THERMAL AND VIBRATIONAL-STATE SELECTED RATES OF THE CH4+CL[--]HCL+CH3 REACTION

We present direct ab initio dynamics studies of thermal and vibrational-state selected rates of the hydrogen abstraction CH4+Cl<->CH3HCl reaction. Rate constants were calculated within the canonical variational transition state theory formalism augmented by multidimensional semiclassical tunneling corrections. A vibrational diabatic model was used for vibrational-state selected rate calculations, particularly for exciting the CH4 symmetric stretching and umbrella bending modes. The potential energy information was calculated by a combined density functional and molecular orbital approach. Becke's half-and-half (BH&H) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BH&HLYP) were used with the 6-311G(d,p) basis set for determining structures and frequencies at the stationary points and along the minimum energy path (MEP). Energetics information was further improved by a series of single point spin-projected fourth-order Moller-Plesset perturbation theory (PMP4(SDTQ)) calculations using the 6-311+G(2df,2pd) basis set. We found that the calculated thermal rate constants have reasonable agreement with experimental results for both the forward and reverse reactions. Our results also predict that exciting the CH4 symmetric stretching mode will greatly enhance the hydrogen atom transfer rate. Surprisingly, exciting the CH4 umbrella bend mode is also predicted to have a noticeable enhancement factor at room temperature. (C) 1995 American Institute of Physics.