Showing papers on "Elementary reaction published in 1985"

Chemical Reaction Dynamics in Solution

Abstract: An impressive renaissance in the study of chemical reactions in solution has occurred during the past decade. Although many of the reactions of interest to chemists occur in solution, most attention in the physical chemistry community has tended in the past 30 or so years to focus, with understandable cause, on gas phase chemical dynamics. But with the advent of experimental techniques such as picosecond spectroscopy, the accessibility of large-scale computer simulation, and the creation of new theories, solution phase reaction theory and experiment have now come into their own. As a consequence, at least a glimmering of understanding (and sometimes more) is now emerging in our appreciation of key features of various reaction types in solution. But it is an important measure of the rate of progress in this frontier in chemistry that new questions are often generated faster than are definitive answers. In contrast to the situation in the gas phase, where reactants (usually) follow their route to products in splendid isolation, the ubiquitous solvent molecules in solution continually perturb the reactants in their course. The first way in which a solvent medium can affect a reaction rate is via a modification of the activation (free) energy. This can be a substantial influence, since a change of only 2 kcal can modify the room temperature rate by a factor of 30. Good examples here are ionic and polar reactions in polar solvents. Calculations based on Transition State Theory (TST) and continuum dielectric picture of the solvent are well known, but have long been recognized as not being very reliable. As a result of progress in theory and computation of the microscopic properties of polar solvents, workers have been able to find more accurate activation parameters. TST predic-

Unimolecular reaction rate theory for transition states of partial looseness. II. Implementation and analysis with applications to NO2 and C2H6 dissociations

Abstract: Implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments. The fragments may be atomic, diatomic, or polyatomic species. Action-angle and internal coordinates for the transitional modes of the reaction, transformations to Cartesian coordinates, and other calculational aspects are described. Results for the NO2-->NO+O reaction are presented, including the dependence of the microcanonical rate constant on the bond fission and bending potentials for model potential energy surfaces. Illustrative calculations for the C2H6-->2CH3 reaction are also given.

Chemistry of high temperature combustion of alkanes up to octane

Abstract: Alkanes are initially attacked by H, O, and OH radicals generated in the oxyhydrogen reaction. The alkyl radical formed in this way decomposes to smaller alkyl radicals by fast thermal elimination of alkenes. Only the relatively slow thermal decomposition of the smallest alkyl radicals, CH3 and C2H5, competes with recombination and with oxidation reactions by O atoms and O2. This part of the mechanism is rate-controlling in the combustion of alkanes (and alkenes) and must be described by a detailed mechanism consiting of elementary reactions. Alkyl radical decomposition and the reactions leading to C1-and C2-fragments are too fast to be rate-limiting and can therefore be described by simplified reaction schemes disregarding alkyl isomeric structures. Simulations of flames of higher alkanes (up to octane) using these simplifying assumptions show agreement with the experimental material available. The mechanism derived by these considerations can then be used to explain of phenomena (like non-Zeldovich NO formation or formation of soot precursors), which can be interpreted from a detailed knowledge of the C1/C2-chemistry.

High temperature reaction rate for H+O2=OH+O and OH+H2=H2O+H

Abstract: Shock heating together with atomic resonance absorption spectrometry (ARAS) was used to record simultaneously H- and O-atom concentration profiles in the post-shock region behind reflected shocks. The dissociation of N2O together with the reaction O + H2 = OH + H was used as a source of H and OH for the reactions H + O2 = OH + O and OH + H2 = H2O + H. The test gas mixtures consisted of argon with relative concentrations of a few ppm N2O and 100 to 500 ppm H2 and O2. The experiments were conducted in the temperature range of 1700 to 2500 K at total densities of 6 · 10−6 to 1.3 · 10−5 mol cm−3. The following rate coefficients were deduced: For temperatures below 2500 K nearly complete agreement with the value for the rate coefficient of reaction R1 as recommended by Baulch [1] was obtained. For reaction R2 a rate coefficient was deduced which is close to the value as given by Gardiner et al. [23].

The Influence of the Metal on the Kinetics of Outer Sphere Redox Reactions

Abstract: The kinetic parameters of the outer sphere redox couple [Ru(NH3)6]2+/3+ were measured on six different metal electrodes. The reaction rate is independent of the substrate. The significance of these results for the electron transfer theory is discussed, and a new model for adiabatic electron transfer reactions is outlined.

A comprehensive chemical kinetic reaction mechanism for the oxidation of N-butane

Abstract: A detailed chemical kinetic reaction mechanism is developed for the intermediate and high temperature oxidation of n-butane. The mechanism, consisting of 238 elementary reactions among 47 chemical species, is validated by comparison between computed and experimental results from shock tubes, turbulent flow reactor, and premixed laminar flames. The turbulent flow reactor experiments are described briefly. The model accurately reproduces n-butane combustion kinetics for wide ranges of pressure, temperature, and fuel-air equivalence ratio. In spite of the large number of species and reactions, it is found that computed results are most sensitive to reactions involving the H/sub 2/-O/sub 2/-CO submechanism, in agreement with other modeling studies of hydrocarbon oxidation. 38 references, 1 figure, 1 table.

Kinetics of the Reactions between CH2(X̃3B1)-Radicals and Saturated Hydrocarbons in the Temperature Range 296 K ≤ T ≤ 707 K

Abstract: The reaction between CH2-radicals in their ground electronic state (X3B1) and n-hexane was studied in a discharge flow system with LMR detection of CH2. In the temperature regime 413 K ≤ T ≤ 707 K the experimental rate constant was found to be The reaction proceeds both via direct H-atom abstraction by CH2(X) and via thermal excitation of CH2(X) to the low-lying singlet state (a1A1) followed by fast consecutive reactions of CH2(a). The contributions due to thermal excitation and singlet reaction were evaluated for the present work as well as for a recent study of the reactions of CH2(X) with a series of other hydrocarbons. Corrected rate constants kT for the direct reactions of CH2(X) with the reactants HR = CH4 (1), C2H6 (2), C3H8 (3), n-C6H14 (4), i-C4H10 (5), and CH3CHO (6) in the temperature range 296 K ≤ T ≤ 707 K were found to be The activation energies for the reactions studied are described by an Evans Polanyi type relation. Arrhenius expressions are proposed for the rate constants of H-atom abstraction by CH2(X3B1) from primary, secondary, tertiary, and aldehydic C -H bonds. The results are compared to the isoelectronic reactions of O(3P).

Shock-tube Studies of N2O Decomposition and N2O–H2 Reaction

Abstract: N2O decomposition and N2O–H2 reaction were studied behind incident and reflected shock waves in the temperature range 1450–2200 K and pressure range 0.6–3.5 atm using both single-pulse and time-resolve techniques. A computer-simulation study was performed to determine the rate-constant expressions of important elementary reactions. Computer calculations showed that the values of the rate constant for N2O+O\xrightarrowk2N2+O2 and N2O+O\xrightarrowk2NO+NO (cited in current papers) are too low. The rate-constant expressions were found to be k2=7.0×1014exp(−28 kcal/RT) cm3mol−1 s−1 and k3=5.6×1014exp (−28 kcal/RT) cm3 mol−1 s−1. The rate constant expression for N2O+H\xrightarrowk2N2+OH, was found to be k4=1.5×1014exp (−15 kcal/RT) cm3 mol−1 s−1.

High Resolution Infrared Studies of Molecular Dynamics

Abstract: Reaction dynamics is one of the principal subjects of chemistry. The traditional way of studying this problem relies on inspection of changes in concentration of reactants and products as the reaction proceeds; the result of chemical analysis is employed to infer the details of intermediate processes. Chemical reactions are obviously not as simple as explained merely by means of classical methods of chemical analysis. Many workers have closely examined the individual steps of gas phase reaction processes, i.e. elementary reactions. Such efforts have resulted in the detection of quite a large number of short-lived molecules as reaction intermediates. Identification of these species is a sort of prerequisite for understanding the reaction mechanism. Because most reaction intermediates involve un­ paired electrons, they have attracted a lot of attention in many fields of molecular science. Increasing development of spectroscopic tools has enabled workers to characterize many of them. The high-resolution spectra of reaction intermediates are extremely valuable in examining, in real time, distributions over quantum states of molecules taking part in reac­ tions, which can deviate considerably from the Boltzmann distribution. High resolution molecular spectroscopy and kinetic studies have thus collaborated closely to bring about recent remarkable progress in the study of reaction dynamics. Electronic spectroscopy in the visible and ultraviolet regions, combined with flash photolysis, has been most widely employed in the study of transient molecules as reaction intermediates (1). The development of the dye laser has accelerated this trend, because this light source has made laser-induced fluorescence (UF) easy to apply. This method provides us

Kinetics of some gas-phase elementary reactions of boron monofluoride

Abstract: : Rate coefficients for some elementary bimolecular reactions involving the molecule BF have been measured in a gas phase flowtube facility. The BF + C12 reaction was studied from 295 to 881 K. The rate coefficient is temperature independent with a value of 1.4 + or - 0.2 x 10 to the -11th power cc/molecule-sec. Reaction of BF with O2 was studied between 675 and 1033 K. The inferred Arrhenius rate is 1.8 x 10 to the -11th power exp(-7240/T) cc/molecule-sec. Room temperature rate coefficients for BF + O and BF + NO2 were determined to be 1.8 + or - 0.4 x 10 to the -10th power and 2.1 + or - 0.3 x 10 to the -13th power/molecule-sec., respectively. Reaction of BF was undetectable upon mixing with H2O, HF, CO2, NO, N2O, SO2, or CH3C1. Upper limits are given for rate coefficients for these reactions. Boron monofluoride proved to be less reactive than other simple boron species which have been studied recently, namely, boron atoms and boron monoxide. Results for BF are compared with available rate information for reactions of isoelectronic CO and N2.

A Direct Study of the Reactions of CH2 (X̃ 3B1)-Radicals with Selected Hydrocarbons in the Temperature Range 296 K ≤ T ≤ 705 K

Abstract: The kinetics of the reactions of CH2 (X 3B1)-radicals with five selected organic compounds has been studied in an isothermal discharge flow system in the temperature range 296 K ≤ T ≤ 705 K. Ground state CH2-radicals have been generated via the reaction O + CH2CO and monitored with a far infrared laser magnetic resonance spectrometer. The experimental results are described by the following Arrhenius expressions: Two basic reaction mechanisms, either direct H-atom abstraction by 3CH2 or thermal excitation of 3CH2 to the low lying a 1A1 state followed by consecutive reactions of 1CH2, are of importance. For acetaldehyde, isobutane, and propane direct H-atom abstraction by 3CH2 predominates. After separation of the small contribution attributed to the singlet reaction the following rate constants for the reactions of CH2 (X3B1) with acetaldehyde, isobutane, and propane are obtained: Presuming the reactions of 1CH2 with hydrocarbons are fast the thermal excitation mechanism dominates the reaction system in the cases of methane and ethane. The activation energy of EA (CH4) = 40 ± 8 kJ/mol measured for methane is concluded to he determined by the singlet-triplet energy splitting in CH2.

Laser Induced Fluorescence Studies on C2O and CH Radicals

Abstract: C2O and CH radicals were quantitatively analysed by LIF in the reaction systems C3O2 + O + H and C2H2 + O + H. The measurements of rate constants and products of the reaction C2O + O as well as C2O + H have been carried out by excimer laser pulse photolysis for the production and by LIF for the detection of the radicals. The results give a basis of a new interpretation of the C2H2 + O + H atom flame system.

The Reaction of OH with C2H2 at Atmospheric Conditions Investigated by CW‐UV‐Laser‐Longpath‐Absorption of OH

Abstract: A sensitive CW-UV-laser absorption method at 308 nm has been used to investigate the pressure dependence of the kinetics of OH reactions with C2H2. OH radicals are generated by excimer laser photolysis of H2O at 193 nm or of H2O2 at 248 nm. Using an absorption light path of 151 m and accumulating 50 decay curves, OH concentrations in the range from 1010 down to 107 cm−3 can be detected in the presence of N2 at 10–1000 mbar. The experimental data on the pressure dependence of the rate constant for the reaction of OH with C2H2 in N2 yield the limiting values (at room temperature): .

Kinetics of the high temperature, low concentration CH4 oxidation verified by H and O atom measurements

Abstract: In the kinetics of methane oxidation at high temperatures H and O atoms are very important reactants. In this study concentrations of both these atoms were directly measured behind reflected shock waves in relatively low concentration CH4−O2 mixtures diluted in Ar. the temperatures ranged 1850 K≤T≤2500 K, the pressures were about 1.8 bar, and the concentrations varied between 5 and 50 ppm for CH4, and 10 and 2000 ppm for O2. Atomic resonance absorption spectroscopy (ARAS) was used to measure H and O atom concentrations in nearly 90 individual experiments. Computer simulation based on a reaction model allowing for 25 elementary reactions and selected rate coefficients, was in good agreement with the experimental results. The influence of the elementary reaction CH3+OH→CH3O/CH2OH+H→CH2O+2H, proposed earlier, was confirmed by these experiments.

Charge Transfer from the n‐Hexadecane Radical Cation to Cycloalkanes, Alkenes and Aromatics

Abstract: Charge transfer from n-hexadecane radical cations C16H+34 to solutes as cycloalkanes, alkenes and aromatics was studied by pulse radiolysis. Using ion-pair kinetics the rate constants ks of the electron transfer reactions were determined. The electron transfer rate constants ks increase from low values for slightly exothermic reactions to a limiting value of 9 · 109 dm3 mol−1 s−1 when the electron transfer reaction is more exothermic than −0.4 eV.

Elementary reactions involved in the oxidation of propene: Arrhenius parameters for the reaction HO2+C3H6=C3H6O+OH

Abstract: The decomposition of tetramethylbutane (TMB) in the presence of O 2 in aged boric-acid-coated vessels has been used as a source of HO 2 radicals to study reaction (13) over the temperature range 400–500° C. HO 2 +C 3 H 6 →C 3 H 6 O+OH Although the decomposition of TMB is the major source of HO 2 , the presence of C 3 H 6 does perturb [HO 2 ] to a small extent. To allow for this, a detailed product analysis for CO, C 2 H 4 , CH 3 CHO, CH 2 =CHCHO, C 3 H 6 O (propene oxide) and i -C 4 H 8 (formed from TMB in 99% yield) has been carried out for several mixtures over the temperature range used. From the results, a basic mechanism has been developed for the oxidation of propene. Values of k 13 / k 7 1/2 have been obtained at 400, 440, 470, and 500° C, and subjected to sensitivity analysis including major changes in the mechanism of propene oxidation. Using k 7 =2.0×10 9 dm 3 mol −1 s −1 , values of A 13 =10 9.02±0.30 dm 3 mol −1 s −1 and E 13 =59.4±4.5 kJ mol −1 are obtained. No other reliable experimental values are available, but the present results are consistent with rate constants for the reaction of HO 2 radicals with C 2 H 4 , i -C 4 H 8 , and 2, 3-dimethyl-butene-2 to form the respective alkene oxide. HO 2 +HO 2 →H 2 O 2 +O 2

Temperature Dependence of the Reaction D + H2 (v = 1) → HD (v = 0,1) + H

Abstract: The temperature dependence of the rate of the reaction was studied in a discharge flow reactor. Although the energy of one vibrational quantum exceeds the energy barrier of the reaction, a distinct temperature dependence of the reaction rate was observed. This indicates that only a moderate fraction of the vibrational energy can be used to surmount the energy barrier of the reaction.

Rates of Elementary Reactions: Measurement and Applications

Abstract: Modern experimental techniques for measuring rate parameters of elementary reactions have transformed the field of gas-phase reaction kinetics from one of indirect inference to one of direct determination. Recent progress in the principal techniques is described, a few examples are given of the hundreds of elementary reactions for which rate information has become available, and comparison with reaction rate theory is briefly discussed. Some generalizations regarding the dependence of rate parameters on structure and thermodynamics are drawn, and successful applications to atmospheric and combustion modeling and measurement are presented.

Kinetic Studies of NH2 and HCO Reactions with Unsaturated Hydrocarbons, by Flash Photolysis and Laser Resonance Absorption

Abstract: The flash photolysis and laser resonance absorption technique was used for the determination of the rate constants of reactions of NH2 with acetylene in the temperature range 340–510 K, NH2 with 1,3-butadiene from 298 to 500 K and HCO with 1,3-butadiene from 298 to 522 K. The Arrhenius expressions for the rate constants are the following (in cm3 · molecule−1 · s−1 units): The rate expressions for k1, and k2 are compared to previous results reported in the literature. The determination of k4 represents the first data concerning the reactivity of HCO with an unsaturated hydrocarbon. A tentative evaluation of the reactivity of HCO with olefins is also given. Product analysis shows that reaction (4) proceeds essentially via an addition process rather than a hydrogen atom transfer from HCO.

Elementary reactions of the NCl radical. Part 2.—Kinetics of the NCl + ClO and NCl + NO reactions

Abstract: The reaction kinetics of ground-state NCl(X3Σ–) radicals have been studied in a discharge–flow system at 295 K. The time-resolved decay of NCl in the presence of an excess of ClO(X2Π) and NO(X2Π) was followed using molecular-beam mass spectrometry and compared with those for analogous radical–radical reactions. NCl + ClO → ClNO + Cl; k1=(2.3 ± 0.2)× 10–11 cm3 s–1, NCl + NO → N2O + Cl; k2=(1.4 ± 0.1)× 10–11 cm3 s–1. Observations supporting the indicated reaction products are discussed. These are the first determinations of the rate constants for the above reactions. The values determined are compared with those for analogous radical–radical reactions.

Morphology of dynamic electron transfer characteristic of chemical reaction dynamics

Abstract: The morphology of the dynamic electron transfer characteristic of chemical reaction dynamics has been studied by introducing the new criterion of an isomorphism of the manifold of electron orbitals at the different points of the reaction coordinate. This criterion is a natural development of the famous Amos–Hall corresponding orbital. Natural orbitals are obtained from the dynamic Fock equation, then it is possible to describe the electronic process of a chemical reaction by the way that the occupation number of an electron orbital changes within the electron orbital manifold which is constructed to have isomorphic equivalency along the reaction coordinate. The electronic process accompanying the chemical reaction CH3+HF → CH4+F has been studied as a demonstration of the present theory. The electron orbital which is transformed by the new criterion is strongly localized, though the transformation operator itself does not contain any explicit localization terms, such as self‐energy integrals. The transitio...

Comments on the linear relation between reaction rate and affinity

Abstract: In the case of catalytic dehydrogenation of cyclohexane, the linear relation between reaction rate and affinity holds at an appreciable distance away from equilibrium on both sides. In fact, in order to explain this observation, it is necessary to invoke a high value of three for the stoichiometric number of the rate determining step. This in turn can be explained by a reasonable mechanism for the reaction.