Ab initio, kinetics, and dynamics study of Cl+CH4→HCl+CH3

Abstract

The Cl+CH4→HCl+CH3 reaction has been studied using different levels of the chemical reaction theory. Ab initio calculations at the fourth-order Møller–Plesset perturbation theory//second-order Møller–Plesset perturbation theory level, employing the 6-311G(2df,2pd) basis set, satisfactorily describe the system stationary points. A kinetics study using variational transition state theory has been accomplished, using ab initio information along the minimum energy reaction path. The agreement with experimental rate constants in the 200–500 K range is the best reported up to date. Reduced-dimensionality quasiclassical trajectory (QCT) calculations of the reaction dynamics have been performed on an analytical many-body potential energy surface, which is fitted to ab initio calculations of the system. The various experiments showing the absence of energy release to the CH3 group has led us to invoke a pseudotriatomic approximation, in which the CH3 moiety is treated as a single particle, so as to carry out dynamics calculations. Our QCT calculations give particular attention to the comparison with the numerous experiments available on this reaction. These include internal state distributions of the nascent diatomic molecule and state specific angular distributions. Further insight into the state specific scattering distributions is provided through analysis of opacity functions. The satisfactory reproduction of a great variety of experimental results by the theoretical study presented here is consistent with the idea that the CH3 modes are weakly coupled to the reaction coordinate, thus validating the model adopted in this work.