individual process alone. The results provide important implications for developing models to scale geochemical/biogeochemical reactions.

Liu, Chongxuan; Shang, Jianying; Kerisit, Sebastien; Zachara, John M.; Zhu, Weihuang

2013-03-01

149

The rate of the reaction between CN and C2H2 at interstellar temperatures  

NASA Technical Reports Server (NTRS)

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.

Woon, D. E.; Herbst, E.

1997-01-01

150

A Study of the ^14C(?,?) Reaction Rate Through the ANC Technique  

NASA Astrophysics Data System (ADS)

The astrophysical significance of the ^14C(?,?) reaction is due to its involvement in the NCO chain. The NCO chain is thought to trigger He flashes in white dwarf stars, and is also thought to be a neutron source for the s-process in low mass stars [L. Buchmann et al., The Astrophys. Journ. 324 (1988), M. Hashimoto et al., The Astrophys. Journ. 307 (1986)]. Recently the ANC of the 3^-, 6.4 MeV, near threshold state in ^18O was measured via a sub-Coulomb, inverse kinematics ?-transfer reaction at the Florida State University J.D. Fox Superconducting Accelerator Laboratory. The ^14C(?,?) reaction rate at stellar temperatures is highly dependent upon the structure of this 3^- state. As shown in E.D. Johnson et al. PRL 97 (2006) the measured ANC can be used to calculate the ^14C(?,?) reaction rate without any dependence on nuclear models.

Johnson, E. D.; Rogachev, G. V.; Mukhamedzhanov, A. M.; Aguilar, A.; Bender, P.; Devore, T.; Goldberg, G. V.; Kemper, K. W.; Lee, S.; Miller, L.; Mitchell, J.; Peplowski, P.; Perry, M.; Reynolds, R.; Rojas, A.

2008-04-01

151

Difference in rates of the reaction of various mammalian oxyhemoglobins with phenylhydrazine.  

PubMed

Second order rate constants for the initial reaction of 12 mammalian oxyhemoglobins (Hb) with equimolar phenylhydrazine (PHZ), a compound inducing Heinz body hemolytic anemia, were determined by recording continuous changes in absorbance with time at 577 nm. The rate constants were varied in a range from 43 m-1.s-1 with pig Hb to 255 m-1.s-1 with dog Hb. On the other hand, isosbestic points at 526 and 587 nm were common to all the reaction processes. The aerobic reaction of Hb with PHZ resulted in denaturation of hemoprotein, and final reaction products were determined to be beta-meso-phenylbiliverdin IX alpha and N-phenylprotoporphyrin IX. These results suggest that the reactivity of PHZ to Hb is influenced by the globin molecule, and the oxidative cleavage of the porphyrin ring causes the denaturation of hemoprotein. PMID:7717880

Kinuta, M; Matteson, J L; Itano, H A

1995-01-01

152

Unbound states of (32)Cl andthe (31)S(p,gamma)(32)Cl reaction rate  

SciTech Connect

The {sup 31}S(p,{gamma}){sup 32}Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the {sup 32}S(3He,t){sup 32}Cl charge-exchange reaction to determine properties of proton-unbound levels in {sup 32}Cl that have previously contributed significant uncertainties to the {sup 31}S(p,{gamma}){sup 32}Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in {sup 32}Cl. Proton-branching ratios were obtained by detecting decay protons from unbound {sup 32}Cl states in coincidence with tritons. An improved {sup 31}S(p,{gamma}){sup 32}Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Matos, M. [Louisiana State University; Blackmon, Jeff C [ORNL; Linhardt, Laura [Louisiana State University; Bardayan, Daniel W [ORNL; Nesaraja, Caroline D [ORNL; Clark, Jason [Argonne National Laboratory (ANL); Diebel, C. [Yale University; O'Malley, Patrick [Rutgers University; Parker, P.D. [Yale University

2011-01-01

153

Rate of browning reaction during preparation of coconut and palm sugar  

Microsoft Academic Search

The objective of this research was to analyse the rate of browning reaction during preparation of palm and coconut sugar. Brown colour formation during preparation of coconut sugar at a laboratory scale followed a zero order reaction with a constant of formation (k) equal to 5.15×10?3 AU min?1 (AU=Absorbance Unit). The amount of glucose, fructose and sucrose decreased during this

Anton Apriyantono; Astrid Aristyani; Nurhayati; Yeni Lidya; Slamet Budiyanto; Soewarno T Soekarto

2002-01-01

154

RATE OF INFORMATION PROCESSING AND REACTION TIME OF AIRCRAFT PILOTS AND NON-PILOTS  

Microsoft Academic Search

Reaction time and rate of information processing are cited as critical components in the make-up of pilots. A need was identified to establish the validity of various chronometric measures in the selection of pilots. Fifty-eight military and commercial pilots and twenty non-pilots were subjected to Schepers' Computerised Information Processing Test Battery, which measures reaction time, form discrimination time, colour discrimination

WERNER BARKHUIZEN; JOHANN SCHEPERS; JOHAN COETZEE

155

The Q-K model for gas-phase chemical reaction rates  

NASA Astrophysics Data System (ADS)

The quantum-kinetic, or Q-K, model is based on the quantum vibration model that is employed in the computation of gas flows at the molecular level by the direct simulation Monte Carlo (DSMC) method. The Q-K procedure for dissociation is physically realistic within the context of the vibration model in that the reaction occurs upon the selection of the vibrational level that corresponds to dissociation. An analogous, but entirely phenomenological, procedure has been presented for endothermic exchange and chain reactions. These procedures for the endothermic reactions have been well validated, but the existing procedures for the corresponding exothermic reactions have proved to be problematic. This paper presents new procedures for the exothermic reactions that are computationally efficient and provide a near exact match with the equilibrium constant of statistical mechanics. The Q-K model does not depend on the availability of continuum rate coefficients. Instead, the simplicity of the new DSMC procedures allows analytical expressions to be written down for the corresponding rate coefficients in an equilibrium gas. These are used to validate the Q-K model for reactions in high temperature air and in hydrogen-oxygen combustion. The development of the Q-K model has been driven by the need for efficient reaction procedures in DSMC applications that often involve the computation of billions of simulated collisions. It is not intended to compete with the modern theories for gas-phase chemical reactions that employ more accurate physical representations of real reactions. At the same time, the degree of validation of the model is such that the analytical expressions for the rate coefficients that correspond to the model should be useful in their own right.

Bird, G. A.

2011-10-01

156

Calculations on the rate of the ion-molecule reaction between NH3(+) and H2  

NASA Technical Reports Server (NTRS)

The rate coefficient for the ion-molecule reaction NH3(+) + H2 yields NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He yields NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.

Herbst, Eric; Defrees, D. J.; Talbi, D.; Pauzat, F.; Koch, W.

1991-01-01

157

Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide  

NASA Technical Reports Server (NTRS)

It is pointed out that the investigated reaction of oxygen with dimethyl sulfide may play an important role in photochemical smog formation and in the chemical evolution of dense interstellar clouds. Kinetic data were obtained with the aid of the flash photolysis-resonance fluorescence method. The photodecomposition of molecular oxygen provided the oxygen atoms for the experiments. The decay of atomic oxygen was studied on the basis of resonance fluorescence observations. Both reactions investigated were found to be fast processes. A negative temperature dependence of the rate constants for reactions with dimethyl sulfide was observed.

Lee, J. H.; Timmons, R. B.; Stief, L. J.

1976-01-01

158

?-Ionone reactions with ozone and OH radical: Rate constants and gas-phase products  

NASA Astrophysics Data System (ADS)

The bimolecular rate constants, kOH rad + ?-ionone (118±30)×10 -12 cm 3 molecule -1 s -1 and k+?-ionone, (0.19±0.05)×10 -16 cm 3 molecule -1 s -1, were measured using the relative rate technique for the reaction of the hydroxyl radical (OH) and ozone (O 3) with 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2-one ( ?-ionone) at 297±3 K and 1 atm total pressure. To more clearly define part of ?-ionone's indoor environment degradation mechanism, the products of the ?-ionone+OH rad and ?-ionone+O 3 reactions were also investigated. The identified ?-ionone+OH rad reaction products were: glyoxal (ethanedial, HC(=O)C(=O)H), and methylglyoxal (2-oxopropanal, CH 3C(=O)C(=O)H) and the identified ?-ionone+O 3 reaction product was 2-oxopropanal. The derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N, O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) were used to propose 2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde as the other major ?-ionone+OH rad and ?-ionone+O 3 reaction product. The elucidation of this other reaction product was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible ?-ionone+OH rad and ?-ionone+O 3 reaction mechanisms based on previously published volatile organic compound+OH rad and volatile organic compound+O 3 gas-phase reaction mechanisms. The additional gas-phase products observed from the ?-ionone+OH rad reaction are proposed to be the result of cyclization through a radical intermediate.

Forester, Crystal D.; Ham, Jason E.; Wells, J. Raymond

159

A study of rates of reaction of some substituted benzoic acids and diphenyldiazomethane  

E-print Network

LIBRAS Y 4 CIA COLLEGE OF TEXA A STUDY OF RATES OF REACTION OF SOME SUBSTITUTED BENZOIC ACIDS AND DIPHENYLDIAZOMETHANE A Thesis By JOHN S. WESTMORELAND Submitted to the Graduate School of the Agricultural and &chanical College of Texas... in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Ja~uara 1~97 N j t tj t t~t& t A STUDY OF REACTION RATES OF SOME SUBSTITUTED BENZOIC ACIDS AND DIPHENYLDIAZOMETHANE A Thesis JOHN S. VESTMORELAND Approved as to style...

Westmoreland, John Sherman

1957-01-01

160

Rate of reaction with nitric oxide determines the hypertensive effect of cell-free hemoglobin.  

PubMed

Administration of extracellular hemoglobin-based oxygen carriers often induces mild increases in blood pressure. In order to test whether nitric oxide (NO) scavenging is responsible for the hypertensive effect, we constructed and tested a set of recombinant hemoglobins that vary in rates of reaction with NO. The results suggest that the rapid reactions of oxy- and deoxyhemoglobin with nitric oxide are the fundamental cause of the hypertension. The magnitude of the blood-pressure effect correlates directly with the in vitro rate of NO oxidation. Hemoglobins with decreased NO-scavenging activity may be more suitable for certain therapeutic applications than those that cause depletion of nitric oxide. PMID:9661203

Doherty, D H; Doyle, M P; Curry, S R; Vali, R J; Fattor, T J; Olson, J S; Lemon, D D

1998-07-01

161

Astrophysical reaction rate for Be9 formation within a three-body approach  

NASA Astrophysics Data System (ADS)

The structure of the Borromean nucleus Be9 (?+?+n) is addressed within a three-body approach using the analytical transformed harmonic oscillator method. The three-body formalism provides an accurate description of the radiative capture reaction rate for the entire temperature range relevant in astrophysics. At high temperatures, results match the calculations based on two-step sequential processes. At low temperatures, where the particles have no access to intermediate two-body resonances, the three-body direct capture leads to reaction rates larger than the sequential processes. These results support the reliability of the method for systems with several charged particles.

Casal, J.; Rodríguez-Gallardo, M.; Arias, J. M.; Thompson, I. J.

2014-10-01

162

Observation of double pygmy resonances in $^{195,196}$Pt and enhanced astrophysical reaction rates  

E-print Network

Our measurements of $^{195,196}$Pt $\\gamma$-strength functions show a double-humped enhancement in the $E_{\\gamma}= 4-8$ MeV region. For the first time, the detailed shape of these resonances is revealed for excitation energies in the quasicontinuum. We demonstrate that the corresponding neutron-capture cross sections and astrophysical reaction rates are increased by up to a factor of 2 when these newly observed pygmy resonances are included. These results lend credence to theoretical predictions of enhanced reaction rates due to such pygmy resonances and hence are important for a better understanding of r-process nucleosynthesis.

Giacoppo, F; Eriksen, T K; Görgen, A; Guttormsen, M; Hagen, T W; Larsen, A C; Kheswa, B V; Klintefjord, M; Koehler, P E; Nyhus, H T; Renstr\\om, T; Sahin, E; Siem, S; Tornyi, T G

2014-01-01

163

Observation of double pygmy resonances in $^{195,196}$Pt and enhanced astrophysical reaction rates  

E-print Network

Our measurements of $^{195,196}$Pt $\\gamma$-strength functions show a double-humped enhancement in the $E_{\\gamma}= 4-8$ MeV region. For the first time, the detailed shape of these resonances is revealed for excitation energies in the quasicontinuum. We demonstrate that the corresponding neutron-capture cross sections and astrophysical reaction rates are increased by up to a factor of 2 when these newly observed pygmy resonances are included. These results lend credence to theoretical predictions of enhanced reaction rates due to such pygmy resonances and hence are important for a better understanding of r-process nucleosynthesis.

F. Giacoppo; F. L. Bello Garrote; T. K. Eriksen; A. Görgen; M. Guttormsen; T. W. Hagen; A. C. Larsen; B. V. Kheswa; M. Klintefjord; P. E. Koehler; H. T. Nyhus; T. Renstr\\om; E. Sahin; S. Siem; T. G. Tornyi

2014-02-11

164

Model Proton-Coupled Electron Transfer Reactions in Solution: Predictions of Rates, Mechanisms, and Kinetic Isotope Effects  

E-print Network

Model Proton-Coupled Electron Transfer Reactions in Solution: Predictions of Rates, Mechanisms isotope effects for proton-coupled electron transfer (PCET) reactions. These studies are based) the proton transfer reaction becomes more exothermic, (4) the electron transfer reaction becomes more

Hammes-Schiffer, Sharon

165

Relation between State-Selected or State-Averaged Cross Sections of Endothermic Reactions and Rate Constants of Exothermic Reactions. Application of Bimolecular Microcanonical Activated Complex Theory  

Microsoft Academic Search

An expression is derived relating state-selected or state-averaged molecular beam reaction cross sections of endothermic reactions to observed rate constants for the inverse (exothermic) reactions. An approximation of Anlauf, Maylotte, Polanyi, and Bernstein is used, together with bimolecular microcanonical activated complex theory [R. A. Marcus, J. Chem. Phys. 45, 2138 (1966)].

R. A. Marcus

1970-01-01

166

Dependence of the Rate of an Interfacial Diels-Alder Reaction on the Steric Environment of the Immobilized Dienophile  

E-print Network

voltammetry was used to measure the rate of the reaction by monitoring the decay of the redox-active quinoneDependence of the Rate of an Interfacial Diels-Alder Reaction on the Steric Environment for an interfacial Diels-Alder reaction and the steric environment around the reacting molecules. The study used

Mrksich, Milan

167

Acid–base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer  

PubMed Central

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid–base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta. PMID:23091030

Chen, Modi; Titcombe, Mari; Jiang, Jingkun; Jen, Coty; Kuang, Chongai; Fischer, Marc L.; Eisele, Fred L.; Siepmann, J. Ilja; Hanson, David R.; Zhao, Jun; McMurry, Peter H.

2012-01-01

168

Impacts of a Newly Calculated 3? Reaction Rate on the Helium Accreting White Dwarfs  

NASA Astrophysics Data System (ADS)

Effects of a new triple-? reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary systems have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type la supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto[4]. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for the lower accretion rate compared to that of dM/dt~=4×1O-8 Msolar yr-1 which has been believed to the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates and the region of carbon deflagration for slow accretion rate is disappeared.

Saruwatari, Motoaki; Hashimoto, Masa-Aki

2010-08-01

169

Effects of a New Triple-? Reaction Rate on the Helium Ignition of Accreting White Dwarfs  

NASA Astrophysics Data System (ADS)

The effects of a new triple-? reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary system have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type Ia supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto [K. Nomoto, Astrophys. J. 253 (1982), 798]. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for lower accretion rate than dM/dt ? 4 × 10^{-8} M_{?} yr^{-1} which has been believed to be the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates, and the region of carbon deflagration for slow accretion rates disappears.

Saruwatari, M.; Hashimoto, M.

2010-11-01

170

Studying exothermic reactions in the Ni-Al system at rapid heating rates using a nanocalorimeter  

NASA Astrophysics Data System (ADS)

Heats of reaction and heat capacity changes were measured using scanning nanocalorimetry for a nickel and aluminum bilayer where initial heating rates of 104 K/s were achieved. Multiple exotherms were observed on the initial heating, but the number of intermediate exotherms decreased with increasing heating rate. The final phase was the B2 NiAl intermetallic. Results from the nanocalorimeter were compared with a conventional differential scanning calorimeter (operating at 0.7 K/s) to understand the effect of significant (10 000×) increases in heating rate on the phase transformation sequence. The high heating rate in the nanocalorimeter delays reaction initiation, causes the exothermic peaks to shift to higher temperatures, and appears to suppress the formation of intermediate, metastable phases. Potential explanations for this apparent suppression are discussed.

Swaminathan, P.; Grapes, M. D.; Woll, K.; Barron, S. C.; LaVan, D. A.; Weihs, T. P.

2013-04-01

171

[Determination of trace amounts of nitrite and its chemical reaction kinetics].  

PubMed

A catalytic kinetic spectrophotometric method for the determination of nitrite, NO2(-)-S2O8(2-)-MR, was developed. It is based on the fading reaction of methyl red (MR) oxidized by potassium persulfate which can be catalyzed by NO2- in the medium of dilute HCl. The optimum experimental conditions were gained by combining single factor experiments with orthogonal experiments. Calibration curve, detection limit, precision, and anti-interference under the optimum experimental conditions were researched. Its kinetics principles and parameters were discussed. Its quantitative principle was investigated. The results show that the optimum experimental conditions of this method should be as follows: 1.0 mL 0.3 mol x L(-1) HCl, 1.0 mL 0.01 mol x L(-1) K2S2O8, 0.6 mL 0.2 g x L(-1) MR, reaction temperature 80 degrees C and reaction time 9 min. The principles for the quantitative determination of trace nitrite is that variation of MR concentration at the maximum absorption wavelength of 518 nm, ln(A0/A), shows a good linear relationship with the concentration of NO2- under the optimum experimental conditions. Its determination range is 0.01-0.80 mg x L(-1) and its detection limit is 0.007 mg x L(-1). The kinetic characteristics are that the reaction order in NO2- is 1 and the fading reaction is a pseudo first order reaction. Its apparent activation energy is 85.04 kJ x mol(-1). Its apparent rate constant is 0.021 4 min(-1), and the half-life is 32.39 min at 80 degrees C. The kinetic principle is that the variation of MR concentration is directly proportional to the concentration of NO2-, ln(A0/A) = kc(NO2-). This new method for the determination of trace nitrite has never previously been reported in the published literature so far. It is highly sensitive and selective. Most of the common ions don't interfere with the determination of nitrite. This method has the advantages of convenient operation and the regents used are cheap and nontoxic. It was applied to the determination of trace nitrite in food and water samples with satisfactory results. PMID:25358175

Luo, Zhi-yong; Zheng, Huai-li

2014-06-01

172

Controlling the rate of organic reactions: rational design of allosteric Diels-Alderase ribozymes  

PubMed Central

Allosteric mechanisms are widely used in nature to control the rates of enzymatic reactions, but little is known about RNA catalysts controlled by these principles. The only natural allosteric ribozyme reported to date catalyzes an RNA cleavage reaction, and so do almost all artificial systems. RNA has, however, been shown to accelerate a much wider range of chemical reactions. Here we report that RNA catalysts for organic reactions can be put under the stringent control of effector molecules by straight-forward rational design. This approach uses known RNA sequences with catalytic and ligand-binding properties, and exploits weakly conserved sequence elements and available structural information to induce the formation of alternative, catalytically inactive structures. The potential and general applicability is demonstrated by the design of three different systems in which the rate of a catalytic carbon–carbon bond forming reaction is positively regulated up to 2100-fold by theophylline, tobramycin and a specific mRNA sequence, respectively. Although smaller in size than a tRNA, all three ribozymes show typical features of allosteric metabolic enzymes, namely high rate acceleration and tight allosteric regulation. Not only do these findings demonstrate RNA's power as a catalyst, but also highlight on RNA's capabilities as signaling components in regulatory networks. PMID:16990253

Amontov, Sergey; Jäschke, Andres

2006-01-01

173

Role of heavy-ion nuclear reactions in determining on-orbit single event error rates  

Microsoft Academic Search

Simulations show that neglecting ion-ion interaction processes (both particles having Z>1) results in an underestimation of the total on-orbit single event upset error rate by more than two orders of magnitude for certain technologies. The inclusion of ion-ion nuclear reactions leads to dramatically different SEU error rates for CMOS devices containing high Z materials compared with direct ionization by the

Christina L. Howe; Robert A. Weller; Robert A. Reed; Marcus H. Mendenhall; Ronald D. Schrimpf; Kevin M. Warren; Dennis R. Ball; Lloyd W. Massengill; Kenneth A. LaBel; Nadim F. Haddad

2005-01-01

174

Electrochemical degradation of sulfonamides at BDD electrode: kinetics, reaction pathway and eco-toxicity evaluation.  

PubMed

The investigation dealt with electrochemical oxidation of five sulfonamides (SNs): sulfadiazine (SDZ), sulfathiazole (STZ), sulfamerazine (SMR), sulfamethazine (SMN) and sulfadimethoxine (SDM) in aqueous solution at boron-doped diamond (BDD) electrode. All studied sulfonamides were degraded according to a pseudo first order kinetics. The structure of SNs had no significant effect on the values of pseudo first order rate constants. Increased degradation efficiency was observed in higher temperature and in acidic pH. Due to the presence of chlorine and nitrate SNs were more effectively oxidized from municipal wastewater treatment plant (WWTP) effluents than from pure supporting electrolyte Na2SO4. The intermediates identified by LC-MS and GC-MS analysis suggested that the hydroxyl radicals attack mainly the SN bond, but also the aromatic ring systems (aniline, pyrimidine or triazole) of SNs. Finally, the toxicity of the SNs solutions and effluents after electrochemical treatment was assessed through the measurement of growth inhibition of green algae (Scenedesmus vacualatus) and duckweed (Lemna minor). Toxicity of SMR, STZ, SMN solutions before and after electrochemical oxidation and SDM solution after the process in L. minor test was observed. No significant toxicity of studied SNs was observed in algae test. PMID:25215656

Fabia?ska, Aleksandra; Bia?k-Bieli?ska, Anna; Stepnowski, Piotr; Stolte, Stefan; Siedlecka, Ewa Maria

2014-09-15

175

Thermonuclear cross section and reaction-rate-parameter data compilation. Final report, Feb 89-Jul 90  

SciTech Connect

With the increasing need to develop new sources of energy for Air Force missions, interest in fusion energy has increased. While the earlier report from this project listed scores of possible thermonuclear reactions, this report concentrates solely on cross section data and reaction rate parameter data for the major fusion reactions under scrutiny. The meanings of terms commonly used in the field of thermonuclear fusion are discussed, along with descriptions of the mechanisms involved in determining the ignition point of plasma. In addition, polynomial-style-curve-fits have been developed for each list of data herein. The characteristics of four advanced fusion fuels: p-11B, p-6Li, d-3He, and 3He-3He, are compared to the fuel d-t. D-3He presents itself as the reaction of choice for fusion space propulsion. Other research efforts using d-3He are mentioned.

Cox, L.T.

1991-07-01

176

The rate constant for the CO+H2O2 reaction  

NASA Astrophysics Data System (ADS)

The rate constant for the reaction CO + H 2O 2 ? HOCO + OH (R1) at 713 K is determined based on the batch reactor experiments of Baldwin et al. [R.R. Baldwin, R.W. Walker, S.J. Webster, Combust. Flame 15 (1970) 167] on decomposition of HO sensitized by CO. The value, k1(713K)=8.1×102cmmols, is consistent with spin-unrestricted density functional theory. Extrapolation to a wider temperature range through ab initio calculations yields the rate constant k1=3.6×104T2.5exp(-14425[K]/T)cmmols. The reaction is probably of minor importance in combustion. The present analysis reconciles the batch reactor data of Baldwin et al. with recent high-level theoretical work on the CO+HO reaction.

Glarborg, Peter; Marshall, Paul

2009-06-01

177

Rate constant for the reaction Cl + HO2NO2 yielding products. [in stratospheric chemistry  

NASA Technical Reports Server (NTRS)

The rates for the reaction of Cl atoms iwth HO2NO2 were calculated from data obtained by the use of the discharge flow/resonance fluorescence (DF/RF) and the discharge flow/mass spectrometric (DF/MS) techniques. The total rate constant, k1, for the overall reaction: 1a (Cl + HO2NO2 yielding HCl + NO2 +O2), 1b (yielding HO2 + ClNO2), and the two possible additional channels was found to be less than 1.0 x 10 to the -13th cu cm/s at 296 K. The value of (k1a + k1b) was found to be 3.4 + or - 1.4) x 10 to the -14th cu cm/s. Thus, the reaction of Cl with peroxynitric acid is too slow, by a factor of 100, to contribute significantly to the hydrogen abstraction by Cl in the stratosphere.

Simonaitis, R.; Leu, M. T.

1985-01-01

178

Rate constant for the reaction of atomic oxygen with phosphine at 298 K  

NASA Technical Reports Server (NTRS)

The rate constant for the reaction of atomic oxygen with phosphine has been measured at 298 K using flash photolysis combined with time-resolved detection of O(3P) via resonance fluorescence. Atomic oxygen was produced by flash photolysis of N2O or NO highly diluted in argon. The results were shown to be independent of (PH3), (O), total pressure and the source of O(3P). The mean value of all the experiments is k1 = (3.6 + or -0.8) x 10 to the -11th cu cm/s (1 sigma). Two previous measurements of k1 differed by more than an order of magnitude, and the results support the higher value obtained in a discharge flow-mass spectrometry study. A comparison with rate data for other atomic and free radical reactions with phosphine is presented, and the role of these reactions in the aeronomy or photochemistry of Jupiter and Saturn is briefly considered.

Stief, L. J.; Payne, W. A.; Nava, D. F.

1987-01-01

179

The Impact of Nuclear Reaction Rate Uncertainties on Evolutionary Studies of the Nova Outburst  

E-print Network

The observable consequences of a nova outburst depend sensitively on the details of the thermonuclear runaway which initiates the outburst. One of the more important sources of uncertainty is the nuclear reaction data used as input for the evolutionary calculations. A recent paper by Starrfield, Truran, Wiescher, & Sparks (1998) has demonstrated that changes in the reaction rate library used within a nova simulation have significant effects, not just on the production of individual isotopes (which can change by an order of magnitude), but on global observables such as the peak luminosity and the amount of mass ejected. We present preliminary results of systematic analyses of the impact of reaction rate uncertainties on nova nucleosynthesis.

W. Raphael Hix; Michael S. Smith; Anthony Mezzacappa; Sumner Starrfield; Donald L. Smith

2000-01-27

180

Measurement of proton transfer reaction rates in a microwave cavity discharge flowing afterglow  

NASA Astrophysics Data System (ADS)

The reaction rate coefficients between the hydronium ion and the molecules ethene (C2H4), propene (C 3H6), 1-butene (C4H8) and hydrogen sulfide (H2S) were measured at 296 K. The measured reaction rates were compared to collision rates calculated using average dipole orientation (ADO) theory. Reaction efficiency depends primarily upon the proton affinity of the molecules. All the measurements were obtained using the newly developed microwave cavity discharge flowing afterglow (MCD-FA) apparatus. This device uses an Asmussen-type microwave cavity discharge ion source that is spatially separated from the flow tube, eliminating many of the problems inherent with the original FA devices. In addition to measuring reaction rate coefficients, the MCD-FA was shown to be an effective tool for measuring trace compounds in atmospheric air. This method has many advantages over current detection techniques since compounds can be detected in almost real time, large mass ranges can be scanned quickly, and repeated calibration is not required. Preliminary measurements were made of car exhaust and exhaled alveolar air. Car exhaust showed the presence of numerous hydrocarbons, such as butene, benzene and toluene while the exhaled alveolar air showed the presence of various volatile organic compounds such as methanol and acetone.

Brooke, George M., IV

181

Fundamental Kinetic Modeling: ab initio rate constant calculations of elementary reactions  

E-print Network

Fundamental Kinetic Model' provides an alternative method for collecting rate constant data of elementary reactions when experimental observation is unavailable. A standard procedure of PMP4/6-3 IG*//UHF/6-3 IG* ab initio calculations of reactants...

Jenks, Richard Lee

2012-06-07

182

Effect of Conceptual Change Approach on Students' Understanding of Reaction Rate Concepts  

ERIC Educational Resources Information Center

The purpose of the present study was to investigate the effect of conceptual change text oriented instruction compared to traditional instruction on 10th grade students' understanding of reaction rate concepts. 45 students from two classes of the same teacher in a public high school participated in this study. Students in the experimental group…

Kingir, Sevgi; Geban, Omer

2012-01-01

183

Metamorphic reaction rates at ˜650-800 °C from diffusion of niobium in rutile  

NASA Astrophysics Data System (ADS)

The ability to quantify the rates at which metamorphic reactions occur is critical to assessing the extent to which equilibrium is achieved and maintained in a variety of dynamic settings. Here we investigate the kinetics of rutile replacement by titanite during amphibolite-facies overprinting of eclogite, garnet amphibolite and anorthosite from Catalina Island, CA, the Tromsø Nappe, Norway, the North Qaidam terrane, China, and the Guichicovi Complex, Mexico. Trace element concentration profiles across rutile rimmed by titanite, as determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), reveal Nb zoning in rutile that we interpret as the result of Nb back-diffusion from the rutile-titanite boundary. We present new field-based reaction rates calculated from grain boundary velocities, which in turn were calculated using a 1-D diffusion model for Nb back-diffusion into rutile during titanite replacement over the temperature range 670-770 °C. Our data are consistent with or slightly faster than previous estimates of field-based reaction rates for regional metamorphism, and extend the temperature and compositional range over which regional metamorphic reaction rates are known.

Cruz-Uribe, Alicia M.; Feineman, Maureen D.; Zack, Thomas; Barth, Matthias

2014-04-01

184

Microscopic theory of protein folding rates. II. Local reaction coordinates and chain dynamics  

E-print Network

Microscopic theory of protein folding rates. II. Local reaction coordinates and chain dynamics John involved in barrier crossing for protein folding are investigated in terms of the chain dynamics of the polymer backbone, completing the microscopic description of protein folding presented in the preceding

Takada, Shoji

185

Reconsideration of the rate constant for the reaction of hydroxyl radicals with nitric acid  

Microsoft Academic Search

The authors report rate coefficients for the reaction of OH with HNOâ, kâ, between 10 and 500 Torr of He, SFâ, Nâ, and Oâ and at 10 different temperatures between 200 and 375 K. They generated OH via pulsed photolysis of HNOâ and monitored the [OH] temporal profile via pulsed laser induced fluorescence. Below 300 K the value of kâ

Steven S. Brown; Ranajit K. Talukdar; A. R. Ravishankara

1999-01-01

186

The Temperature Dependence of the Rate Constant for the Reaction of Hydroxyl Radicals With Nitric Acid  

Microsoft Academic Search

The flash photolysis resonance fluorescence technique has been used to measure the rate constant for the reaction OH + HONO2 - products over the temperature range 225-443 K. The data below room temperature can be fit to the expression k = (1.05 _+ 0.40) x 10 TM exp [(759 -+ 100)\\/T] cm 3 molec - s -. This equation, however,

Michael J. Kurylo; Kenneth D. Cornett; Jennifer L. Murphy

1982-01-01

187

Probing the Rate-Determining Step of the Claisen-Schmidt Condensation by Competition Reactions  

ERIC Educational Resources Information Center

Competition experiments are a useful tool for preliminary study of the linear free energy relationship of organic reactions. This article describes a physical organic experiment for upper-level undergraduates to identify the rate-determining step of the Claisen-Schmidt condensation of benzaldehyde and acetophenone by studying the linear free…

Mak, Kendrew K. W.; Chan, Wing-Fat; Lung, Ka-Ying; Lam, Wai-Yee; Ng, Weng-Cheong; Lee, Siu-Fung

2007-01-01

188

Thermal and Microcanonical Rates of Unimolecular Reactions from an Energy Diffusion Theory Approach  

E-print Network

Thermal and Microcanonical Rates of Unimolecular Reactions from an Energy Diffusion Theory Approach; In Final Form: September 13, 1999 We present an energy diffusion theory approach for computing thermal compared to the thermal energy. The weak-collision limit has been extensively studied.1-9 However

Miller, William H.

189

Atmospheric degradation of saturated alcohols: Room temperature rate coefficients for NO3 radical reactions  

NASA Astrophysics Data System (ADS)

Rate coefficients for the reactions of NO3 radicals with a series of saturated alcohols are reported here using the relative rate technique. Experiments were performed using air as bath gas in a 50 L glass-pyrex reaction chamber at room temperature (298 ± 2) K with long-path FTIR spectroscopy used to monitor the reaction at atmospheric pressure (708 ± 8) Torr. The reference compounds used and their rate coefficients are: propanal kNO3 = (6.0 ± 0.6) × 10-15, methyl methacrylate kNO3 = (3.55 ± 0.62) × 10-15, acetaldehyde kNO3 = (2.62 ± 0.29) × 10-15 and propene kNO3 = (9.50 ± 1.9) × 10-15, in cm3 molecule-1 s-1. Rate coefficients obtained were (in units cm3 molecule-1 s-1): (1.87 ± 0.14) × 10-15, (2.39 ± 0.20) × 10-15, (2.28 ± 0.17) × 10-15, (1.80 ± 0.13) × 10-15 and (3.52 ± 0.19) × 10-15 for 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol respectively. Reactivity trend can be explained in terms of the different types of hydrogen inside the hydrocarbon chain. The reaction occurs by an initial H-atom abstraction mainly from C-H groups of the alcohols by the NO3 radical being NO3 more reactive towards an H atom attached to a tertiary carbon than that attached to a secondary or primary carbon. Reactivity trend is compared with their similar structural 2-butanol and with the corresponding alkanes. Atmospheric implications are also discussed calculating lifetimes of the saturated alcohols studied here due to the reaction with NO3 radicals in comparison to their reactions with the other major atmospheric oxidants.

Moreno, Alberto; Salgado, Sagrario; Taccone, Raul; Martín, Pilar; Cabañas, Beatriz

2014-10-01

190

X-ray structure and reactivity of (Ru(bpy)/sub 2/(CO)H)PF/sub 6/ x 1/2 C/sub 3/H/sub 6/O (bpy = 2,2'-bipyridyl), a possible intermediate in the water-gas shift reaction catalyzed by ruthenium polypyridyl complexes  

SciTech Connect

The structure of (Ru(C/sub 10/H/sub 8/N/sub 2/)/sub 2/(CO)H)PF/sub 6/ x 1/2 C/sub 3/H/sub 6/O is reported. The compound crystallizes in the monoclinic C2/c space group with unit cell parameters a = 26.680 (4) A, b = 7.005 (3) A, c = 25.936 (2) A, and ..beta.. = 93.96 (2)/sup 0/ (V = 4836.04 A/sup 3/, Z = 8). The cationic species has a cis geometry for CO and H. The ruthenium-hydride and ruthenium-carbon distances are 1.68 (4) and 1.804 (6) A, respectively. The average ruthenium-nitrogen distance is 2.109 A. In acidic solution the title compound acts as a hydride donor to produce dihydrogen gas in a pseudo-first-order reaction; the second-order rate constant for this reaction is 0.105 -/+ 0.01 dm/sup 3/ mol/sup -1/ s/sup -1/ at 303 K with an activation energy 56 -/+ 2 kJ mol/sup -1/. A value of -100 -/+ 8 J mol/sup -1/ deg/sup -1/ has been obtained for the entropy of activation. The consequences of these results for the possible reaction mechanism of the homogeneous water-gas shift reaction catalyzed by (Ru(bpy)/sub 2/(CO)Cl)/sup +/ are discussed. 15 references, 4 figures, 6 tables.

Haasnoot, J.G.; Hinrichs, W.; Weir, O.; Vos, J.G.

1986-11-05

191

Product distributions, rate constants, and mechanisms of LiH+H reactions.  

PubMed

We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H-->Li+H2 and of the H exchange reaction LiH+H'-->LiH'+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v',j') rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j' preferred with respect to even j'. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results. PMID:15974734

Defazio, Paolo; Petrongolo, Carlo; Gamallo, Pablo; González, Miguel

2005-06-01

192

HO + CO reaction rates and H/D kinetic isotope effects: master equation models with ab initio SCTST rate constants.  

PubMed

Ab initio microcanonical rate constants were computed using Semi-Classical Transition State Theory (SCTST) and used in two master equation formulations (1D, depending on active energy with centrifugal corrections, and 2D, depending on total energy and angular momentum) to compute temperature-dependent rate constants for the title reactions using a potential energy surface obtained by sophisticated ab initio calculations. The 2D master equation was used at the P = 0 and P = ? limits, while the 1D master equation with centrifugal corrections and an empirical energy transfer parameter could be used over the entire pressure range. Rate constants were computed for 75 K ? T ? 2500 K and 0 ? [He] ? 10(23) cm(-3). For all temperatures and pressures important for combustion and for the terrestrial atmosphere, the agreement with the experimental rate constants is very good, but at very high pressures and T ? 200 K, the theoretical rate constants are significantly smaller than the experimental values. This effect is possibly due to the presence in the experiments of dimers and prereactive complexes, which were not included in the model calculations. The computed H/D kinetic isotope effects are in acceptable agreement with experimental data, which show considerable scatter. Overall, the agreement between experimental and theoretical H/D kinetic isotope effects is much better than in previous work, and an assumption of non-RRKM behavior does not appear to be needed to reproduce experimental observations. PMID:23317151

Weston, Ralph E; Nguyen, Thanh Lam; Stanton, John F; Barker, John R

2013-02-01

193

Reaction and internal energy relaxation rates in viscous thermochemically non-equilibrium gas flows  

NASA Astrophysics Data System (ADS)

In the present paper, reaction and energy relaxation rates as well as the normal stress are studied for viscous gas flows with vibrational and chemical non-equilibrium. Using the modified Chapman-Enskog method, multi-temperature models based on the Treanor and Boltzmann vibrational distributions are developed for the general case taking into account all kinds of vibrational energy transitions, exchange reactions, dissociation, and recombination. Integral equations specifying the first-order corrections to the normal mean stress and reaction rates are derived, as well as approximate systems of linear equations for their numerical computation. Generalized thermodynamic driving forces associated with all non-equilibrium processes are introduced. It is shown that normal stresses and rates of non-equilibrium processes can be expressed in terms of the same driving forces; the symmetry of kinetic coefficients in these expressions is proven. The developed general model is applied to a particular case of a pure N2 viscous flow with slow VT relaxation. Normal stress and rates of vibrational relaxation are studied for various ratios of vibrational and translational temperatures. The cross effects between different vibrational transitions in viscous flows are evaluated, along with the influence of anharmonicity and flow compressibility on the first-order corrections to the relaxation rate. Limits of validity for the widely used Landau-Teller model of vibrational relaxation are indicated.

Kustova, E. V.; Oblapenko, G. P.

2015-01-01

194

Thermal release rate studies of nuclear reaction products from polycrystalline metal matrices  

NASA Astrophysics Data System (ADS)

The thermal release rate of nuclear reaction products was investigated in offline annealing experiments. This work was motivated by the search for a high melting catcher material for recoiling products from heavy ion induced nuclear fusion reactions. Polycrystalline refractory metal foils of Ni, Y, Zr, Nb, Mo, Hf, W, and Re were investigated as catcher metals. Diffusion data for various tracer/host combinations were deduced from the measured release rates. This work focuses on the diffusion and the release rate of volatile p-elements from row 5 and 6 of the periodic table as lighter homologues of the superheavy elements with Z ? 113 to be studied in future experiments. A massive radiation damage enhancement of the diffusion velocity was observed. Diffusion trends have been established along the groups and rows of the periodic table based on the dependence of diffusion velocity on atomic sizes.

Wittwer, D.; Dressler, R.; Eichler, R.; Gäggeler, H. W.; Piguet, D.; Türler, A.

2013-02-01

195

Quantum three-body calculation of nonresonant triple-{alpha} reaction rate at low temperatures  

SciTech Connect

Triple-{alpha} reaction rate is re-evaluated by directly solving the three-body Schroedinger equation. The resonant and nonresonant processes are treated on the same footing using the continuum-discretized coupled-channels method for three-body scattering. An accurate description of the {alpha}-{alpha} nonresonant states significantly quenches the Coulomb barrier between the first two {alpha}-particles and the third {alpha}-particle. Consequently, the{alpha}-{alpha} nonresonant continuum states give a markedly larger contribution at low temperatures than that reported in previous studies. We show that Nomoto's method for three-body nonresonant capture processes, which is adopted in the NACRE compilation and many other studies, is a crude approximation of the accurate quantum three-body model calculation. We find an increase in triple-{alpha} reaction rate by about 20 orders of magnitude around 10{sup 7} K compared with the rate of NACRE.

Ogata, Kazuyuki; Kan, Masataka [Department of Physics, Kyushu University, Fukuoka 812-8581 (Japan); Kamimura, Masayasu [Department of Physics, Kyushu University, Fukuoka 812-8581 (Japan); RIKEN Nishina Center, Wako 351-0198 (Japan)

2010-08-12

196

Estimating Reaction Rate Coefficients Within a Travel-Time Modeling Framework  

SciTech Connect

A generalized, efficient, and practical approach based on the travel-time modeling framework is developed to estimate in situ reaction rate coefficients for groundwater remediation in heterogeneous aquifers. The required information for this approach can be obtained by conducting tracer tests with injection of a mixture of conservative and reactive tracers and measurements of both breakthrough curves (BTCs). The conservative BTC is used to infer the travel-time distribution from the injection point to the observation point. For advection-dominant reactive transport with well-mixed reactive species and a constant travel-time distribution, the reactive BTC is obtained by integrating the solutions to advective-reactive transport over the entire travel-time distribution, and then is used in optimization to determine the in situ reaction rate coefficients. By directly working on the conservative and reactive BTCs, this approach avoids costly aquifer characterization and improves the estimation for transport in heterogeneous aquifers which may not be sufficiently described by traditional mechanistic transport models with constant transport parameters. Simplified schemes are proposed for reactive transport with zero-, first-, nth-order, and Michaelis-Menten reactions. The proposed approach is validated by a reactive transport case in a two-dimensional synthetic heterogeneous aquifer and a field-scale bioremediation experiment conducted at Oak Ridge, Tennessee. The field application indicates that ethanol degradation for U(VI)-bioremediation is better approximated by zero-order reaction kinetics than first-order reaction kinetics.

Gong, R [Georgia Institute of Technology; Lu, C [Georgia Institute of Technology; Luo, Jian [Georgia Institute of Technology; Wu, Wei-min [Stanford University; Cheng, H. [Stanford University; Criddle, Craig [Stanford University; Kitanidis, Peter K. [Stanford University; Gu, Baohua [ORNL; Watson, David B [ORNL; Jardine, Philip M [ORNL; Brooks, Scott C [ORNL

2011-03-01

197

Quantification of rate constants for successive enzymatic reactions with DNP hyperpolarized MR.  

PubMed

A kinetic model is provided to obtain reaction rate constants in successive enzymatic reactions that are monitored using NMR spectroscopy and hyperpolarized substrates. The model was applied for simulation and analysis of the successive oxidation of choline to betaine aldehyde, and further to betaine, by the enzyme choline oxidase. This enzymatic reaction was investigated under two different sets of conditions: two different choline molecular probes were used, [1,1,2,2-D4 , 1-(13) C]choline chloride and [1,1,2,2-D4 , 2-(13) C]choline chloride, in different MR systems (clinical scanner and high-resolution spectrometer), as well as in different reactors and reaction volumes (4.8 and 0.7 mL). The kinetic analysis according to the model yielded similar results in both set-ups, supporting the robustness of the model. This was achieved despite the complex and negating influences of reaction kinetics and polarization decay, and in the presence of uncontrolled mixing characteristics, which may introduce uncertainties in both effective timing and effective pulses. The ability to quantify rate constants using hyperpolarized MR in the first seconds of consecutive enzyme activity is important for further development of the utilization of dynamic nuclear polarization-MR for biological determinations. PMID:24639024

Allouche-Arnon, Hyla; Hovav, Yonatan; Friesen-Waldner, Lanette; Sosna, Jacob; Moshe Gomori, J; Vega, Shimon; Katz-Brull, Rachel

2014-06-01

198

Benchmark experiments for validation of reaction rates determination in reactor dosimetry  

NASA Astrophysics Data System (ADS)

The precision of Monte Carlo calculations of quantities of neutron dosimetry strongly depends on precision of reaction rates prediction. Research reactor represents a very useful tool for validation of the ability of a code to calculate such quantities as it can provide environments with various types of neutron energy spectra. Especially, a zero power research reactor with well-defined core geometry and neutronic properties enables precise comparison between experimental and calculated data. Thus, at the VR-1 zero power research reactor, a set of benchmark experiments were proposed and carried out to verify the MCNP Monte Carlo code ability to predict correctly the reaction rates. For that purpose two frequently used reactions were chosen: He-3(n,p)H-3 and Au-197(n,?)Au-198. The benchmark consists of response measurement of small He-3 gas filled detector in various positions of reactor core and of activated gold wires placed inside the core or to its vicinity. The reaction rates were calculated in MCNP5 code utilizing a detailed model of VR-1 reactor which was validated for neutronic calculations at the reactor. The paper describes in detail the experimental set-up of the benchmark, the MCNP model of the VR-1 reactor and provides a comparison between experimental and calculated data.

Rataj, J.; Huml, O.; Heraltova, L.; Bily, T.

2014-11-01

199

The rate of the F + H2 reaction at very low temperatures.  

PubMed

The prototypical F + H2 ? HF + H reaction possesses a substantial energetic barrier (~800 K) and might therefore be expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been detected in a wide range of cold (10-100 K) environments. In fact, the reaction does take place efficiently at low temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H2, and hence a more accurate estimation of the total mass of astronomical objects. PMID:24451590

Tizniti, Meryem; Le Picard, Sébastien D; Lique, François; Berteloite, Coralie; Canosa, André; Alexander, Millard H; Sims, Ian R

2014-02-01

200

Rapid heartbeat, but dry palms: reactions of heart rate and skin conductance levels to social rejection  

PubMed Central

Background: Social rejection elicits negative mood, emotional distress, and neural activity in networks that are associated with physical pain. However, studies assessing physiological reactions to social rejection are rare and results of these studies were found to be ambiguous. Therefore, the present study aimed to examine and specify physiological effects of social rejection. Methods: Participants (n = 50) were assigned to either a social exclusion or inclusion condition of a virtual ball-tossing game (Cyberball). Immediate and delayed physiological [skin conductance level (SCL) and heart rate] reactions were recorded. In addition, subjects reported levels of affect, emotional states, and fundamental needs. Results: Subjects who were socially rejected showed increased heart rates. However, social rejection had no effect on subjects' SCLs. Both conditions showed heightened arousal on this measurement. Furthermore, psychological consequences of social rejection indicated the validity of the paradigm. Conclusions: Our results reveal that social rejection evokes an immediate physiological reaction. Accelerated heart rates indicate that behavior activation rather than inhibition is associated with socially threatening events. In addition, results revealed gender-specific response patterns suggesting that sample characteristics such as differences in gender may account for ambiguous findings of physiological reactions to social rejection. PMID:25221535

Iffland, Benjamin; Sansen, Lisa M.; Catani, Claudia; Neuner, Frank

2014-01-01

201

Relativistic equilibrium velocity distribution, nuclear fusion reaction rate and the solar neutrino problem  

E-print Network

In solar interior, it is the equilibrium velocity distribution of few high-energy protons and nuclei that participates in determining nuclear fusion reaction rates. So, it is inappropriate to use the Maxwellian velocity distribution to calculate the rates of solar nuclear fusion reactions. We have to use the relativistic equilibrium velocity distribution for the purpose. The nuclear fusion reaction rate based on the relativistic equilibrium velocity distribution has a reduction factor with respect to that based on the Maxwellian distribution. The reduction factor depends on the temperature, reduced mass and atomic numbers of the studied nuclear fusion reactions, in other words, it varies with the sort of neutrinos. Substituting the relativistic equilibrium velocity distribution for the Maxwellian distribution is not important for the calculation of solar sound speeds. The relativistic equilibrium velocity distribution, if adopted in standard solar models, will lower solar neutrino fluxes and change solar neutrino energy spectra but maintain solar sound speeds. This velocity distribution is possibly a solution to the solar neutrino problem.

Jian-Miin Liu

2003-07-07

202

?-Terpineol reactions with the nitrate radical: Rate constant and gas-phase products  

NASA Astrophysics Data System (ADS)

The bimolecular rate constant of k rad +?-terpineol (16 ± 4) × 10 -12 cm 3 molecule -1 s -1 was measured using the relative rate technique for the reaction of the nitrate radical (NO 3rad ) with ?-terpineol (2-(4-methyl-1-cyclohex-3-enyl)propan-2-ol) at 297 ± 3 K and 1 atmosphere total pressure. To more clearly define part of ?-terpineol's indoor environment degradation mechanism, the products of ?-terpineol + NO 3rad reaction were investigated. The identified reaction products were: acetone, glyoxal (HC( dbnd O)C( dbnd O)H), and methylglyoxal (CH 3C( dbnd O)C( dbnd O)H). The use of derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N, O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) were used to propose the other major reaction products: 6-hydroxyhept-5-en-2-one, 4-(1-hydroxy-1-methylethyl)-1-methyl-2-oxocyclohexyl nitrate, 5-(1-hydroxy-1-methylethyl)-2-oxocyclohexyl nitrate, 1-formyl-5-hydroxy-4-(hydroxymethyl)-1,5-dimethylhexyl nitrate, and 1,4-diformyl-5-hydroxy-1,5-dimethylhexyl nitrate. The elucidation of these products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible ?-terpineol + NO 3rad reaction mechanisms based on previously published volatile organic compound + NO 3rad gas-phase mechanisms. The additional gas-phase products (2,6,6-trimethyltetrahydro-2 H-pyran-2,5-dicarbaldehyde and 2,2-dimethylcyclohexane-1,4-dicarbaldehyde) are proposed to be the result of cyclization through a reaction intermediate.

Jones, Brian T.; Ham, Jason E.

203

Aqueous Complexation Reactions Governing the Rate and Extent of Biogeochemical U(VI) Reduction  

SciTech Connect

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. ? Elucidate the controls on the rate and extent of contaminant reactivity. (2) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.

Scott C. Brooks; Wenming Dong; Sue Carroll; Jim Fredrickson; Ken Kemner; Shelly Kelly

2006-06-01

204

Aqueous Complexation Reactions Governing the Rate and Extent of Biogeochemical U(VI) Reduction  

SciTech Connect

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. (2) Elucidate the controls on the rate and extent of contaminant reactivity. (3) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.

Scott C. Brooks; Wenming Dong; Sue Carroll; James K. Fredrickson; Kenneth M. Kemner; Shelly D. Kelly

2006-06-01

205

Reaction of atomic bromine with acetylene and loss rate of atmospheric acetylene due to reaction with OH, Cl, O, and Br  

NASA Technical Reports Server (NTRS)

The first-order, diffusion, and bimolecular rate constants for the reaction Br + C2H2 yields C2H3Br are evaluated. The rate constants are measured at 210, 248, 298, and 393 K and at pressures between 15-100 torr Ar using flash photolysis combined with time-resolved detection of atomic bromine via Br resonance radiation. It is observed that the reaction is not affected by pressure or temperature and the bimolecular constant = (4.0 + or - 0.8) x 10 to the -15th cu cm/sec with an error of two standard deviations. The C2H2 + Br reaction rates are compared with reactions of C2H2 with Cl, OH, NH2, and H. The loss rates for atmospheric C2H2 for reactions with OH, Cl, O, and Br are calculated as a function of altitude.

Payne, W. A.; Nava, D. F.; Brunning, J.; Stief, L. J.

1986-01-01

206

Rates for neutron-capture reactions on tungsten isotopes in iron meteorites. [Abstract only  

NASA Technical Reports Server (NTRS)

High-precision W isotopic analyses by Harper and Jacobsen indicate the W-182/W-183 ratio in the Toluca iron meteorite is shifted by -(3.0 +/- 0.9) x 10(exp -4) relative to a terrestrial standard. Possible causes of this shift are neutron-capture reactions on W during Toluca's approximately 600-Ma exposure to cosmic ray particles or radiogenic growth of W-182 from 9-Ma Hf-182 in the silicate portion of the Earth after removal of W to the Earth's core. Calculations for the rates of neutron-capture reactions on W isotopes were done to study the first possibility. The LAHET Code System (LCS) which consists of the Los Alamos High Energy Transport (LAHET) code and the Monte Carlo N-Particle(MCNP) transport code was used to numerically simulate the irradiation of the Toluca iron meteorite by galactic-cosmic-ray (GCR) particles and to calculate the rates of W(n, gamma) reactions. Toluca was modeled as a 3.9-m-radius sphere with the composition of a typical IA iron meteorite. The incident GCR protons and their interactions were modeled with LAHET, which also handled the interactions of neutrons with energies above 20 MeV. The rates for the capture of neutrons by W-182, W-183, and W-186 were calculated using the detailed library of (n, gamma) cross sections in MCNP. For this study of the possible effect of W(n, gamma) reactions on W isotope systematics, we consider the peak rates. The calculated maximum change in the normalized W-182/W-183 ratio due to neutron-capture reactions cannot account for more than 25% of the mass 182 deficit observed in Toluca W.

Masarik, J.; Reedy, R. C.

1994-01-01

207

Relationships between enzymatic flux capacities and metabolic flux rates: Nonequilibrium reactions in?muscle?glycolysis  

PubMed Central

The rules that govern the relationships between enzymatic flux capacities (Vmax) and maximum physiological flux rates (v) at enzyme-catalyzed steps in pathways are poorly understood. We relate in vitro Vmax values with in vivo flux rates for glycogen phosphorylase, hexokinase, and phosphofructokinase, enzymes catalyzing nonequilibrium reactions, from a variety of muscle types in fishes, insects, birds, and mammals. Flux capacities are in large excess over physiological flux rates in low-flux muscles, resulting in low fractional velocities (%Vmax = v/Vmax × 100) in vivo. In high-flux muscles, close matches between flux capacities and flux rates (resulting in fractional velocities approaching 100% in vivo) are observed. These empirical observations are reconciled with current concepts concerning enzyme function and regulation. We suggest that in high-flux muscles, close matches between enzymatic flux capacities and metabolic flux rates (i.e., the lack of excess capacities) may result from space constraints in the sarcoplasm. PMID:9192692

Suarez, R. K.; Staples, J. F.; Lighton, J. R. B.; West, T. G.

1997-01-01

208

Determination of relative rate constants for in vitro RNA processing reactions by internal competition.  

PubMed

Studies of RNA recognition and catalysis typically involve measurement of rate constants for reactions of individual RNA sequence variants by fitting changes in substrate or product concentration to exponential or linear functions. A complementary approach is determination of relative rate constants by internal competition, which involves quantifying the time-dependent changes in substrate or product ratios in reactions containing multiple substrates. Here, we review approaches for determining relative rate constants by analysis of both substrate and product ratios and illustrate their application using the in vitro processing of precursor transfer RNA (tRNA) by ribonuclease P as a model system. The presence of inactive substrate populations is a common complicating factor in analysis of reactions involving RNA substrates, and approaches for quantitative correction of observed rate constants for these effects are illustrated. These results, together with recent applications in the literature, indicate that internal competition offers an alternate method for analyzing RNA processing kinetics using standard molecular biology methods that directly quantifies substrate specificity and may be extended to a range of applications. PMID:25173512

Lin, Hsuan-Chun; Yandek, Lindsay E; Gjermeni, Ino; Harris, Michael E

2014-12-15

209

SIMULTANEOUS MEASUREMENT OF ANISOTROPIC SOLUTE DIFFUSIVITY AND BINDING REACTION RATES IN BIOLOGICAL TISSUES BY FRAP  

PubMed Central

Several solutes (e.g., growth factors, cationic solutes, etc.) can reversibly bind to the extracellular matrix (ECM) of biological tissues. Binding interactions have significant implications on transport of such solutes through the ECM. In order to fully delineate transport phenomena in biological tissues, knowledge of binding kinetics is crucial. In this study, a new method for the simultaneous determination of solute anisotropic diffusivity and binding reaction rates was presented. The new technique was solely based on Fourier analysis of fluorescence recovery after photobleaching (FRAP) images. Computer simulated FRAP tests were used to assess the sensitivity and the robustness of the method to experimental parameters, such as anisotropic solute diffusivity and rates of binding reaction. The new method was applied to the determination of diffusivity and binding rates of 5-dodecanoylaminofluorescein (DAF) in bovine coccygeal annulus fibrosus (AF). Our findings indicate that DAF reversibly binds to the ECM of AF. In addition, it was found that DAF diffusion in AF is anisotropic. The results were in agreement with those reported in previous studies. This study provides a new tool for the simultaneous determination of solute anisotropic diffusion tensor and rates of binding reaction that can be used to investigate diffusive-reactive transport in biological tissues and tissue engineered constructs. PMID:20686922

Travascio, Francesco; Gu, Wei Yong

2010-01-01

210

Substituent effects on the reaction rates of hydrogen abstraction in the pyrolysis of phenethyl phenyl ethers  

SciTech Connect

We report reaction profiles and forward rate constants for hydrogen abstraction reactions occurring in the pyrolysis of methoxy-substituted derivatives of phenethyl phenyl ether (PhCH{sub 2}CH{sub 2}OPh, PPE), where the substituents are located on the aryl ether ring (PhCH{sub 2}CH{sub 2}OPh-X). We use density functional theory in combination with transition-state theory, and anharmonic corrections are included within the independent mode approximation. PPE is the simplest model of the abundant {beta}-O-4 linkage in lignin. The mechanism of PPE pyrolysis and overall product selectivities have been studied experimentally by one of us, which was followed by computational analysis of key individual hydrogen-transfer reaction steps. In the previous work, we have been able to use a simplified kinetic model based on quasi-steady-state conditions to reproduce experimental {alpha}/{beta} selectivities for PPE and PPEs with substituents on the phenethyl ring (X-PhCH{sub 2}CH{sub 2}OPh). This model is not applicable to PPE derivatives where methoxy substituents are located on the phenyl ring adjacent to the ether oxygen because of the strongly endothermic character of the hydrogen abstraction by substituted phenoxy radicals as well as the decreased kinetic chain lengths resulting from enhanced rates of the initial C?O homolysis step. Substituents decelerate the hydrogen abstraction by the phenoxy radical, while the influence on the benzyl abstraction is less homogeneous. The calculations provide insight into the contributions of steric and polar effects in these important hydrogen-transfer steps. We emphasize the importance of an exhaustive conformational space search to calculate rate constants and product selectivities. The computed rate constants will be used in future work to numerically simulate the pyrolysis mechanism, pending the calculation of the rate constants of all participating reactions.

Beste, Ariana [ORNL; Buchanan III, A C [ORNL

2010-01-01

211

Rate coefficients of hydroxyl radical reactions with pesticide molecules and related compounds: A review  

NASA Astrophysics Data System (ADS)

Rate coefficients published in the literature on hydroxyl radical reactions with pesticides and related compounds are discussed together with the experimental methods and the basic reaction mechanisms. Recommendations are made for the most probable values. Most of the molecules whose rate coefficients are discussed have aromatic ring: their rate coefficients are in the range of 2×109-1×1010 mol-1 dm3 s-1. The rate coefficients show some variation with the electron withdrawing-donating nature of the substituent on the ring. The rate coefficients for triazine pesticides (simazine, atrazine, prometon) are all around 2.5×109 mol-1 dm3 s-1. The values do not show variation with the substituent on the s-triazine ring. The rate coefficients for the non-aromatic molecules which have C=C double bonds or several C-H bonds may also be above 1×109 mol-1 dm3 s-1. However, the values for molecules without C=C double bonds or several C-H bonds are in the 1×107-1×109 mol-1 dm3 s-1 range.

Wojnárovits, László; Takács, Erzsébet

2014-03-01

212

Laboratory Experiments and Investigations on the Reaction Rates of Mg-sulfates Under Mars Relevant Conditions  

NASA Astrophysics Data System (ADS)

Large deposits of hydrous Mg-sulfates was identified on Mars by orbital remote sensing (OMEGA on Mars Express and CRISM on Mars Reconnaissance Orbiter). Kieserite (MgSO4.H2O) and a non-specific “polyhydrated sulfates” are among the most observed and widely distributed sulfates (Bibring et al., 2005, Murchie et al., 2007). They frequently co-exist (Gendrin et al., 2005) and sometimes occur in alternative stratigraphic layers (Roach et al., 2008). Mg-sulfates were suggested, by compositional correlations and mineral models, to exist in Meridiani outcrops (Clark et al., 2005) and in rocks and regolith at Gusev (Squyres et al., 2006, Haskin et al., 2005, Wang et al., 2006, 2008); but no information on the hydration state of these sulfates can be extracted. We have conducted 188 experiments to investigate the stability fields and phase transition pathways of hydrous Mg-sulfates (Wang et al., 2009). In addition, we can extract the information on the reaction rates of five important dehydration and rehydration processes involved in these experiments. Our experiments were done at four temperatures (50°C, 21°C, 5°C, and -10°C) and ten relative humidity levels, with five hydrous Mg-sulfate species as starting phases. The rate information was extracted from the mineral identifications of the intermediate reaction products, measured by non-invasive Raman spectroscopy at regular time intervals during the entire duration of experiments (tens’ thousands hours). The rates for five processes are all strongly controlled by temperatures. We found that the experimental results match Arrhenius equation very well, thus the rate constants for dehydration and rehydration processes of Mg-sulfates at lower temperatures (down to 180K) can be approximately estimated by using the experimentally derived pre-exponential factor(s) and activation energy(s). In this study, only the orders of magnitudes for reaction rate ratios at different temperatures were considered. The estimated reaction rate ratios at different temperatures for five important processes helped us to understand the stable, especially the metastable, Mg-sulfate species that could be seen at Mars surface in non-polar regions during a moderate obliquity period. Therefore in addition to exam the spectral similarity, we now can use the knowledge gained through the laboratory experiments on stability field, phase transition pathway, and reaction rate of Mg-sulfates to evaluate the realistic mineral candidates for “polyhydrated sulfates”, that were so widely observed on Mars by OMEGA and CRISM. Furthermore, we will be able to investigate the formation mechanism of alternative stratigraphic layers of sulfates on Mars and the paleo-climatic conditions that they may imply.

Wang, A.; Freeman, J. J.

2009-12-01

213

The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates  

NASA Astrophysics Data System (ADS)

The role of coupled physical and geochemical heterogeneities in hydro-geochemical transport is investigated by simulating three-dimensional transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: 1) spatially homogeneous reactive mineral surface area, 2) reactive surface area positively correlated to hydraulic heterogeneity, and 3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (?). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical dis-equilibrium between fluids and the mineral galena that resulted in increased aqueous lead (Pb2 +) concentrations. Calcite dissolution buffered the pH change and created conditions of galena oversaturation, which then reduced lead concentrations along the flow path. Near the leak kinetic geochemical reactions control the release of solutes into the fluid, but further along the flow path mineral solubility controls solute concentrations. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface area in coordination with physical heterogeneity on the effective reaction rate (Krxn,eff) and Pb2 + concentrations within the plume. Dissimilarities between ensemble Pb2 + concentration and Krxn,eff are attributed to how geochemical heterogeneity affects the time (?eq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn,eff and Pb2 + concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in ?eq and Leq.

Atchley, Adam L.; Navarre-Sitchler, Alexis K.; Maxwell, Reed M.

2014-09-01

214

The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates.  

PubMed

The role of coupled physical and geochemical heterogeneities in hydro-geochemical transport is investigated by simulating three-dimensional transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: 1) spatially homogeneous reactive mineral surface area, 2) reactive surface area positively correlated to hydraulic heterogeneity, and 3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (?). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical dis-equilibrium between fluids and the mineral galena that resulted in increased aqueous lead (Pb(2+)) concentrations. Calcite dissolution buffered the pH change and created conditions of galena oversaturation, which then reduced lead concentrations along the flow path. Near the leak kinetic geochemical reactions control the release of solutes into the fluid, but further along the flow path mineral solubility controls solute concentrations. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface area in coordination with physical heterogeneity on the effective reaction rate (Krxn,eff) and Pb(2+) concentrations within the plume. Dissimilarities between ensemble Pb(2+) concentration and Krxn,eff are attributed to how geochemical heterogeneity affects the time (?eq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn,eff and Pb(2+) concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in ?eq and Leq. PMID:25113426

Atchley, Adam L; Navarre-Sitchler, Alexis K; Maxwell, Reed M

2014-09-01

215

Up-Scaling Geochemical Reaction Rates for Carbon Dioxide (CO2) in Deep Saline Aquifers  

SciTech Connect

Geochemical reactions in deep subsurface environments are complicated by the consolidated nature and mineralogical complexity of sedimentary rocks. Understanding the kinetics of these reactions is critical to our ability to make long-term predictions about subsurface processes such as pH buffering, alteration in rock structure, permeability changes, and formation of secondary precipitates. In this project, we used a combination of experiments and numerical simulation to bridge the gap between our knowledge of these reactions at the lab scale and rates that are meaningful for modeling reactive transport at core scales. The focus is on acid-driven mineral dissolution, which is specifically relevant in the context of CO2-water-rock interactions in geological sequestration of carbon dioxide. The project led to major findings in three areas. First, we modeled reactive transport in pore-network systems to investigate scaling effects in geochemical reaction rates. We found significant scaling effects when CO2 concentrations are high and reaction rates are fast. These findings indicate that the increased acidity associated with geological sequestration can generate conditions for which proper scaling tools are yet to be developed. Second, we used mathematical modeling to investigate the extent to which SO2, if co-injected with CO2, would acidify formation brines. We found that there exist realistic conditions in which the impact on brine acidity will be limited due to diffusion rate-limited SO2 dissolution from the CO2 phase, and the subsequent pH shift may also be limited by the lack of availability of oxidants to produce sulfuric acid. Third, for three Viking sandstones (Alberta sedimentary basin, Canada), we employed backscattered electron microscopy and energy dispersive X-ray spectroscopy to statistically characterize mineral contact with pore space. We determined that for reactive minerals in sedimentary consolidated rocks, abundance alone is not a good predictor of mineral accessible surface area, and should not be used in reactive transport modeling. Our work showed that reaction rates would be overestimated by three to five times.

Peters, Catherine A

2013-02-28

216

Temporal and spatial variation of surface reaction rates in porous media: Applications to silicate weathering  

NASA Astrophysics Data System (ADS)

Percolation theory provides a promising framework for modeling transport in heterogeneous porous media, including hydraulic and electrical conductivity, air permeability, gas diffusivity, and solute transport. Using percolation concepts (e.g., critical path analysis, fractal scaling of percolation clusters, and cluster statistics), we developed a physically-based model for predicting solute transport. Our model predicted spatial solute distributions as a function of time, and arrival time distributions as a function of system size. Our solute transport predictions gave good matches to a wide range of experiments. We now apply our solute transport model to silicate weathering. We assume that surface chemical reactions are at equilibrium at the scale of a single pore, but that at larger length scales, reactions are limited by transport of reactants or products. Using results from published field experiments, we find that the temporal and spatial dependence derived from solute velocity successfully predicts the measured time- and length-dependence of reaction rates and weathering of silicate minerals over a wide range of time and length scale. A similar analysis of lab experiments (uranium breakthrough curves measured in two short and long columns from the Hanford site) indicates that normalized reaction rate versus normalized time follow 2D invasion and 3D random percolation.

Ghanbarian, B.; Hunt, A. G.; Skinner, T. E.; Ewing, R. P.

2013-12-01

217

Mechanism and rate constant of the reaction of atomic hydrogen with propyne  

NASA Astrophysics Data System (ADS)

The potential energy surface for the reaction of atomic hydrogen with propyne has been studied at the G3//UB3LYP/6-31G(d) level of theory. Three reaction entrances were revealed, namely, terminal addition, nonterminal addition, and direct H-abstraction, leading to CH3CCH2, CH3CHCH, and H2+C3H3, respectively. The respective activation barriers are 1.7, 3.9, and 8.4 kcal/mol. The CH3-extrusion from CH3CHCH forms C2H2 via a barrier of about 32 kcal/mol. Several H-shift paths along the CCC skeleton were also examined for three C3H5 isomers. Multichannel RRKM and TST calculations have been carried out for the total and individual rate constants over a wide range of temperatures and pressures. The total rate constants possess both positive temperature dependence and typical "S" shaped fall-off behavior. At atmospheric pressure, the collisional stabilization of the initial adducts dominates the H+CH3CCH reaction at temperatures lower than 500 K, and at T>1000 K, CH3 and C2H2 are the major products. Moreover, the direct H-abstraction channel also contributes significantly to the overall reaction. The theoretical results are compared with those of previous studies.

Wang, Baoshan; Hou, Hua; Gu, Yueshu

2000-05-01

218

The injection rate of intravenous midazolam significantly influences the occurrence of paradoxical reaction in pediatric patients  

PubMed Central

Background: Paradoxical reactions to benzodiazepines including restlessness, anxiety and sometimes violent behavior sometimes occur. Most of the known predicting factors of disinhibitory reactions such as age, gender, genetic or the psychological background are not modifiable. This study was conducted to evaluate the effect of rate of midazolam administration, as a controllable factor, on the occurrence of paradoxical reaction to midazolam (PRM) in pediatric patients. Materials and Methods: In a randomized, double-blind clinical trial 98 American Society of Anesthesiologists physical status I, II, aged from 6 months to 6 years, and undergoing elective surgery, were enrolled in the study. Patients were randomly allocated to receive midazolam 0.1 mg/kg as a 0.1% solution at an injection rate of 0.2 ml/s or 1 ml/s. The occurrence of PRM was compared between the two groups with Chi-square test. Results: The occurrence of PRM in the rapid injection group was significantly higher than the slow injection group (20.4% vs. 4.1%, P < 0.05, relative risk CI: 95% 6.03 (1.24-29.4)). Conclusion: Slow intravenous administration of midazolam significantly reduces the occurrence of paradoxical reactions and should be respected in premedication of pediatric patients.

Moallemy, Abbas; Teshnizi, Saeed Hoseini; Mohseni, Masood

2014-01-01

219

Absolute rate of the reaction of bromine atoms with ozone from 200-360 K  

NASA Technical Reports Server (NTRS)

The rate constant for the reaction Br + O3 yields BrO + O2 was measured from 200 to 360 K by the technique of flash photolysis coupled to time resolved detection of bromine atoms by resonance fluorescence (FP-RF). Br atoms were produced by the flash photolysis of CH3Br at lambda 165nm.O3 was monitored continuously under reaction conditions by absorption at 253.7 nm. At each of five temperatures the results were independent of substantial variations in O3, total pressure and limited variations in flash intensity. The measured rate constants obeyed the Arrhenius expression, where the error quoted is two standard deviations. Results are compared with previous determinations which employed the discharge flow-mass spectrometric technique.

Michael, J. V.; Lee, J. H.; Payne, W. A.; Stief, L. J.

1978-01-01

220

Absolute rate of the reaction of bromine atoms with ozone from 200 to 360 K  

NASA Technical Reports Server (NTRS)

The rate constant for the reaction Br + O3 yields BrO + O2 has been measured from 200 to 360 K by the technique of flash photolysis coupled to time resolved detection of bromine atoms by resonance fluorescence (FP-RF). Br atoms were produced by the flash photolysis of CH3Br at a wavelength of 165 nm. O3 concentration was monitored continuously under reaction conditions by absorption at 253.7 nm. At each of five temperatures the results were independent of substantial variations in O3 concentration, total pressure (Ar), and limited variations in flash intensity (i.e., initial Br concentration). The measured rate constants obey the Arrhenius expression, k = (7.74 plus or minus 0.50) x 10 to the -12th exp(-603 plus or minus 16/T) cu cm/molecule/sec, where the error quoted is two standard deviations.

Michael, J. V.; Lee, J. H.; Payne, W. A.; Stief, L. J.

1978-01-01

221

Oxygen consumption rates in subseafloor basaltic crust derived from a reaction transport model.  

PubMed

Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass; yet, empirical analysis of reaction rates in basaltic crust is lacking. Here we report the first assessment of oxygen consumption in young (~8?Ma) and cool (<25?°C) basaltic crust, which we calculate from modelling dissolved oxygen and strontium pore water gradients in basal sediments collected during Integrated Ocean Drilling Program Expedition 336 to 'North Pond' on the western flank of the Mid-Atlantic Ridge. Dissolved oxygen is completely consumed within the upper to middle section of the sediment column, with an increase in concentration towards the sediment-basement interface, indicating an upward supply from oxic fluids circulating within the crust. A parametric reaction transport model of oxygen behaviour in upper basement suggests oxygen consumption rates of 1?nmol? cm(-3)ROCK d(-1) or less in young and cool basaltic crust. PMID:24071791

Orcutt, Beth N; Wheat, C Geoffrey; Rouxel, Olivier; Hulme, Samuel; Edwards, Katrina J; Bach, Wolfgang

2013-01-01

222

Monte Carlo uncertainty of the He3(?,?)Be7 reaction rate  

NASA Astrophysics Data System (ADS)

Background: The He3(?,?)Be7 reaction is of critical importance in determining the flux of solar neutrinos through the pp-II and pp-III chains. For this reason and others, the description of the cross section and its extrapolation towards low-energy has always been a matter of intense debate. While large systematic differences have been present in the past, several recent measurements are all in excellent statistical agreement. Purpose: The convergence of the recent individual experimental measurements of the He3(? ,?)Be7 reaction prompts a global analysis of the reaction data. From the combined data, a more precise estimate of the low-energy cross section can be determined. Results: A global R-matrix fit is used to describe the He3(?,?)Be7 data as well as scattering data over a similar energy range. The R-matrix fit is then subjected to a Monte Carlo analysis to extract the uncertainties on the cross section and corresponding reaction rate. Conclusion: By combining several recent measurements of the He3(?,?)Be7 reaction, the combined data yield a zero energy S factor of S(0)=0.542±0.011(MCfit)±0.006(model)-0.011+0.019(phaseshifts) keV b. This gives a total uncertainty in S (0) of +0.023/-0.017 keV b.

deBoer, R. J.; Görres, J.; Smith, K.; Uberseder, E.; Wiescher, M.; Kontos, A.; Imbriani, G.; Di Leva, A.; Strieder, F.

2014-09-01

223

Temperature dependence of the rate constant for the reaction HS + NO/sub 2/  

SciTech Connect

Two reactions of atmospheric importance involving the HS radical have been studied by using a discharge flow laser magnetic resonance technique: (1) HS + NO/sub 2/ ..-->.. products; and (2) HS + O/sub 2/ ..-->.. products. The rate constant for reaction 1 was measured at low pressure (approx. 1 Torr) between 221 and 415 K and a negative temperature dependence was observed: k/sub 1/ = (2.9 +/- 0.5) x 10/sup -11/ exp((240 +/- 50)/T) cm/sup 3/ molecule/sup -1/ s/sup -1/. The room temperature value for k/sub 1/ from this study is higher than three previous values. Secondary chemistry associated with high H/sub 2/S concentrations in the earlier studies is believed to contribute to the discrepancies. DS was used to investigate the isotope effect for reaction 1 and no evidence has been observed for a primary isotope effect, k(DS + NO/sub 2/) = (7.3 +/- 1.1) x 10/sup -11/ cm/sup 3/ molecule/sup 1/ s/sup -1/, at 299 K. No reaction was observed with O/sub 2/ and an upper limit 1.5 x 10/sup -17/ cm/sup 3/ molecule/sup -1/ s/sup -1/ was assigned to reaction 2.

Wang, N.S.; Lovejoy, E.R.; Howard, C.J.

1987-10-22

224

Absolute rate of the reaction of hydrogen atoms with ozone from 219-360 K  

NASA Technical Reports Server (NTRS)

Absolute rate constants for the reaction of atomic hydrogen with ozone were obtained over the temperature range 219-360 K by the flash photolysis-resonance fluorescence technique. The results can be expressed in Arrhenius form by K = (1.33 plus or minus 0.32)x10 to the minus 10 power exp (-449 plus or minus 58/T) cu cm/molecule/s (two standard deviations). The present work is compared to two previous determinations and is discussed theoretically.

Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief, L. J.

1978-01-01

225

Equation of state and reaction rate for condensed-phase explosives  

Microsoft Academic Search

The wide-ranging equation of state is a nonideal equation of state based on empirical fitting forms argued from thermodynamic considerations that yield the proper physical features of detonation. The complete equation of state forms are presented and the equation of state and a reaction rate are calibrated for the condensed-phase explosive PBX-9502. Experimental overdriven Hugoniot data are used to calibrate

B. L. Wescott; D. Scott Stewart; W. C. Davis

2005-01-01

226

Enzymatic reaction of silent substrates: kinetic theory and application to the serine protease chymotrypsin.  

PubMed

Investigating the selectivity that an enzyme expresses toward its substrates can be technically challenging if reaction of these substrates is not accompanied by a conveniently monitored change in some physicochemical property. In this paper, we describe a simple method for determining steady-state kinetic parameters for enzymatic turnover of such "silent" substrates. According to this method, silent substrate S is allowed to compete for enzymic reaction with signal-generating substrate S*, whose conversion to product can be conveniently monitored. Full reaction progress curves are collected under conditions of [S*](o) < K(m)* and [S](o) >or= 3K(m). Progress curves collected under these conditions are characterized by an initial lag phase of duration tau that is followed by the pseudo-first-order reaction of S. Steady-state kinetic parameters for the silent substrate can be obtained by one of two methods. One method combines least-squares fitting with numerical integration of appropriate rate equations to analyze the progress curves, while the other method relies on direct graphical analysis in which K(m) is the value of [S](o) that reduces the control velocity by a factor of 2 and V(max) is shown to simply equal the ratio [S](o)/tau. We use these methods to analyze the alpha-chymotrypsin-catalyzed hydrolysis of silent substrate Suc-Ala-Phe-AlaNH(2) with signal generator Suc-Ala-Phe-pNA. From the curve-fitting method, k(c) = 0.9 +/- 0.2 s(-1) and K(m) = 0.4 +/- 0.1 mM, while by direct graphical analysis, k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.51 +/- 0.03 mM. As validation of this new method, we show agreement of these values with those determined independently by HPLC analysis of the hydrolysis of Suc-Ala-Phe-AlaNH(2) by alpha-CT, where k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.5 +/- 0.1 mM. PMID:12705836

Case, April; Huskey, W Phillip; Stein, Ross L

2003-04-29

227

A reinvestigation of the rate of the C/+/ + H2 radiative association reaction. [interstellar chemistry  

NASA Technical Reports Server (NTRS)

It is noted that new experimental results and statistical theories have prompted a reinvestigation of the rate coefficient of the interstellar reaction in which C(+) + H2 yields CH2(+) + h(nu) in the 10-100 K temperature range. The results presented here indicate a rate coefficient between 10 to the -16 and 10 to the -15 cu cm/s at all temperatures studied. In applying the modified thermal and phase space theories, it is expected that they will be as accurate as for radiative association as for three-body association, provided the ab initio value for the radiative decay rate is correct. It is expected that the calculated values of the rate coefficient will be accurate to within an order of magnitude and will have the correct temperature dependence.

Herbst, E.

1982-01-01

228

Rates and mechanisms of the atomic oxygen reaction with nickel at elevated temperatures  

NASA Technical Reports Server (NTRS)

The oxidation of nickel by atomic oxygen at pressure from 1 to 45 N/sq m between 1050 and 1250 K was investigated. In these ranges, the oxidation was found to follow the parobolic rate law, viz., K sub p = 0.0000114 exp(-13410/T) g squared/cm4/sec for films of greater than 1 micron thickness and was pressure independent. The activation enthalpy for the oxidation reaction was 112 + or - 11 kj/mole (27 + or - 3 kcal/mole). Of a number of possible mechanisms and defect structures considered, it was shown that the most likely was a saturated surface defect model for atomic oxidation, based on reaction activation enthalpies, impurity effects, pressure independence, and magnitudes of rates. A model judged somewhat less likely was one having doubly ionized cationic defects rate controlling in both atomic and molecular oxygen. From comparisons of the appropriate processes, the following enthalpy values were derived: enthalpy of activation (Ni diffusion in Ni0) = 110 + or - 30 kj/mole and standard enthalpy change for reaction formation (doubly ionized cation vacancies in Ni0 from atomic oxygen)= -9 + or - 25 kj/mole.

Christian, J. D.; Gilbreath, W. P.

1973-01-01

229

The CH + CO reaction: Rate coefficient for carbon atom exchange at 294 K  

SciTech Connect

A fast-flow reactor equipped with isotope-specific laser-excited fluorescence detection of CH radicals has been used to study carbon atom exchange in the reaction between CH and CO at 294 K and 2 Torr of total pressure. The rate coefficient for exchange, k{sub 3} = (2.1 {times} 0.3) {times} 10{sup {minus}12} cm{sup 3} s{sup {minus}1}, is about an order of magnitude larger than the bimolecular rate for the addition reaction, k{sub 2} = (2.7 {plus minus} 0.4) {times} 10{sup {minus}13}. High-pressure limiting bimolecular and low-pressure termolecular recombination rate coefficients of 1.1 {times} 10{sup {minus}10} cm{sup 3} s{sup {minus}1} and 4.9 {times} 10{sup {minus}30} cm{sup 6} s{sup {minus}1} are derived. The results are discussed in the context of previous work on the title reaction and on the chemistry of singlet CH{sub 2}.

Anderson, S.M.; McCurdy, K.E.; Kolb, C.E. (Aerodyne Research, Inc., Billerica, MA (USA))

1989-02-09

230

Experimental and theoretical rate coefficients for the gas phase reaction of ?-Pinene with OH radical  

NASA Astrophysics Data System (ADS)

Rate coefficient for the reaction of ?-pinene with OH radicals was determined at 298 K and 800 Torr of N2 using the relative rate technique. Isobutylene was used as a reference compound and the concentrations of the organics were followed by gas chromatographic analysis. The rate coefficient for the reaction of ?-pinene with OH radical was measured to be (9.35 ± 2.79) × 10-11 cm3 molecule-1 s-1. Theoretical kinetic calculations were also performed for the title reaction using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT). The kinetics data obtained over the temperature range of 200-400 K were used to derive the Arrhenius expression: k(T) = 8.24 × 10-23T3.41 exp[2421/T] cm3 molecule-1 s-1. The OH-driven atmospheric lifetime (?) and global warming potential (GWP) for ?-pinene were computed and concluded that ?-pinene is very short lived (2.5 h) in the Earth's atmosphere with a GWP of 1.6 × 10-2 at 20 years horizon of time and which is negligible. The ozone formation potential of ?-pinene was also calculated and reported in this present work.

Dash, Manas Ranjan; Rajakumar, B.

2013-11-01

231

Rate coefficients for reaction of OH with acetone between 202 and 395 K  

SciTech Connect

The kinetics of the title reaction were investigated between 202 and 395 K and at 20, 50, and 100 Torr of Ar or N{sub 2} bath gas using pulsed laser photolysis (PLP) generation of OH combined with both resonance fluorescence (RF) and laser-induced fluorescence (LIF) detection. OH was generated either by the sequential 439 nm, two-photon dissociation of NO{sub 2} in the presence of H{sub 2}, or by HONO photolysis at 351 nm. The accuracy of the rate constants obtained was enhanced by optical absorption measurements of acetone concentrations both before and after the photolysis reactor. The temperature dependence is not describe by a simple Arrhenius expression but by k{sub 1} (202--395 K) = 8.8 x 10{sup {minus}12} exp({minus}1,320/T) + 1.7 x 10{sup {minus}14} exp(423/T) cm{sup 3} s{sup {minus}1}, indicating that a simple H atom abstraction may not be the only reaction mechanism. The estimated total error (95% confidence) associated wit the rate coefficient derived from this expression is estimated as 5% and is independent of temperature. The curvature in the Arrhenius plot results in a significantly larger rate coefficient at low temperatures than obtained by extrapolation of the previous measurement and implies greater significance for the reaction with OH as a sink for acetone in the upper troposphere than presently assumed.

Wollenhaupt, M.; Carl, S.A.; Horowitz, A.; Crowley, J.N.

2000-03-30

232

Irreversible bimolecular reactions with inertia: from the trapping to the target setting at finite densities.  

PubMed

We investigate numerically pseudo-first-order irreversible bimolecular reactions of the type A + B ? B between hard spheres undergoing event-driven Brownian dynamics. We study the encounter rate and the survival probability of A particles as functions of the packing fraction ? in the trapping (a single particle diffusing among static non-overlapping traps) and target (many traps diffusing in the presence of a single static target particle) settings, as well as in the case of diffusing traps and particles (full mobility). We show that, since inertial effects are accounted for in our simulation protocol, the standard Smoluchowski theory of coagulation of non-interacting colloids is recovered only at times greater than a characteristic time ?t, marking the transition from the under-damped to the over-damped regime. We show that the survival probability S(t) decays exponentially during this first stage, with a rate 1/?0 is proportional to ?. Furthermore, we work out a simple analytical expression that is able to capture to an excellent extent the numerical results for t < ?t at low and intermediate densities. Moreover, we demonstrate that the time constant of the asymptotic exponential decay of S(t) for diffusing traps and particles is k(S)(-1), where kS = 4?(DA + DB)R? is the Smoluchowski rate. Detailed analyses of the effective decay exponent ? = d [log(-logS(t))]/d (logt) and of the steady-state encounter rate reveal that the full mobility and trapping problem are characterized by very similar kinetics, rather different from the target problem. Our results do not allow one to ascertain whether the prediction S(t) is proportional to exp(-at(3/2)) (a = const.) as t ? ? for the trapping problem in 3D is indeed recovered. In fact, at high density, S(t) is dominated by short encounter times, which makes it exceedingly hard to record the events corresponding to the exploration of large, trap-free regions. As a consequence, at high densities the steady-state rate simply tends to 1/?0. Finally, we work out an analytical formula for the rate that shows a remarkable agreement with the numerics up ? = 0.4. PMID:23670209

Piazza, Francesco; Foffi, Giuseppe; De Michele, Cristiano

2013-06-19

233

Variable-Temperature Rate Coefficients of Proton-Transfer Equilibrium Reaction C2H4 + H3O+  

E-print Network

Variable-Temperature Rate Coefficients of Proton-Transfer Equilibrium Reaction C2H4 + H3O+ C2H5: The rate coefficients for the forward and reverse proton-transfer reactions C2H4 + H3O+ C2H5 + + H2O and S = (-15.0 ± 0.9) J·mol-1 ·K-1 , respectively. INTRODUCTION Proton-transfer reactions of simple molecules

Sanov, Andrei

234

Absolute rate of the reaction of Cl(p-2) with molecular hydrogen from 200 - 500 K  

NASA Technical Reports Server (NTRS)

Rate constants for the reaction of atomic chlorine with hydrogen are measured from 200 - 500 K using the flash photolysis-resonance fluorescence technique. The results are compared with previous work and are discussed with particular reference to the equilibrium constant for the reaction and to relative rate data for chlorine atom reactions. Theoretical calculations, using the BEBO method with tunneling, give excellent agreement with experiment.

Whytock, D. A.; Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief, L. J.

1976-01-01

235

Ultrasonic degradation of trichloroethylene and chlorobenzene at micromolar concentrations: kinetics and modelling  

Microsoft Academic Search

Although most papers in the field of sonochemical degradation of volatile organics in aqueous media describe experiments at the millimolar concentration range, this study focuses on the degradation kinetics of chlorobenzene (CB) and trichloroethylene (TCE) in the micromolar range. It was found that the reaction kinetics increase with decreasing initial substrate concentrations. For example, the pseudo-first-order reaction rate constant of

J. Dewulf; H. Van Langenhove; A. De Visscher; S. Sabbe

2001-01-01

236

Upscaling of reaction rates in reactive transport using pore-scale reactive transport model  

NASA Astrophysics Data System (ADS)

Dissolved CO2 during geological CO2 storage may react with minerals in fractured rocks, confined aquifers, or faults, resulting in mineral precipitation and dissolution. The overall rate of reaction can be affected by coupled processes among hydrodynamics, transport, and reactions at the (sub) pore-scale. In this research pore-scale modeling of coupled fluid flow, reactive transport, and heterogeneous reaction at the mineral surface is applied to account for permeability alterations caused by precipitation-induced pore-blocking. This work is motivated by the observed CO2 seeps from a natural analog to geologic CO2 sequestration at Crystal Geyser, Utah. A key observation is the lateral migration of CO2 seep sites at a scale of ~ 100 meters over time. A pore-scale model provides fundamental mechanistic explanations of how calcite precipitation alters flow paths by pore plugging under different geochemical compositions and pore configurations. In addition, response function of reaction rates will be constructed from pore-scale simulations which account for a range of reaction regimes characterized by the Damkohler and Peclet numbers. Newly developed response functions will be used in a continuum scale model that may account for large-scale phenomena mimicking lateral migration of surface CO2 seeps. Comparison of field observations and simulations results will provide mechanistic explanations of the lateral migration and enhance our understanding of subsurface processes associated with the CO2 injection. This work is supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Yoon, H.; Dewers, T. A.; Arnold, B. W.; Major, J. R.; Eichhubl, P.; Srinivasan, S.

2013-12-01

237

A study of the photocatalytic effects of aqueous suspensions of platinized semiconductor materials on the reaction rates of candidate redox reactions  

NASA Technical Reports Server (NTRS)

The effectiveness of powdered semiconductor materials in photocatalyzing candidate redox reactions was investigated. The rate of the photocatalyzed oxidation of cyanide at platinized TiO2 was studied. The extent of the cyanide reaction was followed directly using an electroanalytical method (i.e. differential pulse polarography). Experiments were performed in natural or artificial light. A comparison was made of kinetic data obtained for photocatalysis at platinized powders with rate data for nonplatinized powders.

Miles, A. M.

1982-01-01

238

Probabilistic models and uncertainty quantification for the ionization reaction rate of atomic Nitrogen  

NASA Astrophysics Data System (ADS)

The objective in this paper is to analyze some stochastic models for estimating the ionization reaction rate constant of atomic Nitrogen (N + e- ? N+ + 2e-). Parameters of the models are identified by means of Bayesian inference using spatially resolved absolute radiance data obtained from the Electric Arc Shock Tube (EAST) wind-tunnel. The proposed methodology accounts for uncertainties in the model parameters as well as physical model inadequacies, providing estimates of the rate constant that reflect both types of uncertainties. We present four different probabilistic models by varying the error structure (either additive or multiplicative) and by choosing different descriptions of the statistical correlation among data points. In order to assess the validity of our methodology, we first present some calibration results obtained with manufactured data and then proceed by using experimental data collected at EAST experimental facility. In order to simulate the radiative signature emitted in the shock-heated air plasma, we use a one-dimensional flow solver with Park's two-temperature model that simulates non-equilibrium effects. We also discuss the implications of the choice of the stochastic model on the estimation of the reaction rate and its uncertainties. Our analysis shows that the stochastic models based on correlated multiplicative errors are the most plausible models among the four models proposed in this study. The rate of the atomic Nitrogen ionization is found to be (6.2 ± 3.3) × 1011 cm3 mol-1 s-1 at 10,000 K.

Miki, K.; Panesi, M.; Prudencio, E. E.; Prudhomme, S.

2012-05-01

239

Clusters of reaction rates and concentrations in protein networks such as the phosphotransferase system.  

PubMed

To understand the functioning of living cells, it is often helpful or even necessary to exploit inherent timescale disparities and focus on long-term dynamic behaviour. In the present study, we explore this type of behaviour for the biochemical network of the phosphotransferase system. We show that, during the slow phase that follows a fast initial transient, the network reaction rates are partitioned into clusters corresponding to connected parts of the reaction network. Rates within any of these clusters assume essentially the same value: differences within each cluster are vastly smaller than that from one cluster to another. This rate clustering induces an analogous clustering of the reactive compounds: only the molecular concentrations on the interface between these clusters are produced and consumed at substantially different rates and hence change considerably during the slow phase. The remaining concentrations essentially assume their steady-state values already by the end of the transient phase. Further, we find that this clustering phenomenon occurs for a large number of parameter values and also for models with different topologies; to each of these models, there corresponds a particular network partitioning. Our results show that, in spite of its complexity, the phosphotransferase system tends to behave in a rather simple (yet versatile) way. The persistence of clustering for the perturbed models we examined suggests that it is likely to be encountered in various environmental conditions, as well as in other signal transduction pathways with network structures similar to that of the phosphotransferase system. PMID:24289186

Härdin, Hanna M; Zagaris, Antonios; Willms, Allan R; Westerhoff, Hans V

2014-01-01

240

Reaction rate prediction for elementary reactions in binary mixed solvents assuming an additive solvation of reactant(s)  

E-print Network

Procedure ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . 14 Method's Application to Reaction . . . . 15 Time of Analysis ~ ~ ~ ~ ~ ~ ~ 16 Calculations ~ ~ ~ . . ~ ~ 17 IV RESULTS ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 18 Data o o o . . o o . . . . o ~ ~ ~ ~ 18 Theory... of reactions occurring in binary mixed solvent systems can only be as correct as the phenomenological perception. To be presented here is another alternative straightforward conception of reactions occurring in binary mixed solvent systems. The explanation...

Van Woert, Howard Clark

2012-06-07

241

The use of the magnetic field effect for studying a chemiluminescent chemical reaction in aqueous solution. Reaction rate constants and lifetimes of intermediate molecules  

NASA Astrophysics Data System (ADS)

The phase shift magnetic field effect technique is applied for investigation of the chemiluminescent (ChL) reaction of luminol oxidation by potassium ferricyanide in aqueous alkali solution. The external modulated magnetic field changed the rate constant of recombination of luminol radicals. Rate constants of intermediate stages of the reaction are obtained: 10 8 M -1 s -1 for diazaquinone reaction with hydrogen peroxide, 2 X 10 6 M -1 s -1 for diazaquinone hydrolysis and 2 X 10 5 s -1 for the decomposition of hydroperoxide, which is a precursor of the light emitter.

Triebel, Michael M.; Totrov, Maxim M.; Zorinyants, George E.; Frankevich, Eugene L.

1993-11-01

242

The rate constant for the reaction of oxygen /3P/ atoms with dichlorine monoxide  

NASA Technical Reports Server (NTRS)

A fast flow discharge apparatus was used to measure the rate constant for the reaction of ground state oxygen atoms with dichlorine monoxide in the temperature range 236-295 K. The air afterflow technique (NO2 chemiluminescence) was used for detection of oxygen atoms. The Arrhenius expression for the rate constant was found to be 2.7 plus or minus 0.3 times 10 to the -11th power exp(-560 plus or minus 80/T) cu cm per molecule per sec. At 295 K the rate constant is 4.1 plus or minus 0.5 times 10 to the -12th power cu cm per molecule per sec.

Miziolek, A. W.; Molina, M. J.

1978-01-01

243

Standard Test Method for Measuring Reaction Rates by Analysis of Barium-140 From Fission Dosimeters  

E-print Network

1.1 This test method describes two procedures for the measurement of reaction rates by determining the amount of the fission product 140Ba produced by the non-threshold reactions 235U(n,f), 241Am(n,f), and 239Pu(n,f), and by the threshold reactions 238U(n,f), 237Np(n,f), and 232Th(n,f). 1.2 These reactions produce many fission products, among which is 140Ba, having a half-life of 12.752 days. 140Ba emits gamma rays of several energies; however, these are not easily detected in the presence of other fission products. Competing activity from other fission products requires that a chemical separation be employed or that the 140Ba activity be determined indirectly by counting its daughter product 140La. This test method describes both procedure (a), the nondestructive determination of 140Ba by the direct counting of 140La several days after irradiation, and procedure (b), the chemical separation of 140Ba and the subsequent counting of 140Ba or its daughter 140La. 1.3 With suitable techniques, fission neutron fl...

American Society for Testing and Materials. Philadelphia

2008-01-01

244

Reaction rate uncertainties and 26Al in AGB silicon carbide stardust  

E-print Network

Stardust is a class of presolar grains each of which presents an ideally uncontaminated stellar sample. Mainstream silicon carbide (SiC) stardust formed in the extended envelopes of carbon-rich asymptotic giant branch (AGB) stars and incorporated the radioactive nucleus 26Al as a trace element. The aim of this paper is to analyse in detail the effect of nuclear uncertainties, in particular the large uncertainties of up to four orders of magnitude related to the 26Al_g+(p,gamma)27Si reaction rate, on the production of 26Al in AGB stars and compare model predictions to data obtained from laboratory analysis of SiC stardust grains. Stellar uncertainties are also briefly discussed. We use a detailed nucleosynthesis postprocessing code to calculate the 26Al/27Al ratios at the surface of AGB stars of different masses (M = 1.75, 3, and 5 M_sun) and metallicities (Z = 0.02, 0.012, and 0.008). For the lower limit and recommended value of the 26Al_g(p,gamma)27Si reaction rate, the predicted 26Al/27Al ratios replicate the upper values of the range of the 26Al/27Al ratios measured in SiC grains. For the upper limit of the 26Al_g(p,gamma)27Si reaction rate, instead, the predicted 26Al/27Al ratios are approximately 100 times lower and lie below the range observed in SiC grains. When considering models of different masses and metallicities, the spread of more than an order of magnitude in the 26Al/27Al ratios measured in stellar SiC grains is not reproduced. We propose two scenarios to explain the spread of the 26Al/27Al ratios observed in mainstream SiC, depending on the choice of the 26Al_g+p reaction rate. One involves different times of stardust formation, the other involves extra-mixing processes. Stronger conclusions will be possible after more information is available from future nuclear experiments on the 26Al_g+p reaction.

M. A. van Raai; M. Lugaro; A. I. Karakas; C. Iliadis

2007-12-21

245

Contribution of 19F resonances on 18O( p, ?)15N reaction rate  

NASA Astrophysics Data System (ADS)

The 18O( p, ?)15N reaction influences the isotopes production such as 19F, 18O, and 15N which can be used to test the models of stellar evolution. 19F is synthesized in both asymptotic giant branch (AGB) and metal-rich Wolf-Rayet (WR) stars. Using R-matrix theory we allow new values of resonances parameters in 19F. We show that the most important contribution to the differential and total cross section at low energies, comes from the levels in 19F situated at resonances energies E R =151, 680 and 840 keV with spin and parity 1/2+. The total width of the 680 keV resonance is badly known. So, we have focused on this broad resonance corresponding to the 8.65 MeV level in 19F. We delimit the temperature range in which each resonance contribution to the total reaction rate occurs by analyzing the ratio ( N A < ??> i / N A < ??>). This allowed us to show that the 680 and 840 keV broad resonances strongly dominate the reaction rate over the stellar temperature range T 9=0.02-0.06 and T 9=0.5-5. Finally, these results were compared to NACRE and Iliadis astrophysical compilations.

Benmeslem, Meriem; Chafa, Azzedine; Barhoumi, Slimane; Tribeche, Mouloud

2014-08-01

246

Modeling microbial reaction rates in a submarine hydrothermal vent chimney wall  

NASA Astrophysics Data System (ADS)

The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction-transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation-reduction reactions associated with fluid mixing as a function of temperature. The metabolisms included in the reaction network are methanogenesis, aerobic oxidation of hydrogen, sulfide and methane and sulfate reduction by hydrogen and methane. Model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney (77-102 °C), and methane and sulfide oxidation peak near the seawater-side of the chimney. The fastest metabolisms are aerobic oxidation of H2 and sulfide and reduction of sulfate by H2 with maximum rates of 140, 900 and 800 pmol cm-3 d-1, respectively. The maximum rate of hydrogenotrophic methanogenesis is just under 0.03 pmol cm-3 d-1, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). These simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature. The model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the role of microbial activity in the deep subsurface.

LaRowe, Douglas E.; Dale, Andrew W.; Aguilera, David R.; L'Heureux, Ivan; Amend, Jan P.; Regnier, Pierre

2014-01-01

247

THE IMPACT OF HELIUM-BURNING REACTION RATES ON MASSIVE STAR EVOLUTION AND NUCLEOSYNTHESIS  

SciTech Connect

We study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the helium-burning reaction rates within the range of their uncertainties. The current solar abundances from Lodders are used for the initial stellar composition. We compute a grid of 12 initial stellar masses and 176 models per stellar mass to explore the effects of independently varying the {sup 12}C({alpha}, {gamma}){sup 16}O and 3{alpha} reaction rates, denoted R{sub {alpha},12} and R{sub 3{alpha}}, respectively. The production factors of both the intermediate-mass elements (A = 16-40) and the s-only isotopes along the weak s-process path ({sup 70}Ge, {sup 76}Se, {sup 80}Kr, {sup 82}Kr, {sup 86}Sr, and {sup 87}Sr) were found to be in reasonable agreement with predictions for variations of R{sub 3{alpha}} and R{sub {alpha},12} of {+-}25%; the s-only isotopes, however, tend to favor higher values of R{sub 3{alpha}} than the intermediate-mass isotopes. The experimental uncertainty (one standard deviation) in R{sub 3{alpha}}(R{sub {alpha},12}) is approximately {+-}10%({+-}25%). The results show that a more accurate measurement of one of these rates would decrease the uncertainty in the other as inferred from the present calculations. We also observe sharp changes in production factors and standard deviations for small changes in the reaction rates, due to differences in the convection structure of the star. The compactness parameter was used to assess which models would likely explode as successful supernovae, and hence contribute explosive nucleosynthesis yields. We also provide the approximate remnant masses for each model and the carbon mass fractions at the end of core-helium burning as a key parameter for later evolution stages.

West, Christopher; Heger, Alexander [Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States); Austin, Sam M., E-mail: west0482@umn.edu, E-mail: alexander.heger@monash.edu, E-mail: austin@nscl.msu.edu [Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556 (United States)

2013-05-20

248

Neutrino-nucleon reaction rates in the supernova core in the relativistic random phase approximation  

NASA Astrophysics Data System (ADS)

We calculate neutrino reaction rates with nucleons via the neutral and charged currents in the supernova core in the relativistic random phase approximation (RPA) and study their effects on the opacity of the supernova core. The formulation is based on the Lagrangian employed in the calculation of the nuclear equation of state (EOS) in the relativistic mean field theory (RMF). The nonlinear meson terms are treated appropriately so that the consistency of the density correlation derived in RPA with the thermodynamic derivative obtained from the EOS by the RMF is satisfied in the static and long wavelength limit. We employ pion and rho meson exchange interactions together with the phenomenological Landau-Migdal parameters for the isospin-dependent nuclear interactions. We find that both the charged and neutral current reaction rates are suppressed from Bruenn's standard approximate formula considerably in the high density regime (?b>~1014 g/cm3 with ?b the baryonic density). In the low density regime (?b<~1014 g/cm3), on the other hand, the vector current contribution to the neutrino-nucleon scattering rate is enhanced in the vicinity of the boundary of the liquid-gas phase transition, while the other contributions are moderately suppressed there also. In the high temperature regime (T>~40 MeV with T the temperature) or in the regime where electrons have a large chemical potential, the latter of which is important only for the electron capture process and its inverse process, the recoil of nucleons cannot be neglected and further reduces the reaction rates with respect to the standard approximate formula which discards any energy transfer in the processes. These issues could have a great impact on the neutrino heating mechanism of collapse-driven supernovae.

Yamada, Shoichi; Toki, Hiroshi

2000-01-01

249

Characterization of the inactivation of rat fatty acid synthase by C75: inhibition of partial reactions and protection by substrates  

PubMed Central

C75, a synthetic inhibitor of FAS (fatty acid synthase), has both anti-tumour and anti-obesity properties. In this study we provide a detailed kinetic characterization of the mechanism of in vitro inhibition of rat liver FAS. At room temperature, C75 is a competitive irreversible inhibitor of the overall reaction with regard to all three substrates, i.e. acetyl-CoA, malonyl-CoA and NADPH, exhibiting pseudo-first-order kinetics of the complexing type, i.e. a weak non-covalent enzyme–inhibitor complex is formed before irreversible enzyme modification. C75 is a relatively inefficient inactivator of FAS, with a maximal rate of inactivation of 1 min?1 and an extrapolated KI (dissociation constant for the initial complex) of approx. 16 mM. The apparent second-order rate constants calculated from these values are 0.06 mM?1·min?1 at room temperature and 0.21 mM?1·min?1 at 37 °C. We also provide experimental evidence that C75 inactivates the ?-ketoacyl synthase (3-oxoacyl synthase) partial activity of FAS. Unexpectedly, C75 also inactivates the enoyl reductase and thioesterase partial activities of FAS with about the same rates as for inactivation of the ?-ketoacyl synthase. In contrast with the overall reaction, the ?-ketoacyl synthase activity and the enoyl reductase activity, substrates do not protect the thioesterase activity of rat liver FAS from inactivation by C75. These results differentiate inactivation by C75 from that by cerulenin, which only inactivates the ?-ketoacyl synthase activity of FAS, by forming an adduct with an active-site cysteine. Interference by dithiothreitol and protection by the substrates, acetyl-CoA, malonyl-CoA and NADPH, further distinguish the mechanism of C75-mediated inactivation from that of cerulenin. The most likely explanation for the multiple effects observed with C75 on rat liver FAS and its partial reactions is that there are multiple sites of interaction between C75 and FAS. PMID:15715522

2005-01-01

250

Determination of sulfite radical (SO.3-) reaction rate constants by means of competition kinetics.  

PubMed

The sulfite radical anion (SO.3-) was found to react rapidly with the flavonoid quercetin (k = 2.5 x 10(8) dm3mol-1s-1) and the carotenoids crocin (k = 1.0 x 10(9) dm3mol-1s-1) and crocetin (k = 1.5 x 10(9) dm3mol-1s-1). The reactions can easily be monitored due to the strong absorptions of the substrates and, in the case of quercetin, the formation of a strongly absorbing transient species. Using these substances, we determined by means of competition kinetics rate constants of SO.3- reactions with nucleic acid components, polyunsaturated fatty acids, and glutathione. PMID:3685252

Erben-Russ, M; Michel, C; Bors, W; Saran, M

1987-01-01

251

Process for the oxidation of materials in water at supercritical temperatures utilizing reaction rate enhancers  

SciTech Connect

A method is described for substantially completely oxidizing combustible materials in which an aqueous stream bearing the combustible materials is reacted in the presence of an oxidant comprising diatomic oxygen and at a temperature greater than the critical temperature of water and at a pressure greater than about 25 bar, within a reactor for a period of less than about 5 minutes to produce a reaction product stream, wherein the reaction is initiated in the presence of a rate enhancer comprising at least one oxidizing agent in addition to said oxidant selected from the group consisting of ozone, hydrogen peroxide, salts containing persulfate, salts containing permanganate, nitric acid, salts containing nitrate, oxyacids of chlorine and their corresponding salts, hypochlorous acid, salts containing hypochlorite, chlorous acid, salts containing chlorite, chloric acid, salts containing chlorate, perchloric acid, and salts containing perchlorate.

Swallow, K.C.; Killilea, W.R.; Hong, G.T.; Bourhis, A.L.

1993-08-03

252

Eight-dimensional quantum reaction rate calculations for the H+CH4 and H2+CH3 reactions on recent potential energy surfaces  

NASA Astrophysics Data System (ADS)

Eight-dimensional (8D) transition-state wave packet simulations have been performed on two latest potential energy surfaces (PES), the Zhou-Fu-Wang-Collins-Zhang (ZFWCZ) PES [Y. Zhou, B. Fu, C. Wang, M. A. Collins, and D. H. Zhang, J. Chem. Phys. 134, 064323 (2011)] and the Xu-Chen-Zhang (XCZ)-neural networks (NN) PES [X. Xu, J. Chen, and D. H. Zhang, Chin. J. Chem. Phys. 27, 373 (2014)]. Reaction rate constants for both the H+CH4 reaction and the H2+CH3 reaction are calculated. Simulations of the H+CH4 reaction based on the XCZ-NN PES show that the ZFWCZ PES predicts rate constants with reasonable high accuracy for low temperatures while leads to slightly lower results for high temperatures, in line with the distribution of interpolation error associated with the ZFWCZ PES. The 8D H+CH4 rate constants derived on the ZFWCZ PES compare well with full-dimensional 12D results based on the equivalent m-ZFWCZ PES, with a maximum relative difference of no more than 20%. Additionally, very good agreement is shown by comparing the 8D XCZ-NN rate constants with the 12D results obtained on the ZFWCZ-WM PES, after considering the difference in static barrier height between these two PESs. The reaction rate constants calculated for the H2+CH3 reaction are found to be in good consistency with experimental observations.

Zhou, Yong; Zhang, Dong H.

2014-11-01

253

Structural and dynamical control of the reaction rate in protein electron transfer  

NASA Astrophysics Data System (ADS)

Electron transfer (ET) reactions in proteins are key steps in many vital bioenergetic processes, and the reaction rate is known to be highly sensitive to the protein structure in some cases. For most bioenergetic reactions, as described by the Fermi Golden rule, the rate is proportional to a product of the average square of the effective electronic donor to acceptor coupling and a Franck-Condon factor, which accounts for the nuclear control of the energy gap. The nuclear factor is reasonably well described in Marcus theory and its modifications, and this work is focused on the mechanisms that control the effective coupling. About ten years ago, the Pathways model described for the first time how protein environment may control the effective coupling. In this work, a novel theoretical approach is developed to explore the mechanisms of structural and dynamical control beyond the qualitative level of the Pathways model. In Chapter 1, the assumptions of the Pathways model, its limitations and effects of the structure and the electronic Hamiltonian are investigated for model chain-like bridges using the Dyson's equations. In Chapter II, the framework to explore the sensitivity of the effective coupling to quality of the electronic Hamiltonian, the interference among the dominant pathways and the bridge dynamics is presented. This analysis employs the Green's function technique and includes combined molecular dynamics and electronic structure calculations. Finally, in Chapter III, this framework is tested on the bacterial photosynthetic reaction center, and the mechanisms of the structural and dynamical control for different ET steps are discussed.

Balabin, Ilya A.

254

Rate Equations and Kinetic Parameters of the Reactions Involved in Pyrite Oxidation by Thiobacillus ferrooxidans  

PubMed Central

Rate equations and kinetic parameters were obtained for various reactions involved in the bacterial oxidation of pyrite. The rate constants were 3.5 ?M Fe2+ per min per FeS2 percent pulp density for the spontaneous pyrite dissolution, 10 ?M Fe2+ per min per mM Fe3+ for the indirect leaching with Fe3+, 90 ?M O2 per min per mg of wet cells per ml for the Thiobacillus ferrooxidans oxidation of washed pyrite, and 250 ?M O2 per min per mg of wet cells per ml for the T. ferrooxidans oxidation of unwashed pyrite. The Km values for pyrite concentration were similar and were 1.9, 2.5, and 2.75% pulp density for indirect leaching, washed pyrite oxidation by T. ferrooxidans, and unwashed pyrite oxidation by T. ferrooxidans, respectively. The last reaction was competitively inhibited by increasing concentrations of cells, with a Ki value of 0.13 mg of wet cells per ml. T. ferrooxidans cells also increased the rate of Fe2+ production from Fe3+ plus pyrite. PMID:16348054

Lizama, Hector M.; Suzuki, Isamu

1989-01-01

255

The 18F(p,a)15O reaction rate for application to nova gamma-ray emission  

E-print Network

The 18F(p,a)15O reaction is recognized as one of the most important reaction for nova gamma-ray astronomy as it governs the early <= 511 keV emission. However, its rate remains largely uncertain at nova temperatures due to unknown low-energy resonance strengths. We report here on our last results concerning the study of the D(18F,pa)15N reaction, as well as on the determination of the 18F(p,a)15O reaction rate using the R-matrix theory. Remaining uncertainties are discussed.

N. de Sereville; E. Berthoumieux; A. Coc

2004-08-02

256

Theory and simulation of the time-dependent rate coefficients of diffusion-influenced reactions.  

PubMed Central

A general formalism is developed for calculating the time-dependent rate coefficient k(t) of an irreversible diffusion-influenced reaction. This formalism allows one to treat most factors that affect k(t), including rotational Brownian motion and conformational gating of reactant molecules and orientation constraint for product formation. At long times k(t) is shown to have the asymptotic expansion k(infinity)[1 + k(infinity) (pie Dt)-1/2 /4 pie D + ...], where D is the relative translational diffusion constant. An approximate analytical method for calculating k(t) is presented. This is based on the approximation that the probability density of the reactant pair in the reactive region keeps the equilibrium distribution but with a decreasing amplitude. The rate coefficient then is determined by the Green function in the absence of chemical reaction. Within the framework of this approximation, two general relations are obtained. The first relation allows the rate coefficient for an arbitrary amplitude of the reactivity to be found if the rate coefficient for one amplitude of the reactivity is known. The second relation allows the rate coefficient in the presence of conformational gating to be found from that in the absence of conformational gating. The ratio k(t)/k(0) is shown to be the survival probability of the reactant pair at time t starting from an initial distribution that is localized in the reactive region. This relation forms the basis of the calculation of k(t) through Brownian dynamics simulations. Two simulation procedures involving the propagation of nonreactive trajectories initiated only from the reactive region are described and illustrated on a model system. Both analytical and simulation results demonstrate the accuracy of the equilibrium-distribution approximation method. PMID:8913584

Zhou, H X; Szabo, A

1996-01-01

257

Rate constants for the reaction of ground state atomic oxygen with methanol  

NASA Astrophysics Data System (ADS)

The reaction of O(3P) with methanol has been studied using the complementary discharge flow and flash photolysis techniques. In both cases, resonance fluorescence detection of atomic oxygen was employed. The discharge flow (DF) apparatus was used in a temperature range of 298-998 K while the flash photolysis (FP) apparatus was used in the overlapping range of 329-527 K. The apparent bimolecular rate constants for the O-atom/methanol reaction obtained from DF experiments at low temperatures (T?450 K) were independent of both the initial O-atom concentration and the mode of O-atom production. In addition, large excesses of O2 were added to the flow to intercept the primary reaction product (CH2OH), but had no apparent effect on the measured rate constant. Results from the two methods were in good agreement within this limited temperature range (˜300-500 K). At temperatures above ˜450 K, the apparent rate constants obtained from DF experiments were increasingly sensitive to the O2 concentration, with the rate constants being smaller when determined in the presence of large [O2]. Since the initial O-atom concentrations were on the order of 1011 or less, a simple stoichiometry effect can be ruled out. However, the results of the present kinetic experiments indicated that heterogeneous pyrolysis of CH3OH may have occurred in the flow system. This observation is consistent with studies of the adsorption of methanol on silica surfaces. This problem was apparently overcome by adding small amounts of O2 and the rate constants obtained in this way were seen to agree well with values extrapolated from the lower temperature DF and FP experiments. The rate data from DF and FP experiments were thus combined to obtain the following Arrhenius expression (298-998 K): k1 (T) = (2.70±0.50)×10-11 exp(-5030±160/RT cm3 molecule-1 s-1. This result is compared with those of previous studies, and kinetic complications (stoichiometry) and heterogeneous effects are further discussed.

Keil, David G.; Tanzawa, Tsuneo; Skolni, Edward G.; Klemm, R. Bruce; Michael, Joe V.

1981-09-01

258

Redox reaction rates in shallow aquifers: Implications for nitrate transport in groundwater and streams  

USGS Publications Warehouse

Groundwater age and water chemistry data along flow paths from recharge areas to streams were used to evaluate the trends and transformations of agricultural chemicals. Results from this analysis indicate that median nitrate recharge concentrations in these agricultural areas have increased markedly over the last 50 years from 4 mg N/L in samples collected prior to 1983 to 7.5 mg N/L in samples collected since 1983. The effect that nitrate accumulation in shallow aquifers will have on drinking water quality and stream ecosystems is dependent on the rate of redox reactions along flow paths and on the age distribution of nitrate discharging to supply wells and streams.

Tesoriero, Anthony J.

2012-01-01

259

Rates of reaction and process design data for the Hydrocarb Process  

SciTech Connect

In support of studies for developing the coprocessing of fossil fuels with biomass by the Hydrocarb Process, experimental and process design data are reported. The experimental work includes the hydropryolysis of biomass and the thermal decomposition of methane in a tubular reactor. The rates of reaction and conversion were obtained at temperature and pressure conditions pertaining to a Hydrocarb Process design. A Process Simulation Computer Model was used to design the process and obtain complete energy and mass balances. Multiple feedstocks including biomass with natural gas and biomass with coal were evaluated. Additional feedstocks including green waste, sewage sludge and digester gas were also evaluated for a pilot plant unit.

Steinberg, M.; Kobayashi, Atsushi [Brookhaven National Lab., Upton, NY (United States); Tung, Yuanki [Hydrocarb Corp., New York, NY (United States)

1992-08-01

260

Study on reaction rates for methanol synthesis from carbon monoxide, carbon dioxide, and hydrogen  

Microsoft Academic Search

The reaction rates for methanol synthesis from carbon monoxide, carbon dioxide, and hydrogen over a copper-zinc-based catalyst were measured under the following conditions: temperature, 210-300°C; pressure, 4-10 MPa, feed gas composition Hâ\\/CO 2.7-720, Hâ\\/COâ, 4.3-120, COâ\\/CO, 0.05-180 (in moles). It was concluded that direct methanol production from COâ takes place under the above conditions in addition to the two well-known

M. Takagawa; M. Ohsugi

1987-01-01

261

Novel technique for constraining r-process (n,$\\gamma$) reaction rates  

E-print Network

A novel technique has been developed, which will open exciting new opportunities for studying the very neutron-rich nuclei involved in the r-process. As a proof-of-principle, the $\\gamma$-spectra from the $\\beta$-decay of $^{76}$Ga have been measured with the SuN detector at the National Superconducting Cyclotron Laboratory. The nuclear level density and $\\gamma$-ray strength function are extracted and used as input to Hauser-Feshbach calculations. The present technique is shown to strongly constrain the $^{75}$Ge($n,\\gamma$)$^{76}$Ge cross section and reaction rate.

Spyrou, A; Larsen, A C; Guttormsen, M; Cooper, K; Dombos, A C; Morrissey, D J; Naqvi, F; Perdikakis, G; Quinn, S J; Renstrøm, T; Rodriguez, J A; Simon, A; Sumithrarachchi, C S; Zegers, R G T

2014-01-01

262

Atmospheric fate of hydrofluoroethanes and hydrofluorochloroethanes: 1. Rate coefficients for reactions with OH  

SciTech Connect

The rate coefficients for the reactions of OH with five halocarbons (CF{sub 3}CH{sub 2} (HFC 134a), CF{sub 3}CHClF (HCFC 124), CF{sub 3}CHCl{sub 2} (HCFC 123), CH{sub 3}CHF{sub 2} (HFC 152a), and CH{sub 3}CF{sub 2}Cl (HCFC 142b)), which are proposed as alternatives to chlorofluoromethanes, have been measured. A pulsed pholtolysis system and a discharge flow apparatus were used to measure the rate coefficients between approximately 210 and 425 K. Use of the complementary techniques enabled identification of systematic errors and minimization of these errors. The obtained values are compared with values previously measured by other groups. This data base is used in the subsequent paper to calculate the atmospheric lifetimes of the five compounds.

Gierczak, T.; Talukdar, R.; Lovejoy, E.R.; Ravishankara, A.R. (National Oceanic and Atmospheric Administration, Boulder, CO (USA) Univ. of Colorado, Boulder (USA)); Vaghjiani, G.L. (Univ. of Dayton Research Inst., Edwards AFB, CA (USA))

1991-03-20

263

Elementary reaction rate measurements at high temperatures by tunable-laser flash-absorption  

SciTech Connect

The major objective of this program is to measure thermal rate coefficients and branching ratios of elementary reactions. To perform these measurements, the authors constructed an ultrahigh-purity shock tube to generate temperatures between 1000 and 5500 K. The tunable-laser flash-absorption technique is used to measure the rate of change of the concentration of species which absorb below 50,000 cm{sup {minus}1} e.g.: OH, CH, and CH{sub 3}. This technique is being extended into the vacuum-ultraviolet spectral region where one can measure atomic species e.g.: H, D, C, O, and N; and diatomic species e.g.: O{sub 2}, CO, and OH.

Hessler, J.P. [Argonne National Laboratory, IL (United States)

1993-12-01

264

RATE CONSTANTS FOR THE REACTION OF OH RADICALS WITH A SERIES OF ALKENES AND DIALKENES AT 295 + OR - 1 K  

EPA Science Inventory

Using a relative rate technique, rate constants for the gas phase reactions of the OH radical with n-butane, n-hexane and a series of alkenes and dialkenes, relative to that for propene, have been determined in one atmosphere of air at 295 + or - 1K. The resulting rate constant d...

265

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. II. Reaction rates and potential energy surface  

NASA Astrophysics Data System (ADS)

Using picosecond excitation in a supersonic jet, we present a full account of our earlier report on the dynamics of state-selective photoisomerization of t-stilbene. Collisionless isomerization in this case indicates the twisting of the molecule about the ethylene bond away from the trans configuration Central to this reaction is the question of vibrational energy redistribution or IVR. From direct (single vibronic level) time-resolved measurements, relative fluorescence quantum yields from relaxed and unrelaxed states, and a thorough vibrational analysis from excitation and dispersed fluorescence spectra (previous paper), the following conclusions are reached: (i) The IVR yield is state selective being more extensive from combination modes than from fundamental modes of similar energy. The IVR yield becomes very significant above ?900-1000 cm-1. The rate is much faster than the reaction at all energies studies. (ii) The barrier to isomerization is observed at 3.3±0.2 kcal/mol (1100-1200 cm-1). The radiative lifetimes, measured from the 0° level fluorescence decays, are 2.7±0.1 ns (h12) and 2.5±0.1 ns (d12). (iii) The observed isomerization rates in the isolated molecule are approximately an order of magnitude less than the calculated RRKM rates and observed solution phase rates. (iv) The apparent non-RRKM behavior in the isolated behavior is explained by considering the nature of IVR and by adopting a diabatic representation of the reactive surface (i.e., an allowed surface) using a Landau-Zener-Stueckelberg model. (v) Finally, we compare t-stilbene with other related isolated molecules and to solution phase t-stilbene results in order to assess the role of mode mixing and the nature of the reactive surface.

Syage, J. A.; Felker, P. M.; Zewail, A. H.

1984-12-01

266

Effect of interfacial reaction rate on the morphogenesis of nanostructured coatings in a simulated electrodeposition process.  

PubMed

Brownian dynamics simulations (BDSs) are performed to investigate the influence of interfacial electrochemical reaction rate on the evolution of coating morphology on circular fibres. The boundary condition for the fluid phase concentration, representing the balance between the rates of interfacial reaction and transport of ions by bulk diffusion, is incorporated into the BDS by using a reaction probability, P(s). Different modes of growth, ranging from diffusion limited ([Formula: see text]) to reaction controlled [Formula: see text], are studied. It is found that, consistent with experimental observations, two distinct morphological regimes exist, with a dense and uniform structure for [Formula: see text] (reaction limited deposition (RLD)) and an open and porous one as [Formula: see text] (diffusion limited deposition (DLD)). An analysis of the fractal dimension indicates that this morphological transition occurs at P(s)?0.3. Long-time power-law scalings for the evolution of thickness [Formula: see text] and roughness (?) of the coating exist, i.e. [Formula: see text] with 0.86???0.91 and 0.56???0.93 for 0.01?P(s)?1. These values are different from those reported for sequential, pseudo-time lattice simulations on planar surfaces, signifying the importance of multiparticle dynamics and surface curvature. The internal structure and porosity of the coating are characterized quantitatively by the radial density profile, pair correlation function, two-point probability function, void distribution function and pore area distribution. For RLD the radial density, ?(n), remains nearly constant, while for DLD ?(n) follows a power law, [Formula: see text]. The coating exhibits short ranged order in the RLD regime while a long range order is created by DLD. The void distribution function becomes broader with increasing P(s), indicating that in the RLD regime the coating consists of small and spherical pores, while in the DLD regime large and elongated pores are obtained. The pore area distribution shows narrower distributions in DLD for small pores, while the area of the largest pore increases by nearly three orders of magnitude as one moves from the RLD to the DLD regime. Such morphological diversity could be potentially exploited for applications such as percolation, catalysis and surface protection. PMID:21727476

Magan, Rahul V; Sureshkumar, Radhakrishna

2005-07-01

267

Rate constant for reaction of vitamin C with protein radicals in ?-irradiated aqueous albumin solution at 295 K  

NASA Astrophysics Data System (ADS)

When an aqueous solution of albumin (0.1 kg dm -3) is irradiated by ?-rays at 295 K, albumin radicals with a long lifetime are observed by ESR. The reaction of vitamin C with the albumin radicals has been studied at 295 K in the albumin solution, which is considered as a model of cells. The rate constant for the reaction of vitamin C with the albumin radicals was measured as 0.014 dm 3 mol -1 s -1, which is much smaller than the reported rate constants (10 6-10 10 dm 3 mol -1 s -1) for the reaction of vitamin C with radicals in a dilute aqueous solution. The small rate constant for the reaction of vitamin C is ascribed to the reaction in polymer coils in the albumin solution, since vitamin C and albumin radicals diffuse very slowly in the coils.

Miyazaki, Tetsuo; Yoshimura, Toru; Mita, Kazuya; Suzuki, Keiji; Watanabe, Masami

1995-02-01

268

Kinetic sonication effects in aqueous acetonitrile solutions. Reaction rate levelling by ultrasound.  

PubMed

The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile-water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (kson/knon) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling. PMID:23643415

Piiskop, Sander; Salmar, Siim; Tuulmets, Ants; Kuznetsov, Aleksei; Järv, Jaak

2013-11-01

269

Reaction Rate Measurement at the Californium User Facility (CUF) for unfolding the neutron spectrum  

NASA Astrophysics Data System (ADS)

Neutron Activation Analysis was used to determine Reaction Rate measurement of several activation detectors at the ORNL Californium User Facility (CUF). The irradiations were performed with 34 mg Cf^252 neutron source strength.. Ten source capsules > 34 mg were positioned concentrically around a sample cavity. We have determined absolute activity per atom of 9 detectors: Au^197 (n,?) Au^198, Al^27(n,?)Na^24, Al^27(n,p) Mg^27, Fe^56(n,p) Mn^5, Fe^54(n,p) Mn^54, In^115 (n,?)In^116, Ti^46(n,p)Sc^46, Ni^60 (n,p) Co^60, Fe^58(n,?) Fe^59. The errors are within 1.5-8% except Ni^60 and Fe^58 have errors of 46% and 32 %. These high errors may be attributed to the counting statistics. These reaction rate values will be used to unfold the neutron spectrum of the CUF using the MAXED 2000, a computer code for the de convolution of multi sphere neutron spectrometer data and the results are discussed.

Hannan, Mohammad; Ortega, Ruben

2011-03-01

270

Reaction rate uncertainties and 26Al in AGB silicon carbide stardust  

E-print Network

Stardust is a class of presolar grains each of which presents an ideally uncontaminated stellar sample. Mainstream silicon carbide (SiC) stardust formed in the extended envelopes of carbon-rich asymptotic giant branch (AGB) stars and incorporated the radioactive nucleus 26Al as a trace element. The aim of this paper is to analyse in detail the effect of nuclear uncertainties, in particular the large uncertainties of up to four orders of magnitude related to the 26Al_g+(p,gamma)27Si reaction rate, on the production of 26Al in AGB stars and compare model predictions to data obtained from laboratory analysis of SiC stardust grains. Stellar uncertainties are also briefly discussed. We use a detailed nucleosynthesis postprocessing code to calculate the 26Al/27Al ratios at the surface of AGB stars of different masses (M = 1.75, 3, and 5 M_sun) and metallicities (Z = 0.02, 0.012, and 0.008). For the lower limit and recommended value of the 26Al_g(p,gamma)27Si reaction rate, the predicted 26Al/27Al ratios replicate t...

van Raai, M A; Karakas, A I; Iliadis, C

2007-01-01

271

The Effects of Changes in Reaction Rates on Simulations of Nova Explosions  

E-print Network

Classical novae participate in the cycle of Galactic chemical evolution in which grains and metal enriched gas in their ejecta, supplementing those of supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for the ISM. Once in the diffuse gas, this material is mixed with the existing gases and then incorporated into young stars and planetary systems during star formation. Infrared observations have confirmed the presence of carbon, SiC, hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that some fraction of the pre-solar grains identified in meteoritic material come from novae. The mean mass returned by a nova outburst to the ISM probably exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar Masses per year of processed matter into the ISM. Novae are expected to be the major source of 15N and 17O in the Galaxy and to contribute to the abundances of other isotopes in this atomic mass range. Here, we report on how changes in the nuclear reaction rates affect the properties of the outburst and alter the predictions of the contributions of novae to Galactic chemical evolution. We also discuss the necessity of including the pep reaction in studies of thermonuclear runaways in material accreted onto white dwarfs.

S. Starrfield; C. Iliadis; W. R. Hix; F. X. Timmes; W. M. Sparks

2007-06-04

272

Thermal behaviors, nonisothermal decomposition reaction kinetics, thermal safety and burning rates of BTATz-CMDB propellant.  

PubMed

The composite modified double base (CMDB) propellants (nos. RB0601 and RB0602) containing 3,6-bis (1H-1,2,3,4-tetrazol-5-yl-amino)-1,2,4,5-tetrazine (BTATz) without and with the ballistic modifier were prepared and their thermal behaviors, nonisothermal decomposition reaction kinetics, thermal safety and burning rates were investigated. The results show that there are three mass-loss stages in TG curve and two exothermic peaks in DSC curve for the BTATz-CMDB propellant. The first two mass-loss stages occur in succession and the temperature ranges are near apart, and the decomposition peaks of the two stages overlap each other, inducing only one visible exothermic peak appear in DSC curve during 350-550 K. The reaction mechanisms of the main exothermal decomposition processes of RB0601 and RB0602 are all classified as chemical reaction, the mechanism functions are f(alpha)=(1-alpha)(2), and the kinetic equations are dalpha/dt = 10(19.24)(1-alpha)(2)e(-2.32x10(4)/T) and dalpha/dt = 10(20.32)(1-alpha)(2)e(-2.32x10(4)/T). The thermal safety evaluation on the BTATz-CMDB propellants was obtained. With the substitution of 26% RDX by BTATz and with the help of the ballistic modifier in the CMDB propellant formulation, the burning rate can be improved by 89.0% at 8 MPa and 47.1% at 22 MPa, the pressure exponent can be reduced to 0.353 at 14-20 MPa. PMID:20542638

Yi, Jian-Hua; Zhao, Feng-Qi; Wang, Bo-Zhou; Liu, Qian; Zhou, Cheng; Hu, Rong-Zu; Ren, Ying-Hui; Xu, Si-Yu; Xu, Kang-Zhen; Ren, Xiao-Ning

2010-09-15

273

Benzoylformate analogues exhibit differential rate-determining steps in the benzoylformate decarboxylase reaction  

SciTech Connect

Benzoylformate decarboxylase from Pseudomonas putida is a thiamine pyrophosphate (TPP)-dependent enzyme which converts benzoylformate to benzaldehyde and CO/sub 2/. The rate-determining step(s) in the benzoylformate decarboxylase reaction for a series of substituted benzoylformates (p-CH/sub 3/O, p-CH/sub 3/, p-Cl, and m-F) were studied using solvent deuterium and /sup 13/C kinetic isotope effects. The normal substrate was found to have two partially rate-determining steps; initial tetrahedral adduct formation (D/sub 2/O-sensitive) and decarboxylation (/sup 13/C-sensitive). D/sub 2/O and /sup 13/C isotope effects indicate that electron-withdrawing substituents (p-Cl and m-F) remove the rate dependence upon decarboxylation such that only a D/sub 2/O effect on (V/K) is observed. Conversely, electron-donating substituents increase the rate-dependence upon decarboxylation such that a larger /sup 13/(V/K) is seen while the D/sub 2/O effects on (V) and (V/K) are not dramatically different from those for benzoylformate. All of the data are consistent with substituent stabilization or destabilization of the carbanionic intermediate formed upon decarboxylation.

Garcia, G.A.; Weiss, P.M.; Cook, P.F.; Kenyon, G.L.; Cleland, W.W.

1987-05-01

274

The unusual effect of reagent vibrational excitation on the rates of endothermic and exothermic elementary combustion reactions  

Microsoft Academic Search

Surprisal analyses are carried out for the reactions H + O2(v) ? OH + O, O + H2(v) ? OH + H, and OH + H2(v) ? H2O + H. The rate constants are taken from published quasiclassical trajectory calculations covering the temperature range 300–4000 K. The dichotomy expected when reagent vibrational energy converts an endoergic reaction into an exothermic

Assa Lifshitz; Heshel Teitelbaum

1997-01-01

275

Rate Constants for Reactions between Iodine-and Chlorine-Containing Species: A Detailed Mechanism of the  

E-print Network

Rate Constants for Reactions between Iodine- and Chlorine-Containing Species: A Detailed Mechanism of the Chlorine Dioxide/Chlorite-Iodide Reaction Istva´n Lengyel, Jing Li, Kenneth Kustin,* and Irving R. Epstein, consisting of four separable parts. Sequentially, beginning with mixing, these parts are the (a) chlorine

Epstein, Irving R.

276

Mixed quantum classical calculation of proton transfer reaction rates: From deep tunneling to over the barrier regimes  

SciTech Connect

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula.

Xie, Weiwei; Xu, Yang; Zhu, Lili; Shi, Qiang, E-mail: qshi@iccas.ac.cn [Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190 (China)] [Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190 (China)

2014-05-07

277

Mixed quantum classical calculation of proton transfer reaction rates: from deep tunneling to over the barrier regimes.  

PubMed

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula. PMID:24811623

Xie, Weiwei; Xu, Yang; Zhu, Lili; Shi, Qiang

2014-05-01

278

Revised Production Rates for Na-22 and Mn-54 in Meteorites Using Cross Sections Measured for Neutron-induced Reactions  

NASA Technical Reports Server (NTRS)

The interactions of galactic cosmic rays (GCR) with extraterrestrial bodies produce small amounts of radionuclides and stable isotopes. The production rates of many relatively short-lived radionuclides, including 2.6-year Na-22 and 312-day Mn-54, have been measured in several meteorites collected very soon after they fell. Theoretical models used to calculate production rates for comparison with the measured values rely on input data containing good cross section measurements for all relevant reactions. Most GCR particles are protons, but secondary neutrons make most cosmogenic nuclides. Calculated production rates using only cross sections for proton-induced reactions do not agree well with measurements. One possible explanation is that the contribution to the production rate from reactions initiated by secondary neutrons produced in primary GCR interactions should be included explicitly. This, however, is difficult to do because so few of the relevant cross sections for neutron-induced reactions have been measured.

Sisterson, J. M.; Kim, K. J.; Reedy, R. C.

2004-01-01

279

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows  

NASA Technical Reports Server (NTRS)

The use of probability theory to determine the effects of turbulent fluctuations on reaction rates in turbulent combustion systems is briefly reviewed. Results are presented for the effect of species fluctuations in particular. It is found that turbulent fluctuations of species act to reduce the reaction rates, in contrast with the temperature fluctuations previously determined to increase Arrhenius reaction rate constants. For the temperature fluctuations, a criterion is set forth for determining if, in a given region of a turbulent flow field, the temperature can be expected to exhibit ramp like fluctuations. Using the above results, along with results previously obtained, a model is described for testing the effects of turbulent fluctuations of temperature and species on reaction rates in computer programs dealing with turbulent reacting flows. An alternative model which employs three variable probability density functions (temperature and two species) and is currently being formulated is discussed as well.

Chinitz, W.; Foy, E.; Rowan, G.; Goldstein, D.

1982-01-01

280

Ground Reaction Forces and Loading Rates Associated with Parkour and Traditional Drop Landing Techniques  

PubMed Central

Due to the relative infancy of Parkour there is currently a lack of empirical evidence on which to base specific technique instruction upon. The purpose of this study was to compare the ground reaction forces and loading rates involved in two Parkour landing techniques encouraged by local Parkour instructors and a traditional landing technique recommended in the literature. Ten male participants performed three different drop landing techniques (Parkour precision, Parkour roll, and traditional) onto a force plate. Compared to the traditional technique the Parkour precision technique demonstrated significantly less maximal vertical landing force (38%, p < 0.01, ES = 1.76) and landing loading rate (54%, p < 0.01, ES = 1.22). Similarly, less maximal vertical landing force (43%, p < 0.01, ES = 2.04) and landing loading rate (63%, p < 0.01, ES = 1.54) were observed in the Parkour roll technique compared to the traditional technique. It is unclear whether or not the Parkour precision technique produced lower landing forces and loading rates than the Parkour roll technique as no significant differences were found. The landing techniques encouraged by local Parkour instructors such as the precision and roll appear to be more appropriate for Parkour practitioners to perform than a traditional landing technique due to the lower landing forces and loading rates experienced. Key points Parkour precision and Parkour roll landings were found to be safer than a traditional landing technique, resulting in lower maximal vertical forces, slower times to maximal vertical force and ultimately lesser loading rates. Parkour roll may be more appropriate (safer) to utilize than the Parkour precision during Parkour landing scenarios. The Parkour landing techniques investigated n this study may be beneficial for landing by non-Parkour practitioners in everyday life. PMID:24149735

Puddle, Damien L.; Maulder, Peter S.

2013-01-01

281

Rate and reaction probability of the surface reaction between ozone and dihydromyrcenol measured in a bench scale reactor and a room-sized chamber  

NASA Astrophysics Data System (ADS)

Low volatility terpenoids emitted from consumer products can react with ozone on surfaces and may significantly alter concentrations of ozone, terpenoids and reaction products in indoor air. We measured the reaction probability and a second-order surface-specific reaction rate for the ozonation of dihydromyrcenol, a representative indoor terpenoid, adsorbed onto polyvinylchloride (PVC), glass, and latex paint coated spheres. The reaction probability ranged from (0.06-8.97) × 10 -5 and was very sensitive to humidity, substrate and mass adsorbed. The average surface reaction probability is about 10 times greater than that for the gas-phase reaction. The second-order surface-specific rate coefficient ranged from (0.32-7.05) × 10 -15 cm 4 s -1 molecule -1and was much less sensitive to humidity, substrate, or mass adsorbed. We also measured the ozone deposition velocity due to adsorbed dihydromyrcenol on painted drywall in a room-sized chamber, Based on that, we calculated the rate coefficient ((0.42-1.6) × 10 -15 cm 4 molecule -1 s -1), which was consistent with that derived from bench-scale experiments for the latex paint under similar conditions. We predict that more than 95% of dihydromyrcenol oxidation takes place on indoor surfaces, rather than in building air.

Shu, Shi; Morrison, Glenn C.

2012-02-01

282

Bimolecular reaction rates from ring polymer molecular dynamics: application to H + CH4 ? H2 + CH3.  

PubMed

In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH(4) ? H(2) + CH(3). The present calculations were carried out using a modified and recalibrated version of the Jordan-Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom-diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. PMID:21280711

Suleimanov, Yury V; Collepardo-Guevara, Rosana; Manolopoulos, David E

2011-01-28

283

Atm. Chem. Phys. 2011 "Rate coefficients for the gas-phase reaction of OH with (Z)-3-hexen-1-ol,  

E-print Network

Atm. Chem. Phys. 2011 "Rate coefficients for the gas-phase reaction of OH with (Z)-3-hexen-1-ol, 1-penten-3-ol, (E)-2-penten-1-ol, and (E)-2-hexen-1-ol between 243 and 404 K" M.E. Davis and J ) 1-penten-3-ol #12;Atm. Chem. Phys. 2011 "Rate coefficients for the gas-phase reaction of OH with (Z

Meskhidze, Nicholas

284

A comparative study of reaction rate, species, and vibration-dissociation coupling models for an AOTV flowfield  

E-print Network

A COMPARATIVE STUDY OF REACTION RATE, SPECIES, AND VIBRATION DISSOCIATION COUPLING MODELS FOR AN AOTV FLOWFIELD A Thesis by GLENN JAMES BOBSKILL Submitted to the Graduate College of Texas A&M UniversitY in partial fulfillment... of the requirements for the degree of MASTER GF SC. BNCH August 1938 Major Subject: Aerospace Engineering A COMPARATIVE STUDY OF REACTION RATE, SPECIES, AND VIBRATION -DISSOCIATION COUPLING MODELS FOR AN AOTV FLOWFIELD A Thesis by GLENN JAMES BOBSKILL...

Bobskill, Glenn James

2012-06-07

285

An Analogy Using Pennies and Dimes to Explain Chemical Kinetics Concepts  

ERIC Educational Resources Information Center

In this article, the authors present an analogy that uses coins and graphical analysis to teach kinetics concepts and resolve pseudo-first-order rate constants related to transition-metal complexes ligand-solvent exchange reactions. They describe an activity that is directed to upper-division undergraduate and graduate students. The activity…

Cortes-Figueroa, Jose E.; Perez, Wanda I.; Lopez, Jose R.; Moore-Russo, Deborah A.

2011-01-01

286

Ion-track reaction-rate model for hydrogen production from polymers  

NASA Astrophysics Data System (ADS)

A theoretical model has been constructed to calculate the early time evolution of hydrogen radicals that form molecular hydrogen in a single track during the ion irradiation of polymers. The calculated G(H 2)-values (number of H 2 molecules formed per 100 eV of absorbed energy) were compared to those recently measured during various ion irradiations of polyethylene (PE), polystyrene (PS), and polycarbonate (PC). In this model the LET (Linear Energy Transfer) of the energetic ion was distributed into two regions: (a) a Gaussian track-core of high energy-density which is caused primarily by atomic excitation, and (b) a long range, low energy-density distribution caused by electron scattering. Following the energy absorption, the interaction of a time dependent distribution of free hydrogen radicals, H ·, originating within the initial track begins. This process is described by a set of chemical rate equations. The effect of radical diffusion out of the original track is also included. Using H · diffusion rates based upon an extrapolation of known values for H 2 and He in polymers, the hydrogen ion reaction rates were estimated. The effective C-H bond (or radical pair) energy was determined to be approximately 10 eV, but varied somewhat with polymer and ion species. The model calculates the residual carbon density profile along the ion track. The implications of this profile for other properties of irradiated polymers are also discussed.

Lewis, M. B.; Coghlan, W. A.

1996-04-01

287

Rate and mechanism of the reactions of OH and Cl with 2-methyl-3-buten-2-ol  

Microsoft Academic Search

An environmental chamber\\/Fourier transform infrared system was used to determine the rate coefficient k1 for the gas-phase reaction of OH with 2-methyl-3-buten-2-ol (MBO, (CH3)2C(OH)CH=CH2), relative to the rate of its reaction with ethylene (k2) and propylene (k3). Experiments performed at 295+\\/-1K, in 700 torr total pressure of air, gave k1=(6.9+\\/-1.0)×10-11cm3molecule-1s-1. At 295+\\/-1K, the reaction of OH with MBO yielded, on

C. Ferronato; J. J. Orlando; G. S. Tyndall

1998-01-01

288

The rate of the reaction between C2H and C2H2 at interstellar temperatures  

NASA Technical Reports Server (NTRS)

The reaction between the radical C2H and the stable hydrocarbon C2H2 is one of the simplest neutral-neutral hydrocarbon reactions in chemical models of dense interstellar clouds and carbon-rich circumstellar shells. Although known to be rapid at temperatures > or = 300 K, the reaction has yet to be studied at lower temperatures. We present here ab initio calculations of the potential surface for this reaction and dynamical calculations to determine its rate at low temperature. Despite a small potential barrier in the exit channel, the calculated rate is large, showing that this reaction and, most probably, more complex analogs contribute to the formation of complex organic molecules in low-temperature sources.

Herbst, E.; Woon, D. E.

1997-01-01

289

Early diagenetic processes, fluxes, and reaction rates in sediments of the South Atlantic  

SciTech Connect

Porewaters recovered from sediment cores (gravity corers, box corers, and multicorers) from various subregions of the South Atlantic (Amazon River mouth, equatorial upwelling, Congo River mouth, Benguela coastal upwelling area, and Angola Basin) were investigated geochemically. Objectives included determination of Eh, pH, oxygen, nitrate, sulfate, alkalinity, phosphate, ammonium, fluoride, sulfide, Ca, Mg, Sr, Fe, Mn, and Si, in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes. Porewater profiles from the eastern upwelling areas of the South Atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation, whereas organic matter degradation by sulfate reduction is restricted to the near-surface sediments. Further, a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m, far below the zone of bioturbation or bioirrigation. Because other sedimentological reasons seem to fail as explanations, an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed. Porewater profiles from the sediments of the Amazon fan area are mainly influenced by reactions with Fe(III)-phases. The remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments. In contrast to the upwelling sediments, the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5.3 m, probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals. A comparison of organic matter degradation rates from the different areas of the South Atlantic show the expected relationship to primary productivity.

Schulz, H.D.; Dahmke, A.; Schinzel, U.; Wallmann, K.; Zabel, M. (Univ. of Bremen (Germany))

1994-05-01

290

Path-integral virial estimator for reaction-rate calculation based on the quantum instanton approximation.  

PubMed

The quantum instanton approximation is a type of quantum transition-state theory that calculates the chemical reaction rate using the reactive flux correlation function and its low-order derivatives at time zero. Here we present several path-integral estimators for the latter quantities, which characterize the initial decay profile of the flux correlation function. As with the internal energy or heat-capacity calculation, different estimators yield different variances (and therefore different convergence properties) in a Monte Carlo calculation. Here we obtain a virial (-type) estimator by using a coordinate scaling procedure rather than integration by parts, which allows more computational benefits. We also consider two different methods for treating the flux operator, i.e., local-path and global-path approaches, in which the latter achieves a smaller variance at the cost of using second-order potential derivatives. Numerical tests are performed for a one-dimensional Eckart barrier and a model proton transfer reaction in a polar solvent, which illustrates the reduced variance of the virial estimator over the corresponding thermodynamic estimator. PMID:16512703

Yang, Sandy; Yamamoto, Takeshi; Miller, William H

2006-02-28

291

Analytical solution of steady-state equations for chemical reaction networks with bilinear rate laws.  

PubMed

True steady states are a rare occurrence in living organisms, yet their knowledge is essential for quasi-steady-state approximations, multistability analysis, and other important tools in the investigation of chemical reaction networks (CRN) used to describe molecular processes on the cellular level. Here, we present an approach that can provide closed form steady-state solutions to complex systems, resulting from CRN with binary reactions and mass-action rate laws. We map the nonlinear algebraic problem of finding steady states onto a linear problem in a higher-dimensional space. We show that the linearized version of the steady-state equations obeys the linear conservation laws of the original CRN. We identify two classes of problems for which complete, minimally parameterized solutions may be obtained using only the machinery of linear systems and a judicious choice of the variables used as free parameters. We exemplify our method, providing explicit formulae, on CRN describing signal initiation of two important types of RTK receptor-ligand systems, VEGF and EGF-ErbB1. PMID:24334389

Halász, Adám M; Lai, Hong-Jian; McCabe Pryor, Meghan; Radhakrishnan, Krishnan; Edwards, Jeremy S

2013-01-01

292

Influence of salts, including amino acids, on the rate and outcome of the in-water prebiotic reactions  

NASA Astrophysics Data System (ADS)

Most prebiotic reactions are hypothesized to have occurred in water. However, organic compounds in general are not water-soluble. This has created a great problem for prebiotic chemistry. However, it has been shown recently that many organic materials which are not soluble in water are still capable of reacting in water, often at faster rates than in the organic solvents. This has provided a new era in the study of the prebiotic reactions. Simulated prebiotic reactions are typically performed in water. However, presence of inorganic salts and the amino acids was ubiquitous in the aqueous media on the prebiotic Earth. We thus address the influence of common salts and selected water-soluble amino acids on the rate and outcome of selected prebiotic reactions in water. We focus on Diels-Alder (DA) reaction and multicomponent Passerini reaction (PR), as models for other prebiotic reactions. Some of the results came from our laboratory, and others are from the literature. DA reaction is influenced by salts and by selected water-soluble amino acids, but generally not to a large extent. The PR is also influenced by salts, but not dramatically. However, concentrations of salts and amino acids could have been extremely high in various local niches on the early Earth, and the influence of such concentrations on these prebiotic reactions awaits further study.

Kolb, Vera M.

2013-09-01

293

Temperature dependence of the NO + O3 reaction rate from 195 to 369 K  

NASA Technical Reports Server (NTRS)

The temperature dependence of the NO + O3 reaction rate was examined by means of the fast flow technique. Several different experimental conditions and detection schemes were employed. With excess NO or excess O3, NO2 chemiluminescence was monitored. In addition, with excess O3, NO was followed by fluorescence induced by an NO microwave discharge lamp. The results of the three independent sets of data are compared and found to agree within experimental error, indicating the absence of secondary chemistry which might complicate the kinetics. The data exhibit curvature on an Arrhenius plot; however, the simple Arrhenius expression k = (2.6 + or - 0.8) x 10 to the -12th exp(-1435 + or - 64/T) cu cm/molecule s is an adequate description for T between 195 and 369 K. This result is compared to earlier determinations.

Michael, J. V.; Allen, J. E., Jr.; Brobst, W. D.

1981-01-01

294

Pressure dependence on the reaction propagation rate of PETN at high pressure  

SciTech Connect

The reaction propagation rate (RPR) of the sensitive high explosive pentaerythritol tetranitrate (PETN) was measured in a diamond anvil cell (DAC) over the pressure range of 2--20 GPa. The experimental technique used is the same as that previously reported. The RPR data shows that it burns one to two orders of magnitude faster in the DAC than 1,3,5,-triamino-2,4,6-trinitrobenzene (TATB) and nitromethane (CH{sub 3}NO{sub 2}) respectively. The PETN RPR curve did not show sample pressure-dependent behavior like that of nitromethane, but instead varied abruptly like the RPR curve of TATB. In order to interpret these changes, static-pressure DAC mid-IR FTIR spectra were taken of micro-pellets of PETN embedded in KBr. The relationship between changes in the spectra, the RPR curve, and published single crystal PETN wedge test data are discussed.

Foltz, M.F.

1993-04-01

295

Absolute Determination of the 22Na(p,g) Reaction Rate in Novae  

E-print Network

Gamma-ray telescopes in orbit around the Earth are searching for evidence of the elusive radionuclide 22Na produced in novae. Previously published uncertainties in the dominant destructive reaction, 22Na(p,g)23Mg, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beamline, which included beam rastering and cold vacuum protection of the 22Na implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~ 20 C bombardment as a result of protective layers. We avoided the need to know the stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for E_p = 213, 288, 454, and 610 keV are stronger by factors of 2.4 to 3.2 than previously reported. Upper limits have been placed on proposed resonances at 198-, 209-, and 232-keV. We have re-evaluated the 22Na(p,g) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to 22Na destruction in novae. Hydrodynamic simulations including our rate indicate that the expected abundance of 22Na ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.

A. L. Sallaska; C. Wrede; A. Garcia; D. W. Storm; T. A. D. Brown; C. Ruiz; K. A. Snover; D. F. Ottewell; L. Buchmann; C. Vockenhuber; D. A. Hutcheon; J. A. Caggiano; J. Jose

2011-02-04

296

Darzens reaction rate enhancement using aqueous media leading to a high level of kinetically controlled diastereoselective synthesis of steroidal epoxyketones.  

PubMed

Darzens reactions between halocarbonyls and aldehydes have been carried out in water in the presence of a Li(+)-containing base, a phase-transfer catalyst, and granular polytetrafluoroethylene under mechanical stirring. Reactions using both aromatic and aliphatic aldehydes produced epoxides stereoselectively in good to excellent yields. This is the first time that aliphatic aldehydes with ?-H have been used in aqueous Darzens reactions. The Darzens reactions were much faster in water than in organic solvents. This aqueous rate enhancement occurred for Darzens reactions between enantiopure steroidal haloketones and aldehydes, yielding enantiopure spiroepoxides with a high level of kinetically controlled diastereoselectivity. Chromatography was avoided in the purifications of the steroidal spiroepoxides. This is an example of preparing enantiopure epoxyketones via aqueous Darzens reaction using chiral ?-haloketone substrates. PMID:25119148

Li, Bo; Li, Chunbao

2014-09-01

297

Reaction rates, depositional history and sources of indium in sediments from Appalachian and Canadian Shield lakes  

NASA Astrophysics Data System (ADS)

Sediment cores were collected at the deepest site of twelve headwater lakes from the Province of Québec, Canada that receive contaminants only from atmospheric deposition, either directly to the lake surface or indirectly from the watershed. Several of the lakes are located within relatively short distance (<40 km) and others at more than 200 km from potential sources of contamination. The sediments were dated and analyzed for In and other elements including Fe, Mn, Al and organic C. Fe-rich authigenic material was collected on Teflon sheets inserted vertically into the sediments at the only study site whose hypolimnion remains perennially oxic. Porewater samples collected at the coring site of four of the lakes were also analyzed for In and other solutes including sulfide, sulfate, Fe, Mn, inorganic and organic C and major ions. The porewater In profiles display concentration gradients at or below the sediment-water interface. Modeling these profiles with a one-dimensional transport-reaction equation assuming steady state allows definition of depth intervals (zones) where In is either released to or removed from porewater and quantification of net In reactions rates in each zone. The position of the In consumption zones, the shape of the vertical profiles of dissolved In, sulfide and iron, as well as thermodynamic calculations of saturation states collectively suggest that In(OH)3(s) and In2S3(s) do not precipitate in the sediments and that adsorption of In onto sedimentary FeS(s) does not occur. However, similarities in the In and Fe porewater profiles, and the presence of In in the authigenic Fe-rich solids, reveal that part of the In becomes associated with authigenic Fe oxyhydroxides in the perennially oxic lake and is coupled to the Fe redox cycling. Comparison of the In/Corg and In/Fe molar ratios in the authigenic Fe-rich material and in surface sediments (0-0.5 cm) of this lake suggests that most non-lithogenic In was bound to humic substances. From the magnitude of the net In reaction rates, we infer that the post-depositional redistribution of this element is quantitatively not important and that the In sedimentary record represents accurately In deposition at the sampling sites. Reconstructed chronologies of the anthropogenic In deposition and comparison of In inventories among lakes point to non-ferrous metal smelters as a past significant source of atmospheric In contamination and to a significant reduction of industrial In emissions into the North American atmosphere in recent decades.

Tessier, André; Gobeil, Charles; Laforte, Lucie

2014-07-01

298

The plutonium\\/hydrogen reaction: The pressure dependence of reaction initiation time and nucleation rate controlled by a plutonium dioxide over-layer  

Microsoft Academic Search

As part of an ongoing programme to quantify those parameters which influence the early stages of the plutonium hydriding reaction, the hydrogen pressure dependence of both plutonium hydriding initiation time (It) and hydriding nucleation rate (Nr) have been determined for plutonium covered in a reproducible dioxide over-layer. The data show that initiation time is inversely proportional to hydrogen pressure, while

Gordon W. McGillivray; John P. Knowles; Ian M. Findlay; Marina J. Dawes

2011-01-01

299

Chemical mechanisms and reaction rates for the initiation of hot corrosion of IN-738  

NASA Technical Reports Server (NTRS)

Sodium-sulfate-induced hot corrosion of preoxidized IN-738 was studied at 975 C with special emphasis placed on the processes occurring during the long induction period. Thermogravimetric tests were run for predetermined periods of time, and then one set of specimens was washed with water. Chemical analysis of the wash solutions yielded information about water soluble metal salts and residual sulfate. A second set of samples was cross sectioned dry and polished in a nonaqueous medium. Element distributions within the oxide scale were obtained from electron microprobe X-ray micrographs. Evolution of SO was monitored throughout the thermogravimetric tests. Kinetic rate studies were performed for several pertinent processes; appropriate rate constants were obtained from the following chemical reactions: Cr2O3 + 2 Na2SO4(1) + 3/2 O2 yields 2 Na2CrO4(1) + 2 SO3(g)n TiO2 + Na2SO4(1) yields Na2O(TiO2)n + SO3(g)n TiO2 + Na2CrO4(1) yields Na2O(TiO2)n + CrO3(g).

Fryburg, G. C.; Kohl, F. J.; Stearns, C. A.

1984-01-01

300

A model to predict the thermal reaction norm for the embryo growth rate from field data.  

PubMed

The incubation of eggs is strongly influenced by temperature as observed in all species studied to date. For example, incubation duration, sexual phenotype, growth, and performances in many vertebrate hatchlings are affected by incubation temperature. Yet it is very difficult to predict temperature effect based on the temperature within a field nest, as temperature varies throughout incubation. Previous works used egg incubation at constant temperatures in the laboratory to evaluate the dependency of growtProd. Type: FTPh rate on temperature. However, generating such data is time consuming and not always feasible due to logistical and legislative constraints. This paper therefore presents a methodology to extract the thermal reaction norm for the embryo growth rate directly from a time series of incubation temperatures recorded within natural nests. This methodology was successfully applied to the nests of the marine turtle Caretta caretta incubated on Dalyan Beach in Turkey, although it can also be used for any egg-laying species, with some of its limitations being discussed in the paper. Knowledge about embryo growth patterns is also important when determining the thermosensitive period for species with temperature-dependent sex determination. Indeed, in this case, sexual phenotype is sensitive to temperature only during this window of embryonic development. PMID:25436957

Girondot, Marc; Kaska, Yakup

2014-10-01

301

Direct Measurement of the Effective Rate Constant for Primary Charge Separation in Isolated Photosystem II Reaction Centers  

SciTech Connect

Transient absorption measurements of the pheophytin a anion band and Qx band bleach region using preferential excitation of P680 are performed on isolated photosystem II reaction centers to determine the effective rate constant for charge separtion. A novel analysis of the Qx band bleach region explicity takes the changing background into account in order to directly measure the rate of growth of the bleach. Both spectral regions reveal biphasic kinetics, with a ca. (8 ps)-1 rate constant for the faster component, and a ca. (50 ps)-1 rate constant for the slower component. We propose that the fster component corresponds to the effective rate constant for charge separation from within the equilibrated reaction center core and provides a lower limit for the intrinsic rate constant for charge separation. The slower component corresponds to charge separation that is limited by slow energy transfer from a long-wavelength accessory chlorophyll a.

Greenfield, S. R.; Seibert, M.; Govindjee; Wasielewski, M. R.

1997-03-27

302

Combination of 31P-NMR magnetization transfer and radioisotope exchange methods for assessment of an enzyme reaction mechanism: rate-determining steps of the creatine kinase reaction.  

PubMed

The theoretical analysis of a reversible enzyme reaction performed in this work shows that the 31P-NMR magnetization (saturation) transfer technique combined with a radioisotope exchange method may potentially provide information on the position of rate-determining step(s). It depends on chemical shifts of NMR signals of nuclei of interest in free and enzyme-bound forms of substrate(s) and product(s) of the reaction. The creatine kinase reaction (MgATP + creatine----MgADP + P-creatine) has been used as a model. Chemical shifts of 31P in binary, ternary and transitional state substrate-enzyme complexes have been estimated by the variable frequency saturation transfer (VFST) method. This method is based on selective irradiation of numerous points in the spectrum and observation of changes in the intensity of visible line(s) which occur due to chemical exchange between it and lines which are not visible in the routine spectrum. Also, dissociation rate constants of MgADP-containing complexes were determined. Magnetization exchange rates, P-creatine----[gamma-P]MgATP and [beta-P]MgADP----[beta-P]MgATP, were compared with radioisotope exchange rates, [gamma-32P-MgATP----P-creatine and [3H]MgADP----MgATP at different [P-creatine]/[creatine] ratios and at different temperatures. All these exchange rates were close to each other at 30-37 degrees C and [PCr]/[Cr] ratios lower than 2. It is concluded that phosphoryl group transfer is the rate-determining step of the overall creatine kinase reaction under these conditions. However, at lower temperatures (below 25 degrees C) or at high [PCr]/[Cr] ratios ([ADP] less than 20 microM) the rate-determining step seems to be shifted toward dissociation of nucleotide substrates from enzyme-substrate complexes, since exchange rates became significantly different. This approach is useful for analysis of mechanism of enzymatic reactions and also can be applied to non-enzymatic reactions and evaluation of small rapidly exchangeable metabolite pools. PMID:2248962

Kupriyanov, V V; Balaban, R S; Lyulina, N V; Steinschneider AYa; Saks, V A

1990-12-01

303

The Reaction Mechanism of Decomposing Chloroform by Bi-Metal Nano-Metallic Particles of Fe/Ni  

SciTech Connect

By adding Ni into the production of Fe/Ni nano-metallic particles, the acceleration of reduction ability of particles to decompose Chloroform is observed. The addition also could inhibit the shielding effect of pure iron compounds. This research studied the production and properties of the nano-particle metallic compounds of Fe and Ni, the decomposition of Chloroform by the particles and the mechanism of the decomposition processes. The experimental results indicated effective and rapid decomposition of chloroform by the Fe/Ni nano-particles on aluminum oxides, comparing to nano particles of iron in other researches. The reaction mechanism of Fe/Ni particles was pseudo first order with the half life about 0.7 hour, which was much shorter than the nano-Fe particles.

Hsieh, Su-Hwei; Horng, Jao-Jia

2004-03-31

304

Chemical reaction rates and non-equilibrium pressure of reacting gas mixtures in the state-to-state approach  

NASA Astrophysics Data System (ADS)

Viscous gas flows with vibrational relaxation and chemical reactions in the state-to-state approach are analyzed. A modified Chapman-Enskog method is used for the determination of chemical reaction and vibrational transition rates and non-equilibrium pressure. Constitutive equations depend on the thermodynamic forces: velocity divergence and chemical reaction/transition affinity. As an application, N2 flow with vibrational relaxation across a shock wave is investigated. Two distinct processes occur behind the shock: for small values of the distance the affinity is large and vibrational relaxation is in its initial stage; for large distances the affinity is small and the chemical reaction is in its final stage. The affinity contributes more to the transition rate than the velocity divergence and the effect of these two contributions are more important for small distances from the shock front. For the non-equilibrium pressure, the term associated with the bulk viscosity increases by a small amount the hydrostatic pressure.

Kustova, Elena V.; Kremer, Gilberto M.

2014-12-01

305

Fast and quantitative differentiation of single-base mismatched DNA by initial reaction rate of catalytic hairpin assembly.  

PubMed

The widely used catalytic hairpin assembly (CHA) amplification strategy generally needs several hours to accomplish one measurement based on the prevailingly used maximum intensity detection mode, making it less practical for assays where high throughput or speed is desired. To make the best use of the kinetic specificity of toehold domain for circuit reaction initiation, we developed a mathematical model and proposed an initial reaction rate detection mode to quantitatively differentiate the single-base mismatch. Using the kinetic mode, assay time can be reduced substantially to 10 min for one measurement with the comparable sensitivity and single-base mismatch differentiating ability as were obtained by the maximum intensity detection mode. This initial reaction rate based approach not only provided a fast and quantitative differentiation of single-base mismatch, but also helped in-depth understanding of the CHA system, which will be beneficial to the design of highly sensitive and specific toehold-mediated hybridization reactions. PMID:24768863

Li, Chenxi; Li, Yixin; Xu, Xiao; Wang, Xinyi; Chen, Yang; Yang, Xiaoda; Liu, Feng; Li, Na

2014-10-15

306

Experimental measurements of low temperature rate coefficients for neutral-neutral reactions of interest for atmospheric chemistry of Titan, Pluto and Triton: reactions of the CN radical.  

PubMed

The kinetics of the reactions of cyano radical, CN (X2sigma+) with three hydrocarbons, propane (CH3CH2CH3), propene (CH3CH=CH2) and 1-butyne (CH[triple band]CCH2CH3) have been studied over the temperature range of 23-298 K using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus combined with the pulsed laser photolysis-laser induced fluorescence technique. These reactions are of interest for the cold atmospheres of Titan, Pluto and Triton, as they might participate in the formation of nitrogen and carbon bearing molecules, including nitriles, that are thought to play an important role in the formation of hazes and biological molecules. All three reactions are rapid with rate coefficients in excess of 10(-10) cm3 molecule(-1) s(-1) at the lowest temperatures of this study and show behaviour characteristic of barrierless reactions. Temperature dependences, different for each reaction, are compared to those used in the most recent photochemical models of Titan's atmosphere. PMID:21302546

Morales, Sébastien B; Le Picard, Sébastien D; Canosa, André; Sims, Ian R

2010-01-01

307

CF3CF=CH2 and (Z)-CF3CF=CHF: temperature dependent OH rate coefficients and global warming potentials.  

PubMed

Rate coefficients over the temperature range 206-380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF(3)CF=CH(2)), k(1)(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF(3)CF=CHF), k(2)(T), which are major components in proposed substitutes for HFC-134a (CF(3)CFH(2)) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N(2)). For CF(3)CF=CH(2), the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k(1)(T)=(1.26+/-0.11) x 10(-12) exp[(-35+/-10)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K)=(1.12+/-0.09) x 10(-12) cm(3) molecule(-1) s(-1). For (Z)-CF(3)CF=CHF, the rate coefficients are given by the non-Arrhenius expression k(2)(T)=(1.6+/-0.2) x 10(-18)T(2) exp[(655+/-50)/T] cm(3) molecule(-1) s(-1) where k(2)(296 K)=(1.29+/-0.06) x 10(-12) cm(3) molecule(-1) s(-1). Over the temperature range most relevant to the atmosphere, 200-300 K, the Arrhenius expression k(2)(T)=(7.30+/-0.7) x 10(-13) exp[(165+/-20)/T] cm(3) molecule(-1) s(-1) reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2sigma (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF(3)CF=CH(2) and (Z)-CF(3)CF=CHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss. PMID:18231683

Papadimitriou, Vassileios C; Talukdar, Ranajit K; Portmann, R W; Ravishankara, A R; Burkholder, James B

2008-02-14

308

Rate constant for the reaction of the H atom with H 2O 2 in aqueous solution  

NASA Astrophysics Data System (ADS)

The rate constant for the reaction H rad +H 2O 2? rad OH+H 2O was newly determined to be 3.5×10 7 dm 3 mol -1 s -1 by observing the Cl 2rad - absorbance in pulse radiolysis experiments involving very pure acidic aqueous solutions of chloride ions and hydrogen peroxide. A reaction mechanism for the generation of Cl 2rad - is described.

Ershov, B. G.; Gordeev, A. V.; Kelm, M.; Janata, E.

2003-07-01

309

Determination of rate constants for the reaction between 2-(2- amino-ethoxy) ethanol and carbonyl sulfide  

E-print Network

of GC Switching Valve Final Position of GC Switching Valve GC Results of Liquid Product using Tenax GC and Chromosorb-107 Columns with Temperature Programming GC Results of Standard Calibration Gas using Tenax-GC and Chzomosozb-107 Columns.... 4 atm). Reactor The reactor was designed to simulate a, single tray in a gas absorption column. A detailed schematic of the reaction assembly is shown in Figure 3. The reaction vessel consisted of a one gallon Autoclave rated at 45. 6 MPa (450...

Singh, Mander

2012-06-07

310

Reaction rates of 21Ne(?, n)24Mg, 17O(?, n)20Ne and 18O(?, n)21Ne.  

NASA Astrophysics Data System (ADS)

The nucleosynthesis of elements heavier than iron occurs mainly by neutron capture in the s- and r-process. In the course of an investigation program on all relevant (?, n)-reactions, the three possible neutron sources 21Ne(?, n)24Mg, 17O(?, n)20 Ne and 18O(?, n)21Ne have been investigated experimentally within the present limits of sensitivity (10-10barn). Cross sections, resonance parameters, S-factors and reaction rates have been deduced from these measurements.

Denker, A.; Drotleff, H. W.; Große, M.; Hammer, J. W.; Knee, H.; Kunz, R.; Mayer, A.; Seidel, R.; Wolf, G.

1993-07-01

311

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins  

Microsoft Academic Search

The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant\\u000a portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation\\u000a rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic\\u000a pyrite is typically framboidal attesting to its recrystallization from precursor

Reinhard Aesse

1987-01-01

312

Increase of Reaction Rate and Sensitivity of Low-Abundance Enzyme Assay using Micro/Nanofluidic Preconcentration Chip  

PubMed Central

We report a novel method of increasing both the reaction rate and the sensitivity of low-abundance enzyme assay using a micro/nanofluidic preconcentration chip. The disposable preconcentration device made out of PDMS with a surface-patterned ion-selective membrane increases local enzyme/substrate concentrations for rapid monitoring of enzyme activity. As a model system, we used trypsin as the enzyme and BODIPY FL casein as the fluorogenic substrate. We demonstrated that the reaction rate of trypsin-BODIPY FL was significantly enhanced by increasing the local concentrations of both trypsin and BODIPY FL casein in the preconcentration chip. The reaction time required to turn over substrates at 1 ng/mL was only ~10 min compared to ~1 h without preconcentration, which demonstrates significantly higher reaction rate through the increase of the concentrations of both the enzyme and substrate. Furthermore, trypsin activity can be measured down to a concentration level of 10 pg/mL, which is ~100 fold enhancement in sensitivity compared to the result without the preconcentration step. This micro/nanofluidic preconcentrator chip could be used as a generic micro reaction platform to study any enzyme-substrate systems, or other biochemical reaction systems in low concentration ranges. PMID:18358012

Lee, Jeong Hoon; Song, Yong-Ak; Tannenbaum, Steven R.; Han, Jongyoon

2009-01-01

313

A significant upper limit for the rate of formation of OCS from the reaction of OH with CS2  

NASA Astrophysics Data System (ADS)

The reaction rate of OH with CS2 to form OCS is examined by measuring the formation of O(C-14)S from mixtures of H2O2 with (C-14)S2 in a photolytic system. The upper limit for the formation of OCS based on the oxidation of CO to CO2 corresponds to a rate constant, where k1 is less than 0.3 x 10 to the -14th cu cm/ molecule-sec. Other chemical reactions in the system are found to form both (C-14)O and (C-14)O2, indicating the existence of a combination of reactions so that all the O(C-14)S observed is not necessarily formed from the reaction of OH with CS2. The experimental reaction rate is significantly slower than those reported by Kurylo (1978) and by Cox and Sheppard (1980), showing that neither is an important atmospheric sink for CS2 nor an important source for atmospheric OCS. It is also suggested that the reaction of OH with OCS is probably not an important atmospheric sink for OCS nor an important source of SO2.

Iyer, R. S.; Rowland, F. S.

1980-10-01

314

Photo- and thermal-oxidation studies on methyl and phenyl linoleate: anti-oxidant behaviour and rates of reaction.  

PubMed

Photo-peroxidation of methyl and phenyl linoleate in methanol solutions at 25 degrees C, in the presence of methylene blue or 5,10,15,20-tetra(4-pyridyl)-porphyrin (TPP) as sensitisers of singlet oxygen, was found to proceed at more than 30 times the rate of the same polyunsaturated fatty acid (PUFA) ester species undergoing thermal-peroxidation in the bulk phase at 50 degrees C. The addition of anti-oxidants such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) quench the thermal-oxidation effectively but appear to only partially inhibit the photosensitized peroxidation reactions. The kinetics of the overall peroxidation reactions were followed by ultraviolet spectroscopy, measurements of hydroperoxide concentration and by high performance liquid chromatography (HPLC). The photo-peroxidation reaction proceeds more rapidly in chloroform solution as the lifetime of singlet oxygen is shown to be over ten times longer in chloroform than methanol. The initial fast reaction kinetics of the photo-peroxidation reactions were evaluated using a pulsed laser technique to show that singlet oxygen reacts competitively with both the anti-oxidants and the polyunsaturated fatty acid ester. Second order kinetic rate constants (in the range 10(5)-10(7) dm(3) mol(-1) s(-1)) were evaluated for the reactivity of singlet oxygen with a range of anti-oxidants and a singlet oxygen quencher, and the results used to explain the effect of anti-oxidants at different concentrations on the rate of the linoleate photo-peroxidation reaction. PMID:10974235

Chacón, J N; Gaggini, P; Sinclair, R S; Smith, F J

2000-09-01

315

Gas-phase chemistry of benzyl alcohol: Reaction rate constants and products with OH radical and ozone  

NASA Astrophysics Data System (ADS)

A bimolecular rate constant, kOH+Benzyl alcohol, of (28 ± 7) × 10 -12 cm 3 molecule -1 s -1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with benzyl alcohol, at (297 ± 3) K and 1 atm total pressure. Additionally, an upper limit of the bimolecular rate constant, kO3+Benzyl alcohol, of approximately 6 × 10 -19 cm 3 molecule -1 s -1 was determined by monitoring the decrease in benzyl alcohol concentration over time in an excess of ozone (O 3). To more clearly define part of benzyl alcohol's indoor environment degradation mechanism, the products of the benzyl alcohol + OH were also investigated. The derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N, O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) were used to positively identify benzaldehyde, glyoxal and 4-oxopentanal as benzyl alcohol/OH reaction products. The elucidation of other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible benzyl alcohol/OH reaction mechanisms based on previously published volatile organic compound/OH gas-phase reaction mechanisms.

Harrison, Joel C.; Wells, J. R.

316

Laboratory Studies of the Kinetics of Aqueous Phase Reactions of SO4- and OH Radicals with Organo-Sulfur Compounds of Atmospheric Interest  

NASA Astrophysics Data System (ADS)

As dimethyl sulfide (DMS) undergoes gas-phase oxidization in the atmosphere, the products become progressively less volatile and can be partially partitioned into the condensed phase. Therefore condensed phase transformations may play an important role in the atmospheric chemistry of these species. As part of a research program aimed at developing a detailed understanding of DMS oxidation and its role in the global climate system, we are investigating the aqueous phase reactions between radicals commonly found in the atmospheric condensed phase and organo-sulfur compounds of atmospheric interest. The present study employs a Laser Flash Photolysis-Long Path UV-Visible Absorption technique to investigate the aqueous phase reactions of SO4- and OH radicals with the organo-sulfur species dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2), methane sulfinate (MSI), and methane sulfonate (MS). Laser flash photolysis of either S2O82- or H2O2 at 248 nm is used to produce the free radicals. Reactions of SO4- radicals are studied by monitoring the time dependence of the SO4- absorption at 450 nm under pseudo-first order conditions with the organo-sulfur compounds in large excess over SO4-. A competitive kinetics technique is employed in the studies of OH reactions, where the reaction rates of the organo-sulfur species with OH are measured relative to the rate of the reaction of SCN- with OH. The product (SCN)2- from reactions of SCN- with OH (SCN- + OH -> SCN + OH-, SCN + SCN- -> (SCN)2- ), whose concentration decreases with increasing concentration of the organo-sulfur species, is monitored as the kinetic probe (at 475 nm). Room temperature (293+/-1 K) rate coefficients for SO4- reactions with DMSO, DMSO2, MSI, and MS in units of M-1s-1 are found to be (2.6 +/- 0.3) x 109, (3.4 +/- 0.3) x 106, > 5 x 108, and (1.2 +/- 0.1) x 104, respectively, while OH reaction rates in the same units are found to be (5.9 +/- 1.0)x 109, (1.4 +/- 0.5) x 107, (1.1 +/- 0.5) x 109, and (1.0 +/- 0.2) x 107, respectively. Rate coefficients for the SO4- + DMSO2 and MS reactions have not previously been reported. Both previous studies and our studies demonstrate that DMSO and MSI are very reactive species, while DMSO2 and MS are much less reactive. The rate coefficient we report for the OH + MS reaction is much smaller than previously reported values. Temperature-dependent kinetics studies are now in progress and updated results from these studies will be reported.

Zhu, L.; Nicovich, J. M.; Wine, P. H.; Wine, P. H.

2001-12-01

317

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

PubMed Central

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. ?fH298o 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

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

2011-01-01

318

Effects of a new 3-alpha reaction rate on X-ray bursts of a helium accreting neutron star  

NASA Astrophysics Data System (ADS)

We investigate the effects of a new 3-? reaction rate (OKK rate) on a He flash of a He accreting neutron star in a binary system. Since ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, dM/dt (?). We find that, at low accretion rates, nuclear burning occurs at the He layers of rather low densities. As a consequence, He ignition would occur at accretion rate lower than M&dot ~=3×10-8Msolaryr-1 Furthermore we find that the OKK rate is consistent with the available observations of X-ray bursts on a He accreting neutron star. We advocate that the OKK rate is better than the previous rate for the astrophysical phenomena of X-ray burst due to He accretion.

Matsuo, Y.; Tsujimoto, H.; Noda, T.; Saruwatari, M.; Ono, M.; Hashimoto, M.; Fujimoto, M. Y.

2012-11-01

319

High-energy threshold reaction rates on 0.8 GeV proton-irradiated thick Pb-target  

E-print Network

This works presents results of activation-aided determination of threshold reaction rates in 92 209Bi, natPb, 197Au, 181Ta, 169Tm, natIn, 93Nb, 64Zn, 65Cu, 63Cu, 59Co, 19F, and 12C samples and in 121 27Al samples. All the samples were aligned with the proton beam axis inside and outside the demountable 92-cm thick Pb target of 15-cm diameter assembled of 23 4-cm thick discs. The samples were placed on 12 target disks to reproduce the long axis distribution of protons and neutrons. In June 2006, the target was exposed for 18 hours to a 800-MeV proton beam extracted from the ITEP U-10 accelerator. The proton fluence and the proton beam shape were determined using the 27Al(p,x)7Be monitor reaction. The reaction rates were determined by the direct gamma-spectrometry techniques. In total, 1196 gamma-spectra have been measured, and about 1500 reaction rates determined. The measured reaction rates were simulated by the MCNPX code using the following databases: ENDF/B6 for neutrons below 20 MeV, MENDL2 for 20-100 MeV neutrons, and MENDL2P for proton cross sections up to 200 MeV. An acceptable agreement of simulations with experimental data has been found.

Yu. E. Titarenko; V. F. Batyaev; A. Yu. Titarenko; M. A. Butko; K. V. Pavlov; R. S. Tikhonov; S. N. Florya; S. G. Mashnik; W. Gudowski

2007-05-08

320

EFFECTS OF PORE STRUCTURE CHANGE AND MULTI-SCALE HETEROGENEITY ON CONTAMINANT TRANSPORT AND REACTION RATE UPSCALING  

SciTech Connect

This project addressed the scaling of geochemical reactions to core and field scales, and the interrelationship between reaction rates and flow in porous media. We targeted reactive transport problems relevant to the Hanford site specifically the reaction of highly caustic, radioactive waste solutions with subsurface sediments, and the immobilization of 90Sr and 129I through mineral incorporation and passive flow blockage, respectively. We addressed the correlation of results for pore-scale fluid-soil interaction with field-scale fluid flow, with the specific goals of (i) predicting attenuation of radionuclide concentration; (ii) estimating changes in flow rates through changes of soil permeabilities; and (iii) estimating effective reaction rates. In supplemental work, we also simulated reactive transport systems relevant to geologic carbon sequestration. As a whole, this research generated a better understanding of reactive transport in porous media, and resulted in more accurate methods for reaction rate upscaling and improved prediction of permeability evolution. These scientific advancements will ultimately lead to better tools for management and remediation of DOE legacy waste problems.

Peters, Catherine A [Princeton University] [Princeton University

2013-05-15

321

Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption  

NASA Astrophysics Data System (ADS)

The development of hydrogen-based energy sources as viable alternatives to fossil-fuel technologies has revolutionized clean energy production using fuel cells. However, to date, the slow rate of the hydrogen oxidation reaction (HOR) in alkaline environments has hindered advances in alkaline fuel cell systems. Here, we address this by studying the trends in the activity of the HOR in alkaline environments. We demonstrate that it can be enhanced more than fivefold compared to state-of-the-art platinum catalysts. The maximum activity is found for materials (Ir and Pt0.1Ru0.9) with an optimal balance between the active sites that are required for the adsorption/dissociation of H2 and for the adsorption of hydroxyl species (OHad). We propose that the more oxophilic sites on Ir (defects) and PtRu material (Ru atoms) electrodes facilitate the adsorption of OHad species. Those then react with the hydrogen intermediates (Had) that are adsorbed on more noble surface sites.

Strmcnik, Dusan; Uchimura, Masanobu; Wang, Chao; Subbaraman, Ram; Danilovic, Nemanja; van der Vliet, Dennis; Paulikas, Arvydas P.; Stamenkovic, Vojislav R.; Markovic, Nenad M.

2013-04-01

322

Proteolysis inside the membrane is a rate-governed reaction not driven by substrate affinity  

PubMed Central

SUMMARY Enzymatic cleavage of transmembrane anchors to release proteins from the membrane controls diverse signaling pathways and is implicated in over a dozen diseases. How catalysis works within the viscous, water-excluding, two-dimensional membrane is unknown. We developed an inducible reconstitution system to interrogate rhomboid proteolysis quantitatively within the membrane in real time. Remarkably, rhomboid proteases displayed no physiological affinity for substrates (Kd ~190 ?M, or 0.1 mol%). Instead, ~10,000-fold differences in proteolytic efficiency with substrate mutants and diverse rhomboid proteases were reflected in kcat values alone. Analysis of gate-open mutant and solvent isotope effects revealed that substrate gating, not hydrolysis, is rate limiting. Ultimately a single proteolytic event within the membrane normally takes minutes. Rhomboid intramembrane proteolysis is thus a slow, kinetically controlled reaction not driven by transmembrane protein-protein affinity. These properties are unlike those of other studied proteases or membrane proteins but strikingly reminiscent of one subset of DNA-repair enzymes, raising important mechanistic and drug-design implications. PMID:24315097

Dickey, Seth W.; Baker, Rosanna P.; Cho, Sangwoo; Urban, Siniša

2013-01-01

323

Modeling kinetic partitioning of secondary organic aerosol and size distribution dynamics: representing effects of volatility, phase state, and particle-phase reaction  

SciTech Connect

Evidence is mounting that the majority of the climatically active aerosols are produced through the growth of smaller particles via secondary organic aerosol (SOA) formation from gas-to-particle conversion of anthropogenic and biogenic volatile organic compounds (VOCs). The timescale of SOA partitioning and the associated size distribution dynamics are expected to depend on the gas-phase oxidation of the precursor VOCs and their products, volatility of these organic solutes, composition and phase state of the pre-existing particles, and diffusivity and reactivity of the solute within the particle phase. This paper describes a new framework for modeling kinetic gas-particle partitioning of SOA, with an analytical treatment for the diffusion-reaction process within the particle phase. The formulation is amenable for eventual use in regional and global climate models, although it currently awaits implementation of the actual particle-phase reactions that are important for SOA formation. In the present work, the model is applied to investigate the competitive growth dynamics of the Aitken and accumulation mode particles while the Kelvin effect and coagulation are neglected for simplicity. The timescale of SOA partitioning and evolution of number and composition size distributions are evaluated for a range of solute volatilities (C*), particle-phase bulk diffusivities (Db), and particle-phase reactivity, as exemplified by a pseudo-first-order rate constant (kc). Results show that irreversible condensation of non-volatile organic vapors (equivalent to ) produces significant narrowing of the size distribution. At the other extreme, non-reactive partitioning of semi-volatile organic vapors is volume-controlled in which the final (equilibrium) size distribution simply shifts to the right on the diameter axis while its shape remains unchanged. However, appreciable narrowing of the size distribution may occur when the pre-existing particles are highly viscous semi-solids such that small particles reach quasi-equilibrium much faster than the large ones. In the case of reactive partitioning (finite ), the size distribution experiences permanent narrowing, which is especially pronounced for Db < 10-13 cm2 s-1 and kc > 0.01 s-1. As a result, both number and composition size distributions are needed to effectively constrain and evaluate the next generation of SOA models that treat phase state thermodynamics, particle-phase diffusion and particle-phase chemical reactions.

Zaveri, Rahul A.; Easter, Richard C.; Shilling, John E.; Seinfeld, J. H.

2014-05-27

324

SISGR - In situ characterization and modeling of formation reactions under extreme heating rates in nanostructured multilayer foils  

SciTech Connect

Materials subjected to extreme conditions, such as very rapid heating, behave differently than materials under more ordinary conditions. In this program we examined the effect of rapid heating on solid-state chemical reactions in metallic materials. One primary goal was to develop experimental techniques capable of observing these reactions, which can occur at heating rates in excess of one million degrees Celsius per second. One approach that we used is x-ray diffraction performed using microfocused x-ray beams and very fast x-ray detectors. A second approach is the use of a pulsed electron source for dynamic transmission electron microscopy. With these techniques we were able to observe how the heating rate affects the chemical reaction, from which we were able to discern general principles about how these reactions proceed. A second thrust of this program was to develop computational tools to help us understand and predict the reactions. From atomic-scale simulations were learned about the interdiffusion between different metals at high heating rates, and about how new crystalline phases form. A second class of computational models allow us to predict the shape of the reaction front that occurs in these materials, and to connect our understanding of interdiffusion from the atomistic simulations to measurements made in the laboratory. Both the experimental and computational techniques developed in this program are expected to be broadly applicable to a wider range of scientific problems than the intermetallic solid-state reactions studied here. For example, we have already begun using the x-ray techniques to study how materials respond to mechanical deformation at very high rates.

Hufnagel, Todd C.

2014-06-09

325

Measurements of the O+ plus N2 and O+ plus O2 reaction rates from 300 to 900 K  

NASA Technical Reports Server (NTRS)

Rate coefficients for the O(+) + N2 atom transfer and O(+) + O2 charge transfer reactions are determined at thermal energies between 300 K and 900 K difference in a heated drift tube mass spectrometer apparatus. At 300 K the values K(O(+) + N2) = (1.2 plus or minus 0.1) x 10 to the negative 12 power cubic cm/sec and k(O(+) + O2) = (2.1 plus or minus 0.2) x 10 to the negative 11 power cubic cm/sec were obtained, with a 50% difference decrease in the reaction rates upon heating to 700 K. These results are in good agreement with heated flowing afterglow results, but the O(+) + O2 thermal rate coefficients are systematically lower than equivalent Maxwellian rates inferred by conversion of nonthermal drift tube and flow drift data.

Chen, A.; Johnsen, R.; Biondi, M. A.

1977-01-01

326

Stochastic theory of large-scale enzyme-reaction networks: Finite copy number corrections to rate equation models  

NASA Astrophysics Data System (ADS)

Chemical reactions inside cells occur in compartment volumes in the range of atto- to femtoliters. Physiological concentrations realized in such small volumes imply low copy numbers of interacting molecules with the consequence of considerable fluctuations in the concentrations. In contrast, rate equation models are based on the implicit assumption of infinitely large numbers of interacting molecules, or equivalently, that reactions occur in infinite volumes at constant macroscopic concentrations. In this article we compute the finite-volume corrections (or equivalently the finite copy number corrections) to the solutions of the rate equations for chemical reaction networks composed of arbitrarily large numbers of enzyme-catalyzed reactions which are confined inside a small subcellular compartment. This is achieved by applying a mesoscopic version of the quasisteady-state assumption to the exact Fokker-Planck equation associated with the Poisson representation of the chemical master equation. The procedure yields impressively simple and compact expressions for the finite-volume corrections. We prove that the predictions of the rate equations will always underestimate the actual steady-state substrate concentrations for an enzyme-reaction network confined in a small volume. In particular we show that the finite-volume corrections increase with decreasing subcellular volume, decreasing Michaelis-Menten constants, and increasing enzyme saturation. The magnitude of the corrections depends sensitively on the topology of the network. The predictions of the theory are shown to be in excellent agreement with stochastic simulations for two types of networks typically associated with protein methylation and metabolism.

Thomas, Philipp; Straube, Arthur V.; Grima, Ramon

2010-11-01

327

Measurements of Fission and Radioactive Capture Reaction Rates Inside the Fuel of the Ipen\\/MB01  

Microsoft Academic Search

This work presents the measures of the nuclear reaction rates along the radial direction of the fuel pellet by irradiation\\u000a and posterior gamma spectrometry of a thin slice of fuel pellet of UO2 at 4.3% enrichment. From its irradiation, the rate of radioactive capture and fission had been measured as a function of\\u000a the radius of the pellet disk using

Luís Felipe L. Mura; Ulysses D'utra Bitelli; Leda C. C. B. Fanaro

2011-01-01

328

Limiting high pressure rate coefficients for the HO+NO 2 &?HONO 2 reaction on ab initio potential energy surfaces  

Microsoft Academic Search

Phase space limiting high pressure rate coefficients for the title reaction on ab initio potential energy surfaces have been calculated at 50-600 K. Calculated rigidity factors at different levels of theory are\\u000a presented. The best limiting high pressure rate coefficient obtained at 300 K, 4.0x10-11cm3 molecule-1 s-1, compares very well with the latest IUPAC recommended value.

Carlos J. Cobos

2003-01-01

329

Including anharmonicity in the calculation of rate constants. II. The OH+H2-->H2O+H reaction.  

PubMed

A recently developed method for calculating anharmonic vibrational energy levels at nonstationary points along a reaction path that is based on second-order perturbation theory in curvilinear coordinates is combined with variational transition state theory with semiclassical multidimensional tunneling approximations to calculate thermal rate constants for the title reaction. Two different potential energy surfaces were employed for these calculations, an improved version of the author's surface 5 and the WSLFH surface of Wu et al. [J. Chem. Phys. 113, 3150 (2000)]. We present detailed comparisons of rate constants computed for the two surfaces with and without anharmonicity and with various approximations for incorporating tunneling along the reaction path. The results for this system are quite sensitive to the surface employed, the choice of coordinates (curvilinear versus rectilinear), and the inclusion of anharmonicity. A comparison with experiment provides information on the accuracy of these surfaces. PMID:18397061

Isaacson, Alan D

2008-04-01

330

Evaluation of the Li insertion/extraction reaction rate at a vacuum-deposited silicon film anode  

NASA Astrophysics Data System (ADS)

We have recognized that a vacuum-deposited silicon film shows very attractive Li insertion/extraction reaction performance, but the reaction rate in view of Li diffusion in the silicon film has not been determined yet. In this study, we performed analysis of the reaction rate by the means of potential-step chronoamperometry (PSCA) method. Samples of the silicon films of several different thicknesses ranging from 500 to 14000 Å were prepared by depositing silicon in vacuum on an etched Ni foil. The current-time curves were recorded under PSCA condition and offered to evaluate the apparent diffusion coefficient Dapp of Li in the silicon films. The obtained Dapp values were as low as 10 -10 to 10 -13 cm 2 s -1, which were dependent on the film thickness.

Yoshimura, Kazutaka; Suzuki, Junji; Sekine, Kyoichi; Takamura, Tsutomu

331

TECHNIQUES AFFECTING PRECISION AND ACCURACY IN HYDROLYSIS RATE CONSTANT DETERMINATIONS OF VOLATILE ORGANIC COMPOUNDS USING JEFFERS' ZERO HEADSPACE REACTION BULBS  

EPA Science Inventory

A recently published method for measurement of hydrolysis rate constants for volatile organic compounds in aqueous samples was adapted for use in our laboratory. n applying the method, we developed the capability to make the zero-headspace reaction bulbs and used them to measure ...

332

Effects of 24-hour sleep deprivation on rate of decrement in a 10-minute auditory reaction time task  

Microsoft Academic Search

Used a reaction time (RT) task of short duration, characterized by a high signal rate (16 signals\\/min) to examine if 1 night of sleep deprivation (SD) in interaction with task duration would transfer and\\/or change the RT distribution. 5 male and 3 female undergraduates participated on 2 occasions: after SD and after normal sleep. From a significant interaction between SD

Hans O. Lisper; Anders Kjellberg

1972-01-01

333

Absolute rate of the reaction of C l(2P) with methane from 200-500 K  

NASA Technical Reports Server (NTRS)

Rate constants for the reaction of atomic chlorine with methane have been measured from 200-500K using the flash photolysis-resonance fluorescence technique. When the results from fourteen equally spaced experimental determinations are plotted in Arrhenius form a definite curvature is noted. The results are compared to previous work and are theoretically discussed.

Whytock, D. A.; Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief, L. J.

1976-01-01

334

Short communication Effect of electrolyte pH on the rate of the anodic and cathodic reactions in an  

E-print Network

Short communication Effect of electrolyte pH on the rate of the anodic and cathodic reactions generation from an air-cathode microbial fuel cell (MFC) with a mixed bacteria culture at different pH showed resistance [8­13]. The electrolyte pH is crucial to the MFCs power output. Generally, bacteria require a p

335

An Interactive Classroom Activity Demonstrating Reaction Mechanisms and Rate-Determining Steps  

ERIC Educational Resources Information Center

An interactive classroom activity that includes two-step reaction of unwrapping and eating chocolate candies is described which brings not only the reaction intermediate, but also the reactants and products into macroscopic view. The qualitative activation barriers of both steps can be adjusted independently.

Jennings, Laura D.; Keller, Steven W.

2005-01-01

336

Charting an Alternate Pathway to Reaction Orders and Rate Laws in Introductory Chemistry Courses  

ERIC Educational Resources Information Center

Reaction kinetics is an axiomatic topic in chemistry that is often addressed as early as the high school course and serves as the foundation for more sophisticated conversations in college-level organic, physical, and biological chemistry courses. Despite the fundamental nature of reaction kinetics, students can struggle with transforming their…

Rushton, Gregory T.; Criswell, Brett A.; McAllister, Nicole D.; Polizzi, Samuel J.; Moore, Lamesha A.; Pierre, Michelle S.

2014-01-01

337

Calculation of the specific rate of catabolic activity (Ac) from the heat flow rate of soil microbial reactions measured by calorimetry: significance and applications.  

PubMed

The calculation of parameters involved in the kinetics of the microbial soil reactions linked to the carbon cycle is strongly limited by the methodologies employed. Hence, a mathematical model is proposed to quantify easily the specific rate of catabolic activity A(c) by microcalorimetry based on Belaich's model. It permits to quantify A(c) from the plots of the heat flow rate vs. time recorded from soil samples amended with glucose. It was applied for several soil samples collected in the Amazon. The results obtained were compared, and statistical and graphical analyses were used to provide the biophysical significance of A(c) in soils. Results suggest that A(c) could be used as an empirical measure of stress. It correlates positively with the heat yield, Y(Q/X), of the soil microbial growth reactions, indicating that higher specific rates of catabolic activity cause higher dissipation of energy per unit of cell, yielding less-efficient metabolic reactions, which could affect negatively the soil quality. It is strongly affected by the initial microbial population and by the percentage of nitrogen in the samples. The statistical analysis also demonstrated that A(c) is more sensitive to changing environmental conditions than Y(Q/X), yielding more-accurate information about the soil metabolic processes. PMID:17191799

Barros, Nieves; Gallego, Marta; Feijóo, Sergio

2004-10-01

338

Inter-and intraspecific variation in excited-state triplet energy transfer rates in reaction centers of photosynthetic bacteria.  

SciTech Connect

In protein-cofactor reaction center (RC) complexes of purple photosynthetic bacteria, the major role of the bound carotenoid (C) is to quench the triplet state formed on the primary electron donor (P) before its sensitization of the excited singlet state of molecular oxygen from its ground triplet state. This triplet energy is transferred from P to C via the bacteriochlorophyll monomer B{sub B}. Using time-resolved electron paramagnetic resonance (TREPR), we have examined the temperature dependence of the rates of this triplet energy transfer reaction in the RC of three wild-type species of purple nonsulfur bacteria. Species-specific differences in the rate of transfer were observed. Wild-type Rhodobacter capsulatus RCs were less efficient at the triplet transfer reaction than Rhodobacter sphaeroides RCs, but were more efficient than Rhodospirillum rubrum RCs. In addition, RCs from three mutant strains of R. capsulatus carrying substitutions of amino acids near P and B{sub B} were examined. Two of the mutant RCs showed decreased triplet transfer rates compared with wild-type RCs, whereas one of the mutant RCs demonstrated a slight increase in triplet transfer rate at low temperatures. The results show that site-specific changes within the RC of R. capsulatus can mimic interspecies differences in the rates of triplet energy transfer. This application of TREPR was instrumental in defining critical energetic and coupling factors that dictate the efficiency of this photoprotective process.

Laible, P. D.; Morris, Z. S.; Thurnauer, M. C.; Schiffer, M.; Hanson, D. K.

2003-08-01

339

Significance of vapor phase chemical reactions on CVD rates predicted by chemically frozen and local thermochemical equilibrium boundary layer theories  

NASA Technical Reports Server (NTRS)

This paper investigates the role played by vapor-phase chemical reactions on CVD rates by comparing the results of two extreme theories developed to predict CVD mass transport rates in the absence of interfacial kinetic barrier: one based on chemically frozen boundary layer and the other based on local thermochemical equilibrium. Both theories consider laminar convective-diffusion boundary layers at high Reynolds numbers and include thermal (Soret) diffusion and variable property effects. As an example, Na2SO4 deposition was studied. It was found that gas phase reactions have no important role on Na2SO4 deposition rates and on the predictions of the theories. The implications of the predictions of the two theories to other CVD systems are discussed.

Gokoglu, Suleyman A.

1988-01-01

340

Gas-Phase Reaction Pathways and Rate Coefficients for the Dichlorosilane-Hydrogen and Trichlorosilane-Hydrogen Systems  

NASA Technical Reports Server (NTRS)

As part of NASA Ames Research Center's Integrated Process Team on Device/Process Modeling and Nanotechnology our goal is to create/contribute to a gas-phase chemical database for use in modeling microelectronics devices. In particular, we use ab initio methods to determine chemical reaction pathways and to evaluate reaction rate coefficients. Our initial studies concern reactions involved in the dichlorosilane-hydrogen (SiCl2H2--H2) and trichlorosilane-hydrogen (SiCl2H-H2) systems. Reactant, saddle point (transition state), and product geometries and their vibrational harmonic frequencies are determined using the complete-active-space self-consistent-field (CASSCF) electronic structure method with the correlation consistent polarized valence double-zeta basis set (cc-pVDZ). Reaction pathways are constructed by following the imaginary frequency mode of the saddle point to both the reactant and product. Accurate energetics are determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations (CCSD(T)) extrapolated to the complete basis set limit. Using the data from the electronic structure calculations, reaction rate coefficients are obtained using conventional and variational transition state and RRKM theories.

Dateo, Christopher E.; Walch, Stephen P.

2002-01-01

341

Rate coefficient for the reaction H+O2 --> OH+O: Results at high temperatures, 2000 to 5300 K  

NASA Astrophysics Data System (ADS)

The tunable-laser flash-absorption technique has been used to study the high-temperature behavior of the reaction H+O2?OH+O by monitoring the absorption of the hydroxyl radical. Sensitivity analysis of a detailed reaction mechanism shows that for fuel rich mixtures only two reactions are sensitive when hydroxyl is monitored: H2+M?2H+M and H+O2?OH+O. Rate coefficients for these reactions have been determined by least-squares analysis of measured absorption profiles. For the rate of dissociation of H2 in krypton we obtain k1(T)=(8.86±0.88)×10-10 exp[-48321/T(K)] cm3 s-1 from 3450 to 5300 K. For the H+O2 reaction we combined our results with previous low temperature measurements and recommend k2(T)=(1.62±0.12)×10-10 exp[-(7474±122)/T(K)] cm3 s-1 from 960 to 5300 K. The uncertainties are at the 95% confidence level. Measured cross sections for rotational and vibrational energy transfer in O2 and OH have been used to show that relaxation effects do not influence the results. We compare our results to recent trajectory calculations. In addition, we calculate the rate of the reverse reaction, OH+O?H+O2, and compare it to trajectory and statistical adiabatic channel calculations. Finally, we point out that the first excited surface of the hydroperoxyl radical, 2A', which correlates with H(2S)+O2(1?g), may be needed to explain very high temperature results.

Du, Hong; Hessler, Jan P.

1992-01-01

342

Effects of Finite Reaction Rates on the Kinetic Phase Transitions in the Catalytic Oxidation of Carbon Monoxide  

NASA Astrophysics Data System (ADS)

Oxidation of carbon monoxide is one of the most extensively studied heterogeneous catalysis reactions, being important among other applications in automobile-emission control. Catalytic oxidation of carbon monoxide on platinum (111) surface was simulated by the Monte Carlo technique following an extended version of the model proposed by Ziff, Gulari and Barshad (ZGB). In the simulation, a simple square two-dimensional lattice of active sites replaces the surface of the catalyst. Finite reaction rates for (i) diffusion of the reactive species on the surface, (ii) reaction of a CO molecule with an oxygen atom in a nearest neighbor site, and (iii) desorption of unreacted CO molecules, have been taken into account. The produced CO2 desorbs instantly. The average coverage of O, CO and the CO2 production rate for a steady state configuration, as a function of the normalized CO partial pressure (PCO), shows two kinetic phase transitions. In the ZGB model these transitions occur at PCO ~ 0.39 and PCO ~ 0.53. For 0.39 < PCO > 0.53 a reactive (CO2 production) steady state is found. Outside of the interval, the only steady state is a poisoned catalyst of pure CO or pure O. Our results show that finite reaction rates shift the values in which these phase transitions occur.

López-Carreño, L. D.

343

Structural Identifiability of the Yield Coefficients in Bioprocess Models When the Reaction Rates  

E-print Network

submodels. The first submodel depends only on the reaction structure and is independent of the kinetics are known, the second submodel can be used for modeling the kinetic structure. This submodel is in a form

Bastin, Georges

344

Reaction times and error rates for “same”-“different” judgments of multidimensional stimull  

Microsoft Academic Search

Each S indicated whether two successively presented rows of letters were “same” or “different.” Reaction times of the “different”\\u000a response seemed to indicate that S examined the stimulus letters in a serial, self-terminating manner. However, the reaction\\u000a times of the “same” response were not consistent with this model. Consequently, it was proposed that S employs simultaneously\\u000a two distinct processes for

Donald Bamber

1969-01-01

345

Spectroscopic factors for proton unbound levels in 26Al and their influence on stellar reaction rates  

Microsoft Academic Search

The reaction 25Mg(3He, d)26Al at a bombarding energy of 15 MeV was used to populate proton unbound states in 26Al. Angular distributions were measured and spectroscopic factors extracted for nine levels from 6343 to 6680 keV excitation in 26Al. This excitation energy range corresponds to resonance energies for the 25Mg(p, gamma)26Al reaction from 37 to 374 keV in the center-of-mass.

A. A. Rollefson; V. Wijekumar; C. P. Browne; M. Wiescher; H. J. Hausman; W. Y. Kim; P. Schmalbrock

1990-01-01

346

The effect of nuclear reaction rates and convective mixing on the evolution of a 6M{sub ?} star  

SciTech Connect

We present the evolution of a 6M{sub ?} star, of solar-like initial metallicity, and investigate the effects of key nuclear reaction rates, as well as the treatment of the convective mixing on its evolution along the Cepheid instability strip. In particular, we study the effect of recent estimates of the {sup 14}N(p,?){sup 15}O reaction on the formation and extension of the blue loop during core helium burning. We also investigate the effects induced on this blue loop by the adoption of non-standard convective mixing prescriptions, as well as the implications of modifying the Mixing Length Theory.

Halabi, Ghina M., E-mail: gfm01@aub.edu.lb [American University of Beirut, Department of Physics, P.O. Box 11-0236, Riad El-Solh, Beirut (Lebanon)

2014-05-09

347

Measurement of the neutron capture rate of the (C-12)(n,gamma)(C-13) reaction at stellar energy  

NASA Astrophysics Data System (ADS)

The reaction (C-12)(u,gamma)(C-13) at stellar energy plays important roles in the nucleosynthesis of intermediate-mass nuclei in inhomogeneous big bang models and in that of heavy elements during stellar evolution. In the present study, the reaction rate was measured by detecting gamma-rays emitted from a captured state; 16.8 + or - 2.1 microbarn was obtained, about 5 times larger than the estimated value from the thermal-neutron capture cross section. The present result favors the nucleosynthesis of intermediate-mass nuclei in inhomogeneous big bang models and also of heavy elements in exploding supernovae.

Nagai, Y.; Igashira, M.; Takeda, K.; Mukai, N.; Motoyama, S.; Uesawa, F.; Kitazawa, H.; Fukuda, T.

1991-05-01

348

A survey of the reaction rate constants for the thermal dissociation and recombination of nitrogen and oxygen  

NASA Technical Reports Server (NTRS)

The objective of the present report is to survey the various values of forward and backward reaction rate constants used by investigators in the field of high-temperature (T greater than 2000 K) gas reactions involving nitrogen and oxygen only. The objective is to find those values that correlate well so that they can be used for the studies of hypersonic flow and supersonic combustion with reasonable confidence. Relatively good agreement among these various values is observed for temperatures lower than 10,000 K.

Marraffa, Lionel; Dulikravich, George S.; Keeney, Timothy C.; Deiwert, George S.

1988-01-01

349

Heterogeneous reaction rates in an ionic liquid: quantitative results from two-dimensional multiple population-period transient spectroscopy.  

PubMed

The hypotheses that ionic liquids are structurally heterogeneous at the molecular level and, even further, that this heterogeneity can transfer to the rates of reactions run in ionic liquids is being actively debated. Here, this hypothesis is tested using multiple population-period transient spectroscopy (MUPPETS), an emerging type of multidimensional measurement that resolves the kinetics of subensembles within a heterogeneous sample. A previous MUPPETS study of the excited-state twisting and electronic relaxation of auramine indicated that an ionic-liquid solvent induces rate dispersion due to a combination of heterogeneous and homogeneous processes, but those data could not quantitatively separate these contributions [Khurmi, C.; Berg, M. A. J. Phys. Chem. Lett.2010, 1, 161]. New MUPPETS data that include phase resolution and subtraction of thermal gratings are presented here and are successfully modeled. The total range of reaction rates (10--90%) is a factor of 70. If the solvent effect is viewed as a set of local viscosities, the viscosity distribution is broad and highly asymmetric. However, if the solvent is viewed as changing a reaction barrier, the data correspond to a Gaussian distribution of barrier heights. The relaxation of each subensemble is nonexponential with an initial induction period, but the shape of the decay is invariant across the rate distribution. A small (2%), long-lived component is identified as a part of the homogeneous kinetic scheme and thus as a secondary channel for excited-state relaxation, not as an impurity or alternative ground-state form of auramine. On the basis of these results, we suggest that the primary cause of rate heterogeneity is a long-lived local electric field acting on the charge redistribution during the reaction. PMID:21630702

Sahu, Kalyanasis; Kern, Sean J; Berg, Mark A

2011-07-14

350

Rate constants and temperature dependences for the reactions of hydroxyl radical with several halogenated methanes, ethanes, and propanes by relative rate measurements  

NASA Technical Reports Server (NTRS)

Rate constants of 15 OH reactions with halogen-substituted alkanes, C1 to C3, were studied using a relative rate technique in the temperature range 283-403 K. Compounds studied were CHF2Cl (22), CHF2Br (22B), CH3F (41), CH2F2 (32), CHF3 (23), CHClFCCl2F (122a), CHCl2CF3 (123), CHClFCF3 (124), CH3CF3 (143a), CH3CH2F (161), CF3CHFCF3 (227ea), CF3CH2CF3 (236fa), CF3CHFCHF2 (236ea), and CHF2CF2CH2F (245ca). Using CH4, CH3CCl3, CF3CF2H, and C2H6 as primary reference standards (JPL 92-20 rate constants), absolute rate constants are derived. Results are in good agreement with previous experimental results for six of the compounds studied, including CHF2Cl, CHF2Br, CH2F2, CH3CF3, CHFClCFCl2, and CF3CHFCF3. For the remainder the relative rate constants are lower than those derived from experiments in which OH loss was used to measure the reaction rate. Comparisons of the derived Arrhenius A factors with previous literature transition-state calculations show order of magnitude agreement in most cases. However, the experimental A factors show a much closer proportionality to the number of H atoms in the molecule than is evident from the transition state calculations. For most of the compounds studied, an A factor of (8 +/- 3)E-13 cm(exp 3)/(molecule s) per C-H bond is observed. A new measurement of the ratio k(CH3CCl3)/k(CH4) is reported that is in good agreement with previous data.

Hsu, K.-J.; DeMore, W. B.

1995-01-01

351

Atmospheric reactions of methylcyclohexanes with Cl atoms and OH radicals: determination of rate coefficients and degradation products.  

PubMed

As the result of biogenic and anthropogenic activities, large quantities of chemical compounds are emitted into the troposphere. Alkanes, in general, and cycloalkanes are an important chemical class of hydrocarbons found in diesel, jet and gasoline, vehicle exhaust emissions, and ambient air in urban areas. In general, the primary atmospheric fate of organic compounds in the gas phase is the reaction with hydroxyl radicals (OH). The oxidation by Cl atoms has gained importance in the study of atmospheric reactions because they may exert some influence in the boundary layer, particularly in marine and coastal environments, and in the Arctic troposphere. The aim of this paper is to study of the atmospheric reactivity of methylcylohexanes with Cl atoms and OH radicals under atmospheric conditions (in air at room temperature and pressure). Relative kinetic techniques have been used to determine the rate coefficients for the reaction of Cl atoms and OH radicals with methylcyclohexane, cis-1,4-dimethylcyclohexane, trans-1,4-dimethylcyclohexane, and 1,3,5-trimethylcyclohexane at 298?±?2 K and 720?±?5 Torr of air by Fourier transform infrared) spectroscopy and gas chromatography-mass spectrometry (GC-MS) in two atmospheric simulation chambers. The products formed in the reaction under atmospheric conditions were investigated using a 200-L Teflon bag and employing the technique of solid-phase microextraction coupled to a GC-MS. The rate coefficients obtained for the reaction of Cl atoms with the studied compounds are the following ones (in units of 10(-10) cm(3) molecule(-1) s(-1)): (3.11?±?0.16), (2.89?±?0.16), (2.89?±?0.26), and (2.61?±?0.42), respectively. For the reactions with OH radicals the determined rate coefficients are (in units of 10(-11) cm(3) molecule(-1) s(-1)): (1.18?±?0.12), (1.49?±?0.16), (1.41?±?0.15), and (1.77?±?0.23), respectively. The reported error is twice the standard deviation. A detailed mechanism for ring-retaining product channels is proposed to justify the observed reaction products. The global tropospheric lifetimes estimated from the reported OH- and Cl-rate coefficients show that the main removal path for the investigated methylcyclohexanes is the reaction with OH radicals. But in marine environments, after sunrise, Cl reactions become more important in the tropospheric degradation. Thus, the estimated lifetimes range from 16 to 24 h for the reactions of the OH radical (calculated with [OH]?=?10(6) atoms cm(-3)) and around 7-8 h in the reactions with Cl atoms in marine environments (calculated with [Cl]?=?1.3?×?10(5) atoms cm(-3)). The reaction of Cl atoms and OH radicals and methylcylohexanes can proceed by H abstraction from the different positions. PMID:24788931

Ballesteros, Bernabé; Ceacero-Vega, Antonio A; Jiménez, Elena; Albaladejo, José

2014-05-01

352

Neutron spectroscopic factors of 7Li and astrophysical 6Li(n,g)7Li reaction rates  

E-print Network

Angular distributions of the 7Li(6Li,6Li)7Li elastic scattering and the 7Li(6Li,7Li_{g.s.})6Li, 7Li(6Li,7Li*_{0.48})6Li transfer reactions at Ec.m. = 23.7 MeV were measured with the Q3D magnetic spectrograph. The optical potential of 6Li+7Li was obtained by fitting the elastic scattering differential cross sections. Based on the distorted wave Born approximation (DWBA) analysis, spectroscopic factors of 7Li=6Li+n were determined to be 0.73 +- 0.05 and 0.90 +- 0.09 for the ground and first exited states in 7Li, respectively. Using the spectroscopic factors, the cross sections of the 6Li(n,g)7Li direct neutron capture reactions and the astrophysical 6Li(n,g)7Li reaction rates were derived.

Jun Su; Zhihong Li; Bing Guo; Xixiang Bai; Zhichang Li; Jiancheng Liu; Youbao Wang; Gang Lian; Sheng Zeng; Baoxiang Wang; Shengquan Yan; Yunju Li; Ertao Li; Qiwen Fan; Weiping Liu

2010-01-25

353

The plutonium/hydrogen reaction: The pressure dependence of reaction initiation time and nucleation rate controlled by a plutonium dioxide over-layer  

NASA Astrophysics Data System (ADS)

As part of an ongoing programme to quantify those parameters which influence the early stages of the plutonium hydriding reaction, the hydrogen pressure dependence of both plutonium hydriding initiation time ( I t) and hydriding nucleation rate ( N r) have been determined for plutonium covered in a reproducible dioxide over-layer. The data show that initiation time is inversely proportional to hydrogen pressure, while nucleation rate is proportional to hydrogen pressure. Both observations are consistent with a model of hydriding attack in which the dioxide over-layer acts as a diffusion barrier, controlling the flow of hydrogen from the gas phase to the oxide/metal interface. The low scatter and reproducibility of the experimental data set demonstrate the importance of synthesising well controlled and characterised oxide layers prior to determining these experimental parameters.

McGillivray, Gordon W.; Knowles, John P.; Findlay, Ian M.; Dawes, Marina J.

2011-05-01

354

Hydrogenation of O and OH on Pt(111): A comparison between the reaction rates of the first and the second hydrogen addition steps  

NASA Astrophysics Data System (ADS)

The formation of water through hydrogenation of oxygen on platinum occurs at a surprisingly low reaction rate. The reaction rate limited process for this catalytic reaction is, however, yet to be settled. In the present work, the reaction rates of the first and the second hydrogen addition steps are compared when hydrogen is obtained through intense synchrotron radiation that induces proton production in a water overlayer on top of the adsorbed oxygen species. A substantial amount of the produced hydrogen diffuses to the platinum surface and promotes water formation at the two starting conditions O/Pt(111) and (H2O+OH)/Pt(111). The comparison shows no significant difference in the reaction rate between the first and the second hydrogen addition steps, which indicates that the rate determining process of the water formation from oxygen on Pt(111) is neither the first nor the second H addition step or, alternatively, that both H addition steps exert rate control.

Näslund, L.-Å.

2014-03-01

355

Hydrogenation of O and OH on Pt(111): A comparison between the reaction rates of the first and the second hydrogen addition steps  

SciTech Connect

The formation of water through hydrogenation of oxygen on platinum occurs at a surprisingly low reaction rate. The reaction rate limited process for this catalytic reaction is, however, yet to be settled. In the present work, the reaction rates of the first and the second hydrogen addition steps are compared when hydrogen is obtained through intense synchrotron radiation that induces proton production in a water overlayer on top of the adsorbed oxygen species. A substantial amount of the produced hydrogen diffuses to the platinum surface and promotes water formation at the two starting conditions O/Pt(111) and (H{sub 2}O+OH)/Pt(111). The comparison shows no significant difference in the reaction rate between the first and the second hydrogen addition steps, which indicates that the rate determining process of the water formation from oxygen on Pt(111) is neither the first nor the second H addition step or, alternatively, that both H addition steps exert rate control.

Näslund, L.-Å., E-mail: lars-ake.naslund@liu.se [Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025 (United States)

2014-03-14

356

Astrophysically Important Reaction Rates For Novae And X-ray Bursts From Proton Breakup At Intermediate Energies  

SciTech Connect

We discuss the use of one-nucleon removal reactions of loosely bound nuclei at intermediate energies as an indirect method in nuclear astrophysics. The breakup reactions are proved to be good spectroscopic tools and can be used to study a large number of loosely bound proton- or neutron-rich nuclei over a wide range of beam energies. As peripheral processes, they can be used to extract asymptotic normalization coefficients (ANCs) from which non-resonant capture reaction rates of astrophysical interest can be calculated parameter free. In this talk, we present results of a proton-breakup experiment carried out at GANIL (France) with a cocktail beam centered around {sup 23}Al at 50 MeV/nucleon. Momentum distributions of the breakup fragments, inclusive and in coincidence with gamma rays detected by EXOGAM Germanium clover array, were measured in the focal plan of SPEG energy-loss spectrometer. We present in particular the investigations of reaction rates for {sup 22}Mg(p,{gamma}){sup 23}Al and {sup 23}Al(p,{gamma}){sup 24}Si important for novae and X-ray bursts, respectively.

Banu, A.; Trache, L. [Cyclotron Institute, Texas A and M University, College Station, Texas 77843-3366 (United States); National Institute of Physics and Nuclear Engineering, 'Horia Hulubei' (IFIN-HH), R-077125 Magurele-Bucharest (Romania); Carstoiu, F.; Negoita, F.; Rotaru, F. [National Institute of Physics and Nuclear Engineering, 'Horia Hulubei' (IFIN-HH), R-077125 Magurele-Bucharest (Romania); Orr, N. A.; Achouri, N. L.; Laurent, B. [Laboratoire de Physique Corpusculaire, IN2P3-CNRS, ISMRA et Universite, F-14050 Caen (France); Bonaccorso, A. [Instituto Nazionale di Fisica Nucleare, Sez. Di Pisa, I-56127 Pisa (Italy); Catford, W. N.; Patterson, N.; Thomas, J. S. [Department of Physics, University of Surrey, Guildford GU2 5XH (United Kingdom); Chartier, M.; Fernandez-Dominguez, B.; Paschalis, S.; Pietras, B. [Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE (United Kingdom); Freer, M. [School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT (United Kingdom); Gaudefroy, L.; Roussel-Chomaz, P. [Grand Accelerateur d'Ions Lourds, BP 55027, 14076 Caen Cedex 5 (France); Horoi, M. [Department of Physics, Central Michigan University, Mount Pleasant, Michigan 48859 (United States)

2010-11-24

357

Why HOC+ is detectable in interstellar clouds: the rate of the reaction between HOC+ and H2  

NASA Technical Reports Server (NTRS)

The recent confirmation by Ziurys and Apponi of the detection of HOC+ toward Sgr B2 (OH), and their identification of the ion in Orion-KL and several other sources show that HOC+ is far more abundant than predicted by previous ion-molecule models. In these models, the reaction HOC(+) + H2 --> HCO(+) + H2 is assumed to rapidly destroy HOC+, based on the results of a prior calculation. We have recalculated the rate of this reaction as a function of temperature using a new ab initio potential surface and a phase space approach to the dynamics which includes tunneling. The newly calculated rate is small (< or = 1 x 10(-10) cm3 s-1) at temperatures under 100 K.

Herbst, E.; Woon, D. E.

1996-01-01

358

Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228-362 K  

NASA Technical Reports Server (NTRS)

Absolute rate constants for the reaction OH ? H2CO measured over the temperature range 228-362 K using the flash photolysis-resonance fluorescence technique are given. The results are independent of variations in H2CO concentration, total pressure Ar concentration, and flash intensity (i.e., initial OH concentration). The rate constant is found to be invariant with temperature in this range, the best representation being k sub 1 = (1.05 ? or - 0.11) x 10 to the 11th power cu cm molecule(-1) s(-1) where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k sub 1. The reaction is also discussed from a theoretical point of view.

Stief, L. J.; Nava, D. F.; Payne, W. A.; Michael, J. V.

1980-01-01

359

Measurement and analysis of the 238U(n, 2n) reaction rate in depleted uranium/polyethylene shells  

NASA Astrophysics Data System (ADS)

In order to check the conceptual design of the subcritical blanket in a fusion-fission hybrid reactor, a depleted uranium/polyethylene simulation device with alternate shells has been established. The measurement of the 238U(n, 2n) reaction rate was carried out using an activation technique, by measuring the 208 keV ? rays emitted from 237U. The self-absorption of depleted uranium foils with different thicknesses was experimentally corrected. The distribution of the 238U(n, 2n) reaction rate at 90° to the incident D+ beam was obtained, with uncertainty between 5.3% and 6.0%. The experiment was analyzed using MCNP5 code with the ENDF/BVI library, and the calculated results are all about 5% higher than the measured results.

Yan, Xiao-Song; Liu, Rong; Lu, Xin-Xin; Jiang, Li; Wen, Zhong-Wei; Han, Zi-Jie

2012-07-01

360

Experimental determination of the rate constant of the reaction between C2H5O2 and OH radicals  

NASA Astrophysics Data System (ADS)

The rate constant of the reaction has been measured at T = 296 K using laser photolysis coupled to CRDS and LIF. C2H5O2 was generated from the reaction of Cl-atoms with C2H6 at 55 Torr O2, whereby Cl-atoms were generated by 248 nm photolysis of (COCl)2. OH radicals were generated simultaneously by photolysing H2O2. The initial Cl-concentration (and hence C2H5O2) was determined in separate experiments by replacing C2H6 by CH3OH and thus conversion of Cl-atoms to HO2. The time-resolved OH decay was detected in excess of C2H5O2. A fast rate constant of k = (1.2 ± 0.3) × 10-10 cm3 s-1 was determined.

Faragó, Eszter P.; Schoemaecker, Coralie; Viskolcz, Béla; Fittschen, Christa

2015-01-01

361

Investigation of the thermonuclear 18Ne(?,p)21Na reaction rate via resonant elastic scattering of 21Na + p  

NASA Astrophysics Data System (ADS)

The 18Ne(?,p)21Na reaction is thought to be one of the key breakout reactions from the hot CNO cycles to the rp process in type I x-ray bursts. In this work, the resonant properties of the compound nucleus 22Mg have been investigated by measuring the resonant elastic scattering of 21Na + p. An 89-MeV 21Na radioactive beam delivered from the CNS Radioactive Ion Beam Separator bombarded an 8.8 mg/cm2 thick polyethylene (CH2)n target. The 21Na beam intensity was about 2×105 pps, with a purity of about 70% on target. The recoiled protons were measured at the center-of-mass scattering angles of ?c.m.?175.2?, 152.2?, and 150.5? by three sets of ?E-E telescopes, respectively. The excitation function was obtained with the thick-target method over energies Ex(22Mg)=5.5-9.2 MeV. In total, 23 states above the proton-threshold in 22Mg were observed, and their resonant parameters were determined via an R-matrix analysis of the excitation functions. We have made several new J? assignments and confirmed some tentative assignments made in previous work. The thermonuclear 18Ne(?,p)21Na rate has been recalculated based on our recommended spin-parity assignments. The astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the 18Ne(?,p)21Na rate significantly affects the peak nuclear energy generation rate, reaction fluxes, and onset temperature of this breakout reaction in these astrophysical phenomena.

Zhang, L. Y.; He, J. J.; Parikh, A.; Xu, S. W.; Yamaguchi, H.; Kahl, D.; Kubono, S.; Mohr, P.; Hu, J.; Ma, P.; Chen, S. Z.; Wakabayashi, Y.; Wang, H. W.; Tian, W. D.; Chen, R. F.; Guo, B.; Hashimoto, T.; Togano, Y.; Hayakawa, S.; Teranishi, T.; Iwasa, N.; Yamada, T.; Komatsubara, T.; Zhang, Y. H.; Zhou, X. H.

2014-01-01

362

Absolute rate parameters for the reaction of ground state atomic oxygen with carbonyl sulfide. [using O(3P) monitoring  

NASA Technical Reports Server (NTRS)

The rate parameters for the reaction of O(3P) with carbonyl sulfide, O(3P) + OCS yields CO + SO have been determined directly by monitoring O(3P) using the flash photolysis-resonance fluorescence technique. The value for k sub 1 was measured over a temperature range of 263 - 502 K and the data were fitted to an Arrhenuis expression with good linearity.

Klemm, R. B.; Stief, L. J.

1974-01-01

363

A theoretical study of the potential energy surface and rate constant for an O(³P) + HOâ reaction  

Microsoft Academic Search

The potential energy surface for the reaction O(³P) + HOâ â HOâ â HO + Oâ and the kinetic rate constant were investigated using ab initio calculations with the multireference Moeller-Plesset second-order perturbation (MRMP2) method and dynamics calculations based on the microcanonical variational theory. The MRMP2 potential energy surface is compared with those of MP2, QCISD, and B3LYP based on

Osamu Setokuchi; Masaru Sato; Sadao Matuzawa

2000-01-01

364

Au and MN Reaction Rate Measurements for High-Energy Neutron Transport Code Validation for Spallation Sources  

Microsoft Academic Search

Spallation-neutron fluence distributions and integral yields for protons of Ep = 300-590MeV incident on thick Pb\\/Bi targets immersed in a water bath have been determined. The results constitute a data-base pertinent to a well-defined, representative configuration for practicable, high-intensity spallation neutron sources at these intermediate beam energies. They include neutron-fluence distributions based on Au and Mn reaction rates around a

K. van der Meer; H. Aït Abderrahim; S. Dekelver; F. van Gestel; E. Malambu; J. Oeyen; M. Willekens; E. H. Lehmann; M. Daum; Y. Foucher; W. Hajdas; H.-P. Linder; D. Bar; D. Berkovits; I. Mardor; D. Saphier; A. Shor; Y. Yariv; M. B. Goldberg; J. Gerber

2003-01-01

365

EFFECTS OF PORE STRUCTURE CHANGE AND MULTI-SCALE HETEROGENEITY ON CONTAMINANT TRANSPORT AND REACTION RATE UPSCALING  

SciTech Connect

This project addressed the scaling of geochemical reactions to core and field scales, and the interrelationship between reaction rates and flow in porous media. We targeted reactive transport problems relevant to the Hanford site ? specifically the reaction of highly caustic, radioactive waste solutions with subsurface sediments, and the immobilization of 90Sr and 129I through mineral incorporation and passive flow blockage, respectively. We addressed the correlation of results for pore-scale fluid-soil interaction with field-scale fluid flow, with the specific goals of (i) predicting attenuation of radionuclide concentration; (ii) estimating changes in flow rates through changes of soil permeabilities; and (iii) estimating effective reaction rates. In supplemental work, we also simulated reactive transport systems relevant to geologic carbon sequestration. As a whole, this research generated a better understanding of reactive transport in porous media, and resulted in more accurate methods for reaction rate upscaling and improved prediction of permeability evolution. These scientific advancements will ultimately lead to better tools for management and remediation of DOE’s legacy waste problems. We established three key issues of reactive flow upscaling, and organized this project in three corresponding thrust areas. 1) Reactive flow experiments. The combination of mineral dissolution and precipitation alters pore network structure and the subsequent flow velocities, thereby creating a complex interaction between reaction and transport. To examine this phenomenon, we conducted controlled laboratory experimentation using reactive flow-through columns. ? Results and Key Findings: Four reactive column experiments (S1, S3, S4, S5) have been completed in which simulated tank waste leachage (STWL) was reacted with pure quartz sand, with and without Aluminum. The STWL is a caustic solution that dissolves quartz. Because Al is a necessary element in the formation of secondary mineral precipitates (cancrinite), conducting experiments under conditions with and without Al allowed us to experimentally separate the conditions that lead to quartz dissolution from the conditions that lead to quartz dissolution plus cancrinite precipitation. Consistent with our expectations, in the experiments without Al, there was a substantial reduction in volume of the solid matrix. With Al there was a net increase in the volume of the solid matrix. The rate and extent of reaction was found to increase with temperature. These results demonstrate a successful effort to identify conditions that lead to increases and conditions that lead to decreases in solid matrix volume due to reactions of caustic tank wastes with quartz sands. In addition, we have begun to work with slightly larger, intermediate-scale columns packed with Hanford natural sediments and quartz. Similar dissolution and precipitation were observed in these colums. The measurements are being interpreted with reactive transport modeling using STOMP; preliminary observations are reported here. 2) Multi-Scale Imaging and Analysis. Mineral dissolution and precipitation rates within a porous medium will be different in different pores due to natural heterogeneity and the heterogeneity that is created from the reactions themselves. We used a combination of X-ray computed microtomography, backscattered electron and energy dispersive X-ray spectroscopy combined with computational image analysis to quantify pore structure, mineral distribution, structure changes and fluid-air and fluid-grain interfaces. ? Results and Key Findings: Three of the columns from the reactive flow experiments at PNNL (S1, S3, S4) were imaged using 3D X-ray computed microtomography (XCMT) at BNL and analyzed using 3DMA-rock at SUNY Stony Brook. The imaging results support the mass balance findings reported by Dr. Um’s group, regarding the substantial dissolution of quartz in column S1. An important observation is that of grain movement accompanying dissolution in the unconsolidated media. The resultant movement

Lindquist, W. Brent; Jones, Keith W.; Um, Wooyong; Rockhold, mark; Peters, Catherine A.; Celia, Michael A.

2013-02-15

366

Experimental determination of the O17(p,?)N14 and O17(p,?)F18 reaction rates  

NASA Astrophysics Data System (ADS)

The O17(p,?)N14 and O17(p,?)F18 reactions are of major importance to hydrogen-burning nucleosynthesis in a number of different stellar sites. In particular, O17 and F18 nucleosynthesis in classical novae is strongly dependent on the thermonuclear rates of these two reactions. The previously estimated rate for O17(p,?)N14 carries very large uncertainties in the temperature range of classical novae (T=0.01 0.4 GK), whereas a recent measurement has reduced the uncertainty of the O17(p,?)F18 rate. We report on the observation of a previously undiscovered resonance at Ec.m.=183.3 keV in the O17(p,?)N14 reaction, with a measured resonance strength ??p?=(1.6±0.2)×10-3 eV. We studied in the same experiment the O17(p,?)F18 reaction by an activation method, and the resonance strength was found to amount to ??p?=(2.2±0.4)×10-6 eV. The excitation energy of the corresponding level in F18 was determined to be 5789.8±0.3 keV in a Doppler shift attenuation method measurement, which yielded a value of ?<2.6 fs for the level lifetime. The O17(p,?)N14 and O17(p,?)F18 reaction rates were calculated using the measured resonance properties and reconsidering some previous analyses of the contributions of other levels or processes. The O17(p,?)N14 rate is now well established below T=1.5 GK, with uncertainties reduced by orders of magnitude in the temperature range T=0.1 0.4 GK. The uncertainty in the O17(p,?)F18 rate is somewhat larger because of remaining obscurities in the knowledge of the direct capture process. These new resonance properties have important consequences for O17 nucleosynthesis and ?-ray emission of classical novae.

Chafa, A.; Tatischeff, V.; Aguer, P.; Barhoumi, S.; Coc, A.; Garrido, F.; Hernanz, M.; José, J.; Kiener, J.; Lefebvre-Schuhl, A.; Ouichaoui, S.; de Séréville, N.; Thibaud, J.-P.

2007-03-01

367

Application of the re-circulating tracer well test method to determine nitrate reaction rates in shallow unconfined aquifers.  

PubMed

Five re-circulating tracer well tests (RCTWTs) have been conducted in a variety of aquifer settings, at four sites across New Zealand. The tests constitute the first practical assessment of the two-well RCTWT methodology described by Burbery and Wang (Journal of Hydrology, 2010; 382:163-173) and were aimed at evaluating nitrate reaction rates in situ. The performance of the RCTWTs differed significantly at the different sites. The RCTWT method performed well when it was applied to determine potential nitrate reaction rates in anoxic, electro-chemically reductive, nitrate-free aquifers of volcanic lithology, on the North Island, New Zealand. Regional groundwater flow was not fast-flowing in this setting. An effective first-order nitrate reaction rate in the region of 0.09 d(-1) to 0.26 d(-1) was determined from two RCTWTs applied at one site where a reaction rate of 0.37 d(-1) had previously been estimated from a push-pull test. The RCTWT method performed poorly, however, in a fast-flowing, nitrate-impacted fluvio-glacial gravel aquifer that was examined on the South Island, New Zealand. This setting was more akin to the hypothetical physiochemical problem described by Burbery and Wang (2010). Although aerobic conditions were identified as the primary reason for failure to measure any nitrate reaction in the gravel aquifer, failure to establish significant interflow in the re-circulation cell due to the heterogeneous nature of the aquifer structure, and natural variability exhibited in nitrate contaminant levels of the ambient groundwater further contributed to the poor performance of the test. Our findings suggest that in practice, environmental conditions are more complex than assumed by the RCTWT methodology, which compromises the practicability of the method as one for determining attenuation rates in groundwater based on tracing ambient contaminant levels. Although limited, there appears to be a scope for RCTWTs to provide useful information on potential attenuation rates when reactants are supplemented to the aquifer system under examination. PMID:23261905

Burbery, Lee F; Flintoft, Mark J; Close, Murray E

2013-02-01

368

Application of the re-circulating tracer well test method to determine nitrate reaction rates in shallow unconfined aquifers  

NASA Astrophysics Data System (ADS)

Five re-circulating tracer well tests (RCTWTs) have been conducted in a variety of aquifer settings, at four sites across New Zealand. The tests constitute the first practical assessment of the two-well RCTWT methodology described by Burbery and Wang (Journal of Hydrology, 2010; 382:163-173) and were aimed at evaluating nitrate reaction rates in situ. The performance of the RCTWTs differed significantly at the different sites. The RCTWT method performed well when it was applied to determine potential nitrate reaction rates in anoxic, electro-chemically reductive, nitrate-free aquifers of volcanic lithology, on the North Island, New Zealand. Regional groundwater flow was not fast-flowing in this setting. An effective first-order nitrate reaction rate in the region of 0.09 d- 1 to 0.26 d- 1 was determined from two RCTWTs applied at one site where a reaction rate of 0.37 d- 1 had previously been estimated from a push-pull test. The RCTWT method performed poorly, however, in a fast-flowing, nitrate-impacted fluvio-glacial gravel aquifer that was examined on the South Island, New Zealand. This setting was more akin to the hypothetical physiochemical problem described by Burbery and Wang (2010). Although aerobic conditions were identified as the primary reason for failure to measure any nitrate reaction in the gravel aquifer, failure to establish significant interflow in the re-circulation cell due to the heterogeneous nature of the aquifer structure, and natural variability exhibited in nitrate contaminant levels of the ambient groundwater further contributed to the poor performance of the test. Our findings suggest that in practice, environmental conditions are more complex than assumed by the RCTWT methodology, which compromises the practicability of the method as one for determining attenuation rates in groundwater based on tracing ambient contaminant levels. Although limited, there appears to be a scope for RCTWTs to provide useful information on potential attenuation rates when reactants are supplemented to the aquifer system under examination.

Burbery, Lee F.; Flintoft, Mark J.; Close, Murray E.

2013-02-01

369

Partial reactions of the Na,K-ATPase: determination of rate constants  

Microsoft Academic Search

A B S T R A C T Experiments were designed to characterize several partial reactions of the Na,K-ATPase and to demonstrate that a model can be defined that reproduces most of the transport features of the pump with a single set of kinetic parameters. We used the fluorescence label 5-iodoacetamidofluorescein, which is thought to be sensitive to conformational changes,

STEPHAN HEYSE; INGO WUDDEL

1994-01-01

370

Aqueous complexation reactions governing the rate and extent of biogeochemical U(VI) reduction  

SciTech Connect

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments.

Kemner, K.M.; Kelly, S.D.; Brooks, Scott C.; Dong, Wenming; Carroll, Sue; Fredrickson, James K.

2006-06-01

371

Variations of boundary reaction rate and particle size on the diffusion-induced stress in a phase separating electrode  

SciTech Connect

In contrast to the case of single-phase delithiation wherein faster discharging leads to higher diffusion-induced stress (DIS), this paper reports nonmonotonous dependency of the boundary reaction rate on the DIS in nanosized spherical electrode accompanying phase separation. It is attributed to a transition from two-phase to single-phase delithiation driven by increase of the boundary reaction rate leading to narrowing and vanishing of the miscibility gap in a range of the particle size. The profiles of lithium concentration and the DIS are identified during the transition based on a continuum model. The resultant maximum DIS first decreases in the region of two-phase delithiation and later returns to increase in the region of single-phase delithiation with the increase of the boundary reaction rate. A map for the failure behavior in the spherical electrode particle is constructed based on the Tresca failure criterion. These results indicate that the failure caused by the DIS can be avoided by appropriate selection of the said parameters in such electrodes.

Zhang, Lei; He, Linghui; Ni, Yong, E-mail: yni@ustc.edu.cn [CAS Key Laboratory of Mechanical Behavior and Design of Materials, and Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026 (China); Song, Yicheng [Department of Mechanics and Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai University, Shanghai 200444 (China)

2014-10-14

372

Enzymatic hydrolysis of nylons: quantification of the reaction rate of nylon hydrolase for thin-layered nylons.  

PubMed

Nylon hydrolase degrades various aliphatic nylons, including nylon-6 and nylon-66. We synthesized a nylon-66 copolymer (M w?=?22,900, M n?=?7,400), in which a part of an adipoyl unit (32 % molar ratio) of nylon-66 was replaced with a succinyl unit by interfacial polymerization. To quantify the reaction rate of the enzymatic hydrolysis of nylons at the surface of solid polymers, we prepared a thin layer of nylons on the bottom surface of each well in a polystyrene-based micro-assay plate. The thickness of the nylon layer was monitored by imaging analysis of the photographic data. More than 99 % of the copolymer with thicknesses of 260 nm (approximately 600 layers of polymer strands) were converted to water-soluble oligomers by nylon hydrolase (3 mg enzyme ml(-1)) at 30 °C within 60 h. These results were further confirmed by TLC analysis of the reaction products and by assay of liberated amino groups in the soluble fractions. The degradation rate of the thin-layered nylon-6 was similarly analyzed. We demonstrate that this assay enables a quantitative evaluation of the reaction rate of hydrolysis at the interface between the solid and aqueous phases and a quantitative comparison of the degradability for various polyamides. PMID:24962117

Nagai, Keisuke; Iida, Kazuki; Shimizu, Kimiaki; Kinugasa, Ryo; Izumi, Motoki; Kato, Dai-Ichiro; Takeo, Masahiro; Mochiji, Kozo; Negoro, Seiji

2014-10-01

373

Rate Coefficients for Reaction of OH with Acetone between 202 and 395 K M. Wollenhaupt, S. A. Carl, A. Horowitz, and J. N. Crowley*  

E-print Network

Rate Coefficients for Reaction of OH with Acetone between 202 and 395 K M. Wollenhaupt, S. A. Carl. The accuracy of the rate constants obtained was enhanced by optical absorption measurements of acetone measurement and implies greater significance for the reaction with OH as a sink for acetone in the upper

Peinke, Joachim

374

Reactions between ?-lactoglobulin and genipin: kinetics and characterization of the products.  

PubMed

In this paper, we present the first detailed study of the reaction kinetics and the characterization of the products from the endothermic reactions between ?-lactoglobulin and genipin. The effects of the concentration, temperature, and pH were investigated. In the temperature range studied, the reaction was approximately a pseudo-first-order with respect to genipin and 0.22-order and -0.24-order with respect to ?-lactoglobulin for pH 6.75 and 10.5 with corresponding activation energy (E(a)) estimated to be 66.2 ± 3.8 and 9.40 ± 0.36 kJ/mol, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis studies, validated by matrix-assisted laser desorption ionization-time of flight mass spectrometry, showed the presence of oligomeric, i.e., di-, tri-, quadri-, and pentameric, forms of cross-linked ?-lactoglobulin by genipin at neutral but not alkaline pH; however, an extensive cross-linked network was not observed, consistent with the atomic force microscopy images. It was demonstrated that the reaction temperature and the concentration of genipin but not that of ?-lactoglobulin positively affected the extent of the cross-linking reactions. PMID:22463374

Qi, Phoebe X; Nuñez, Alberto; Wickham, Edward D

2012-05-01

375

Iodine atom production rates by electron impact versus post discharge reactions for pulsed COIL  

NASA Astrophysics Data System (ADS)

Pulsed discharge is effective means to achieve high-peak lasing in COIL. Numerical model is developed for simulation of pulsed discharge in gas stream from the singlet oxygen generator mixed with CF3I. The model comprises a system of kinetic equations for neutral and charged species, electric circuit equation, and gas thermal balance equation. Sources of iodine atoms under discharge and post-discharge conditions are analyzed. The dominant source in the discharge is electron-impact dissociation of CF3I molecules. In post-discharge phase chemical reactions are identified giving notable input into I production. Deformation of laser pulse waveform observed experimentally is explained by influence of these reactions.

Napartovich, A. P.; Kochetov, I. V.; Vagin, N. P.; Yuryshev, N. N.

2009-02-01

376

Material interactions with the Low Earth Orbital (LEO) environment: Accurate reaction rate measurements  

NASA Technical Reports Server (NTRS)

To resolve uncertainties in estimated LEO atomic oxygen fluence and provide reaction product composition data for comparison to data obtained in ground-based simulation laboratories, a flight experiment has been proposed for the space shuttle which utilizes an ion-neutral mass spectrometer to obtain in-situ ambient density measurements and identify reaction products from modeled polymers exposed to the atomic oxygen environment. An overview of this experiment is presented and the methodology of calibrating the flight mass spectrometer in a neutral beam facility prior to its use on the space shuttle is established. The experiment, designated EOIM-3 (Evaluation of Oxygen Interactions with Materials, third series), will provide a reliable materials interaction data base for future spacecraft design and will furnish insight into the basic chemical mechanisms leading to atomic oxygen interactions with surfaces.

Visentine, James T.; Leger, Lubert J.

1987-01-01

377

Heart Rate and Blood Pressure Quantitative Trait Loci for the Airpuff Startle Reaction  

Microsoft Academic Search

The airpuff startle reaction is a probe of sensori-autonomic processing and is useful for studies of genetic control of stress-induced cardiovascular activity. Using a Wistar-Kyoto-Spontaneously Hypertensive Rat F2 cross, we reported an airpuff-elicited strain-dependent and trial-dependent bradycardia, the absence of which cosegregated with hyperten- sion. Here, we use the mapping power of the HXB-BXH recombinant inbred rat strains (n23) to

Rebecca L. Jaworski; Martin Jirout; Shamara Closson; Laura Breen; Pamela L. Flodman; M. Anne Spence; Vladimir Kren; Drahomira Krenova; Michal Pravenec; Morton P. Printz

378

Temperature Profiles in Endothermic and Exothermic Reactions and the Interpretation of Experimental Rate Data  

Microsoft Academic Search

Any endothermic or exothermic reaction is accompanied by self-cooling or self-heating. In reacting systems in which heat transfer is controlled by conduction, non-uniform temperature-position profiles are established. Examples of this situation are the exothermic decomposition of gaseous diethyl peroxide and the endothermic decomposition of nitrosyl chloride at low pressures (when convection is unimportant). In kinetic studies, allowance must be made

T. Boddington; P. Gray

1970-01-01

379

The Q-K model for gas-phase chemical reaction rates  

Microsoft Academic Search

The quantum-kinetic, or Q-K, model is based on the quantum vibration model that is employed in the computation of gas flows at the molecular level by the direct simulation Monte Carlo (DSMC) method. The Q-K procedure for dissociation is physically realistic within the context of the vibration model in that the reaction occurs upon the selection of the vibrational level

G. A. Bird

2011-01-01

380

Temperature dependence of the rate constant and branching ratio for the OH + ClO reaction  

E-print Network

attempts to model ozone levels in the upper strato- sphere have resulted in an overprediction of ozone loss as 40%,7 possibly indicating that the models are missing a source of HCl production in the stratosphere of the OH ] ClO reaction (1b) as a source of HCl production in the models.2,3,5,6,8 In the most recent study

Elrod, Matthew J.

381

A Kinetic and Product Study of the Cl + HO2 Reaction  

NASA Technical Reports Server (NTRS)

Absolute rate data and product branching ratios for the reactions Cl + HO2 to HCl + O2 (k1a) and Cl + HO2 to OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10-11 exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10-11 exp[(123 +/- 15)/T] cm3 molecule-1 s-1, respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10-11 cm3 molecule-1 s-1. Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10-11 exp[-(708 +/- 29)/T] cm3 molecule-1 s-1. These expressions for k1a and k1b are valid for 226 K T 336 and 256 K T 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K T 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K T 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10-11 exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10-11 exp[-(733 +/- 41)/T] cm3 molecule-1 s-1, respectively. These expressions for k1a and k1b are valid for 256 K T 296 and 226 K T 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10-11 exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10-11 exp[-(801 +/- 94)/T] cm3 molecule-1 s-1, respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 to OH for detection purposes.

Hickson, Kevin M.; Keyser, Leon F.

2005-01-01

382

Solubility and Reaction Rates of Aluminum Solid Phases Under Geothermal Conditions  

SciTech Connect

Experimental studies involving equilibrium solubility and dissolution/precipitation rates were initiated on aluminum hydroxide phases prevalent under geothermal reservoir conditions. A large capacity, hydrogen-electrode concentration cell (HECC) was constructed specifically for this purpose.

Benezeth, P.; Palmer, D.A.; Wesolowski, D.J.; Anovitz, L.M.

2000-05-28

383

Controlling hydrolysis reaction rates with binary ionic liquid mixtures by tuning hydrogen-bonding interactions.  

PubMed

The ability of a binary ionic liquid (IL) system consisting of a phosphonium transition state analogue (TSA) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf(2)]) to accelerate the rate of the well-studied hydrolysis of a tert-alkyl picolinium salt by influencing the solvent structure was investigated. A significant rate enhancement was observed in the presence of the TSA; however, comparison with other cations illustrated that this enhancement was not unique to the chosen TSA. Instead, the rate enhancements were correlated with the dilution of hydrogen bonding by the added cations. This phenomenon was further examined by the use of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BMMIM][NTf(2)]) as a cosolvent and the use of Reichardt's dye to measure the extent of hydrogen bonding on solutes in these systems. The rate increases are rationalized in terms of weaker hydrogen bonding from the solvent system to water. PMID:22263886

Weber, Cameron C; Masters, Anthony F; Maschmeyer, Thomas

2012-02-16

384

New constraints on the 18F(p,?)15O rate in novae from the (d,p) reaction  

NASA Astrophysics Data System (ADS)

The degree to which the (p,?) and (p,?) reactions destroy 18F at temperatures (1-4)×108K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of ?-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (Ex=6.411MeV in 19Ne). To gain further information about these resonances, we used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse 2H(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0 9MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,?)19Ne and 18F(p,?)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

Kozub, R. L.; Bardayan, D. W.; Batchelder, J. C.; Blackmon, J. C.; Brune, C. R.; Champagne, A. E.; Cizewski, J. A.; Davinson, T.; Greife, U.; Gross, C. J.; Jewett, C. C.; Livesay, R. J.; Ma, Z.; Moazen, B. H.; Nesaraja, C. D.; Sahin, L.; Scott, J. P.; Shapira, D.; Smith, M. S.; Thomas, J. S.; Woods, P. J.

2005-03-01

385

New Constraints on the 18F(p,alpha) 15O Rate in Novae from the (d,p) Reaction  

E-print Network

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

R. L. Kozub; D. W. Bardayan; J. C. Batchelder; J. C. Blackmon; C. R. Brune; A. E. Champagne; J. A. Cizewski; T. Davinson; U. Greife; C. J. Gross; C. C. Jewett; R. J. Livesay; Z. Ma; B. H. Moazen; C. D. Nesaraja; L. Sahin; J. P. Scott; D. Shapira; M. S. Smith; J. S. Thomas; P. J. Woods

2004-05-27

386

An investigation of the effect of pore scale flow on average geochemical reaction rates using direct numerical simulation  

SciTech Connect

The scale-dependence of geochemical reaction rates hinders their use in continuum scale models intended for the interpretation and prediction of chemical fate and transport in subsurface environments such as those considered for geologic sequestration of CO{sub 2}. Processes that take place at the pore scale, especially those involving mass transport limitations to reactive surfaces, may contribute to the discrepancy commonly observed between laboratory-determined and continuum-scale or field rates. Here, the dependence of mineral dissolution rates on the pore structure of the porous media is investigated by means of pore scale modeling of flow and multicomponent reactive transport. The pore scale model is composed of high-performance simulation tools and algorithms for incompressible flow and conservative transport combined with a general-purpose multicomponent geochemical reaction code. The model performs direct numerical simulation of reactive transport based on an operator-splitting approach to coupling transport and reactions. The approach is validated with a Poiseuille flow single-pore experiment and verified with an equivalent 1-D continuum-scale model of a capillary tube packed with calcite spheres. Using the case of calcite dissolution as an example, the high-resolution model is used to demonstrate that nonuniformity in the flow field at the pore scale has the effect of decreasing the overall reactivity of the system, even when systems with identical reactive surface area are considered. The effect becomes more pronounced as the heterogeneity of the reactive grain packing increases, particularly where the flow slows sufficiently such that the solution approaches equilibrium locally and the average rate becomes transport-limited.

Molins, Sergi; Trebotich, David; Steefel, Carl; Shen, Chaopeng

2012-01-01

387

An investigation of the effect of pore scale flow on average geochemical reaction rates using direct numerical simulation  

SciTech Connect

The scale-dependence of geochemical reaction rates hinders their use in continuum scale models intended for the interpretation and prediction of chemical fate and transport in subsurface environments such as those considered for geologic sequestration of CO{sub 2}. Processes that take place at the pore scale, especially those involving mass transport limitations to reactive surfaces, may contribute to the discrepancy commonly observed between laboratory-determined and continuum-scale or field rates. Here, the dependence of mineral dissolution rates on the pore structure of the porous media is investigated by means of pore scale modeling of flow and multicomponent reactive transport. The pore scale model is comprised of high performance simulation tools and algorithms for incompressible flow and conservative transport combined with a general-purpose multicomponent geochemical reaction code. The model performs direct numerical simulation of reactive transport based on an operator-splitting approach to coupling transport and reactions. The approach is validated with a Poiseuille flow single-pore experiment and verified with an equivalent 1D continuum-scale model of a capillary tube packed with calcite spheres. Using the case of calcite dissolution as an example, the high resolution model is used to demonstrate that non-uniformity in the flow field at the pore scale has the effect of decreasing the overall reactivity of the system, even when systems with identical reactive surface area are considered. The effect becomes more pronounced as the heterogeneity of the reactive grain packing increases, particularly where the flow slows sufficiently such that the solution approaches equilibrium locally and the average rate becomes transport-limited.

Rafa, S. Molins; Trebotich, D.; Steefel, C. I.; Shen, C.

2012-02-01

388

Aroxyl-radical-scavenging rate increases remarkably under the coexistence of ?-tocopherol and ubiquinol-10 (or vitamin C): finding of synergistic effect on the reaction rate.  

PubMed

Measurements of aroxyl radical (ArO(•))-scavenging rate constants (ks(AOH)) of antioxidants (AOHs) (?-tocopherol (?-TocH), ubiquinol-10 (UQ10H2), and sodium ascorbate (Na(+)AsH(-))) were performed in 2-propanol/water (2-PrOH/H2O, 5/1, v/v) solution using stopped-flow spectrophotometry. ks(AOH) values were measured not only for each AOH but also for the mixtures of two AOHs ((i) ?-TocH and UQ10H2 and (ii) ?-TocH and Na(+)AsH(-)). A notable synergistic effect that the ks(AOH) values increase 1.6, 2.5, and 6.8 times for ?-TocH, UQ10H2, and Na(+)AsH(-), respectively, was observed for the solutions including two kinds of AOHs. Furthermore, measurements of the regeneration rates of ?-tocopheroxyl radical (?-Toc(•)) to ?-TocH by UQ10H2 and Na(+)AsH(-) were performed in 2-PrOH/H2O using double-mixing stopped-flow spectrophotometry. Second-order rate constants (kr) obtained for UQ10H2 and Na(+)AsH(-) were 2.01 × 10(5) and 1.19 × 10(6) M(-1) s(-1), respectively. In fact, UV-vis absorption of ?-Toc(•) (?max = 428 nm), which had been produced by reaction of ?-TocH with ArO(•), disappeared under the existence of UQ10H2 or Na(+)AsH(-) due to the above fast regeneration reaction. The result indicates that the prooxidant effect of ?-Toc(•) is suppressed by the coexistence of UQ10H2 or Na(+)AsH(-). As ?-TocH, UQ10H2, and ascorbate monoanion (AsH(-)) coexist in relatively high concentrations in plasma, blood, and various tissues, the above synergistic effect, that is, the increase of the free-radical-scavenging rate and suppression of the prooxidant reaction, may function in biological systems. PMID:23777398

Mukai, Kazuo; Ouchi, Aya; Nakaya, Saori; Nagaoka, Shin-ichi

2013-07-18

389

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution  

Microsoft Academic Search

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was

Z. Y. Liu; X. G. Li; Y. F. Cheng

2011-01-01

390

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution  

Microsoft Academic Search

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was

Z. Y. Liu; X. G. Li; Y. F. Cheng

2010-01-01

391

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution  

Microsoft Academic Search

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH\\u000a solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as\\u000a well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain\\u000a rate in electrochemical cathodic reaction. It was

Z. Y. Liu; X. G. Li; Y. F. Cheng

392

Ozone-induced dissociation of conjugated lipids reveals significant reaction rate enhancements and characteristic odd-electron product ions.  

PubMed

Ozone-induced dissociation (OzID) is an alternative ion activation method that relies on the gas phase ion-molecule reaction between a mass-selected target ion and ozone in an ion trap mass spectrometer. Herein, we evaluated the performance of OzID for both the structural elucidation and selective detection of conjugated carbon-carbon double bond motifs within lipids. The relative reactivity trends for [M + X](+) ions (where X = Li, Na, K) formed via electrospray ionization (ESI) of conjugated versus nonconjugated fatty acid methyl esters (FAMEs) were examined using two different OzID-enabled linear ion-trap mass spectrometers. Compared with nonconjugated analogues, FAMEs derived from conjugated linoleic acids were found to react up to 200 times faster and to yield characteristic radical cations. The significantly enhanced reactivity of conjugated isomers means that OzID product ions can be observed without invoking a reaction delay in the experimental sequence (i.e., trapping of ions in the presence of ozone is not required). This possibility has been exploited to undertake neutral-loss scans on a triple quadrupole mass spectrometer targeting characteristic OzID transitions. Such analyses reveal the presence of conjugated double bonds in lipids extracted from selected foodstuffs. Finally, by benchmarking of the absolute ozone concentration inside the ion trap, second order rate constants for the gas phase reactions between unsaturated organic ions and ozone were obtained. These results demonstrate a significant influence of the adducting metal on reaction rate constants in the fashion Li > Na > K. PMID:23292977

Pham, Huong T; Maccarone, Alan T; Campbell, J Larry; Mitchell, Todd W; Blanksby, Stephen J

2013-02-01

393

Ozone-Induced Dissociation of Conjugated Lipids Reveals Significant Reaction Rate Enhancements and Characteristic Odd-Electron Product Ions  

NASA Astrophysics Data System (ADS)

Ozone-induced dissociation (OzID) is an alternative ion activation method that relies on the gas phase ion-molecule reaction between a mass-selected target ion and ozone in an ion trap mass spectrometer. Herein, we evaluated the performance of OzID for both the structural elucidation and selective detection of conjugated carbon-carbon double bond motifs within lipids. The relative reactivity trends for [M + X]+ ions (where X = Li, Na, K) formed via electrospray ionization (ESI) of conjugated versus nonconjugated fatty acid methyl esters (FAMEs) were examined using two different OzID-enabled linear ion-trap mass spectrometers. Compared with nonconjugated analogues, FAMEs derived from conjugated linoleic acids were found to react up to 200 times faster and to yield characteristic radical cations. The significantly enhanced reactivity of conjugated isomers means that OzID product ions can be observed without invoking a reaction delay in the experimental sequence (i.e., trapping of ions in the presence of ozone is not required). This possibility has been exploited to undertake neutral-loss scans on a triple quadrupole mass spectrometer targeting characteristic OzID transitions. Such analyses reveal the presence of conjugated double bonds in lipids extracted from selected foodstuffs. Finally, by benchmarking of the absolute ozone concentration inside the ion trap, second order rate constants for the gas phase reactions between unsaturated organic ions and ozone were obtained. These results demonstrate a significant influence of the adducting metal on reaction rate constants in the fashion Li > Na > K.

Pham, Huong T.; Maccarone, Alan T.; Campbell, J. Larry; Mitchell, Todd W.; Blanksby, Stephen J.

2013-02-01

394

Polycyclic aromatic hydrocarbon reaction rates with peroxy-acid treatment: prediction of reactivity using local ionization potential.  

PubMed

Property-Encoded Surface Translator (PEST) descriptors were found to be correlated with the degradation rates of polycyclic aromatic hydrocarbons (PAHs) by the peroxy-acid process. Reaction rate constants (k) in hr(-1) for nine PAHs (acenaphthene, anthracene, benzo[a]pyrene, benzo[k]fluoranthene, fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene) were determined by a peroxy-acid treatment method that utilized acetic acid, hydrogen peroxide, and a sulphuric acid catalyst to degrade the polyaromatic structures. Molecular properties of the selected nine PAHs were derived from structures optimized at B3LYP/6-31G(d) and HF/6-31G(d) levels of theory. Properties of adiabatic and vertical ionization potential (IP), highest occupied molecular orbitals (HOMO), HOMO/lowest unoccupied molecular orbital (LUMO) gap energies and HOMO/singly occupied molecular orbital (SOMO) gap energies were not correlated with rates of peroxy-acid reaction. PEST descriptors were calculated from B3LYP/6-31G(d) optimized structures and found to have significant levels of correlation with k. PIP Min described the minimum local IP on the surface of the molecule and was found to be related to k. PEST technology appears to be an accurate method in predicting reactivity and could prove to be a valuable asset in building treatment models and in remediation design for PAHs and other organic contaminants in the environment. PMID:23734862

Shoulder, J M; Alderman, N S; Breneman, C M; Nyman, M C

2013-08-01

395

Accurate prediction of rate constants of Diels-Alder reactions and application to design of Diels-Alder ligation.  

PubMed

Bioorthogonal reactions are useful tools to gain insights into the structure, dynamics, and function of biomolecules in the field of chemical biology. Recently, the Diels-Alder reaction has become a promising and attractive procedure for ligation in bioorthogonal chemistry because of its higher rate and selectivity in water. However, a drawback of the previous Diels-Alder ligation is that the widely used maleimide moiety as a typical Michael acceptor can readily undergo Michael addition with nucleophiles in living systems. Thus, it is important to develop a nucleophile-tolerant Diels-Alder system in order to extend the scope of the application of Diels-Alder ligation. To solve this problem, we found that the theoretical protocol M06-2X/6-31+G(d)//B3LYP/6-31G(d) can accurately predict the activation free energies of Diels-Alder reactions with a precision of 1.4 kcal mol(-1) by benchmarking the calculations against the 72 available experimental data. Subsequently, the electronic effect and ring-strain effect on the Diels-Alder reaction were studied to guide the design of the new dienophiles. The criteria of the design is that the designed Diels-Alder reaction should have a lower barrier than the Michael addition, while at the same time it should show a similar (or even higher) reactivity as compared to the maleimide-involving Diels-Alder ligation. Among the designed dienophiles, three substituted cyclopropenes (i.e. 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes) meet our requirements. These substituted cyclopropene analogs could be synthesized and they are thermodynamically stable. As a result, we propose that 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes may be potential candidates for efficient and selective Diels-Alder ligation in living systems. PMID:22370563

Tang, Shi-Ya; Shi, Jing; Guo, Qing-Xiang

2012-04-01

396

Determination of the Rate Coefficients of the SO2 plus O plus M yields SO3 plus M Reaction  

NASA Technical Reports Server (NTRS)

Rate coefficients of the title reaction R(sub 31) (SO2 +O+M yields SO3 +M) and R(sub 56) (SO2 + HO2 yields SO3 +OH), important in the conversion of S(IV) to S(VI),were obtained at T =970-1150 K and rho (sub ave) = 16.2 micro mol/cubic cm behind reflected shock waves by a perturbation method. Shock-heated H2/ O2/Ar mixtures were perturbed by adding small amounts of SO2 (1%, 2%, and 3%) and the OH temporal profiles were then measured using laser absorption spectroscopy. Reaction rate coefficients were elucidated by matching the characteristic reaction times acquired from the individual experimental absorption profiles via simultaneous optimization of k(sub 31) and k(sub 56) values in the reaction modeling (for satisfactory matches to the observed characteristic times, it was necessary to take into account R(sub 56)). In the experimental conditions of this study, R(sub 31) is in the low-pressure limit. The rate coefficient expressions fitted using the combined data of this study and the previous experimental results are k(sub 31,0)/[Ar] = 2.9 10(exp 35) T(exp ?6.0) exp(?4780 K/T ) + 6.1 10(exp 24) T(exp ?3.0) exp(?1980 K/T ) cm(sup 6) mol(exp ?2)/ s at T = 300-2500 K; k(sub 56) = 1.36 10(exp 11) exp(?3420 K/T ) cm(exp 3)/mol/s at T = 970-1150 K. Computer simulations of typical aircraft engine environments, using the reaction mechanism with the above k(sub 31,0) and k(sub 56) expressions, gave the maximum S(IV) to S(VI) conversion yield of ca. 3.5% and 2.5% for the constant density and constant pressure flow condition, respectively. Moreover, maximum conversions occur at rather higher temperatures (?1200 K) than that where the maximum k(sub 31,0) value is located (approximately 800 K). This is because the conversion yield is dependent upon not only the k(sup 31,0) and k(sup 56) values (production flux) but also the availability of H, O, and HO2 in the system (consumption flux).

Hwang, S. M.; Cooke, J. A.; De Witt, K. J.; Rabinowitz, M. J.

2010-01-01

397

Reaction mechanism for cocaine esterase-catalyzed hydrolyses of (+)- and (-)-cocaine: unexpected common rate-determining step.  

PubMed

First-principles quantum mechanical/molecular mechanical free energy calculations have been performed to examine the catalytic mechanism for cocaine esterase (CocE)-catalyzed hydrolysis of (+)-cocaine in comparison with CocE-catalyzed hydrolysis of (-)-cocaine. It has been shown that the acylation of (+)-cocaine consists of nucleophilic attack of the hydroxyl group of Ser117 on the carbonyl carbon of (+)-cocaine benzoyl ester and the dissociation of (+)-cocaine benzoyl ester. The first reaction step of deacylation of (+)-cocaine, which is identical to that of (-)-cocaine, is rate-determining, indicating that CocE-catalyzed hydrolyses of (+)- and (-)-cocaine have a common rate-determining step. The computational results predict that the catalytic rate constant of CocE against (+)-cocaine should be the same as that of CocE against (-)-cocaine, in contrast with the remarkable difference between human butyrylcholinesterase-catalyzed hydrolyses of (+)- and (-)-cocaine. The prediction has been confirmed by experimental kinetic analysis on CocE-catalyzed hydrolysis of (+)-cocaine in comparison with CocE-catalyzed hydrolysis of (-)-cocaine. The determined common rate-determining step indicates that rational design of a high-activity mutant of CocE should be focused on the first reaction step of the deacylation. Furthermore, the obtained mechanistic insights into the detailed differences in the acylation between the (+)- and (-)-cocaine hydrolyses provide indirect clues for rational design of amino acid mutations that could more favorably stabilize the rate-determining transition state in the deacylation and, thus, improve the catalytic activity of CocE. This study provides a valuable mechanistic base for rational design of an improved esterase for therapeutic treatment of cocaine abuse. PMID:21486046

Liu, Junjun; Zhao, Xinyun; Yang, Wenchao; Zhan, Chang-Guo

2011-05-01

398

Direct measurement of neon production rates by (?,n) reactions in minerals  

NASA Astrophysics Data System (ADS)

The production of nucleogenic neon from alpha particle capture by 18O and 19F offers a potential chronometer sensitive to temperatures higher than the more widely used (U-Th)/He chronometer. The accuracy depends on the cross sections and the calculated stopping power for alpha particles in the mineral being studied. Published 18O(?,n)21Ne production rates are in poor agreement and were calculated from contradictory cross sections, and therefore demand experimental verification. Similarly, the stopping powers for alpha particles are calculated from SRIM (Stopping Range of Ions in Matter software) based on a limited experimental dataset. To address these issues we used a particle accelerator to implant alpha particles at precisely known energies into slabs of synthetic quartz (SiO2) and barium tungstate (BaWO4) to measure 21Ne production from capture by 18O. Within experimental uncertainties the observed 21Ne production rates compare favorably to our predictions using published cross sections and stopping powers, indicating that ages calculated using these quantities are accurate at the ?3% level. In addition, we measured the 22Ne/21Ne ratio and (U-Th)/He and (U-Th)/Ne ages of Durango fluorapatite, which is an important model system for this work because it contains both oxygen and fluorine. Finally, we present 21Ne/4He production rate ratios for a variety of minerals of geochemical interest along with software for calculating neon production rates and (U-Th)/Ne ages.

Cox, Stephen E.; Farley, Kenneth A.; Cherniak, Daniele J.

2015-01-01

399

Acidbase chemical reaction model for nucleation rates in the polluted atmospheric boundary layer  

E-print Network

- spheric boundary layer are positively correlated with concentra- tions of sulfuric acid vapor. However or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model

400

Convective mixing and chemical reactions in microchannels with high flow rates  

Microsoft Academic Search

This work presents a theoretical and experimental investigation of convective micromixing in various mixer structures and combinations with the aim of high mixing intensity and a high throughput. Different mixing elements are integrated on a silicon chip to achieve a device for high flow rates above 20kg\\/h. Test structures are fabricated and characterized according to their flow behavior and mixing

Norbert Kockmann; Thomas Kiefer; Michael Engler; Peter Woias

2006-01-01

401

Measurements of Fission and Radioactive Capture Reaction Rates Inside the Fuel of the Ipen/MB-01  

NASA Astrophysics Data System (ADS)

This work presents the measures of the nuclear reaction rates along the radial direction of the fuel pellet by irradiation and posterior gamma spectrometry of a thin slice of fuel pellet of UO2 at 4.3% enrichment. From its irradiation, the rate of radioactive capture and fission had been measured as a function of the radius of the pellet disk using a Ortec GMX HPGe detector. Lead collimators had been used for this purpose. Simulating the fuel pellet in the pin fuel of the IPEN/MB-01 reactor, a thin UO2 disk is used, being inserted in the interior of a dismountable fuel rod. This fuel rod is then placed in the central position of the IPEN/MB-01 reactor core and irradiated during 1 h under a neutron flux of 5 ×108 n/cm2 s. In gamma spectrometry, 10 collimators with different diameters have been used; consequently, the nuclear reactions of radioactive capture that occurs in atoms of 238U and the fission that occurs on both 235U and 238U are measured in function of 10 different regions (diameter of collimator) of the UO2 fuel pellet disk. Nuclear fission produces different fission products such as 143Ce with a yield fission of 5.9% which decay is monitored in this work. Corrections in geometric efficiency due to introduction of collimators on HPGe detection system were estimated using photon transport of MCNP-4C code. Some calculated values of nuclear reaction rate of radioactive capture and fission along the radial direction of the fuel pellet obtained by Monte Carlo methodology, using the MCNP-4C code, are presented and compared to the experimental data showing very good agreement.

Mura, Luís Felipe L.; Bitelli, Ulysses d'Utra; Fanaro, Leda C. C. B.

2011-05-01

402

Rate coefficients for the gas-phase reaction of chlorine atoms with a series of methoxylated aromatic compounds.  

PubMed

The reaction of a series of oxygenated aromatics (two methoxybenzene and six methoxyphenol isomers) with chlorine atoms has been studied in two simulation chambers with volumes of 1080 and 480 L at the University of Wuppertal. Experiments were performed at 295 ± 2 K and a total pressure of synthetic air of 1 bar using the relative kinetic method with in situ Fourier transform infrared spectroscopy for chemical analysis. The following rate coefficients (in units of cubic centimeter per molecule per second) were determined: (1.07 ± 0.24) × 10(-10) for methoxybenzene, (1.20 ± 0.24) × 10(-10) for 1-methoxy-2-methylbenzene, (2.97 ± 0.66) × 10(-10) for 2-methoxyphenol (guaiacol), (2.99 ± 0.62) × 10(-10) for 3-methoxyphenol, (2.86 ± 0.58) × 10(-10) for 4-methoxyphenol, (3.35 ± 0.68) × 10(-10) for 2-methoxy-4-methylphenol, (4.73 ± 1.06) × 10(-10) for 2,3-dimethoxyphenol, and (2.71 ± 0.61) × 10(-10) for 2,6-dimethoxyphenol (syringol). To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of the chlorine atoms with the methoxy-aromatic compounds investigated. The reactivity of the methoxylated aromatics toward Cl is compared with that of other substituted aromatic compounds, and the differences in the rate coefficients are interpreted in terms of the type, number, and position of the different substituents on the aromatic ring. The atmospheric implications of the studied reactions are also discussed. PMID:24571131

Lauraguais, Amélie; Bejan, Iustinian; Barnes, Ian; Wiesen, Peter; Coeur-Tourneur, Cécile; Cassez, Andy

2014-03-13

403

On the introduction of {sup 17}O+p reaction rates evaluated through the THM in AGB nucleosynthesis calculations  

SciTech Connect

The rates for the {sup 17}O(p,??{sup 14}N, {sup 17}O(p,?){sup 18}F and {sup 18}O(p,?){sup 15}N reactions deduced trough the Trojan Horse Method (THM) have been introduced into a state-of-the-art asymptotic giant branch (AGB) models for proton-capture nucleosynthesis and cool bottom process. The predicted abundances have been compared with isotopic compositions provided by geochemical analysis of presolar grains. As a result, an improved agreement is found between the models and the isotopic mix of oxide grains of AGB origins, whose composition is the signature of low-temperature proton-capture nucleosynthesis.

Palmerini, S.; Sergi, M. L.; La Cognata, M.; Pizzone, R. G. [I.N.F.N. Laboratori Nazionali del Sud, via Santa Sofia 62, Catania (Italy); Lamia, L.; Spitaleri, C. [Dipartimento di Fisica e Astronomia, Universitá degli Studi di Catania (Italy)

2014-05-09

404

The loss rates of O{sup +} in the inner magnetosphere caused by both magnetic field line curvature scattering and charge exchange reactions  

SciTech Connect

With consideration of magnetic field line curvature (FLC) pitch angle scattering and charge exchange reactions, the O{sup +} (>300?keV) in the inner magnetosphere loss rates are investigated by using an eigenfunction analysis. The FLC scattering provides a mechanism for the ring current O{sup +} to enter the loss cone and influence the loss rates caused by charge exchange reactions. Assuming that the pitch angle change is small for each scattering event, the diffusion equation including a charge exchange term is constructed and solved; the eigenvalues of the equation are identified. The resultant loss rates of O{sup +} are approximately equal to the linear superposition of the loss rate without considering the charge exchange reactions and the loss rate associated with charge exchange reactions alone. The loss time is consistent with the observations from the early recovery phases of magnetic storms.

Ji, Y., E-mail: yji@spaceweather.ac.cn [State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190 (China); College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049 (China); Shen, C. [State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190 (China)] [State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190 (China)

2014-03-15

405

Measurement and estimation of rate constants for the reactions of hydroxyl radical with several alkanes and cycloalkanes.  

PubMed

Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with isobutane, isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,3,4-trimethylpentane, n-heptane, n-octane, cyclopentane, cyclohexane, and cycloheptane. The results have been used to calibrate a structure-reactivity rate constant estimation method for k(298 K) which, when combined with previously determined relationships between k(298 K) and the Arrhenius parameters, is capable of determining the temperature dependence accurately. The estimation method reproduces most of the observed rate data within experimental accuracy but appears to fail for 2,3-dimethylbutane, which has an anomalously high rate constant. Curvature in the Arrhenius plots at low temperatures is not present for compounds with a single type of C-H bond and, for compounds with different C-H bonds, is shown to be consistent with effects due to different group sites on the molecule. PMID:16526640

Wilson, Edmond W; Hamilton, Wesley A; Kennington, Hillary R; Evans, Bill; Scott, Nathan W; DeMore, William B

2006-03-16

406

Reaction of lymphoid organs to laser radiation with different pulsation rates  

NASA Astrophysics Data System (ADS)

Experimental studies were performed on 220 male rats of Wistar line to reveal optimal parameters of laser radiation causing positive changes in biotissues and to select methods of laser therapy. Irradiation of the ventral abdominal wall performed by arsenide-gallium injector (710 - 890 nm, exposure - 128 sec) in pulse rate: 3000 Hz, 1500 Hz, 80 Hz. Content of lymphoblasts, medium and small lymphocytes, plasmocytes, T-lymphocytes and T-helpers as well as the activity of chromatin and lysosomal enzymes were determined in the dynamics of thymus, spleen and lymph nodes. During irradiation with the rate of 3000 Hz prevailing inhibiting influence on the immumocytopoesis and functional activity of lymphocytes in all organs studied was state, the effect being manifested by the decrease in the number of all forms of lymphocytes particular on the 3rd-5th-7th day followed by normalization on the 15th- 21st-30th day. Irradiation with the rate of 1500 Hz produced stimulating effect on the immune organs accompanied by reliable excess of control indices of lymphocyte content particularly of poorly differentiated forms (blasts and medium ones), as well as by the increase of the number of plasmocytes, T-lymphocytes, T-helpers with maximum manifestation on the 7th day. On the 15th day there is a decrease, and on the 21st-30th day--there is normalization. Irradiation with the rate of 80 Hz produced the smallest but most marked effect, particularly on the number of lymphoblasts. Peculiarities in kinetics of cellular elements studied were revealed in different lymphoid organs and in different functional zones of these organs.

Kapinosov, Ivan K.; Bugaeva, Irine O.; Kolokolov, George R.; Provozina, Helen J.

1996-05-01

407

Effects of Triple-$\\alpha$ and $^{12}\\rm C(\\alpha,\\gamma)^{16}O$ Reaction Rates on the Supernova Nucleosynthesis in a Massive Star of 25 $M_{\\odot}$  

E-print Network

We investigate effects of triple-$\\alpha$ and $^{12}\\rm C(\\alpha,\\gamma) ^{16}O$ reaction rates on the production of supernova yields for a massive star of 25 $M_{\\odot}$ whose helium core is considered to be around a 8 $M_{\\odot}$. We combine the reaction rates to see the rate dependence, where the rates are considered to cover the possible variation of the rates. We adopt four combinations of the reaction rates from the two triple-$\\alpha$ reaction rates and two $^{12}\\rm C(\\alpha,\\gamma)^{16}O$ ones. We compare the calculated abundance ratios with the solar system abundances and deduce some constraints to the reaction rates. As the results, the conventional rate is adequate for the triple-$\\alpha$ reaction rate and rather higher value of the reaction rate is favorable for the $^{12}\\rm C(\\alpha,\\gamma)^{16}O$ rate.

Kikuchi, Yukihiro; Ono, Masaomi; Fukuda, Ryohei

2014-01-01

408

The chemistry of bromine in the stratosphere: Influence of a new rate constant for the reaction BrO + HO2  

NASA Technical Reports Server (NTRS)

The impact of new laboratory data for the reaction BrO + HO2 yields HOBr + O2 in the depletion of global stratospheric ozone has been estimated using a one-dimensional photochemical model taking into account the heterogeneous reaction on sulphate aerosols which converts N2O5 into HNO3. Assuring an aerosol loading 2 times as large as the 'background' and a reaction probability of 0.1 for the above heterogeneous reaction, the 6 fold increase in the measured rate constant for the reaction of BrO with HO2 increases the computed depletion of global ozone produced by 20 ppt of total bromine from 2.01 percent to 2.36 percent. The use of the higher rate constant increases the HOBr mixing ratio and makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction.

Pirre, Michel; Marceau, Francois J.; Lebras, Georges; Maguin, Francoise; Poulet, Gille; Ramaroson, Radiela

1994-01-01

409

Rate constant for the reaction NH2 + NO from 216 to 480 K  

NASA Technical Reports Server (NTRS)

The absolute rate constant was measured by the technique of flash photolysis-laser induced fluorescence (FP-LIF). NH2 radicals were produced by the flash photolysis of ammonia and the fluorescent NH2 photons were measured by multiscaling techniques. At each temperature, the results were independent of variations in total pressure, and flash intensity. The results are compared with previous determinations using the techniques of mass spectrometry, absorption spectroscopy, laser absorption spectroscopy, and laser induced fluorescence. The implications of the results are discussed with regard to combustion, post combustion, and atmospheric chemistry. The results are also discussed theoretically.

Stief, L. J.; Brobst, W. D.; Nava, D. F.; Borkowski, R. P.; Michael, J. V.

1982-01-01

410

Estimation of the nitrogen ionization reaction rate using electric arc shock tube data and Bayesian model analysis  

NASA Astrophysics Data System (ADS)

In this paper, we apply a Bayesian analysis to calibrate the parameters of a model for atomic nitrogen ionization using experimental data from the electric arc shock tube (EAST) wind-tunnel at NASA. We use a one-dimensional plasma flow solver coupled with a radiation solver for the simulation of the radiative signature emitted in the shock-heated air plasma as well as a Park's two-temperature model for the thermal and chemical non-equilibrium effects. We simultaneously quantify model parameter uncertainties and physical model inadequacies when solving the statistical inverse problem. Prior to the solution of such a problem, we perform a sensitivity analysis of the radiative heat flux in order to identify important sources of uncertainty. This analysis clearly shows the importance of the direct ionization of atomic nitrogen as it mostly influences the radiative heating. We then solve the statistical inverse problem and compare the calibrated reaction rates against values available in the literature. Our calculations estimate the reaction rate of the atomic nitrogen ionization to be (3.7 ± 1.5) × 1011 cm3 mol-1 s-1 at 10 000 K, a range consistent with Park's estimation. Finally, in order to assess the validity of the estimated parameters, we propagate their uncertainties through a statistical forward problem defined on a prediction scenario different from the calibration scenarios and compare the model predictions against other experimental data.

Miki, K.; Panesi, M.; Prudencio, E. E.; Prudhomme, S.

2012-02-01

411

Predicting gaseous reaction rates of short chain chlorinated paraffins with ·OH: overcoming the difficulty in experimental determination.  

PubMed

Short chain chlorinated paraffins (SCCPs) are under evaluation for inclusion in the Stockholm Convention on persistent organic pollutants. However, information on their reaction rate constants with gaseous ·OH (kOH) is unavailable, limiting the evaluation of their persistence in the atmosphere. Experimental determination of kOH is confined by the unavailability of authentic chemical standards for some SCCP congeners. In this study, we evaluated and selected density functional theory (DFT) methods to predict kOH of SCCPs, by comparing the experimental kOH values of six polychlorinated alkanes (PCAs) with those calculated by the different theoretical methods. We found that the M06-2X/6-311+G(3df,2pd)//B3LYP/6-311 +G(d,p) method is time-effective and can be used to predict kOH of PCAs. Moreover, based on the calculated kOH of nine SCCPs and available experimental kOH values of 22 PCAs with low carbon chain, a quantitative structure-activity relationship (QSAR) model was developed. The molecular structural characteristics determining the ·OH reaction rate were discussed. logkOH was found to negatively correlate with the percentage of chlorine substitutions (Cl%). The DFT calculation method and the QSAR model are important alternatives to the conventional experimental determination of kOH for SCCPs, and are prospective in predicting their persistence in the atmosphere. PMID:25372845

Li, Chao; Xie, Hong-Bin; Chen, Jingwen; Yang, Xianhai; Zhang, Yifei; Qiao, Xianliang

2014-12-01

412

Pulsed laser photolysis and quantum chemical-statistical rate study of the reaction of the ethynyl radical with water vapor  

NASA Astrophysics Data System (ADS)

The rate coefficient of the gas-phase reaction C2H+H2O?products has been experimentally determined over the temperature range 500-825K using a pulsed laser photolysis-chemiluminescence (PLP-CL) technique. Ethynyl radicals (C2H) were generated by pulsed 193nm photolysis of C2H2 in the presence of H2O vapor and buffer gas N2 at 15Torr. The relative concentration of C2H radicals was monitored as a function of time using a CH * chemiluminescence method. The rate constant determinations for C2H+H2O were k1(550K)=(2.3±1.3)×10-13cm3s-1, k1(770cm3s-1, and k1(825cm3s-1. The error in the only other measurement of this rate constant is also discussed. We have also characterized the reaction theoretically using quantum chemical computations. The relevant portion of the potential energy surface of C2H3O in its doublet electronic ground state has been investigated using density functional theory B3LYP /6-311++G(3df,2p) and molecular orbital computations at the unrestricted coupled-cluster level of theory that incorporates all single and double excitations plus perturbative corrections for the triple excitations, along with the 6-311++G(3df,2p) basis set [(U)CCSD(T)/6-311++G(3df,2p)] and using UCCSD(T )/6-31G(d,p) optimized geometries. Five isomers, six dissociation products, and sixteen transition structures were characterized. The results confirm that the hydrogen abstraction producing C2H2+OH is the most facile reaction channel. For this channel, refined computations using (U)CCSD(T)/6-311++G(3df,2p)//(U)CCSD(T)/6-311++G(d,p) and complete-active-space second-order perturbation theory/complete-active-space self-consistent-field theory (CASPT2/CASSCF) [B. O. Roos, Adv. Chem. Phys. 69, 399 (1987)] using the contracted atomic natural orbitals basis set (ANO-L) [J. Almlöf and P. R. Taylor, J. Chem. Phys.86, 4070 (1987)] were performed, yielding zero-point energy-corrected potential energy barriers of 17kJmol-1 and 15kJmol-1, respectively. Transition-state theory rate constant calculations, based on the UCCSD(T) and CASPT2/CASSCF computations that also include H-atom tunneling and a hindered internal rotation, are in perfect agreement with the experimental values. Considering both our experimental and theoretical determinations, the rate constant can best be expressed, in modified Arrhenius form as k1(T)=(2.2±0.1)×10-21T3.05exp[-(376±100)/T]cm3s-1 for the range 300-2000K. Thus, at temperatures above 1500K, reaction of C2H with H2O is predicted to be one of the dominant C2H reactions in hydrocarbon combustion.

Carl, Shaun A.; Minh Thi Nguyen, Hue; Elsamra, Rehab M. I.; Tho Nguyen, Minh; Peeters, Jozef

2005-03-01

413

Metabolic Rate and Ground Reaction Force During Motorized and Non-Motorized Treadmill Exercise  

NASA Technical Reports Server (NTRS)

PURPOSE: To measure vertical ground reaction force (vGRF) and oxygen consumption (VO2) at several velocities during exercise using a ground-based version of the ISS treadmill in the M and NM modes. METHODS: Subjects (n = 20) walked or ran at 0.89, 1.34, 1.79, 2.24, 2.68, and 3.12 m/s while VO2 and vGRF data were collected. VO2 was measured using open-circuit spirometry (TrueOne 2400, Parvo-Medics). Data were averaged over the last 2 min of each 5-min stage. vGRF was measured in separate 15-s bouts at 125 Hz using custom-fitted pressure-sensing insoles (F-Scan Sport Sensors, Tekscan, Inc). A repeated-measures ANOVA was used to test for differences in VO2 and vGRF between M and NM and across speeds. Significance was set at P < 0.05. RESULTS: Most subjects were unable to exercise for 5 min at treadmill speeds above 1.79 m/s in the NM mode; however, vGRF data were obtained for all subjects at each speed in both modes. VO2 was approx.40% higher during NM than M exercise across treadmill speeds. vGRF increased with treadmill speed but was not different between modes. CONCLUSION: Higher VO2 with no change in vGRF suggests that the additional metabolic cost associated with NM treadmill exercise is accounted for in the horizontal forces required to move the treadmill belt. Although this may limit the exercise duration at faster speeds, high-intensity NM exercise activates the hamstrings and plantarflexors, which are not specifically targeted or well protected by other in-flight countermeasures.

Everett, Meghan E.; Loehr, James A.; DeWitt, John K.; Laughlin, Mitzi; Lee, Stuart M. C.

2010-01-01

414

An "Aufbau" Approach to Understanding How the King-Altman Method of Deriving Rate Equations for Enzyme-Catalyzed Reactions Works  

ERIC Educational Resources Information Center

The King-Altman method of deriving rate equations for enzymatic reactions is applied to the derivation of the Michaelis-Menten equation, along with an explanation for how (or why) the King-Altman method works in this case. The slightly more complicated cases of competitive inhibition and a two-substrate enzyme-catalyzed reaction are then treated…

Sims, Paul A.

2009-01-01

415

Quantum instanton calculation of rate constant for CH4 + OH ? CH3 + H2O reaction: torsional anharmonicity and kinetic isotope effect.  

PubMed

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, wh