Kinetics of Reactions of H Atoms With Methane and Chlorinated Methanes

Abstract

The reactions of H atoms with methane, four chlorinated methanes, and isobutene have been studied experimentally using the discharge flow/resonance fluorescence technique over wide ranges of temperatures. The rate constants were obtained in direct experiments as functions of temperature. The experimentally obtained activation energies of the reactions of H atoms with chlorinated methanes demonstrate a correlation with the enthalpies of the reactions. Transition state theory reaction models were created on the basis of ab initio calculations, the Marcus expression for correlation between reaction barriers and reaction energetics, and analysis of experimental data. It is demonstrated that the formalism based on the Marcus expression adequately describes the observed temperature dependencies of the rate constants of the overall reactions. According to the models, abstraction by H atoms of hydrogen atoms from chloromethanes is an important process accounting for significant fractions of the overall rate constants. The models result in expressions for the rate constants of Cl and H atom abstraction channels and the corresponding reverse reactions over wide ranges of temperatures.

Thermochemistry data from kinetics results: A test of quality of the potential energy surface

“…1. These values agree with previous theoretical results [12][13][14] in the common temperature range, the largest differences being with Knyazev's results [13]. Note that experimental data are not available for these reactions, and only the overall rate constants are known, i.e., the sum of the two channels.…”

Direct Combined ab Initio/Transition State Theory Study of the Kinetics of the Abstraction Reactions of Halogenated Methanes with Hydrogen Atoms

“…These discrepancies can be attributed at the interference caused by secondary reactions in the experiment that tends to consume the molecular reactant as well as H atoms, as previously discussed by Bryukov et al 6 Finally, the computed overall rate constants for the reaction of H atoms with CCl 4 were found to be systematically higher than those measured by Bryukov et al 6 by a factor of 2.5 ( Figure 5). However, over the temperature range (300-900 K), the slope (-E a /R) of our predicted Arrhenius plot (-2985 K) is very close to the experimental one reported in ref 6 (-2937 K).…”

“…Berry et al 12 reported low level calculations using the G2(MP2) and the semiempirical BAC-MP4 approach to compute rate constants in the temperature range 298-2500 K for the reaction CHF 3 + H f products. Takahashi et al 9 performed G2(MP2) calculations adjusting the barriers to their experimental data in order to predict rate constants for all elementary reactions in the temperature range 1100-1350 K. More recently, Maity et al 13 6 In this work, highly correlated ab initio quantum chemical calculations were performed in order to directly compute the reaction barriers for the title reactions without any further adjustments of the energy. The energetics of these reactions was used together with TST calculations to compute rate constants in the temperature range 300-2500 K. For all of the reactions, all possible elementary pathways are computed and their relative importance under thermal oxidation and combustion conditions (700-2500 K) are discussed.…”

Rate coefficients and kinetic isotope effects of the X + CH4 → CH3 + HX (X = H, D, Mu) reactions from ring polymer molecular dynamics

“…1 The rate coefficient of the reaction is well known over a large temperature range. [2][3][4][5][6][7][8] It has also served as a benchmark for understanding bimolecular reaction dynamics involving polyatomic molecules. [9][10][11] Due to the involvement of light hydrogenic atoms, the reaction is intrinsically quantum mechanical.…”