Showing papers on "Elementary reaction published in 2000"

Mechanism of reaction of myeloperoxidase with hydrogen peroxide and chloride ion

Abstract: The reaction of myeloperoxidase compound I (MPO-I) with chloride ion is widely assumed to produce the bacterial killing agent after phagocytosis. Two values of the rate constant for this important reaction have been published previously: 4.7 x 106 M-1.s-1 measured at 25 degrees C [Marquez, L.A. and Dunford, H.B. (1995) J. Biol. Chem. 270, 30434-30440], and 2.5 x 104 M-1.s-1 at 15 degrees C [Furtmuller, P.G., Burner, U. & Obinger, C. (1998) Biochemistry 37, 17923-17930]. The present paper is the result of a collaboration of the two groups to resolve the discrepancy in the rate constants. It was found that the rate constant for the reaction of compound I, generated from myeloperoxidase (MPO) and excess hydrogen peroxide with chloride, decreased with increasing chloride concentration. The rate constant published in 1995 was measured over a lower chloride concentration range; the 1998 rate constant at a higher range. Therefore the observed conversion of compound I to native enzyme in the presence of hydrogen peroxide and chloride ion cannot be attributed solely to the single elementary reaction MPO-I + Cl- --> MPO + HOCl. The simplest mechanism for the overall reaction which fit the experimental data is the following: MPO+H2O2 rk-1k1 MPO-I+H2O MPO-I+Cl- rk-2k2 MPO-I-Cl- MPO-I-Cl- -->k3 MPO+HOCl where MPO-I-Cl- is a chlorinating intermediate. We can now say that the 1995 rate constant is k2 and the corresponding reaction is rate-controlling at low [Cl-]. At high [Cl-], the reaction with rate constant k3 is rate controlling. The 1998 rate constant for high [Cl-] is a composite rate constant, approximated by k2k3/k-2. Values of k1 and k-1 are known from the literature. Results of this study yielded k2 = 2.2 x 106 M-1.s-1, k-2 = 1.9 x 105 s-1 and k3 = 5.2 x 104 s-1. Essentially identical results were obtained using human myeloperoxidase and beef spleen myeloperoxidase.

Reaction class transition state theory: Hydrogen abstraction reactions by hydrogen atoms as test cases

Abstract: We present a new method called Reaction Class Transition State Theory (RC-TST) for estimating thermal rate constants of a large number of reactions in a class. This method is based on the transition state theory framework within the reaction class approach. Thermal rate constants of a given reaction in a class relative to those of its principal reaction can be efficiently predicted from only its differential barrier height and reaction energy. Such requirements are much less than what is needed by the conventional TST method. Furthermore, we have shown that the differential energetic information can be calculated at a relatively low level of theory. No frequency calculation beyond those of the principal reaction is required for this theory. The new theory was applied to a number of hydrogen abstraction reactions. Excellent agreement with experimental data shows that the RC-TST method can be very useful in design of fundamental kinetic models of complex reactions.

Chemical kinetics of NO removal by pulsed corona discharges

Abstract: Nitric oxides (NO and NO2) and SO2 emissions are a major environmental problem because of their negative influence on human health and vegetation. The federal regulations on limiting the pollution emitted by the engines of motor vehicles have triggered intense research on new techniques for the removal of these pollutants. New methods for exhaust gas cleaning are needed and among the several approaches to reduce the pollutant emissions, the non-thermal plasma technique shows promise (Luo J, Suib S L, Marques M, Hayashi Y and Matsumoto H 1998 J. Phys. Chem. A 102 7954). In this work, a volume-averaged model is presented that can describe the removal of NOx by the multi-pulse treatment of the exhaust gases at low temperatures and at atmospheric pressure in corona reactors. The model takes into account the production of active radicals after every discharge and the removal of NO by these radicals. Furthermore, the effect of ethene, one of the most important unburnt hydrocarbons in the exhaust gas, on the removal of NO is also investigated in this study. The effect of ethene has been investigated experimentally by several authors (Mizuno A, Chakrabati A and Okazaki K 1993 Non-Thermal Plasma Techniques for Pollution Control ed B M Penetrante and S E Schultheis (Berlin: Springer) p 165, Prather M J and Logan J A 1994 Proc. Combustion Institute 25 1513), but there are almost no studies which try to explain, in detail, the chemical processes in such a system. The detailed reaction mechanism used in this study consists of 443 elementary reactions and 50 chemical species. The results of our numerical simulations show good agreement with the experimental results published in the literature. Reaction flow analysis and sensitivity analysis are performed in order to identify the specific reaction paths and the rate-limiting reactions for typical operating conditions of pulsed corona reactors.

Laser Flash Photolysis Studies on the First Superoxide Thermal Source. First Direct Measurements of the Rates of Solvent-Assisted 1,2-Hydrogen Atom Shifts and a Proposed New Mechanism for This Unusual Rearrangement1

Abstract: The thermal decomposition of bis(4-carboxybenzyl)hyponitrite (SOTS-1) in aerated water under physiological conditions has previously been shown to give the superoxide radical anion in a yield of 40 mol % (Ingold, K. U.; et al. J. Am. Chem. Soc. 1997, 119, 12364). The absolute kinetics of the elementary reactions involved in the cascade of events leading from the first-formed water-soluble benzyloxyl radical to superoxide have been determined by laser flash photolysis. On the basis of these kinetics it is concluded that SOTS-1 will be suitable for studies of superoxide-induced oxidative stress in most biological systems. A water-assisted 1,2-H shift converting benzyloxyl into the benzyl ketyl radical is an important step in the above reaction cascade. The kinetics of the 1,2-H shift assisted by H2O, D2O, and a number of nucleophilic alcohols have been measured for the first time. These data have led to a proposed new mechanism involving the initial formation of a ketyl radical anion and an oxonium cation w...

Multiple dynamical pathways in the O(1D)+CH4 reaction: A comprehensive crossed beam study

Abstract: In this report, the O(1D)+CH4 reaction has been reinvestigated using universal crossed molecular beam methods. Angular resolved time-of-flight spectra have been measured for various reaction channels of the title reaction: OH+CH3, H+H2COH/H3CO, and H2+HCOH/H2CO. Different product angular distributions have been observed for these product channels, indicating that these reaction channels occur via distinctive dynamical pathways. This study provides an excellent example of multiple dynamical pathways in a single chemical reaction, which opens enormous opportunities in investigating the dynamics of complicated chemical reactions that are important in combustion and atmospheric chemistry, and also provides a link between kinetics studies and dynamical research.

Elementary Reaction Mechanism for Benzene Oxidation in Supercritical Water

Abstract: Supercritical water (SCW) benzene oxidation data were modeled using a published, low-pressure ( 220 bar) conditions, new reaction pathways were added, and quantum Rice−Ramsperger−Kassel theory was used to calculate the rate coefficients and, hence, product selectivities for pressure dependent reactions. The most important difference between the benzene oxidation mechanism for SCW conditions and those for combustion conditions is reactions in SCW involving C6H5OO predicted to be formed by C6H5 reacting with O2. Through the adjustment of the rate coefficients of two thermal decomposition pathways of C6H5OO, whose values are unknown, the model accurately predicts the measured benzene and phenol concentration profiles at 813 K (540 °C), 246 bar, stoichiometric oxygen, and 3...

Transition states for surface-catalyzed chemistry.

Abstract: Fluorine substituent effects have been used to probe the nature of the transition states for the several elementary reaction steps occurring on metal surfaces. The reactions described include β-hydrogen elimination in adsorbed alkoxide and alkyl groups, coupling of alkyl groups, and dehalogenation of alkyl chloride and iodides. The substituent effect method can provide a connection between heterogeneous catalysis, surface science, and computational molecular simulation of surface reactions.

Thermochemical Property, Pathway and Kinetic Analysis on the Reactions of Allylic Isobutenyl Radical with O2: an Elementary Reaction Mechanism for Isobutene Oxidation

Abstract: Kinetics for the reactions of allylic isobutenyl radical (CC(C)C) with molecular oxygen are analyzed by using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) and master equation analysis for falloff. Thermochemical properties and reaction path parameters are determined by ab initio−Moller−Plesset (MP2(full)/6-31g(d) and MP4(full)/6-31g(d,p)//MP2(full)/6-31g(d)), complete basis set model chemistry (CBS-4 and CBS-q with MP2(full)/6-31g(d) and B3LYP/6-31g(d) optimized geometries), and density functional (B3LYP/6-31g(d) and B3LYP/6-311+g(3df,2p)//B3LYP/6-31g(d)) calculations. An elementary reaction mechanism is constructed to model the experimental system, isobutene oxidation. The forward and reverse rate constants for initiation reaction C2CC + O2 ↔ CC(C)C + HO2 are determined to be 1.86 × 109 T1.301 exp(−40939 cal/RT) (cm3 mol-1 s-1) and 6.39 × 108 T0.944 exp(−123.14 cal/RT) (cm3 mol-1 s-1), respectively. Calculations on 2,5-dimethylhexa-1,5-diene, methacrolein, isobutene oxides, and acetone product f...

Site-Selective Surface Reactions: Hydrocarbon Oxidation Processes on Oxidized Mo(110)

Abstract: The site specificity for elementary steps important in hydrocarbon oxidation are investigated on oxidized Mo(110). Our studies illustrate how detailed mechanistic information is obtained about surface reactions by separating complex problems into elementary reaction steps on well-defined materials. This methodology is particularly powerful for the study of fundamental structure−reactivity relationships, including issues of site specificity. Methods for synthesizing and characterizing oxidized surfaces with specific types of oxygen coordination are described to demonstrate how a model system can be constructed. We have focused on the C−O bond formation step in the oxidation of methane via our investigations of methyl radical reactions with surface oxygen and the microscopic reverse, methyl evolution from adsorbed methoxy. A combination of infrared and electron energy loss spectroscopies is used to identify specific intermediates on the surface. Our results are discussed in terms of hydrocarbon oxidation on...

Influence of coke formation on the conversion of hydrocarbons I. Alkanes on a USY-zeolite.

Abstract: The conversion of n -hexane and 2,2,4 triMe-pentane was studied at 723 K on a USY-zeolite (Si/Al: frame=30; bulk=2.7) in an electrobalance reactor with external recirculation. The influence of the coke content on the reaction paths involved in the conversion of paraffinic model components was evaluated. Coke formation from propylene and i-butene, the main olefinic products formed during the conversion of the two paraffinic model components, was investigated. Under the conditions used, an increase in the molar H/C ratio of the products as a function of coke yield can be observed due to hydride transfer reactions with coke. Coke molecules cannot be considered as being inert with respect to the cracking reactions and their formation leads to the occurrence of reactions that can strongly influence the catalyst performance. In particular, the potential of coke molecules to form highly delocalized carbenium ions and their ability to act as hydride donor towards surface carbenium ions provides reaction paths to paraffinic reaction products. The effect of coke is not identical for all the elementary reactions involved in the conversion of the paraffinic model components.

A master equation model for bimolecular reaction via multi-well isomerizing intermediates

Abstract: A reversible linear master equation model is presented for pressure- and temperature-dependent bimolecular reactions proceeding via multiple long-lived intermediates. This kinetic treatment, which applies when the reactions are measured under pseudo-first-order conditions, facilitates accurate and efficient simulation of the time dependence of the populations of reactants, intermediate species and products. Detailed exploratory calculations have been carried out to demonstrate the capabilities of the approach, with applications to the bimolecular association reaction C3H6+H⇌C3H7 and the bimolecular chemical activation reaction C2H2+1CH2→C3H3+H. The efficiency of the method can be dramatically enhanced through use of a diffusion approximation to the master equation, and a methodology for exploiting the sparse structure of the resulting rate matrix is established.

Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X 1A′) formation from reaction of C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1)

Abstract: The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X 1A′), via reactive collisions of the phenyl radical C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X 1A1), was used to identify the site of the carbon–hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol−1. Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the π el...

Analysis of an elementary reaction mechanism for benzene oxidation in supercritical water

Abstract: A benzene supercritical water oxidation (SCWO) mechanism, based on published low-pressure benzene combustion mechanisms and submechanisms describing the oxidation of key intarmediates, was developed and analyzed to determine the controlling reactions under SCWO conditions of 750–860 K, 139–278 bar, and equivalence ratios from 0.5 to 2.5. To adapt the combustion mechanims to the lower temperature ( 220 bar) conditions, new reaction pathways were added, and quantum Rice-Ramsperger-Kassel theory was used to calculate the rate coefficients and, hence, product selectivities for pressure-dependent reactions. The most important difference between the benzene oxidation mechanism for supercritical water conditions and those for combustion conditions is reactions in supercritical water involving C 6 H 5 OO predicted to be formed by C 6 H 5 reacting with O 2 . Through the adjustment of the rate coefficients of two thermal decomposition pathways of C 6 H 5 OO, whose values are unknown, the model accurately predicts the measured benzene and phenol concentration profiles at 813 K, 246 bar, stoichiometric oxygen, and 3–7 s residence time and reproduces the finding that the carbon dioxide concentration exceeds that of carbon monoxide at all reaction conditions and levels of benzene conversion. Comparison of the model predictions to benzene SCWO data measured at several different conditions reveals that the model qualitatively explains the trends of the data and gives good quantitative agreement without further adjustment of rate coefficients.

Observation of laser speckle effects and nonclassical kinetics in an elementary chemical reaction.

Abstract: An experimental demonstration is provided for memory-based, nonclassical reaction kinetics in a homogeneous system with an elementary reaction, $A+B\ensuremath{\rightarrow}C$. A new reaction-kinetics regime is observed which is a direct consequence of speckles in the laser beam. However, in spite of the nonrandom, speckled initial distribution of reactant $B$, the long-time regime gives the first experimental demonstration of the asymptotic self-segregation (``Zeldovich'') effect. Monte Carlo simulation results are consistent with the experiments.

Dynamic monte carlo simulations of catalytic surface reactions

Abstract: A general-purpose program for dynamic Monte Carlo (DMC) simulations of catalytic surface reactions has been developed. The stochastic model is based on the master equation. Inputs for the program are the catalytic surface, the adsorbates, the elementary reaction steps, and the adsorbate-adsorbate interactions, which can be defined for a variety of systems. The computer code is especially applicable to structured surfaces, which can be defined as bitmap by common drawing software. Dissociative or non-dissociative adsorption, associative or single desorption. Langmuir-Hinshelwood and Eley-Ridel reactions as well as surface diffusion can be used in the surface reaction mechanism. The performance of the algorithm is optimized by a combination of two different solution methods. We present three example models, all of them showing a strong connection between surface structures and chemical reactions: inhomogenities on Pt(111) may act as nucleation centers either for the formation of CO or oxygen islands during kinetic phase transitions of CO oxidation. Model calculations reproduce experimentally observed structure formation at ultra high vacuum conditions. Adsorbate-induced changes of the surface structure leads to spatiotemporal pattern formation during CO oxidation on Pt(100) and Pt(110). DMC simulations based on the reconstruction model generate target pattern and cellular structures similar to experimental findings. A common class of real catalysts consist of nanoparticles deposited on a support. Mechanistic steps on facet boundaries of supported nanoparticles significantly influence the catalytic behavior. DMC calculations can be used to simulate different scenarios to get an insight into the principles of real catalysts.

Two-dimensional simulation of an oxy-acetylene torch diamond reactor with a detailed gas-phase and surface mechanism

Abstract: A two-dimensional model is presented for the hydrodynamics and chemistry of an oxy-acetylene torch reactor for chemical vapor deposition of diamond, and it is validated against spectroscopy and growth rate data from the literature. The model combines the laminar equations for flow, heat, and mass transfer with combustion and deposition chemistries, and includes multicomponent diffusion and thermodiffusion. A two-step solution approach is used. In the first step, a lumped chemistry model is used to calculate the flame shape, temperatures and hydrodynamics. In the second step, a detailed, 27 species / 119 elementary reactions gas phase chemistry model and a 41 species / 67 elementary reactions surface chemistry model are used to calculate radicals and intermediates concentrations in the gas phase and at the surface, as well as growth rates. Important experimental trends are predicted correctly, but there are some discrepancies. The main problem lies in the use of the Miller‐Melius hydrocarbon combustion mechanism for rich oxy-acetylene flames. @J. A. Miller and C. F. Melius, Combustion and Flame 91 ,2 1~1992!#. Despite this problem, some aspects of the diamond growth process are clarified. It is demonstrated that gas-phase diffusion limitations play a minor role in the diamond growth process, which is determined by surface kinetics. Except for atomic hydrogen, gas phase diffusion is also of minor importance for the transport of species in and behind the flame front. Finally, it is shown that penetration of nitrogen from the ambient air into the flame cannot explain the observed changes at the center of the diamond films as reported in the literature. © 2000 American Institute of Physics.@S0021-8979~00!06420-3#

Simulation and Analysis of Laminar Flow Reactors

Abstract: Laminar flow reactors are frequently used to experimentally study an isolated elementary reaction step as well as chemical kinetic mechanisms of many coupled reactions. This classical method is effective in measuring kinetic rate parameters when the effects of mass diffusion and wall surface reactions can be neglected or accurately assessed. We perform a series of two-dimensional direct numerical simulations to investigate issues related to the operation of this classical apparatus. By utilizing a well-established gas phase kinetic mechanism for moist CO oxidation and a commonly used sub-model for multi-component diffusive transport, we investigate a virtual elementary kinetic experiment. In particular, we extract data from the simulations and evaluate the rate parameters of the reaction CO + OH ⇄ CO2 + H as one would in an actual experiment. We show that under appropriate operating conditions, the desired elementary reaction rate parameters can be recovered accurately with minimal efforts in ana...

A simple model for product rovibrational distributions in elementary chemical reactions

Abstract: We explore the application of a simple model of collisional processes, developed initially for inelastic collisions, to the analysis of product rovibrational states in elementary chemical reactions. The model depicts collisional transfer as a process of momentum exchange ~predominantly linear-to-angular momentum! and is modified to take account of change in center-of-mass and enthalpy change that accompany reaction. The kinematics of center-of-mass shift derived by Elsum and Gordon @J. Chem. Phys. 76, 3009 ~1982!# lead to two limiting cases based on the parameter b. The kinematic extremes alternatively may be specified in terms of the molecular torque arm about which interconversion of linear and angular momentum is effected. This torque arm length approximates to the product bond length when b.0 and the reactant bond length when b.90°. Our approach shares elements in common with the classical kinematic model of Elsum and Gordon but is somewhat simpler and more transparent. The method is shown to give accurate peak values of v, j states of the products of a wide range of elementary reactions for which experimental data is available. Monte Carlo trajectory calculations based on the physical principles described here give excellent fits to experimental v, j distributions in F+I2-->IF+I, H+D2-->HD+D, and Cl+H2-->HCl+H using input data consisting of atomic radii, atomic masses, velocities, and reaction enthalpies.

Reaction Rate Theory

Abstract: IN CHAPTERS 15 AND 16, WE SAW THAT ELEMENTARY REACTIONS may be unimolecular, bimolecular, or termolecular. In a unimolecular step, a molecule with excess energy concentrates enough energy in the bond or bonds to be broken and movement to product occurs. In a bimolecular or termolecular step, the reactant molecules come together in an inelastic collision. When enough kinetic energy is transformed to potential energy, movement to products occurs. In most cases, these processes involve passage over a potential energy barrier, as figureindicates.