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1

Programmable calculator and kinetics of chemical reactions  

Microsoft Academic Search

This paper suggests the use of simulation techniques using a programmable calculator in the study of chemical reactions.† This article does not deal with the use of programmable calculations for producing ‘experimental’ data (i.e. as a substitute for the laboratory). Readers interested in that direction may refer to [1]. Also, I would like to thank E. W. Jenkins of the

Amos Ehrlich

1980-01-01

2

APOLLO: A computer program for the calculation of chemical equilibrium and reaction kinetics of chemical systems  

SciTech Connect

Several of the technologies being evaluated for the treatment of waste material involve chemical reactions. Our example is the in situ vitrification (ISV) process where electrical energy is used to melt soil and waste into a glass like'' material that immobilizes and encapsulates any residual waste. During the ISV process, various chemical reactions may occur that produce significant amounts of products which must be contained and treated. The APOLLO program was developed to assist in predicting the composition of the gases that are formed. Although the development of this program was directed toward ISV applications, it should be applicable to other technologies where chemical reactions are of interest. This document presents the mathematical methodology of the APOLLO computer code. APOLLO is a computer code that calculates the products of both equilibrium and kinetic chemical reactions. The current version, written in FORTRAN, is readily adaptable to existing transport programs designed for the analysis of chemically reacting flow systems. Separate subroutines EQREACT and KIREACT for equilibrium ad kinetic chemistry respectively have been developed. A full detailed description of the numerical techniques used, which include both Lagrange multiplies and a third-order integrating scheme is presented. Sample test problems are presented and the results are in excellent agreement with those reported in the literature.

Nguyen, H.D.

1991-11-01

3

APOLLO: A computer program for the calculation of chemical equilibrium and reaction kinetics of chemical systems  

SciTech Connect

Several of the technologies being evaluated for the treatment of waste material involve chemical reactions. Our example is the in situ vitrification (ISV) process where electrical energy is used to melt soil and waste into a ``glass like`` material that immobilizes and encapsulates any residual waste. During the ISV process, various chemical reactions may occur that produce significant amounts of products which must be contained and treated. The APOLLO program was developed to assist in predicting the composition of the gases that are formed. Although the development of this program was directed toward ISV applications, it should be applicable to other technologies where chemical reactions are of interest. This document presents the mathematical methodology of the APOLLO computer code. APOLLO is a computer code that calculates the products of both equilibrium and kinetic chemical reactions. The current version, written in FORTRAN, is readily adaptable to existing transport programs designed for the analysis of chemically reacting flow systems. Separate subroutines EQREACT and KIREACT for equilibrium ad kinetic chemistry respectively have been developed. A full detailed description of the numerical techniques used, which include both Lagrange multiplies and a third-order integrating scheme is presented. Sample test problems are presented and the results are in excellent agreement with those reported in the literature.

Nguyen, H.D.

1991-11-01

4

Calculation of hypersonic laminar boundary layers with chemical kinetics  

NASA Astrophysics Data System (ADS)

CLIM, a computer program developed to calculate the laminar boundary layer in axisymmetric steady hypersonic flows such as those on reentry vehicles, is described and demonstrated. CLIM takes into account the effects of transverse or longitudinal curvature, wall injection, and chemical reactions of equilibrium or nonequilibrium gases, permitting the treatment of ablation phenomena. The basic hypotheses are outlined; the program equations are derived; the finite-difference discretization is explained; and results are presented graphically for test problems involving a 10-deg-half-angle cone, a Shuttle-like hyperboloid, and the ablation of a carbon wall. When programmed on an IBM 3081, CLIM comprises about 3000 lines and can compute the wall stresses and the boundary-layer thickness, velocity, temperature, density, and concentration profiles for a 50-section body and 10 gas species in less than 1 min for nondissociated air, about 1 min for equilibrium air, and less than 10 min for nonequilibrium air.

Noel, F.; Boukhobza, P.

1986-11-01

5

Chemical Kinetics  

Microsoft Academic Search

\\u000a While thermodynamics provides steady-state information of the combustion process, chemical kinetics describes the transient\\u000a states of the system during the combustion process. Particularly important is information related to the rate at which species\\u000a are consumed and produced, and the rate at which the heat is released. Combustion chemistry has two important characteristics\\u000a not commonly observed in other chemical systems. First,

Sara McAllister; Jyh-Yuan Chen; A. Carlos Fernandez-Pello

6

Effect of chemical kinetics uncertainties on calculated constituents in a tropospheric photochemical model  

Microsoft Academic Search

Random photochemical reaction rates are employed in a 1D photochemical model to examine uncertainties in tropospheric concentrations and thereby determine critical kinetic processes and significant correlations. Monte Carlo computations are used to simulate different chemical environments and their related imprecisions. The most critical processes are the primary photodissociation of O3 (which initiates ozone destruction) and NO2 (which initiates ozone formation),

Anne M. Thompson; Richard W. Stewart

1991-01-01

7

Chemical and Biological Kinetics  

NASA Astrophysics Data System (ADS)

Examples of the application of the methods and ideas of chemical kinetics in various branches of chemistry and biology are considered and the results of studies on the kinetics and mechanisms of autoxidation and inhibited and catalysed oxidation of organic substances in the liquid phase are surveyed. Problems of the kinetics of the ageing of polymers and the principles of their stabilisation are discussed and certain trends in biological kinetics (kinetics of tumour growth, kinetic criteria of the effectiveness of chemotherapy, problems of gerontology, etc.) are considered. The bibliography includes 281 references.

Emanuel', N. M.

1981-10-01

8

Evaluation of the catalytic mechanism of AICAR transformylase by pH-dependent kinetics, mutagenesis, and quantum chemical calculations.  

PubMed

The catalytic mechanism of 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (AICAR Tfase) is evaluated with pH dependent kinetics, site-directed mutagenesis, and quantum chemical calculations. The chemistry step, represented by the burst rates, was not pH-dependent, which is consistent with our proposed mechanism that the 4-carboxamide of AICAR assists proton shuttling. Quantum chemical calculations on a model system of 5-amino-4-carboxamide imidazole (AICA) and formamide using the B3LYP/6-31G level of theory confirmed that the 4-carboxamide participated in the proton-shuttling mechanism. The result also indicated that the amide-assisted mechanism is concerted such that the proton transfers from the 5-amino group to the formamide are simultaneous with nucleophilic attack by the 5-amino group. Because the process does not lead to a kinetically stable intermediate, the intramolecular proton transfer from the 5-amino group through the 4-carboxamide to the formamide proceeds in the same transition state. Interestingly, the calculations predicted that protonation of the N3 of the imidazole of AICA would reduce the energy barrier significantly. However, the pK(a) of the imidazole of AICAR was determined to be 3.23 +/- 0.01 by NMR titration, and AICAR is likely to bind to the enzyme with its imidazole in the free base form. An alternative pathway was suggested by modeling Lys266 to have a hydrogen-bonding interaction with the N3 of the imidazole of AICAR. Lys266 has been implicated in catalysis based on mutagenesis studies and the recent X-ray structure of AICAR Tfase. The quantum chemical calculations on a model system that contains AICA complexed with CH3NH3+ as a mimic of the Lys residue confirmed that such an interaction lowered the activation energy of the reaction and likewise implicated the 4-carboxamide. To experimentally verify this hypothesis, we prepared the K266R mutant and found that its kcat is reduced by 150-fold from that of the wild type without changes in substrate and cofactor Km values. The kcat-pH profile indicated virtually no pH-dependence in the pH range 6-10.5. The results suggest that the ammonium moiety of Lys or Arg is important in catalysis, most likely acting as a general acid catalyst with a pK(a) value greater than 10.5. The H267A mutant was also prepared since His267 has been found in the active site and implicated in catalysis. The mutant enzyme showed no detectable activity while retaining its binding affinity for substrate, indicating that it plays a critical role in catalysis. We propose that His267 interacts with Lys266 to aid in the precise positioning of the general acid catalyst to the N3 of the imidazole of AICAR. PMID:11457277

Shim, J H; Wall, M; Benkovic, S J; Díaz, N; Suárez, D; Merz, K M

2001-05-23

9

Chemical Kinetics: Isolation Method  

NSDL National Science Digital Library

This site offers an interactive tutorial that guides the student through the Method of Isolation used for the determination of chemical reaction rate laws and rate constants. This tutorial is coupled to others to further guide the student to a better understanding of chemical kinetics.

Blauch, David N.

10

Reversible Chemical Kinetics Model  

NSDL National Science Digital Library

The Reversible Chemical Kinetics Model illustrates the time evolution of a reversible chemical reaction, from a given set of initial reactants concentration until chemical equilibrium is reached. The concentrations are plotted against time and the respective numerical values are periodically displayed at the data Table. As chemical equilibrium is attained, the concentrations become constant, the reaction quotient (Q) equalizes the equilibrium constant and Gibbs's energy change (G/RT) approaches zero. The Reversible Chemical Kinetics Model was developed using the Easy Java Simulations (EJS) modeling tool. It is distributed as a ready-to-run (compiled) Java archive. Double clicking the jar file will run the program if Java is installed. You can modify this simulation if you have EJS installed by right-clicking within the map and selecting "Open Ejs Model" from the pop-up menu item.

Fernandes, Fernando S.

2012-10-20

11

LLNL Chemical Kinetics Modeling Group  

SciTech Connect

The LLNL chemical kinetics modeling group has been responsible for much progress in the development of chemical kinetic models for practical fuels. The group began its work in the early 1970s, developing chemical kinetic models for methane, ethane, ethanol and halogenated inhibitors. Most recently, it has been developing chemical kinetic models for large n-alkanes, cycloalkanes, hexenes, and large methyl esters. These component models are needed to represent gasoline, diesel, jet, and oil-sand-derived fuels.

Pitz, W J; Westbrook, C K; Mehl, M; Herbinet, O; Curran, H J; Silke, E J

2008-09-24

12

Stochastic Simulation of Chemical Kinetics  

Microsoft Academic Search

Stochastic chemical kinetics describes the time evolution of a well- stirred chemically reacting system in a way that takes into account the fact that molecules come in whole numbers and exhibit some degree of randomness in their dynamical behavior. Researchers are increasingly using this approach to chemical kinetics in the analysis of cellular systems in biology, where the small molecular

Daniel T. Gillespie

2007-01-01

13

Chemical Kinetics: Rate of Reaction  

NSDL National Science Digital Library

This site offers an interactive tutorial that emphasizes graphical interpretation of chemical kinetics. The stoichiometric coefficients for a chemical equation are determined by comparing the slopes of concentration-time plots for the reactants and products. This tutorial is coupled to others to further guide the student to a better understanding of chemical kinetics.

Blauch, David N.

14

Chemical Looping Combustion Kinetics  

SciTech Connect

One of the most promising methods of capturing CO{sub 2} emitted by coal-fired power plants for subsequent sequestration is chemical looping combustion (CLC). A powdered metal oxide such as NiO transfers oxygen directly to a fuel in a fuel reactor at high temperatures with no air present. Heat, water, and CO{sub 2} are released, and after H{sub 2}O condensation the CO{sub 2} (undiluted by N{sub 2}) is ready for sequestration, whereas the nickel metal is ready for reoxidation in the air reactor. In principle, these processes can be repeated endlessly with the original nickel metal/nickel oxide participating in a loop that admits fuel and rejects ash, heat, and water. Our project accumulated kinetic rate data at high temperatures and elevated pressures for the metal oxide reduction step and for the metal reoxidation step. These data will be used in computational modeling of CLC on the laboratory scale and presumably later on the plant scale. The oxygen carrier on which the research at Utah is focused is CuO/Cu{sub 2}O rather than nickel oxide because the copper system lends itself to use with solid fuels in an alternative to CLC called 'chemical looping with oxygen uncoupling' (CLOU).

Edward Eyring; Gabor Konya

2009-03-31

15

Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations.  

PubMed

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm3 molecule(-1) s(-1)) = -13.25 +/- 0.16 and E(a) = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented. PMID:18193853

Becerra, Rosa; Boganov, Sergey E; Egorov, Mikhail P; Faustov, Valery I; Krylova, Irina V; Nefedov, Oleg M; Promyslov, Vladimir M; Walsh, Robin

2008-01-15

16

Center Manifold and Lie Symmetry Calculations on a Quasi-chemical Model for Growth-death Kinetics in Food  

Microsoft Academic Search

Food scientists at the U.S. Army's Natick Solider Center have developed a model for the lifecyle of the bacteria Staphylococcus aureus in intermediate moisture bread. In this article, we study this model using dynamical systems and Lie symmetry methods. We calculate center manifolds and Lie symmetries for different cases of parameter values and compare our results to those of the

Rachelle C. DeCoste; Louis Piscitelle

2007-01-01

17

Chemical Kinetic Modeling of Hydrogen Combustion Limits  

SciTech Connect

A detailed chemical kinetic model is used to explore the flammability and detonability of hydrogen mixtures. In the case of flammability, a detailed chemical kinetic mechanism for hydrogen is coupled to the CHEMKIN Premix code to compute premixed, laminar flame speeds. The detailed chemical kinetic model reproduces flame speeds in the literature over a range of equivalence ratios, pressures and reactant temperatures. A series of calculation were performed to assess the key parameters determining the flammability of hydrogen mixtures. Increased reactant temperature was found to greatly increase the flame speed and the flammability of the mixture. The effect of added diluents was assessed. Addition of water and carbon dioxide were found to reduce the flame speed and thus the flammability of a hydrogen mixture approximately equally well and much more than the addition of nitrogen. The detailed chemical kinetic model was used to explore the detonability of hydrogen mixtures. A Zeldovich-von Neumann-Doring (ZND) detonation model coupled with detailed chemical kinetics was used to model the detonation. The effectiveness on different diluents was assessed in reducing the detonability of a hydrogen mixture. Carbon dioxide was found to be most effective in reducing the detonability followed by water and nitrogen. The chemical action of chemical inhibitors on reducing the flammability of hydrogen mixtures is discussed. Bromine and organophosphorus inhibitors act through catalytic cycles that recombine H and OH radicals in the flame. The reduction in H and OH radicals reduces chain branching in the flame through the H + O{sub 2} = OH + O chain branching reaction. The reduction in chain branching and radical production reduces the flame speed and thus the flammability of the hydrogen mixture.

Pitz, W J; Westbrook, C K

2008-04-02

18

Calculating Kinetics of a Student Designed Machine  

NSDL National Science Digital Library

This activity is a project-based inquiry where students design and create a machine to complete a task. Then students will be asked to calculate velocities, kinetic energy and potential energies of various parts of their machine.

19

Detailed Chemical Kinetic Modeling of Cyclohexane Oxidation  

SciTech Connect

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Reaction rate constant rules are developed for the low temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Since cyclohexane produces only one type of cyclohexyl radical, much of the low temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical + O{sub 2} through five, six and seven membered ring transition states. The direct elimination of cyclohexene and HO{sub 2} from RO{sub 2} is included in the treatment using a modified rate constant of Cavallotti et al. Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies on cyclohexane. Calculations indicated that the production of 1,2-epoxycyclohexane observed in the experiments can not be simulated based on the current understanding of low temperature chemistry. Possible 'alternative' H-atom isomerizations leading to different products from the parent O{sub 2}QOOH radical were included in the low temperature chemical kinetic mechanism and were found to play a significant role.

Silke, E J; Pitz, W J; Westbrook, C K; Ribaucour, M

2006-11-10

20

Enhancing Thai students' learning of chemical kinetics  

Microsoft Academic Search

Chemical kinetics is an extremely important concept for introductory chemistry courses. The literature suggests that instruction in chemical kinetics is often teacher?dominated at both the secondary school and tertiary levels, and this is the case in Thailand – the educational context for this inquiry. The work reported here seeks to shift students from passive learning to more active, student?centred learning

Sanoe Chairam; Ekasith Somsook; Richard K. Coll

2009-01-01

21

Enhancing Thai Students' Learning of Chemical Kinetics  

ERIC Educational Resources Information Center

|Chemical kinetics is an extremely important concept for introductory chemistry courses. The literature suggests that instruction in chemical kinetics is often teacher-dominated at both the secondary school and tertiary levels, and this is the case in Thailand--the educational context for this inquiry. The work reported here seeks to shift…

Chairam, Sanoe; Somsook, Ekasith; Coll, Richard K.

2009-01-01

22

Incorporating Diffusion in Complex Geometries into Stochastic Chemical Kinetics Simulations  

Microsoft Academic Search

A method is developed for incorporating diffusion of chemicals in complex geometries into stochastic chemical kinetics simulations. Systems are modeled using the reaction-diffusion master equation, with jump rates for diffusive motion between mesh cells calculated from the discretization weights of an embedded boundary method. Since diffusive jumps between cells are treated as first order reactions, individual realizations of the stochastic

Samuel A. Isaacson; Charles S. Peskin

2006-01-01

23

Polypyrroles as antioxidants: kinetic studies on reactions of bilirubin and biliverdin dimethyl esters and synthetic model compounds with peroxyl radicals in solution. Chemical calculations on selected typical structures.  

PubMed

[reaction: see text] Rate constants for hydrogen-atom transfer (HAT) from bilirubin dimethyl ester (BRDE) and biliverdin dimethyl ester (BVDE) to peroxyl radicals during inhibited autoxidation of styrene initiated by azo-bisisobutyronitrile (AIBN) were k(inh)(BRDE) = 22.5 x 10(4) and k(inh)(BVDE) = 10.2 x 10(4) M(-1) s(-1), and the stoichiometric factors (n) were 2.0 and 2.7, respectively. A synthetic tetrapyrrole (bis(dipyrromethene)) containing the alpha-central (2,2') CH2 linkage gave k(inh) = 39.9 x 10(4) M(-1) s(-1) and n = 2.3, whereas the beta-linked (3,3') isomer was not an active antioxidant. Several dipyrrinones were synthesized as mimics of the two outer heterocyclic rings of bilirubin and biliverdin. The dipyrrinones containing N-H groups in each ring were active antioxidants, whereas those lacking two such "free" N-H groups, such as N-CH3 dipyrrinones and dipyrromethenes, did not exhibit antioxidant activity. Overall, the relative k(inh) values compared to those of phenolic antioxidants, 2,6-di-tert-butyl-4-methoxyphenol (DBHA) and 2,6-di-tert-butyl-4-methylphenol (BHT), were 2,2'-bis(dipyrromethene) > BRDE > DBHA > dipyrrinones > BVDE > BHT. This general trend in antioxidant activities was also observed for the inhibited autoxidation of cumene initiated by AIBN. Chemical calculations of the N-H bond dissociation enthalpies (BDEs) of the typical structures support a HAT mechanism from N-H groups to trap peroxyl radicals. Intramolecular hydrogen bonding of intermediate nitrogen radicals has a major influence on the antioxidant activities of all compounds studied. Indeed, chemical calculations showed that the initial nitrogen radical from a dipyrrinone is stabilized by 9.0 kcal/mol because of H-bonding between the N-H remaining on one ring and the ground-state pyrrolyl radical of the adjacent ring in the natural zusammen structure. The calculated minimum structure of bilirubin shows strong intramolecular H-bonding of the N-H groups with carbonyl groups resulting in the known "ridge-tile" structure which is not an active HAT antioxidant. The calculated minimum structure of biliverdin is planar. BRDE is readily converted into BVDE by reaction with the electron-deficient DPPH* radical under argon in chlorobenzene. An electron-transfer mechanism is proposed for the initiating step in this reaction, and this is supported by the relatively low ionizing potential of a model dipyrrole representing the two central rings of bilirubin. PMID:16388613

Chepelev, Leonid L; Beshara, Cory S; MacLean, Patricia D; Hatfield, Gillian L; Rand, Amy A; Thompson, Alison; Wright, James S; Barclay, L Ross C

2006-01-01

24

Chemical kinetics of Estane aging in PBX  

SciTech Connect

The Plastic-Bonded Explosive PBX 9501 is about 95% HMX, 2.5% Estane 5703, 2.5% nitroplasticizer (NP), and 0.1% stabilizer by weight. The NP, BDNPA/F, is a eutectic mixture of bis(2,2-dinitropropyl) acetal and bis(2,2-dinitropropyl)formal. The stabilizer is diphenylamine (DPA) or Irganox 1010. The Estane, a polyester-polyurethane, slowly degrades with time. Knowledge of the effect of the Estane aging on the mechanical properties of the PBX 9501 is required to predict with confidence the useful lifetime of the explosive with respect to safety and reliability. A detailed master equation model of the chemical mechanisms and kinetics of the aging of Estane 5703 in PBX 9501 is being developed. Its output will be used as input into other models being developed to calculate the changes in the mechanical properties of the PBX.

Pack, R.T.; Hanson, D.E.; Redondo, A. [Los Alamos National Lab., NM (United States). Theoretical Div.

1997-12-01

25

Chemical kinetics and oil shale process design  

SciTech Connect

Oil shale processes are reviewed with the goal of showing how chemical kinetics influences the design and operation of different processes for different types of oil shale. Reaction kinetics are presented for organic pyrolysis, carbon combustion, carbonate decomposition, and sulfur and nitrogen reactions.

Burnham, A.K.

1993-07-01

26

Modeling of Chemical Kinetics in Pulse Combustors.  

National Technical Information Service (NTIS)

Chemical kinetic modeling of the successive reignition of fuel-air mixtures is described, under conditions of pressure and temperature very similar to those encountered in conventional pulse combustion systems. The overall system model is described and th...

C. K. Westbrook

1985-01-01

27

Chemical kinetic modelling of hydrocarbon ignition  

SciTech Connect

Chemical kinetic modeling of hydrocarbon ignition is discussed with reference to a range of experimental configurations, including shock tubes, detonations, pulse combustors, static reactors, stirred reactors and internal combustion engines. Important conditions of temperature, pressure or other factors are examined to determine the main chemical reaction sequences responsible for chain branching and ignition, and kinetic factors which can alter the rate of ignition are identified. Hydrocarbon ignition usually involves complex interactions between physical and chemical factors, and it therefore is a suitable and often productive subject for computer simulations. In most of the studies to be discussed below, the focus of the attention is placed on the chemical features of the system. The other physical parts of each application are generally included in the form of initial or boundary conditions to the chemical kinetic parts of the problem, as appropriate for each type of application being addressed.

Westbrook, C.K.; Pitz, W.J.; Curran, H.J.; Gaffuri, P.; Marinov, N.M.

1995-08-25

28

Chemical Kinetics: Method of Initial Rates  

NSDL National Science Digital Library

This site offers an interactive tutorial to assist in the understanding of how the Method of Initial Rates is employed to determine rate constants for chemical reactions. The student runs a series of experiments to create graphical data that is analyzed. This tutorial is coupled to others to further guide the student to a better understanding of chemical kinetics.

Blauch, David N.

29

Chemical kinetics models for semiconductor processing  

SciTech Connect

Chemical reactions in the gas-phase and on surfaces are important in the deposition and etching of materials for microelectronic applications. A general software framework for describing homogeneous and heterogeneous reaction kinetics utilizing the Chemkin suite of codes is presented. Experimental, theoretical and modeling approaches to developing chemical reaction mechanisms are discussed. A number of TCAD application modules for simulating the chemically reacting flow in deposition and etching reactors have been developed and are also described.

Coltrin, M.E.; Creighton, J.R. [Sandia National Labs., Albuquerque, NM (United States); Meeks, E.; Grcar, J.F.; Houf, W.G. [Sandia National Labs., Livermore, CA (United States); Kee, R.J. [Colorado School of Mines, Golden, CO (United States)

1997-12-31

30

Chemical Kinetic Modeling of Advanced Transportation Fuels  

SciTech Connect

Development of detailed chemical kinetic models for advanced petroleum-based and nonpetroleum based fuels is a difficult challenge because of the hundreds to thousands of different components in these fuels and because some of these fuels contain components that have not been considered in the past. It is important to develop detailed chemical kinetic models for these fuels since the models can be put into engine simulation codes used for optimizing engine design for maximum efficiency and minimal pollutant emissions. For example, these chemistry-enabled engine codes can be used to optimize combustion chamber shape and fuel injection timing. They also allow insight into how the composition of advanced petroleum-based and non-petroleum based fuels affect engine performance characteristics. Additionally, chemical kinetic models can be used separately to interpret important in-cylinder experimental data and gain insight into advanced engine combustion processes such as HCCI and lean burn engines. The objectives are: (1) Develop detailed chemical kinetic reaction models for components of advanced petroleum-based and non-petroleum based fuels. These fuels models include components from vegetable-oil-derived biodiesel, oil-sand derived fuel, alcohol fuels and other advanced bio-based and alternative fuels. (2) Develop detailed chemical kinetic reaction models for mixtures of non-petroleum and petroleum-based components to represent real fuels and lead to efficient reduced combustion models needed for engine modeling codes. (3) Characterize the role of fuel composition on efficiency and pollutant emissions from practical automotive engines.

PItz, W J; Westbrook, C K; Herbinet, O

2009-01-20

31

Improved wood–kirkwood detonation chemical kinetics  

Microsoft Academic Search

We report an improved implementation of the Wood–Kirkwood kinetic detonation model based on a multi-species Buckingham exponential-6\\u000a equation of state (EOS) and multiple reaction rate laws. The exp-6 EOS allows for treatment of chemical systems at a statistical\\u000a mechanics level, instead of an atomistic level. Finite global rate laws are used for the slowest chemical reactions. Other\\u000a reactions are given

Kurt R. Glaesemann; Laurence E. Fried

2008-01-01

32

Kinetics of Chemical Reactions in Foods  

Microsoft Academic Search

Chemical reactions occur in foods during processing and storage. Some reactions result in a quality loss and must be minimized,\\u000a whereas others result in the formation of a desired flavor or color and must be optimized to obtain the best product quality.\\u000a Kinetics is a science that involves the study of chemical reaction rates and mechanisms. An understanding of reaction

Romeo T. Toledo

33

Chemical Dosing and First-Order Kinetics  

ERIC Educational Resources Information Center

|College students encounter a variety of first-order phenomena in their mathematics and science courses. Introductory chemistry textbooks that discuss first-order processes, usually in conjunction with chemical kinetics or radioactive decay, stop at single, discrete dose events. Although single-dose situations are important, multiple-dose events,…

Hladky, Paul W.

2011-01-01

34

Detailed chemical kinetic model for ethanol oxidation  

SciTech Connect

A detailed chemical kinetic model for ethanol oxidation has been developed and validated against a variety of experimental data sets. Laminar flame speed data obtained from a constant volume bomb, ignition delay data behind reflected shock waves, and ethanol oxidation product profiles from a turbulent flow reactor were used in this study. Very good agreement was found in modeling the data sets obtained from the three different experimental systems. The computational modeling results show that high temperature ethanol oxidation exhibits strong sensitivity to the fall-off kinetics of ethanol decomposition, branching ratio selection for c2h5oh+oh=products, and reactions involving the hydroperoxyl (HO2) radical.

Marinov, N.

1997-04-01

35

Promoting Graphical Thinking: Using Temperature and a Graphing Calculator to Teach Kinetics Concepts  

ERIC Educational Resources Information Center

|A combination of graphical thinking with chemical and physical theories in the classroom is encouraged by using the Calculator-Based Laboratory System (CBL) with a temperature sensor and graphing calculator. The theory of first-order kinetics is logically explained with the aid of the cooling or heating of the metal bead of the CBL's temperature…

Cortes-Figueroa, Jose E.; Moore-Russo, Deborah A.

2004-01-01

36

A Detailed Chemical Kinetic Model for TNT  

Microsoft Academic Search

A detailed chemical kinetic mechanism for 2,4,6-tri-nitrotoluene (TNT) has been developed to explore problems of explosive performance and soot formation during the destruction of munitions. The TNT mechanism treats only gas-phase reactions. Reactions for the decomposition of TNT and for the consumption of intermediate products formed from TNT are assembled based on information from the literature and on current understanding

W J Pitz; C K Westbrook

2005-01-01

37

Sensitivity, robustness, and identifiability in stochastic chemical kinetics models  

PubMed Central

We present a novel and simple method to numerically calculate Fisher information matrices for stochastic chemical kinetics models. The linear noise approximation is used to derive model equations and a likelihood function that leads to an efficient computational algorithm. Our approach reduces the problem of calculating the Fisher information matrix to solving a set of ordinary differential equations. This is the first method to compute Fisher information for stochastic chemical kinetics models without the need for Monte Carlo simulations. This methodology is then used to study sensitivity, robustness, and parameter identifiability in stochastic chemical kinetics models. We show that significant differences exist between stochastic and deterministic models as well as between stochastic models with time-series and time-point measurements. We demonstrate that these discrepancies arise from the variability in molecule numbers, correlations between species, and temporal correlations and show how this approach can be used in the analysis and design of experiments probing stochastic processes at the cellular level. The algorithm has been implemented as a Matlab package and is available from the authors upon request.

Komorowski, Michal; Costa, Maria J.; Rand, David A.; Stumpf, Michael P. H.

2011-01-01

38

Kinetic theory of steady chemical nucleation in the gas phase  

Microsoft Academic Search

We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the effective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in

T. Yamamoto; T. Chigai; S. Watanabe; T. Kozasa

2001-01-01

39

Perspective: Stochastic algorithms for chemical kinetics.  

PubMed

We outline our perspective on stochastic chemical kinetics, paying particular attention to numerical simulation algorithms. We first focus on dilute, well-mixed systems, whose description using ordinary differential equations has served as the basis for traditional chemical kinetics for the past 150 years. For such systems, we review the physical and mathematical rationale for a discrete-stochastic approach, and for the approximations that need to be made in order to regain the traditional continuous-deterministic description. We next take note of some of the more promising strategies for dealing stochastically with stiff systems, rare events, and sensitivity analysis. Finally, we review some recent efforts to adapt and extend the discrete-stochastic approach to systems that are not well-mixed. In that currently developing area, we focus mainly on the strategy of subdividing the system into well-mixed subvolumes, and then simulating diffusional transfers of reactant molecules between adjacent subvolumes together with chemical reactions inside the subvolumes. PMID:23656106

Gillespie, Daniel T; Hellander, Andreas; Petzold, Linda R

2013-05-01

40

Calculation of Kinetics Parameters for the NBSR  

SciTech Connect

The delayed neutron fraction and prompt neutron lifetime have been calculated at different times in the fuel cycle for the NBSR when fueled with both high-enriched uranium (HEU) and low-enriched uranium (LEU) fuel. The best-estimate values for both the delayed neutron fraction and the prompt neutron lifetime are the result of calculations using MCNP5-1.60 with the most recent ENDFB-VII evaluations. The best-estimate values for the total delayed neutron fraction from fission products are 0.00665 and 0.00661 for the HEU fueled core at startup and end-of-cycle, respectively. For the LEU fuel the best estimate values are 0.00650 and 0.00648 at startup and end-of-cycle, respectively. The present recommendations for the delayed neutron fractions from fission products are smaller than the value reported previously of 0.00726 for the HEU fuel. The best-estimate values for the contribution from photoneutrons will remain as 0.000316, independent of the fuel or time in the cycle.The values of the prompt neutron lifetime as calculated with MCNP5-1.60 are compared to values calculated with two other independent methods and the results are in reasonable agreement with each other. The recommended, conservative values of the neutron lifetime for the HEU fuel are 650 {micro}s and 750 {micro}s for the startup and end-of-cycle conditions, respectively. For LEU fuel the recommended, conservative values are 600 {micro}s and 700 {micro}s for the startup and end-of-cycle conditions, respectively. In all three calculations, the prompt neutron lifetime was determined to be longer for the end-of-cycle equilibrium condition when compared to the startup condition. The results of the three analyses were in agreement that the LEU fuel will exhibit a shorter prompt neutron lifetime when compared to the HEU fuel.

Hanson A. L.; Diamond D.

2012-03-06

41

The Deterministic Limit of Stochastic Chemical Kinetics  

PubMed Central

An analysis is presented of the approximating assumptions that underlie a recently proposed derivation of the traditional deterministic reaction rate equation from a discrete-stochastic formulation of chemical kinetics. It is shown that if the system is close enough to the thermodynamic limit, in which the molecular populations and the containing volume all approach infinity in such a way that the molecular concentrations remain finite, then the required approximating assumptions will be justified for practically all spatially homogeneous systems that one is likely to encounter.

Gillespie, Daniel T.

2009-01-01

42

Kinetics Calculations for RIA Experiments in the Capsule Driver Core.  

National Technical Information Service (NTIS)

Space-time kinetics calculations have been done for transient fuel behavior tests that were conducted during the late 1960's in the Capsule Driver Core in PBF. The purpose of the calculations was to determine the amount of energy deposited by delayed-neut...

A. J. Scott D. W. Nigg J. L. Judd S. A. Easson

1981-01-01

43

Using Excel to Calculate Mineral Chemical Analyses  

NSDL National Science Digital Library

In this assignment, students are provided instructions to make an Excel spreadsheet to calculate mineral chemical analyses from weight percent to atoms per given number of oxygen atoms. This skill will be useful for the major rock-forming mineral groups.

44

Chemical Kinetic Modeling of Biofuel Combustion  

NASA Astrophysics Data System (ADS)

Bioalcohols, such as bioethanol and biobutanol, are suitable replacements for gasoline, while biodiesel can replace petroleum diesel. Improving biofuel engine performance requires understanding its fundamental combustion properties and the pathways of combustion. This study's contribution is experimentally validated chemical kinetic combustion mechanisms for biobutanol and biodiesel. Fundamental combustion data and chemical kinetic mechanisms are presented and discussed to improve our understanding of biofuel combustion. The net environmental impact of biobutanol (i.e., n-butanol) has not been studied extensively, so this study first assesses the sustainability of n-butanol derived from corn. The results indicate that technical advances in fuel production are required before commercializing biobutanol. The primary contribution of this research is new experimental data and a novel chemical kinetic mechanism for n-butanol combustion. The results indicate that under the given experimental conditions, n-butanol is consumed primarily via abstraction of hydrogen atoms to produce fuel radical molecules, which subsequently decompose to smaller hydrocarbon and oxygenated species. The hydroxyl moiety in n-butanol results in the direct production of the oxygenated species such as butanal, acetaldehyde, and formaldehyde. The formation of these compounds sequesters carbon from forming soot precursors, but they may introduce other adverse environmental and health effects. Biodiesel is a mixture of long chain fatty acid methyl esters derived from fats and oils. This research study presents high quality experimental data for one large fatty acid methyl ester, methyl decanoate, and models its combustion using an improved skeletal mechanism. The results indicate that methyl decanoate is consumed via abstraction of hydrogen atoms to produce fuel radicals, which ultimately lead to the production of alkenes. The ester moiety in methyl decanoate leads to the formation of low molecular weight oxygenated compounds such as carbon monoxide, formaldehyde, and ketene. The study concludes that the oxygenated molecules in biofuels follow similar combustion pathways to the hydrocarbons in petroleum fuels. The oxygenated moiety's ability to sequester carbon from forming soot precursors is highlighted. However, the direct formation of oxygenated hydrocarbons warrants further investigation into the environmental and health impacts of practical biofuel combustion systems.

Sarathy, Subram Maniam

45

The multiplexed chemical kinetic photoionization mass spectrometer: A new approach to isomer-resolved chemical kinetics  

Microsoft Academic Search

We have developed a multiplexed time- and photon-energy-resolved photoionization mass spectrometer for the study of the kinetics and isomeric product branching of gas phase, neutral chemical reactions. The instrument utilizes a side-sampled flow tube reactor, continuously tunable synchrotron radiation for photoionization, a multimass double-focusing mass spectrometer with 100% duty cycle, and a time- and position-sensitive detector for single ion counting.

David L. Osborne; Peng Zou; Howard Johnsen; Carl C. Hayden; Craig A. Taatjes; Vadim D. Knyazev; Simon W. North; Darcy S. Peterka; Musahid Ahmed; Stephen R. Leone

2008-01-01

46

Chemkin-II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics  

Microsoft Academic Search

This document is the user's manual for the second-generation Chemkin package. Chemkin is a software package for whose purpose is to facilitate the formation, solution, and interpretation of problems involving elementary gas-phase chemical kinetics. It provides an especially flexible and powerful tool for incorporating complex chemical kinetics into simulations of fluid dynamics. The package consists of two major software components:

R. J. Kee; F. M. Rupley; J. A. Miller

1989-01-01

47

A Detailed Chemical Kinetic Model for TNT  

SciTech Connect

A detailed chemical kinetic mechanism for 2,4,6-tri-nitrotoluene (TNT) has been developed to explore problems of explosive performance and soot formation during the destruction of munitions. The TNT mechanism treats only gas-phase reactions. Reactions for the decomposition of TNT and for the consumption of intermediate products formed from TNT are assembled based on information from the literature and on current understanding of aromatic chemistry. Thermodynamic properties of intermediate and radical species are estimated by group additivity. Reaction paths are developed based on similar paths for aromatic hydrocarbons. Reaction-rate constant expressions are estimated from the literature and from analogous reactions where the rate constants are available. The detailed reaction mechanism for TNT is added to existing reaction mechanisms for RDX and for hydrocarbons. Computed results show the effect of oxygen concentration on the amount of soot precursors that are formed in the combustion of RDX and TNT mixtures in N{sub 2}/O{sub 2} mixtures.

Pitz, W J; Westbrook, C K

2005-01-13

48

REDUCED KINETICS AND COUPLING FUNCTIONS FOR CALCULATING CO AND NO EMISSIONS IN GAS-TURBINE COMBUSTION  

Microsoft Academic Search

A reduced chemical-kinetic mechanism consisting of two global steps for fuel oxidation and an additional step for NO production is proposed as the minimal chemistry description for calculating CO and NO emissions in gas-turbine combustion. Carbon monoxide is seen to emerge as the main intermediate during the fuel-oxidation process, which takes place in two steps: fast partial hydrocarbon oxidation to

ALAIN LÉPINETTE; AMABLE LIÑÁN; BENIGNO LÁZARO; ANTONIO L. SÁNCHEZ

2005-01-01

49

Laboratory Kinetics of Chemical Reactions occurring in Astrophysical Environments  

Microsoft Academic Search

To model the dynamics and molecular evolution of interstellar clouds, a great deal of information on chemical kinetics is required, preferably under the appropriate conditions of kinetic temperature and internal excitation. This kinetics includes (i) gas phase ionic (ion-molecule and electron-ion) and (ii) neutral-neutral reactions and (iii) heterogeneous surface catalysis, all from simple species to those as complex as polyaromatic

N. G. Adams; L. M. Babcock

2005-01-01

50

Understanding Chemical Reaction Kinetics and Equilibrium with Interlocking Building Blocks  

ERIC Educational Resources Information Center

|Chemical reaction kinetics and equilibrium are essential core concepts of chemistry but are challenging topics for many students, both at the high school and undergraduate university level. Visualization at the molecular level is valuable to aid understanding of reaction kinetics and equilibrium. This activity provides a discovery-based method to…

Cloonan, Carrie A.; Nichol, Carolyn A.; Hutchinson, John S.

2011-01-01

51

On differential geometric formalization of chemical reaction kinetics statical aspects  

Microsoft Academic Search

In this article the basic mathematical structure of chemical reaction kinetics is investigated. For this purpose the methods of modern differential geometry are used. Using this approach the various aspects of species, reactions and stoichiometry can be separately treated.

L. Ropolyi; P. Réti

1984-01-01

52

Detailed and global chemical kinetics model for hydrogen.  

National Technical Information Service (NTIS)

Detailed and global chemical kinetic computations for hydrogen-air mixtures have been performed to describe flame propagation, flame structure and ignition phenomena. Simulations of laminar flame speeds, flame compositions and shock tube ignition delay ti...

N. M. Marinov C. K. Westbrook W. J. Pitz

1995-01-01

53

A hydrocode calculation coupled with reaction kinetics of carbon compounds within an impact vapor plume and its implications for cometary impacts on Galilean satellites  

Microsoft Academic Search

The synthesis of organic molecules via chemical reactions within impact vapor plumes has been proposed as a mechanism to supply organics on a planet. However, the kinetics of chemical reactions within a rapidly expanding vapor plume or quenching process of the reactions has not been studied extensively. In this study, we constructed a new numerical model that calculates kinetics of

Ryo Ishimaru; Hiroki Senshu; Seiji Sugita; Takafumi Matsui

2010-01-01

54

CHEMKIN-III: A FORTRAN chemical kinetics package for the analysis of gas-phase chemical and plasma kinetics.  

National Technical Information Service (NTIS)

This document is the user's manual for the third-generation CHEMKIN package. CHEMKIN is a software package whose purpose is to facilitate the formation, solution, and interpretation of problems involving elementary gas-phase chemical kinetics. It provides...

R. J. Kee F. M. Rupley E. Meeks J. A. Miller

1996-01-01

55

Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics.  

National Technical Information Service (NTIS)

This document is the user's manual for the second-generation Chemkin package. Chemkin is a software package whose purpose is to facilitate the formation, solution, and interpretation of problems involving elementary gas-phase chemical kinetics. It provide...

R. J. Kee F. M. Rupley J. A. Miller

1989-01-01

56

The Multiplexed Chemical Kinetic Photoionization Mass Spectrometer: A New Approach To Isomer-resolved Chemical Kinetics  

SciTech Connect

We have developed a multiplexed time- and photon-energy?resolved photoionizationmass spectrometer for the study of the kinetics and isomeric product branching of gasphase, neutral chemical reactions. The instrument utilizes a side-sampled flow tubereactor, continuously tunable synchrotron radiation for photoionization, a multi-massdouble-focusing mass spectrometer with 100percent duty cycle, and a time- and positionsensitive detector for single ion counting. This approach enables multiplexed, universal detection of molecules with high sensitivity and selectivity. In addition to measurement of rate coefficients as a function of temperature and pressure, different structural isomers can be distinguished based on their photoionization efficiency curves, providing a more detailed probe of reaction mechanisms. The multiplexed 3-dimensional data structure (intensity as a function of molecular mass, reaction time, and photoionization energy) provides insights that might not be available in serial acquisition, as well as additional constraints on data interpretation.

Osborne, David L.; Zou, Peng; Johnsen, Howard; Hayden, Carl C.; Taatjes, Craig A.; Knyazev, Vadim D.; North, Simon W.; Peterka, Darcy S.; Ahmed, Musahid; Leone, Stephen R.

2008-08-28

57

ACFM, A Computational Framework for Magnetofluid and Kinetic Calculations  

NASA Astrophysics Data System (ADS)

We present a general computational framework, ACFM, for continuum calculations that can be used to address many of the computational physics problems in plasma physics/magnetic fusion research. The explicit goal of the project is to bridge the gap between the physical/mathematical formulation of a problem and its computational implementation in a simple and efficient manner, thus removing "computing" from the purview of a few specialists to a more general community of user/theorists. The framework is being developed with modern objected oriented methods with an attempt to provide a full set of tools that would enable a user to write code in a manner that closely resembles the mathematical physics language he is already familiar with. The framework and some of the ideas will be discussed with examples from such disparate topics as collisionless reconnection, kinetic calculations with a Vlasov-Poisson system, magnetofluid "closure" problem and self-organized states.

Aydemir, Ahmet Y.

2002-11-01

58

Using CSP to Understand Complex Chemical Kinetics  

Microsoft Academic Search

The conventional methods of simplified kinetics modeling through the use of partial-equilibrium and quasi-steady approximations are re- viewed and critiqued. The method of computational singular pertur- bation (CSP) is then presented with special emphasis on the interpre- tation of CSP data to obtain physical insights on massively complex reaction systems. A simple example is used to demonstrate how CSP deals

S. H. Lam

1992-01-01

59

PLASMAKIN: A chemical kinetics library for plasma physics modeling  

Microsoft Academic Search

PLASMAKIN is a software library to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces - particle generation and loss rates, photon emission rates and energy exchange rates. PLASMAKIN supports any number of chemical species and reactions, is independent of problem dimensions

2001-01-01

60

PLASMAKIN: A chemical kinetics library for plasma physics modeling  

Microsoft Academic Search

PLASMAKIN is a software library to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces — particle generation and loss rates, photon emission rates and energy exchange rates. PLASMAKIN supports any number of chemical species and reactions, is independent of problem dimensions

Nuno R. Pinhão

2001-01-01

61

New Spectroscopic Methods in Gas-Phase Chemical Kinetics  

Microsoft Academic Search

The possibilities provided by the application of advances in laser technology to the solution of problems in gas-phase chemical kinetics have been analysed. The basic principles of various laser methods for producing and recording active chemical species have been described. Examples of the use of lasers to study the reactions of radicals and excited species, in the diagnostics of complex

Oleg M. Sarkisov; S. G. Cheskis

1985-01-01

62

Hungarian University Students' Misunderstandings in Thermodynamics and Chemical Kinetics  

ERIC Educational Resources Information Center

|The misunderstandings related to thermodynamics (including chemical equilibrium) and chemical kinetics of first and second year Hungarian students of chemistry, environmental science, biology and pharmacy were investigated. We demonstrated that Hungarian university students have similar misunderstandings in physical chemistry to those reported in…

Turanyi, Tamas; Toth, Zoltan

2013-01-01

63

An efficient chemical kinetics solver using high dimensional model representation  

Microsoft Academic Search

A high dimensional model representation (HDMR) technique is introduced to capture the input-output behavior of chemical kinetic models. The HDMR expresses the output chemical species concentrations as a rapidly convergent hierarchical correlated function expansion in the input variables. In this paper, the input variables are taken as the species concentrations at time t{sub i} and the output is the concentrations

Jeffrey A. Shorter; Precila C. Ip; Herschel A. Rabitz

1999-01-01

64

Fluid flow and chemical reaction kinetics in metamorphic systems  

SciTech Connect

The treatment and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow, diffusion, and thermal evolution. The interplay of fluid flow and surface reaction rates, the distinction between steady state and equilibrium, and the possible overstepping of metamorphic reactions are discussed using a simple analytic model. This model serves as an introduction to the second part of the paper, which develops a reaction model that solves the coupled temperature-fluid flow-chemical composition differential equations relevant to metamorphic processes. Consideration of stable isotopic evidence requires that such a kinetic model be considered for the chemical evolution of a metamorphic aureole. A general numerical scheme is discussed to handle the solution of the model. The results of this kinetic model allow us to reach several important conclusions regarding the factors controlling the chemical evolution of mineral assemblages during a metamorphic event. 41 refs., 19 figs., 5 tabs.

Lasaga, A.C.; Rye, D.M. (Yale Univ., New Haven, CT (United States))

1993-05-01

65

Application of evaluated fission-product delayed-neutron precursor data in reactor kinetics calculations  

SciTech Connect

Evaluated fission-product yield and decay data have been used to describe 105 delayed neutron precursors explicitly in point reactor kinetics calculations. Results calculated for /sup 235/U thermal fission show that rod-drop reactivity values obtained from kinetics calculations with 6-group precursor data are considerably higher than those calculated with explicit delayed-neutron precursor data. The calculated kinetics associated with positive reactivity steps are significantly different.

Perry, R.T.; Wilson, W.B.; England, T.R.; Brady, M.C.

1985-01-01

66

Direct first-principles chemical potential calculations of liquids  

NASA Astrophysics Data System (ADS)

We propose a scheme that drastically improves the efficiency of Widom's particle insertion method by efficiently sampling cavities while calculating the integrals providing the chemical potentials of a physical system. This idea enables us to calculate chemical potentials of liquids directly from first-principles without the help of any reference system, which is necessary in the commonly used thermodynamic integration method. As an example, we apply our scheme, combined with the density functional formalism, to the calculation of the chemical potential of liquid copper. The calculated chemical potential is further used to locate the melting temperature. The calculated results closely agree with experiments.

Hong, Qi-Jun; van de Walle, Axel

2012-09-01

67

Chemical kinetics of discharge-driven oxygen-iodine lasers  

Microsoft Academic Search

Oxygen-iodine lasers that utilize electrical discharges to produce O2(a1Delta) are currently being developed. The discharge generators differ from those used in chemical oxygen-iodine lasers in that they produce significant amounts of atomic oxygen and traces of ozone. As a consequence of these differences, the chemical kinetics of the discharge laser are markedly different from those of a conventional chemical oxygen-iodine

Valeriy N. Azyazov; Ivan O. Antonov; Michael C. Heaven

2007-01-01

68

Chemical kinetics study of a nanosecond pulsed He-O2 plasma Jet  

NASA Astrophysics Data System (ADS)

Atmospheric-pressure, nanosecond pulsed plasma jets have shown promising applications in biomedical and dental fields. In order to better understand the mechanisms associated to the plasma processes, a zero-dimensional plasma chemistry model was used to study the chemical kinetics of an atmospheric-pressure, nanosecond pulsed He-O2 plasma jet. Density kinetics of the charged and neutral species in the plasma jet as a function of the input parameters including the oxygen concentration and the electric field are calculated. The roles played by different reactions in the kinetics schemes are assessed. In addition, both of He-O2 and He-O2-N2 plasma chemical kinetics reaction schemes are discussed to examine the effects of air entrainment on the characteristics of the plasma jet.

Xia, Shengguo; Jiang, Chunqi

2011-11-01

69

Calculation of the atomic kinetic energy from a density functional virial relationship  

Microsoft Academic Search

The kinetic energies of a series of atoms and ions have been calculated with a high degree of accuracy using some known exchange energy functional and the electrostatic potential. They are related to the kinetic energy functional through the virial equations. The kinetic energy contribution to the ionization potential has also been calculated to a very high degree of accuracy.

P. Fuentealba

1997-01-01

70

Modeling of homogeneous mercury speciation using detailed chemical kinetics  

Microsoft Academic Search

Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model consisting of 107 reactions and 30 species. This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with new reaction rate constants calculated neither from experimental data nor by estimated, which was commonly used by other

Minghou Xu; Yu Qiaoa; Chuguang Zheng; Laicai Lib; Jing Liu

2003-01-01

71

Combustion Research Program: Flame studies, laser diagnostics, and chemical kinetics  

SciTech Connect

This project has comprised laser flame diagnostic experiments, chemical kinetics measurements, and low pressure flame studies. Collisional quenching has been investigated for several systems: the OH radical, by H{sub 2}0 in low pressure flames; the rotational level dependence for NH, including measurements to J=24; and of NH{sub 2} at room temperature. Transition probability measurements for bands involving v{prime} = 2 and 3 of the A-X system of OH were measured in a flame. Laser-induced fluorescence of vinyl radicals was unsuccessfully attempted. RRKM and transition state theory calculations were performed on the OH + C{sub 2}H{sub 4} reaction, on the t-butyl radical + HX; and transition state theory has been applied to a series of bond scission reactions. OH concentrations were measured quantitatively in low pressure H{sub 2}/N{sub 2}O and H{sub 2}/O{sub 2} flames, and the ability to determine spatially precise flame temperatures accurately using OH laser-induced fluorescence was studied.

Crosley, D.R.

1992-09-01

72

Kinetic and Chemical Mechanism of Malate Synthase from Mycobacterium tuberculosis  

PubMed Central

Malate synthase catalyzes the Claisen-like condensation of acetyl-coenzyme A and glyoxylate in the glyoxylate shunt of the citric acid cycle. The Mycobacterium tuberculosis malate synthase G gene, glcB, was cloned, and the N-terminal His6 tagged 80 kDa protein was expressed in soluble form and purified by metal affinity chromatography. A chromogenic 4,4?-dithiodipyridine assay did not yield linear kinetics, but the generation of an active site directed mutant, C619S, gave an active enzyme and linear kinetics. The resulting mutant exhibited comparable kinetics to wild type and was used for the full kinetic analysis. Initial velocity studies were intersecting suggesting a sequential mechanism, which was confirmed by product and dead-end inhibition. The inhibition studies delineated the ordered binding of glyoxylate followed by AcCoA and the ordered release of CoA followed by malate. The pH dependence of kcat and kcat/Kgly are both bell-shaped and catalysis depends on a general base (pK 5.3) and a general acid (pK 9.2). Primary kinetic isotope effects determined using [C2H3-methyl] acetyl-CoA suggested that proton removal and carbon-carbon bond formation were partially rate-limiting. Solvent kinetic isotope effects on kcat suggested the hydrolysis of the malyl-CoA intermediate was also partially rate-limiting. Multiple kinetic isotope effects, utilizing D2O and [C2H3-methyl] acetyl-CoA, confirmed a stepwise mechanism in which the step exhibiting primary kinetic isotope effects precedes the step exhibiting the solvent isotope effects. The kinetic data and the pH dependence of the kinetic parameters were combined with existing structural and mutagenesis data to propose a chemical mechanism for malate synthase from Mycobacterium tuberculosis.

Quartararo, Christine E.; Blanchard, John S.

2011-01-01

73

Formulation of kinetic equations for chemically reacting gases  

NASA Astrophysics Data System (ADS)

The article gives a brief discussion of problems arising with the derivation and formulation of kinetic equations for chemically reacting gases. It is remarked that, for an adequate description of the kinetics of chemical reactions of monatomic molecules, in such equations metastable bound states of the particles must be taken into consideration. In general form, equations of low approximation are formulated for an arbitrary set of reactions between the molecules of a mixture; these are a generalization of the Waldmann-Snyder equations and make it possible to describe the hydrodynamics and the kinetics of reactions in polarized media. The article discusses the transition from these equations to equations for the diagonal elements of matrices of the density (generalized Boltzmann equations), and the latter are formulated. The laws of conservation of mass, momentum, and energy are obtained for all the proposed equations and the H-theorem is proved.

Kolesnichenko, E. G.

1980-07-01

74

Model reduction for chemical kinetics: An optimization approach  

SciTech Connect

The kinetics of a detailed chemically reacting system can potentially be very complex. Although the chemist may be interested in only a few species, the reaction model almost always involves a much larger number of species. Some of those species are radicals, which are very reactive species and can be important intermediaries in the reaction scheme. A large number of elementary reactions can occur among the species; some of these reactions are fast and some are slow. The aim of simplified kinetics modeling is to derive the simplest reaction system which retains the essential features of the full system. An optimization-based method for reduction of the number of species and reactions in chemical kinetics model is described. Numerical results for several reaction mechanisms illustrate the potential of this approach.

Petzold, L. [Univ. of California, Santa Barbara, CA (United States). Dept. of Mechanical and Environmental Engineering; Zhu, W. [Univ. of Minnesota, Minneapolis, MN (United States). Dept. of Computer Science

1999-04-01

75

Chemical Kinetics at the Single-Molecule Level  

ERIC Educational Resources Information Center

|For over a century, chemists have investigated the rates of chemical reactions using experimental conditions involving huge numbers of molecules. As a consequence, the description of the kinetics of the reaction in terms of average values was good enough for all practical purposes. From the pedagogical point of view, such a description misses the…

Levitus, Marcia

2011-01-01

76

Chemical Kinetic Studies on Dry Sorbents. Final Report.  

National Technical Information Service (NTIS)

The scope of this research investigation has included a review of potential additives suitable for dry flue-gas desulfurization (FGD) and a bench scale laboratory study to determine the chemical kinetics for the reaction of five different sorbents with su...

T. C. Keener W. T. Davis

1982-01-01

77

Evaluated Chemical Kinetic Rate Constants for Various Gas Phase Reactions  

Microsoft Academic Search

The available information, up to mid-1972, for the rate constants of a series of gas phase chemical reactions has been evaluated critically. For each reaction, relevant thermodynamic data are presented and values for the equilibrium constant expressed in mathematical form. Kinetic data are presented in tabular and graphical form together with a discussion of the pertinent details. Recommended rate constant

Keith Schofield

1973-01-01

78

Prospective Chemistry Teachers' Conceptions of Chemical Thermodynamics and Kinetics  

ERIC Educational Resources Information Center

|This study aimed at identifying specifically prospective chemistry teachers' difficulties in determining the differences between the concepts of chemical thermodynamics and kinetics. Data were collected from 67 prospective chemistry teachers at Kazim Karabekir Education Faculty of Ataturk University in Turkey during 2005-2006 academic year. Data…

Sozbilir, Mustafa; Pinarbasi, Tacettin; Canpolat, Nurtac

2010-01-01

79

Model reduction for chemical kinetics: An optimization approach  

Microsoft Academic Search

The kinetics of a detailed chemically reacting system can potentially be very complex. Although the chemist may be interested in only a few species, the reaction model almost always involves a much larger number of species. Some of those species are radicals, which are very reactive species and can be important intermediaries in the reaction scheme. A large number of

Linda Petzold; Wenjie Zhu

1999-01-01

80

Infrared Absorption Spectroscopy and Chemical Kinetics of Free Radicals  

Microsoft Academic Search

This research was directed at the detection, monitoring, and study of the chemical kinetic behavior by infrared absorption spectroscopy of small free radical species thought to be important intermediates in combustion. Work on the reaction of OH with acetaldehyde has been completed and published and work on the reaction of O(¹D) with CHâ has been completed and submitted for publication.

Robert F. Curl; Graham Glass

1994-01-01

81

Chemkin-II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics  

SciTech Connect

This document is the user's manual for the second-generation Chemkin package. Chemkin is a software package for whose purpose is to facilitate the formation, solution, and interpretation of problems involving elementary gas-phase chemical kinetics. It provides an especially flexible and powerful tool for incorporating complex chemical kinetics into simulations of fluid dynamics. The package consists of two major software components: an Interpreter and Gas-Phase Subroutine Library. The Interpreter is a program that reads a symbolic description of an elementary, user-specified chemical reaction mechanism. One output from the Interpreter is a data file that forms a link to the Gas-Phase Subroutine Library. This library is a collection of about 100 highly modular Fortran subroutines that may be called to return information on equation of state, thermodynamic properties, and chemical production rates.

Kee, R.J.; Rupley, F.M.; Miller, J.A.

1989-09-01

82

A pared-down gas-phase kinetics for the chemical oxygen-iodine laser medium  

NASA Astrophysics Data System (ADS)

Kinetic data obtained in the last decade has resulted in revisions of some mechanisms of excitation and deactivation of excited states in the chemical oxygen-iodine laser (COIL) medium. This review considers new kinetic data and presents analyses of the mechanisms of pumping and quenching of electronically and vibrationally excited states in the oxygen-iodine laser media. An effective three-level model of I2 molecule excitation and relaxation has been developed. The calculated effective rate constants for deactivation of I2(X,11 ? ? ? 24) by O2, N2, He and CO2 are presented. A simplified kinetic package for the COIL active medium is recommended. This model consists of a 30-reaction set with 14 species. The results of calculations utilizing simplified model are in good agreement with the experimental data.

Pichugin, S. Yu.; Heaven, M. C.

2013-11-01

83

Chemical Kinetics of Hydrocarbon Ignition in Practical Combustion Systems  

SciTech Connect

Chemical kinetic factors of hydrocarbon oxidation are examined in a variety of ignition problems. Ignition is related to the presence of a dominant chain branching reaction mechanism that can drive a chemical system to completion in a very short period of time. Ignition in laboratory environments is studied for problems including shock tubes and rapid compression machines. Modeling of the laboratory systems are used to develop kinetic models that can be used to analyze ignition in practical systems. Two major chain branching regimes are identified, one consisting of high temperature ignition with a chain branching reaction mechanism based on the reaction between atomic hydrogen with molecular oxygen, and the second based on an intermediate temperature thermal decomposition of hydrogen peroxide. Kinetic models are then used to describe ignition in practical combustion environments, including detonations and pulse combustors for high temperature ignition, and engine knock and diesel ignition for intermediate temperature ignition. The final example of ignition in a practical environment is homogeneous charge, compression ignition (HCCI) which is shown to be a problem dominated by the kinetics intermediate temperature hydrocarbon ignition. Model results show why high hydrocarbon and CO emissions are inevitable in HCCI combustion. The conclusion of this study is that the kinetics of hydrocarbon ignition are actually quite simple, since only one or two elementary reactions are dominant. However, there are many combustion factors that can influence these two major reactions, and these are the features that vary from one practical system to another.

Westbrook, C.K.

2000-07-07

84

Calculation of Chemical Shift Anisotropy in Proteins  

PubMed Central

Individual peptide groups in proteins must exhibit some variation in the chemical shift anisotropy (CSA) of their constituent atoms, but not much is known about the extent or origins of this dispersion. Direct spectroscopic measurement of CSA remains technically challenging, and theoretical methods can help to overcome these limitations by estimating shielding tensors for arbitrary structures. Here we use an automated fragmentation quantum mechanics/molecular mechanics (AF-QM/MM) approach to compute 15N, 13C? and 1H chemical shift tensors for human ubiquitin and the GB1 and GB3 fragments of staphylococcal protein G. The average and range of variation of the anisotropies is in good agreement with experimental estimates from solid-state NMR, and the variation among residues is somewhat smaller than that estimated from solution-state measurements. Hydrogen-bond effects account for much of the variation, both between helix and sheet regions, and within elements of secondary structure, but other effects (including variations in torsion angles) may play a role as well.

Tang, Sishi; Case, David A.

2011-01-01

85

Numerical Simulation of SNCR Technology with Simplified Chemical Kinetics Model  

NASA Astrophysics Data System (ADS)

The paper deals with numerical simulation of SNCR method. For numerical modelling was used CFD code Ansys/CFX. SNCR method was described by dominant chemical reaction, which were look up NIST Chemical database. The reactions including reduction of NOx and concentration change of pollutants, like N2O and CO in flue gas too. Proposed chemical kinetics and CFD model was applied to two boilers. Both simulations were compared with experimental measurements. First simulation was used to validation of chemical mechanism. Second simulation was based on first simulation and it was used to verification of compiled SNCR chemical mechanism. Next the new variant of the reagent penetration lance was proposed and compared with the original variants.

Blejcha?, T.; Dolní?ková, D.

2013-04-01

86

DNA as a universal substrate for chemical kinetics  

PubMed Central

Molecular programming aims to systematically engineer molecular and chemical systems of autonomous function and ever-increasing complexity. A key goal is to develop embedded control circuitry within a chemical system to direct molecular events. Here we show that systems of DNA molecules can be constructed that closely approximate the dynamic behavior of arbitrary systems of coupled chemical reactions. By using strand displacement reactions as a primitive, we construct reaction cascades with effectively unimolecular and bimolecular kinetics. Our construction allows individual reactions to be coupled in arbitrary ways such that reactants can participate in multiple reactions simultaneously, reproducing the desired dynamical properties. Thus arbitrary systems of chemical equations can be compiled into real chemical systems. We illustrate our method on the Lotka–Volterra oscillator, a limit-cycle oscillator, a chaotic system, and systems implementing feedback digital logic and algorithmic behavior.

Soloveichik, David; Seelig, Georg; Winfree, Erik

2010-01-01

87

Accelerating the Computation of Detailed Chemical Reaction Kinetics for Simulating Combustion of Complex Fuels  

SciTech Connect

Combustion of hydrocarbon fuels has been a very challenging scientific and engineering problem due to the complexity of turbulent flows and hydrocarbon reaction kinetics. There is an urgent need to develop an efficient modeling capability to accurately predict the combustion of complex fuels. Detailed chemical kinetic models for the surrogates of fuels such as gasoline, diesel and JP-8 consist of thousands of chemical species and Arrhenius reaction steps. Oxygenated fuels such as bio-fuels and heavier hydrocarbons, such as from newer fossil fuel sources, are expected to have a much more complex chemistry requiring increasingly larger chemical kinetic models. Such models are beyond current computational capability, except for homogeneous or partially stirred reactor type calculations. The advent of highly parallel multi-core processors and graphical processing units (GPUs) promises a steep increase in computational performance in the coming years. This paper will present a software framework that translates the detailed chemical kinetic models to high-performance code targeted for GPU accelerators.

Sankaran, R.; Grout, R.

2012-01-01

88

Accelerating the Computation of Detailed Chemical Reaction Kinetics for Simulating Combustion of Complex Fuels  

SciTech Connect

Combustion of hydrocarbon fuels has been a very challenging scientific and engineering problem due to the complexity of turbulent flows and hydrocarbon reaction kinetics. There is an urgent need to develop an efficient modeling capability to accurately predict the combustion of complex fuels. Detailed chemical kinetic models for the surrogates of fuels such as gasoline, diesel and JP-8 consist of thousands of chemical species and Arrhenius reaction steps. Oxygenated fuels such as bio-fuels and heavier hydrocarbons, such as from newer fossil fuel sources, are expected to have a much more complex chemistry requiring increasingly larger chemical kinetic models. Such models are beyond current computational capability, except for homogeneous or partially stirred reactor type calculations. The advent of highly parallel multi-core processors and graphical processing units (GPUs) promises a steep increase in computational performance in the coming years. This paper will present a software framework that translates the detailed chemical kinetic models to high- performance code targeted for GPU accelerators.

Grout, Ray W [ORNL

2012-01-01

89

Parallel in Time Simulation of Multiscale Stochastic Chemical Kinetics  

Microsoft Academic Search

A version of the time-parallel algorithm parareal is analyzed and applied to\\u000astochastic models in chemical kinetics. A fast predictor at the macroscopic\\u000ascale (evaluated in serial) is available in the form of the usual reaction rate\\u000aequations. A stochastic simulation algorithm is used to obtain an exact\\u000arealization of the process at the mesoscopic scale (in parallel).\\u000a The underlying

Stefan Engblom

2009-01-01

90

Chemical Kinetic Modeling of Combustion of Automotive Fuels  

SciTech Connect

The objectives of this report are to: (1) Develop detailed chemical kinetic reaction models for components of fuels, including olefins and cycloalkanes used in diesel, spark-ignition and HCCI engines; (2) Develop surrogate mixtures of hydrocarbon components to represent real fuels and lead to efficient reduced combustion models; and (3) Characterize the role of fuel composition on production of emissions from practical automotive engines.

Pitz, W J; Westbrook, C K; Silke, E J

2006-11-10

91

Kinetic and chemical mechanisms of shikimate dehydrogenase from Mycobacterium tuberculosis.  

PubMed

Mycobacterium tuberculosis shikimate dehydrogenase (MtbSD) catalyzes the fourth reaction in the shikimate pathway, the NADPH-dependent reduction of 3-dehydroshikimate. To gather information on the kinetic mechanism, initial velocity patterns, product inhibition, and primary deuterium kinetic isotope effect studies were performed and the results suggested a steady-state ordered bi-bi kinetic mechanism. The magnitudes of both primary and solvent kinetic isotope effects indicated that the hydride transferred from NADPH and protons transferred from the solvent in the catalytic cycle are not significantly rate limiting in the overall reaction. Proton inventory analysis indicates that one proton gives rise to solvent isotope effects. Multiple isotope effect studies indicate that both hydride and proton transfers are concerted. The pH profiles revealed that acid/base chemistry takes place in catalysis and substrate binding. The MtbSD 3D model was obtained in silico by homology modeling. Kinetic and chemical mechanisms for MtbSD are proposed on the basis of experimental data. PMID:17178095

Fonseca, Isabel O; Silva, Rafael G; Fernandes, Claudia L; de Souza, Osmar N; Basso, Luiz A; Santos, Diógenes S

2006-11-21

92

Quantum-chemical Calculations in the Study of Antitumour Compounds  

NASA Astrophysics Data System (ADS)

The results of quantum-chemical calculations on antitumour preparations concerning the mechanism of their action at the electronic and molecular levels and structure-activity correlations are discussed in this review. Preparations whose action involves alkylating and free-radial mechanisms, complex-forming agents, and antimetabolites are considered. Modern quantum-chemical methods for calculations on biologically active substances are described. The bibliography includes 106 references.

Luzhkov, V. B.; Bogdanov, G. N.

1986-01-01

93

Chemical Kinetic Models for HCCI and Diesel Combustion  

SciTech Connect

Hydrocarbon fuels for advanced combustion engines consist of complex mixtures of hundreds or even thousands of different components. These components can be grouped into a number of chemically distinct classes, consisting of n-paraffins, branched paraffins, cyclic paraffins, olefins, oxygenates, and aromatics. Biodiesel contains its own unique chemical class called methyl esters. The fractional amounts of these chemical classes are quite different in gasoline, diesel fuel, oil-sand derived fuels and bio-derived fuels, which contributes to the very different combustion characteristics of each of these types of combustion systems. The objectives of this project are: (1) Develop detailed chemical kinetic models for fuel components used in surrogate fuels for diesel and HCCI engines; (2) Develop surrogate fuel models to represent real fuels and model low temperature combustion strategies in HCCI and diesel engines that lead to low emissions and high efficiency; and (3) Characterize the role of fuel composition on low temperature combustion modes of advanced combustion engines.

Pitz, W J; Westbook, C K; Mehl, M

2008-10-30

94

Kinetic theory of steady chemical nucleation in the gas phase  

NASA Astrophysics Data System (ADS)

We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the effective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in a multi-component vapor, in which nucleation occurs via the chemical reactions yielding a condensate having a stoichiometric composition. An exact expression of the steady nucleation rate is given together with its approximate formulas for practical applications. The present formulation is not concerned with any particular cluster model. The supersaturation ratio for a many-component vapor is defined so as to be a natural extension of that for a one-component vapor. It is shown that the transition probabilities due to growth and decay of the clusters are of the same form as the growth and evaporation rates in a one-component vapor.

Yamamoto, T.; Chigai, T.; Watanabe, S.; Kozasa, T.

2001-12-01

95

Free energy calculations: Applications to chemical and biochemical phenomena  

SciTech Connect

The author will review the applications of free energy calculations employing molecular dynamics or Monte Carlo methods to a variety of chemical and biochemical phenomena. The focus is on the applications of such calculations to molecular solvation, molecular association, macromolecular stability, and enzyme catalysis. The molecules discussed range from monovalent ions and small molecules to proteins and nucleic acids.

Kollman, P. [Univ. of California, San Francisco, CA (United States). Dept. of Pharmaceutical Chemistry

1993-11-01

96

Carbon Footprint Calculations: An Application of Chemical Principles  

ERIC Educational Resources Information Center

|Topics commonly taught in a general chemistry course can be used to calculate the quantity of carbon dioxide emitted into the atmosphere by various human activities. Each calculation begins with the balanced chemical equation for the reaction that produces the CO[subscript 2] gas. Stoichiometry, thermochemistry, the ideal gas law, and dimensional…

Treptow, Richard S.

2010-01-01

97

Free energy calculations: Applications to chemical and biochemical phenomena  

Microsoft Academic Search

The author will review the applications of free energy calculations employing molecular dynamics or Monte Carlo methods to a variety of chemical and biochemical phenomena. The focus is on the applications of such calculations to molecular solvation, molecular association, macromolecular stability, and enzyme catalysis. The molecules discussed range from monovalent ions and small molecules to proteins and nucleic acids.

Peter. Kollman

1993-01-01

98

Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Supplement II. CODATA Task Group on Gas Phase Chemical Kinetics  

Microsoft Academic Search

This paper updates and extends previous critical evaluations of the kinetics and photochemistry of gas phase chemical reactions of neutral species involved in atmosphere chemistry [J. Phys. Chem. Ref. Data 9, 295 (1980); 11 327 (1982)]. The work has been carried out by the authors under the auspices of the CODATA Task Group on Gas Phase Chemical Kinetics. Data sheets

D. L. Baulch; R. A. Cox; R. F. Hampson; J. Troe; R. T. Watson

1984-01-01

99

Chemical-kinetic prediction of critical parameters in gaseous detonations  

SciTech Connect

A theoretical model including a detailed chemical kinetic reaction mechanism for hydrogen and hydrocarbon oxidation is used to examine the effects of variations in initial pressure and temperature on the detonation properties of gaseous fuel-oxidizer mixtures. Fuels considered include hydrogen, methane, ethane, ethylene, and acetylene. Induction lengths are computed for initial pressures between 0.1 and 10.0 atmospheres and initial temperatures between 200K and 500K. These induction lengths are then compared with available experimental data for critical energy and critical tube diameter for initiation of spherical detonation, as well as detonation limits in linear tubes. Combined with earlier studies concerning variations in fuel-oxidizer equivalence ratio and degree of dilution with N/sub 2/, the model provides a unified treatment of fuel oxidation kinetics in detonations. 4 figures, 1 table.

Westbrook, C.K.; Urtiew, P.A.

1982-01-12

100

Chemical kinetics in chemical vapor deposition: growth of silicon dioxide from tetraethoxysilane (TEOS)  

Microsoft Academic Search

Chemical reactions in the gas-phase and on surfaces are important in the chemical vapor deposition (CVD) of materials for microelectronic applications. General approaches for modeling the homogeneous and heterogeneous kinetics in CVD are discussed. A software framework for implementing the theory utilizing the CHEMKIN suite of codes is presented. Specific examples are drawn from the CVD of SiO2 using tetraethoxysilane

Michael E. Coltrin; Pauline Ho; Harry K. Moffat; Richard J. Buss

2000-01-01

101

Chemical compounds in the remote Pacific troposphere: Comparison between MLOPEX measurements and chemical transport model calculations  

Microsoft Academic Search

A global three-dimensional chemical transport model, called MOZART (Model of OZone And Related species in the Troposphere), is used to compare calculated abundances of chemical species and their seasonal evolution in the remote Pacific troposphere near Hawaii with values observed during the Mauna Loa Observatory Photochemistry Experiments (MLOPEX 1 and 2). MOZART is a fully diurnal model which calculates the

G. P. Brasseur; D. A. Hauglustaine; S. Walters

1996-01-01

102

PLASMAKIN: A chemical kinetics library for plasma physics modeling  

NASA Astrophysics Data System (ADS)

PLASMAKIN is a software library to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces - particle generation and loss rates, photon emission rates and energy exchange rates. PLASMAKIN supports any number of chemical species and reactions, is independent of problem dimensions and can be used both in stationary and time-dependent problems. PLASMAKIN supports a large number of species properties and reaction types. Namely gas or electron temperature dependent collision rate coefficients, vibrational and cascade levels, evaluation of branching ratios, superelastic and other reverse processes, three-body collisions, radiation imprisonment and photoelectric emission. Non-standard rate coefficient functions can be handled by a user supplied routine. The interaction of the user with the library is limited to the preparation of an input file characterizing the interacting species and reactions, accessing the data or the chemical kinetics results through a reduced number of procedure calls with similar interfaces. PLASMAKIN allows the user to concentrate on the physical nature of the problems independently of the gas mixture properties and reactions model. As an example a program has been prepared to evaluate the maintenance field, equilibrium populations, relative contribution of each reaction, photon emission spectra and the energy losses rates on the positive column region of a discharge in electropositive gases. The program is applied to a discharge in neon. The library is written in Fortran 95 and has error support.

Pinha~o, N. R.

2001-03-01

103

Progress in Chemical Kinetic Modeling for Surrogate Fuels  

SciTech Connect

Gasoline, diesel, and other alternative transportation fuels contain hundreds to thousands of compounds. It is currently not possible to represent all these compounds in detailed chemical kinetic models. Instead, these fuels are represented by surrogate fuel models which contain a limited number of representative compounds. We have been extending the list of compounds for detailed chemical models that are available for use in fuel surrogate models. Detailed models for components with larger and more complicated fuel molecular structures are now available. These advancements are allowing a more accurate representation of practical and alternative fuels. We have developed detailed chemical kinetic models for fuels with higher molecular weight fuel molecules such as n-hexadecane (C16). Also, we can consider more complicated fuel molecular structures like cyclic alkanes and aromatics that are found in practical fuels. For alternative fuels, the capability to model large biodiesel fuels that have ester structures is becoming available. These newly addressed cyclic and ester structures in fuels profoundly affect the reaction rate of the fuel predicted by the model. Finally, these surrogate fuel models contain large numbers of species and reactions and must be reduced for use in multi-dimensional models for spark-ignition, HCCI and diesel engines.

Pitz, W J; Westbrook, C K; Herbinet, O; Silke, E J

2008-06-06

104

STANDARD CURVES AND FORMULAE FOR NEUTRON KINETICS CALCULATIONS  

Microsoft Academic Search

The response of the neutron kinetic equations to a wide range of step ; and ramp additions of reactivity has been evaluated on the PACE 231R analogue ; computer for two fuels, U²³⁵ and Pu²³⁹, with a full range of ; neutron lifetimes. The results are presented in the form of standard curves that ; may be readily used to

1962-01-01

105

An automatic chemical lumping method for the reduction of large chemical kinetic mechanisms  

Microsoft Academic Search

A novel approach to the lumping of species in large chemical kinetic mechanisms is presented. Species with similar composition and functionalities are lumped into one single representative species. Simulations using the detailed scheme are used to gather statistical information on the distribution of the isomers within each lump group. These distributions are functions of space and time. Closure is performed

P. Pepiot-Desjardins; H. Pitsch

2008-01-01

106

Thermodynamic and Kinetic Properties of Metal Hydrides from First-Principles Calculations  

NASA Astrophysics Data System (ADS)

In an effort to minimize the worldwide dependence on fossil fuels, much research has focused on the development of hydrogen fuel cell vehicles. Among the many challenges currently facing the transition to such an alternative energy economy is the storage of hydrogen in an economical and practical way. One class of materials that has presented itself as a possible candidate is solid metal hydrides. These materials chemically bind hydrogen and on heating, release the gas which can then be used to generate power as needed for the vehicle. In order to meet guidelines that have been set for such a storage system, hydrogen must be released rapidly in a narrow temperature range of -40 to 80°C with all reactions being reversible. This sets both thermodynamic and kinetic requirements for the design of candidate metal hydrides. First-principles calculations are well-suited for the task of exploring reactions involving metal hydrides. Here, density-functional theory is used to calculate properties of these materials at the quantum mechanical level of accuracy. In particular, three systems have been investigated: 1. Li-Mg-N-H. Reactions between all known compounds in this system are systematically investigated in order to predict thermodynamically allowed reactions that release hydrogen. The properties of these reactions are compared to the requirements set for hydrogen storage systems. Additionally, ground-state structures are predicted for Li2Mg(NH)2 and Li 4Mg(NH)3. 2. Na-Al-H. The kinetics of mass transport during the (de)hydrogenation of the well-known metal hydride NaAlH4 are investigated. A model is developed to study the flux of native defects through phases involved in these reactions. Since it is also known that titanium is an effective catalyst for both dehydrogenation and rehydrogenation, the effect of Ti substitution in bulk lattices on the kinetics of mass transport is investigated. Results are compared to experiments in order to determine if mass transport represents the rate-limiting process during de- or rehydrogenation and what the effect of Ti may be. 3. Si-H. Properties of the recently synthesized compound SiH4(H 2)2 are investigated. Under high pressures, hydrogen binding to SiH4 exhibits characteristics of both physical and chemical bonds. A ground-state structure is predicted for this phase and the vibrational and bonding properties are investigated in order to determine the origin of the unusual binding between H2 and SiH4.

Michel, Kyle Jay

107

Reflected kinetics model for nuclear space reactor kinetics and control scoping calculations  

SciTech Connect

The objective of this research is to develop a model that offers an alternative to the point kinetics (PK) modelling approach in the analysis of space reactor kinetics and control studies. Modelling effort will focus on the explicit treatment of control drums as reactivity input devices so that the transition to automatic control can be smoothly done. The proposed model is developed for the specific integration of automatic control and the solution of the servo mechanism problem. The integration of the kinetics model with an automatic controller will provide a useful tool for performing space reactor scoping studies for different designs and configurations. Such a tool should prove to be invaluable in the design phase of a space nuclear system from the point of view of kinetics and control limitations.

Washington, K.E.

1986-05-01

108

ACFM, A Computational Framework for Magnetofluid and Kinetic Calculations  

Microsoft Academic Search

We present a general computational framework, ACFM, for continuum calculations that can be used to address many of the computational physics problems in plasma physics\\/magnetic fusion research. The explicit goal of the project is to bridge the gap between the physical\\/mathematical formulation of a problem and its computational implementation in a simple and efficient manner, thus removing \\

Ahmet Y. Aydemir

2002-01-01

109

Gas phase chemical kinetics of sodium in the upper atmosphere  

Microsoft Academic Search

More reasonable rate constants are estimated for the chemical reactions which involve atomic sodium and atmospheric oxygen species and which are responsible for upper-atmospheric chemiluminescence. The sodium-oxygen atmospheric reaction sequence is outlined, and reactive cross sections as well as rate constants for an ambient temperature of 185 K are calculated for reactions between Na and O3, NaO and O, and

C. E. Kolb; J. B. Elgin

1976-01-01

110

Grain-growth kinetics in wadsleyite: Effects of chemical environment  

NASA Astrophysics Data System (ADS)

Grain-growth kinetics in wadsleyite was investigated using a multianvil high-pressure apparatus. Fine-grained wadsleyite aggregates were synthesized by isostatic hot-pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment. Wadsleyite samples show normal grain-growth characterized by a log-normal grain-size distribution following the relation, L-L0n=kt where n is a constant, L the grain-size at time t, L0 the grain-size at time t = 0 and k is a rate constant that depends on temperature T and chemical environments ( f: oxygen fugacity in Pa, COH: water content in H/10 6Si) as: k=ADfOr exp{-HD*}/{RT}+AWfOrCOHq exp{-HW*}/{RT} with AD=10 (m s Pa), rD = 0.12 ± 0.11(0.20 ± 0.14), HD* = 410 ± 230(500 ± 270) kJ/mol, AW=10 (m s Pa), rW = 0.14 ± 0.05(0.22 ± 0.06), q = 1.7 ± 0.3(2.2 ± 0.3) and HW* = 120 ± 60(160 ± 70) kJ/mol with assumed value of n = 2(3) (values in parentheses denote parameters for n = 3). Both water and oxygen fugacities significantly enhance grain-growth kinetics. The large value of the parameter describing the water fugacity dependence, q ˜ 1.5-2.5, cannot be explained solely by a simple model in which grain-growth is controlled by diffusion of atoms (defects) across the grain-boundaries The interaction of grain-boundaries with charged defects or the density of hydrated ledges may be important factors that control the grain-growth kinetics of wadsleyite. When compared at similar thermo-chemical conditions, grain-growth of wadsleyite is found to be more sluggish than grain-growth of olivine. The present results show that a small wadsleyite grain-size (<1 mm) in subducting slabs can be maintained for a significant geological time (˜1 My) under "dry" (<200 H/10 6Si) conditions when the temperature is lower than 1500 K, whereas when a large amount of water (>100,000 H/10 6Si) is present, a small grain-size (<1 mm) can only be maintained for a significant time at low temperatures (<600 K).

Nishihara, Yu; Shinmei, Toru; Karato, Shun-ichiro

2006-01-01

111

Free-Electron Lasers, Thermal Diffusion, Chemical Kinetics, and Surgery  

NASA Astrophysics Data System (ADS)

Experiments demonstrate that the Mark-III FEL is a particularly effective tool for etching soft matter with remarkably little damage surrounding the site when tuned to wavelengths near 6.45 microns. Based on these observatons, human neuorsurgical and ophthalmic procedures were developed and have been performed successfully. A thermodynamic model was proposed to account for the wavelength dependence; however, the dynamics have not been well understood. We have theoretically investigated thermal diffusion and chemical kinetics in a system of alternating layers of protein and water as heated by a Mark-III FEL. The model is representative of cornea and the exposure conditions are comparable to previous experimental FEL investigations. A substantial temperature enhancement develops in the surface layer on the ten-nanosecond time scale. We consider the onset of both the helix-coil transition and chemical bond breaking of collagen in terms of the thermal, chemical, and structural properties of the system as well as laser wavelength and pulse structure.

Edwards, Glenn; Hutson, M. Shane

2001-11-01

112

Quantum effects in ab-initio calculations of rate constants for chemical reactions occuring in the condensed phase  

Microsoft Academic Search

An overview of recent advances in the development of methods designed to calculate rate constants for chemical reactions obeying mass action kinetic equations in condensed phases is presented. A general framework addressing mixed quantum-classical systems is elaborated that enables quantum features such as tunneling effects, zero-point vibrations, dynamic quantum coherence, and non-adiabatic effects to be calculated. An efficient Monte Carlo

Jeremy Schofield

2006-01-01

113

GPU Accelerated Chemical Similarity Calculation for Compound Library Comparison  

PubMed Central

Chemical similarity calculation plays an important role in compound library design, virtual screening, and “lead” optimization. In this manuscript, we present a novel GPU-accelerated algorithm for all-vs-all Tanimoto matrix calculation and nearest neighbor search. By taking advantage of multi-core GPU architecture and CUDA parallel programming technology, the algorithm is up to 39 times superior to the existing commercial software that runs on CPUs. Because of the utilization of intrinsic GPU instructions, this approach is nearly 10 times faster than existing GPU-accelerated sparse vector algorithm, when Unity fingerprints are used for Tanimoto calculation. The GPU program that implements this new method takes about 20 minutes to complete the calculation of Tanimoto coefficients between 32M PubChem compounds and 10K Active Probes compounds, i.e., 324G Tanimoto coefficients, on a 128-CUDA-core GPU.

Ma, Chao; Wang, Lirong; Xie, Xiang-Qun

2012-01-01

114

Exact non-additive kinetic potentials in realistic chemical systems  

NASA Astrophysics Data System (ADS)

In methods based on frozen-density embedding theory or subsystem formulation of density functional theory, the non-additive kinetic potential (vtnad(r)) needs to be approximated. Since vtnad(r) is defined as a bifunctional, the common strategies rely on approximating vtnad[?A,?B](r). In this work, the exact potentials (not bifunctionals) are constructed for chemically relevant pairs of electron densities (?A and ?B) representing: dissociating molecules, two parts of a molecule linked by a covalent bond, or valence and core electrons. The method used is applicable only for particular case, where ?A is a one-electron or spin-compensated two-electron density, for which the analytic relation between the density and potential exists. The sum ?A + ?B is, however, not limited to such restrictions. Kohn-Sham molecular densities are used for this purpose. The constructed potentials are analyzed to identify the properties which must be taken into account when constructing approximations to the corresponding bifunctional. It is comprehensively shown that the full von Weizsäcker component is indispensable in order to approximate adequately the non-additive kinetic potential for such pairs of densities.

de Silva, Piotr; Wesolowski, Tomasz A.

2012-09-01

115

Infrared Absorption Spectroscopy and Chemical Kinetics of Free Radicals  

SciTech Connect

This research was directed at the detection, monitoring, and study of the chemical kinetic behavior by infrared absorption spectroscopy of small free radical species thought to be important intermediates in combustion. Work on the reaction of OH with acetaldehyde has been completed and published and work on the reaction of O({sup 1}D) with CH{sub 4} has been completed and submitted for publication. In the course of our investigation of branching ratios of the reactions of O({sup 1}D) with acetaldehyde and methane, we discovered that hot atom chemistry effects are not negligible at the gas pressures (13 Torr) initially used. Branching ratios of the reaction of O({sup 1}D) with CH{sub 4} have been measured at a tenfold higher He flow and fivefold higher pressure.

Curl, Robert F; Glass, Graham

2004-11-01

116

Advanced software for the calculation of thermochemistry, kinetics, and dynamics.  

SciTech Connect

The Born-Oppenheimer separation of the Schroedinger equation allows the electronic and nuclear motions to be solved in three steps. (1) The solution of the electronic wave function at a discrete set of molecular conformations; (2) the fitting of this discrete set of energy values in order to construct an analytical approximation to the potential energy surface (PES) at all molecular conformations; (3) the use of this analytical PES to solve for the nuclear motion using either time-dependent or time-independent formulations to compute molecular energy values, chemical reaction rates, and cumulative reaction probabilities. This project involves the development of technology to address all three of these steps. This report focuses on our recent work on the optimization of nonlinear wave function parameters for the electronic wave functions.

Shepard, R.; Chemistry

2007-01-01

117

Improve Claus simulation by integrating kinetic limitations into equilibrium calculations  

SciTech Connect

The Claus process is the most widely used commercial process for element sulfur recovery from acid gas streams. The interest in this process is not only for economic reasons but because of the general concern for air pollution control to protect the environment. Computer-aided simulation is suited for designing a new Claus unit and can be used to search for better operating conditions (for an existing Claus plant). This simulation program is also useful for designing the upstream amine unit and the downstream tail gas clean-up unit. Modern current Claus simulators are all based on calculating the equilibrium compositions using the free energy minimization technique. The simulation results including the overall sulfur yield, air to acid gas ratio, and stream compositions do not satisfactorily represent plant data. This is not unexpected since no plant is operated under equilibrium conditions but under rate limitations.

Wen, T.C.; Chen, D.H.; Hopper, J.R.; Maddox, R.N.

1986-01-01

118

Detailed Chemical Kinetic Reaction Mechanisms for Incineration of Organophosphorus and Fluoro-Organophosphorus Compounds.  

National Technical Information Service (NTIS)

A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previ...

P. A. Glaude C. Melius W. J. Pitz C. K. Westbrook

2001-01-01

119

Relevance of thermodynamic and kinetic parameters of chemical vapor deposition precursors.  

PubMed

We have studied various metallorganic and organometallic compounds by simultaneous nonisothermal thermogravimetric and differential thermogravimetric analyses to confirm their volatility and thermal stability. The equilibrium vapor pressures of the metallorganic and organometallic compounds were determined by horizontal dual arm single furnace thermoanalyzer as transpiration apparatus. Antoine coefficients were calculated from the temperature dependence equilibrium vapor pressure data. The model-fitting solid-state kinetic analyses of Al(acac)3, (acac = acetylacetonato), Cr(CO)6, Fe(Cp)2, (Cp-cyclopentadienyl), Ga(acac)3, Mn(tmhd)3, and Y(tmhd)3 (tmhd = 2,2,6,6,-tetramethyl-3,5-heptanedionato) revealed that the processes follow diffusion controlled, contracting area and zero order model sublimation or evaporation kinetics. The activation energy for the sublimation/evaporation processes were calculated by model-free kinetic methods. Thin films of nickel and lanthanum-strontium-manganite (LSM) are grown on silicon substrate at 573 K using selected metallorganic complexes of Ni[(acac)2en], La(tmhd)3, Sr(tmhd)2 and Mn(tmhd)3 as precursors by plasma assisted liquid injection chemical vapor deposition (PA-LICVD). The deposited films were characterized by scanning electron microscopy and energy dispersive X-ray analysis for their composition and morphology. PMID:22097553

Selvakumar, J; Nagaraja, K S; Sathiyamoorthy, D

2011-09-01

120

Thermodynamic and kinetic consistency of calculated binary nucleation rates  

SciTech Connect

To establish the accuracy and applicability of analytical expressions for the steady state rate of binary nucleation, we numerically solved the birth-death equations for the vapor-to-liquid transition. These calculations were performed using rate coefficients that are consistent with the principle of detailed balance and a new self-consistent form of the equilibrium distribution function for binary cluster concentrations. We found that the customary saddle point and growth path approximations are almost always valid and can fail only if the nucleating solution phase is significantly nonideal. For example, problems can arise when the vapor composition puts the system on the verge of partial liquid phase miscibility. When this occurs for comparable monomer impingement rates, nucleation still occurs through the saddle point, but the usual quadratic expansion for the cluster free energy is inadequate. When the two impingement rates differ significantly, however, the major particle flux may bypass the saddle point and cross a low ridge on the free energy surface. The dependence of the saddle point location on the gas phase composition is also important in initiating or terminating ridge crossing nucleation.

Wilemski, G. [Lawrence Livermore National Lab., CA (United States); Wyslouzil, B.E. [Worcester Polytechnic Institute, MA (United States)

1996-04-02

121

Calculating kinetics parameters and reactivity changes with continuous-energy Monte Carlo  

SciTech Connect

The iterated fission probability interpretation of the adjoint flux forms the basis for a method to perform adjoint weighting of tally scores in continuous-energy Monte Carlo k-eigenvalue calculations. Applying this approach, adjoint-weighted tallies are developed for two applications: calculating point reactor kinetics parameters and estimating changes in reactivity from perturbations. Calculations are performed in the widely-used production code, MCNP, and the results of both applications are compared with discrete ordinates calculations, experimental measurements, and other Monte Carlo calculations.

Kiedrowski, Brian C [Los Alamos National Laboratory; Brown, Forrest B [Los Alamos National Laboratory; Wilson, Paul [UNIV. WISCONSIN

2009-01-01

122

Identifying Alternative Conceptions of Chemical Kinetics among Secondary School and Undergraduate Students in Turkey  

ERIC Educational Resources Information Center

|This study identifies some alternative conceptions of chemical kinetics held by secondary school and undergraduate students (N = 191) in Turkey. Undergraduate students who participated are studying to become chemistry teachers when they graduate. Students' conceptions about chemical kinetics were elicited through a series of written tasks and…

Cakmakci, Gultekin

2010-01-01

123

Uptake and release kinetics of 22 polar organic chemicals in the Chemcatcher passive sampler.  

PubMed

The Chemcatcher passive sampler, which uses Empore™ disks as sampling phase, is frequently used to monitor polar organic chemicals in river water and effluents. Uptake kinetics need to be quantified to calculate time-weighted average concentrations from Chemcatcher field deployments. Information on release kinetics is needed if performance reference compounds (PRCs) are used to quantify the influence of environmental conditions on the uptake. In a series of uptake and elimination experiments, we used Empore™ SDB disks (poly(styrenedivinylbenzene) copolymer modified with sulfonic acid groups) as a sampling phase and 22 compounds with a logK(ow) (octanol-water partitioning coefficient) range from -2.6 to 3.8. Uptake experiments were conducted in river water or tap water and lasted up to 25 days. Only 1 of 22 compounds (sulfamethoxazole) approached equilibrium in the uptake trials. Other compounds showed continuing non-linear uptake, even after 25 days. All compounds could be released from SDB disks, and desorption was proportionally higher in disks loaded for shorter periods. Desorption showed two-phase characteristics, and desorption was proportionally higher for passively sorbed compounds compared to actively loaded compounds (active loading was performed by pulling spiked river water over SDB disks using vacuum). We hypothesise that the two-phase kinetics and better retention of actively loaded compounds--and compounds loaded for a longer period--may be caused by slow diffusion of chemicals within the polymer. As sorption and desorption did not show isotropic kinetics, it is not possible to develop robust PRCs for adsorbent material like SDB disks. PMID:23532391

Vermeirssen, Etiënne L M; Dietschweiler, Conrad; Escher, Beate I; van der Voet, Jürgen; Hollender, Juliane

2013-03-27

124

Calculation of the chemical behavior of highly concentrated geothermal brines  

SciTech Connect

In this report, the authors describe the development of a chemical equilibrium model for hydrothermal waters based on the semiempirical equations of aqueous electrolyte solutions recently introduced by Pitzer and co-workers. Accurate solubility predictions (usually within 5-10% of experimental results) can be made for ionic strengths 0-20m. Comparison with experimental data indicates that a theory parameterized by binary and ternary data can be used to accurately predict solubilities in much more complex mixtures. Mineral solubilities calculated with this model are compared with those calculated from other currently available models. Whereas the predictions are typically within 5%, the ion pairing models are substantially in error at low ionic strengths. Recent results parameterizing the H{sub 2}S and SiO{sub 2} systems will be presented.

Moller-Weare, Nancy; Weare, John H.

1982-10-08

125

Application of a programmable pocket calculator to a single compartment mathematical model of solute kinetics.  

PubMed

A single compartment mathematical model has been applied to kinetics of small solutes (urea and creatinine) in dialysis therapy. The model can be described by two equations requiring iterative solution of calculated values, given several measurable variables. The equations have been programmed onto a Hewlett-Packard 65 pocket calculator and recorded on 3 X 1/2" magnetic strips, facilitating clinical application to dialysis therapy. PMID:1201656

Walker, W E; Hall, D A; Sanfelippo, M L; Swenson, R S

1975-12-01

126

Method of extrapolating inelastic deformations for the accelerated calculation of the kinetics of cyclic deformation  

Microsoft Academic Search

;I'hr calculation of the kinetics ryf inelastie deformation of structures makes it possible to use the most adecluate m\\/~dels of the medium and to obt,~ ~he most complete Information on the inelastic operation of structures .with ~real, often very complex programs of cyclic ~t, kermornechanlcaI actto~. Itowever, such a calculation is also the most l~:~0us one: before the cyclic de%rmatton

V. É. Gokhberg; O. S. Sadakov

1987-01-01

127

EQ6 Calculations for Chemical Degradation of Navy Waste Packages  

SciTech Connect

The Monitored Geologic Repository Waste Package Operations of the Civilian Radioactive Waste Management System Management & Operating Contractor (CRWMS M&O) performed calculations to provide input for disposal of spent nuclear fuel (SNF) from the Navy (Refs. 1 and 2). The Navy SNF has been considered for disposal at the potential Yucca Mountain site. For some waste packages, the containment may breach (Ref. 3), allowing the influx of water. Water in the waste package may moderate neutrons, increasing the likelihood of a criticality event within the waste package. The water may gradually leach the fissile components and neutron absorbers out of the waste package. In addition, the accumulation of silica (SiO{sub 2}) in the waste package over time may further affect the neutronics of the system. This study presents calculations of the long-term geochemical behavior of waste packages containing the Enhanced Design Alternative (EDA) II inner shell, Navy canister, and basket components. The calculations do not include the Navy SNF in the waste package. The specific study objectives were to determine the chemical composition of the water and the quantity of silicon (Si) and other solid corrosion products in the waste package during the first million years after the waste package is breached. The results of this calculation will be used to ensure that the type and amount of criticality control material used in the waste package design will prevent criticality.

S. LeStrange

1999-11-15

128

PLASMAKIN: A chemical kinetics library for plasma physics modeling  

NASA Astrophysics Data System (ADS)

PLASMAKIN is a package to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces: particle production and loss rates, photon spectra and energy exchange rates. It has no limits on the number of species and reactions that can be handled, is independent of problem dimensions and can be used in both steady-state and time-dependent problems. A broad range of species properties and reaction types are supported: gas or electron temperature dependent rate coefficients, vibrational and cascade levels, branching ratios, superelastic and other reverse processes, three-body collisions, radiation imprisonment and photoelectric emission. Non-standard rate coefficient functions can be handled by a user-supplied shared library. Reaction data is supplied in text files and is independent of the user's program. Recent additions include the simulation of emission spectra taking line broadening into account; reactions with excited ionic species; 3-body reactions with species with different efficiencies as 3rd body; a species properties database and a Python interface for rapid scripting and debugging.

Pinhao, Nuno

2007-10-01

129

Detailed and global chemical kinetics model for hydrogen  

SciTech Connect

Detailed and global chemical kinetic computations for hydrogen-air mixtures have been performed to describe flame propagation, flame structure and ignition phenomena. Simulations of laminar flame speeds, flame compositions and shock tube ignition delay times have been successfully performed. Sensitivity analysis was applied to determine the governing rate-controlling reactions for the experimental data sets examined. In the flame propagation and structure studies, the reactions, OH + H{sub 2} = H{sub 2}0 + H, 0 + H{sub 2} = OH + H and 0 + OH = 0{sub 2} + H were the most important in flames. The shock tube ignition delay time study indicated the H + 0{sub 2} + M = H0{sub 2} + M (M = N{sub 2}, H{sub 2}) and 0 + OH = 0{sub 2} + H reactions controlled ignition. A global rate expression for a one-step overall reaction was developed and validated against experimental hydrogen-air laminar flame speed data. The global reaction expression was determined to be 1.8 {times} 10{sup 13} exp({minus}17614K/T)[H{sub 2}]{sup 1.0}[O{sub 2}]{sup 0.5} for the single step reaction H{sub 2} + 1/2O{sub 2} = H{sub 2}O.

Marinov, N.M.; Westbrook, C.K.; Pitz, W.J.

1995-03-01

130

An EXCEL template for calculation of enzyme kinetic parameters by non-linear regression.  

PubMed

MOTIVATION: An EXCEL template has been developed for the calculation of enzyme kinetic parameters by non-linear regression techniques. The tool is accurate, inexpensive, as well as easy to use and modify. AVAILABILITY: The program is available from http://www.ebi.ac.uk/biocat/biocat.html CONTACT: agustin. hernandez@bio.kuleuven.ac.be PMID:9545460

Hernández, A; Ruiz, M T

1998-01-01

131

Calculation of rates for enzyme and microbial kinetics via a spline technique  

Technology Transfer Automated Retrieval System (TEKTRAN)

In biocatalysis research, determination of enzyme kinetics, microbial growth rates, substrate utilization rates, and product accumulation rates sometime require derivatives to be calculated with a method that can be duplicated and yields consistent results. In this paper, several methods that have ...

132

Calculation of 13C Chemical Shifts in RNA Nucleosides: Structure 13C Chemical Shift Relationships  

Microsoft Academic Search

Isotropic 13C chemical shifts of the ribose sugar in model RNA nucleosides are calculated using SCF and DFT-GIAO ab initio methods for different combinations of ribose sugar pucker, exocyclic torsion angle, and glycosidic torsion angle. Idealized conformations were obtained using structures that were fully optimized by ab initio DFT methods starting with averaged parameters from a collection of crystallographic data.

Paolo Rossi; Gerard S. Harbison

2001-01-01

133

One-dimensional hydro-kinetic modelling of the decaying arc in air-PA66-copper mixtures: I. Chemical kinetics, thermodynamics, transport and radiative properties  

NASA Astrophysics Data System (ADS)

A one-dimensional hydro-kinetic model is developed to study arc extinction in a low-voltage breaking device. In part 1 of this double paper, the data necessary for the implementation of the hydro-kinetic model are presented. The equilibrium composition of the plasma is derived from the law of mass action. Mixtures of air and materials ablated from the thermoplastic wall (PA66, monomer C12H22O2N2) and from the contacts (copper) are considered. Thermodynamic properties such as mass density, enthalpy and specific heat at constant pressure are then calculated. Transport coefficients (viscosity, electrical conductivity and thermal conductivity) are determined with the Chapman-Enskog theory. Concerning radiation, the net emission coefficient, corresponding to the power radiated per unit volume and per unit solid angle, is calculated. Finally, for chemical kinetics, a reliable set of reaction rate coefficients linking the chemical species of the plasma is compiled. Particular attention is paid to ionization and recombination of atomic species which are obtained with the 'bottleneck' method.

Teulet, Ph; Gonzalez, J. J.; Mercado-Cabrera, A.; Cressault, Y.; Gleizes, A.

2009-09-01

134

Quantum chemical and kinetics study of the thermal gas phase decomposition of 2-chloropropene.  

PubMed

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant. PMID:24032406

Tucceri, María E; Badenes, María P; Cobos, Carlos J

2013-10-01

135

A kinetic model for chemical reactions without barriers: transport coefficients and eigenmodes  

NASA Astrophysics Data System (ADS)

The kinetic model of the Boltzmann equation proposed in the work of Kremer and Soares 2009 for a binary mixture undergoing chemical reactions of symmetric type which occur without activation energy is revisited here, with the aim of investigating in detail the transport properties of the reactive mixture and the influence of the reaction process on the transport coefficients. Accordingly, the non-equilibrium solutions of the Boltzmann equations are determined through an expansion in Sonine polynomials up to the first order, using the Chapman-Enskog method, in a chemical regime for which the reaction process is close to its final equilibrium state. The non-equilibrium deviations are explicitly calculated for what concerns the thermal-diffusion ratio and coefficients of shear viscosity, diffusion and thermal conductivity. The theoretical and formal analysis developed in the present paper is complemented with some numerical simulations performed for different concentrations of reactants and products of the reaction as well as for both exothermic and endothermic chemical processes. The results reveal that chemical reactions without energy barrier can induce an appreciable influence on the transport properties of the mixture. Oppositely to the case of reactions with activation energy, the coefficients of shear viscosity and thermal conductivity become larger than those of an inert mixture when the reactions are exothermic. An application of the non-barrier model and its detailed transport picture are included in this paper, in order to investigate the dynamics of the local perturbations on the constituent number densities, and velocity and temperature of the whole mixture, induced by spontaneous internal fluctuations. It is shown that for the longitudinal disturbances there exist two hydrodynamic sound modes, one purely diffusive hydrodynamic mode and one kinetic mode.

Alves, Giselle M.; Kremer, Gilberto M.; Marques, Wilson, Jr.; Jacinta Soares, Ana

2011-03-01

136

Detailed and reduced chemical-kinetic descriptions for hydrocarbon combustion  

NASA Astrophysics Data System (ADS)

Numerical and theoretical studies of autoignition processes of fuels such as propane are in need of realistic simplified chemical-kinetic descriptions that retain the essential features of the detailed descriptions. These descriptions should be computationally feasible and cost-effective. Such descriptions are useful for investigating ignition processes that occur, for example, in homogeneous-charge compression-ignition engines, for studying the structures and dynamics of detonations and in fields such as multi-dimensional Computational Fluid Dynamics (CFD). Reduced chemistry has previously been developed successfully for a number of other hydrocarbon fuels, however, propane has not been considered in this manner. This work focuses on the fuels of propane, as well propene, allene and propyne, for several reasons. The ignition properties of propane resemble those of other higher hydrocarbons but are different from those of the lower hydrocarbons (e.g. ethylene and acetylene). Propane, therefore, may be the smallest hydrocarbon that is representative of higher hydrocarbons in ignition and detonation processes. Since the overall activation energy and ignition times for propane are similar to those of other higher hydrocarbons, including liquid fuels that are suitable for many applications, propane has been used as a model fuel for several numerical and experimental studies. The reason for studying elementary chemistry of propene and C3H4 (allene or propyne) is that during the combustion process, propane breaks down to propene and C3H4 before proceeding to products. Similarly, propene combustion includes C3H4 chemistry. In studying propane combustion, it is therefore necessary to understand the underlying combustion chemistry of propene as well as C3H 4. The first part of this thesis focuses on obtaining and testing a detailed chemical-kinetic description for autoignition of propane, propene and C 3H4, by comparing predictions obtained with this detailed mechanism against numerous experimental data available from shock-tube studies and flame-speed measurements. To keep the detailed mechanism small, attention is restricted to pressures below about 100 atm, temperatures above about 1000 K and equivalence ratios less than about 3. Based on this detailed chemistry description, short (or skeletal) mechanisms are then obtained for each of the three fuels by eliminating reactions that are unimportant for the autoignition process under conditions presented above. This was achieved by utilizing tools such as sensitivity and reaction pathway analyses. Two distinct methodologies were then used in order to obtain a reduced mechanism for autoignition from the short mechanisms. A Systematic Reduction approach is first taken that involves introducing steady-state approximations to as many species as analytically possible. To avoid resorting to numerical methods, the analysis for obtaining ignition times for heptane, presented by Peters and co-workers is followed in order to obtain a rough estimate for an expression of propane ignition time. The results from this expression are then compared to the ignition times obtained computationally with the detailed mechanism. The second method is an Empirical Approach in which chemistry is not derived formally, but rather postulated empirically on the basis of experimental, computational and theoretical observations. As a result, generalized reduced mechanisms are proposed for autoignition of propane, propene and C3H 4. Expressions for ignition times obtained via this empirical approach are compared to the computational results obtained from the detailed mechanism.

Petrova, Maria V.

137

Calculations of kinetic isotope effects in the Hofmann eliminations of substituted (2-phenylethyl)trimethylammonium ions  

SciTech Connect

Theoretical calculations of kinetic isotope effects (KIE) for the Hofmann elimination of the (2-phenylethyl)trimethylammonium ion (I,Z = H) have been carried out for an extensive series of transition-state models encompassing the Elcb-like region of the E2 mechanistic spectrum. The reaction coordinate employed corresponded to the irreversible fragmentation of the base-H'-C/sub ..beta../-C/sub ..cap alpha../-N system, with proton transfer being the dominant contributor. Structural parameters (bond distances and angles) were related to the independent bond orders n/sub ..cap alpha..-N/ and n/sub ..beta..-H'/ by empirical and semiempirical relationships. The most probable transition-state structure for the reaction was determined by interpolation of the experimental values for the ..beta..-D/sub 2/ and /sup 15/N KIE into plots of the trends of the calculated KIE. The nonsolvated models obtained in this manner gave only poor agreement between calculated and experimental secondary deuterium (..cap alpha..-D/sub 2/) and leaving group deuterium (N(CD/sub 3/)/sub x/(CH/sub 3/)/sub 3-x/, x = 1 to 3) KIE; explicit consideration of differential solvation of the reactant and transition state afforded the most chemically reasonable resolution of these discrepancies. Using solvated models, transition-state structures were also determined for the Hofmann elimination of parasubstituted derivatives of I (Z = OCH/sub 3/, Cl, CF/sub 3/). These transition states are related by a shift parallel to the central E2 diagogonal of an O'Ferrall-Jencks reaction diagram, as predicted by Thorton, indicating that, in the absence of other factors (differing solvent or base, etc.), the extent to which negative charge is accumulated at C..beta.. in the transition state is solely a factor of the leaving group. Both independent bond orders (n/sub ..cap alpha..-N/ and n/sub ..beta..-H'/) exhibit a linear dependence on the sigma value of the substituent, allowing for the first time prediction of transition states.

Lewis, D.E. (Univ. of Arkansas, Fayetteville); Sims, L.B.; Yamataka, H.; McKenna, J.

1980-12-03

138

Application of Photoacoustic Effect to Chemical Kinetics in Solution.  

NASA Astrophysics Data System (ADS)

The application of the pulsed photoacoustic technique (PA) to the study of chemical kinetics in very dilute solutions is demonstrated with three different reactions: A bidental ligand substitution of 1,2-bi(diphenylphosphino) ethane on the five-coordinate cobalt dithiolene complex, a nucleophilic addition of tributyl phosphite to (p-(dimethylamino)triphenyl)methyl cation, and an enzyme catalyzed reduction of nicotinamide adenine dinucleotide by glucose-6-phosphate. Photoacoustic detection is shown to be 100 times more sensitive than spectrophotometric detection. As a consequence, it allows one to use lower reactant concentrations so that the time scale for the corresponding fast reactions can be lengthened. The pulsed PA technique also provides an alternative method for the direct determination of reverse rate constants and equilibrium constants of reactions. Moreover, Michaelis constants of biochemical reactions, especially for enzymes whose K_{rm m} values is below 1.0 muM, can be measured more accurately using photoacoustic detection than using spectrophotometric detection. Additionally, the pulsed PA method can be very valuable for the study of reactions that are restricted by the low solubility, availability, or high cost of reactants. The sensitivity of the PA method is found to be limited by the background signal resulting from solvents. Photoacoustic signals generated in strongly absorbing media are also studied and results are in agreement with the theory developed by G. J. Diebold. From the time profile of the acoustic signal, the absorption coefficient of strongly absorbing medium can be determined provided the sound speed is known. Since the pulse shape of the acoustic transient depends upon the relative density and the acoustic velocity of the transparent and the strongly absorbing media, the physical properties of the transparent medium can also be determined with the use of the PA technique.

Cheng, Huy-Zu.

139

Power modulation study of chemical kinetics in rf discharges  

Microsoft Academic Search

A method for measuring and modeling dominant kinetic rates and mechanisms in situ for an rf plasma has been developed. The coupling of small periodic power ] modulation(1)) of the discharge with transfer function analysis(2) provides a novel way to study the dominant reaction kinetics in the plasma environment. In this paper we review relaxation techniques used previously to study

L. D. B. Kiss; H. H. Sawin

1992-01-01

140

Application of Chemical Kinetics to Deterioration of Foods.  

ERIC Educational Resources Information Center

|Possible modes of food deterioration (such as microbial decay, nonenzymatic browning, senescence, lipid oxidation) are reviewed. A basic mathematical approach to the kinetics of food deterioration, kinetic approach to accelerating shelf-life deterioration, and shelf-life predictions are discussed. (JN)|

Labuza, T. P.

1984-01-01

141

Oxidation kinetics of chemically vapor-deposited silicon carbide in wet oxygen  

Microsoft Academic Search

The oxidation kinetics of chemically vapor-deposited SiC in dry oxygen and wet oxygen at temperatures between 1,200 and 1,400 C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the

Elizabeth J. Opila

1994-01-01

142

Detailed Chemical Kinetic Reaction Mechanism for Oxidation of Four Small Alkyl Esters in Laminar Premixed Flames.  

National Technical Information Service (NTIS)

A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate and ethyl acetate. This mechanism is validated by comparisons between computed resul...

C. K. Westbrook F. L. Dryer M. Chaos P. R. Westmoreland W. J. Pitz

2008-01-01

143

Recent Advances in Detailed Chemical Kinetic Models for Large Hydrocarbon and Biodiesel Transportation Fuels  

SciTech Connect

n-Hexadecane and 2,2,4,4,6,8,8-heptamethylnonane represent the primary reference fuels for diesel that are used to determine cetane number, a measure of the ignition property of diesel fuel. With the development of chemical kinetics models for these two primary reference fuels for diesel, a new capability is now available to model diesel fuel ignition. Also, we have developed chemical kinetic models for a whole series of large n-alkanes and a large iso-alkane to represent these chemical classes in fuel surrogates for conventional and future fuels. Methyl decanoate and methyl stearate are large methyl esters that are closely related to biodiesel fuels, and kinetic models for these molecules have also been developed. These chemical kinetic models are used to predict the effect of the fuel molecule size and structure on ignition characteristics under conditions found in internal combustion engines.

Westbrook, C K; Pitz, W J; Curran, H J; Herbinet, O; Mehl, M

2009-03-30

144

A mathematical model of the coupled fluid mechanics and chemical kinetics in a chemical vapor deposition reactor  

Microsoft Academic Search

The authors describe a numerical model of the coupled gas-phase hydrodynamics and chemical kinetics in a silicon chemical vapor deposition (CVD) reactor. The model, which includes a 20-step elementary reaction mechanism for the thermal decomposition of silane, predicts gas-phase temperature, velocity, and chemical species concentration profiles. It also predicts silicon deposition rates at the heated reactor wall as a function

M. E. Coltrin; R. J. Kee; J. A. Miller

1983-01-01

145

Uridine phosphorylase from Trypanosoma cruzi: kinetic and chemical mechanisms†  

PubMed Central

The reversible phosphorolysis of uridine to generate uracil and ribose 1-phosphate is catalyzed by uridine phosphorylase and is involved in the pyrimidine salvage pathway. We define the reaction mechanism of uridine phosphorylase from Trypanosoma cruzi by steady-state and pre-steady-state kinetics, pH-rate profiles, kinetic isotope effects from uridine and solvent deuterium isotope effects. Initial rate and product inhibition patterns suggest a steady-state random kinetic mechanism. Pre-steady state kinetics indicated no rate-limiting step after formation of the enzyme-products ternary complex, as no burst in product formation is observed. The limiting single-turnover rate constant equals the steady-state turnover number, thus chemistry is partially or fully rate limiting. Kinetic isotope effects with [1?-3H]-, [1?-14C]-, and [5?-14C,1,3-15N2]uridine gave experimental values of ?-T(V/K)uridine = 1.063, 14(V/K)uridine = 1.069, and 15,?-15(V/K)uridine = 1.018, in agreement with an ANDN (SN2) mechanism where chemistry contributes significantly to the overall rate-limiting step of the reaction. Density functional theory modeling of the reaction in gas phase supports an ANDN mechanism. Solvent deuterium kinetic isotope effects were unity, indicating that no kinetically significant proton transfer step is involved at the transition state. In this N-ribosyl transferase, proton transfer to neutralize the leaving group is not part of transition state formation, consistent with an enzyme-stabilized anionic uracil as the leaving group. Kinetic analysis as a function of pH indicates one protonated group essential for catalysis and for substrate binding.

Silva, Rafael G.; Schramm, Vern L.

2011-01-01

146

Data quality in predictive toxicology: identification of chemical structures and calculation of chemical properties.  

PubMed Central

Every technique for toxicity prediction and for the detection of structure-activity relationships relies on the accurate estimation and representation of chemical and toxicologic properties. In this paper we discuss the potential sources of errors associated with the identification of compounds, the representation of their structures, and the calculation of chemical descriptors. It is based on a case study where machine learning techniques were applied to data from noncongeneric compounds and a complex toxicologic end point (carcinogenicity). We propose methods applicable to the routine quality control of large chemical datasets, but our main intention is to raise awareness about this topic and to open a discussion about quality assurance in predictive toxicology. The accuracy and reproducibility of toxicity data will be reported in another paper.

Helma, C; Kramer, S; Pfahringer, B; Gottmann, E

2000-01-01

147

Chemical kinetic considerations for postflame synthesis of carbon nanotubes in premixed flames using a support catalyst  

SciTech Connect

Multiwalled carbon nanotubes (MWCNTs) on a grid supported cobalt nanocatalyst were grown, by exposing it to combustion gases from ethylene/air rich premixed flames. Ten equivalence ratios ({phi}) were investigated, as follows: 1.37, 1.44, 1.47, 1.50, 1.55, 1.57, 1.62, 1.75, 1.82, and 1.91. MWCNT growth could be observed for the range of equivalence ratios between 1.45 and 1.75, with the best yield restricted to the range 1.5-1.6. A one-dimensional premixed flame code with a postflame heat loss model, including detailed chemistry, was used to estimate the gas phase chemical composition that favors MWCNT growth. The results of the calculations show that the mixture, including the water gas shift reaction, is not even in partial chemical equilibrium. Therefore, past discussions of compositional parameters that relate to optimum carbon nanotube (CNT) growth are revised to include chemical kinetic effects. Specifically, rapid departures of the water gas shift reaction from partial equilibrium and changes in mole fraction ratios of unburned C{sub 2} hydrocarbons to hydrogen correlate well with experimentally observed CNT yields. (author)

Gopinath, Prarthana; Gore, Jay [School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 (United States)

2007-11-15

148

Critical evaluation of Jet-A spray combustion using propane chemical kinetics in gas turbine combustion simulated by KIVA-2  

SciTech Connect

Jet-A spray combustion has been evaluated in gas turbine combustion with the use of propane chemical kinetics as the first approximation for the chemical reactions. Here, the numerical solutions are obtained by using the KIVA-2 computer code. The KIVA-2 code is the most developed of the available multidimensional combustion computer programs for application of the in-cylinder combustion dynamics of internal combustion engines. The released version of KIVA-2 assumes that 12 chemical species are present; the code uses an Arrhenius kinetic-controlled combustion model governed by a four-step global chemical reaction and six equilibrium reactions. Researchers efforts involve the addition of Jet-A thermophysical properties and the implementation of detailed reaction mechanisms for propane oxidation. Three different detailed reaction mechanism models are considered. The first model consists of 131 reactions and 45 species. This is considered as the full mechanism which is developed through the study of chemical kinetics of propane combustion in an enclosed chamber. The full mechanism is evaluated by comparing calculated ignition delay times with available shock tube data. However, these detailed reactions occupy too much computer memory and CPU time for the computation. Therefore, it only serves as a benchmark case by which to evaluate other simplified models. Two possible simplified models were tested in the existing computer code KIVA-2 for the same conditions as used with the full mechanism. One model is obtained through a sensitivity analysis using LSENS, the general kinetics and sensitivity analysis program code of D. A. Bittker and K. Radhakrishnan. This model consists of 45 chemical reactions and 27 species. The other model is based on the work published by C. K. Westbrook and F. L. Dryer.

Nguyen, H.L.; Ying, S.J.

1990-07-01

149

The Development of a Detailed Chemical Kinetic Mechanism for Diisobutylene and Comparison to Shock Tube Ignition Times  

Microsoft Academic Search

There is much demand for chemical kinetic models to represent practical fuels such as gasoline, diesel and aviation fuel. These blended fuels contain hundreds of components whose identity and amounts are often unknown. A chemical kinetic mechanism that would represent the oxidation of all these species with accompanying chemical reactions is intractable with current computational capabilities, chemical knowledge and manpower

W Metcalfe; H J Curran; J M Simmie; W J Pitz; C K Westbrook

2005-01-01

150

Ab Initio Calculation of Nuclear Magnetic Resonance Chemical Shift Anisotropy Tensors 1. Influence of Basis Set on the Calculation of 31P Chemical Shifts  

Microsoft Academic Search

The influence of changes in the contracted Gaussian basis set used for ab initio calculations of nuclear magnetic resonance (NMR) phosphorous chemical shift anisotropy (CSA) tensors was investigated. The isotropic chemical shitl and chemical shift anisotropy were found to converge with increasing complexity of the basis set at the Hartree-Fock @IF) level. The addition of d polarization function on the

Todd M. Alam

1998-01-01

151

Mechanistic, kinetic, and processing aspects of tungsten chemical mechanical polishing  

NASA Astrophysics Data System (ADS)

This dissertation presents an investigation into tungsten chemical mechanical polishing (CMP). CMP is the industrially predominant unit operation that removes excess tungsten after non-selective chemical vapor deposition (CVD) during sub-micron integrated circuit (IC) manufacture. This work explores the CMP process from process engineering and fundamental mechanistic perspectives. The process engineering study optimized an existing CMP process to address issues of polish pad and wafer carrier life. Polish rates, post-CMP metrology of patterned wafers, electrical test data, and synergy with a thermal endpoint technique were used to determine the optimal process. The oxidation rate of tungsten during CMP is significantly lower than the removal rate under identical conditions. Tungsten polished without inhibition during cathodic potentiostatic control. Hertzian indenter model calculations preclude colloids of the size used in tungsten CMP slurries from indenting the tungsten surface. AFM surface topography maps and TEM images of post-CMP tungsten do not show evidence of plow marks or intergranular fracture. Polish rate is dependent on potassium iodate concentration; process temperature is not. The colloid species significantly affects the polish rate and process temperature. Process temperature is not a predictor of polish rate. A process energy balance indicates that the process temperature is predominantly due to shaft work, and that any heat of reaction evolved during the CMP process is negligible. Friction and adhesion between alumina and tungsten were studied using modified AFM techniques. Friction was constant with potassium iodate concentration, but varied with applied pressure. This corroborates the results from the energy balance. Adhesion between the alumina and the tungsten was proportional to the potassium iodate concentration. A heuristic mechanism, which captures the relationship between polish rate, pressure, velocity, and slurry chemistry, is presented. In this mechanism, the colloid reacts with the chemistry of the slurry to produce active sites. These active sites become inactive by removing tungsten from the film. The process repeats when then inactive sites are reconverted to active sites. It is shown that the empirical form of the heuristic mechanism fits all of the data obtained. The mechanism also agrees with the limiting cases that were investigated.

Stein, David

152

Trajectory calculation of a trapped particle in electro-dynamic balance for study of chemical reaction of aerosol particles  

NASA Astrophysics Data System (ADS)

Electrodynamic balance (EDB) is a powerful tool for investigating the chemical reactions between a fine particle and gaseous species. But the EDB device alone is inadequate to match the rapid weight change of a fine particle caused by chemical reactions, because it takes a few seconds to set a fine particle at null point. The particle trajectory calculation for the trapped particle added to the EDB is thus a very useful tool for the measurement of the transient response of a particle weight change with no need to adjust the applied DC voltage to set the null point. The purpose of this study is to develop the trajectory calculation method to track the particle oscillation pattern in the EDB and examine the possibility for kinetic studies on the reaction of a single aerosol particle with gaseous species. The results demonstrated the feasibility of applying particle trajectory calculation to realize the research purpose.

Okuma, Miho; Itou, Takahiro; Harano, Azuchi; Takarada, Takayuki; James, Davis E.

2013-03-01

153

Equilibrium, chemical kinetic, and transport limitations to diamond growth  

NASA Astrophysics Data System (ADS)

Because of their extreme properties, diamond films have found some industrial applications, i.e., heat sinks and tool coatings. However, to increase their economic attractiveness, the growth rate must be increased, the deposition temperature must be lowered, and single crystal films must be achieved. We have studied two types of chemical vapor deposition systems, hot-filament and microwave assisted, in order to understand the factors limiting diamond growth rate. From simultaneous microbalance growth rate measurements and mass spectrometer measurements, changes in growth rate are correlated with changes in gas phase composition. Measured reaction orders support the proposal that diamond growth occurs through a single-carbon-atom species, e.g., CHsb3. When a two-carbon atom source gas is used, it is likely that the dissociation to two, single-carbon atom species occurs on the substrate surface (dissociative adsorption). Furthermore, a shift to zero-order suggests that the diamond growth is a surface-site limited process at higher hydrocarbon concentrations. The diamond growth rate maximum with pressure is explained by transport limitations of species within the reaction zone. The reported diamond growth rates in the hot-filament reactor are several times higher than those reported by other research groups. These higher growth rates result from surrounding the substrate with the filament. We have used the measured growth rates, filament temperatures, and thermocouple measurements to calculate activation energies for diamond growth. When the filament temperature is used for the calculation, an activation energy of 73 kcal per mole is obtained; however, based on estimated substrate temperatures, an activation energy of 18 kcal per mole is determined. A dimensional analysis approach was developed to select the most important gas phase reactions occurring during diamond CVD. Steady-state analysis of these reactions and the application of mass transport equations lead to the conclusion that diamond growth, in current hot-filament and microwave assisted CVD processes, is occurring in a partial equilibrium environment in which diffusion of atomic hydrogen controls the overall diamond growth rate. The initial stages of diamond growth on non-diamond substrates correspond to carburization, nucleation and growth. When polycrystalline or single crystal diamond is used as a substrate, the carburization and nucleation stages are not observed and growth begins immediately. The nucleation rate depends sensitively on the radiative heat transfer to the substrate. Adding ozone to the hot-filament CVD charge increases the production of carbon monoxide and carbon dioxide; this increase is observed with or without the filament being activated. A consistent effect on the diamond growth rate was not observed when ozone was added to the hot-filament reactor.

Evans, Edward Anthony

154

Kinetic model for the chemical dissolution of multiparticle systems  

NASA Astrophysics Data System (ADS)

A kinetic model is designed to interpret fraction reacted/time curves obtained from leaching multiparticle concentrates. The effect of individual particle shape on fraction reacted/time curves was determined by comparing data obtained from leaching a sphere and numerous irregular particles of malachite made by breaking up blocks of ore. All the leaching processes were carried out under the same experimental conditions. Finally, a kinetic model is presented which includes both size and shape of the particles. Experimental results show that fraction reacted/time curves are nearly the same for all particles with the same initial size/shape factor ratio. The importance of the solid's shape in interpreting heterogeneous kinetic data is clearly shown.

Núñez, C.; Espiell, F.

1985-09-01

155

Calculating Low Pressure Chemical Activation Atmospheric Hydrocarbon Synthesis Rates  

NASA Astrophysics Data System (ADS)

The formation of larger hydrocarbons, polyacetylenes, aromatics, and nitriles in the reducing atmospheres of the outer planets and moons occurs by recombination and addition reactions of smaller fragments at low temperatures and pressures, difficult to reach in the laboratory. However, because C-C bonds are usually weaker than C-H bonds, cleavage reactions can occur which interrupt this synthesis. The competing dissociation and recombination channels in these chemical activation reactions will be energy and pressure dependent. (Similar competition can occur during photolysis.) The rate constants can be determined computationally using statistical rate theory with parameters available from experiments and quantum calculations. We have applied this approach to generate temperature and pressure dependent rate constant expressions for several reactions that help determine atmospheric concentrations of larger hydrocarbon species in jovian atmospheres. Attention must be paid to isomers and H tunneling. Examples include: H + C3H7 -> CH3 + C2H5 C2H3 + C2H5 -> C4H8, H + C4H7, or CH3 + C3H5 (allyl isomer) H + C3H5 -> C3H6 or CH3 + C2H3 CH3 + CN -> H + CH2CN In addition, if photolysis is followed by internal conversion of electronic energy to vibrationally excited ground state molecules, the same approach may apply to computing C-C scission product yields. Product experiments to test this are underway. Research supported by grants from the NASA Outer Planets Research Program and Planetary Atmospheres Program.

Smith, Gregory P.

2008-09-01

156

Approaching chemical accuracy with density functional calculations: Diatomic energy corrections  

NASA Astrophysics Data System (ADS)

Density functional theory (DFT) is widely used to predict materials properties, but the local density approximation (LDA) and generalized gradient approximation (GGA) exchange-correlation functionals are known to poorly predict the energetics of reactions involving molecular species. In this paper, we obtain corrections for the O2, H2, N2, F2, and Cl2 molecules within the Perdew-Burke-Enzerhof GGA, Perdew-Wang GGA, and Perdew-Zunger LDA exchange-correlation functionals by comparing DFT-calculated formation energies of oxides, hydrides, nitrides, fluorides, and chlorides to experimental values. We also show that the choice of compounds used to obtain the correction is significant, and we use a leave-one-out cross-validation approach to rigorously determine the proper fit set. We report confidence intervals with our correction values, which quantifies the variation caused by the choice of fit set after outlier removal. The remaining variation in the correction values is of the order of 1 kcal/mol, which indicates that chemical accuracy is a realistic goal for these systems.

Grindy, Scott; Meredig, Bryce; Kirklin, Scott; Saal, James E.; Wolverton, C.

2013-02-01

157

New approaches for quantum chemical calculations on very large molecules  

SciTech Connect

This is the final report for a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Ab initio quantum chemistry has proven to be a valuable tool for understanding the electronic structure of small molecules containing 5--50 atoms. Calculations on larger systems have been prohibitive because of the forbidding scaling properties of the methods used. In the simplest traditional many-body method, second-order perturbation theory, the work scales as the fifth power of N, where N is proportional to the number of electrons in the system. New methods for the many-body problem, which scale as the third power of N and are well suited to utilize parallel computation, have been studied. The combination of these two developments enable the treatment of molecules in the range of 50--500 atoms with first principles methods. The ability to treat molecules containing 50--500 atoms with quantum chemistry techniques would open new vistas for theoretical chemistry. Large organic molecules of pharmaceutical interest and novel species such as fullerenes would be amenable to study. Inorganic complexes containing transition metals and actinides could be treated using realistic ligands. Important problems such as the chemistry at active sites in enzymes and the photochemistry involved in photosynthesis of chlorophyll could be studied. Catalytic reactions of hydrocarbons on zeolites and other supports of interest to the petrochemical and chemical industries could be examined.

Martin, R.L.; Russo, T.V.; Hay, P.J.

1996-04-01

158

Detailed Chemical Kinetic Reaction Mechanisms for Incineration of Organophosphorus and Fluoro-Organophosphorus Compounds  

SciTech Connect

A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previous kinetic models of organophosphorus compounds such as TMP, DMMP and DIMP that are often used as surrogates to predict incineration of GB. Kinetic models of the three surrogates and GB are then used to predict their consumption in a perfectly stirred reactor fueled by natural gas to simulate incineration of these chemicals. Computed results indicate that DIMP is the only one of these surrogates that adequately describes combustion of GB under comparable conditions. The kinetic pathways responsible for these differences in reactivity are identified and discussed. The most important reaction in GB and DIMP that makes them more reactive than TMP or DMMP is found to be a six-center molecular elimination reaction producing propene.

Glaude, P A; Melius, C; Pitz, W J; Westbrook, C K

2001-12-13

159

Calculation of 13C Chemical Shifts in RNA Nucleosides: Structure-13C Chemical Shift Relationships  

NASA Astrophysics Data System (ADS)

Isotropic 13C chemical shifts of the ribose sugar in model RNA nucleosides are calculated using SCF and DFT-GIAO ab initio methods for different combinations of ribose sugar pucker, exocyclic torsion angle, and glycosidic torsion angle. Idealized conformations were obtained using structures that were fully optimized by ab initio DFT methods starting with averaged parameters from a collection of crystallographic data. Solid-state coordinates of accurate crystal or neutron diffraction structures were also examined directly without optimization. The resulting 13C chemical shifts for the two sets of calculations are then compared. The GIAO-DFT method overestimates the shifts by an average of 5 ppm while the GIAO-SCF underestimates the shifts by the same amount. However, in the majority of cases the errors appear to be systematic, as the slope of a plot of calculated vs experimental shifts is very close to unity, with minimal scatter. The values of the 13C NMR shifts of the ribose sugar are therefore sufficiently precise to allow for statistical separation of sugar puckering modes and exocyclic torsion angle conformers, based on the canonical equation model formulated in a previous paper.

Rossi, Paolo; Harbison, Gerard S.

2001-07-01

160

Froth flotation: kinetic models based on chemical analogy  

Microsoft Academic Search

This paper studies several kinetic aspects of flotation of celestite and calcite in a mechanical cell with sodium dodecylsulphate as the collector agent, using pure minerals ranging in average particle size between 137 and 74 ?m, respectively, with a constant pH 3 in the flotation bath in all experiments. The results obtained indicate that both minerals float rapidly and that

F Hernáinz; M Calero

2001-01-01

161

Uncovering Oscillations, Complexity, and Chaos in Chemical Kinetics Using Mathematica  

Microsoft Academic Search

Unlike reactions with no peculiar temporal behavior, in oscillatory reactions concentrations can rise and fall spontaneously in a cyclic or disorganized fashion. In this article, the software Mathematica is used for a theoretical study of kinetic mechanisms of oscillating and chaotic reactions. A first simple example is introduced through a three-step reaction, called the Lotka model, which exhibits a temporal

M. M. C. Ferreira; W. C. Ferreira Jr.; A. C. S. Lino; M. E. G. Porto

1999-01-01

162

A Student Experiment in Non-Isothermal Chemical Kinetics  

Microsoft Academic Search

An advanced level experiment in physical chemistry is described in which the kinetics of the reaction between aqueous bromine and formic acid are studied spectrophotometrically under non-isothermal, pseudo-first order conditions. A dynamic temperature profile is achieved conveniently by heating the reactants prior to mixing and allowing the reaction mixture to cool in accordance with Newton's law of cooling. The equations

Steven C. Hodgson; Lawrence N. Ngeh; John D. Orbell; Stephen W. Bigger

1998-01-01

163

Chemical kinetics model for sulfur dioxide removal in flue gas using corona discharge  

Microsoft Academic Search

Plasma remediation is being investigated for the removal of sulfur dioxide from automotive exhausts and gases generated by combustion of fossil fuels. Modeling is playing an increasing vital role in process optimization and understanding of governing physical and chemical process. In this paper, a chemical kinetics model is developed to analyze the time evolution of the different main species involved

L. M. Dong; Z. Wu; J. X. Yang; X. C. Chi

2003-01-01

164

Integration of large chemical kinetic mechanisms via exponential methods with Krylov approximations to Jacobian matrix functions  

Microsoft Academic Search

Recent trends in hydrocarbon fuel research indicate that the number of species and reactions in chemical kinetic mechanisms is rapidly increasing in an effort to provide predictive capabilities for fuels of practical interest. In order to cope with the computational cost associated with the time integration of stiff, large chemical systems, a novel approach is proposed. The approach combines an

Fabrizio Bisetti

2012-01-01

165

Integration of large chemical kinetic mechanisms via exponential methods with Krylov approximations to Jacobian matrix functions  

Microsoft Academic Search

Recent trends in hydrocarbon fuel research indicate that the number of species and reactions in chemical kinetic mechanisms is rapidly increasing in an effort to provide predictive capabilities for fuels of practical interest. In order to cope with the computational cost associated with the time integration of stiff, large chemical systems, a novel approach is proposed. The approach combines an

Fabrizio Bisetti

2011-01-01

166

Time-resolved IR chemiluminescence in gas-phase chemical kinetics  

Microsoft Academic Search

The time-resolved IR chemiluminescence method based on the study of the chemiluminescence of vibrationally excited molecules formed in chemical reactions is discussed. The approaches used in the study of elementary reactions and complex processes are systematised. The technique is compared with other methods for the study of gas-phase chemical kinetics. The bibliography includes 76 references.

Evgenii N. Chesnokov; Viktor N. Panfilov

1999-01-01

167

Theory of chemical kinetics. Technical report, January 16, 1980-January 15, 1981  

SciTech Connect

Progress made in research in the theory of chemical kinetics is described. The work in the past year has been primarily concerned with the efficiency of thermal and chemical engines, the role of inertial effect in thermal engines, the behavior of harmonically driven oscillatory reactions and a detailed analysis of glycolysis including its mechanism, its control features and its resonance effects.

Ross, J.

1981-01-01

168

A Detailed Chemical Kinetic Analysis of Low Temperature Non-Sooting Diesel Combustion  

Microsoft Academic Search

We have developed a model of the diesel fuel injection process for application to analysis of low temperature non-sooting combustion. The model uses a simplified mixing correlation and detailed chemical kinetics, and analyzes a parcel of fuel as it moves along the fuel jet, from injection into evaporation and ignition. The model predicts chemical composition and soot precursors, and is

S M Aceves; D L Flowers

2004-01-01

169

ON THE KINETICS OF POTENTIAL, ELECTROMOTANCE, AND CHEMICAL CHANGE IN THE EXCITABLE SYSTEM OF NERVE  

Microsoft Academic Search

The kinetics of interaction between potential, chemical equilibrium, and electro- motance in the excitable system of nerve are analyzed. The theoretical system has the following properties: It gives rise to two electromotances each of which depends directly on a chemical equilibrium. The equilibria are determined by the potential across the system. After a sudden potential shift the equilibria reach their

PAUL MUELLER

1958-01-01

170

ON THE KINETICS OF POTENTIAL, ELECTROMOTANCE, AND CHEMICAL CHANGE IN THE EXCITABLE SYSTEM OF NERVE  

PubMed Central

The kinetics of interaction between potential, chemical equilibrium, and electromotance in the excitable system of nerve are analyzed. The theoretical system has the following properties: It gives rise to two electromotances each of which depends directly on a chemical equilibrium. The equilibria are determined by the potential across the system. After a sudden potential shift the equilibria reach their new value with an exponential time course, the time constant of which is determined by the rate constants of the two reactions. The rate constants are different due to different activation energies. The two electromotances give rise to potentials of opposite sign. The total potential produced by the system is equal to the sum of the two potentials. The two equilibria are thus determined by any externally applied potential as well as by the sum of the internally produced potentials. The dependence of the equilibria on the potential is calculated from first principles. The equations which describe this system are solved by an analogue computer, which gives instantaneous solutions of the total internal potential as a function of time and any voltage applied from an external source. Comparison between recorded and computed action potentials shows excellent agreement under all experimental conditions. The electromotances might originate from a Ca++—Na+—K+ exchange at fixed negative sites in the Schwann cell.

Mueller, Paul

1958-01-01

171

Kinetics of SiC chemical vapor deposition from methylsilane  

Microsoft Academic Search

The kinetics of heterogeneous decomposition of methylsilane in a pumped hot-wall reactor were studied just below the critical\\u000a partial pressure for homogeneous decomposition. The activation energies of the process derived from gas-phase conversion measurements\\u000a and from deposition rates of SiC were found to be equal (about 230 kJ\\/mol). The nature of the diluent (H2, He, or Ar) has only a

A. M. Abyzov; E. P. Smirnov

2000-01-01

172

Plasma drift-kinetic equation calculations in three-dimensional magnetic geometries  

SciTech Connect

A new code to simulate three-dimensional plasmas in complex toroidal geometries is presented. It solves drift-kinetic equations for the one-particle distribution function f based on their projection onto a functional basis consisting of an arbitrary number of Legendre-Laguerre polynomials. In this paper, these theoretical aspects of the code are exposed together with their relation with the standard formalism. Comparisons with neoclassical theory for the large aspect ratio case and first calculations in the geometry of the TJ-II Heliac are also presented.

Reynolds, J. M. [Instituto de Biocomputacion y Fisica de Sistemas Complejos, Universidad de Zaragoza, 50018 Zaragoza (Spain); Lopez-Bruna, D. [Laboratorio Nacional de Fusion, CIEMAT, 28040 Madrid (Spain)

2010-07-15

173

The model of macroscopic kinetics in non-equilibrium plasma chemical reactions. I - General considerations and basic relations  

Microsoft Academic Search

The reaction of a chemical active nonequilibrium plasma system is studied within simple kinetic models in principle. Of special interest are steady states of the system with the meaning of a reversible chemical equilibrium. Starting from a schematized description of the nonisothermal plasma chemical reactor the basic relations of the so-called macroscopic kinetics are developed and in the case of

A. Rutscher; H. E. Wagner

1985-01-01

174

Including the relativistic kinetic energy in a spline-augmented plane-wave band calculation  

SciTech Connect

The first-order relativistic correction to the kinetic energy of an electron, the mass-velocity term, is not bounded from below. It can, therefore, not be used within a variational framework. To overcome this deficiency we developed a method to include the entire relativistic kinetic energy {radical}(p{sup 2}c{sup 2}+m{sub 0}{sup 2}c{sup 4}){minus}m{sub 0}c{sup 2} in a spline-augmented plane-wave band calculation. The first results for silver are quite promising, especially for d and p states: The analysis of the energies of the core states as well as of the valence band structure suggests that the energies of d bands are reproduced within 1 mRy. However, the combination of the relativistic kinetic energy with the Darwin term leads to energies which are too low for s-like valence states by 10 mRy. Therefore, the s and d valence band complex is spread out and the Fermi level is lowered by the same amount as the s states. We expect to overcome these deficiencies in future investigations by using a alternative form of the relativistic potential correction along the lines proposed by Douglas and Kroll. {copyright} {ital 1997} {ital The American Physical Society}

Fehrenbach, G.M.; Schmidt, G. [Sektion Physik der Ludwig Maximilians Universitaet Muenchen, Theresienstrae 37, 80333 Munich (Federal Republic of Germany)

1997-03-01

175

Hydrogenation of linolenate. IV. Kinetics of catalytic and homogeneous chemical reduction  

Microsoft Academic Search

Kinetics for consecutive reactions of octadecatrienoate to octadecadienoate to octadecenoate have been studied with the aid\\u000a of radioisotopic tracers and gas chromatography. Evidence for a triene to monoene shunt has been obtained. Similarly, the\\u000a chemical reduction with hydrazine has been studied, but no evidence for this anomalous behavior was obtained. Methods to determine\\u000a reaction rates from these kinetic measurements are

C. R. Scholfield; Janina Nowakowska; H. J. Dutton

1962-01-01

176

Kinetics of the tungsten hexafluoride-silane reaction for the chemical vapor deposition of tungsten  

Microsoft Academic Search

In this study, the kinetics of the low-pressure chemical vapor deposition (LPCVD) of tungsten by silane reduction of tungsten hexafluoride on Si(100) surfaces was studied. A single-wafer, cold-wall reactor was sued for the experiments. The SiH[sub 4]\\/WF[sub 6] ratio was 1.0. The pressure and temperature range were 1-10 torr and 137-385[degree]C, respectively. Kinetic data were obtained in the absence of

Gokce; Huseyin

1991-01-01

177

Reactive molecular dynamics simulation and chemical kinetic modeling of pyrolysis and combustion of n-dodecane  

Microsoft Academic Search

The initiation mechanisms and kinetics of pyrolysis and combustion of n-dodecane are investigated by using the reactive molecular dynamics (ReaxFF MD) simulation and chemical kinetic modeling. From ReaxFF MD simulations, we find the initiation mechanisms of pyrolysis of n-dodecane are mainly through two pathways, (1) the cleavage of C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction

Quan-De Wang; Jing-Bo Wang; Juan-Qin Li; Ning-Xin Tan; Xiang-Yuan Li

2011-01-01

178

Reactive molecular dynamics simulation and chemical kinetic modeling of pyrolysis and combustion of n-dodecane  

Microsoft Academic Search

The initiation mechanisms and kinetics of pyrolysis and combustion of n-dodecane are investigated by using the reactive molecular dynamics (ReaxFF MD) simulation and chemical kinetic modeling. From ReaxFF MD simulations, we find the initiation mechanisms of pyrolysis of n-dodecane are mainly through two pathways, (1) the cleavage of C–C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction

Quan-De Wang; Jing-Bo Wang; Juan-Qin Li; Ning-Xin Tan; Xiang-Yuan Li

2011-01-01

179

Deuterium isotope effects during HMX combustion: Chemical kinetic burn-rate control mechanism verified  

Microsoft Academic Search

The appearance of a significant deuterium isotope effect during the combustion of the solid HMX compound verifies that the chemical reaction kinetics is a major contributor in determining the experimentally observed or global burn rate. Burn rate comparison of HMX and its deuterium labeled HMX-d(8) analogue reveals a primary kinetic deuterium isotope effect (1 deg. KDIE) at 500 psig (3.55

S. A. Shackelford; B. B. Goshgarian; R. D. Chapman; R. E. Askins; D. A. Flanigan; R. N. Rogers

1989-01-01

180

Calculation of propulsive nozzle flowfields in multidiffusing chemically reacting environments  

Microsoft Academic Search

An advanced engineering model has been developed to aid in the analysis and design of hydrogen\\/oxygen chemical rocket engines. The complete multispecies, chemically reacting and multidiffusing Navier-Stokes equations are modelled, including the Soret thermal diffusion and the Dufour energy transfer terms. In addition to the spectrum of multispecies aspects developed, the model developed in this study is also conservative in

Kenneth John Kacynski

1994-01-01

181

Constrained-equilibrium calculations for chemical systems subject to generalized linear constraints using the NASA and STANJAN equilibrium programs  

NASA Astrophysics Data System (ADS)

Fast efficient numerical programs for determining the equilibrium composition of large chemical systems subject to generalized linear constraints are needed for chemical kinetic calculations involving both the conventional local thermodynamic equilibrium (LTE) and the more general rate-controlled constrained-equilibrium RCCE assumptions. For this purpose two callable subroutines based on the well known NASA and STANJAN equilibrium codes have been developed by a simple modification of their input files. To evaluate the performance of these subroutines, test calculations have been made for the hydrogen - oxygen (H - O) and carbon - hydrogen - oxygen - nitrogen (C - H - O - N) systems with various combinations of constraints on the elements, the total moles and the free valence (number of unpaired electrons) in the system. The allowed domain of the constraints was determined and both interior and boundary points were investigated for several temperature and pressure conditions. The results showed that STANJAN was superior to NASA both in convergence and speed under all conditions investigated.

Bishnu, Partha S.; Hamiroune, Djamel; Metghalchi, Mohamad; Keck, James C.

1997-09-01

182

Anomalous Kinetic Roughening by Chemical Reaction-Limited Aggregation  

NASA Astrophysics Data System (ADS)

The evolution of rough fronts in the early growth stage of poly(chloro-p-xylylene) films on Si/SiO2 substrates was measured using atomic force microscopy, and analyzed in detail according to the dynamic scaling theories of kinetic roughening. While the growth of the interface width and the correlation length were reduced as the system entered into the three-dimensional growth mode, both the global and the local roughness exponents remained at ?=1.39 and ?loc=0.74. The scaling behavior, ?=?s\

Lee, InJae; Park, Euldoo

2011-12-01

183

New velocity-space discretization for continuum kinetic calculations and Fokker-Planck collisions  

NASA Astrophysics Data System (ADS)

Numerical techniques for discretization of velocity space in continuum kinetic calculations are described. An efficient spectral collocation method is developed for the speed coordinate - the radius in velocity space - employing a novel set of non-classical orthogonal polynomials. For problems in which Fokker-Planck collisions are included, a common situation in plasma physics, a procedure is detailed to accurately and efficiently treat the field term in the collision operator (in the absence of gyrokinetic corrections). When species with disparate masses are included simultaneously, a careful extrapolation of the Rosenbluth potentials is performed. The techniques are demonstrated in several applications, including neoclassical calculations of the bootstrap current and plasma flows in a tokamak.

Landreman, Matt; Ernst, Darin R.

2013-06-01

184

Propanil in a Manitoba soil: an interactive spreadsheet model based on conventional chemical kinetics.  

PubMed

An interactive spreadsheet model has been created for quantitative predictions of propanil sorption and reaction in a slurried Manitoba clay soil. Based on experimental values for the numbers of empty and filled sorption sites as reactants and products, the reaction mechanism has been described with conventional chemical kinetics. The on line HPLC ? extraction method revealed labile sorption, intraparticle diffusion, and a chemical reaction. Laidler's integral rate law for second order kinetics describes the labile sorption. Desorption, intraparticle diffusion, and the chemical reaction are all described by first order kinetics. The time dependent effects of initial concentration and amount of slurried soil can be predicted for sorption, intraparticle diffusion, and the amount of reaction product. Suggested applications include storm runoff and inputs for fate and transport hydrology models. PMID:22354380

Gamble, Donald S; Webster, G R Barrie; Lamoureux, Marc

2012-02-21

185

CHEMKIN2. General Gas-Phase Chemical Kinetics  

SciTech Connect

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary, user-specified chemical reaction mechanism. A data file is generated which forms a link to the Gas-phase Subroutine Library, a collection of about 200 modular subroutines which may be called to return thermodynamic properties, chemical production rates, derivatives of thermodynamic properties, derivatives of chemical production rates, or sensitivity parameters. Both single and double precision versions of CHEMKIN are included. Also provided is a set of FORTRAN subroutines for evaluating gas-phase transport properties such as thermal conductivities, viscosities, and diffusion coefficients. These properties are an important part of any computational simulation of a chemically reacting flow. The transport properties subroutines are designed to be used in conjunction with the CHEMKIN Subroutine Library. The transport properties depend on the state of the gas and on certain molecular parameters. The parameters considered are the Lennard-Jones potential well depth and collision diameter, the dipole moment, the polarizability, and the rotational relaxation collision number.

Rupley, F.M. [Sandia National Labs., Livermore, CA (United States)

1992-01-24

186

CHEMKIN2. General Gas-Phase Chemical Kinetics  

SciTech Connect

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. CHEMKIN was designed as an easily transportable program for use on CDC, Cray, and DEC VAX systems. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary, user-specified chemical reaction mechanism. A data file is generated which forms a link to the Gas-phase Subroutine Library, a collection of about 200 modular subroutines which may be called to return thermodynamic properties, chemical production rates, derivatives of thermodynamic properties, derivatives of chemical production rates, or sensitivity parameters. Both single and double precision versions of CHEMKIN are included. Also provided is a set of FORTRAN subroutines for evaluating gas-phase transport properties such as thermal conductivities, viscosities, and diffusion coefficients. These properties are an important part of any computational simulation of a chemically reacting flow. The transport properties subroutines are designed to be used in conjunction with the CHEMKIN Subroutine Library. The transport properties depend on the state of the gas and on certain molecular parameters. The parameters considered are the Lennard-Jones potential well depth and collision diameter, the dipole moment, the polarizability, and the rotational relaxation collision number.

Kee, R.J. [Sandia National Labs., Livermore, CA (United States); Warnatz, J. [Institut fuer Angewandte Physikalische Chemie, Universitaet Heidleberg, Heidleberg (Germany)

1980-03-01

187

Recovering tubewise power from three-dimensional nodal kinetics calculation during material relocation in an HWR  

SciTech Connect

To analyze severe accidents in some special-purpose heavy-water reactors made of assemblies consisting of a number of coaxial tubes of aluminum-clad U-Al fuel and aluminum-clad neutron-capturing material, a mechanistic model, MARTINS, for tube beatup, melting, and molten material relocation has been developed and integrated with the DIF3D nodal hexagonal-z reactor kinetics and other phenomenological modules. The DIF3D kinetics homogenizes all materials located and computes the total power produced in an axial segment of a fuel assembly. This paper presents an approximate method, used in MARTINS, to calculate the distribution of this total nodal power into the intact fuel and capturing material tubes and the meat-cladding mixtures relocating during tube disruption. The method accounts for the change in intraassembly radial power profile due to assembly geometry change with the progress of segment-by-segment disruption of different tubes. Earlier methods to recover pinwise power from nodal calculation for liquid-metal-cooled reactors and light water reactors (X-Y and hexagonal unit cells) are not practical for a disrupting assembly having material relocation. Figure 1 shows the assembly`s end view, divided into rings for modeling and analysis. A ring is a coolant subchannel plus the outer surrounding tube. The present method for distributing the nodal power consists of two parts: (a) calculation of the relative values of ring-by-ring power per unit uranium mass and power per unit mass of neutron-capturing material in a given assembly segment, and (b) normalization of these relative values such that the total power of all rings (intact tubes and U-Al-Cp meat-cladding mixtures, where Cp implies the neutron-capturing material) equals the DIF3D-calculated nodal power for the assembly axial segment.

Kalimullah; Morris, E.E.; Yang, W.S. [Argonne National Lab., IL (United States)

1994-12-31

188

Elimination of the translational kinetic energy contamination in pre-Born-Oppenheimer calculations  

NASA Astrophysics Data System (ADS)

In this paper we present a simple strategy for the elimination of the translational kinetic energy contamination of the total energy in pre-Born--Oppenheimer calculations carried out in laboratory-fixed Cartesian coordinates (LFCCs). The simple expressions for the coordinates and the operators are thus preserved throughout the calculations, while the mathematical form and the parametrisation of the basis functions are chosen so that the translational and rotational invariances are respected. The basis functions are constructed using explicitly correlated Gaussian functions (ECGs) and the global vector representation. First, we observe that it is not possible to parametrise the ECGs so that the system is at rest in LFCCs and at the same time the basis functions are square-integrable with a non-vanishing norm. Then, we work out a practical strategy to circumvent this problem by making use of the properties of the linear transformation between the LFCCs and translationally invariant and center-of-mass Cartesian coordinates as well as the transformation properties of the corresponding basis function parameter matrices. By exploiting these formal mathematical relationships we can identify and separate the translational contamination terms in the matrix representation of the kinetic energy operator in the LFCC formalism. We present numerical examples for the translational contamination and its elimination for the two lowest rotational energy levels of the singlet hydrogen molecule, corresponding to para- and ortho-H2, respectively, treated as four-particle quantum systems.

Simmen, Benjamin; Mátyus, Edit; Reiher, Markus

2013-08-01

189

Role of plasma activation in kinetics of carbon nanotube growth in plasma-enhanced chemical vapor deposition  

NASA Astrophysics Data System (ADS)

The effect of the acceleration of carbon nanotube (CNT) growth from methane via modification of the gas composition with plasma discharge is studied by means of kinetic modeling. A model of CNT growth describing the detailed gas-phase and surface kinetics of hydrocarbons in catalytic chemical vapor deposition is used. It is shown that the effect of plasma is related to the formation of active species in the gas phase that can easily adsorb and dissociate on the catalyst surface. It is also demonstrated that in addition to the adsorption of carbon precursors, the CNT growth rate can be limited by the gas-phase diffusion of carbon species to the catalyst surface. The critical energy input above which the dominant contribution to CNT growth is provided by non-radical neutral species is calculated and confirmed by analytic estimates.

Lebedeva, Irina V.; Knizhnik, Andrey A.; Gavrikov, Alexey V.; Baranov, Alexey E.; Potapkin, Boris V.; Smith, David J.; Sommerer, Timothy J.

2012-04-01

190

Selecting the optimum quasi-steady-state species for reduced chemical kinetic mechanisms using a genetic algorithm  

SciTech Connect

A genetic optimization algorithm has been applied to the selection of quasi-steady-state (QSS) species in reduced chemical kinetic mechanisms. The algorithm seeks to minimize the error between reduced and detailed chemistry for simple reactor calculations approximating conditions of interest for a computational fluid dynamics simulation. The genetic algorithm does not guarantee that the global optimum will be found, but much greater accuracy can be obtained than by choosing QSS species through a simple kinetic criterion or by human trial and error. The algorithm is demonstrated for methane-air combustion over a range of temperatures and stoichiometries and for homogeneous charge compression ignition engine combustion. The results are in excellent agreement with those predicted by the baseline mechanism. A factor of two reduction in the number of species was obtained for a skeletal mechanism that had already been greatly reduced from the parent detailed mechanism.

Montgomery, Christopher J.; Yang, Chongguan [Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, UT 84101 (United States); Parkinson, Alan R. [Department of Mechanical Engineering, Brigham Young University, 270 Clyde Building, Provo, UT 84602 (United States); Chen, J.-Y. [Department of Mechanical Engineering, University of California-Berkeley, 6163 Etcheverry Hall, Berkeley, CA 94720 (United States)

2006-01-01

191

Chemical Kinetics of Triplet Methylene from Infrared Diode Laser Flash Kinetic Spectroscopy  

Microsoft Academic Search

A flash kinetic spectrometer based on a tunable infrared diode laser has been constructed. This spectrometer can measure Doppler limited spectra in the frequency region from 300-3000 cm^{-1}, allowing for the detection of nearly any molecular species. The spectrometer can be used in a spectroscopy mode to obtain the high resolution (<=q10^ {-3} cm^{-1})^ectrum of the species, or in a

David Charles Darwin

1989-01-01

192

Chemical kinetic mechanism for the oxidation of paraffinic hydrocarbons needed for primary reference fuels  

SciTech Connect

A detailed chemical kinetic reaction mechanism is described which simulates the oxidation of the primary reference fuels n-heptane and iso-octane. The high temperature subset of these mechanisms is identified, and the extensions to deal with low temperature conditions are also explained. The algorithms used to assign reaction rates to elementary steps in the reaction mechanism are described, and the means of identifying the different chemical species and the relevant reactions are outlined. Finally, we show how interested kinetic modeling researchers can obtain copies of this reaction mechanism.

Westbrook, C.K.; Pitz, W.J.

1993-03-01

193

KEMOD: A mixed chemical kinetic and equilibrium model of aqueous and solid phase geochemical reactions  

SciTech Connect

This report presents the development of a mixed chemical Kinetic and Equilibrium MODel in which every chemical species can be treated either as a equilibrium-controlled or as a kinetically controlled reaction. The reaction processes include aqueous complexation, adsorption/desorption, ion exchange, precipitation/dissolution, oxidation/reduction, and acid/base reactions. Further development and modification of KEMOD can be made in: (1) inclusion of species switching solution algorithms, (2) incorporation of the effect of temperature and pressure on equilibrium and rate constants, and (3) extension to high ionic strength.

Yeh, G.T.; Iskra, G.A. [Pennsylvania State Univ., University Park, PA (United States). Dept. of Civil Engineering; Szecsody, J.E.; Zachara, J.M.; Streile, G.P. [Pacific Northwest Lab., Richland, WA (United States)

1995-01-01

194

Inactivation of Escherichia coli with ozone: chemical and inactivation kinetics  

Microsoft Academic Search

The apparent chemical and inactivation reactions taking place during the disinfection of Escherichia coli with ozone in the presence of humic acid were investigated with continuous-flow tubular reactors. The apparent decomposition of dissolved ozone in the presence of humic acid and E. coli cells was modeled successfully with mixed second-order rate expressions within a time scale relevant to E. coli

Nimrata K Hunt; Benito J Mariñas

1999-01-01

195

Equilibrium, chemical kinetic, and transport limitations to diamond growth  

Microsoft Academic Search

Because of their extreme properties, diamond films have found some industrial applications, i.e., heat sinks and tool coatings. However, to increase their economic attractiveness, the growth rate must be increased, the deposition temperature must be lowered, and single crystal films must be achieved. We have studied two types of chemical vapor deposition systems, hot-filament and microwave assisted, in order to

Edward Anthony Evans

1998-01-01

196

Parameter Estimates in Differential Equation Models for Chemical Kinetics  

ERIC Educational Resources Information Center

|We discuss the need for devoting time in differential equations courses to modelling and the completion of the modelling process with efforts to estimate the parameters in the models using data. We estimate the parameters present in several differential equation models of chemical reactions of order n, where n = 0, 1, 2, and apply more general…

Winkel, Brian

2011-01-01

197

Chemical Kinetics for Modeling Silicon Epitaxy from Chlorosilanes  

SciTech Connect

A reaction mechanism has been developed that describes the gas-phas 0971 and surface reactions involved in the chemical vapor deposition of Si from chlorosilanes. Good agreement with deposition rate data from a single wafer reactor with no wafer rotation has been attained over a range of gas mixtures, total flow rates, and reactor temperatures.

Balakrishna, A.; Chacin, J.M.; Comita, P.B.; Haas, B.; Ho, P.; Thilderkvist, A.

1998-11-24

198

CHEMKIN2. General Gas-Phase Chemical Kinetics  

Microsoft Academic Search

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. CHEMKIN was designed as an easily transportable program for use on CDC, Cray, and DEC VAX systems. It consists of two major

R. J. Kee; J. Warnatz

1980-01-01

199

Infrared absorption spectroscopy and chemical kinetics of free radicals  

SciTech Connect

Propargyl radical has recently attracted interest because of its possible role in combustion and soot formation. At high temperatures it is not easily destroyed by dissociation nor by reaction with oxygen thus, it has been observed in significant concentrations in numerous pyrolysis and oxidation processes. During the last year, we have obtained the high resolution spectrum of the v{sub 1} acetylenic CH stretch of propargyl radical (HCCCH{sub 2}) near 3322 cm{sup {minus}1} using infrared laser kinetic spectroscopy at Doppler limited resolution. Propargyl is prepared by flash photolysis of propargyl bromide (or propargyl chloride) at 193 nm (ArFexcimer) and its transient infrared absorption probed by a cw color center laser. We are beginning to investigate the kinetics of propargyl radical. The decay of the radical after the flash appears to be second order. The fine structure transition of the Br atom is accessible and when monitored under the same conditions appears to exhibit a simple first order decay suggesting that the Br atom is reacting with precursor propargyl bromide. Ketenyl radicals were produced by 193 nm excimer laser photolysis of ketene and probed with a tunable diode laser operating at 2014 cm{sup {minus}1}. Under these conditions, any singlet methylene which may be formed should react with the precursor, ketene, at a rate fast enough to ensure its total removal from the photolysis cell within 1 {mu}s. In the presence of 2 to 8 Torr of O{sub 2}, the ketenyl radical was observed to decay exponentially with time constants that ranged fro 20 to 5 {mu}s.

Curl, R.F.; Glass, G.P.

1992-04-01

200

The Role of Comprehensive Detailed Chemical Kinetic Reaction Mechanisms in Combustion Research  

SciTech Connect

Recent developments by the authors in the field of comprehensive detailed chemical kinetic reaction mechanisms for hydrocarbon fuels are reviewed. Examples are given of how these mechanisms provide fundamental chemical insights into a range of combustion applications. Practical combustion consists primarily of chemical heat release from reactions between a fuel and an oxidizer, and computer simulations of practical combustion systems have become an essential tool of combustion research (Westbrook et al., 2005). At the heart of most combustion simulations, the chemical kinetic submodel frequently is the most detailed, complex and computationally costly part of a system model. Historically, the chemical submodel equations are solved using time-implicit numerical algorithms, due to the extreme stiffness of the coupled rate equations, with a computational cost that varies roughly with the cube of the number of chemical species in the model. While early mechanisms (c. 1980) for apparently simple fuels such as methane (Warnatz, 1980) or methanol (Westbrook and Dryer, 1979) included perhaps 25 species, current detailed mechanisms for much larger, more complex fuels such as hexadecane (Fournet et al., 2001; Ristori et al., 2001; Westbrook et al., 2008) or methyl ester methyl decanoate (Herbinet et al., 2008) have as many as 2000 or even 3000 species. Rapid growth in capabilities of modern computers has been an essential feature in this rapid growth in the size and complexity of chemical kinetic reaction mechanisms.

Westbrook, C K; Pitz, W J; Curran, H J; Mehl, M

2008-07-16

201

The subtle business of model reduction for stochastic chemical kinetics.  

PubMed

This paper addresses the problem of simplifying chemical reaction networks by adroitly reducing the number of reaction channels and chemical species. The analysis adopts a discrete-stochastic point of view and focuses on the model reaction set S(1)<=>S(2)-->S(3), whose simplicity allows all the mathematics to be done exactly. The advantages and disadvantages of replacing this reaction set with a single S(3)-producing reaction are analyzed quantitatively using novel criteria for measuring simulation accuracy and simulation efficiency. It is shown that in all cases in which such a model reduction can be accomplished accurately and with a significant gain in simulation efficiency, a procedure called the slow-scale stochastic simulation algorithm provides a robust and theoretically transparent way of implementing the reduction. PMID:19222263

Gillespie, Dan T; Cao, Yang; Sanft, Kevin R; Petzold, Linda R

2009-02-14

202

CHEMKIN2. General Gas-Phase Chemical Kinetics  

Microsoft Academic Search

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary,

Rupley

1992-01-01

203

Kinetically-induced hexagonality in chemically grown silicon nanowires  

Microsoft Academic Search

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically\\u000a synthesized nanowires. The structures are typified by mixed stacking mismatches of closely packed Si dimers. Instead of viewing\\u000a them as defects, we define the concept of hexagonality and describe these structures as Si polymorphs. The small transverse\\u000a dimensions of a nanowire make this

Xiaohua Liu; Dunwei Wang

2009-01-01

204

Kinetics of halide chemical vapor deposition of silicon carbide film  

Microsoft Academic Search

Halide chemical vapor deposition (HCVD) emerges as a promising method for growing thick silicon carbide (SiC) epilayers, due to its ability to deposit SiC film at a high growth rate. In this paper, we develop a comprehensive model for HCVD of SiC film using silicon tetrachloride and propane as precursors. The model includes gas flow, heat and mass transfer, gas-phase

Rong Wang; Ronghui Ma

2007-01-01

205

Analysis of Premixed Charge Compression Ignition Combustion with a Sequential Fluid Mechanics-Multizone Chemical Kinetics Model  

SciTech Connect

We have developed a methodology for analysis of PCCI engines that applies to conditions in which there is some stratification in the air-fuel distribution inside the cylinder at the time of combustion. Our analysis methodology consists of two stages: first, a fluid mechanics code is used to determine temperature and equivalence ratio distributions as a function of crank angle, assuming motored conditions. The distribution information is then used for grouping the mass in the cylinder into a two-dimensional (temperature-equivalence ratio) array of zones. The zone information is then handed on to a detailed chemical kinetics model that calculates combustion, emissions and engine efficiency information. The methodology applies to situations where chemistry and fluid mechanics are weakly linked. The results of the multi-zone model have been compared to the results obtained from a fully integrated code, in which a chemical kinetics code is directly linked into a fluid mechanics code to calculate chemistry in every cell of the grid. The results show that the multi-zone model predicts burn duration and peak cylinder pressure with good accuracy. However, ignition timing predicted by the multi-zone model is sensitive to the transition angle between the fluid mechanics code and the chemical kinetics code. The paper explores the possibility of using three different criteria for determining the transition angle: fraction of heat release at the time of ignition, temperature of the hottest cell at the time of ignition, and a fixed crank angle of transition. The results show that the three criteria have some validity as transition criteria. Further research is necessary to investigate the effect of fuel properties and operating conditions on transition angle.

Aceves, S M; Flowers, D L; Espinosa-Loza, F; Babajimopoulos, A; Assanis, D N

2004-09-30

206

Engineering of protein thermodynamic, kinetic, and colloidal stability: Chemical Glycosylation with monofunctionally activated glycans.  

PubMed

In this work we establish the relationship between chemical glycosylation and protein thermodynamic, kinetic, and colloidal stability. While there have been reports in the literature that chemical glycosylation modulates protein stability, mechanistic details still remain uncertain. To address this issue, we designed and coupled monofunctional activated glycans (lactose and dextran) to the model protein alpha-chymotrypsin (alpha-CT). This resulted in a series of glycoconjugates with variations in the glycan size and degree of glycosylation. Thermodynamic unfolding, thermal inactivation, and temperature-induced aggregation experiments revealed that chemical glycosylation increased protein thermodynamic (Delta G(25 degrees C)), kinetic (t(1/2)(45 degrees C)), and colloidal stability. These results highlight the potential of chemical glycosylation with monofunctional activated glycans as a technology for increasing the long-term stability of liquid protein formulations for industrial and biotherapeutic applications. PMID:16586505

Solá, Ricardo J; Al-Azzam, Wasfi; Griebenow, Kai

2006-08-20

207

Thermochemical analysis and kinetics aspects for a chemical model for camphene ozonolysis  

NASA Astrophysics Data System (ADS)

In this work, a chemical model for the camphene ozonolysis, leading to carbonyl final products, is proposed and discussed on the basis of the thermochemical properties and kinetic data obtained at density functional theory levels of calculation. The mechanism is initiated by the electrophilic attack of ozone to the double bond in camphene leading to a 1,2,3-trioxolane intermediate, which decomposes to peroxy radicals and carbonyl compounds in a total of 10 elementary reactions. The thermodynamic properties (enthalpy and entropies differences) are calculated at 298 K. For the thermochemical evaluation, theoretical calculations are performed with the B3LYP, MPW1PW91, and mPW1K density functionals and the basis sets 6-31G(d), 6-31G(2d,2p), 6-31+G(d,p), and 6-31+G(2d,2p). Eventually, single point calculations adopting the 6-311++G(2d,2p) basis set are performed in order to improve the electronic energies. The enthalpy profiles suggest highly exothermic reactions for the individual steps, with a global enthalpy difference of -179.18 kcal mol-1, determined at the B3LYP/6-31+G(2d,2p) level. The Gibbs free energy differences for each step, at 298 K, calculated at the B3LYP/6-311++G(2d,2p)//B3LYP/6-31+G(2d,2p) level, are used to estimate the composition of a final product mixture under equilibrium conditions as 58% of camphenilone and 42% of 6,6-dimethyl-?-caprolactone-2,5-methylene. For the reaction kinetics, the bimolecular O3 + camphene step is assumed to be rate determining in the global mechanism. A saddle point for the ozone addition to the double bond is located and rate constants are determined on the basis of the transition state theory. This saddle point is well represented by a loosely bound structure and corrections for the basis set superposition error (BSSE) are calculated, either by considering the effect over the geometry optimization procedure (here referred as CP1 procedure), or the effect of the BSSE over the electronic energy of a previously optimized geometry, included a posteriori (here referred as CP2). The rate constants, calculated at 298 K from the data obtained at the mPW1K/6-31+G(d,p), CP1/B3LYP//6-31+G(2d,2p), and CP2/B3LYP//6-31+G(2d,2p) levels (3.62 × 10-18, 1.12 × 10-18, and 1.39 × 10-18 cm3 molecule-1 s-1), are found in good agreement with the available experimental data at the same temperature, 0.9 × 10-18 cm3 molecule-1 s-1 [R. Atkinson, S. M. Aschmann, and J. Arey, Atmos. Environ. 24, 2647 (1990)]. The importance of the BSSE corrections for the final rate constants must be pointed out. Furthermore, this work will contribute to a better understanding of the chemistry of monoterpenes in the atmosphere, as well as the implications for the phenomena of pollution.

Oliveira, R. C. de M.; Bauerfeldt, G. F.

2012-10-01

208

Thermochemical analysis and kinetics aspects for a chemical model for camphene ozonolysis.  

PubMed

In this work, a chemical model for the camphene ozonolysis, leading to carbonyl final products, is proposed and discussed on the basis of the thermochemical properties and kinetic data obtained at density functional theory levels of calculation. The mechanism is initiated by the electrophilic attack of ozone to the double bond in camphene leading to a 1,2,3-trioxolane intermediate, which decomposes to peroxy radicals and carbonyl compounds in a total of 10 elementary reactions. The thermodynamic properties (enthalpy and entropies differences) are calculated at 298 K. For the thermochemical evaluation, theoretical calculations are performed with the B3LYP, MPW1PW91, and mPW1K density functionals and the basis sets 6-31G(d), 6-31G(2d,2p), 6-31+G(d,p), and 6-31+G(2d,2p). Eventually, single point calculations adopting the 6-311++G(2d,2p) basis set are performed in order to improve the electronic energies. The enthalpy profiles suggest highly exothermic reactions for the individual steps, with a global enthalpy difference of -179.18 kcal mol(-1), determined at the B3LYP?6-31+G(2d,2p) level. The Gibbs free energy differences for each step, at 298 K, calculated at the B3LYP?6-311++G(2d,2p)??B3LYP?6-31+G(2d,2p) level, are used to estimate the composition of a final product mixture under equilibrium conditions as 58% of camphenilone and 42% of 6,6-dimethyl-?-caprolactone-2,5-methylene. For the reaction kinetics, the bimolecular O(3) + camphene step is assumed to be rate determining in the global mechanism. A saddle point for the ozone addition to the double bond is located and rate constants are determined on the basis of the transition state theory. This saddle point is well represented by a loosely bound structure and corrections for the basis set superposition error (BSSE) are calculated, either by considering the effect over the geometry optimization procedure (here referred as CP1 procedure), or the effect of the BSSE over the electronic energy of a previously optimized geometry, included a posteriori (here referred as CP2). The rate constants, calculated at 298 K from the data obtained at the mPW1K?6-31+G(d,p), CP1?B3LYP??6-31+G(2d,2p), and CP2?B3LYP??6-31+G(2d,2p) levels (3.62 × 10(-18), 1.12 × 10(-18), and 1.39 × 10(-18) cm(3) molecule(-1) s(-1)), are found in good agreement with the available experimental data at the same temperature, 0.9 × 10(-18) cm(3) molecule(-1) s(-1) [R. Atkinson, S. M. Aschmann, and J. Arey, Atmos. Environ. 24, 2647 (1990)]. The importance of the BSSE corrections for the final rate constants must be pointed out. Furthermore, this work will contribute to a better understanding of the chemistry of monoterpenes in the atmosphere, as well as the implications for the phenomena of pollution. PMID:23039598

Oliveira, R C de M; Bauerfeldt, G F

2012-10-01

209

Stability and Control of Lean Blowout in Chemical Kinetics-Controlled Combustion Systems  

Microsoft Academic Search

This study is motivated by lean-blowout (LBO) detection and control in dry-low-emission (DLE) combustion systems. However, this analysis is confined to chemical kinetics-controlled combustion. Despite its simplicity, some useful insight may still be shed on near-LBO combustion systems, as the chemical reaction rates are rather low near LBO. A third-order linear well-stirred reactor (WSR) model is derived to examine a

Tongxun Yi; Ephraim J. Gutmark; Bruce K. Walker

2009-01-01

210

Microfluidic sub-millisecond mixers for the study of chemical reaction kinetics  

Microsoft Academic Search

We present a micromachined, high Reynolds number (2000-6000), sub-millisecond liquid mixer for the study of chemical reaction kinetics. This 1 cm×1 cm×1 mm bulk micromachined silicon mixer is capable of initiating and quenching (starting and stopping) chemical reactions in intervals as short as 100 ?s. The centimeter sized mixer chip contains two tee mixers connected by one channel which serves

Amish Desai; D. Bokenkamp; Xing Yang; Yu-Chong Tai; E. Marzluff; S. Mayo

1997-01-01

211

Long-term bioconcentration kinetics of hydrophobic chemicals in Selenastrum capricornutum and Microcystis aeruginosa  

SciTech Connect

The bioconcentration of two chlorobenzenes (CBs) and of seven polychlorobiphenyls (PCBs) to Selenastrum capricornutum and Microcystis aeruginosa was studied with accumulation experiments followed by gas purge elimination experiments. Henry's law constants at 10 C were needed to interpret the gas purge results and were measured in control experiments. For the M. aerogunisa culture, steady-state uptake was reached within days, whereas uptake by S. capricornutum took several weeks. The relationships between the log bioconcentration factors (BCF) and log octanol-water partition coefficients (K[sub OW]) were nonlinear, with relatively low values for the more hydrophobic PCBs. Rate constants for the elimination of CBs and PCBs from the algal cells were shown to be larger than 1 per day when calculated with a one-compartment model. With such large rate constants, it is unlikely that the curvature observed for these species is caused by slow kinetics or that algal growth affects BCF by dilution of CB or PCB concentrations. The log BCF-log K[sub OW] relationships could be described by a simple three-phase model that accounted for the binding of CBs and PCBs to dissolved organic carbon (DOC). Modeling bioconcentration of hydrophobic chemicals in phytoplankton should account for the binding to DOC.

Koelmans, A.A.; Woude, H. van der; Hattink, J.; Niesten, D.J.M. (Wageningen Agricultural Univ. (Netherlands). Aquatic Ecology and Water Quality Management Group)

1999-06-01

212

Interactions of multiphase hydrodynamics, droplet evaporation, and chemical kinetics in FCC riser reactors.  

SciTech Connect

A computational fluid dynamics (CFD) computer code, ICRKFLO, has been developed for flow simulation of fluid catalytic cracking (FCC) riser reactors, which convert crude oil into gasoline and other valuable products. The FCC flow, especially in the entry region, is a three-phase reacting flow including hot catalyst particles, inert lift gas, and feed oil droplets. The impact of the hydrodynamics processes of heat transfer, droplet evaporation, and mixing on the chemical kinetics or riser performance can be significant. ICRKFLO was used to evaluate the impact of these processes on the performance of an advanced FCC unit. The code solves for major flow properties of all three phases in an FCC riser, with models governing the transport of catalyst particles and feed oil droplet, the vaporization of the feed oil droplets, the cracking of the oil vapor, and the formation and deposition of coke on particles. First, the code was validated against available test data of a pilot-scale FCC unit. Then, flow calculations for the FCC unit were performed. Computational results indicate that the heat transfer and droplet vaporization processes have a significant impact on the performance of a pilot-scale FCC unit. The impact is expected to be even greater on commercial scale units.

Chang, S. L.

1998-02-17

213

Calculation of propulsive nozzle flowfields in multidiffusing chemically reacting environments  

NASA Astrophysics Data System (ADS)

An advanced engineering model has been developed to aid in the analysis and design of hydrogen/oxygen chemical rocket engines. The complete multispecies, chemically reacting and multidiffusing Navier-Stokes equations are modelled, including the Soret thermal diffusion and the Dufour energy transfer terms. In addition to the spectrum of multispecies aspects developed, the model developed in this study is also conservative in axisymmetric flow for both inviscid and viscous flow environments and the boundary conditions employ a viscous, chemically reacting, reference plane characteristics method. Demonstration cases are presented for a 1030:1 area ratio nozzle, a 25 lbf film cooled nozzle, and a transpiration cooled plug and spool rocket engine. The results indicate that the thrust coefficient predictions of the 1030:1 and the 25 lbf film cooled nozzle are within 0.2 to 0.5 percent, respectively, of experimental measurements when all of the chemical reaction and diffusion terms are considered. Further, the model's predictions agree very well with the heat transfer measurements made in all of the nozzle test cases. The Soret thermal diffusion term is demonstrated to have a significant effect on the predicted mass fraction of hydrogen along the wall of the nozzle in both the laminar flow 1030:1 nozzle and the turbulent flow plug and spool nozzle analysis cases performed. Further, the Soret term was shown to represent an important fraction of the diffusion fluxes occurring in a transpiration cooled rocket engine.

Kacynski, Kenneth John

1994-04-01

214

Chemical Cycle Kinetics: Removing the Limitation of Linearity of a Non-equilibrium Thermodynamic Description  

NASA Astrophysics Data System (ADS)

Chemical cycle kinetics is customarily analyzed by means of the law of mass action which describes how the concentrations of the substances vary with time. The connection of this approach with non-equilibrium thermodynamics (NET) has traditionally been restricted to the linear domain close to equilibrium in which the reaction rates are linear functions of the affinities. We show, by a pertinent formulation of the concept of local equilibrium in the mesoscopic description along the reaction coordinates, that the connection between kinetic and thermodynamic approaches is deeper than thought and holds in the nonlinear domain far from equilibrium, for higher values of the affinity. This new perspective indicates how to overcome the inherent limitation of classical NET in treating cyclic reactions, providing a description of closed and open cycles operating far from equilibrium, in accordance with thermodynamic principles. We propose that the new set of equations are tested and used for data reduction in chemical reaction kinetics.

Rubi, J. M.; Bedeaux, D.; Kjelstrup, S.; Pagonabarraga, I.

2013-07-01

215

Chemical kinetic modeling of dimethyl carbonate in an opposed-flow diffusion flame  

Microsoft Academic Search

Dimethyl carbonate (DMC) has been of interest as an oxygenate additive to diesel fuel because of its high oxygen content. In this study, a chemical kinetic mechanism for DMC was developed for the first time and used to understand its combustion under conditions in an opposed flow diffusion flame. Computed results were compared to previously published experimental results from an

Pierre A. Glaude; William J. Pitz; Murray J. Thomson

2005-01-01

216

Cooperative Learning Instruction for Conceptual Change in the Concepts of Chemical Kinetics  

ERIC Educational Resources Information Center

|Learning is a social event and so the students need learning environments that enable them to work with their peers so that they can learn through their interactions. This study discusses the effectiveness of cooperative learning compared to traditional instruction in terms of students' motivation and understanding of chemical kinetics in a high…

Kirik, Ozgecan Tastan; Boz, Yezdan

2012-01-01

217

A. G. Vernon Harcourt: A Founder of Chemical Kinetics and a Friend of "Lewis Carroll."  

ERIC Educational Resources Information Center

|Outlines the life of A. G. Vernon Harcourt, a founder of chemical kinetics, contributor to the purification of coal gas from sulfur compounds, inventor of the percentage chloroform inhaler, friend to Lewis Carroll, and instructor to the Prince of Wales. (CS)|

Shorter, John

1980-01-01

218

Post-processing of detailed chemical kinetic mechanisms onto CFD simulations  

Microsoft Academic Search

A new general method to combine computational fluid dynamics tools and detailed chemical kinetic mechanisms is presented. The method involves post-processing of data extracted from computational fluid dynamics (CFD) simulations obtained by using a simple reaction model to generate an overall estimate of the temperature and flow field in the computational domain. In post-processing of the data, the individual cells

M. S. Skjøth-rasmussen; O. Holm-christensen; M. Østberg; T. S. Christensen; T. Johannessen; A. D. Jensen; P. Glarborg; H. Livbjerg

2004-01-01

219

A detailed chemical kinetic model of high-temperature ethylene glycol gasification  

Microsoft Academic Search

In recent experimental investigations, ethylene glycol is used as a model substance for biomass-based pyrolysis oil in an entrained flow gasifier. In order to gain a deeper insight into process sequences and to conduct parametric analysis, this study describes the development and validation of a detailed chemical kinetic model of high-temperature ethylene glycol gasification. A detailed reaction mechanism based on

Simon Hafner; Arash Rashidi; Georgiana Baldea; Uwe Riedel

2011-01-01

220

Analog Computation in Fast Chemical Kinetics Using a Digital Transient Recorder  

Microsoft Academic Search

An analog computation method for chemical kinetics is described. An exponentially varying transient signal from a stopped-flow or temperature-jump experiment is digitized and stored in a transient recorder. It is repetitively reconverted to an analog signal whose logarithm is displayed on an oscilloscope. The operator subtracts the equilibrium value of the original signal by adjusting a potentiometer until the logarithmic

James E. Stewart

1972-01-01

221

Detailed Chemical Kinetic Reaction Mechanism for Oxidation of n-Octane and ISO-Octane.  

National Technical Information Service (NTIS)

The development of detailed chemical kinetic reaction mechanisms for oxidation of n-octane and iso-octane is described, with emphasis on the factors which are specific to many large hydrocarbon fuel molecules. Elements which are of particular importance a...

E. I. Axelsson K. Brezinsky F. L. Dryer W. J. Pitz C. K. Westbrook

1986-01-01

222

Kinetic effect of Pd additions on the hydrogen uptake of chemically activated, ultramicroporous carbon  

SciTech Connect

The effect of mixing chemically-activated ultramicroporous carbon (UMC) with Pd nanopowder is investigated. Results show that Pd addition doubles the rate of hydrogen uptake, but does not enhance the hydrogen capacity or improve desorption kinetics. The effect of Pd on the rate of hydrogen adsorption supports the occurrence of the hydrogen spillover mechanism in the Pd - UMC system.

Bhat, Vinay V [ORNL; Contescu, Cristian I [ORNL; Gallego, Nidia C [ORNL

2010-01-01

223

INFLUENCE OF ORGANIC COSOLVENTS ON THE SORPTION KINETICS OF HYDROPHOBIC ORGANIC CHEMICALS  

EPA Science Inventory

A quantitative examination of the kinetics of sorption of hydrophobic organic chemicals by soils from mixed solvents reveals that the reverse sorption rate constant (k2) increases log-linearly with increasing volume fraction of organic cosolvent (fc). This relationship was expec...

224

A Chemical Kinetics Model to Predict Lubricant Performance in a Diesel Engine. Part I: Simulation Methodology  

Microsoft Academic Search

The ability of a lubricant to protect increasingly complex diesel engines directly affects engine durability and warranty costs and is becoming increasingly costly to validate. This paper presents a novel approach combining a chemical kinetic model using rate constants determined by a set of laboratory bench tests and a finite-difference computer program to predict lubricant performance in a given diesel

Chun-I Chen; Stephen M. Hsu

2003-01-01

225

Chemical kinetics simulation for atmospheric pressure air plasmas in a streamer regime  

Microsoft Academic Search

We present a theoretical analysis of air discharges at high pressure which operates in a streamer regime. The aim is to provide a simplified framework to study the chemical kinetics in the discharge gas phase. We have tried to model the characteristics of actually existing devices, such as dc and ac discharges in sharp point-plane, point to point, or coaxial

R. Barni; P. Esena; C. Riccardi

2005-01-01

226

SENKIN: A Fortran program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis  

Microsoft Academic Search

SENKIN is a Fortran computer program that computes the time evolution of a homogeneous reacting gas mixture in a closed system. The model accounts for finite-rate elementary chemical reactions, and performs kinetic sensitivity analysis with respect to the reaction rates. The program considers five problem types: an adiabatic system with constant pressure; an adiabatic system with constant volume; an adiabatic

A. E. Lutz; R. J. Kee; J. A. Miller

1988-01-01

227

Gas-Phase and Heterogeneous Chemical Kinetics of the Troposphere and Stratosphere  

Microsoft Academic Search

This review involves chemical kinetics of atmospheric reactions, with emphasis on gas phase and heterogeneous processes related to the following topics: (a) in the troposphere, chemistry of CFC\\/Halon replacement compounds and chemistry of sulfur species and (b) in the stratosphere, chemistry of fluorine, bromine, and iodine species and of heterogeneous processes occurring on sulfuric acid aerosols. The results of laboratory

Mario J. Molina; Luisa T. Molina; Charles E. Kolb

1996-01-01

228

Gas-Phase Oxidation of Nitric Oxide: Chemical Kinetics and Rate Constant  

Microsoft Academic Search

Inhaled nitric oxide (NO) is gaining popularity as a selective pulmonary vasodilator. Because of the potential toxicity of NO and its oxidizing product nitrogen dioxide (NO2), any system for the delivery of inhaled NO must aim at predictable and reproducible levels of NO and at as low concentrations of NO2 as possible. This review describes the chemical kinetics and rate

Hirokazu Tsukahara; Takanobu Ishida; Mitsufumi Mayumi

1999-01-01

229

Bimolecular gas-phase reactions in the Kramers theory of chemical kinetics @fa@f)  

Microsoft Academic Search

An expression for the rate constant is obtained for a bimolecular gas?phase reaction via an extended version of the Kramers theory of chemical kinetics. The multicomponent Fokker–Planck equation is transformed into a suitable set of relative coordinates that facilitated the derivation. The derived expression for the rate constant shows explicitly how the rate of reaction depends on the potential energy

R. S. Larson; M. D. Kostin

1984-01-01

230

Reactive molecular dynamics simulation and chemical kinetic modeling of pyrolysis and combustion of n-dodecane  

SciTech Connect

The initiation mechanisms and kinetics of pyrolysis and combustion of n-dodecane are investigated by using the reactive molecular dynamics (ReaxFF MD) simulation and chemical kinetic modeling. From ReaxFF MD simulations, we find the initiation mechanisms of pyrolysis of n-dodecane are mainly through two pathways, (1) the cleavage of C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding n-C{sub 12}H{sub 25} radical. Another pathway is the H-abstraction reactions by small radicals including H, CH{sub 3}, and C{sub 2}H{sub 5}, which are the products after the initiation reaction of n-dodecane pyrolysis. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis of n-dodecane pyrolysis. The density/pressure effects on the pyrolysis of n-dodecane are also analyzed. By appropriate mapping of the length and time from macroscopic kinetic modeling to ReaxFF MD, a simple comparison of the conversion of n-dodecane from ReaxFF MD simulations and that from kinetic modeling is performed. In addition, the oxidation of n-dodecane is studied by ReaxFF MD simulations. We find that formaldehyde molecule is an important intermediate in the oxidation of n-dodecane, which has been confirmed by kinetic modeling, and ReaxFF leads to reasonable reaction pathways for the oxidation of n-dodecane. These results indicate that ReaxFF MD simulations can give an atomistic description of the initiation mechanism and product distributions of pyrolysis and combustion for hydrocarbon fuels, and can be further used to provide molecular based robust kinetic reaction mechanism for chemical kinetic modeling of hydrocarbon fuels. (author)

Wang, Quan-De [College of Chemistry, Sichuan University, Chengdu (China); Wang, Jing-Bo; Li, Juan-Qin; Tan, Ning-Xin; Li, Xiang-Yuan [College of Chemical Engineering, Sichuan University, Chengdu (China)

2011-02-15

231

Calculating chemical concentrations during the initiation of crevice corrosion  

Microsoft Academic Search

Numerical simulations of solution chemistry and electrochemical potential variations within crevice corrosion sites on stainless steel have been carried out using a method recently developed by Friedly and Rubin. Potential differences in the crevice solution have been calculated via the Nernst–Planck equation rather than by making the usual assumption of electroneutrality at any point, although this assumption is proven to

Stephen P. White; Graham J. Weir; N. J. Laycock

2000-01-01

232

The nature of chemical bonds from PNOF5 calculations.  

PubMed

Natural orbital functional theory (NOFT) is used for the first time in the analysis of different types of chemical bonds. Concretely, the Piris natural orbital functional PNOF5 is used. It provides a localization scheme that yields an orbital picture which agrees very well with the empirical valence shell electron pair repulsion theory (VSEPR) and Bent's rule, as well as with other theoretical pictures provided by valence bond (VB) or linear combination of atomic orbitals-molecular orbital (LCAO-MO) methods. In this context, PNOF5 provides a novel tool for chemical bond analysis. In this work, PNOF5 is applied to selected molecules that have ionic, polar covalent, covalent, multiple (? and ?), 3c-2e, and 3c-4e bonds. PMID:22615195

Matxain, Jon M; Piris, Mario; Uranga, Jon; Lopez, Xabier; Merino, Gabriel; Ugalde, Jesus M

2012-05-21

233

Errors in the Calculation of 27Al Nuclear Magnetic Resonance Chemical Shifts  

PubMed Central

Computational chemistry is an important tool for signal assignment of 27Al nuclear magnetic resonance spectra in order to elucidate the species of aluminum(III) in aqueous solutions. The accuracy of the popular theoretical models for computing the 27Al chemical shifts was evaluated by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. In order to differentiate the error due to the chemical shielding tensor calculation from that due to the inadequacy of the molecular geometry prediction, single-crystal X-ray diffraction determined structures were used to build the isolated molecule models for calculating the chemical shifts. The results were compared with those obtained using the calculated geometries at the B3LYP/6-31G(d) level. The isotropic chemical shielding constants computed at different levels have strong linear correlations even though the absolute values differ in tens of ppm. The root-mean-square difference between the experimental chemical shifts and the calculated values is approximately 5 ppm for the calculations based on the X-ray structures, but more than 10 ppm for the calculations based on the computed geometries. The result indicates that the popular theoretical models are adequate in calculating the chemical shifts while an accurate molecular geometry is more critical.

Wang, Xianlong; Wang, Chengfei; Zhao, Hui

2012-01-01

234

Errors in the Calculation of 27Al Nuclear Magnetic Resonance Chemical Shifts.  

PubMed

Computational chemistry is an important tool for signal assignment of 27Al nuclear magnetic resonance spectra in order to elucidate the species of aluminum(III) in aqueous solutions. The accuracy of the popular theoretical models for computing the 27Al chemical shifts was evaluated by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. In order to differentiate the error due to the chemical shielding tensor calculation from that due to the inadequacy of the molecular geometry prediction, single-crystal X-ray diffraction determined structures were used to build the isolated molecule models for calculating the chemical shifts. The results were compared with those obtained using the calculated geometries at the B3LYP/6-31G(d) level. The isotropic chemical shielding constants computed at different levels have strong linear correlations even though the absolute values differ in tens of ppm. The root-mean-square difference between the experimental chemical shifts and the calculated values is approximately 5 ppm for the calculations based on the X-ray structures, but more than 10 ppm for the calculations based on the computed geometries. The result indicates that the popular theoretical models are adequate in calculating the chemical shifts while an accurate molecular geometry is more critical. PMID:23203134

Wang, Xianlong; Wang, Chengfei; Zhao, Hui

2012-11-21

235

Chemical kinetic analysis of hydrogen-air ignition and reaction times  

Microsoft Academic Search

An anaytical study of hydrogen air kinetics was performed. Calculations were made over a range of pressure from 0.2 to 4.0 atm, temperatures from 850 to 2000 K, and mixture equivalence ratios from 0.2 to 2.0. The finite rate chemistry model included 60 reactions in 20 species of the H2-O2-N2 system. The calculations also included an assessment of how small

R. C. Rogers; C. J. Jr. Schexnayder

1981-01-01

236

Chemical kinetic modeling of a methane opposed flow diffusion flame and comparison to experiments  

SciTech Connect

The chemical structure of an opposed flow, methane diffusion flame is studied using a chemical kinetic model and the results are compared to experimental measurements. The chemical kinetic paths leading to aromatics and polycyclic aromatics hydrocarbons (PAHs) in the diffusion flame are identified. These paths all involve resonantly stabilized radicals which include propargyl, allyl, cyclopentadienyl, and benzyl radicals. The modeling results show reasonable agreement with the experimental measurements for the large hydrocarbon aliphatic compounds, aromatics, and PAHs. the benzene was predicted to be formed primarily by the reaction sequence of Allyl plus Propargyl equals Fulvene plus H plus H followed by fulvene isomerization to benzene. Naphthalene was modeled using the reaction of benzyl with propargyl, while the combination of cyclopentadienyl radicals were shown to be a minor contributor in the diffusion flame. The agreement between the model and experiment for the four-ring PAHs was poor.

Marinov, N.M., Pitz, W.J.; Westbrook, C.K. [Lawrence Livermore National Lab., CA (United States); Vincitore, A.M.; Senka, S.M. [Univ. of California, Los Angeles, CA (United States); Lutz, A.E. [Sandia National Labs., Livermore, CA (United States)

1998-01-01

237

Calculations of the Chemical Composition of the Sacramento Urban Plume  

NASA Astrophysics Data System (ADS)

Recent measurements within the Sacramento urban plume have provided a detailed benchmark for testing our understanding of tropospheric chemistry. Available measurements include a wide suite of VOC and BVOC, NOy,i, O3, and CO at the source and at a receptor site five hours downwind. Further, the meteorology in the region is extremely regular making it possible to evaluate effects of temperature or day-of-week patterns with a single season of measurements. Here we use a Lagrangian model representing transport from Granite Bay, a suburb at the eastern edge of Sacramento, to the University of California Blodgett Forest Research Station (UC- BFRS). The model represents chemistry based on MCM v3.1 along with mixing and dilution. The model is initiated with concentrations of NOx, peroxynitrates, alkyl and multifunctional nitrates, HNO3, VOCs and O3 based on measurements at the edge of the Sacramento suburban sprawl east of the city. Biogenic VOC emissions throughout the transect are included. The outputs of the model are compared with ozone measurements at Cool three hours downwind, and detailed measurements of VOC, the speciation of the nitrogen oxides and O3 at UC-BFRS, 5 hours downwind of the Sacramento suburbs in the center of the Mountain counties air basin. The comparisons indicate 1) O3 at UC-BFRS and Cool is largely driven by the combination of rural biogenic emissions and urban NOx emissions, 2) that OH is underestimated by standard chemical models, 3) that partitioning of NOy is dominated by peroxy and other multifunctional nitrates that are not represented in standard chemical models and which have a strong impact on how much NO2 is available for ozone production. We also investigate model representation of temperature and weekend/weekday effects.

Perez, I. M.; Cohen, R. C.

2007-12-01

238

Can accurate kinetic laws be created to describe chemical weathering?  

NASA Astrophysics Data System (ADS)

Knowledge of the mechanisms and rates of mineral dissolution and growth, especially close to equilibrium, is essential for describing the temporal and spatial evolution of natural processes like weathering and its impact on CO2 budget and climate. The Surface Complexation approach (SC) combined with Transition State Theory (TST) provides an efficient framework for describing mineral dissolution over wide ranges of solution composition, chemical affinity, and temperature. There has been a large debate for several years, however, about the comparative merits of SC/TS versus classical growth theories for describing mineral dissolution and growth at near-to-equilibrium conditions. This study considers recent results obtained in our laboratory on oxides, hydroxides, silicates, and carbonates on near-equilibrium dissolution and growth via the combination of complementary microscopic and macroscopic techniques including hydrothermal atomic force microscopy, hydrogen-electrode concentration cell, mixed flow and batch reactors. Results show that the dissolution and precipitation of hydroxides, kaolinite, and hydromagnesite powders of relatively high BET surface area closely follow SC/TST rate laws with a linear dependence of both dissolution and growth rates on fluid saturation state (?) even at very close to equilibrium conditions (|?G| < 500 J/mol). This occurs because sufficient reactive sites (e.g. at kink, steps, and edges) are available at the exposed faces for dissolution and/or growth, allowing reactions to proceed via the direct and reversible detachment/attachment of reactants at the surface. In contrast, for magnesite and quartz, which have low surface areas, fewer active sites are available for growth and dissolution. Such minerals exhibit rates dependencies on ? at near equilibrium conditions ranging from linear to highly non-linear functions of ?, depending on the treatment of the crystals before the reaction. It follows that the form of the f(?G) function describing the growth and dissolution of minerals with low surface areas depends on the availability of reactive sites at the exposed faces and thus on the history of the mineral-fluid interaction and the hydrodynamic conditions under which the crystals are reacted. It is advocated that the crystal surface roughness could serve as a proxy of the density of reactive sites. The consequences of the different rate laws on the quantification of loess weathering along the Mississippi valley for the next one hundred years are examined.

Schott, Jacques; Oelkers, Eric H.; Bénézeth, Pascale; Goddéris, Yves; François, Louis

2012-11-01

239

Tocopheramines and tocotrienamines as antioxidants: ESR spectroscopy, rapid kinetics and DFT calculations.  

PubMed

Tocopheramines (TNH2) and tocotrienamines (T3NH2) are analogues of tocopherols (TOH) and tocotrienols in which phenolic OH is replaced by NH2. It was shown in previous studies that TNH2 and T3NH2 act as potent antioxidants. In this study we compared the one-electron oxidation of TNH2/T3NH2 by diphenyl picryl hydrazyl (DPPH) and galvinoxyl (GOX) radicals with the one of ?-TOH as a reference compound using ESR spectroscopy, stopped flow spectrophotometry and density functional theory (DFT) calculations. ESR spectroscopy revealed the presence of tocopheramine radicals during electrochemical oxidation of ?-TNH2. Kinetic measurements demonstrated that in apolar n-hexane TNH2/T3NH2 derivatives reacted two to three orders of magnitude slower than ?-TOH with the model radicals. DFT calculations indicated that this correlates well with the higher bond dissociation energy (BDE) for N-H in TNH2 than for O-H in ?-TOH in pure H-atom transfer (HAT). In the more polar medium ethanol TNH2/T3NH2 derivatives partially reacted faster than ?-TOH depending on the reaction partner. DFT calculations suggest that this is due to reaction mechanisms alternative to HAT. According to thermochemistry data sequential proton loss and electron transfer (SPLET) is more favored for ?-TOH in ethanol than for TNH2. Therefore, for TNH2 a contribution of the alternative mechanism of sequential electron transfer-proton transfer (SET-PT) could be a possible explanation. These data show that the antioxidant reactivity strongly depends on the structure, reaction partners and environment. According to these findings TNH2/T3NH2 should be superior as antioxidants over ?-TOH in polar head group regions of membranes but not in the apolar core of lipid bilayers. PMID:23876337

Bamonti, Luca; Hosoya, Takashi; Pirker, Katharina F; Böhmdorfer, Stefan; Mazzini, Francesco; Galli, Francesco; Netscher, Thomas; Rosenau, Thomas; Gille, Lars

2013-07-02

240

Perspective on Free-Energy Perturbation Calculations for Chemical Equilibria  

PubMed Central

An overview is provided on the computation of free energy changes in solution using perturbation theory, overlap sampling, and related approximate methods. As a specific application, extensive results are provided for free energies of hydration of substituted benzenes using the OPLS-AA force field in explicit TIP4P water. For a similar amount of computer time, the double-wide sampling and overlap sampling methods yield very similar results in the free-energy perturbation calculations. With standard protocols, the average statistical uncertainty in computed differences in free energies of hydration is 0.1 – 0.2 kcal/mol. Application of the power-series expansion in the Peierls equation was also tested. Use of the first-order term is generally reliable, while inclusion of the slowly-convergent, second-order fluctuation term causes deterioration in the results for strongly hydrogen-bonded solutes.

Jorgensen, William L.; Thomas, Laura L.

2009-01-01

241

Analysis of the computational singular perturbation reduction method for chemical kinetics.  

SciTech Connect

This article is concerned with the asymptotic accuracy of the Computational Singular Perturbation (CSP) method developed by Lam and Goussis [The CSP method for simplifying kinetics, Int. J. Chem. Kin. 26 (1994) 461-486] to reduce the dimensionality of a system of chemical kinetics equations. The method, which is generally applicable to multiple-time scale problems arising in a broad array of scientific disciplines, exploits the presence of disparate time scales to model the dynamics by an evolution equation on a lower-dimensional slow manifold. In this article it is shown that the successive applications of the CSP algorithm generate, order by order, the asymptotic expansion of a slow manifold. The results are illustrated on the Michaelis-Menten-Henri equations of enzyme kinetics.

Zagaris, A.; Kaper, H. G.; Kaper, T. J.; Mathematics and Computer Science; Boston Univ.

2004-01-01

242

Thermoluminescence kinetic parameters of microwave chemically vapour-deposited diamond films at different gamma dose rates  

NASA Astrophysics Data System (ADS)

Microwave chemically vapour-deposited (MWCVD) diamond films have been successfully used in dosimetric applications including radiotherapy and industrial dose assessments. The thermoluminescence (TL) properties and dose response, in a suitable MWCVD diamond film thermoluminescent dosimeter, should be independent of the irradiation dose rate used. Dose rate effects have been reported to exist, affecting the TL performance of polycrystalline CVD diamond. In the present work, dose rate effects on the TL kinetic parameters of two MWCVD diamond films of 6 and 12 ?m thickness, exposed to 60Co gamma dose rates of 2.4, 5.94 and 13.1 Gy/min, were investigated using a computerised glow curve deconvolution method. The TL glow curves featured a single TL peak at different dose rates, which were best fitted with a first-order kinetic curve having 2-3% figure of merit. No significant dose rate effects on the TL kinetic properties of the studied samples were observed.

Cruz-Zaragoza, E.; Favalli, A.; Gastélum, S.; Furetta, C.; Meléndrez, R.; Barboza-Flores, M.

243

1H NMR chemical shift calculations as a probe of supramolecular host-guest geometry.  

PubMed

The self-assembled supramolecular host [Ga(4)L(6)](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) can encapsulate cationic guest molecules within its hydrophobic cavity and catalyze the chemical transformations of bound guests. The cavity of host 1 is lined with aromatic naphthalene groups, which create a magnetically shielded interior environment, resulting in upfield shifted (1-3 ppm) NMR resonances for encapsulated guest molecules. Using gauge independent atomic orbital (GIAO) DFT computations, we show that (1)H NMR chemical shifts for guests encapsulated in 1 can be efficiently and accurately calculated and that valuable structural information is obtained by comparing calculated and experimental chemical shifts. The (1)H NMR chemical shift calculations are used to map the magnetic environment of the interior of 1, discriminate between different host-guest geometries, and explain the unexpected downfield chemical shift observed for a particular guest molecule interacting with host 1. PMID:21714506

Mugridge, Jeffrey S; Bergman, Robert G; Raymond, Kenneth N

2011-06-29

244

Chemical Kinetic Reaction Mechanisms for Combustion of Hydrocarbon and Other Types of Chemical Fuels  

DOE Data Explorer

Reaction mechanisms have been tested and validated extensively through comparisons between computed results and measured data from laboratory experiments (e.g., shock tubes, laminar flames, rapid compression machines, flow reactors, stirred reactors) and from practical systems (e.g., diesel engines, spark-ignition engines, homogeneous charge, compression ignition (HCCI) engines). These kinetic models are used to examine a wide range of combustion systems.[Taken from https://www-pls.llnl.gov/?url=science_and_technology-chemistry-combustion

245

Detailed Chemical Kinetic Reaction Mechanisms for Autoignition of Isomers of Heptane Under Rapid Compression  

SciTech Connect

Detailed chemical kinetic reaction mechanisms are developed for combustion of all nine isomers of heptane (C{sub 7}H{sub 16}), and these mechanisms are tested by simulating autoignition of each isomer under rapid compression machine conditions. The reaction mechanisms focus on the manner in which the molecular structure of each isomer determines the rates and product distributions of possible classes of reactions. The reaction pathways emphasize the importance of alkylperoxy radical isomerizations and addition reactions of molecular oxygen to alkyl and hydroperoxyalkyl radicals. A new reaction group has been added to past models, in which hydroperoxyalkyl radicals that originated with abstraction of an H atom from a tertiary site in the parent heptane molecule are assigned new reaction sequences involving additional internal H atom abstractions not previously allowed. This process accelerates autoignition in fuels with tertiary C-H bonds in the parent fuel. In addition, the rates of hydroperoxyalkylperoxy radical isomerization reactions have all been reduced so that they are now equal to rates of analogous alkylperoxy radical isomerizations, significantly improving agreement between computed and experimental ignition delay times in the rapid compression machine. Computed ignition delay times agree well with experimental results in the few cases where experiments have been carried out for specific heptane isomers, and predictive model calculations are reported for the remaining isomers. The computed results fall into three general groups; the first consists of the most reactive isomers, including n-heptane, 2-methyl hexane and 3-methyl hexane. The second group consists of the least reactive isomers, including 2,2-dimethyl pentane, 3,3-dimethyl pentane, 2,3-dimethyl pentane, 2,4-dimethyl pentane and 2,2,3-trimethyl butane. The remaining isomer, 3-ethyl pentane, was observed computationally to have an intermediate level of reactivity. These observations are generally consistent with knocking tendencies of these isomers, as measured by octane ratings, although the correlations are only approximate.

Westbrook, C K; Pitz, W J; Boercker, J E; Curran, H J; Griffiths, J F; Mohamed, C; Ribaucour, M

2001-12-17

246

History and Philosophy of Science through Models: The Case of Chemical Kinetics  

NASA Astrophysics Data System (ADS)

The case for a greater role for the history and philosophy of science in science education is reviewed. It is argued that such a role can only be realised if it is based on both a credible analytical approach to the history and philosophy of science and if the evolution of a sufficient number of major themes in science is known in suitable detail. Adopting Lakatos' Theory of Scientific Research Programmes as the analytical approach, it is proposed that the development, use, and replacement, of specific models forms the core of such programmes.Chemical kinetics was selected as an exemplar major topic in chemistry. Eight models which have played a central role in the evolution of the study of chemical kinetics were identified by an analysis of the literature. The implications that these models have for the teaching and learning of chemistry today are discussed.

Justi, Rosária; Gilbert, John K.

247

Calculations of kinetic isotope effects in the syn-eliminations of (2-phenylethyl)dimethylamine oxides  

SciTech Connect

Transition state theory (TST) calculations of kinetic isotope effects (KIE) for the syn-elimination of (2-phenylethyl)dimethylamine oxides have been carried out for a series of transition state (TS) models encompassing both E1-like and E1cB-like regions of the E2 mechanistic spectrum. A large number of different reaction coordinates were explored for both unsolvated and for coordination of solvent dimethylsulfoxide in the cyclic transition state models. The models of reaction for both solvated and unsolvated models of proton transfer are presented. A simplified method for easier initial screening of reaction coordinate contributions is developed, discussed, and found to produce accurate approximations to the full model KIE values. Both unsolvated and solvated models show E1-like E2 mechanism and the calculated values from both models are in extremely good agreement with experimentally measured KIE. Both models were used to investigate para-substituted derivatives (Z = CL, OCH/sub 3/) of the parent compound (Z = H). The transition states are related by a shift in structure parallel to the central E2 diagonal of an O'Ferrall-Jencks-Fry reaction diagram, as predicted by Thornton, indicating that in the absence of other factors, the extent to which negative charge is accumulated at C/sub ..beta../ in the transition state is a function primarily of the leaving group. All of the structural parameters such as bond distances and bond angles were related to independent bond orders. Beta-deuterium isotope effects produced by both solvated and nonsolvated models are temperature dependent.

Shafiei-Kermani, H.R.

1987-01-01

248

A detailed chemical kinetic model for gas phase combustion of TNT  

Microsoft Academic Search

A detailed chemical kinetic mechanism for gas phase combustion of 2,4,6-tri-nitrotoluene (TNT) has been developed to explore problems of explosive performance and of soot formation during the destruction of munitions. Thermodynamic properties of intermediate and radical species are estimated by group additivity. Reactions for the decomposition and oxidation of TNT and its intermediate products are assembled, based on information from

William J. Pitz; Charles K. Westbrook

2007-01-01

249

Natural gas autoignition under diesel conditions. Experiments and chemical kinetic modeling  

Microsoft Academic Search

The effects of ambient gas thermodynamic state and fuel composition on the autoignition of natural gas under direct-injection diesel conditions were studied experimentally in a constant-volume combustion vessel and computationally using a detailed chemical kinetic model. Natural gas compositions representative of variations observed across the U.S. were considered. These results extend previous observations to more realistic natural gas compositions and

J. D. Naber; D. L. Siebers; C. K. Westbrook; J. A. Caton; S. S. Di Julio

1994-01-01

250

Modeling Instabilities in Lean Premixed Turbulent Combustors Using Detailed Chemical Kinetics  

Microsoft Academic Search

The development of non-conventional combustion technology with ultra-low emissions and the safe operation of combustion systems require a thorough understanding of the mechanisms of combustion instabilities. The objective of the present work is to investigate the role of unmixedness and chemical kinetics in driving combustion instabilities. The reaction-rate responses of different species to inlet flow variations have been studied using

Bjørn Lilleberg; Ivar S. Ertesvåg; Kjell Erik Rian

2009-01-01

251

Feasibility of Reduced Chemical Kinetic Mechanisms of Methane in Internal Combustion Engine Simulations  

NASA Astrophysics Data System (ADS)

Three reduced chemical kinetic mechanisms of methane combustion were tested and compared with the standard detailed scheme GriMech 3.0., using the internal combustion engine (ICE) model of Chemkin 4.02 [1]. This study shows acceptable concordances in the prediction of temperature and main species profiles. But reduced schemes were incapables to predict all polluant emissions in an internal combustion engine.

Ennetta, Ridha; Said, Rachid

2008-09-01

252

Analysis of RDX-TAGzT pseudo-propellant combustion with detailed chemical kinetics  

Microsoft Academic Search

A detailed model of steady-state combustion of a pseudo-propellant containing cyclotrimethylene trinitramine (RDX) and triaminoguanidinium azotetrazolate (TAGzT) is presented. The physicochemical processes occurring within the foam layer, comprised of a liquid and gas bubbles, and a gas-phase region above the burning surface are considered. The chemical kinetics is represented by a global thermal decomposition mechanism within the liquid by considering

Neeraj Kumbhakarna; Stefan T. Thynell; Arindrajit Chowdhury; Ping Lin

2011-01-01

253

Quantum-chemical evaluation of energy quantities governing electron transfer kinetics: applications to intramolecular processes  

Microsoft Academic Search

Important energy quantities governing electron transfer (ET) kinetics in polar solutions (reorganization energy, Er, and net free energy change, ?U) are evaluated on the basis of quantum-chemical self-consistent reaction-field (SCRF) models. Either self-consistent field (SCF) or configuration interaction (CI) wavefunctions are used for the solute, which occupies a molecular cavity of realistic shape in a dielectric continuum. A classical SCRF

M. V. Basilevsky; G. E. Chudinov; I. V. Rostov; Yi-Ping Liu; M. D. Newton

1996-01-01

254

ESR Intensity Relations and Some Gas-Phase Chemical Kinetics of the OD Radical  

Microsoft Academic Search

The theoretical intensity relations for the quantitative determination of gas-phase OD radical and D atom concentrations by ESR spectroscopy are derived. OD was generated by the fast reaction of D atoms with NO2, and chemical kinetic rate constants for the reactions OD+OD?D2O+O,OD+CO?CO2+D, were measured at 300°K by following the OD decay in a fast flow system. The values obtained were

A. A. Westenberg; W. E. Wilson

1966-01-01

255

“State defining” experiment in chemical kinetics—primary characterization of catalyst activity in a TAP experiment  

Microsoft Academic Search

This paper presents a new strategy, “state-by-state transient screening”, for kinetic characterization of states of a multicomponent catalyst as applied to TAP pulse-response experiments. The key idea is to perform an insignificant chemical perturbation of the catalytic system so that the known essential characteristics of the catalyst (e.g. oxidation degree) do not change during the experiment. Two types of catalytic

Sergiy O. Shekhtman; Gregory S. Yablonsky; John T. Gleaves; Rebecca Fushimi

2003-01-01

256

Kinetics of chemical vapor deposition of SiC from methyltrichlorosilane and hydrogen  

Microsoft Academic Search

In this study, the dependence of the deposition rate on processing parameters, such as temperature, and partial pressure is studied by chemical vapor deposition from mixture of methyltrichlorosilane (CH3SiCl3, MTS) and hydrogen. The kinetics investigation is carried out in a tubular, hot-wall reactor coupled to a sensitive magnetic suspension microbalance. The results show that the active energy limited by surface

Cuiying Lu; Laifei Cheng; Chunnian Zhao; Litong Zhang; Yongdong Xu

2009-01-01

257

Thermodynamics and kinetics of apoazurin folding under macromolecular crowding effect and chemical interference  

NASA Astrophysics Data System (ADS)

Proteins fold in a cellular milieu crowded by different kinds of macromolecules. They exert volume exclusion impacting protein folding processes in vivo. Folding processes, however, has been studied by chemical denaturation under in vitro conditions. The impact of the two factors as an attempt to advance the understanding of folding mechanism in vivo is not understood. Here, we investigate the folding mechanisms of apoazurin affected by the macromolecular crowding and chemical interference by using coarse-grained molecular simulations. Crowding agents are modeled as hard-spheres and the chemical denaturation effects are implemented into an energy function of the side chain and backbone interactions. Protein folding stability, mechanism, and kinetics rates of apoazurin under chemical interference and macromolecular crowding conditions are being investigated.

Zegarra, Fabio; Cheung, Margaret

2013-03-01

258

Calculation of elastic nonlinearity parameter in crystals with different chemical bonds  

Microsoft Academic Search

Definition of the new elastic nonlinearity parameter ? has been carried out. The computer calculations of ?'s anisotropy for more then sixty crystals with all chemical bonding types and with any symmetry have made. The definite correlation between ? values and chemical bonding types in crystals investigated has been found. Some peculiarities of results obtained have been marked

B. P. Sorokin; P. P. Turchin; D. A. Glushkov

1995-01-01

259

Numerical Simulation of a Reacting Vortex Ring Using Detailed Chemical Kinetics  

NASA Astrophysics Data System (ADS)

A DNS study is conducted to examine the laminar flame-vortex interactions in a reacting vortex ring using `realistic' chemical kinetics. The set of equations solved is the compressible multi-species reacting flow equations comprising conservation of mass, linear momentum, energy, and species mass fractions. Transport properties for pure species were evaluated using thermo-molecular databases provided by the Chemkin library.(R.J. Kee et al.,) SANDIA Report, SAND86-8246, 1994. The mixture average formulation was used to evaluate the transport properties for the mixture. Methane combustion was simulated using GRI-Mech v1.2 kinetic model. The vortex ring was generated by a brief discharge of fuel through a round orifice which enters a quiescent ambient with the chemical composition of air. By adjusting the ratio of the ambient and fuel temperatures, the ignition delay time was controlled. The detailed kinetic mechanism will be examined to determine the ignition paths for this unsteady configuration. Time dependent correlations between fundamental parameters such as stoichiometry, heat release rate, hydrodynamic and chemical variables will be investigated to find the most appropriate flame observables for unsteady methane diffusion flames.

Safta, C.; Madnia, C. K.

1999-11-01

260

Biologically-Based Kinetic Models for Chemical Kinetics in the Isolated Perfased Rat Liver system: BBKM-IPRLR (Basic Reclrculation Model for Water Soluble I Lipid Insoluble Chemicals (LOG Pow <0)).  

National Technical Information Service (NTIS)

The objective of this report is to describe a generic biologically- based kinetic (BBK) model to simulate the kinetics of water soluble - lipid insoluble chemicals (log Po/w < 0) in the isolated perfused rat liver (IPRL) system. Such a model should have s...

J. M. Frazier

1997-01-01

261

Monte Carlo calculation of showers produced by electrons and. gamma. rays in layered chemically inhomogeneous media  

Microsoft Academic Search

The procedure for calculation of the interaction ranges of electrons and ..gamma.. rays proposed by Messel and Crawford (Electron-Photon Shower Distribution Functions, Pergamon Press, 1970) for homogeneous semi-infinite media is generalized to include the case of layered and chemically inhomogeneous materials. The correctness of the proposed improved procedure is verified by comparing the calculated efficiency of electron detection with the

N. A. Gunko; B. A. Dmitriev

1978-01-01

262

Compactness and robustness: Applications in the solution of integral equations for chemical kinetics and electromagnetic scattering  

NASA Astrophysics Data System (ADS)

This thesis employs the topological concept of compactness to deduce robust solutions to two integral equations arising from chemistry and physics: the inverse Laplace problem in chemical kinetics and the vector wave scattering problem in dielectric optics. The inverse Laplace problem occurs in the quantitative understanding of biological processes that exhibit complex kinetic behavior: different subpopulations of transition events from the "reactant" state to the "product" state follow distinct reaction rate constants, which results in a weighted superposition of exponential decay modes. Reconstruction of the rate constant distribution from kinetic data is often critical for mechanistic understandings of chemical reactions related to biological macromolecules. We devise a "phase function approach" to recover the probability distribution of rate constants from decay data in the time domain. The robustness (numerical stability) of this reconstruction algorithm builds upon the continuity of the transformations connecting the relevant function spaces that are compact metric spaces. The robust "phase function approach" not only is useful for the analysis of heterogeneous subpopulations of exponential decays within a single transition step, but also is generalizable to the kinetic analysis of complex chemical reactions that involve multiple intermediate steps. A quantitative characterization of the light scattering is central to many meteoro-logical, optical, and medical applications. We give a rigorous treatment to electromagnetic scattering on arbitrarily shaped dielectric media via the Born equation: an integral equation with a strongly singular convolution kernel that corresponds to a non-compact Green operator. By constructing a quadratic polynomial of the Green operator that cancels out the kernel singularity and satisfies the compactness criterion, we reveal the universality of a real resonance mode in dielectric optics. Meanwhile, exploiting the properties of compact operators, we outline the geometric and physical conditions that guarantee a robust solution to the light scattering problem, and devise an asymptotic solution to the Born equation of electromagnetic scattering for arbitrarily shaped dielectric in a non-perturbative manner.

Zhou, Yajun

263

Calculation of the characteristics of coplanar resonators for kinetic inductance detectors  

Microsoft Academic Search

Photon detectors based on the change of kinetic inductance of a thin superconducting film have a number of applications, particularly in astronomy, owing to their high sensitivity and ease of integration into large arrays. Here we discuss in detail the analysis of kinetic inductance detectors that use thin film microwave coplanar resonators. Photon absorption decreases the electron pair density, increasing

Adrian Porch; Phil Mauskopf; Simon Doyle; Chris Dunscombe

2005-01-01

264

A Computer Generated Reduced Iso-Octane Chemical Kinetic Mechanism Applied to Simulation of HCCI Combustion  

SciTech Connect

This paper shows how a computer can systematically remove non-essential chemical reactions from a large chemical kinetic mechanism. The computer removes the reactions based upon a single solution using a detailed mechanism. The resulting reduced chemical mechanism produces similar numerical predictions significantly faster than predictions that use the detailed mechanism. Specifically, a reduced chemical kinetics mechanism for iso-octane has been derived from a detailed mechanism by eliminating unimportant reaction steps and species. The reduced mechanism has been developed for the specific purpose of fast and accurate prediction of ignition timing in an HCCI engine. The reduced mechanism contains 199 species and 383 reactions, while the detailed mechanism contains 859 species and 3606 reactions. Both mechanisms have been used in numerical simulation of HCCI combustion. The simulations show that the reduced mechanism predicts pressure traces and heat release with good accuracy, similar to the accuracy obtained with the detailed mechanism. As may be expected, emissions of hydrocarbon and carbon monoxide are not as well predicted with the reduced mechanism as with the detailed mechanism, since the reduced mechanism was targeted for predicting HCCI ignition and not HC and CO emissions. Considering that the reduced mechanism requires about 25 times less computational time than the detailed mechanism (2 hours vs. 2 days), the ability to automatically generate a problem specific reduced mechanism is an important new tool for combustion research in general.

Aceves, S M; Martinez-Frias, J; Flowers, D; Smith, J R; Dibble, R; Chen, J Y

2002-08-12

265

Mycobacterium tuberculosis beta-ketoacyl-acyl carrier protein (ACP) reductase: kinetic and chemical mechanisms.  

PubMed

Beta-ketoacyl-acyl carrier protein (ACP) reductase from Mycobacterium tuberculosis (MabA) is responsible for the second step of the type-II fatty acid elongation system of bacteria, plants, and apicomplexan organisms, catalyzing the NADPH-dependent reduction of beta-ketoacyl-ACP to generate beta-hydroxyacyl-ACP and NADP(+). In the present work, the mabA-encoded MabA has been cloned, expressed, and purified to homogeneity. Initial velocity studies, product inhibition, and primary deuterium kinetic isotope effects suggested a steady-state random bi-bi kinetic mechanism for the MabA-catalyzed reaction. The magnitudes of the primary deuterium kinetic isotope effect indicated that the C(4)-proS hydrogen is transferred from the pyridine nucleotide and that this transfer contributes modestly to the rate-limiting step of the reaction. The pH-rate profiles demonstrated groups with pK values of 6.9 and 8.0, important for binding of NADPH, and with pK values of 8.8 and 9.6, important for binding of AcAcCoA and for catalysis, respectively. Temperature studies were employed to determine the activation energy of the reaction. Solvent kinetic isotope effects and proton inventory analysis established that a single proton is transferred in a partially rate-limiting step and that the mechanism of carbonyl reduction is probably concerted. The observation of an inverse (D)2(O)V/K and an increase in (D)2(O)V when [4S-(2)H]NADPH was the varied substrate obscured the distinction between stepwise and concerted mechanisms; however, the latter was further supported by the pH dependence of the primary deuterium kinetic isotope effect. Kinetic and chemical mechanisms for the MabA-catalyzed reaction are proposed on the basis of the experimental data. PMID:17059223

Silva, Rafael G; de Carvalho, Luiz Pedro S; Blanchard, John S; Santos, Diógenes S; Basso, Luiz A

2006-10-31

266

The effects of different experimental designs on parameter es- timation in the kinetics of a reversible chemical reaction  

Microsoft Academic Search

Using the e xperimental design methods, instead o f deterministic ones commonly used by chemists, to estimate the kinetic constants is a very important subject in chemical re- searches, since such-obtained kinetic parameters have much better statistical properties. D-optimum design is a popular statistical technique, which emphasizes to obtain the es- timated parameters with the smallest content of the confidence

Qing-Song Xu; Yi-Zeng Liang; Kai-Tai Fang

2000-01-01

267

Surftherm: A program to analyze thermochemical and kinetic data in gas-phase and surface chemical reaction mechanisms.  

National Technical Information Service (NTIS)

This report documents the Surftherm program that analyzes transport coefficient, thermochemical- and kinetic rate information in complex gas-phase and surface chemical reaction mechanisms. The program is designed for use with the Chemkin (gas-phase chemis...

M. E. Coltrin H. K. Moffat

1994-01-01

268

Communication: Kinetics of chemical ordering in Ag-Au and Ag-Ni nanoalloys  

NASA Astrophysics Data System (ADS)

The energy landscape and kinetics of medium-sized Ag-Au and Ag-Ni nanoalloy particles are explored via a discrete path sampling approach, focusing on rearrangements connecting regions differing in chemical order. The highly miscible Ag27Au28 supports a large number of nearly degenerate icosahedral homotops. The transformation from reverse core-shell to core-shell involves large displacements away from the icosahedron through elementary steps corresponding to surface diffusion and vacancy formation. The immiscible Ag42Ni13 naturally forms an asymmetric core-shell structure, and about 10 eV is required to extrude the nickel core to the surface. The corresponding transformation occurs via a long and smooth sequence of surface displacements. For both systems the rearrangement kinetics exhibit Arrhenius behavior. These results are discussed in the light of experimental observations.

Calvo, F.; Fortunelli, A.; Negreiros, F.; Wales, D. J.

2013-09-01

269

Communication: Kinetics of chemical ordering in Ag-Au and Ag-Ni nanoalloys.  

PubMed

The energy landscape and kinetics of medium-sized Ag-Au and Ag-Ni nanoalloy particles are explored via a discrete path sampling approach, focusing on rearrangements connecting regions differing in chemical order. The highly miscible Ag27Au28 supports a large number of nearly degenerate icosahedral homotops. The transformation from reverse core-shell to core-shell involves large displacements away from the icosahedron through elementary steps corresponding to surface diffusion and vacancy formation. The immiscible Ag42Ni13 naturally forms an asymmetric core-shell structure, and about 10 eV is required to extrude the nickel core to the surface. The corresponding transformation occurs via a long and smooth sequence of surface displacements. For both systems the rearrangement kinetics exhibit Arrhenius behavior. These results are discussed in the light of experimental observations. PMID:24070271

Calvo, F; Fortunelli, A; Negreiros, F; Wales, D J

2013-09-21

270

Effects of pressure on the thermal decomposition kinetics, chemical reactivity and phase behavior of RDX  

SciTech Connect

The effects of pressure on the thermal decomposition kinetics, chemical reactivity, and phase behavior of RDX have been studied by a combination of measurement techniques in conjunction with a high-pressure diamond anvil cell. These techniques include Fourier transform infrared (FTIR) spectroscopy for kinetic measurements and phase identification, energy dispersive x-ray powder diffraction for identification of the observed polymorphic forms and also compression measurements, and optical polarizing microscopy for visual detection and confirmation of phase transformations and determinations of transition pressures. The ruby method of pressure measurement was used in all methods employed. Studies were generally limited to the region where decomposition rates could be measured within reasonable laboratory time, i.e., below 10 GPa and 573 K. The P-T phase diagram for RDX was determined to 573 K and 7.0/GPa, delineating the stability fields of three solid phases, and the liquidus.

Miller, P.J. (Naval Surface Warfare Center, Silver Spring, MD (USA)); Block, S.; Piermarini, G.J. (Inst. for Materials Science and Engineering, National Inst. of Standards and Technology, Gaithersburg, MD (US))

1991-01-01

271

System Size Dependence of Chemical and Kinetic Freeze-Out in Heavy Ion Collisions  

NASA Astrophysics Data System (ADS)

Heavy Ion collisions have been performed at RHIC with Cu and Au nuclei to date. The two systems differ in initial energy density and provide further understanding of the quark matter phase diagram. A statistical thermal model study of charged hadron ratios provides chemical freeze-out properties of the two systems. Boltzmann blast-wave studies of the two systems at top RHIC energies show that the kinetic freeze-out properties scale with total charged multiplicity at midrapidity. We present results from two low energy runs at RHIC in Au+Au collisions at 19.6 GeV and Cu+Cu collisions at 22.4 GeV. Particle spectra for &±circ;, K^±, p and p are studied as a function of (mT-m0) up to ˜1 GeV measured using ionization energy loss in the STAR Time Projection Chamber. We report the chemical freeze-out temperature Tch vs. the baryon chemical potential ?B, and the kinetic freeze-out temperature Tkin vs. the blast velocity , and other results. Our results are compared with RHIC data at various energies and world data.

Mall, Orpheus

2011-04-01

272

Quantum chemical 13Calpha chemical shift calculations for protein NMR structure determination, refinement, and validation  

Microsoft Academic Search

A recently determined set of 20 NMR-derived conformations of a 48-residue all-alpha-helical protein, (PDB ID code 2JVD), is validated here by comparing the observed 13Calpha chemical shifts with those computed at the density functional level of theory. In addition, a recently introduced physics-based method, aimed at determining protein structures by using NOE-derived distance constraints together with observed and computed 13Calpha

Jorge A. Vila; James M. Aramini; Paolo Rossi; Alexandre Kuzin; Min Su; Jayaraman Seetharaman; Rong Xiao; Liang Tong; Gaetano T. Montelione; Harold A. Scheraga

2008-01-01

273

A Detailed Chemical Kinetic Analysis of Low Temperature Non-Sooting Diesel Combustion  

SciTech Connect

We have developed a model of the diesel fuel injection process for application to analysis of low temperature non-sooting combustion. The model uses a simplified mixing correlation and detailed chemical kinetics, and analyzes a parcel of fuel as it moves along the fuel jet, from injection into evaporation and ignition. The model predicts chemical composition and soot precursors, and is applied at conditions that result in low temperature non-sooting combustion. Production of soot precursors is the first step toward production of soot, and modeling precursor production is expected to give insight into the overall evolution of soot inside the engine. The results of the analysis show that the model has been successful in describing many of the observed characteristics of low temperature combustion. The model predicts results that are qualitatively similar to those obtained for soot formation experiments at conditions in which the EGR rate is increased from zero to very high values as the fueling rate is kept constant. The model also describes the two paths to achieve non-sooting combustion. The first is smokeless rich combustion and the second is modulated kinetics (MK). The importance of the temperature after ignition and the equivalence ratio at the time of ignition is demonstrated, as these parameters can be used to collapse onto a single line all the results for soot precursors for multiple fueling rates. A parametric analysis indicates that precursor formation increases considerably as the gas temperature in the combustion chamber and the characteristic mixing time are increased. The model provides a chemical kinetic description of low temperature diesel combustion that improves the understanding of this clean and efficient regime of operation.

Aceves, S M; Flowers, D L

2004-10-01

274

Application of the multi-standard methodology for calculating 1H NMR chemical shifts.  

PubMed

Gauge including atomic orbitals (GIAO) (1)H NMR chemical shift calculations have been performed for 66 organic compounds at 72 different levels of theory using the multi-standard approach (MSTD) previously developed for (13)C NMR. This straightforward computational technique involves the combination of methanol and benzene as standards. The studied methodology has been shown to predict (1)H NMR chemical shifts efficiently at different levels of theory. PMID:22713105

Sarotti, Ariel M; Pellegrinet, Silvina C

2012-07-03

275

Calculation of carbon-14, chlorine-37, and deuterium kinetic isotope effects in the solvolysis of tert-butyl chloride  

Microsoft Academic Search

In the solvolysis of tert-butyl chloride, satisfactory ..cap alpha..-carbon-14, ..beta..-deuterium, and chlorine kinetic isotope effects (KIE) may be calculated for a productlike transition state characterized by bond orders n\\/sub C Cl\\/ = 0.2, n\\/sub C C\\/ = 1.18, and n\\/sub C H\\/ = 0.94, employing a diagonal valence force field, provided that allowance is made for hydrogen-bonded solvation of the

Graham W. Burton; Leslie B. Sims; Joe C. Wilson; Arthur Fry

1977-01-01

276

Robust automated mass spectra interpretation and chemical formula calculation using mixed integer linear programming.  

PubMed

Untargeted metabolite profiling using liquid chromatography and mass spectrometry coupled via electrospray ionization is a powerful tool for the discovery of novel natural products, metabolic capabilities, and biomarkers. However, the elucidation of the identities of uncharacterized metabolites from spectral features remains challenging. A critical step in the metabolite identification workflow is the assignment of redundant spectral features (adducts, fragments, multimers) and calculation of the underlying chemical formula. Inspection of the data by experts using computational tools solving partial problems (e.g., chemical formula calculation for individual ions) can be performed to disambiguate alternative solutions and provide reliable results. However, manual curation is tedious and not readily scalable or standardized. Here we describe an automated procedure for the robust automated mass spectra interpretation and chemical formula calculation using mixed integer linear programming optimization (RAMSI). Chemical rules among related ions are expressed as linear constraints and both the spectra interpretation and chemical formula calculation are performed in a single optimization step. This approach is unbiased in that it does not require predefined sets of neutral losses and positive and negative polarity spectra can be combined in a single optimization. The procedure was evaluated with 30 experimental mass spectra and was found to effectively identify the protonated or deprotonated molecule ([M + H](+) or [M - H](-)) while being robust to the presence of background ions. RAMSI provides a much-needed standardized tool for interpreting ions for subsequent identification in untargeted metabolomics workflows. PMID:24032353

Baran, Richard; Northen, Trent R

2013-09-27

277

Extracting chemical information from plane wave calculations by a 3D 'fuzzy atoms' analysis  

NASA Astrophysics Data System (ADS)

Bond order and valence indices have been calculated by the method of the three-dimensional 'fuzzy atoms' analysis, using the numerical molecular orbitals obtained from plane wave DFT calculations, i.e., without introducing any external atom-centered functions. Weight functions of both Hirshfeld and Becke types have been applied. The results are rather close to the similar 'fuzzy atoms' ones obtained by using atom-centered basis sets and agree well with the chemical expectations, stressing the power of the genuine chemical concepts.

Bakó, I.; Stirling, A.; Seitsonen, A. P.; Mayer, I.

2013-03-01

278

Comparison of two gas-phase chemical kinetic mechanisms of ozone formation over Europe  

Microsoft Academic Search

Two recent gas-phase chemical kinetic mechanisms for tropospheric ozone formation, one based on the lumped-structure approach\\u000a (CB05) and the other based on the lumped-molecule approach (RACM2), are compared for simulations of ozone over Europe. The\\u000a host air quality model is POLAIR3D of the Polyphemus modeling platform. A one-month period (15 July to 15 August 2001) is\\u000a simulated. Model performance is

Youngseob Kim; Karine Sartelet; Christian Seigneur

2009-01-01

279

Application of a Genetic Algorithm to the Optimization of Rate Constants in Chemical Kinetic Models for Combustion Simulation of HCCI Engines  

Microsoft Academic Search

For numerically predicting the combustion processes in homogeneous charge compression ignition (HCCI) engines, practical chemical kinetic models have been explored. A genetic algorithm (GA) has been applied to the optimization of the rate constants in detailed chemical kinetic models, and a detailed kinetic model (592 reactions) for gasoline reference fuels with arbitrary octane number between 60 and 100 has been

Sang-Kyu Kim; Kazuma Ito; Daisuke Yoshihara; Tomoyuki Wakisaka

2005-01-01

280

Density functional calculations of 15 N chemical shifts in solvated dipeptides  

Microsoft Academic Search

We performed density functional calculations to examine the effects of solvation, hydrogen bonding, backbone conformation,\\u000a and the side chain on 15N chemical shielding in proteins. We used N-methylacetamide (NMA) and N-formyl-alanyl-X (with X being one of the 19 naturally occurring amino acids excluding proline) as model systems. In addition,\\u000a calculations were performed for selected fragments from protein GB3. The conducting

Ling Cai; David Fushman; Daniel S. Kosov

2008-01-01

281

Kinetics  

NSDL National Science Digital Library

Conceptual questions about kinetics. For example, "[w]hat are the reaction velocity, the rates of formation of N2 and H2, and the rate of decomposition of ammonia for the decomposition of ammonia on a tungsten surface under the conditions reflected in the figure."

Nurrenbern, Susan C.; Robinson, William R.

2008-02-27

282

Finite volume method for the calculation of compressible chemically reacting flows  

SciTech Connect

Several efficient pseudo time techniques have been developed for calculating steady state chemically reacting flows. The techniques include the implicit treatment of the chemical source term, point implicit multiple grid accelerator and a constant CFL condition. It turns out that these methods can be viewed as ways of rescaling the equations in time such that all chemical and convective phenomena evolve at comparable pseudo time scales. Consequently the number of iterations needed to solve reacting problems is approximately the same as for non-reacting problems. The techniques are demonstrated for a simple dissociation model and a nontrivial H2 - Air combustion model.

Bussing, T.R.A.; Murman, E.M.

1983-01-01

283

Dissecting the mechanisms of a class of chemical glycosylation using primary ¹³C kinetic isotope effects.  

PubMed

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary (13)C kinetic isotope effects have now been determined for the formation of ?- and ?-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the ?-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity. PMID:22824899

Huang, Min; Garrett, Graham E; Birlirakis, Nicolas; Bohé, Luis; Pratt, Derek A; Crich, David

2012-07-22

284

SENKIN: A Fortran program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis  

SciTech Connect

SENKIN is a Fortran computer program that computes the time evolution of a homogeneous reacting gas mixture in a closed system. The model accounts for finite-rate elementary chemical reactions, and performs kinetic sensitivity analysis with respect to the reaction rates. The program considers five problem types: an adiabatic system with constant pressure; an adiabatic system with constant volume; an adiabatic system with the volume a specified function of time; a system where the pressure and temperature are constant; and a system where the pressure is constant and the temperature is a specified function of time. The program uses the DASAC software to solve both the nonlinear ordinary differential equations that describe the temperature and species mass fractions and the set of linear differential equations that describe the first-order sensitivity coefficients of temperature and species composition with respect to the individual reaction rates. The program runs in conjunction with the CHEMKIN package, which handles the chemical reaction mechanism.

Lutz, A.E.; Kee, R.J.; Miller, J.A.

1988-02-01

285

Kinetics of gas-phase reactions relevant to the chemical vapor deposition of indium compounds  

SciTech Connect

Compounds containing indium are of interest for electronic and optical applications. These compounds include III-V semiconductors such as InP and InAs used in both electronic devices and solar cells, and indium tin oxide, which can be used for optical memory and antireflection coatings. Chemical vapor deposition (CVD) techniques can be used to deposit these materials on a variety of substrates. At the temperatures typically employed (550--900 K), gas-phase chemical reactions involving the indium-containing precursor can occur. The kinetics of trimethylindium pyrolysis are investigated in a flow reactor equipped with a molecular-beam mass-spectrometric sampling system. Data are analyzed using a new computational approach that accounts for heat and mass transport in the reactor. The measured activation energy, 46.2 kcal/mol, is in good agreement with previously reported values.

Allendorf, M.D.; McDaniel, A.H.

1998-03-01

286

Dissecting the mechanisms of a class of chemical glycosylation using primary 13C kinetic isotope effects  

NASA Astrophysics Data System (ADS)

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary 13C kinetic isotope effects have now been determined for the formation of ?- and ?-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the ?-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity.

Huang, Min; Garrett, Graham E.; Birlirakis, Nicolas; Bohé, Luis; Pratt, Derek A.; Crich, David

2012-08-01

287

Kinetic and chemical characterization of thermal decomposition of dicumylperoxide in cumene.  

PubMed

Dicumylperoxide (DCP) is one of the most used peroxides in the polymer industry. It has been reported that its thermal decomposition can result in runaway phenomena and thermal explosions with significant economic losses and injuries to people. In the present paper thermal behaviour of dicumylperoxide in cumene was investigated over the temperature range of 393-433 K under aerated and de-aerated conditions. The results indicated that when oxygen was present, the decomposition rate did not follow a simple pseudo-first order kinetic as previously reported in literature. A satisfactory fit of the experimental data was, in this case, achieved by means of kinetic expression derived under the assumption of an autocatalytic scheme of reaction. The reaction rate was, on the contrary, correctly described by a pseudo-first order kinetic in absence of oxygen. Under both aerated and de-aerated conditions, chemical analysis showed that the decomposition mainly resulted in the formation of acetophenone and dimethylphenylcarbinol with minor occurrence of 2,3-dimethyl-2,3-diphenylbutane. The formation of methane and ethane was also invariably observed while the appearance of cumylhydroperoxide as a reaction intermediate was detected under only aerated conditions. Therefore, two reaction schemes were proposed to explain system behaviour in the presence of oxygen and after its purging. PMID:21277085

Di Somma, Ilaria; Marotta, Raffaele; Andreozzi, Roberto; Caprio, Vincenzo

2011-01-13

288

Unusual mechanism for H{sub 3}{sup +} formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations  

SciTech Connect

The formation of H{sub 3}{sup +} from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H{sub 3}{sup +} formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H{sub 3}{sup +} ions with kinetic energies on the order of 4 to 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H{sub 2} molecule attached to a C{sub 2}H{sub 4}{sup 2+} entity but IRC calculations show that it belongs to the reaction channel yielding C{sub 2}H{sub 3}{sup +}+ H{sub 3}{sup +}. Experimentally measured kinetic energies of the correlated H{sub 3}{sup +} and C{sub 2}H{sub 3}{sup +} ions confirm the reaction path suggested by theory.

Kraus, Peter M.; Schwarzer, Martin C.; Schirmel, Nora; Urbasch, Gunter; Frenking, Gernot; Weitzel, Karl-Michael [Fachbereich Chemie, Physikalische Chemie, Philipps-Universitaet Marburg, Hans-Meerwein-Strasse, D-35032 Marburg (Germany)

2011-03-21

289

Isobutane ignition delay time measurements at high pressure and detailed chemical kinetic simulations  

SciTech Connect

Rapid compression machine and shock-tube ignition experiments were performed for real fuel/air isobutane mixtures at equivalence ratios of 0.3, 0.5, 1, and 2. The wide range of experimental conditions included temperatures from 590 to 1567 K at pressures of approximately 1, 10, 20, and 30 atm. These data represent the most comprehensive set of experiments currently available for isobutane oxidation and further accentuate the complementary attributes of the two techniques toward high-pressure oxidation experiments over a wide range of temperatures. The experimental results were used to validate a detailed chemical kinetic model composed of 1328 reactions involving 230 species. This mechanism has been successfully used to simulate previously published ignition delay times as well. A thorough sensitivity analysis was performed to gain further insight to the chemical processes occurring at various conditions. Additionally, useful ignition delay time correlations were developed for temperatures greater than 1025 K. Comparisons are also made with available isobutane data from the literature, as well as with 100% n-butane and 50-50% n-butane-isobutane mixtures in air that were presented by the authors in recent studies. In general, the kinetic model shows excellent agreement with the data over the wide range of conditions of the present study. (author)

Healy, D.; Curran, H.J. [Combustion Chemistry Centre, School of Chemistry, NUI Galway (Ireland); Donato, N.S.; Aul, C.J.; Petersen, E.L. [Department of Mechanical Engineering, Texas A and M University, College Station, TX (United States); Zinner, C.M. [Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL (United States); Bourque, G. [Rolls-Royce Canada Limited, 9500 Cote de Liesse, Lachine, Quebec (Canada)

2010-08-15

290

LASERS, ACTIVE MEDIA: Calculation of the mixing chamber of an ejector chemical oxygen --- iodine laser  

Microsoft Academic Search

Gas parameters are calculated at the outlet of the mixing chamber of an ejector chemical oxygen---iodine laser with a nozzle unit consisting of nozzles of three types, which provides a total pressure of the active medium that substantially exceeds a pressure in the generator of singlet oxygen. This technique of forming the laser active medium substantially facilitates the ejection of

M. V. Zagidullin; V. D. Nikolaev

2001-01-01

291

Calculation of the mixing chamber of an ejector chemical oxygen - iodine laser  

Microsoft Academic Search

Gas parameters are calculated at the outlet of the mixing chamber of an ejector chemical oxygen-iodine laser with a nozzle unit consisting of nozzles of three types, which provides a total pressure of the active medium that substantially exceeds a pressure in the generator of singlet oxygen. This technique of forming the laser active medium substantially facilitates the ejection of

M V Zagidullin; V D Nikolaev

2001-01-01

292

Calculation of equation of state of QCD at zero temperature and finite chemical potential  

NASA Astrophysics Data System (ADS)

In this paper we calculate the equation of state (EOS) of QCD at zero temperature and finite chemical potential by using several models of quark propagators including the Dyson-Schwinger equations (DSEs) model, the hard-dense-loop (HDL) approximation and the quasi-particle model. The results are analyzed and compared with the known results in the literature.

Jiang, Yu; Li, Ning; Sun, Wei-Min; Zong, Hong-Shi

2010-09-01

293

Systematic Approach to Calculate the Concentration of Chemical Species in Multi-Equilibrium Problems  

ERIC Educational Resources Information Center

|A general systematic approach is proposed for the numerical calculation of multi-equilibrium problems. The approach involves several steps: (i) the establishment of balances involving the chemical species in solution (e.g., mass balances, charge balance, and stoichiometric balance for the reaction products), (ii) the selection of the unknowns…

Baeza-Baeza, Juan Jose; Garcia-Alvarez-Coque, Maria Celia

2011-01-01

294

STUDY OF CHEMICAL EQUILIBRIA IN METHANOL SYNTHESIS II. Calculation of Equilibrium Constant Kp  

Microsoft Academic Search

Based on the experimental data of Kp measured In this paper, the thermodynamic calculation of Kp from Kr and equation of state was studied and it was found that the chemical equilibrium of methanol synthesis can be described well by using Cheredeichenko (1968)'s Kr, 00 and Blssett (1977)' s Kr. rs relationship and a modified SRK equation.Meanwhile, the equilibrium constant

Liu Hui; Luo Zanchun; Zhu Bingchen

1994-01-01

295

Kinetic and Chemical Mechanism of the Dihydrofolate Reductase from Mycobacterium tuberculosis  

PubMed Central

Dihydrofolate reductase from Mycobacterium tuberculosis catalyzes the NAD(P)H dependent reduction of dihydrofolate, yielding NAD(P)+ and tetrahydrofolate, the primary one carbon unit carrier in biology. Tetrahydrofolate needs to be recycled so that reactions involved in dTMP synthesis and purine metabolism are maintained. In this work, we report the kinetic characterization of the MtDHFR. This enzyme has a sequential steady-state random kinetic mechanism, probably with a preferred pathway with NADPH binding first. A pKa value for an enzymic acid of approximately 7.0 was identified from the pH dependence of V, and the analysis of the primary kinetic isotope effects revealed that the hydride transfer step is at least partly rate limiting throughout the pH range analyzed. Additionally, the determination and analysis of solvent, and multiple kinetic isotope effects was conducted, and equilibrium isotope effects were measured on the equilibrium constant. D2OV and D2OV/K[4R-4-2H]-NADH were slightly inverse at pH 6.0, and inverse values for D2OV[4R-4-2H]-NADH and D2OV/K[4R-4-2H]-NADH suggested that a pre-equilibrium protonation is occurring before the hydride transfer step, indicating a stepwise mechanism for proton and hydride transfer. The same value was obtained for DkH at pH values of 5.5 and 7.5, reaffirming the rate-limiting nature of the hydride transfer step. A chemical mechanism is proposed based on the results obtained here.

Czekster, Clarissa M.; Vandemeulebroucke, An; Blanchard, John S.

2010-01-01

296

Decay-ratio calculation in the frequency domain with the LAPUR code using 1D-kinetics  

SciTech Connect

This paper deals with the problem of computing the Decay Ratio in the frequency domain codes as the LAPUR code. First, it is explained how to calculate the feedback reactivity in the frequency domain using slab-geometry i.e. 1D kinetics, also we show how to perform the coupling of the 1D kinetics with the thermal-hydraulic part of the LAPUR code in order to obtain the reactivity feedback coefficients for the different channels. In addition, we show how to obtain the reactivity variation in the complex domain by solving the eigenvalue equation in the frequency domain and we compare this result with the reactivity variation obtained in first order perturbation theory using the 1D neutron fluxes of the base case. Because LAPUR works in the linear regime, it is assumed that in general the perturbations are small. There is also a section devoted to the reactivity weighting factors used to couple the reactivity contribution from the different channels to the reactivity of the entire reactor core in point kinetics and 1D kinetics. Finally we analyze the effects of the different approaches on the DR value. (authors)

Munoz-Cobo, J. L.; Escriva, A.; Garcia, C.; Berna, C. [Instituto de Ingenieria Energetica, Universitat Politecnica de Valencia, Camino de Vera s/n, Valencia 46022 (Spain); Melara, J. [IBERDROLA Ingenieria Y Construccion, C/ Jose Bardasano Baos 9, 28016 Madrid (Spain)

2012-07-01

297

ChemSage—A computer program for the calculation of complex chemical equilibria  

NASA Astrophysics Data System (ADS)

An extensive computer program called ChemSage, based upon the SOLGASMIX Gibbs energy minimizer, is presented together with several examples which illustrate its use. ChemSage was designed to perform three types of thermochemical calculations in complex systems involving phases exhibiting nonideal mixing properties. These are the calculation of thermodynamic functions, heterogeneous phase equilibria, and steady-state conditions for the simulation of simple multistage reactors. The thermodynamic functions module calculates specific heat, enthalpy, entropy, and Gibbs energy with respect to a chosen reference state for a given phase and, if this phase is a mixture, the partial properties of its components. Chemical equilibrium calculations can be made for a system which has been uniquely defined with respect to temperature, pressure (or volume), and composition. One of these quantities may also be replaced by an extensive property or phase target, e.g., for the calculation of adiabatic and liquidus temperatures, respectively.

Eriksson, Gunnar; Hack, Klaus

1990-12-01

298

New chemical kinetics for description of chemical noise in small, heterogeneous biological systems: Beyond the paradigm of the rate constant concept  

NASA Astrophysics Data System (ADS)

We introduce a novel chemical kinetics for quantitative description of chemical fluctuations in a small, heterogeneous biological reaction system. At first, we discuss the recently proposed renewal chemical kinetics, and its application to quantitative interpretation of the randomness in fluctuating enzymatic turnover times of a-galactosidase. From the analysis of the randomness parameter data of the single enzyme reaction, one can extract valuable quantitative information about the enzyme reaction system, beyond the reach of the conventional Michaelis-Menten analysis. Next, we discuss a new universal behavior in the time dependence of the chemical fluctuation of product density for a small, heterogeneous reaction system, which is predicted from an exact analytic study for a general reaction model and confirmed by stochastic simulation results. We also discuss the dependence of the chemical noise on substrate concentrations for a heterogeneous enzyme reaction system, which turns out qualitatively different from that for a homogeneous enzyme reaction system.

Sung, Jaeyoung

2012-02-01

299

Spectroscopic investigations, quantum chemical calculations, HOMO-LUMO and NBO/NLMO analysis of 4-pyridinecarbohydrazide  

NASA Astrophysics Data System (ADS)

The FT-IR and FT-Raman spectra of 4-pyridinecarbohydrazide have been recorded. The complete vibrational assignment and analysis of the fundamental modes was carried out using the experimental data and quantum chemical studies. The observed vibrational data were compared with the wavenumbers derived theoretically for the optimized geometry of the compound from the DFT-B3LYP gradient calculations employing 6-311++G(d,p) basis set. The 1H and 13C NMR chemical shifts have been simulated. Thermodynamic properties have been calculated at different temperatures. HOMO-LUMO energy gap has been calculated. The intramolecular contacts have been interpreted using Natural Bond Orbital (NBO) and Natural Localized Molecular Orbital (NLMO) analysis.

Gobinath, E.; John Xavier, R.

2013-11-01

300

A Sequential Fluid-mechanic Chemical-kinetic Model of Propane HCCI Combustion  

SciTech Connect

We have developed a methodology for predicting combustion and emissions in a Homogeneous Charge Compression Ignition (HCCI) Engine. This methodology combines a detailed fluid mechanics code with a detailed chemical kinetics code. Instead of directly linking the two codes, which would require an extremely long computational time, the methodology consists of first running the fluid mechanics code to obtain temperature profiles as a function of time. These temperature profiles are then used as input to a multi-zone chemical kinetics code. The advantage of this procedure is that a small number of zones (10) is enough to obtain accurate results. This procedure achieves the benefits of linking the fluid mechanics and the chemical kinetics codes with a great reduction in the computational effort, to a level that can be handled with current computers. The success of this procedure is in large part a consequence of the fact that for much of the compression stroke the chemistry is inactive and thus has little influence on fluid mechanics and heat transfer. Then, when chemistry is active, combustion is rather sudden, leaving little time for interaction between chemistry and fluid mixing and heat transfer. This sequential methodology has been capable of explaining the main characteristics of HCCI combustion that have been observed in experiments. In this paper, we use our model to explore an HCCI engine running on propane. The paper compares experimental and numerical pressure traces, heat release rates, and hydrocarbon and carbon monoxide emissions. The results show an excellent agreement, even in parameters that are difficult to predict, such as chemical heat release rates. Carbon monoxide emissions are reasonably well predicted, even though it is intrinsically difficult to make good predictions of CO emissions in HCCI engines. The paper includes a sensitivity study on the effect of the heat transfer correlation on the results of the analysis. Importantly, the paper also shows a numerical study on how parameters such as swirl rate, crevices and ceramic walls could help in reducing HC and CO emissions from HCCI engines.

Aceves, S M; Flowers, D L; Martinez-Frias, J; Smith, J R; Westbrook, C; Pitz, W; Dibble, R; Wright, J F; Akinyemi, W C; Hessel, R P

2000-11-29

301

Gas phase chemical kinetics at high temperature of carbonaceous molecules: application to circumstellar envelopes  

NASA Astrophysics Data System (ADS)

Circumstellar shells of evolved stars are a theater of extremely rich physical and chemical processes. More than seventy molecules of varied nature have been identified in the envelopes through their spectral fingerprints in the microwave or far infrared regions. Many of them are carbon chain molecules and radicals and a significant number are unique to the circumstellar medium. However, observational data remain scarce and more than half of the detected species have been observed in only one object, the nearby carbon star IRC + 10216. Chemical kinetic models are needed to describe the formation of molecules in evolved circumstellar outflows. Upcoming terrestrial telescopes such as ALMA will increase the spatial resolution by several orders of magnitude and provide a wealth of data. The determination of relevant laboratory kinetics data is critical to keep up with the development of the observations and of the refinement of chemical models. Today, the majority of reactions studied in the laboratory are the ones involved in combustion and concerning light hydrocarbons. Our objective is to provide the scientific community with rate coefficients of reactions between abundant species in these warm environments. Cyanopolyynes from HC_2N to HC_9N have all been detected in carbon rich circumstellar envelopes in up to 10 sources for HC_3N. Neutral-neutral reactions of the CN radical with unsaturated hydrocarbons could be a dominant route in the formation of cyanopolyynes, even at low temperatures. Our approach aims to bridge the temperature gap between resistively heated flow tubes and shock tubes. The present kinetic measurements are obtained using a new reactor combining a high enthalpy source (Moudens et al. 2011) with a flow tube and a pulsed laser photolysis and laser induced fluorescence system to probe the undergoing chemical reactions. The high enthalpy flow tube has been used to measure the rate constant of the reaction of the CN radical with propane, propene, propadiene, 1,3-butadiene, and butyne over a temperature range extending from 300 to 1200 K. The majority of the reactions studied are rapid, with rate constants greater than 10-10 cm^3 molecule-1 s-1.

Biennier, L.; Gardez, A.; Saidani, G.; Georges, R.; Rowe, B.; Reddy, K. P. J.

2011-05-01

302

Premixed hydrocarbon stagnation flames : experiments and simulations to validate combustion chemical-kinetic models  

NASA Astrophysics Data System (ADS)

A methodology based on the comparison of flame simulations relying on reacting flow models with experiment is applied to C1-C3 stagnation flames. The work reported targets the assessment and validation of the modeled reactions and reaction rates relevant to (C1-C3)-flame propagation in several detailed combustion kinetic models. A concensus does not, as yet, exist on the modeling of the reasonably well-understood oxidation of C1-C2 flames, and a better knowledge of C3 hydrocarbon combustion chemistry is required before attempting to bridge the gap between the oxidation of C1-C2 hydrocarbons and the more complex chemistry of heavier hydrocarbons in a single kinetic model. Simultaneous measurements of velocity and CH-radical profiles were performed in atmospheric propane(C3H8)- and propylene(C3H6)-air laminar premixed stagnation flames stabilized in a jet-wall configuration. These nearly-flat flames can be modeled by one-dimensional simulations, providing a means to validate kinetic models. Experimental data for these C3 flames and similar experimental data for atmospheric methane(CH4)-, ethane(C2H6)-, and ethylene(C2H4)-air flames are compared to numerical simulations performed with a one-dimensional hydrodynamic model, a multi-component transport formulation including thermal diffusion, and different detailed-chemistry models, in order to assess the adequacy of the models employed. A novel continuation technique between kinetic models was developed and applied successfully to obtain solutions with the less-robust models. The 2005/12 and 2005/10 releases of the San Diego mechanism are found to have the best overall performance in C3H8 and C3H6 flames, and in CH4, C2H6, and C2H4 flames, respectively. Flame position provides a good surrogate for flame speed in stagnation-flow stabilized flames. The logarithmic sensitivities of the simulated flame locations to variations in the kinetic rates are calculated via the "brute-force" method for fifteen representative flames covering the five fuels under study and the very lean, stoichiometric, and very rich burning regimes, in order to identify the most-important reactions for each flame investigated. The rates of reactions identified in this manner are compared between the different kinetic models. Several reaction-rate differences are thus identified that are likely responsible for the variance in flame-position (or flame-speed) predictions in C1-C2 flames.

Benezech, Laurent Jean-Michel

303

Uncertainties in Kinetic and Photochemical Parameters of Ozone Depleting Substances and the Impact on Atmospheric Lifetime Calculations  

NASA Astrophysics Data System (ADS)

The SPARC (Stratospheric Processes And their Role in Climate) initiative "Reevaluation of the Lifetimes of Dominant Stratospheric Ozone-Depleting Substances (ODSs)" is aimed toward providing a critical evaluation of atmospheric lifetimes for a number of key compounds. The atmospheric lifetimes of ODSs and greenhouse gases play a key role in the evaluation of ozone loss/recovery and climate change. Therefore, it is critically important to have accurate estimates of lifetimes as well as their uncertainty. The initiative has identified 26 key compounds to be included in this study. The compounds were chosen primarily due to their contribution to ozone depletion, their use as ODS replacements, and their contribution as climate forcing agents. Model calculated atmospheric lifetimes rely on kinetic and photochemical input parameters that are obtained from laboratory studies. Important gas-phase loss processes include reaction with OH, O(1D), and Cl atoms as well as EUV (Lyman-?) and UV photolysis where the relative importance of these processes depends on the properties of the individual compound. As part of the initiative, an evaluation of the model kinetic input parameters and their uncertainties was performed. Here, an assessment of the atmospheric loss processes and associated uncertainties for the target compounds will be presented. Two-dimensional (2D) atmospheric model calculations are used to characterize the globally/annually averaged vertical profiles of the target compounds, loss processes and uncertainties, and their atmospheric lifetimes. The 2D model results are used to highlight the limitations of atmospheric lifetime calculations due to uncertainties in the kinetic and photochemical input parameters, which in some cases are substantial. This component of the initiative also identifies areas where more accurate laboratory studies are needed in order to reduce uncertainties in calculated atmospheric lifetimes for these key species.

Burkholder, J. B.; Kurylo, M. J.; Wallington, T.; Fleming, E. L.; Jackman, C. H.

2011-12-01

304

Numerical Modeling of Lead Oxidation in Controlled Lead Bismuth Eutectic Systems: Chemical Kinetics and Hydrodynamic Effects  

SciTech Connect

Using liquid Lead-Bismuth Eutectic (LBE) as coolant in nuclear systems has been studied for more than 50 years. And LBE has many unique nuclear, thermo physical and chemical attributes which are attractive for practical application. But, corrosion is one of the greatest concerns in using liquid Lead-Bismuth Eutectic (LBE) as spallation target in the Accelerator-driven Transmutation of Waste (ATW) program. Los Alamos National Laboratory has designed and built the Liquid Lead-Bismuth Materials Test Loop (MTL) to study the materials behavior in a flow of molten LBE. A difference of 100 deg. C was designed between the coldest and the hottest parts at a nominal flow rate of 8.84 GPM. Liquid LBE flow was activated by a mechanical sump pump or by natural convection. In order to maintain a self-healing protective film on the surface of the stainless steel pipe, a certain concentration of oxygen has to be maintained in the liquid metal. Therefore, it is of importance to understand what the oxygen concentrations are in the LBE loop related to the corrosion effects on the metal surface, the temperature profiles, the flow rates, and diffusion rates through the metal surface. The chemical kinetics also needs to be fully understood in the corrosion processes coupled with the hydrodynamics. The numerical simulation will be developed and used to analyze the system corrosion effects with different kind of oxygen concentrations, flow rates, chemical kinetics, and geometries. The hydrodynamics modeling of using computational fluid dynamics will provide the necessary the levels of oxygen and corrosion products close to the boundary or surface. This paper presents an approach towards the above explained tasks by analyzing the reactions between the Lead and oxygen at a couple of sections in the MTL. Attempt is also made to understand the surface chemistry by choosing an example model and estimating the near wall surface concentration values for propane and oxygen. (authors)

Wu, Chao; Kanthi Kiran Dasika; Chen, Yitung; Moujaes, Samir [Department of Mechanical Engineering, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas NV, 89154-4027 (United States)

2002-07-01

305

Micro-HCCI combustion: experimental characterization and development of a detailed chemical kinetic model with coupled piston motion  

Microsoft Academic Search

Recent experimental and modeling work concerning Homogeneous Charge Compression Ignition (HCCI) combustion in small scales is presented. A zero-dimensional numerical model incorporating detailed chemical kinetics, heat transfer, blow-by, and a force balance is developed to interpret the experimental results and to explore HCCI combustion with a free-piston. The model consists of a new “Reactor Problem” for the sensitivity and kinetics

H. T. Aichlmayr; D. B. Kittelson; M. R. Zachariah

2003-01-01

306

Calculation of solid-propellant burning rates from condensed-phase decomposition kinetics  

Microsoft Academic Search

Kinetic rates of thermal decomposition reactions of three ammonium perchlorate composite propellants have been determined\\u000a by the techniques of differential scanning calorimetry and thermogravimetric analysis at pressures to 500 psi (1 psi=6894.7\\u000a Pa=0.068 atm; 1 atm=14.7 psi). The results of these experiments were extrapolated to give heat-evolution rates at temperatures\\u000a that are believed to prevail at the surface of burning

R. H. W. Waesche; J. Wenograd

2000-01-01

307

Chemically non-equilibrium modelling of N2 thermal ICP at atmospheric pressure using reaction kinetics  

NASA Astrophysics Data System (ADS)

A two-dimensional hydrodynamic model for an N2 inductively coupled thermal plasma (ICTP or thermal ICP) at atmospheric pressure was developed using reaction rates without the chemical equilibrium (CE) assumption. Particle composition distribution in the N2 ICTP was derived by solving the mass conservation equations for each of the particles, considering diffusion, convection and production terms. The electrical conductivity, mass density and diffusion coefficient were calculated at each of the calculation steps with the derived particle composition distributions. Using this model, the influence of gas flow rate on chemical composition distribution was investigated. The dependence of mass flow of N atom on gas flow rate was obtained. From the result, a large deviation from CE in the distribution of the particle composition was found, especially near the wall of the ICTP.

Tanaka, Y.; Sakuta, T.

2002-03-01

308

Thermal Decomposition of Some Chemical Compounds Used As Food Preservatives and Kinetic Parameters of This Process  

Microsoft Academic Search

Thermal decomposition processes of selected chemicals used as food preservatives such as sodium formate, sodium propionate,\\u000a sodium nitrates(V and III) and sodium sulphate(IV) were examined by the derivatographic method. Based on the curves obtained,\\u000a the number of decomposition stages and characteristic temperatures of these compounds have been found. Mass decrements calculated\\u000a from TG curves ranged from 28.9% for sodium formate

J. Mas?owska; M. Wi?d?ocha

2000-01-01

309

An efficient error-propagation-based reduction method for large chemical kinetic mechanisms  

SciTech Connect

Production rates obtained from a detailed chemical mechanism are analyzed in order to quantify the coupling between the various species and reactions involved. These interactions can be represented by a directed relation graph. A geometric error propagation strategy applied to this graph accurately identifies the dependencies of specified targets and creates a set of increasingly simplified kinetic schemes containing only the chemical paths deemed the most important for the targets. An integrity check is performed concurrently with the reduction process to avoid truncated chemical paths and mass accumulation in intermediate species. The quality of a given skeletal model is assessed through the magnitude of the errors introduced in the target predictions. The applied error evaluation is variable-dependent and unambiguous for unsteady problems. The technique yields overall monotonically increasing errors, and the smallest skeletal mechanism that satisfies a user-defined error tolerance over a selected domain of applicability is readily obtained. An additional module based on life-time analysis identifies a set of species that can be modeled accurately by quasi-steady state relations. An application of the reduction procedure is presented for autoignition using a large iso-octane mechanism. The whole process is automatic, is fast, has moderate CPU and memory requirements, and compares favorably to other existing techniques. (author)

Pepiot-Desjardins, P.; Pitsch, H. [Department of Mechanical Engineering, Stanford University, Stanford, CA (United States)

2008-07-15

310

Reactibodies generated by kinetic selection couple chemical reactivity with favorable protein dynamics  

PubMed Central

Igs offer a versatile template for combinatorial and rational design approaches to the de novo creation of catalytically active proteins. We have used a covalent capture selection strategy to identify biocatalysts from within a human semisynthetic antibody variable fragment library that uses a nucleophilic mechanism. Specific phosphonylation at a single tyrosine within the variable light-chain framework was confirmed in a recombinant IgG construct. High-resolution crystallographic structures of unmodified and phosphonylated Fabs display a 15-Å-deep two-chamber cavity at the interface of variable light (VL) and variable heavy (VH) fragments having a nucleophilic tyrosine at the base of the site. The depth and structure of the pocket are atypical of antibodies in general but can be compared qualitatively with the catalytic site of cholinesterases. A structurally disordered heavy chain complementary determining region 3 loop, constituting a wall of the cleft, is stabilized after covalent modification by hydrogen bonding to the phosphonate tropinol moiety. These features and presteady state kinetics analysis indicate that an induced fit mechanism operates in this reaction. Mutations of residues located in this stabilized loop do not interfere with direct contacts to the organophosphate ligand but can interrogate second shell interactions, because the H3 loop has a conformation adjusted for binding. Kinetic and thermodynamic parameters along with computational docking support the active site model, including plasticity and simple catalytic components. Although relatively uncomplicated, this catalytic machinery displays both stereo- and chemical selectivity. The organophosphate pesticide paraoxon is hydrolyzed by covalent catalysis with rate-limiting dephosphorylation. This reactibody is, therefore, a kinetically selected protein template that has enzyme-like catalytic attributes.

Smirnov, Ivan; Carletti, Eugenie; Kurkova, Inna; Nachon, Florian; Nicolet, Yvain; Mitkevich, Vladimir A.; Debat, Helene; Avalle, Berangere; Belogurov, Alexey A.; Kuznetsov, Nikita; Reshetnyak, Andrey; Masson, Patrick; Tonevitsky, Alexander G.; Ponomarenko, Natalia; Makarov, Alexander A.; Friboulet, Alain; Tramontano, Alfonso; Gabibov, Alexander

2011-01-01

311

Stretching-based diagnostics and reduction of chemical kinetic models with diffusion  

SciTech Connect

A new method for diagnostics and reduction of dynamical systems and chemical kinetic models is proposed. The method makes use of the local structure of the normal stretching rates by projecting the dynamics onto the local directions of maximal stretching. The approach is computationally very simple as it implies the spectral analysis of a symmetric matrix. Notwithstanding its simplicity, stretching-based analysis derives from a geometric basis grounded on the pointwise applications of concepts of normal hyperbolicity theory. As a byproduct, a simple reduction method is derived, equivalent to a 'local embedding algorithm', which is based on the local projection of the dynamics onto the 'most unstable and/or slow modes' compared to the time scale dictated by the local tangential dynamics. This method provides excellent results in the analysis and reduction of dynamical systems displaying relaxation towards an equilibrium point, limit cycles and chaotic attractors. Several numerical examples deriving from typical models of reaction/diffusion kinetics exhibiting complex dynamics are thoroughly addressed. The application to typical combustion models is also analyzed.

Adrover, A. [Dipartimento di Ingegneria Chimica, Facolta di Ingegneria, Universita di Roma 'La Sapienza', via Eudossiana 18, 00184 Roma (Italy)]. E-mail: alex@giona.ing.uniroma1.it; Creta, F. [Dipartimento di Meccanica e Aeronautica, Facolta di Ingegneria, Universita di Roma 'La Sapienza', via Eudossiana 18, 00184 Roma (Italy); Giona, M. [Dipartimento di Ingegneria Chimica, Facolta di Ingegneria, Universita di Roma 'La Sapienza', via Eudossiana 18, 00184 Roma (Italy); Valorani, M. [Dipartimento di Meccanica e Aeronautica, Facolta di Ingegneria, Universita di Roma 'La Sapienza', via Eudossiana 18, 00184 Roma (Italy)

2007-08-10

312

Kinetics of the tungsten hexafluoride-silane reaction for the chemical vapor deposition of tungsten  

SciTech Connect

In this study, the kinetics of the low-pressure chemical vapor deposition (LPCVD) of tungsten by silane reduction of tungsten hexafluoride on Si(100) surfaces was studied. A single-wafer, cold-wall reactor was sued for the experiments. The SiH[sub 4]/WF[sub 6] ratio was 1.0. The pressure and temperature range were 1-10 torr and 137-385[degree]C, respectively. Kinetic data were obtained in the absence of mass-transfer effects. The film thicknesses were measured by gravimetry. Scanning electron microscopy (SEM), Auger electron spectroscopy (AES), x-ray diffraction (XRD), and resistivity measurements were used to analyze the W films. For the horizontal substrate position and 4-minute reaction times, the apparent activation energies were determined to be 0.35 eV/atom for 10 torr, 0.17 eV/atom for 3 torr, and 0.08 eV/atom for 1 torr. Lower temperatures and higher pressures produced porous films, while higher temperatures and lower pressures resulted in continuous films with smoother surfaces. As the Si-W interface, a W(110) preferential orientation was observed. As the W films grew thicker, W orientation switched from (110) to (100). Apparent activation energy seems to change with thickness.

Gokce, Huseyin.

1991-01-01

313

Chemical kinetic modeling of high pressure propane oxidation and comparison to experimental results. Revision 1  

SciTech Connect

A pressure dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high pressure flow reactor. Experimental conditions range from 10--15 atm, 650--800 K, and a residence time of 198 ms for propane-air mixtures at an equivalence ratio of 0.4. The experimental results clearly indicate a negative temperature coefficient (NTC) behavior. The chemistry describing this phenomena is critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor. Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals and important isomers (radicals) with O{sub 2} were estimated using thermodynamic properties, with multifrequency quantum Kassel Theory for k(E) coupled with modified strong collision analysis for fall-off. Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. Sensitivity analysis identified the key reaction steps that control the rate of oxidation in the NTC region. The model reasonably simulates the profiles for many of the major and minor species observed in the experiments.

Koert, D.N. [Wichita State Univ., KS (United States). Mechanical Engineering Dept.; Pitz, W.J. [Lawrence Livermore National Lab., CA (United States); Bozzelli, J.W. [New Jersey Inst. of Tech., Newark, NJ (United States). Chemistry and Chemical Engineering Dept.; Cernansky, N.P. [Drexel Univ., Philadelphia, PA (United States). Dept. of Mechanical Engineering and Mechanics

1996-02-01

314

Interactions between a hydrophobic organic chemical and natural organic matter: Equilibrium and kinetic studies  

SciTech Connect

Remediation of polluted soils or sediments may be hindered by the binding of hydrophobic organic chemicals (HOCs) on or into the soil matrix. Interactions between a well-characterized, purified, peat humic acid and pentachlorobenzene were studied in equilibrium and kinetic experiments. The kinetic experiments, performed with a gas-purge method, showed the presence of a fast-desorbing, labile fraction and a slow-desorbing, nonlabile fraction. Increased contact time did not change the measured continuous desorption curves. A desorption experiment with interrupted flow and the measured isotherm suggested nonlinear sorption behavior. A first-order model consisting of two parallel sites could describe the continuous desorption curves but failed to describe the adsorption isotherm and the desorption curve with interrupted flow. A Langmuir model with one site was able to describe the characteristics of the desorption experiments and of the adsorption isotherm, but it did not describe the observed nonlabile fraction. Expanding the model with an additional Langmuir or first-order site did not lead to a good description of the various experiments. The authors speculate that the nonlabile fraction is a consequence of a change of conformation of the humic acid after the binding of pentachlorobenzene. This structural change leads to entrapment of pentachlorobenzene within the humic acid structure.

Schlebaum, W.; Badora, A.; Schraa, G.; Van Riemsdijk, W.H. [Wageningen Agricultural Univ. (Netherlands)

1998-08-01

315

n-Butane: Ignition delay measurements at high pressure and detailed chemical kinetic simulations  

SciTech Connect

Ignition delay time measurements were recorded at equivalence ratios of 0.3, 0.5, 1, and 2 for n-butane at pressures of approximately 1, 10, 20, 30 and 45 atm at temperatures from 690 to 1430 K in both a rapid compression machine and in a shock tube. A detailed chemical kinetic model consisting of 1328 reactions involving 230 species was constructed and used to validate the delay times. Moreover, this mechanism has been used to simulate previously published ignition delay times at atmospheric and higher pressure. Arrhenius-type ignition delay correlations were developed for temperatures greater than 1025 K which relate ignition delay time to temperature and concentration of the mixture. Furthermore, a detailed sensitivity analysis and a reaction pathway analysis were performed to give further insight to the chemistry at various conditions. When compared to existing data from the literature, the model performs quite well, and in several instances the conditions of earlier experiments were duplicated in the laboratory with overall good agreement. To the authors' knowledge, the present paper presents the most comprehensive set of ignition delay time experiments and kinetic model validation for n-butane oxidation in air. (author)

Healy, D.; Curran, H.J. [Combustion Chemistry Centre, School of Chemistry, NUI Galway (Ireland); Donato, N.S.; Aul, C.J.; Petersen, E.L. [Department of Mechanical Engineering, Texas A and M University, College Station, TX (United States); Zinner, C.M. [Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL (United States); Bourque, G. [Rolls-Royce Canada Limited, 9500 Cote de Liesse, Lachine, Quebec, H8T 1A2 (Canada)

2010-08-15

316

Use of detailed chemical kinetics to study HCCI engine combustionwith consideration of turbulent mixing effects  

Microsoft Academic Search

Detailed chemical kinetics was used in an engine CFD code to study\\u000d\\u000a\\u0009the combustion process in HCCI engines. The CHEMKIN code was implemented\\u000d\\u000a\\u0009in KIVA such that the chemistry and flow solutions were coupled.\\u000d\\u000a\\u0009The reaction mechanism consists of hundreds of reactions and species\\u000d\\u000a\\u0009and is derived from fundamental flame chemistry. Effects of turbulent\\u000d\\u000a\\u0009mixing on the reaction rates were

S.-C. Kong; R. D. Reitz

2002-01-01

317

Kinetic Monte Carlo modeling of chemical reactions coupled with heat transfer  

NASA Astrophysics Data System (ADS)

In this paper, we describe two types of effective events for describing heat transfer in a kinetic Monte Carlo (KMC) simulation that may involve stochastic chemical reactions. Simulations employing these events are referred to as KMC-TBT and KMC-PHE. In KMC-TBT, heat transfer is modeled as the stochastic transfer of ``thermal bits'' between adjacent grid points. In KMC-PHE, heat transfer is modeled by integrating the Poisson heat equation for a short time. Either approach is capable of capturing the time dependent system behavior exactly. Both KMC-PHE and KMC-TBT are validated by simulating pure heat transfer in a rod and a square and modeling a heated desorption problem where exact numerical results are available. KMC-PHE is much faster than KMC-TBT and is used to study the endothermic desorption of a lattice gas. Interesting findings from this study are reported.

Castonguay, Thomas C.; Wang, Feng

2008-03-01

318

Desorption kinetics of hydrophobic organic chemicals from sediment to water: a review of data and models.  

PubMed

Resuspension of contaminated sediment can lead to the release of toxic compounds to surface waters where they are more bioavailable and mobile. Because the timeframe of particle resettling during such events is shorter than that needed to reach equilibrium, a kinetic approach is required for modeling the release process. Due to the current inability of common theoretical approaches to predict site-specific release rates, empirical algorithms incorporating the phenomenological assumption of biphasic, or fast and slow, release dominate the descriptions of nonpolar organic chemical release in the literature. Two first-order rate constants and one fraction are sufficient to characterize practically all of the data sets studied. These rate constants were compared to theoretical model parameters and functionalities, including chemical properties of the contaminants and physical properties of the sorbents, to determine if the trends incorporated into the hindered diffusion model are consistent with the parameters used in curve fitting. The results did not correspond to the parameter dependence of the hindered diffusion model. No trend in desorption rate constants, for either fast or slow release, was observed to be dependent on K(OC) or aqueous solubility for six and seven orders of magnitude, respectively. The same was observed for aqueous diffusivity and sediment fraction organic carbon. The distribution of kinetic rate constant values was approximately log-normal, ranging from 0.1 to 50 d(-1) for the fast release (average approximately 5 d(-1)) and 0.0001 to 0.1 d(-1) for the slow release (average approximately 0.03 d(-1)). The implications of these findings with regard to laboratory studies, theoretical desorption process mechanisms, and water quality modeling needs are presented and discussed. PMID:17373505

Birdwell, Justin; Cook, Robert L; Thibodeaux, Louis J

2007-03-01

319

Chemical kinetic modeling of high pressure propane oxidation and comparison to experimental results  

SciTech Connect

A pressure dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high pressure flow reactor. The experiment conditions range from 10--15 atm, 650--800 K, and were performed at a residence time of 200 {micro}s for propane-air mixtures at an equivalence ratio of 0.4. The experimental results include data on negative temperature coefficient (NTC) behavior, where the chemistry describing this phenomena is considered critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor. Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals and important isomers with O{sub 2} were estimated using thermodynamic properties, with multifrequency quantum Kassel Theory for k(E) coupled with modified strong collision analysis for fall-off. Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. The model simulates properly the production of many of the major and minor species observed in the experiments. Numerical simulations show many of the key reactions involving propylperoxy radicals are in partial equilibrium at 10--15 atm. This indicates that their relative concentrations are controlled by a combination of thermochemistry and rate of minor reaction channels (bleed reactions) rather than primary reaction rates. This suggests that thermodynamic parameters of the oxygenated species, which govern equilibrium concentrations, are important. The modeling results show propyl radical and hydroperoxy-propyl radicals reaction with O{sub 2} proceeds, primarily, through thermalized adducts, not chemically activated channels.

Koert, D.N. [Wichita State Univ., KS (United States). Mechanical Engineering Dept.; Pitz, W.J. [Lawrence Livermore National Lab., CA (United States); Bozzelli, J.W. [New Jersey Inst. of Tech., Newark, NJ (United States). Chemistry and Chemical Engineering Dept.; Cernansky, N.P. [Drexel Univ., Philadelphia, PA (United States). Dept. of Mechanical Engineering and Mechanics

1995-11-08

320

Chemical kinetics of the gas-phase reaction between uranium hexafluoride and hydrogen  

SciTech Connect

A study was made of the chemical kinetics of the homogeneous gas-phase reaction between uranium hexafluoride and hydrogen by measuring the rate of disappearance of UF/sub 6/. It has been concluded that the rate-limiting step for which the kinetics have been measured is UF/sub 6/ + HF + H (2). The reaction has been studied in a steady-state flow system over a temperature range of approx.625 to 825 K. Various surface-to-volume ratios were employed to aid in distinguishing gas-phase reactions from surface reactions. The steady-state concentration of the UF/sub 6/ after reaction with H/sub 2/ was monitored in a special multipass infrared spectrophotometer at the 626-cm/sup -1/ absorption band of UF/sub 6/. The principal problems were corrosion, plugging, surface intrusion, and the deleterious effects of minute traces of water; these problems have greatly slowed progress in this field. Several series of measurements involving different initial species concentrations and residence times, with each series at constant temperature, show that the rate is first order in UF/sub 6/. Our measurements yield a dependable Arrhenius curve in terms of a second-order expression for k, the bimolecular specific reaction rate constant for the disappearance of UF/sub 6/. It is believed that this overall rate of the disappearance of UF/sub 6/ is somewhat less than twice that of the critical reaction step 2, indicated above, so that the specific reaction rate constant k/sub 2/ is approx.8.7 c 10/sup 14/ exp(-34550 kcal/RT) cm/sup 3/ mol/sup -1/ s/sup -1/. Conclusions have been reached concerning the relative importance of the various elementary reaction steps involved in the chemical mechanism.

Myerson, A.L. (Mote Marine Lab., Sarasota, FL); Chludzinski, J.J. Jr.

1981-12-10

321

Use of a pressuremeter to measure the kinetics of carbon dioxide evolution in chemically leavened wheat flour dough.  

PubMed

Among a number of impediments to a wider use of chemical leavening agents in bakery applications is the lack of standardized instrumentation capable of providing information on the rates of CO2 production from chemical leaveners in a format that is meaningful to both the technologist (i.e., the dough rate of reaction or DRR) and the researcher (e.g., in terms of fundamental unitskmol CO2 per kg of dough per s). This paper presents an original methodology to carry out the DRR test using a commercial pressuremeter, the Gassmart apparatus, and to model the kinetics of CO2 evolution of chemically leavened dough. Lean formula doughs were leavened at 27 and 39 degrees C with four chemical leavening systems containing sodium bicarbonate and one of four leavening acids, sodium acid pyrophosphate 40 (SAPP), adipic acid (ADA), potassium acid tartrate (KAT), and glucono-delta-lactone (GDL). Chemical kinetics theory was used to gain an insight into the reaction mechanisms responsible for the evolution of carbon dioxide from the leaveners. A first-order reaction kinetics model was found to be suitable for describing the neutralizing properties of GDL and ADA leavening systems, whereas a first-order reaction kinetics model for irreversible parallel reactions better described the leavening properties of the acidic salts KAT and SAPP. PMID:18841986

Bellido, Guillermo G; Scanlon, Martin G; Sapirstein, Harry D; Page, John H

2008-10-09

322

Molecular orbital calculation on the chemical bonding character of phosphate glasses  

Microsoft Academic Search

Chemical bonding character of various bonds in phosphate glasses was investigated on the basis of bond overlap population which was evaluated from molecular orbital calculations. Cluster models were constructed from various phosphate crystals. There are two terminal PO bonds in phosphates, that is, P=O double bonds and PNBO (non-bridging oxygen) bonds. In bond overlap population, P=O bonds were much larger

Y. Asano; S. Sakida; T. Nanba; Y. Miura

323

Validation of hypersonic chemical equilibrium flow calculations using ballistic-range data  

Microsoft Academic Search

.   The hypersonic flow field over a sphere flying in a ballistic-range is numerically simulated for the purpose of validating\\u000a a hypersonic chemical equilibrium flow solver. The numerical results obtained are compared with available experimental data\\u000a on the stand-off distance and the shape of the detached bow shock wave. In the calculation, an adaptive mesh is employed for\\u000a a crisp

K. Sawada; E. Dendou

2001-01-01

324

SIML: A Fast SIMD Algorithm for Calculating LINGO Chemical Similarities on GPUs and CPUs  

PubMed Central

LINGOs are a holographic measure of chemical similarity based on text comparison of SMILES strings. We present a new algorithm for calculating LINGO similarities amenable to parallelization on SIMD architectures (such as GPUs and vector units of modern CPUs). We show that it is nearly 3 times as fast as existing algorithms on a CPU, and over 80 times faster than existing methods when run on a GPU.

Haque, Imran S.; Walters, W. Patrick

2010-01-01

325

Respiratory uptake kinetics of neutral hydrophobic organic chemicals in a marine benthic fish, Pseudopleuronectes yokohamae.  

PubMed

We investigated the respiratory uptake kinetics of polychlorinated biphenyls (PCBs), organohalogen pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), and 2,2',4,4'-tetrabrominated diphenyl ether (BDE #47) in a marine benthic fish, Pseudopleuronectes yokohamae. The respiratory uptake efficiencies (EW) of the chemicals, of which there have been no reports for the majority of persistent organic pollutants (POPs), were obtained by measuring the respiratory uptake rate constants (k1) and the oxygen consumption rates of fish. Fish were exposed to water in which these chemicals were dissolved at environmentally relevant concentrations for 28d, followed by 168d of depuration in clean seawater. The k1 and EW values for 99 compounds were obtained, and they ranged from 2000 to 42000Lkg-lipid(-1)d(-1) and from 0.060 to 1.3, respectively. The EW values of the chemicals, except for PAHs, tended to increase with increasing values of the log octanol-water partition coefficients (KOW) of the chemicals up to a log KOW of 5. For log KOW in the range 3-5, the EW values in this study were much lower than those in a published study (about one-third). As a result of analysis by a two-phase resistance model, the resistance of transport rates to the lipid phase in this study was lower than was the case in the published study. These findings indicate that the EW predicted by the published study for log KOW in the range 3-5 may differ among fish species and water temperature, and further study is needed. PMID:23962382

Kobayashi, Jun; Sakurai, Takeo; Mizukawa, Kaoruko; Kinoshita, Kyoko; Ito, Nozomi; Hashimoto, Shunji; Nakajima, Daisuke; Kawai, Toru; Imaizumi, Yoshitaka; Takada, Hideshige; Suzuki, Noriyuki

2013-08-17

326

Research in chemical kinetics. Progress report, July 20, 1988--August 30, 1989  

SciTech Connect

A major aspect of our research over the past decade under this contract has been the application of radioisotopes generated by nuclear reactions for the study of various kinetic mechanisms. Two general theoretical concepts have been explored in detail by this technique: (a) The addition of halogen atoms to olefins, which have been described for fifty years by the phrase {open_quotes}anti-Markownikoff{close_quotes} to indicate that the preference for one or the other end of an unsymmetric olefin is opposite to that ({open_quotes}Markownikoff addition{close_quotes}) for hydrogen halide addition. (b) The redistribution of internal energy within a molecule after an energetic addition reaction, for which the usual assumption is rapid equilibration into all available degrees of freedom, as calculated by the Rice-Rarnsperger-Kassel-Marcus (RRKM) model. In both instances, significant results have been obtained which expand the overall view of each of these two concepts.

Rowland, F.S.

1996-09-01

327

Surftherm: A program to analyze thermochemical and kinetic data in gas-phase and surface chemical reaction mechanisms  

Microsoft Academic Search

This report documents the Surftherm program that analyzes transport coefficient, thermochemical- and kinetic rate information in complex gas-phase and surface chemical reaction mechanisms. The program is designed for use with the Chemkin (gas-phase chemistry) and Surface Chemkin (heterogeneous chemistry) programs. It was developed as a ``chemist`s companion`` in using the Chemkin packages with complex chemical reaction mechanisms. It presents in

M. E. Coltrin; H. K. Moffat

1994-01-01

328

Critical evaluation of Jet-A spray combustion using propane chemical kinetics in gas turbine combustion simulated by KIVA2  

Microsoft Academic Search

Jet-A spray combustion has been evaluated in gas turbine combustion with the use of propane chemical kinetics as the first approximation for the chemical reactions. Here, the numerical solutions are obtained by using the KIVA-2 computer code. The KIVA-2 code is the most developed of the available multidimensional combustion computer programs for application of the in-cylinder combustion dynamics of internal

H. L. Nguyen; S.-J. Ying

1990-01-01

329

The chemical mechanism generation programme CHEMATA—Part 2: Comparison of four chemical mechanisms for mesoscale calculation of atmospheric pollution  

NASA Astrophysics Data System (ADS)

RACM and the three new mechanisms described in the companion paper (the extended, reduced and small mechanisms) are implemented in a mesoscale 3D transport-chemistry model (TAPOM for Transport and Air POllution Model) in order to find an optimum between calculation speed and mechanism detail. The 3D tests are performed in the domains of Milan, Mexico City and Bogota. The three domains present different chemical and meteorological conditions, which are used to test the behaviour of the four mechanisms in different situations. Three emission scenarios are simulated: the whole emission inventory, 35% NO x reduction and 35% VOC reduction. The comparison of the four mechanisms is performed for O 3, NO x , aldehydes and peroxy radicals. Only the small mechanism presents significant differences in ozone concentrations. RO 2 and aldehyde differences are important with the reduced and the small mechanism, which share a new RO 2 parameterisation. Compared to RACM, the small mechanism shows very large differences for aldehydes and RO 2. The extended mechanism and RACM show almost the same ozone response to emissions reduction strategies, and the reduced mechanism presents differences in the range of 10% with respect to the extended mechanisms. The small mechanism is found to be the most VOC sensitive and therefore presents very different results from the other when emissions are modified. The results indicate a strong restriction to the use of the small mechanism in 3D models. Finally, the calculation time required for the calculation of a simulation with the four mechanisms is compared.

Junier, Martin; Kirchner, Frank; Clappier, Alain; van den Bergh, Hubert

330

Combustion Research Program: Flame studies, laser diagnostics, and chemical kinetics. Final report, 15 July 1987--15 June 1992  

SciTech Connect

This project has comprised laser flame diagnostic experiments, chemical kinetics measurements, and low pressure flame studies. Collisional quenching has been investigated for several systems: the OH radical, by H{sub 2}0 in low pressure flames; the rotational level dependence for NH, including measurements to J=24; and of NH{sub 2} at room temperature. Transition probability measurements for bands involving v{prime} = 2 and 3 of the A-X system of OH were measured in a flame. Laser-induced fluorescence of vinyl radicals was unsuccessfully attempted. RRKM and transition state theory calculations were performed on the OH + C{sub 2}H{sub 4} reaction, on the t-butyl radical + HX; and transition state theory has been applied to a series of bond scission reactions. OH concentrations were measured quantitatively in low pressure H{sub 2}/N{sub 2}O and H{sub 2}/O{sub 2} flames, and the ability to determine spatially precise flame temperatures accurately using OH laser-induced fluorescence was studied.

Crosley, D.R.

1992-09-01

331

Numerical Study on Spark Ignition Characteristics of a Methane-Air Mixture Using Detailed Chemical Kinetics  

NASA Astrophysics Data System (ADS)

Spark ignition is considered one of the most difficult and complex problems because it involves complicated physical and chemical processes, and it has not yet been explained sufficiently. The minimum ignition energy (MIE) is an important parameter for judging the ignition ability of combustion systems. In the present study, the spark ignition characteristics of a methane-air mixture were investigated by numerical analysis using detailed chemical kinetics consisting of 53 species and 325 elementary reactions. Two different analytical models, with and without electrodes, were applied to research the effect of electrode temperature and energy channel length on flame propagation and the relationship between the MIE and equivalence ratio. The electrode temperature was set as 300 K, 1000 K, and 2000 K for analytical models with electrodes, and the energy channel length was set as 1 mm, 2 mm, and 3 mm for analytical models without electrodes. The obtained computational results showed good agreement with experimental results. We determined that with increasing electrode temperature, the minima of the curve indicating the relationship between the MIE and equivalence ratio move toward the leaner side, that a leaner mixture is more sensitive to heat loss to the cold surrounding gas, and that heat loss to the electrodes is an unignorable factor for the initial formation of the flame kernel.

Han, Jilin; Yamashita, Hiroshi; Yamamoto, Kazuhiro

332

A Detailed Chemical Kinetic Reaction Mechanism for Oxidation of Four Small Alkyl Esters in Laminar Premixed Flames  

SciTech Connect

A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate and ethyl acetate. This mechanism is validated by comparisons between computed results and recently measured intermediate species mole fractions in fuel-rich, low pressure, premixed laminar flames. The model development employs a principle of similarity of functional groups in constraining the H atom abstraction and unimolecular decomposition reactions in each of these fuels. As a result, the reaction mechanism and formalism for mechanism development are suitable for extension to larger oxygenated hydrocarbon fuels, together with an improved kinetic understanding of the structure and chemical kinetics of alkyl ester fuels that can be extended to biodiesel fuels. Variations in concentrations of intermediate species levels in these flames are traced to differences in the molecular structure of the fuel molecules.

Westbrook, C K; Pitz, W J; Westmoreland, P R; Dryer, F L; Chaos, M; Osswald, P; Kohse-Hoinghaus, K; Cool, T A; Wang, J; Yang, B; Hansen, N; Kasper, T

2008-02-08

333

Calculated vs measured scattering and kinetic data for the Li+HCl reaction  

Microsoft Academic Search

A study of the Li+HCl reaction has been carried out using classical trajectories. A comparison with detailed scattering data has given sufficient indications on the accuracy of the overall shape of a previous bond order potential energy surface derived from abinitio values. Calculations performed on a new bond order surface having a lower transition state has led to a more

A. Laganà; P. Palmieri; J. M. Alvariño; E. Garcia

1990-01-01

334

A Simple Model for Calculating the Kinetics of Protein Folding from Three-Dimensional Structures  

Microsoft Academic Search

An elementary statistical mechanical model was used to calculate the folding rates for 22 proteins from their known three-dimensional structures. In this model, residues come into contact only after all of the intervening chain is in the native conformation. An additional simplifying assumption is that native structure grows from localized regions that then fuse to form the complete native molecule.

Victor Munoz; William A. Eaton

1999-01-01

335

Phosphorus chemical shifts in a nucleic acid backbone from combined molecular dynamics and density functional calculations.  

PubMed

A comprehensive quantum chemical analysis of the influence of backbone torsion angles on (31)P chemical shifts in DNAs has been carried out. An extensive DFT study employed snapshots obtained from the molecular dynamics simulation of [d(CGCGAATTCGCG)]2 to construct geometries of a hydrated dimethyl phosphate, which was used as a model for the phosphodiester linkage. Our calculations provided differences of 2.1 ± 0.3 and 1.6 ± 0.3 ppm between the B(I) and B(II) chemical shifts in two B-DNA residues of interest, which is in a very good agreement with the difference of 1.6 ppm inferred from experimental data. A more negative (31)P chemical shift for a residue in pure BI conformation compared to residues in mixed B(I)/B(II) conformation states is provided by DFT, in agreement with the NMR experiment. Statistical analysis of the MD/DFT data revealed a large dispersion of chemical shifts in both B(I) and B(II) regions of DNA structures. ?P ranges within 3.5 ± 0.8 ppm in the B(I) region and within 4.5 ± 1.5 ppm in the B(II) region. While the (31)P chemical shift becomes more negative with increasing ? in B(I)-DNA, it has the opposite trend in B(II)-DNA when both ? and ? increase simultaneously. The (31)P chemical shift is dominated by the torsion angles ? and ?, while an implicit treatment of ? and ? is sufficient. The presence of an explicit solvent leads to a damping and a 2-3 ppm upfield shift of the torsion angle dependences. PMID:21073198

P?ececht?lová, Jana; Novák, Petr; Munzarová, Markéta L; Kaupp, Martin; Sklená?, Vladimír

2010-11-12

336

Kinetics, growth, structure, and atmospheric chemical vapor deposition of ferrimagnetic iron oxide thin films from metallorganic acetyl acetonate precursor  

Microsoft Academic Search

Growth of magnetic iron oxide thin films by a chemical vapor deposition process from metallorganic iron acetyl acetonate precursor under atmospheric Ar-Oâ transport is described. Kinetics of film growth depends on deposition parameters, particularly substrate temperature, gas flow rate, and location of substrate away from the source which is a direct effect of mass transfer and pyrolyzing efficiency of precursor

A. C. Rastogi; S. Dhara; B. K. Das

1995-01-01

337

A Microscale Approach to Chemical Kinetics in the General Chemistry Laboratory: The Potassium Iodide Hydrogen Peroxide Iodine-Clock Reaction  

ERIC Educational Resources Information Center

|A microscale laboratory for teaching chemical kinetics utilizing the iodine clock reaction is described. Plastic pipets, 3 mL volume, are used to store and deliver precise drops of reagents and the reaction is run in a 24 well plastic tray using a total 60 drops of reagents. With this procedure, students determine the rate of reaction and the…

Sattsangi, Prem D.

2011-01-01

338

Kinetics of Structural and Chemical Transformations of Adsorbates Obtained with a Time-Resolved Electron Energy Loss Spectrometer  

Microsoft Academic Search

This thesis traces the development of the first time-resolved electron energy loss spectrometer (TREELS), and its application to study the kinetics of chemical and structural transformations of adsorbed molecules on single crystal transition metal surfaces. This advancement in EEL spectroscopy was accomplished by outfitting the analyzer of the EEL spectrometer with a multi-detector possessing 96 anodes which were monitored in

Bruce Alvin Gurney

1987-01-01

339

Chemical Kinetic Modeling of the Oxidation of Large Alkane Fuels: N-Octane and ISO-Octane.  

National Technical Information Service (NTIS)

The development of detailed chemical kinetic reaction mechanisms for oxidation of n-octane and iso-octane is described, with emphasis on the factors which are specific to many large hydrocarbon fuel molecules. Elements which are of particular importance a...

E. I. Axelsson K. Brezinsky F. L. Dryer W. J. Pitz C. K. Westbrook

1986-01-01

340

Is case-based learning an effective teaching strategy to challenge students’ alternative conceptions regarding chemical kinetics?  

Microsoft Academic Search

Background: Case-based learning (CBL) is simply teaching the concept to the students based on the cases. CBL involves a case, which is a scenario based on daily life, and study questions related to the case, which allows students to discuss their ideas. Chemical kinetics is one of the most difficult concepts for students in chemistry. Students have generally low levels

Eylem Yalç?nkaya; Özgecan Ta?tan-K?r?k; Yezdan Boz; Demet Y?ld?ran

2012-01-01

341

Project Title: Modeling Corrosion in Oxygen Controlled LBE Systems with Coupling of Chemical Kinetics and Hydrodynamics - Phase Three  

Microsoft Academic Search

The proposed work will combine chemical kinetics and hydrodynamics in target and test-loop lead-bismuth eutectic (LBE) systems to model system corrosion effects. This approach will result in a predicative tool that can be validated with corrosion test data, used to systematically design tests and interpret the results, and provide guidance for optimization in LBE system designs. The task includes two

Ning Li

342

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics  

Microsoft Academic Search

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of

Chengku Liu; Ghazi A. Karim

2008-01-01

343

Is Case-Based Learning an Effective Teaching Strategy to Challenge Students' Alternative Conceptions regarding Chemical Kinetics?  

ERIC Educational Resources Information Center

|Background: Case-based learning (CBL) is simply teaching the concept to the students based on the cases. CBL involves a case, which is a scenario based on daily life, and study questions related to the case, which allows students to discuss their ideas. Chemical kinetics is one of the most difficult concepts for students in chemistry. Students…

Yalcinkaya, Eylem; Tastan-Kirik, Ozgecan; Boz, Yezdan; Yildiran, Demet

2012-01-01

344

Thermal diffusion and chemical kinetics in laminar biomaterial due to heating by a free-electron laser  

Microsoft Academic Search

We have theoretically investigated the role of thermal diffusion and chemical kinetics as a possible dynamic explanation for the preferential ablative properties of infrared radiation from a free-electron laser ~FEL!. The model is based on a laminar system composed of alternating layers of protein and saline. We have compared exposure to 3 mm where water is the main absorber and

M. Shane Hutson; Susanne A. Hauger; Glenn Edwards

2002-01-01

345

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.

Jung, Moon Chul; Weber, Stephen G.

2006-01-01

346

DFT calculations of 1H and 13C NMR chemical shifts in transition metal hydrides.  

PubMed

Transition metal hydrides are of great interest in chemistry because of their reactivity and their potential use as catalysts for hydrogenation. Among other available techniques, structural properties in transition metal (TM) complexes are often probed by NMR spectroscopy. In this paper we will show that it is possible to establish a viable methodological strategy in the context of density functional theory, that allows the determination of 1H NMR chemical shifts of hydride ligands attached to transition metal atoms in mononuclear systems and clusters with good accuracy with respect to experiment. 13C chemical shifts have also been considered in some cases. We have studied mononuclear ruthenium complexes such as Ru(L)(H)(dppm)2 with L = H or Cl, cationic complex [Ru(H)(H2O)(dppm)2]+ and Ru(H)2(dppm)(PPh3)2, in which hydride ligands are characterized by a negative 1H NMR chemical shift. For these complexes all calculations are in relatively good agreement compared to experimental data with errors not exceeding 20% except for the hydrogen atom in Ru(H)2(dppm)(PPh3)2. For this last complex, the relative error increases to 30%, probably owing to the necessity to take into account dynamical effects of phenyl groups. Carbonyl ligands are often encountered in coordination chemistry. Specific issues arise when calculating 1H or 13C NMR chemical shifts in TM carbonyl complexes. Indeed, while errors of 10 to 20% with respect to experiment are often considered good in the framework of density functional theory, this difference in the case of mononuclear carbonyl complexes culminates to 80%: results obtained with all-electron calculations are overall in very satisfactory agreement with experiment, the error in this case does not exceed 11% contrary to effective core potentials (ECPs) calculations which yield errors always larger than 20%. We conclude that for carbonyl groups the use of ECPs is not recommended, although their use could save time for very large systems, for instance in cluster chemistry. The reliance of NMR chemical shielding on dynamical effects, such as intramolecular rearrangements or trigonal twists, is also examined for H2Fe(CO)4, K+[HFe(CO)](-), HMn(CO)5 and HRe(CO)5. The accuracy of the theory is also examined for complexes with two dihydrogen ligands (Tp*RuH(H2)2 and [FeH(H2)(DMPE)2]+) and a ruthenium cluster, [H3Ru4(C6H6)4(CO)]+. It is shown that for all complexes studied in this work, the effect of the ligands on the chemical shielding of hydrogen coordinated to metal is suitably calculated, thus yielding a very good correlation between experimental chemical shifts and theoretical chemical shielding. PMID:18648699

del Rosal, I; Maron, L; Poteau, R; Jolibois, F

2008-05-07

347

Chemical kinetics of nitric oxide in the CFFF furnace without a slag screen  

SciTech Connect

The nitric oxide (NO) data for the LMF1E test series conducted without the slag screen in the Coal-Fired Flow Facility (CFFF) radiant furnace were analyzed by comparing the experimental results with values calculated with a gas kinetics code. The results of the analysis indicate that the accuracy of the gas temperature data is more important in predicting NO concentration than are the differences in reaction rates, reaction models, or gas flow models considered. Without the slag screen, the NO decomposition in the furnace inlet was only slightly affected by the small fraction of the gas that circulates through the lower furnace. Correct modeling of the gas flow at the furnace inlet was also important but less important than when the slag screen was in place. Adjusting the rate for the reaction, N/sub 2/ + O = NO + N, also improved the predictions; however, the adjustment needs to be justified. When the gas temperatures were input into a gas flow model, which approximates the gas flow in the furnace inlet and through the upper furnace, good agreement was obtained between the calculated and measured NO concentrations in the CFFF furnace. 12 refs., 14 figs.

Chow, L.S.H.; Wang, C.S.; Lyczkowski, R.W.

1985-01-01

348

A dynamic multi-timescale method for combustion modeling with detailed and reduced chemical kinetic mechanisms  

SciTech Connect

A new on-grid dynamic multi-timescale (MTS) method is presented to increase significantly the computation efficiency involving multi-physical and chemical processes using detailed and reduced kinetic mechanisms. The methodology of the MTS method using the instantaneous timescales of different species is introduced. The definition of the characteristic time for species is examined and compared with that of the computational singular perturbation (CSP) and frozen reaction rate methods by using a simple reaction system. A hybrid multi-timescale (HMTS) algorithm is constructed by integrating the MTS method with an implicit Euler scheme, respectively, for species with and without the requirement of accurate time histories at sub-base timescales. The efficiency and the robustness of the MTS and HMTS methods are demonstrated by comparing with the Euler and VODE solvers for homogenous ignition and unsteady flame propagation of hydrogen, methane, and n-decane-air mixtures. The results show that both MTS and HMTS reproduce well the species and temperature histories and are able to decrease computation time by about one-order with the same kinetic mechanism. Compared to MTS, HMTS has slightly better computation efficiency but scarifies the stability at large base time steps. The results also show that with the increase of mechanism size and the decrease of time step, the computation efficiency of multi-timescale method increases compared to the VODE solver. In addition, it is shown that the integration of the multi-timescale method with the path flux analysis based mechanism reduction approach can further increase the computation efficiency. Unsteady simulations of outwardly propagating spherical n-decane-air premixed flames demonstrate that the multi-timescale method is rigorous for direct numerical simulations with both detailed and reduced chemistry and can dramatically improve the computation efficiency. (author)

Gou, Xiaolong [Department of Mechanical and Aerospace Engineering, Princeton University, NJ 08544 (United States); College of Power Engineering, Chongqing University, Chongqing 400030 (China); Sun, Wenting; Ju, Yiguang [Department of Mechanical and Aerospace Engineering, Princeton University, NJ 08544 (United States); Chen, Zheng [Department of Mechanical and Aerospace Engineering, Princeton University, NJ 08544 (United States); School of Engineering, Peking University, Beijing 100871 (China)

2010-06-15

349

Analysis of chemical reaction kinetics of depredating organic pollutants from secondary effluent of wastewater treatment plant in constructed wetlands.  

PubMed

Four subsurface constructed wetlands were built to treat the secondary effluent of a wastewater treatment plant in Tangshan, China. The chemical pollutant indexes of chemical oxygen demand (COD) were analyzed to evaluate the removal efficiency of organic pollutants from the secondary effluent of the wastewater treatment plant. In all cases, the subsurface constructed wetlands were efficient in treating organic pollutants. Under the same hydraulic loading condition, the horizontal flow wetlands exhibited better efficiency of COD removal than vertical flow wetlands: the removal rates in horizontal flow wetlands could be maintained at 68.4 ± 2.42% to 92.2 ± 1.61%, compared with 63.8 ± 1.19% to 85.0 ± 1.25% in the vertical flow wetlands. Meanwhile, the chemical reaction kinetics of organic pollutants was analyzed, and the results showed that the degradation courses of the four subsurface wetlands all corresponded with the first order reaction kinetics to a large extent. PMID:23168635

Wang, Hao; Jiang, Dengling; Yang, Yong; Cao, Guoping

2013-01-01

350

Topology maps of bond descriptors based on the kinetic energy density and the essence of chemical bonding.  

PubMed

Analysis of the kinetic energy density within a molecule identifies patterns in its electronic structure that are intuitively linked to familiar concepts of chemical bonding. The function ?(r), termed localized-orbital locator and based on the positive-definite kinetic energy density ?+, is employed to characterize classes of covalent bonds in terms of its full topology of all critical points of rank three. Not only does ?(r) reveal patterns in chemical bonding, it also discloses features and the influence of extended electronic cores. Gradient paths define the extension of the valence space around an atomic centre; they separate various core level regions from bonding domains, and partition molecules in sensible bonded subunits. Location and ?(r)-values of critical points add a quantitative aspect to the bond characterization; profiles of unconventional chemical linkages such as charge-shift bonds emerge in a natural way. PMID:23450168

Jacobsen, Heiko

2013-04-14

351

Effect of kinetic and configurational thermostats on calculations of the first normal stress coefficient in nonequilibrium molecular dynamics simulations  

NASA Astrophysics Data System (ADS)

Thermostats for homogeneous nonequilibrium molecular dynamics simulations are usually designed to control the kinetic temperature, but it is now possible control any combination of many different types of temperature, including the configurational and kinetic temperatures and their directional components. It is well known that these temperatures can become unequal in homogeneously thermostatted shearing steady states. The microscopic expressions for these temperatures are all derived from equilibrium distribution functions, and it is pertinent to ask, what are the consequences of using these equilibrium microscopic expressions for temperature in thermostats for shearing nonequilibrium steady states? Here we show that the answer to this question depends on which properties are being investigated. We present numerical results showing that the value of the zero shear rate viscosity obtained by extrapolating results of nonequilibrium molecular dynamics simulations of shearing steady states is the same, regardless of the type of temperature that is controlled. It also agrees with the value obtained from the equilibrium stress autocorrelation function via the Green-Kubo relation. However, the values of the limiting zero shear rate first normal stress coefficient obtained from nonequilibrium molecular dynamics simulations of shearing steady states are strongly dependent on the choice of temperature being controlled. They also differ from the value of the first normal stress coefficient that is calculated from the equilibrium stress autocorrelation function. We show that even when all of the directional components of the kinetic and configurational temperatures are simultaneously controlled to the same value, the agreement with the result obtained from the equilibrium stress autocorrelation function is poor.

Daivis, Peter J.; Dalton, Benjamin A.; Morishita, Tetsuya

2012-11-01

352

Theoretical and practical aspects of chemical functionalization of carbon nanofibers (CNFs): DFT calculations and adsorption study.  

PubMed

The nitric acid-functionalized commercial carbon nanofibers (CNFs) were comprehensively studied by instrumental (XRD, BET, SEM, TGA) and theoretical (DFT calculations) methods. The detailed surface study revealed the variation in the characteristics of functionalized CNFs, such as a decreased (up to 34%) surface area and impacted structural, electronic and chemical properties. The effects of functional groups were studied by comparison with pristine nanofibers. The results showed that the C-C bond lengths of the modified CNFs varied significantly. Chemical functionalization altered the frontier orbitals of the pristine material, and therefore altered the nature of their interactions with other substances. Moreover, the pristine and modified CNFs were tested for the removal of phenol from aqueous solutions. It was observed that surface modification tuned the adsorption capacity of carbon nanofibers (up to 0.35 mmol g(-1)), whereas original fibers did not demonstrate any adsorption capacity of phenol. PMID:22209137

Rokhina, Ekaterina V; Lahtinen, Manu; Makarova, Katerina; Jegatheesan, Veeriah; Virkutyte, Jurate

2011-12-14

353

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations  

SciTech Connect

The gas-phase reaction between calcium monocation and fluoromethane: Ca{sup +}+CH{sub 3}F{yields}CaF{sup +}+CH{sub 3} was theoretically analyzed. The potential energy hypersurface was explored by using density functional theory methodology with different functionals and Pople's, Dunning's, Ahlrichs', and Stuttgart-Dresden basis sets. Kinetics calculations (energy and total angular momentum resolved microcanonical variational/conventional theory) were accomplished. The theoretically predicted range for the global kinetic rate constant values at 295 K (7.2x10{sup -11}-5.9x10{sup -10} cm{sup 3} molecule{sup -1} s{sup -1}) agrees reasonably well with the experimental value at the same temperature [(2.6{+-}0.8)x10{sup -10} cm{sup 3} molecule{sup -1} s{sup -1}]. Explicit consideration of a two transition state model, where the formation of a weakly bounded prereactive complex is preceded by an outer transition state (entrance channel) and followed by an inner transition state connecting with a second intermediate that finally leads to products, is mandatory. Experimental observations on the correlation, or lack of correlation, between reaction rate constants and second ionization energies of the metal might well be rationalized in terms of this two transition state model.

Varela-Alvarez, Adrian; Sordo, Jose A. [Departamento de Quimica Fisica y Analitica, Laboratorio de Quimica Computacional, Facultad de Quimica, Universidad de Oviedo, Julian Claveria, 8. 33006 Oviedo, Principado de Asturias (Spain); Rayon, V. M.; Redondo, P.; Barrientos, C. [Departamento de Quimica Fisica y Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid (Spain)

2009-10-14

354

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations  

NASA Astrophysics Data System (ADS)

The gas-phase reaction between calcium monocation and fluoromethane: Ca++CH3F-->CaF++CH3 was theoretically analyzed. The potential energy hypersurface was explored by using density functional theory methodology with different functionals and Pople's, Dunning's, Ahlrichs', and Stuttgart-Dresden basis sets. Kinetics calculations (energy and total angular momentum resolved microcanonical variational/conventional theory) were accomplished. The theoretically predicted range for the global kinetic rate constant values at 295 K (7.2×10-11-5.9×10-10 cm3 molecule-1 s-1) agrees reasonably well with the experimental value at the same temperature [(2.6+/-0.8)×10-10 cm3 molecule-1 s-1]. Explicit consideration of a two transition state model, where the formation of a weakly bounded prereactive complex is preceded by an outer transition state (entrance channel) and followed by an inner transition state connecting with a second intermediate that finally leads to products, is mandatory. Experimental observations on the correlation, or lack of correlation, between reaction rate constants and second ionization energies of the metal might well be rationalized in terms of this two transition state model.

Varela-Álvarez, Adrián; Rayón, V. M.; Redondo, P.; Barrientos, C.; Sordo, José A.

2009-10-01

355

Validation of hypersonic chemical equilibrium flow calculations using ballistic-range data  

NASA Astrophysics Data System (ADS)

The hypersonic flow field over a sphere flying in a ballistic-range is numerically simulated for the purpose of validating a hypersonic chemical equilibrium flow solver. The numerical results obtained are compared with available experimental data on the stand-off distance and the shape of the detached bow shock wave. In the calculation, an adaptive mesh is employed for a crisp capturing of the shock wave. Comparison with the experimental data reveals that the equilibrium flow solver can yield a fairly accurate prediction of the flow field.

Sawada, K.; Dendou, E.

356

Calculation of the mixing chamber of an ejector chemical oxygen - iodine laser  

SciTech Connect

Gas parameters are calculated at the outlet of the mixing chamber of an ejector chemical oxygen-iodine laser with a nozzle unit consisting of nozzles of three types, which provides a total pressure of the active medium that substantially exceeds a pressure in the generator of singlet oxygen. This technique of forming the laser active medium substantially facilitates the ejection of the exhaust gas to the atmosphere by using a diffuser and single-stage vacuum systems based on water circulating pumps. (lasers, active media)

Zagidullin, M V; Nikolaev, V D [Samara Branch of the P.N. Lebedev Physics Institute, Russian Academy of Sciences, Samara (Russian Federation)

2001-06-30

357

A reaction-based paradigm to model reactive chemical transport in groundwater with general kinetic and equilibrium reactions  

NASA Astrophysics Data System (ADS)

This paper presents a reaction-based water quality transport model in subsurface flow systems. Transport of chemical species with a variety of chemical and physical processes is mathematically described by M partial differential equations (PDEs). Decomposition via Gauss Jordan column reduction of the reaction network transforms M species reactive transport equations into two sets of equations: a set of thermodynamic equilibrium equations representing NE equilibrium reactions and a set of reactive transport equations of M NE kinetic-variables involving no equilibrium reactions (a kinetic-variable is a linear combination of species). The elimination of equilibrium reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A variety of numerical methods are investigated for solving the coupled transport and reaction equations. Simulation comparisons with exact solutions were performed to verify numerical accuracy and assess the effectiveness of various numerical strategies to deal with different application circumstances. Two validation examples involving simulations of uranium transport in soil columns are presented to evaluate the ability of the model to simulate reactive transport with complex reaction networks involving both kinetic and equilibrium reactions.

Zhang, Fan; Yeh, Gour-Tsyh; Parker, Jack C.; Brooks, Scott C.; Pace, Molly N.; Kim, Young-Jin; Jardine, Philip M.; Watson, David B.

2007-06-01

358

The influence of chemical mechanisms on PDF calculations of non-premixed turbulent flames  

NASA Astrophysics Data System (ADS)

A series of calculations is reported of the Barlow & Frank non-premixed piloted jet flames D, E and F, with the aim of determining the level of description of the chemistry necessary to account accurately for the turbulence-chemistry interactions observed in these flames. The calculations are based on the modeled transport equation for the joint probability density function of velocity, turbulence frequency and composition (enthalpy and species mass fractions). Seven chemical mechanisms for methane are investigated, ranging from a five-step reduced mechanism to the 53-species GRI 3.0 mechanism. The results show that, for C-H-O species, accurate results are obtained with the GRI 2.11 and GRI 3.0 mechanisms, as well as with 12 and 15-step reduced mechanisms based on GRI 2.11. But significantly inaccurate calculations result from use of the 5-step reduced mechanism (based on GRI 2.11), and from two different 16-species skeletal mechanisms. As has previously been observed, GRI 3.0 over-predicts NO by up to a factor of two; whereas NO is calculated reasonably accurately by GRI 2.11 and the 15-step reduced mechanism.

Pope, Stephen B.

2005-11-01

359

Ab initio calculations of deuterium isotope effects on chemical shifts of salt-bridged lysines.  

PubMed

Deuterium isotope effects measure the change in chemical shift on substitution of a proton by deuterium. They have been calculated by direct treatment of the H/D nuclear quantum effect using a multicomponent ab initio molecular orbital method based on a non-Born-Oppenheimer approximation. This method enables the determination of both the electronic and the protonic (deuteronic) wave functions simultaneously and can directly calculate the geometrical difference induced by H/D isotope effects. The calculations show that the one-bond deuterium isotope effects on (15)N nuclear shielding, (1)?(15)N(D), in ammonium and amines decrease as a counterion or water molecule moves closer to the nitrogen. (1)?(15)N(D) and (2)?(1)H(D) of the NH(3)(+) groups of lysine residues in the B1 domain of protein G have been calculated using truncated side chains and also determined experimentally by NMR. Comparisons show that the structures in solution are different from those in the crystal and that solvation plays an important role in weakening the hydrogen bonds. PMID:21388110

Ullah, Saif; Ishimoto, Takayoshi; Williamson, Mike P; Hansen, Poul Erik

2011-03-09

360

Spectroscopic Studies of Azide compounds: Thermochemistry, Chemical Kinetics and Photodissociation Dynamics  

NASA Astrophysics Data System (ADS)

Some of the most difficult chemical systems, either to observe or produce in significant quantities, are polynitrogen molecules. One example of this type of molecules in the early stages of investigation is cyclic-N3, whose molecular geometry and promising technological applications have attracted our attention to define optimal experimental conditions for being photoproduced. High-resolution synchrotron-radiation-based Photoionization Mass Spectrometry (PIMS) was applied to study the dissociative photoionization of three azide precursors for cyclic-N3; chlorine azide (ClN3), hydrogen azide (HN3), and methyl azide (CH3N3). In our attempts to detect cyclic-N3, the thermochemistry derived in the PIMS studies stimulated our work to perform photodissociation dynamics experiments of CH3N3 at 193 nm using Photofragment Translational Spectroscopy (PTS) with electron impact (EI) detection under collision-free conditions, and chemical kinetic studies based on Infrared Spectroscopy (IR) in matrix-isolated ices formed from rare gases (Argon, Nitrogen and Xenon). PTS experiments lead us to derive the branching ratio between reactions CH 3+N3 (radical channel) vs CH3N+N2 (molecular channel), and to conclude that cyclic-N3 is the dominant product in the radical channel. In contrast, in the matrix isolation experiments we found no evidence of the radical channel, possibly due to barrier-less recombination. However, since no mechanistic reports of methyl azide dissociation exist at these conditions, these studies could have significant implications for future experiments addressed to detect cylic-N3 under matrix environments.

Quinto Hernandez, Alfredo

361

Statistical analysis of chemical transformation kinetics using Markov-Chain Monte Carlo methods.  

PubMed

For the risk assessment of chemicals intentionally released into the environment, as, e.g., pesticides, it is indispensable to investigate their environmental fate. Main characteristics in this context are transformation rates and partitioning behavior. In most cases the relevant parameters are not directly measurable but are determined indirectly from experimentally determined concentrations in various environmental compartments. Usually this is done by fitting mathematical models, which are usually nonlinear, to the observed data and such deriving estimates of the parameter values. Statistical analysis is then used to judge the uncertainty of the estimates. Of particular interest in this context is the question whether degradation rates are significantly different from zero. Standard procedure is to use nonlinear least-squares methods to fit the models and to estimate the standard errors of the estimated parameters from Fisher's Information matrix and estimated level of measurement noise. This, however, frequently leads to counterintuitive results as the estimated probability distributions of the parameters based on local linearization of the optimized models are often too wide or at least differ significantly in shape from the real distribution. In this paper we identify the shortcoming of this procedure and propose a statistically valid approach based on Markov-Chain Monte Carlo sampling that is appropriate to determine the real probability distribution of model parameters. The effectiveness of this method is demonstrated on three data sets. Although it is generally applicable to different problems where model parameters are to be inferred, in the present case for simplicity we restrict the discussion to the evaluation of metabolic degradation of chemicals in soil. It is shown that the method is successfully applicable to problems of different complexity. We applied it to kinetic data from compounds with one and five metabolites. Additionally, using simulated data, it is shown that the MCMC method estimates the real probability distributions of parameters well and much better than the standard optimization approach. PMID:21526818

Görlitz, Linus; Gao, Zhenglei; Schmitt, Walter

2011-04-28

362

Charge transfer in porphyrin-calixarene complexes: ultrafast kinetics, cyclic voltammetry, and DFT calculations.  

PubMed

Transient absorption spectroscopy, cyclic voltammetry, and DFT calculations were used to describe charge transfer processes in a series of 5,10,15,20-tetrakis(N-methylpyridinium-n-yl) porphyrins (TMPyPn, n = 4,3,2) and TMPyPn/p-sulfonatocalix[m]arene (clxm, m = 4,6,8) complexes. Excitation of TMPyPn is accompanied by an increasing electron density at the methylpyridinium substituents in the order TMPyP2 < TMPyP3 < TMPyP4. The quenching of the excited singlet states of the complexes increases with the number of ionized phenolic groups of clxm and can be correlated with the partial transfer of the electron density from O(-) to the peripheral methylpyridinium substituents rather than to the porphyrin ring. PMID:21399801

Kubát, Pavel; Šebera, Jakub; Záliš, Stanislav; Langmaier, Jan; Fuciman, Marcel; Polívka, Tomáš; Lang, Kamil

2011-03-11

363

Quasiclassical calculation of the chemical reaction Ba+C3H7Br?BaBr+C3H7  

NASA Astrophysics Data System (ADS)

The quasi-classical trajectory (QCT) method based on the extended London—Eyring—Polanyi—Sato potential energy surface is used to investigate the product vibrational distribution, angular distribution and angle-resolved kinetic distribution of the reaction Ba+C3H7Br?BaBr+C3H7 at 2.58 kcal/mol. The calculated results show that the product BaBr vibrational distribution is quite hot, the vibrational population peaks are located at v = 12, and the angular product distribution tends to backward scattering. The calculated angle-resolved kinetic distribution shows that the kinetic distribution is obviously related to angle. The QCT results are always qualitatively acceptable and sometimes even quantitatively.

Jiang, Yong-Chao; Li, Gui-Xia; Shi, Xiao-Feng; Huang, Shu-Lai

2012-12-01

364

Temperature rule for the speed of sound in water: a chemical kinetics model  

PubMed

Water forms three-dimensional polymeric structures due to the influence of hydrogen bonds and is fundamentally different from other substances. One of the simplest ways to analyze the structure of water in any system, such as hydration, is to measure the degree of compressibility, which can be determined from the speed of sound, by making use of the physical laws established by Newton and later perfected by Laplace. Although the speed of sound is strongly dependent on the temperature of a liquid, Laplace's equation does not refer to temperature in any of its terms. It is necessary, therefore, to determine the degree of temperature dependency. However, only approximate expressions of a fifth-order polynomial have been reported so far in the literature. In this paper, a universal method for describing the speed of sound from the perspective of physicochemical reaction kinetics is presented. It is shown that the speed of sound U [ms(-1)] changes with temperature T [K] according to a thermodynamically-derived formula given as U= exp(-A/T-BlnT+C) and that the motion and propagation phenomena of sound energy can also be regarded as chemical reactions. PMID:11039524

Okazaki

2000-09-15

365

The Coupling of Related Demonstrations to Illustrate Principles in Chemical Kinetics and Equilibrium  

NASA Astrophysics Data System (ADS)

Two very simple lecture demonstrations, both involving the reaction of magnesium with one or more dilute acids, are linked together to illustrate principles in chemical kinetics and equilibrium. In the first, crumpled Mg ribbon is placed in the nipple of a baby bottle holding 200 mL of 0.40 M HCl. The bottle is inverted into a large beaker of water, and the volume of H2 gas generated in one minute is measured. the experiment is repeated with 0.60 M HCl. The rate law, Rate = k[H+]n, is developed from the data. In the second, equal lengths of Mg ribbon are placed in small beakers or Petri dishes, on an overhead projector, containing equal (0.80 to 1.0 M) concentrations of HCl, H3BO3, and CH3CO2H. Acids are not identified; students are merely told that 'Acids A, B, and C are of the same molarity.' Students are then asked to explain why the rates are so different, which serves as a lead-in for the instructor to explain the meaning of a Ka value. Students readily conclude that one of the acids must be a strong acid, but are puzzled by the other two. [The enormous difference in the Ka values of acetic and boric acids results in a striking difference in their reaction rates.

Pacer, Richard A.

1997-05-01

366

Detailed chemical kinetic reaction mechanism for oxidation of n-octane and iso-octane  

SciTech Connect

The development of detailed chemical kinetic reaction mechanisms for oxidation of n-octane and iso-octane is described, with emphasis on the factors which are specific to many large hydrocarbon fuel molecules. Elements which are of particular importance are found to include site-specific abstraction of H atoms, radical isomerization of alkyl radicals by internal H atom abstraction, and rapid ..beta..-scission of the alkyl radicals. These features, combined with distinctions in the types of intermediate olefin species produced, are used to explain the significant differences in the rate of oxidation between n-octane and iso-octane. Experimental results from the turbulent flow reactor and low pressure laminar flames, using both n-octane and iso-octane as fuels, are used to test the reaction mechanisms and indicate those parts of the total mechanisms which are in greatest need of further development and refinement. It is found that the submechanisms for consumption of the C/sub 2/ - C/sub 4/ olefins need further attention, particularly the identification of the major product species distributions and their temperature dependence for reactions of these olefins and radicals including O and OH. 24 refs., 3 figs., 4 tabs.

Axelsson, E.I.; Brezinsky, K.; Dryer, F.L.; Pitz, W.J.; Westbrook, C.K.

1986-04-15

367

Effect of excluded volume on 2D discrete stochastic chemical kinetics  

SciTech Connect

The stochastic simulation algorithm (SSA) is widely used in the discrete stochastic simulation of chemical kinetics. The propensity functions which play a central role in this algorithm have been derived under the point-molecule assumption, i.e., that the total volume of the molecules is negligible compared to the volume of the container. It has been shown analytically that for a one-dimensional system and the A + A reaction, when the point-molecule assumption is relaxed, the propensity function need only be adjusted by replacing the total volume of the system with the free volume of the system. In this paper we investigate via numerical simulations the impact of relaxing the point-molecule assumption in two dimensions. We find that the distribution of times to the first collision is close to exponential in most cases, so that the formalism of the propensity function is still applicable. In addition, we find that the area excluded by the molecules in two dimensions is usually higher than their close-packed area, requiring a larger correction to the propensity function than just the replacement of the total volume by the free volume.

Lampoudi, Sotiria [Dept. of Computer Science, University of California, Santa Barbara, CA 93106 (United States)], E-mail: slampoud@cs.ucsb.edu; Gillespie, Dan T. [Dan T Gillespie Consulting, 30504 Cordoba Place, Castaic, CA 91384 (United States); Petzold, Linda R. [Dept. of Computer Science, University of California, Santa Barbara, CA 93106 (United States)

2009-06-01

368

A multiple shock tube and chemical kinetic modeling study of diethyl ether pyrolysis and oxidation.  

PubMed

The pyrolysis and oxidation of diethyl ether (DEE) has been studied at pressures from 1 to 4 atm and temperatures of 900-1900 K behind reflected shock waves. A variety of spectroscopic diagnostics have been used, including time-resolved infrared absorption at 3.39 mum and time-resolved ultraviolet emission at 431 nm and absorption at 306.7 nm. In addition, a single-pulse shock tube was used to measure reactant, intermediate, and product species profiles by GC samplings at different reaction times varying from 1.2 to 1.8 ms. A detailed chemical kinetic model comprising 751 reactions involving 148 species was assembled and tested against the experiments with generally good agreement. In the early stages of reaction the unimolecular decomposition and hydrogen atom abstraction of DEE and the decomposition of the ethoxy radical have the largest influence. In separate experiments at 1.9 atm and 1340 K, it is shown that DEE inhibits the reactivity of an equimolar mixture of hydrogen and oxygen (1% of each). PMID:20690588

Yasunaga, K; Gillespie, F; Simmie, J M; Curran, H J; Kuraguchi, Y; Hoshikawa, H; Yamane, M; Hidaka, Y

2010-09-01

369

Kinetics of silicon nitride chemical vapor deposition from silicon tetrafluoride and ammonia  

SciTech Connect

Rate laws for the chemical vapor deposition of Si[sub 3]N[sub 4] from SiF[sub 4] and NH[sub 3] are obtained by fitting the results of parametric reactor experiments with a one-dimensional steady-state model for the reactor. The model includes axial mass transport by both convection and multicomponent diffusion, and allowance is made for the use of an experimentally determined temperature profile. Global kinetic expressions with adjustable constants are used to account for deposition on both crystalline and amorphous surfaces, as well as the heterogeneous decomposition of NH[sub 3]. In addition, there are mechanisms that determine the actual degree of surface crystallinity at any location. Optimum values for the rate constants are found by searching for the best overall fit to the experimental deposition rate and crystallinity data. It is shown that the model, with simple second-order expressions for the deposition rates, is quite successful in reproducing the experimentally observed effects of temperature, flow rate, reactant mole ratio, and axial position.

Larson, R.S. (Sandia National Labs., Livermore, CA (United States))

1993-08-01

370

Kinetic and thermodynamic control of protonation in atmospheric pressure chemical ionization.  

PubMed

For p-(dimethylamino)chalcone (p-DMAC), the N atom is the most basic site in the liquid phase, whereas the O atom possesses the highest proton affinity in the gas phase. A novel and interesting observation is reported that the N- and O-protonated p-DMAC can be competitively produced in atmospheric pressure chemical ionization (APCI) with the change of solvents and ionization conditions. In neat methanol or acetonitrile, the protonation is always under thermodynamic control to form the O-protonated ion. When methanol/water or acetonitrile/water was used as the solvent, the protonation is kinetically controlled to form the N-protonated ion under conditions of relatively high infusion rate and high concentration of water in the mixed solvent. The regioselectivity of protonation of p-DMAC in APCI is probably attributed to the bulky solvent cluster reagent ions (S(n)H(+)) and the analyte having different preferred protonation sites in the liquid phase and gas phase. PMID:23633014

Chai, Yunfeng; Hu, Nan; Pan, Yuanjiang

2013-04-30

371

Kinetic and Thermodynamic Control of Protonation in Atmospheric Pressure Chemical Ionization  

NASA Astrophysics Data System (ADS)

For p-(dimethylamino)chalcone (p-DMAC), the N atom is the most basic site in the liquid phase, whereas the O atom possesses the highest proton affinity in the gas phase. A novel and interesting observation is reported that the N- and O-protonated p-DMAC can be competitively produced in atmospheric pressure chemical ionization (APCI) with the change of solvents and ionization conditions. In neat methanol or acetonitrile, the protonation is always under thermodynamic control to form the O-protonated ion. When methanol/water or acetonitrile/water was used as the solvent, the protonation is kinetically controlled to form the N-protonated ion under conditions of relatively high infusion rate and high concentration of water in the mixed solvent. The regioselectivity of protonation of p-DMAC in APCI is probably attributed to the bulky solvent cluster reagent ions (SnH+) and the analyte having different preferred protonation sites in the liquid phase and gas phase.

Chai, Yunfeng; Hu, Nan; Pan, Yuanjiang

2013-04-01

372

Kinetic and Thermodynamic Control of Protonation in Atmospheric Pressure Chemical Ionization  

NASA Astrophysics Data System (ADS)

For p-(dimethylamino)chalcone ( p-DMAC), the N atom is the most basic site in the liquid phase, whereas the O atom possesses the highest proton affinity in the gas phase. A novel and interesting observation is reported that the N- and O-protonated p-DMAC can be competitively produced in atmospheric pressure chemical ionization (APCI) with the change of solvents and ionization conditions. In neat methanol or acetonitrile, the protonation is always under thermodynamic control to form the O-protonated ion. When methanol/water or acetonitrile/water was used as the solvent, the protonation is kinetically controlled to form the N-protonated ion under conditions of relatively high infusion rate and high concentration of water in the mixed solvent. The regioselectivity of protonation of p-DMAC in APCI is probably attributed to the bulky solvent cluster reagent ions (SnH+) and the analyte having different preferred protonation sites in the liquid phase and gas phase.

Chai, Yunfeng; Hu, Nan; Pan, Yuanjiang

2013-07-01

373

A path flux analysis method for the reduction of detailed chemical kinetic mechanisms  

SciTech Connect

A direct path flux analysis (PFA) method for kinetic mechanism reduction is proposed and validated by using high temperature ignition, perfect stirred reactors, and steady and unsteady flame propagations of n-heptane and n-decane/air mixtures. The formation and consumption fluxes of each species at multiple reaction path generations are analyzed and used to identify the important reaction pathways and the associated species. The formation and consumption path fluxes used in this method retain flux conservation information and are used to define the path indexes for the first and the second generation reaction paths related to a targeted species. Based on the indexes of each reaction path for the first and second generations, different sized reduced chemical mechanisms which contain different number of species are generated. The reduced mechanisms of n-heptane and n-decane obtained by using the present method are compared to those generated by the direct relation graph (DRG) method. The reaction path analysis for n-decane is conducted to demonstrate the validity of the present method. The comparisons of the ignition delay times, flame propagation speeds, flame structures, and unsteady spherical flame propagation processes showed that with either the same or significantly less number of species, the reduced mechanisms generated by the present PFA are more accurate than that of DRG in a broad range of initial pressures and temperatures. The method is also integrated with the dynamic multi-timescale method and a further increase of computation efficiency is achieved. (author)

Sun, Wenting; Ju, Yiguang [Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544 (United States); Chen, Zheng [State Key Laboratory for Turbulence and Complex Systems, College of Engineering, Peking University, Beijing 100871 (China); Gou, Xiaolong [School of Power Engineering, Chongqing University, Chongqing 400044 (China)

2010-07-15

374

Numerical Simulation of Compressible Viscous Magneto-Hydrodynamics Equations with Chemical Kinetics  

NASA Astrophysics Data System (ADS)

Most of the objectives of this research project have been achieved. In this first phase of the AFOSR grant (1 October 1998 - 30 September 2002), the principal investigator and his students have developed a 2-D unsteady compressible viscous magnetohydrodynamic code designated MHD2D which has been validated for 2-D internal and external flow & The code solves the coupled MHD equations (mass. momentum and energy equations of fluid flow including MHD effects (Lorentz force and Joule heating), magnetic induction equations and Maxwell equations) and includes an equilibrium air model for re gas effects, a finite-rate chemical kinetics model for dissociated air, several electrical conductivity models and a bi-temperature model. I-his code has been employed to investigate the concept of supersonic drag and heat transfer reduction by modification/dissipation of shock waves in the presence of strong magnetic fields. A series of numerical experiments for blunt body flows and scramjet inlet flow fields have been conducted by varying the Mach number, Reynolds number, degree of ionization of the air plasma and the intensity of the magnetic field to understand the physics of the phenomena and its potential for supersonic drag reduction in practical applications.

Agarwal, Ramesh K.

2002-09-01

375

Features in chemical kinetics. II. A self-emerging definition of slow manifolds  

NASA Astrophysics Data System (ADS)

In the preceding paper of this series (Part I [P. Nicolini and D. Frezzato, J. Chem. Phys. 138, 234101 (2013)]) we have unveiled some ubiquitous features encoded in the systems of polynomial differential equations normally applied in the description of homogeneous and isothermal chemical kinetics (mass-action law). Here we proceed by investigating a deeply related feature: the appearance of so-called slow manifolds (SMs) which are low-dimensional hyper-surfaces in the neighborhood of which the slow evolution of the reacting system occurs after an initial fast transient. Indeed a geometrical definition of SM, devoid of subjectivity, ``naturally'' follows in terms of a specific sub-dimensional domain embedded in the peculiar region of the concentrations phase-space that in Part I we termed as ``attractiveness region.'' Numerical inspections on simple low-dimensional model cases are presented, including the benchmark case of Davis and Skodje [J. Chem. Phys. 111, 859 (1999)] and the preliminary analysis of a simplified model mechanism of hydrogen combustion.

Nicolini, Paolo; Frezzato, Diego

2013-06-01

376

Features in chemical kinetics. II. A self-emerging definition of slow manifolds.  

PubMed

In the preceding paper of this series (Part I [P. Nicolini and D. Frezzato, J. Chem. Phys. 138, 234101 (2013)]) we have unveiled some ubiquitous features encoded in the systems of polynomial differential equations normally applied in the description of homogeneous and isothermal chemical kinetics (mass-action law). Here we proceed by investigating a deeply related feature: the appearance of so-called slow manifolds (SMs) which are low-dimensional hyper-surfaces in the neighborhood of which the slow evolution of the reacting system occurs after an initial fast transient. Indeed a geometrical definition of SM, devoid of subjectivity, "naturally" follows in terms of a specific sub-dimensional domain embedded in the peculiar region of the concentrations phase-space that in Part I we termed as "attractiveness region." Numerical inspections on simple low-dimensional model cases are presented, including the benchmark case of Davis and Skodje [J. Chem. Phys. 111, 859 (1999)] and the preliminary analysis of a simplified model mechanism of hydrogen combustion. PMID:23802946

Nicolini, Paolo; Frezzato, Diego

2013-06-21

377

An evaluation of the equilibrium calculations within acidification models: The effect of uncertainty in measured chemical components  

NASA Astrophysics Data System (ADS)

There is much concern over the effects of acidic deposition on soils and drainage waters. To better understand the processes regulating the acidification of surface waters, computer models have been developed which utilize equilibrium calculations to predict the concentration of chemical parameters such as pH, acid neutralizing capacity, dissolved inorganic carbon, Al and SO42-. A simple chemical equilibrium model (ALCHEMI) was used to evaluate the effect of uncertainty in the measurement of chemical constituents on thermodynamic calculations. For calculations where pH was not allowed to vary (fixed pH calculations), uncertainty in Al speciation was small (maximum of 7%) and largely due to imprecision in the measurement of total F and pH. When calculations were made from electroneutrality based on measured constituents (variable pH calculations), most of the uncertainty associated with the values of output parameters was due to imprecision in the determination of SO42-.

Schecher, William D.; Driscoll, Charles T.

1988-04-01

378

From the Cover: Force-dependent chemical kinetics of disulfide bond reduction observed with single-molecule techniques  

NASA Astrophysics Data System (ADS)

The mechanism by which mechanical force regulates the kinetics of a chemical reaction is unknown. Here, we use single-molecule force-clamp spectroscopy and protein engineering to study the effect of force on the kinetics of thiol/disulfide exchange. Reduction of disulfide bonds through the thiol/disulfide exchange chemical reaction is crucial in regulating protein function and is known to occur in mechanically stressed proteins. We apply a constant stretching force to single engineered disulfide bonds and measure their rate of reduction by DTT. Although the reduction rate is linearly dependent on the concentration of DTT, it is exponentially dependent on the applied force, increasing 10-fold over a 300-pN range. This result predicts that the disulfide bond lengthens by 0.34 Å at the transition state of the thiol/disulfide exchange reaction. Our work at the single bond level directly demonstrates that thiol/disulfide exchange in proteins is a force-dependent chemical reaction. Our findings suggest that mechanical force plays a role in disulfide reduction in vivo, a property that has never been explored by traditional biochemistry. Furthermore, our work also indicates that the kinetics of any chemical reaction that results in bond lengthening will be force-dependent. atomic force microscopy | mechanochemistry

Wiita, Arun P.; Rama Koti Ainavarapu, Sri; Huang, Hector H.; Fernandez, Julio M.

2006-05-01

379

First-principles Calculations of Nuclear Magnetic Resonance Chemical Shielding Tensors in Complex Ferroelectric Perovskites  

NASA Astrophysics Data System (ADS)

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important experimental probes of local atomistic structure, chemical ordering, and dynamics. Recently, NMR has increasingly been used to study complex ferroelectric perovskite alloys, where spectra can be difficult to interpret. First-principles calculations of NMR spectra can greatly assist in this task. In this work, oxygen, titanium, and niobium NMR chemical shielding tensors, ? , were calculated with first-principles methods for ferroelectric transition metal prototypical ABO3 perovskites [SrTiO3, BaTiO 3, PbTiO3 and PbZrO3] and A(B,B')O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). The principal findings are 1) a large anisotropy between deshielded sigma xx(O) ? sigmayy(O) and shielded sigma zz(O) components; 2) a nearly linear dependence on nearest-distance transition-metal/oxygen bond length, rs, was found for both isotropic deltaiso(O) and axial deltaax(O) chemical shifts ( d?=? reference- ? ), across all the systems studied, with deltaiso(O) varying by ? 400 ppm; 3) the demonstration that the anisotropy and linear variation arise from large paramagnetic contributions to sigmaxx(O) and sigmayy(O), due to virtual transitions between O(2p) and unoccupied B(nd) states. Using these results, an argument against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements, is made. The linear dependence of the chemical shifts on rs provides a scale for determining transition-metal/oxygen bond lengths from experimental 17O NMR spectra. As such, it can be used to assess the degree of local tetragonality in perovskite solid solutions for piezoelectric applications. Results for transition metal atoms show less structural sensitivity, compared to 17O NMR, in homovalent B-site materials, but could be more useful in heterovalent B-site perovskite alloys. This work shows that both 17O and B-site NMR spectroscopy, coupled with first principles calculations, can be an especially useful probe of local structure in complex perovskite alloys.

Pechkis, Daniel Lawrence

380

Analysis of Ter-Ter enzyme kinetic mechanisms by computer simulation of isotope exchange at chemical equilibrium: development and application of ISOTER, a personal-computer-based program.  

PubMed

A convenient, personal-computer-based program has been developed that allows simulation of isotopic exchange kinetics at chemical equilibrium catalyzed by a three reactant-three product (TerTer) enzyme system: A + B + C integral of P + Q + R. This program, ISOTER, utilizes a rapid algebraic method to calculate the exchange rate between any reactant-product pair as a function of the substrate concentration and avoids altogether the necessity of deriving an explicit (but cumbersome and impractical) equation for exchange rate. ISOTER was used to generate model saturation patterns for 16 different TerTer kinetic mechanisms, varying different combinations of reactant-product pairs in constant ratio at equilibrium: [all substrates], [A, P], [B, Q], and [C, R], while holding the nonvaried components constant. These model studies indicate that virtually every one of these mechanisms can be distinguished from the others. In addition, ISOTER has been used to fit multiple sets of experimental data for Escherichia coli glutamine synthetase, which produced a set of rate constants consistent with the previously proposed "preferred order random" kinetic mechanism. PMID:2729545

Wedler, F C; Barkley, R W

1989-03-01

381

Kinetics of chemical ordering in a Ag-Pt nanoalloy particle via first-principles simulations  

SciTech Connect

The energetics and kinetic energy barriers of vacancy/atom exchange in a 37-atom truncated octahedron Ag-Pt binary cluster in the Ag-rich range of compositions are investigated via a first-principles atomistic approach. The energy of the local minima obtained considering various distributions of a single vacancy and a few Pt atoms within the cluster and the energy barriers connecting them are evaluated using accurate density-functional calculations. The effects of the simultaneous presence of a vacancy and Pt atoms are found to be simply additive when their distances are larger than first-neighbors, whereas when they can be stabilizing at low Pt content due to the release of strain by the Pt/vacancy interaction or destabilizing close to a perfect Pt(core)/Ag(shell) arrangement. It is found that alloying with Pt appreciably increases the barriers for homotops transformations, thus rationalizing the issues encountered at the experimental level in producing Ag-Pt equilibrated nanoparticles and bulk phase diagram.

Negreiros, F. R.; Fortunelli, A. [CNR-IPCF, Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche, Molecular Modeling Laboratory, via G. Moruzzi 1, Pisa I56124 (Italy); Taherkhani, F. [Department of Chemistry, Razi University, Kermanshah (Iran, Islamic Republic of); Parsafar, G. [Department of Chemistry and Nanotechnology Center, Sharif University of Technology, Tehran (Iran, Islamic Republic of); Caro, A. [Los Alamos National Laboratories, Division of Materials Science and Technology, Los Alamos, New Mexico 87545 (United States)

2012-11-21

382

Kinetics of chemical ordering in a Ag-Pt nanoalloy particle via first-principles simulations  

NASA Astrophysics Data System (ADS)

The energetics and kinetic energy barriers of vacancy/atom exchange in a 37-atom truncated octahedron Ag-Pt binary cluster in the Ag-rich range of compositions are investigated via a first-principles atomistic approach. The energy of the local minima obtained considering various distributions of a single vacancy and a few Pt atoms within the cluster and the energy barriers connecting them are evaluated using accurate density-functional calculations. The effects of the simultaneous presence of a vacancy and Pt atoms are found to be simply additive when their distances are larger than first-neighbors, whereas when they can be stabilizing at low Pt content due to the release of strain by the Pt/vacancy interaction or destabilizing close to a perfect Pt(core)/Ag(shell) arrangement. It is found that alloying with Pt appreciably increases the barriers for homotops transformations, thus rationalizing the issues encountered at the experimental level in producing Ag-Pt equilibrated nanoparticles and bulk phase diagram.

Negreiros, F. R.; Taherkhani, F.; Parsafar, G.; Caro, A.; Fortunelli, A.

2012-11-01

383

A Chemical Kinetic Modeling Study of the Effects of Oxygenated Hydrocarbons on Soot Emissions from Diesel Engines  

SciTech Connect

A detailed chemical kinetic modeling approach is used to examine the phenomenon of suppression of sooting in diesel engines by addition of oxygenated hydrocarbon species to the fuel. This suppression, which has been observed experimentally for a few years, is explained kinetically as a reduction in concentrations of soot precursors present in the hot products of a fuel-rich diesel ignition zone when oxygenates are included. Oxygenates decrease the overall equivalence ratio of the igniting mixture, producing higher ignition temperatures and more radical species to consume more soot precursor species, leading to lower soot production. The kinetic model is also used to show how different oxygenates, ester structures in particular, can have different soot-suppression efficiencies due to differences in molecular structure of the oxygenated species.

Westbrook, C K; Pitz, W J; Curran, H J

2005-11-14

384

Chemical enthalpies of hydration calculated for the ions of astatine, francium, and radium by the hyperbola method  

Microsoft Academic Search

Calculations by the hyperbola method are presented on the chemical enthalpies of hydration for the ions of astatine, francium, and radium. Values are also given for the enthalpies of hydration for ions of the hypothetical elements 117, 119, and 120.

A. I. Cherkesov; N. P. Sadovnikov

2009-01-01

385

Silicon Oxysulfide, OSiS: Rotational Spectrum, Quantum-Chemical Calculations, and Equilibrium Structure  

NASA Astrophysics Data System (ADS)

Silicon oxysulfide, OSiS, and seven of its minor isotopic species have been characterized for the first time in the gas phase at high spectral resolution by means of Fourier transform microwave spectroscopy. The equilibrium structure of OSiS has been determined from the experimental data using calculated vibration-rotation interaction constants. The structural parameters (rO-Si = 1.5064 Angstrom and rSi-S = 1.9133 Angstrom) are in very good agreement with values from high-level quantum chemical calculations using coupled-cluster techniques together with sophisticated additivity and extrapolation schemes. The bond distances in OSiS are very short in comparison with those in SiO and SiS. This unexpected finding is explained by the partial charges calculated for OSiS via a natural population analysis. The results suggest that electrostatic effects rather than multiple bonding are the key factors in determining bonding in this triatomic molecule. The data presented provide the spectroscopic information needed for radio astronomical searches for OSiS.

Thorwirth, S.; Mück, L. A.; Gauss, J.; Tamassia, F.; Lattanzi, V.; McCarthy, M. C.

2011-05-01

386

Quantum instanton calculation of rate constants for the C2H6 + H ? C2H5 + H2 reaction: anharmonicity and kinetic isotope effects.  

PubMed

Thermal rate constants and kinetic isotope effects for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The obtained results are in good agreement with experimental measurements at high temperatures. The detailed investigation reveals that the anharmonicity of the hindered internal rotation motion does not influence the rate too much compared to its harmonic oscillator approximation. However, the motion of the nonreactive methyl group in C(2)H(6) significantly enhances the rates compared to its rigid case, which makes conventional reduced-dimensionality calculations a challenge. In addition, the temperature dependence of kinetic isotope effects is also revealed. PMID:21960019

Wang, Wenji; Zhao, Yi

2011-09-29

387

A Comparison of CH4\\/NO\\/02 and CH4\\/N20 Flames by LIF Diagnostics and Chemical Kinetic Modeling  

Microsoft Academic Search

Laser-induced fluorescence (LIF) diagnostics and chemical kinetic modeling have been performed on two nitrogen-containing flames al low pressure: CH4\\/NO\\/02 and CH4\\/N20. Relative concentration profiles of Ihe species OH, CH, NO, NH, and CN wereoblained by LIF above a premixed, laminar, flat-flame burner at a pressure of 63Torr. The OH, NH, and CN profiles were converted to absolute concentrations by absorption

STEVEN ZABARNICK

1992-01-01

388

Numerical simulation of laminar co-flow methane-oxygen diffusion flames: effect of chemical kinetic mechanisms  

Microsoft Academic Search

Laminar co-flow methane–oxygen flames issuing into the unconfined atmosphere have been studied. A numerical model, which employs different chemical kinetics sub-models, including a skeletal mechanism with 43 reaction steps and 18 species and four global reaction mechanisms (two 2-steps and two 4-steps mechanisms), and an optically thin radiation sub-model, has been employed in the simulations. Numerical model has been validated

Karnam Bhadraiah; Vasudevan Raghavan

2011-01-01

389

A kinetic-theory approach for computing chemical-reaction rates in upper-atmosphere hypersonic flows  

Microsoft Academic Search

Recently proposed 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 investigated for chemical reactions occurring in upper-atmosphere hypersonic flows. The new models are in good agreement with the measured Arrhenius rates for near-equilibrium conditions and with both measured rates and other theoretical models for

Michael A. Gallis; Ryan B. Bond; John R. Torczynski

2009-01-01

390

Calculating the dermal flux of chemicals with OELs based on their molecular structure: An attempt to assign the skin notation  

Microsoft Academic Search

Our objectives included calculating the permeability coefficient and dermal penetration rates (flux value) for 112 chemicals with occupational exposure limits (OELs) according to the LFER (linear free-energy relationship) model developed using published methods. We also attempted to assign skin notations based on each chemical's molecular structure. There are many studies available where formulae for coefficients of permeability from saturated aqueous

Ma?gorzata Kupczewska-Dobecka; Marek Jakubowski; S?awomir Czerczak

2010-01-01

391

Intramolecular hydrogen bonding in 2-nitromalonaldehyde: Infrared spectrum and quantum chemical calculations  

NASA Astrophysics Data System (ADS)

2-Nitromalonaldehyde (NO2MA), a simple compound to study the intramolecular hydrogen bond, has been synthesized and deuterated at the enolated proton. Molecular structure and vibrational frequencies of NO2MA have been investigated by means of density functional theory (DFT) calculations. The geometrical parameters obtained in the B3LYP level using 6-31G**, 6-311G**, and 6-311++G** basis sets and compared with the corresponding parameters of malonaldehyde (MA). Frequencies calculated at B3LYP level using the 6-311G** and 6-311++G** basis sets are in good agreement with the corresponding experimental results for light and deuterated compounds in CCl4/CS2 solution. The percentage of deviation of the bond lengths and bond angles was used to give a picture of the normal modes, and serves as a basis for the assignment of the wavenumbers. Theoretical calculations show that the hydrogen bond strength of NO2MA is slightly stronger than that of MA, which is in agreement with the spectroscopic results. The observed ?OH/?OD and ?OH/?OD appears at about 2880/2100 and 911/695 cm-1, respectively, are consistent with the calculated geometry and proton chemical shift results. To investigate the effect of NO2 group on the hydrogen bond strength, the charge distributions, steric effects, and electron delocalization in NO2MA and MA were studied by the Natural Bond Orbital (NBO) method for optimized model compounds at B3LYP/6-311++G** level of theory. The results of NBO analysis indicate that the electron-withdrawing effect of NO2 group decreases the hydrogen bond strength, but the steric and resonance effects increase the strength of the bond.

Tayyari, S. F.; Moosavi-Tekyeh, Z.; Zahedi-Tabrizi, M.; Eshghi, H.; Emampour, J. S.; Rahemi, H.; Hassanpour, M.

2006-01-01

392

Surftherm: A program to analyze thermochemical and kinetic data in gas-phase and surface chemical reaction mechanisms  

SciTech Connect

This report documents the Surftherm program that analyzes transport coefficient, thermochemical- and kinetic rate information in complex gas-phase and surface chemical reaction mechanisms. The program is designed for use with the Chemkin (gas-phase chemistry) and Surface Chemkin (heterogeneous chemistry) programs. It was developed as a ``chemist`s companion`` in using the Chemkin packages with complex chemical reaction mechanisms. It presents in tabular form detailed information about the temperature and pressure dependence of chemical reaction rate constants and their reverse rate constants, reaction equilibrium constants, reaction thermochemistry, chemical species thermochemistry and transport properties. This report serves as a user`s manual for use of the program, explaining the required input and the output.

Coltrin, M.E.; Moffat, H.K.

1994-06-01

393

Combustion in Homogeneous Charge Compression Ignition Engines: Experiments and Detailed Chemical Kinetic Simulations  

SciTech Connect

Homogeneous charge compression ignition (HCCI) engines are being considered as an alternative to diesel engines. The HCCI concept involves premixing fuel and air prior to induction into the cylinder (as is done in current spark-ignition engine) then igniting the fuel-air mixture through the compression process (as is done in current diesel engines). The combustion occurring in an HCCI engine is fundamentally different from a spark-ignition or Diesel engine in that the heat release occurs as a global autoignition process, as opposed to the turbulent flame propagation or mixing controlled combustion used in current engines. The advantage of this global autoignition is that the temperatures within the cylinder are uniformly low, yielding very low emissions of oxides of nitrogen (NO{sub x}, the chief precursors to photochemical smog). The inherent features of HCCI combustion allows for design of engines with efficiency comparable to, or potentially higher than, diesel engines. While HCCI engines have great potential, several technical barriers exist which currently prevent widespread commercialization of this technology. The most significant challenge is that the combustion timing cannot be controlled by typical in-cylinder means. Means of controlling combustion have been demonstrated, but a robust control methodology that is applicable to the entire range of operation has yet to be developed. This research focuses on understanding basic characteristics of controlling and operating HCCI engines. Experiments and detailed chemical kinetic simulations have been applied to the characterize some of the fundamental operational and design characteristics of HCCI engines. Experiments have been conducted on single and multi-cylinder engines to investigate general features of how combustion timing affects the performance and emissions of HCCI engines. Single-zone modeling has been used to characterize and compare the implementation of different control strategies. Multi-zone modeling has been applied to investigate combustion chamber design with respect to increasing efficiency and reducing emissions in HCCI engines.

Flowers, D L

2002-06-07

394

Ozonolysis of Mixed Oleic-Acid/Stearic-Acid Particles: Reaction Kinetics and Chemical Morphology  

NASA Astrophysics Data System (ADS)

Atmospheric particles directly and indirectly affect global climate and have a primary role in regional issues of air pollution, visibility, and human health. Atmospheric particles have a variety of shapes, dimensions, and chemical compositions, and these physicochemical properties evolve (i.e., "age") during transport of the particles through the atmosphere, in part because of the chemical reactions of particle-phase organic molecules with gas-phase atmospheric oxidants. As a global average, hydroxyl radical (OH) and ozone (O3) are responsible quantitatively for most oxidant aging of atmospheric particles. The reactions of the hydroxyl radical occur in the surface region of a particle because of the nearly diffusion-limited bimolecular rate constant of OH with a variety of organic molecules. Ozone, on the other hand, is a selective agent for the unsaturated bonds of organic molecules and may diffuse a considerable distance into particles prior to reaction. The reaction of oleic acid with ozone has recently emerged as a model system to better understand the atmospheric chemical oxidation processes affecting organic particles. The ozonolysis of mixed oleic-acid/stearic-acid (OL/SA) aerosol particles from 0/100 to 100/0 weight percent composition is studied. The magnitude of the divergence of the particle beam inside an aerosol mass spectrometer shows that, in the concentration range 100/0 to 60/40, the mixed OL/SA particles are liquid prior to reaction. Upon ozonolysis, particles with SA composition greater than 25% change shape, indicating that they have solidified. Transmission electron micrographs show that SA(s) forms needles. For SA compositions greater than 10%, the reaction kinetics exhibit an initial fast decay of OL for low O3 exposure with no further loss of OL at higher O3 exposures. For compositions from 50/50 to 10/90, the residual OL concentration remains at 28+/-2% of its initial value. The initial reactive uptake coefficient for O3, as determined by OL loss, decreases linearly from 1.25(+/-0.2) 10-3 to 0.60(+/-0.15) 10-3 for composition changes of 100/0 to 60/40. At 50/50 composition, the uptake coefficient drops abruptly to 0.15(+/-0.1) 10-3, and there are no further changes with increased SA content. The amount of SA in the particles also decreases during OL ozonolysis. The stabilized Criegee intermediate (SCI) formed by OL ozonolysis attacks the carboxylic acid group of SA to yield an acyloxyalkyl hydroperoxide product. The experimental observations of this study can be explained by the following two postulates: (1) unreacted mixed particles remain as supersatured liquids up to 60/40 composition and (2) SA, as it solidifies, locks in a significant amount of oleic acid. The results of this study point out the important effects of particle phase, composition, and morphology on chemical reactivity. Oleic acid in liquid regions of a particle reacts rapidly with O3 whereas OL trapped inside solid SA is essentially unavailable for reaction with O3. These results contribute to the continuing development of the scientific community's understanding of particle aging process in the atmosphere, for which the ultimate goal is to provide quantitative mechanistic models of physicochemical transformations under atmospheric conditions.

Martin, S. T.; Katrib, Y.; Biskos, G.; Buseck, P. R.; Davidovits, P.; Jayne, J. T.; Mochida, M.; Wise, M. E.; Worsnop, D. R.

2005-12-01

395

The Study of a Simple Redox Reaction as an Experimental Approach to Chemical Kinetics.  

ERIC Educational Resources Information Center

|Recommends using iodide ions and peroxodisulfate ions for studying rate laws instead of the standard iodine clock for kinetic study. Presents the methodology and a discussion of the kinetics involved for a laboratory experiment for a high school or introductory college course. (ML)|

Elias, Horst; Zipp, Arden P.

1988-01-01

396

A study of the accuracy of moment-closure approximations for stochastic chemical kinetics  

NASA Astrophysics Data System (ADS)

Moment-closure approximations have in recent years become a popular means to estimate the mean concentrations and the variances and covariances of the concentration fluctuations of species involved in stochastic chemical reactions, such as those inside cells. The typical assumption behind these methods is that all cumulants of the probability distribution function solution of the chemical master equation which are higher than a certain order are negligibly small and hence can be set to zero. These approximations are ad hoc and hence the reliability of the predictions of these class of methods is presently unclear. In this article, we study the accuracy of the two moment approximation (2MA) (third and higher order cumulants are zero) and of the three moment approximation (3MA) (fourth and higher order cumulants are zero) for chemical systems which are monostable and composed of unimolecular and bimolecular reactions. We use the system-size expansion, a systematic method of solving the chemical master equation for monostable reaction systems, to calculate in the limit of large reaction volumes, the first- and second-order corrections to the mean concentration prediction of the rate equations and the first-order correction to the variance and covariance predictions of the linear-noise approximation. We also compute these corrections using the 2MA and the 3MA. Comparison of the latter results with those of the system-size expansion shows that: (i) the 2MA accurately captures the first-order correction to the rate equations but its first-order correction to the linear-noise approximation exhibits the wrong dependence on the rate constants. (ii) the 3MA accurately captures the first- and second-order corrections to the rate equation predictions and the first-order correction to the linear-noise approximation. Hence while both the 2MA and the 3MA are more accurate than the rate equations, only the 3MA is more accurate than the linear-noise approximation across all of parameter space. The analytical results are numerically validated for dimerization and enzyme-catalyzed reactions.

Grima, Ramon

2012-04-01

397

A study of the accuracy of moment-closure approximations for stochastic chemical kinetics.  

PubMed

Moment-closure approximations have in recent years become a popular means to estimate the mean concentrations and the variances and covariances of the concentration fluctuations of species involved in stochastic chemical reactions, such as those inside cells. The typical assumption behind these methods is that all cumulants of the probability distribution function solution of the chemical master equation which are higher than a certain order are negligibly small and hence can be set to zero. These approximations are ad hoc and hence the reliability of the predictions of these class of methods is presently unclear. In this article, we study the accuracy of the two moment approximation (2MA) (third and higher order cumulants are zero) and of the three moment approximation (3MA) (fourth and higher order cumulants are zero) for chemical systems which are monostable and composed of unimolecular and bimolecular reactions. We use the system-size expansion, a systematic method of solving the chemical master equation for monostable reaction systems, to calculate in the limit of large reaction volumes, the first- and second-order corrections to the mean concentration prediction of the rate equations and the first-order correction to the variance and covariance predictions of the linear-noise approximation. We also compute these corrections using the 2MA and the 3MA. Comparison of the latter results with those of the system-size expansion shows that: (i) the 2MA accurately captures the first-order correction to the rate equations but its first-order correction to the linear-noise approximation exhibits the wrong dependence on the rate constants. (ii) the 3MA accurately captures the first- and second-order corrections to the rate equation predictions and the first-order correction to the linear-noise approximation. Hence while both the 2MA and the 3MA are more accurate than the rate equations, only the 3MA is more accurate than the linear-noise approximation across all of parameter space. The analytical results are numerically validated for dimerization and enzyme-catalyzed reactions. PMID:22519313

Grima, Ramon

2012-04-21

398

Algorithm for calculating the thermophysical characteristics and chemical composition of the incomplete-combustion products of natural gas  

Microsoft Academic Search

An algorithm for calculating the thermophysical characteristics and chemical composition of incomplete-combustion products of natural gas has been developed on the basis of the method in [1, 2]. The structure of the algorithm is shown in Fig. 1. It may be used in calculations both of furnaces for the open lowoxidative heating of wire and of hot zinc-plating furnace baths.

R. A. Yudin; A. V. Vinogradov; I. R. Yudin

2008-01-01

399

The calculation of the kinetic rate constants for LiF and BeF[sub 2  

SciTech Connect

In the HYLIFE inertial confinement fusion reactor occurs in pulses several times every second. X-rays ablate material from the molten LiF-BeF[sub 2] salt (Flibe) jets used to protect the reactor vessel, generating a hot dissociated partially ionized vapor. Further evaporation occurs as the vapor radiates to the jets. Eventually the vapor must be condensed to restore sufficient vacuum for the next shot. Rate of condensation determines the permissible fusion repetition rate. Analysis here shows that recombination will be fast making it possible to assume quasi-equilibrium in vapor phase. Scaling shows that convective transport in the reactor dominates over diffusive transport over length scales the order of salt droplet diameters, and thus no significant separation or concentration of species will occur, except in very thin (micron scale) layers next to condensing surfaces. Without diffusion, the local stoichiometry of the mixture will remain fixed at the initial composition of 33%BeF[sub 2] and 67%LiF, with traces of tritium, helium, and other species. The gas dynamics in the reactor cavity depend upon the equation of state of gaseous Flibe. Because the chemical kinetics are fast, and the stoichiometry is fixed, a single relationship between pressure, temperature, and specific volume can be used to describe the state of the vapor phase, even at high temperatures where dissociation and ionization occur.

Chen, X.M.; Peterson, P.F.; Schrock, V.E.

1992-03-10

400

The calculation of the kinetic rate constants for LiF and BeF{sub 2}  

SciTech Connect

In the HYLIFE inertial confinement fusion reactor occurs in pulses several times every second. X-rays ablate material from the molten LiF-BeF{sub 2} salt (Flibe) jets used to protect the reactor vessel, generating a hot dissociated partially ionized vapor. Further evaporation occurs as the vapor radiates to the jets. Eventually the vapor must be condensed to restore sufficient vacuum for the next shot. Rate of condensation determines the permissible fusion repetition rate. Analysis here shows that recombination will be fast making it possible to assume quasi-equilibrium in vapor phase. Scaling shows that convective transport in the reactor dominates over diffusive transport over length scales the order of salt droplet diameters, and thus no significant separation or concentration of species will occur, except in very thin (micron scale) layers next to condensing surfaces. Without diffusion, the local stoichiometry of the mixture will remain fixed at the initial composition of 33%BeF{sub 2} and 67%LiF, with traces of tritium, helium, and other species. The gas dynamics in the reactor cavity depend upon the equation of state of gaseous Flibe. Because the chemical kinetics are fast, and the stoichiometry is fixed, a single relationship between pressure, temperature, and specific volume can be used to describe the state of the vapor phase, even at high temperatures where dissociation and ionization occur.

Chen, X.M.; Peterson, P.F.; Schrock, V.E.

1992-03-10

401

Large scale application of kinetic ART, a near order-1 topologically- based off-lattice kinetic Monte-Carlo algorithm with on-the-fly calculation of events  

Microsoft Academic Search

Using a topological classification of eventsfootnotetextB. D. McKay, Congressus Numerantium 30, 45 (1981). combined with the Activation-Relaxation Technique (ART nouveau) for the generation of diffusion pathways, the kinetic ART (k-ART)footnotetextF. El-Mellouhi, N. Mousseau and L. J. Lewis, Phys Rev B, 78,15 (2008). lifts many restrictions generally associated with standard kinetic Monte Carlo algorithms. In particular, it can treat on and

Laurent Karim Beland; Fedwa El-Mellouhi; Normand Mousseau

2010-01-01

402

Burnup calculations and chemical analysis of irradiated fuel samples studied in LWR-PROTEUS phase II  

SciTech Connect

The isotopic compositions of 5 UO{sub 2} samples irradiated in a Swiss PWR power plant, which were investigated in the LWR-PROTEUS Phase II programme, were calculated using the CASMO-4 and BOXER assembly codes. The burnups of the samples range from 50 to 90 MWd/kg. The results for a large number of actinide and fission product nuclides were compared to those of chemical analyses performed using a combination of chromatographic separation and mass spectrometry. A good agreement of calculated and measured concentrations is found for many of the nuclides investigated with both codes. The concentrations of the Pu isotopes are mostly predicted within {+-}10%, the two codes giving quite different results, except for {sup 242}Pu. Relatively significant deviations are found for some isotopes of Cs and Sm, and large discrepancies are observed for Eu and Gd. The overall quality of the predictions by the two codes is comparable, and the deviations from the experimental data do not generally increase with burnup. (authors)

Grimm, P.; Guenther-Leopold, I. [Paul Scherrer Inst., CH-5232 Villigen PSI (Switzerland); Berger, H. D. [AREVA NP GmbH, FEEP, Bunsenstrasse 43, D-91058 Erlangen (Germany)

2006-07-01

403

Quantum chemical calculations of bond dissociation energies for COOH scission and electronic structure in some acids  

NASA Astrophysics Data System (ADS)

Quantum chemical calculations are performed to investigate the equilibrium C—COOH bond distances and the bond dissociation energies (BDEs) for 15 acids. These compounds are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3PW91, B3P86, PBE1PBE) and the complete basis set (CBS—Q) method in conjunction with the 6-311G** basis as DFT methods have been found to have low basis sets sensitivity for small and medium molecules in our previous work. Comparisons between the computational results and the experimental values reveal that CBS—Q method, which can produce reasonable BDEs for some systems in our previous work, seems unable to predict accurate BDEs here. However, the B3P86 calculated results accord very well with the experimental values, within an average absolute error of 2.3 kcal/mol. Thus, B3P86 method is suitable for computing the reliable BDEs of C—COOH bond for carboxylic acid compounds. In addition, the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of studied compounds are estimated, based on which the relative thermal stabilities of the studied acids are also discussed.

Zeng, Hui; Zhao, Jun; Xiao, Xun

2013-02-01

404

Vibrational spectra, structural conformations, scaled quantum chemical calculations and NBO analysis of 3-acetyl-7-methoxycoumarin  

NASA Astrophysics Data System (ADS)

The powder form NIR-FT Raman and FT-IR spectra of 3-acetyl-7-methoxycoumarin (3A7MC) have been recorded in the regions 4000-400 and 3500-100 cm-1, respectively. The equilibrium geometry, vibrational frequencies, band intensities, NMR spectra, NBO analysis and UV-Vis spectral studies of the most stable conformer have been calculated by density functional B3LYP method with the 6-311G(d,p) basis set. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra, the calculated wavenumber and intensity of the vibrational bands and the potential energy distribution over the internal coordinates. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping the electron density isosurface with electrostatic potential surfaces (ESP). Natural bond orbital analysis has been carried out to understand the nature of different interactions responsible for the electron delocalization and the intramolecular charge transfer between the orbitals (n ? ??, n ? ??, ? ? ??).

Joseph, Lynnette; Sajan, D.; Reshmy, R.; Arun Sasi, B. S.; Erdogdu, Y.; Thomas, K. Kurien

2012-12-01

405

Estimation of applicability of perturbation theory to solution of the kinetic Boltzmann equation in calculations of charge-carrier relaxation time in isotropic polycrystalline p-silicon  

Microsoft Academic Search

A condition is formulated for application of perturbation theory to solution of the kinetic Boltzmann equation in calculations of charge-carrier relaxation time in an isotropic silicon polycrystal, where holes are scattered both by a disordered system of potential barriers formed on crystallite surfaces and by a disordered lattice of silicon atoms characterized by local ordering. The total specific resistance of

A. G. Moiseev

2009-01-01

406

Probability distribution of the chemical states of a closed system and thermodynamic law of mass action from kinetics: The RNA example  

Microsoft Academic Search

We show some thermodynamic results concerning chemical kinetics, either from statistical physics, under the strong assumption (EXS) of the statistical exponential distribution of the physical states, or from kinetics. In that frame, the reaction rates are modeled according to the stochastic activation collision theory, under the weaker assumption (EXW) of the statistical exponential distribution not on the whole space but

Christine Jacob; Nicolas Breton; Patrick Daegelen; Jean Peccoud

1997-01-01

407

Kinetics of first stage of formation magnetite nanoparticles prepared by chemical precipitation method  

NASA Astrophysics Data System (ADS)

First stage of formation magnetite nanoparticles is described by kinetics equation. Solution of kinetics equation is Gauss function. Mean value and standard deviation of Gauss function are , Critical size, determinant of collapse and growth of nanoparticles are determined. It's showed nanoparticles less critical size may be is collapsed, bigger — grow. The dependence of the size and speed of the growth size on time are determined.

Ali-zade, R. A.

2009-03-01

408

From the Cover: Inaugural Article: Chemical, physical, and theoretical kinetics of an ultrafast folding protein  

Microsoft Academic Search

An extensive set of equilibrium and kinetic data is presented and analyzed for an ultrafast folding protein---the villin subdomain. The equilibrium data consist of the excess heat capacity, tryptophan fluorescence quantum yield, and natural circular-dichroism spectrum as a function of temperature, and the kinetic data consist of time courses of the quantum yield from nanosecond-laser temperature-jump experiments. The data are

Jan Kubelka; Eric R. Henry; Troy Cellmer; James Hofrichter; William A. Eaton

2008-01-01

409

Chemical kinetics of radio-frequency driven atmospheric-pressure helium-oxygen plasmas in humid air  

NASA Astrophysics Data System (ADS)

We describe the chemical kinetics of radio-frequency (rf) driven atmospheric-pressure helium-based plasmas in ambient air as determined through a zero-dimensional time-dependent global model. The effects of humid-air admixtures on the plasma-induced chemical reactions and the evolution of species concentrations in the helium-oxygen mixture (He-O2, helium with 5000 ppm admixture of oxygen) are studied for wide air impurity levels of 1-5000 ppm with the relative humidity of 0-100%. Comparisons made with experiments using an rf driven micro-scale atmospheric pressure plasma jet and one-dimensional simulations suggest that the plausible air impurity level in the experiments is not more than hundreds ppm. Effects of the air impurity containing water-humidity on electro-negativity and chemical activity are clarified with particular emphasis on reactive oxygen species.

Murakami, Tomoyuki; Niemi, Kari; Gans, Timo; O'Connell, Deborah; Graham, William

2011-11-01

410

Efficient stochastic simulation of chemical kinetics networks using a weighted ensemble of trajectories  

NASA Astrophysics Data System (ADS)

We apply the ``weighted ensemble'' (WE) simulation strategy, previously employed in the context of molecular dynamics simulations, to a series of systems-biology models that range in complexity from a one-dimensional system to a system with 354 species and 3680 reactions. WE is relatively easy to implement, does not require extensive hand-tuning of parameters, does not depend on the details of the simulation algorithm, and can facilitate the simulation of extremely rare events. For the coupled stochastic reaction systems we study, WE is able to produce accurate and efficient approximations of the joint probability distribution for all chemical species for all time t. WE is also able to efficiently extract mean first passage times for the systems, via the construction of a steady-state condition with feedback. In all cases studied here, WE results agree with independent ``brute-force'' calculations, but significantly enhance the precision with which rare or slow processes can be characterized. Speedups over ``brute-force'' in sampling rare events via the Gillespie direct Stochastic Simulation Algorithm range from ~1012 to ~1018 for characterizing rare states in a distribution, and ~102 to ~104 for finding mean first passage times.

Donovan, Rory M.; Sedgewick, Andrew J.; Faeder, James R.; Zuckerman, Daniel M.

2013-09-01

411

Efficient stochastic simulation of chemical kinetics networks using a weighted ensemble of trajectories.  

PubMed

We apply the "weighted ensemble" (WE) simulation strategy, previously employed in the context of molecular dynamics simulations, to a series of systems-biology models that range in complexity from a one-dimensional system to a system with 354 species and 3680 reactions. WE is relatively easy to implement, does not require extensive hand-tuning of parameters, does not depend on the details of the simulation algorithm, and can facilitate the simulation of extremely rare events. For the coupled stochastic reaction systems we study, WE is able to produce accurate and efficient approximations of the joint probability distribution for all chemical species for all time t. WE is also able to efficiently extract mean first passage times for the systems, via the construction of a steady-state condition with feedback. In all cases studied here, WE results agree with independent "brute-force" calculations, but significantly enhance the precision with which rare or slow processes can be characterized. Speedups over "brute-force" in sampling rare events via the Gillespie direct Stochastic Simulation Algorithm range from ?10(12) to ?10(18) for characterizing rare states in a distribution, and ?10(2) to ?10(4) for finding mean first passage times. PMID:24070313

Donovan, Rory M; Sedgewick, Andrew J; Faeder, James R; Zuckerman, Daniel M

2013-09-21

412

Characterization of the Active Site of DNA Polymerase Beta by Molecular Dynamics and Quantum Chemical Calculation  

SciTech Connect

It is well established that the fully formed polymerase active site of the DNA repair enzyme, polymerase b (pol b), including two bound Mg2+ cations and the nucleoside triphosphate (dNTP) substrate, exists at only one point in the catalytic cycle just prior to the chemical nucleotidyl transfer step. The structure of the active conformation has been the subject of much interest as it relates to the mechanism of the chemical step and also to the question of fidelity assurance. While crystal structures of ternary pol b - (primer-template) DNA - dNTP complexes have provided the main structural features of the active site, they are necessarily incomplete due to intentional alterations, e.g. removal of the 3?OH groups from primer and substrate, that were needed to obtain a structure from mid-cycle. Working from the crystal structure closest to the fully formed active site, (pdb: 1bpy), two MD simulations of the solvated ternary complex were performed; one with the missing 3?OH?s restored, via modeling, to the primer and substrate, and the other without restoration of the 3?OH?s. The results of the simulations, taken together with ab initio optimizations on simplified active site models, indicate that the missing primer 3?OH in the crystal structure is responsible for a significant perturbation in the coordination sphere of the catalytic cation and allow us to suggest several corrections and additions to the active site structure as observed by crystallography. In addition, the calculations help to resolve questions that have been raised regarding the protonation states of coordinating ligands.

Rittenhouse, Robert C.; Apostoluk, Wlodzimierz K.; Miller, John H.; Straatsma, TP

2003-11-15

413

Nuclei-selected NMR shielding calculations: a sublinear-scaling quantum-chemical method.  

PubMed

An ab initio method for the direct calculation of NMR shieldings for selected nuclei at the Hartree-Fock and density-functional theory level is presented. Our method shows a computational effort scaling only sublinearly with molecular size, as it is motivated by the physical consideration that the chemical shielding is dominated by its local environment. The key feature of our method is to avoid the conventionally performed calculation of all NMR shieldings but instead to solve directly for specific nuclear shieldings. This has important implications not only for the study of large molecules, but also for the simulation of solvent effects and molecular dynamics, since often just a few shieldings are of interest. Our theory relies on two major aspects both necessary to provide a sublinear scaling behavior: First, an alternative expression for the shielding tensor is derived, which involves the response density matrix with respect to the nuclear magnetic moment instead of the response to the external magnetic field. Second, as unphysical long-range contributions occur within the description of distributed gauge origin methods that do not influence the final expectation value, we present a screening procedure to truncate the B-field dependent basis set, which is crucial in order to ensure an early onset of the sublinear scaling. The screening is in line with the r(-2) distance decay of Biot-Savarts law for induced magnetic fields. Our present truncation relies on the introduced concept of "individual gauge shielding contributions" applied to a reformulated shielding tensor, the latter consisting of gauge-invariant terms. The presented method is generally applicable and shows typical speed-ups of about one order of magnitude; moreover, due to the reduced scaling behavior of O(1) as compared to O(N), the wins become larger with increasing system size. We illustrate the validity of our method for several test systems, including ring-current dominated systems and biomolecules with more than 1000 atoms. PMID:21341823

Beer, Matthias; Kussmann, Jörg; Ochsenfeld, Christian

2011-02-21

414

Gas-particle interactions of tropospheric aerosols: Kinetic and thermodynamic perspectives of multiphase chemical reactions, amorphous organic substances, and the activation of cloud condensation nuclei  

NASA Astrophysics Data System (ADS)

Aerosols are of central importance in the Earth system, influencing the atmosphere, biosphere, climate, and public health. This article outlines recent advances and perspectives in the investigation and effects of gas-particle interactions in tropospheric aerosols, including physical, chemical and biological aspects. In particular, it addresses how multiphase processes and heterogeneous chemical reactions can be efficiently described by kinetic models, how amorphous organic substances can influence the kinetics of water uptake and phase transitions, and how the abundance and activation of cloud condensation nuclei (CCN) depend on thermodynamic and kinetic parameters.

Pöschl, Ulrich

2011-08-01

415

Chemical-state analysis of organic semiconductors using soft X-ray absorption spectroscopy combined with first-principles calculation.  

PubMed

The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS) and first-principles calculations. The organic semiconductors, pentacene (C(22)H(14)) and pentacenequinone (C(22)H(12)O(2)), were subjected to TEY-XAS and the experimental spectra obtained were compared with the 1s core-level excited spectra of C and O atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C?O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation. PMID:22217144

Natsume, Yutaka; Kohno, Teiichiro; Minakata, Takashi; Konishi, Tokuzo; Gullikson, Eric M; Muramatsu, Yasuji

2012-02-02

416

Quantum chemical calculation and experimental measurement of the 13C chemical shift tensors of vanillin and 3,4-dimethoxybenzaldehyde  

NASA Astrophysics Data System (ADS)

The principal values of the 13C nuclear magnetic resonance chemical shift tensors in vanillin and 3,4-dimethoxybenzaldehyde are reported. Theoretical results of the 13C chemical shift tensors were obtained by employing the gauge included atomic orbital (GIAO) approach. The geometrical parameters were optimized by using the MNDO method. The observed chemical shifts of these two compounds were determined in powders by using the recently introduced magic angle turning (MAT) experiment. The results presented in this paper clearly demonstrate the importance of using tensor information in the study of molecular structures.

Zheng, Guang; Hu, Jianzhi; Zhang, Xiaodong; Shen, Lianfang; Ye, Chaohui; Webb, Graham A.

1997-03-01

417

Conformational composition of cyclopentadienylphosphine investigated by microwave spectroscopy and quantum chemical calculations.  

PubMed

The properties of cyclopentadienylphosphine have been investigated by means of Stark-modulation microwave spectroscopy and quantum chemical calculations at the MP2/aug-cc-pVTZ, B3LYP/6-311++G(d,p), and G3 levels of theory. Spectra attributable to two rotamers denoted conformers I and II have been assigned. Conformer I has a symmetry plane (Cs symmetry) consisting of the bisectors of the cyclopentadiene ring and of the phosphino group with the lone electron pair of phosphorus pointing toward the carbon ring. In conformer II, the phosphino group is rotated approximately 120 degrees out of this plane. Relative intensity measurements have been made, and it was found that conformer II is more stable than I by 1.3(4) kJ/mol. The preferred conformer represents a borderline case of intramolecular hydrogen bond stabilization. The experimental and MP2/ aug-cc-pVTZ rotational constants differ by several percent, which indicates that the aug-cc-pVTZ basis set is not large enough to be able to predict an accurate structure for the two conformers that are close to the equilibrium geometries. 5-Substituted 1,3-cyclopentadienyl derivatives may undergo circumambulatory rearrangements. However, there is no manifestation of this effect in the microwave spectrum of cyclopentadienylphosphine. PMID:16419990

Møllendal, Harald; Cole, George C; Guillemin, Jean-Claude

2006-01-26

418

Computational modeling of polyoxotungstates by relativistic DFT calculations of (183)W NMR chemical shifts.  

PubMed

The (183)W nuclear shielding in a variety of tungsten polyoxometalates (POM) (Lindqvist, Anderson, decatungstates, Keggin) of different shapes and charges has been modeled by DFT calculations that take into account relativistic effects, by means of the zero-order regular approximation (ZORA), and solvent effects, by the conductor-like screening model (COSMO) continuum method. The charge/surface area ratio (q/A) is proposed as an indicator of the charge density to which the solvation energies of all POMs are correlated in a satisfactory way. Among the various theoretical levels tested (ZORA scalar or spin-orbit, frozen-core or all-electron basis set, geometry optimization in the gas phase or in the continuum solvent, etc.), the best results are obtained when both geometry optimization in solvent and spin-orbit shielding are included (mean absolute error of delta=35 ppm). The quality of the computed chemical shifts depends systematically on the charge density as expressed by q/A; thus, POMs with low q/A ratios display the best agreement with experimental data. The performance of the method is such that computed values can aid the assignment of the (183)W NMR spectra of polyoxotungstates, as shown by the case of alpha-[PW(11)TiO(40)](5-), whose six signals are ranked computationally so as to almost reproduce the experimental ordering even though the signals are spaced by as little as 5 ppm. PMID:16927351

Bagno, Alessandro; Bonchio, Marcella; Autschbach, Jochen

2006-11-15

419

Interactions of ionic liquids and acetone: thermodynamic properties, quantum-chemical calculations, and NMR analysis.  

PubMed

The interactions between ionic liquids (ILs) and acetone have been studied to obtain a further understanding of the behavior of their mixtures, which generally give place to an exothermic process, mutual miscibility, and negative deviation of Raoult's law. COSMO-RS was used as a suitable computational method to systematically analyze the excess enthalpy of IL-acetone systems (>300), in terms of the intermolecular interactions contributing to the mixture behavior. Spectroscopic and COSMO-RS results indicated that acetone, as a polar compound with strong hydrogen bond acceptor character, in most cases, establishes favorable hydrogen bonding with ILs. This interaction is strengthened by the presence of an acidic cation and an anion with dispersed charge and non-HB acceptor character in the IL. COSMO-RS predictions indicated that gas-liquid and vapor-liquid equilibrium data for IL-acetone systems can be finely tuned by the IL selection, that is, acting on the intermolecular interactions between the molecular and ionic species in the liquid phase. NMR measurements for IL-acetone mixtures at different concentrations were also carried out. Quantum-chemical calculations by using molecular clusters of acetone and IL species were finally performed. These results provided additional evidence of the main role played by hydrogen bonding in the behavior of systems containing ILs and HB acceptor compounds, such as acetone. PMID:23688030

Ruiz, Elia; Ferro, Victor R; Palomar, Jose; Ortega, Juan; Rodriguez, Juan Jose

2013-06-05

420

LocalSCF method for semiempirical quantum-chemical calculation of ultralarge biomolecules  

NASA Astrophysics Data System (ADS)

A linear-scaling semiempirical method, LocalSCF, has been proposed for the quantum-chemical calculations of ultralarge molecular systems by treating the large-scale molecular task as a variational problem. The method resolves the self-consistent field task through the finite atomic expansion of weakly nonorthogonal localized molecular orbitals. The inverse overlap matrix arising from the nonorthogonality of the localized orbitals is approximated by preserving the first-order perturbation term and applying the second-order correction by means of a penalty function. This allows for the separation of the orbital expansion procedure from the self-consistent field optimization of linear coefficients, thereby maintaining the localized molecular orbital size unchanged during the refinement of linear coefficients. Orbital normalization is preserved analytically by the variation of virtual degrees of freedom, which are orthogonal to the initial orbitals. Optimization of linear coefficients of localized orbitals is performed by a gradient procedure. The computer program running on a commodity personal computer was applied to the GroEL-GroES chaperonin complex containing 119 273 atoms.

Anikin, N. A.; Anisimov, V. M.; Bugaenko, V. L.; Bobrikov, V. V.; Andreyev, A. M.

2004-07-01

421

Molecular simulations of Crussard curves of detonation product mixtures at chemical equilibrium: Microscopic calculation of the Chapman-Jouguet state  

Microsoft Academic Search

The simultaneous use of the Reaction Ensemble Monte Carlo (ReMC) method and the Adaptative Erpenbeck EOS (AE-EOS) method allows us to calculate direclty the thermodynamical and chemical equilibrium of a mixture on the hugoniot curve. The ReMC method allow to reach chemical equilibrium of detonation products and the AE-EOS method constraints ths system to satisfy the Hugoniot relation. Once the

Emeric Bourasseau; Vincent Dubois; Nicolas Desbiens; Jean-Bernard Maillet

2007-01-01

422

Molecular simulations of Hugoniots of detonation product mixtures at chemical equilibrium: Microscopic calculation of the Chapman-Jouguet state  

Microsoft Academic Search

In this work, we used simultaneously the reaction ensemble Monte Carlo (ReMC) method and the adaptive Erpenbeck equation of state (AE-EOS) method to directly calculate the thermodynamic and chemical equilibria of mixtures of detonation products on the Hugoniot curve. The ReMC method [W. R. Smith and B. Triska, J. Chem. Phys. 100, 3019 (1994)] allows us to reach the chemical

Emeric Bourasseau; Vincent Dubois; Nicolas Desbiens; Jean-Bernard Maillet

2007-01-01

423

Features in chemical kinetics. I. Signatures of self-emerging dimensional reduction from a general format of the evolution law  

NASA Astrophysics Data System (ADS)

Simplification of chemical kinetics description through dimensional reduction is particularly important to achieve an accurate numerical treatment of complex reacting systems, especially when stiff kinetics are considered and a comprehensive picture of the evolving system is required. To this aim several tools have been proposed in the past decades, such as sensitivity analysis, lumping approaches, and exploitation of time scales separation. In addition, there are methods based on the existence of the so-called slow manifolds, which are hyper-surfaces of lower dimension than the one of the whole phase-space and in whose neighborhood the slow evolution occurs after an initial fast transient. On the other hand, all tools contain to some extent a degree of subjectivity which seems to be irremovable. With reference to macroscopic and spatially homogeneous reacting systems under isothermal conditions, in this work we shall adopt a phenomenological approach to let self-emerge the dimensional reduction from the mathematical structure of the evolution law. By transforming the original system of polynomial differential equations, which describes the chemical evolution, into a universal quadratic format, and making a direct inspection of the high-order time-derivatives of the new dynamic variables, we then formulate a conjecture which leads to the concept of an ``attractiveness'' region in the phase-space where a well-defined state-dependent rate function ? has the simple evolution ??=-?2 along any trajectory up to the stationary state. This constitutes, by itself, a drastic dimensional reduction from a system of N-dimensional equations (being N the number of chemical species) to a one-dimensional and universal evolution law for such a characteristic rate. Step-by-step numerical inspections on model kinetic schemes are presented. In the companion paper [P. Nicolini and D. Frezzato, J. Chem. Phys. 138, 234102 (2013)] this outcome will be naturally related to the appearance (and hence, to the definition) of the slow manifolds.

Nicolini, Paolo; Frezzato, Diego

2013-06-01

424

Features in chemical kinetics. I. Signatures of self-emerging dimensional reduction from a general format of the evolution law.  

PubMed

Simplification of chemical kinetics description through dimensional reduction is particularly important to achieve an accurate numerical treatment of complex reacting systems, especially when stiff kinetics are considered and a comprehensive picture of the evolving system is required. To this aim several tools have been proposed in the past decades, such as sensitivity analysis, lumping approaches, and exploitation of time scales separation. In addition, there are methods based on the existence of the so-called slow manifolds, which are hyper-surfaces of lower dimension than the one of the whole phase-space and in whose neighborhood the slow evolution occurs after an initial fast transient. On the other hand, all tools contain to some extent a degree of subjectivity which seems to be irremovable. With reference to macroscopic and spatially homogeneous reacting systems under isothermal conditions, in this work we shall adopt a phenomenological approach to let self-emerge the dimensional reduction from the mathematical structure of the evolution law. By transforming the original system of polynomial differential equations, which describes the chemical evolution, into a universal quadratic format, and making a direct inspection of the high-order time-derivatives of the new dynamic variables, we then formulate a conjecture which leads to the concept of an "attractiveness" region in the phase-space where a well-defined state-dependent rate function ? has the simple evolution ?[over dot]=-?(2) along any trajectory up to the stationary state. This constitutes, by itself, a drastic dimensional reduction from a system of N-dimensional equations (being N the number of chemical species) to a one-dimensional and universal evolution law for such a characteristic rate. Step-by-step numerical inspections on model kinetic schemes are presented. In the companion paper [P. Nicolini and D. Frezzato, J. Chem. Phys. 138, 234102 (2013)] this outcome will be naturally related to the appearance (and hence, to the definition) of the slow manifolds. PMID:23802945

Nicolini, Paolo; Frezzato, Diego

2013-06-21

425

Theoretical calculations of 95 Mo-NMR chemical shifts for compounds [MoO 4? n S n ] 2?  

Microsoft Academic Search

Summary Theoretical calculation of95Mo-NMR chemical shifts for [MoO4-nSn]2- (n=0-4) compounds is reported here for the first time on the basis of Fenske-Hall method and Sum-Over-State (SOS) perturbation theory. A systematic decrease in shielding of95Mo nuclei with increase of number of sulfur in [MoO4-nSn]2-, which is observed experimentally, can be reasonably explained by our calculation. A good linear relationship between chemical

Sun Yue-ming; Zhu Long-geng; You Xiao-zeng; Jiang Yuang-sheng

1992-01-01

426

Comparative kinetic study between native and chemically modified Cu,Zn superoxide dismutases.  

PubMed Central

The kinetic behaviour of native bovine erythrocyte Cu,Zn superoxide dismutase (N-SOD) and of its derivatives by reaction with polyethylene glycol, acetic and succinic anhydrides has been investigated here in detail. Their responses to changes of pH and ionic strength (I) have been used as a probe for quantitatively displaying the relevance to kinetic rate constant of superficial positive charges driving the superoxide ion (O2-) toward the enzyme's active site. Overall kinetic trends indicate that this long-range O2- electrostatic guidance is essentially due to the positive charges of the amino-acid residues Lys-120 and Lys-134 which are strategically located around the active site. The comparison between the kinetic data obtained from N-SOD and those from polyethylene-glycolated SOD (PEG-SOD) enabled us to state that in PEG-SOD an O2(-)-steering positive electrostatic force, halved in comparison with N-SOD, is still operating, and that only Lys-120 is linked in the reaction of N-SOD with PEG. Elimination of the electrostatic driving force, carried out either by deprotonation of lysine amino groups at high pH, or by their neutralization with succinic anhydride and acetic anhydride, or by ionic screening at high ionic strength, always lowered the kinetic rate constant to a value of approx. 3 x 10(8) M-1.s-1. This value is about 15 times smaller than that measured in the presence of the reactant-steering mechanism and represents the k value of the reaction limited by pure diffusion. Finally, the kinetic behaviour of acetylated SOD and succinylated SOD demonstrated the inhibitor effect of OH- at strongly alkaline pH.

Argese, E; Girotto, R; Orsega, E F

1993-01-01

427

Comparative kinetic study between native and chemically modified Cu,Zn superoxide dismutases.  

PubMed

The kinetic behaviour of native bovine erythrocyte Cu,Zn superoxide dismutase (N-SOD) and of its derivatives by reaction with polyethylene glycol, acetic and succinic anhydrides has been investigated here in detail. Their responses to changes of pH and ionic strength (I) have been used as a probe for quantitatively displaying the relevance to kinetic rate constant of superficial positive charges driving the superoxide ion (O2-) toward the enzyme's active site. Overall kinetic trends indicate that this long-range O2- electrostatic guidance is essentially due to the positive charges of the amino-acid residues Lys-120 and Lys-134 which are strategically located around the active site. The comparison between the kinetic data obtained from N-SOD and those from polyethylene-glycolated SOD (PEG-SOD) enabled us to state that in PEG-SOD an O2(-)-steering positive electrostatic force, halved in comparison with N-SOD, is still operating, and that only Lys-120 is linked in the reaction of N-SOD with PEG. Elimination of the electrostatic driving force, carried out either by deprotonation of lysine amino groups at high pH, or by their neutralization with succinic anhydride and acetic anhydride, or by ionic screening at high ionic strength, always lowered the kinetic rate constant to a value of approx. 3 x 10(8) M-1.s-1. This value is about 15 times smaller than that measured in the presence of the reactant-steering mechanism and represents the k value of the reaction limited by pure diffusion. Finally, the kinetic behaviour of acetylated SOD and succinylated SOD demonstrated the inhibitor effect of OH- at strongly alkaline pH. PMID:8503879

Argese, E; Girotto, R; Orsega, E F

1993-06-01

428

Kinetic roughening during hot-wire chemical vapor deposition of hydrogenated amorphous silicon  

NASA Astrophysics Data System (ADS)

Despite the widespread use of hydrogenated amorphous silicon (a-Si:H), the fundamental surface processes during film growth are not well understood. One approach to studying these mechanisms is to analyze the surface morphology that results from their action. In this dissertation, hot-wire chemical vapor deposition is used to deposit a-Si:H thin films. Both post-deposition atomic force microscopy (AFM) and in situ spectroscopic ellipsometry (SE) are used to characterize the surface morphology and its dynamics. Results of this work indicate that the surface morphology is shaped by geometric shadowing of growth particles and thermally-activated smoothening mechanisms. For films grown at low temperature, the local slope of the surface is found to exhibit power law scaling with time that is consistent with anomalous roughening behavior also observed in models that include shadowing. A temperature-dependent transition in roughening behavior is observed, and an activation energy is extracted that agrees with previous estimates for SiH3 surface diffusion. Additionally, a-Si:H grown on rough substrates is examined. Smoothening at short lateral length scales is observed simultaneously with global roughening. Behavior is found to generally agree with deterministic models in the literature. This work also explores the difference between SE and AFM in how roughness is measured. Rayleigh-Rice theory (vector perturbation theory) is used to calculate ellipsometric data that is subsequently compared to the usual method of using an effective medium layer to approximate roughness. SE measurements are found to critically depend on both the vertical extent of roughness and the root-mean-squared slope of the surface.

Sperling, Brent Andrew

429

Quantum chemical calculations, vibrational studies, HOMO-LUMO and NBO/NLMO analysis of 2-bromo-5-nitrothiazole  

NASA Astrophysics Data System (ADS)

The complete vibrational assignment and analysis of the fundamental modes of 2-bromo-5-nitrothiazole (BNT) was carried out using the experimental FTIR and FT-Raman data and quantum chemical studies. The observed vibrational data were compared with the wavenumbers derived theoretically for the optimized geometry of the compound from the ab initio HF and DFT-B3LYP gradient calculations employing 6-311++G(d,p) basis set. Thermodynamic properties like entropy, heat capacity and zero point energy have been calculated for the molecule. HOMO-LUMO energy gap has been calculated. The intramolecular contacts have been interpreted using Natural Bond Orbital (NBO) and Natural Localized Molecular Orbital (NLMO) analysis. Important non-linear properties such as electric dipole moment and first hyperpolarizability of BNT have been computed using B3LYP quantum chemical calculation.

Gobinath, E.; Xavier, R. John

2013-03-01

430

Chemical kinetic modeling of the oxidation of large alkane fuels: n-octane and iso-octane  

SciTech Connect

The development of detailed chemical kinetic reaction mechanisms for oxidation of n-octane and iso-octane is described, with emphasis on the factors which are specific to many large hydrocarbon fuel molecules. Elements which are of particular importance are found to include site-specific abstraction of H atoms, radical isomerization of alkyl radicals by internal H atom abstraction, and rapid ..beta..-scission of the alkyl radicals. These features, combined with distinctions in the types of intermediate olefin species produced, are used to explain the significant differences in the rate of oxidation between n-octane and iso-octane. 24 refs., 3 figs., 1 tab.

Axelsson, E.I.; Brezinsky, K.; Dryer, F.L.; Pitz, W.J.; Westbrook, C.K.

1986-01-13

431

Molecular structure of tris(cyclopropylsilyl)amine as determined by gas electron diffraction and quantum-chemical calculations  

Microsoft Academic Search

The molecular structure and conformation of tris(cyclopropylsilyl)amine (TCPSA) has been studied by means of gas-phase electron diffraction at 338K and quantum-chemical calculations. A total of 12 relatively stable conformations of TCPSA molecule were considered. According to the experimental results and the DFT calculations the most stable conformer corresponds to a configuration (according to the Prelog–Klyne notation) of the type (?ac)(?ac)(+ac)-(?ac)(?ac)(+ac),

Yuri V. Vishnevskiy; Maxim A. Abaev; Arkadii A. Ivanov; Lev V. Vilkov; Marwan Dakkouri

2008-01-01

432

A Method for Calculating the Shapes of Peaks Resulting from Fragmentations of Metastable Ions in a Mass Spectrometer. II. Peak Shapes Arising from a Distribution of Kinetic Energy Releases: Determination of Distribution Function  

Microsoft Academic Search

Following the calculations described in the preceding paper (part I), which determine the MIKE peak shape arising from a discrete kinetic energy release, a method is presented for extending the calculations for the determination of the kinetic energy release distribution, n(T), from any experimental peak shape. This new approach has the advantage, compared to previous work, that the distribution can

A. Mendez Amaya; A. G. Brenton; J. E. Szulejko; J. H. Beynon

1980-01-01