Showing papers on "Elementary reaction published in 1979"

Comprehensive Mechanism for Methanol Oxidation

Abstract: A detailed chemical kinetic mechanism, involving 26 chemical species and 84 elementary reactions, is proposed for the oxidation of methanol Within the scope of this mechanism, turbulent flow reactor and shock tube experimental data are used to determine rate expressions for several of the important reactions involving CH3OH and its intermediate product species, CH2OH Calculations using the proposed mechanism and elementary reaction rates accurately reproduce experimental results over a combined temperature range of 1000-2180K, for fuel-air equivalence ratios between 005 and 30 and for pressures between 1 and 5 atmospheres The resulting chemical kinetic model is then employed, together with an unsteady, one-dimensional numerical model for flame propagation, to predict the laminar flame speed of a stoichiometric methanol-air mixture The calculated laminar flame speed is 44+ 2 cm/ sec and is in good agreement with experimentally observed values

Kinetic mechanism, structure and properties of premixed flames in hydrogen—oxygen—nitrogen mixtures

Abstract: The composite flux method described by Dixon-Lewis, Goldsworthy & Greenberg (1.975 a )for the computation of detailed temperature and composition profiles in suitable flames has been applied to the simulation of the properties of a number of fuel-rich and fuel-lean hydrogen-oxygen-nitrogen flame systems. The reaction mechanism proposed by Day, Dixon-Lewis & Thompson (1972), extended to include all the reverse reactions, has been used in the simulation, together with assumed sets of reaction rate and transport parameters. The computed profiles have then been compared with published measurements in flames, covering a wide range of experimental conditions, in order to arrive iteratively at an optimum, self-consistent set of rate parameters which also takes full account of the available elementary reaction rate data from sources other than flames. The flame properties considered in this part of the investigation were ( a ) radical recombination profiles in both fuel-rich and fuel-lean flames, and ( b ) the burning velocities and properties of the main reaction zones of several low temperature, slow burning, fuel-rich flames. Three sets of rate parameters which satisfy all the constraints, and which differ only in detail, are given as sets 1, 2 and 3 in table 4 of the paper. Measurements by Kaskan (1958 b ) of radical recombination in the hydrogen-lean systems have used the (0, 0) band ultraviolet absorption of the hydroxyl radical in order to measure its concentration. The interpretation of the measurements so as also to be consistent with the remaining flame measurements by other methods additionally allows a determination of the oscillator strength associated with the transition. A band oscillator strengths f 00 — 9.5 x 10 -4 was found. Following the establishment of the reaction rate parameters, one set of these (table 9) was used to calculate the expected properties of the whole composition range of hydrogen-air premixed flames. In these cases, as well as in the calculations already summarized, either partial equilibrium or kinetic quasi-steady state assumptions must be used in conjunction with the composite flux method. Partial equilibrium assumptions on the reactions OH + H 2 ⇌ H 2 O + H , ( i ) H + O 2 ⇌ OH + O , ( ii ) O + H 2 ⇌ OH + H , ( iii ) may be employed to relate the concentrations of H, OH, O and O 2 in calculations where only the concentration profiles in the recombination regions of the flames are required. In the calculation of complete flame properties, quasi-steady state assumptions must be used to relate the concentrations either of O, OH and HO 2 with that of H (rich flame formulation), or of H, O and HO 2 with that of OH (lean flame formulation). Subsequent investigation showed that the quasi-steady state assumptions were not completely valid for oxygen atoms everywhere in the flames. Nevertheless, further calculations on several flames by the completely different approach of implicit finite difference solution of the time-dependent flame equations, which does not involve any quasi-steady state assumptions, led to results essentially identical with the original computations. The departures from the quasi-steady state do not therefore significantly affect the flame properties computed by the composite flux method. The general pattern of flame structure which emerges from the complete flame calculations is one in which radicals are produced by chain branching reactions in the hotter regions of the flames, while the major heat releasing reactions occur at lower temperatures. Ahead of the heat release zone there is only a very small preheat zone where heating occurs purely by thermal conduction. This behaviour is different from that of flame models which assume a large preheat zone coupled with a single global exothermic reaction of high activation energy. Comparison of the results of calculations which employed respectively the partial equilibrium and quasi-steady state assumptions showed that the former were valid in the ‘recombination zones’ of the flames for predicting the concentrations of those species which are present in significant amounts. Except in lower temperature flames, for example the 15% hydrogen-air flame and to some extent the 70% hydrogen-air flame, the ‘recombination zones’ extend almost back from the hot boundaries of the flames to the maxima in the hydrogen atom mole fraction profiles. On continuing the flame integrations back from the recombination zones into the main reaction zones, the quasi-steady state overall radical concentrations, represented by X H + 2 X O + X OH , where X is mole fraction, fall below those calculated with the partial equilibrium assumptions. On the other hand, the distribution of the radical pool between H, O and OH is such that in fuel-rich flames the comparatively small quasi-steady state oxygen atom concentration, and to a lesser extent also the hydroxyl radical concentration, appreciably overshoot their partial equilibrium values. This is referred to as kinetic overshoot . It is observable only in sufficiently fuel-rich flames, and for example, there is no observable hydroxyl radical kinetic overshoot in hydrogen-air flames containing less than about 50 % hydrogen, and no similar oxygen atom overshoot in those with less than 30 % hydrogen. A fundamental feature of the flame model used is that it assumes a state of thermal equilibrium to exist at each point in the flames, so that the properties of the gas at each point can be represented by a single temperature. This assumption may not be valid in faster flames, because of the finite velocities of relaxation of thermal disequilibrium between the various degrees of freedom in the system. Properly carried out, a comparison of the computed burning velocities of the hydrogen—air flames with experimental observation should throw light on the possible effect of such slow relaxation on the flame properties. However, an attempt at such a comparison initially raised several questions about the interpretation of burning velocity measurements. These are fully discussed. The hydrogen—air flame having the maximum burning velocity is that containing 41 % hydrogen. At this composition it is concluded that the true burning velocity lies in the range (285 ± 10) cms -1 , and hence is not more than 4 or 5 % above the computed value. Finally, the effect on the computed flame properties of ( a ) changes in the diffusion coefficient and ( b ) neglect of thermal diffusion of hydrogen atoms was investigated. Other conditions being equal in fuel-rich hydrogen-air flames near stoichiometric, the neglect of thermal diffusion caused an increase of 5-6% in the computed burning velocity.

Kinetic study of some elementary reactions of sulfur compounds including reactions of S and SO with OH radicals

Abstract: The reactions of S + OH SO + H (1) and SO + OH SO2 + H (2) were studied in a discharge flow reactor coupled to an EPR spectrometer. The rate constants obtained under the pseudo-first-order conditions with an excess of S or SO were found to be k1 = (6.6 ± 1.4) × 10−11 and k2 = (8.4 ± 1.5) × 10−11 at room temperature. Units are cm3/molec·sec. Besides no reactivity was observed between S and CO2 at 298 K and between CIO and SO2 up to 711 K.

A Computer Program Package for the Analysis, Creation, and Estimation of Generalized Reactions—GRACE. I. Generation of Elementary Reaction Network in Radical Reactions—GRACE(I)

Abstract: The system GRACE generates elementary reaction networks for simple reactions including free radicals, ions and even active sites in heterogeneous catalysis, and also predicts the overall reaction rates and the product distributions in radical reactions in the gas phase where the Arrhenius parameters of elementary reactions are available. In this paper, A/GRACE (part A of GRACE) is explained, which prepares elementary reaction networks for radical reactions. The reactant and the product are represented by square symmetric matrices. The off-diagonal elements represent bond multiplicity, or the number of localized electrons, between corresponding atoms, whereas the diagonal elements imply the number of unshared electrons on radical atoms. A reacting system, an ensemble of the participating molecules, is composed of atom groups, each of which consists of a center atom (usually non-hydrogen) and attached hydrogen atom(s), if any. Firstly, all the possible changes in the atom groups are selected, and the permit...

Ab initio approach to organic reaction rates. Kinetic isotope effects in the reaction H + C/sub 2/H/sub 4/. -->. C/sub 2/H/sub 5/

Abstract: An ab Initio Approach to Organic Reaction Rates. Kinetic Isotope Effects in the Reaction H + C/sub 2/H/sub 4/..-->..C/sub 2/H/sub 5/. The results of a transition state theoretical (TST) study, reinforced by the relevant ab initio molecular orbital computation, of the addition reaction H+C/sub 2/H/sub 4/..-->..C/sub 2/H/sub 5/ are reported. The TST procedure has proved to be valuable as a quantitative tool for kinetic considerations. Kinetic isotope effects of various deuterium isotope combinations were considered. The vibrational frequencies of the deuterated reactants as well as the transition states were calculated in exactly the same manner as for the nondeuterated case. The resulting rate constants are included. The ab initio molecular orbital calculation seems to be capable of supplying sufficient information on the characteristics of the transition state for elementary reactions of moderately complex organic compounds. The importance of the entropy or preexponential factor in the rate processe should be emphasized in this connection. (3 tables)

Reaction kinetics involving ground X2Π and excited A2Σ+hydroxyl radicals. Part 1.—Quenching kinetics of OH A2Σ+and rate constants for reactions of OH X2Π with CH3CCl3and CO

Abstract: Resonance fluorescence has been used to determine the rate constants k1 and k2(cm3 molecule–1 s–1)(1 σ) of elementary reactions of ground state OH X2Π radicals from 293 to 430 K: OH + CH3CCl3[graphic omitted]CH2CCl3+ H2O (1), log10k1=(–1162 ± 014)–(1394 ± 113)/2303 T; OH + CO [graphic omitted] CO2+ H (2), k2=(–1266 ± 012)–(88 ± 40)/2303 TIn addition, steady-state measurements of resonance fluorescence have been used to determine the rate constants at 293 K for electronic (or reactive) quenching of excited OH A2Σ+ radicals with a variety of halogenocarbon molecules Most of these excited-state quenching reactions occur with collision efficiencies approaching unity at 293 K, unlike the corresponding reactions involving ground state OH radicals

Solid-solid reactions, II: Mechanism of barium metatitanate synthesis

Abstract: The authors propose a mechanism for the solid-solid reaction BaCO3+ TiO2→BaTiO3+CO2. This mechanism is based on the real structure of the present semiconductors. The reactions at different interfaces and the diffusing species are identified. The reaction rates are calculated and the dependence of the reaction rate upon O2, N2, and CO2 gas pressure is interpreted and discussed.

Relative reactivities of substituted phenyl radicals in elementary reactions

Abstract: The relative reactivities of phenyl, p-chlorophenyl, and p-nitrophenyl radicals, derived from arylazotriphenyl-methane, towards chlorine abstraction from carbon tetrachloride were estimated from the results of the competitive reaction of the radicals with carbon tetrachloride and iodine, assuming that the rate of scavenging of the radicals by iodine is almost invariant, irrespective of the substituent on the radical. On the basis of the results, the relative reactivities of these radicals in other elementary reactions such as hydrogen abstraction, bromine abstraction, addition to benzene or allyl sulphide, and SH2 displacement on disulphide were estimated. These data show that p-chloro- and p-nitro-phenyl radicals are more reactive than phenyl towards electron-donating substrates and less reactive towards elecron-accepting substrates.

A comparison between ethylene dimerization and Diels-Alder reaction

Abstract: The reaction pathways for the two reactions 2C2H4↔C4H8 and C2H4+C4H6↔ C6H10 were investigated. The transition state geometries and activation energies were determined with the SINDO method and compared. Both reactions were found concerted with different significance of diradical character at the transition state. Whereas the ethylene dimerization showed that a diradical will be encountered along the reaction pathway, we did not find one in the Diels-Alder reaction.

Shock‐tube study of thermal decomposition of nitric oxide between 2700 and 3500 K

Abstract: The decomposition of nitric oxide at temperatures ranging from 2700 to 3500 K was studied by means of the shock tube. The experimental data were reduced by the method described in a preceding paper and explained consistently by a set of the elementary reactions. The rate constant of the initiation reaction 2NO N2O + O, which was not well known in this temperature range, was deduced precisely. k1 was one order of magnitude lower than that reported previously in similar shock-tube experiments, and was consistent with results obtained below 2000 K and from the reverse reaction.

Ionic Mechanisms of Carbon Formation in Flames.

Abstract: The formation of incipient carbon in flames can be described by a series of elementary reactions in which the precursor of soot is the ion C3H3+, and the major building block is acetylene, polyacetylenes, or other hydrocarbon fragments which are present in large concentrations. The precursor ion is produced by nonequilibrium chemi-ionization reactions, e.g., CH asterisk plus C2H2 yields C3H3+ plus e or CH asterisk plus O yields CHO+ plus e. The CHO+ rapidly becomes C3H3+ through a series of ion-molecule reactions. The chemi-ion then adds acetylene in a series of very rapid ion-molecule reactions producing larger and larger ions. Ions isomerize very rapidly to produce aromatic structures overcoming one of the major problems with neutral species mechanisms: how to form the first carbon ring. As the ions grow, their recombination rate coefficients with electrons increase so that the larger ions are removed by dissociative recombination. These neutral species now continue to grow by the addition of more acetylene producing even larger neutral species. With increasing size, the rate coefficient of electron attachment also increases causing the reaction to become important, depending upon the temperature. Appreciable concentrations of negative ions shift the ion removal process to dissociative ion-ion recombination, which is orders of magnitude slower than ion-electron recombination. As the neutral species continue to grow by the addition of acetylene, they gradually take on the aspect of particles, i.e., the bulk properties of the substance CxHy dominate over the chemical properties. This change is gradual and there is no distinct size at which a large molecule becomes a particle. Evidence will be presented supporting the above mechanism. All of the ions proposed in the mechanism have been observed by mass spectrometric analysis, their concentrations measured, and their rates of reaction in the flame environment determined to be adequate to account for the observed rate of formation of carbon.

Some problems of chemical physics of ageing and stabilization of polymers

Abstract: Ageing of polymers was examined as a complex process taking place by a sole mechanism. The major role of free-radical reactions in this process was indicated. A study was made of the effect of molecular mobility of polymers on reaction kinetics in a polymer matrix. Results were examined of the three dimensional heterogeneity of reaction kinetics in polymers, which is related to structural heterogeneity (“kinetic interruption” of the reaction). Results were presented of a study of elementary reaction kinetics in polymers. A study was made of the relation between ageing kinetics and the variation of practically important properties of polymers. Mechanisms of stabilization of polymer materials were described and new principles of stabilization proposed. Data were given of an investigation into ageing kinetics of polymers under extremal conditions, by the action of UV light, in corrosive media and in tissues of living organism. Some problems of polymer combustion were discussed.

Elementary reactions in the cationic oligomerization of 2-alkoxy-1,3,2-dioxaphospholane with alkyl iodide as initiator.

Abstract: Elementary reactions in the oligomerization reaction of 2-alkoxy-1, 3, 2-dioxaphospholanes by alkyl iodides were studied. The initial reactions were ring-opening reactions leading to open-chain phosphonate and isomerization reactions with loss of an external alkyl group leading to cyclic phosphonate. The ratio of these two elementary reactions was 2.7: 1.0 in the reaction of 2-methoxy-1, 3, 2-dioxaphospholane with methyl iodide at 70°C. The reaction with ethyl iodide was much slower compared to that with methyl iodide. This suggests the slow propagation of the polymerization of 1, 3, 2-dioxaphospholanes by methyl iodide. The isomerization reaction could be a chain-transfer reaction in the growing chain. This might be the reason why high polymers could not be obtained from 2-methoxy-1, 3, 2-dioxaphospholane. The reaction of 2-t-butoxy-1, 3, 2-dioxaphospholane with methyl iodide was exclusively the isomerization process via the Arbuzov reaction, with removal of the external t-butyl group, which led to isobutylene. In the reaction of 2-neopentoxy-1, 3, 2-dioxaphospholane with methyl iodide, the only product resulted from the addition of methyl iodide to the dioxaphospholane ring, followed by ring opening.

Kinetics of the gas-phase addition reactions of trichlorosilyl radicals. Part 3.—Additions to 2-olefins

Abstract: The following Arrhenius parameters for the forward and reverse steps of trichlorosilyl radical additions to trans-but-2-ene, cis-but-2-ene, cis-pent-2-ene, 2-methyl-but-2-ene and cyclopentene have been obtained by a competitive method. The relevant elementary reactions are ·SiCl3+ CH3COCH3→(CH3)2ĊOSiCl3(3), ·SiCl3+CC⇌Ċ—[graphic omitted]—SiCl3(5, –5), and —[graphic omitted]—SiCl3+ HSiCl3→ H[graphic omitted]—[graphic omitted]SiCl3+·SiCl3(6), [graphic omitted].The rate parameters of reaction (5) are expressed per reaction site; an asterisk indicates the site of addition in an unsymmetrical olefin. Evaluated values of A–5 and A5 imply a fairly ‘loose’ transition state in reaction (5). The Si—C bond energy has been estimated. ·SiCl3 radicals have been revealed to be electrophilic and susceptible to steric hindrance.

Recent Developments and Applications of Pressure Jump Methods

Abstract: Pressure shifts a chemical equilibrium in a manner which is described by van’t Hoff’s equation $$ {(\frac{{\partial \ln k}}{{\partial p}})_T} = \frac{{\vartriangle {V^O}}}{{RT}} $$ (1) This relation is applicable to elementary reaction steps with equilibrium constant K and reaction volume ΔV° = ∑ νiV°i where “°” stands for standard conditions, νi is the stoichiometric factor and Vi the partial molar volume of reactant i. Most reactions in solution imply volume changes due to changes in conformation or solvation and can consequently be influenced by pressure.

MECHANISM OF SOLID STATE REACTION BETWEEN SrCO3 AND SiO2

Abstract: Metastable SrSiO3 was formed during the course of the solid state reaction of SrCO3 and SiO2. The reaction mechanism was investigated by means of DTA and X-ray diffraction analysis. It was found that stable SrSiO3 is formed via two processes: (a) transformation of metastable into stable SrSiO3; (b) solid state reaction between Sr2Sio4 and Sio2.

Measurements of some elementary hydrocarbon reactions at high temperatures

Abstract: The direct concentration measurement of H- and O-atoms in high temperature hydrocarbon reaction systems has been shown to be a promising addition to shock tube technology. Because of the high sensitivity, measurements, in highly diluted reaction systems are possible. The interpretation of elementary reaction rate coefficients is also facilitated.

Modeling of flame properties of methanol

Abstract: Many fuel utilization concepts involve detailed information on flame properties. Processes such as ignition, flame propagation, flame quenching, pollutant formation and others all require the type of fundamental understanding which numerical modeling can help to provide. Computational flame modeling, using a recently developed detailed chemical kinetic reaction mechanism for methanol oxidation, provides a means of obtaining a great deal of data on the structure and propagation of methanol flames. The effects of stoichiometry, pressure, unburned gas temperature, and trace constitutants such as water vapor on flame speed and flammability have been determined and are discussed in this paper. In each distinct physical regime, different elementary reactions tend to dominate the reaction mechanism, and in each case the interactions between fluid mechanical and chemical kinetic processes are responsible for the observed behavior.

Reaction of 1,1,3,3-tetramethylthiourea with methyl iodide: kinetic and thermodynamic aspects

Abstract: In the reaction of 1,1,3,3-tetramethylthiourea with methyl iodide limiting forms of the rate law arise in solvents in which the concentration of free ions and ion-pairs is small in comparison with higher aggregates. The solvent effects on the rate of the forward reaction were linearly correlated with those on the Menschutkin reaction. The effects were quite close to those on the pyridine–methyl iodide reaction. The value (Δ2V‡/Δ2°), which is expected to be an index of the position of the transition state along the reaction co-ordinate, is ca. 0.27 in acetonitrile and propylene carbonate. The position of the transition state, nT, calculated from the enthalpy term, is ca. 0.30 for the present reaction and for the NN-dimethylaniline–methyl iodide reaction. These indices of the transition state position are qualitatively in agreement with each other. The results of CNDO/2 calculations, performed for the assumed transition state structures, are in accord with the results of the experimental observations on the solvent effects on reaction rates.

Shock-tube study of thermal decomposition of nitric oxide between 2700 and 3500 k

Abstract: The decomposition of nitric oxide at temperatures ranging from 2700 to 3500 K was studied by means of the shock tube. The experimental data were reduced by the method described in a preceding paper and explained consistently by a set of the elementary reactions. The rate constant of the initiation reaction 2NO N2O + O, which was not well known in this temperature range, was deduced precisely. k1 was one order of magnitude lower than that reported previously in similar shock-tube experiments, and was consistent with results obtained below 2000 K and from the reverse reaction.

Laser-Induced Fluorescence Study of the Reactions of F Atoms with CH3I, CF3I and ICl

Abstract: The laser-induced fluorescence method for product state analysis developed by ZARE et al. has been extensively applied so far to the oxides and halides of the group II and III elements and the OH radical. In this paper we report on the extension of this technique to the measurement of the internal product state distributions of interhalogen molecules formed under single collision conditions in a crossed beam experiment. A first series of experiments has been carried out for three elementary reactions forming iodine monofluoride, which is, from the spectroscopic point of view, the most suitable interhalogen molecule to be studied by laser-induced fluorescence.