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

Theoretical calculations were carried out on the H-, Cl-, and F-atom abstraction reactions from a series of seven substituted halogenated methanes (CH 3 Cl, CH 2 Cl 2, CHCl 3, CCl 4, CHF 3, CHF 2 Cl, and CHFCl 2 ) by H atom attacks. Geometry optimizations and vibrational frequency calculations were performed using unrestricted Møller−Plesset second-order perturbation theory (UMP2) with the 6-311++G(d,p) basis set. Single-point energy calculations were performed with the highly correlated ab initio coupled cluster method in the space of single, double and triple (pertubatively) electron excitations CCSD(T) using the 6-311++G(3df,3pd) basis set. Canonical transition-state theory with a simple tunneling correction was used to predict the rate constants as a function of temperature (700−2500 K), and three-parameter Arrhenius expressions were obtained by fitting to the computed rate constants for elementary channels and overall reaction.