Reaction dynamics of chlorine atom with methane:: Dual-level ab initio analytic potential energy surface and isotope effects

An analytic potential energy surface for the Cl+CH(4)reversible arrow HCl+CH3 reaction in C-3V symmetry has been obtained by fitting to 1136 energy points from a dual-level MP2/SAC (Moller-Plesset second order perturbation/scaling all correlation) calculation using the 6-311G(2d,d,p) basis set. A zero-point energy correction is made to account for all modes not explicitly treated with the time-independent quantum scattering rotating line umbrella (RLU) model, which is used for the dynamics calculations. The effective potential gives a vibrationally adiabatic ground-state barrier height of 3.36 kcal/mol and an endothermicity (0 K) of 1.19 kcal/mol for the Cl + CH4 reaction, and 4.43 kcal/mol and 2.29 kcal/mol, respectively, for Cl + CD4. Thermal rate constants, tunneling and kinetic isotope effects have been investigated in detail. Calculated differential cross sections for Cl+CD4-->Cl+CD3, with reactants and products in their vibrational ground states, show that the DCl product is strongly backward scattered. Further, ground state Cl + CD4 reacts to give the CD3 product predominantly unexcited at collision energies ranging from 0.15 eV to 0.25 eV. Generally, good agreement with experimental measurements and previous theoretical work is obtained. (C) 1999 American Institute of Physics. [S0021-9606(99)30439-6].