Computational Study of the Reactions of H Atoms with Chlorinated Alkanes. Isodesmic Reactions for Transition States

Abstract

Reactions of H atoms with methane, ethane, and chlorinated methanes and ethanes were studied by computational methods. An approach to estimating reaction rate constants based on the use of isodesmic reactions for transition states was developed and implemented. Reactions of H atoms with CH4 and CCl4 (clear cases of H and Cl abstraction, respectively) were used as reference reactions. Rate constants of all other reactions from the series were calculated using the isodesmic reactions approach. For the seven reactions for which directly obtained experimental data are available, the resultant calculated temperature dependences of the rate constants demonstrate agreement with experiment. Average deviations between calculations and experiment are 17−24%, depending on the quantum chemical method used, although channel-specific rates show larger divergence. Rate constants of all 30 reaction members of the H + chloromethane and H + chloroethane classes are calculated as functions of temperature using the described approach. Individual channels of H and Cl abstraction and the corresponding reverse reactions are quantitatively characterized. In a separate part of the investigation, it is demonstrated that correlations between the energy barriers and reaction enthalpies do not provide a good predictive tool for evaluating temperature dependences of the reaction rate constants of the H + chloroalkanes class.