Showing papers on "Elementary reaction published in 1984"

Rate Coefficients in the C/H/O System

Abstract: This chapter is a critical survey of reaction rate coefficient data important in describing high-temperature combustion of H2, CO, and small hydrocarbons up to C4. A recommended reaction mechanism and rate coefficient set is presented. The approximate temperature range for this mechanism is from 1200 to 2500 K, which therefore excludes detailed consideration of cool flames, low-temperature ignition, or reactions of organic peroxides or peroxy radicals. Low-temperature rate-coefficient data are presented, however, when they contribute to defining or understanding high-temperature rate coefficients. Because our current knowledge of reaction kinetics is incomplete, this mechanism is inadequate for very fuel-rich conditions (see Warnatz et al., 1982). For the most part, reactions are considered only when their rates may be important for modeling combustion processes. This criterion eliminates considering many reactions among minor species present at concentrations so low that reactions of these species cannot play an essential part in combustion processes. The philosophy in evaluating the rate-coefficient data was to be selective rather than exhaustive: Recent results obtained with experimental methods capable of measuring isolated elementary reaction rate parameters directly were preferred, while results obtained using computer simulations of complex reacting systems were considered only when sensitivity to a particular elementary reaction was demonstrated or when direct measurements are not available. Theoretical results were not considered.

A Mathematical Model of the Coupled Fluid Mechanics and Chemical Kinetics in a Chemical Vapor Deposition Reactor

Abstract: We 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 of susceptor temperature, carrier gas, pressure, and flow velocity We find excellent agreement with experimental deposition rates, with no adjustment of parameters The model indicates that gas‐phase chemical kinetic processes are important in describing silicon CVD

Location of transition states in reaction mechanisms

Abstract: A general method is described for the location of transition states in reaction mechanisms. Transition states for unimolecular and bimolecular reactions can be identified. The procedure is fully automatic, once reactants and products are characterised, and no assumptions as to the geometry of the transition state or of the mechanism are necessary. Examples are given of the Cope and Claisen reactions.

A Comprehensive Chemical Kinetic Reaction Mechanism for Oxidation and Pyrolysis of Propane and Propene

Abstract: Abstract—A detailed chemical kinetic reaction mechanism is developed to describe the oxidation and pyrolysis of propane and propene. The mechanism consists of 163 elementary reactions among 4l chemical species. New rate expressions are developed for a number of reactions of propane, propene, and intermediate hydrocarbon species with radicals including H, 0, and OH. The mechanism is tested by comparisons between computed and experimental results in shock tubes and the turbulent flow reactor. The resulting comprehensive mechanism accurately reproduces experimental data for pressures from 1 to 15 atmospheres, temperatures from 1000 to 1700 K, and fuel-oxidizer equivalence ratios from 0.066 to pyrolysis conditions. The mechanism also predicts correctly laminar flame properties for propane and propene, and detonation properties for propane.

Rate Coefficients of Thermal Dissociation, Isomerization, and Recombination Reactions

Abstract: In this chapter we describe the estimation and interpretation of rate coefficients for thermal dissociation, isomerization, and recombination reactions. While such reactions are only a small fraction of the elementary reactions of combustion mechanisms, some of them do play essential roles. Their kinetic behavior is governed by the competition of unimolecular “chemical” changes in molecular structure with bimolecular “physical” collisional energization and deenergization processes.

Direct determination of the rate constant for the reaction CH2+O2 with a LMR-spectrometer

Abstract: The reaction was studied at room temperature in an isothermal discharge flow system. CH2-radicals were produced either in a microwave discharge of CH2CO highly diluted in Helium or via the reaction O + CH2CO CH2 + CO2. The CH2-concentration was measured by laser magnetic resonance. From the pseudo first order decay of CH2 in the presence of a large excess of oxygen the rate constant for reaction (1) was found to be

Direct Determination of the Rate Constant for the Reaction CH2 + H → CH + H2

Abstract: The reaction was investigated at room temperature in an isothermal discharge flow reactor with laser magnetic resonance detection of CH2 and CH. Direct measurements of the decay of CH2 in the presence of an excess of H-atoms yielded a rate constant of Combined with the known rate constant for the reverse reaction CH + H2 + CH2 + H the heat of formation of CH2 was found to be

On the Mechanism of Reversible Unimolecular Reactions and the Canonical (“High Pressure”) Limit of the Rate Coefficient at Low Pressures*)

Abstract: Jost's [1] treatment of generalized first order kinetics is used for a new discussion of the mechanism of reversible unimolecular reactions. A detailed analysis of time dependent average microscopic rate coefficients and level populations is presented using exact solutions for a realistic model of an isomerization reaction. It is shown that unimolecular fall-off for the macroscopic relaxation is not mainly due to a reduced population of reactive levels as suggested by the irreversible Lindemann mechanism, but rather due to a fast reverse reaction even at early times. It is furthermore shown that the average microscopic rate coefficients can be measured directly, in principle. At equilibrium they are equal to the canonical (“high pressure”) limit of the rate coefficient even for arbitrarily low pressures. Practical experimental schemes are suggested for thus obtaining high pressure limiting rate coefficients without extrapolation procedures.

Rate Constants and Vinoxy Product Yield in the Reaction OH + Ethylene Oxide

Abstract: Absolute rate constants and the vinoxy (CH2CHO) product yield for the reaction of OH radicals with ethylene oxide (C2H4O) have been determined using a combined laser photolysis/resonance fluorescence (laser induced fluorescence) technique. The rate constant for OH decay is k1 = (1.1 ± 0.4)- 10−11 exp[-(1460 ± 150) K/T] cm3/s over the temperature range T = 298–435 K. The branching ratio for vinoxy (VO) formation at 298 K is found to be ϕvo = 0.08 and 0.23 at 13 and 80 mbar, respectively. A potential energy scheme accounting for reagent ring opening and the formation of vinoxy is presented.

Studies of the decomposition of oxirane and of its addition to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Abstract: Elementary reactions involved in the oxidation and decomposition of oxirane have been studied in two ways at 480 °C. First, by adding small amounts of oxirane to slowly reacting mixtures of H2+O2, rate constants have been determined for H and OH attack on the additive: H + C2H4O → C2H3O + H2(22H) OH + C2H4O → C2H3O + H2O. (21H) Combination with independent low-temperature data gives values of log10(A22H/dm3 mol–1 s–1)= 10.9±0.2, E22H= 41±2 kJ mol–1 and log10(A21H/dm3 mol–1 s–1)= 10.25±0.15, E21H= 15.1 ± 1.9 kJ mol–1. The major products are CH4 and CO, and CO, and a value of (7.5 ± 2)× 105 dm3 mol–1 s–1 is obtained for the rate constant of the CH3+H2 reaction. Secondly, decomposition studies have been carried out at total pressures (with N2) of 60 and 500 Torr with oxirane pressures varied between 1 and 20 Torr. The major products are CH4, CO, CH3CHO, C2H6, C2H4 and H2, and a comprehensive mechanism is presented to account for their occurrence. In addition to consumption in free-radical processes through H and CH3 attack, oxirane isomerises to give excited CH3CHO* molecules which undergo three competing reactions: [graphic omitted] Little effect of total pressure is observed on the relative rate of each process. From the relative yields of CH4 and C2H6, a value of k4=(4.0 ± 0.2)× 105 dm3 mol–1 s–1 is obtained, which in combination with low-temperature data gives log10(A4/dm3 mol–1 s–1)= 9.03±0.15 and E4= 49.5±1.3 kJ mol–1. It is suggested that C2H4 is formed in radical–radical reactions of the CH2CHO radical. Evidence is presented to support the view that the C—H bond dissociation energy in oxirane is very similar to that in C2H6, and values of D([graphic omitted])= 420.2±4.0 kJ mol–1 and ΔfHo298(C2H3O)= 149.6 kJ mol–1 are suggested: CH3+ C2H4O → CH4+ C2H3O. (4)

Elementary reactions of the SF radical. Part 1.—Rate constants for the reactions F + OCS → SF + CO and SF + SF → SF2+ S

Abstract: Elementary reactions involving ground-state SF(X2Π) radicals have been studied at 295 K in a discharge-flow system near 1 Torr‡ using mass-spectrometric detection with collision-free sampling. SF radicals were generated by the reaction of F(2PJ) atoms with OCS: F + COS [graphic omitted] SF + CO (1). Rate constants are reported for reaction (1) as well as for the bimolecular disproportionation of SF SF + SF [graphic omitted] SF2+ S (2a)k1=(1.81 ± 0.16)× 10–11; k2a=(2.52 ± 0.19)× 10–11 cm3 s–1(1σ).In addition, the N(4S)+ NO reaction has been studied as a reference reaction N + NO [graphic omitted] N2+ O and the following result obtained: k3=(2.03 ± 0.17)× 10–11 cm3 s–1(1σ).

Rate Constant of the Reaction HO + CO in the Presence of N2 and O2

Abstract: The kinetics of the reaction HO + CO was studied in the presence and in the absence of O2 using VUV flash photolysis of H2O and kinetic absorption spectroscopy of HO. In the absence of O2, the value of the rate constant was found to vary from 1.4 · 10−13 cm3 s−1 at low total pressures to (2.30 ± 0.16) · 10−13 cm3 s−1 at atmospheric pressure of N2. In the presence of traces of O2, a complex reaction mechanism is necessary to explain the HO decays. With recent rate data, the mechanism allows us to reinterpret the experimental data of Biermann et al. [9] which were significantly influenced by secondary reactions thus simulating the previously reported O2-effect.

Unimolecular reaction kinetics

Abstract: We use generating functions to analyze the kinetics of the reaction $A+B\ensuremath{\rightarrow}(1+\ensuremath{\epsilon})A+B$ where $\ensuremath{-}1l~\ensuremath{\epsilon}l0$ and $\ensuremath{\epsilon}g0$ correspond, respectively, to $B$ particles that are traps or "sources." For an arbitrary configuration and strengths of static $B$'s and mobile $A$'s, an exact formal expression for the kinetics of the reaction is derived. This solution yields either exponential growth, power-law growth or decay, or no growth asymptotically, depending on the configuration and strength of the $B$'s.

The reaction of CO+2 with atomic hydrogen

Abstract: Using a quasistatic drift tube, a constant rate coefficient k=2.9×10−10 cm3 s−1±30% has been obtained for the reaction of CO+2 with atomic hydrogen at relative energies 0.06 eV≤KEcm≤0.14 eV. Due to the presence of molecular hydrogen in the reaction region, the title reaction is competing with the reaction of CO+2 with H2. A method of separating the two reactions is presented.

Reaction ergodography for unimolecular decomposition of ethanol

Abstract: Some unimolecular decomposition paths of ethanol were analyzed by ab initio MO (Molecular Orbital) calculations in relation to the mode-selective chemical reactions. The geometries and energies of the reactants, transition states, reaction intermediates, and products have been determined for five elementary reaction processes on the singlet potential energy surface of the ground state. The most energetically preferable path is the one for the formation of ethylene. Reaction ergodography along the intrinsic reaction coordinate (IRC) for the decomposition path leading to ethylene clarified two distinct role of the vibrational modes of ethanol: (1) the vibrational mode excited by the CO/sub 2/ laser corresponds to the direction of the reaction path to ethylene and (2) the vibrational mode excited by the HF laser plays a role of energy transfer to the decomposing path to ethylene.

Direct Investigation of the Reaction CH2(X̃3B1) + O(3P) with the LMR

Abstract: The reaction has been studied at room temperature in an isothermal discharge flow reactor. Methylene-radicals and oxygen-atoms have been monitored with a Far-Infrared Laser Magnetic Resonance spectrometer. The rate constant of reaction (1) was evaluated from the pseudo first order decay of CH2 in the presence of a large excess of O-atoms. The influence of side and consecutive reactions has been investigated in a computer simulation of the reaction system. — The final result for the rate constant of reaction (1) is The experimental value is compared to theoretical predictions for the high pressure recombination rate constant of CH2 and O.

Decomposition of 2,3-dimethylbutane in the presence of oxygen. Part 2.—Elementary reactions involved in the formation of products

Abstract: A detailed product analysis has been carried out over a wide range of mixture composition on the decomposition of 2,3-dimethylbutane (DMB) in the presence of O2 in KCl-coated vessels between 480 and 540 °C. Propene, 2-methylbut-2-ene, CH4, HCHO and 2,3-dimethylbut-1-ene are the major initial products, with smaller amounts of 2,3-dimethylbut-2-ene, 3-methylbut-1-ene, but-2-ene and, at high O2 pressures, tetramethyloxirane. By extrapolation of product yields to zero O2 and DMB pressures to remove contributions from the chain processes, values of k0 of 0.0211 ± 0.0027 dm3 mol–1 S–1 at 500 °C and 0.163 ± 0.011 dm3 mol–1 S–1 at 540 °C are obtained: (CH3)2 CHCH(CH3)2+ O2→(CH3)2 CHC(CH3)2+ HO2(0), No other values of k0 available for DMB or any analogous reaction involving a hydrocarbon, despite the importance of this type of reaction in combustion. The recommended Arrhenius parameters are log10(A0/dm3 mol–1 S–1)= 10.31 ± 0.25 and E0= 173 ± 3.5 kJ mol–1. These parameters are consistent with a dissociation energy of 387 kJ mol–1 for the tertiary C—H bond in DMB, in excellent agreement with recent values for that in i-butane.Detailed studies of the relative yields of the products have permitted rate constants to be obtained, and Arrhenius parameters to be recommended, for a number of the elementary steps in the DMB + O2 system. Patterns in the variation of rate constants for different alkyl radical homolyses and alkyl + O2 reactions are discussed.

Negative Temperature Coefficient in Chemical Reactions

Abstract: A systematic analysis of reactions whose rate decreases with increase of temperature is presented. The possibility of a negative temperature coefficient in the elementary reactions is examined from the standpoint of the transition state theory and of collision theory. The mechanisms of complex reactions in which the temperature dependence of the rate is anomalous are discussed, and possible reasons for the anomaly are examined. The bibliography contains 175 references.

Kinetics and the mechanism of the thermal reaction between trifluoromethylhypofluorite and hexafluoropropene

Abstract: The kinetics of the thermal reaction between CF3OF and C3F6 have been investigated between 20 and 75°C. It is a homogeneous chain reaction of moderate length where the main product is a mixture of the two isomers 1-C3F7OCF3 (68%) and 2-C3F7OCF3 (32%). Equimolecular amounts of CF3OOF3 and C6F14 are formed in much smaller quantities. Inert gases and the reaction products have no influence on the reaction, whereas only small amounts of oxygen change the course of reaction and larger amounts produce explosions. The rate of reaction can be represented by eq. (I): The following mechanism explains the experimental results: Reaction (5) can be replaced by reactions (5a) and (5b), without changing the result: Reaction (4) is possibly a two-step reaction: For ∣CF3 = ∣C3F6∣, ν20°C = 36.8, ν50°C = 24.0, and ν70°C = 14.2.

Kinetics of the reaction H + C2H6 → H2 + C2H5 in the temperature region of 1000 K

Abstract: A system for the measurement of rate constants for elementary reactions of hydrogen atoms in the temperature region of 1000 K is described. The concentration of hydrogen atoms is controlled by the ...

Pressure and Temperature Dependence of the Reaction ClO + NO2 (+ N2) → ClONO2 (+ N2)

Abstract: Rate constants for the recombination reaction (1) ClO + NO2 (+ N2) ClONO2 (+ N2) have been measured in the pressure range 23 – 1040 mbar and at temperatures of 264, 298 and 343 K using flash photolysis to generate ClO radicals and time dependent UV absorption in the A-X system as their monitor. k1, is found to show a strong pressure dependence with an onset to “fall-off” at pressures above ∼ 50 mbar. The “fall-off” behaviour is interpreted in terms of complete and symmetrical Kassel integrals and is found to be in agreement with the accepted low pressure limit as well as with a theoretical estimate of the high pressure limiting rate constant, k1∞ = 1.2 · 10−11 cm3/s.

Synthesis of macromolecular antioxidants by reaction of aromatic amines with epoxidized polyisoprenes, 2. Kinetics and mechanism of model reactions

Abstract: Kinetic and thermodynamic parameters were determined for the reaction of the unsymmetrical oxiranes 3,4-epoxy-methylheptane (1) and 1,2-epoxy-3-ethyl-2-methylpentane (4) with aniline in o-dichlorobenzene. A kinetic scheme involving two elementary reactions agrees well with the experimental results obtained for the phenol catalysed reactions. The thermodynamic parameters, especially the entropy, lead to the proposal of a concerted transfer mechanism allowing for the observed normal and abnormal addition reactions.