Ab initio ground potential energy surface, VTST and QCT study of the O(3P)+CH4(X1A1)→OH(X2Π)+CH3(X2A2") reaction

An ab initio study of the ground potential energy surface (PES) of the O(P-3)+CH4-->OH+CH3 reaction has been performed using the second-and fourth-order Moller-Plesset methods with a large basis set. A triatomic analytical ground PES with the methyl group treated as an atom of 15.0 a.m.u. has been derived. This PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) rate constants] and dynamics (QCT method) of the reaction. The ab initio points have also been used directly to calculate the VTST rate constant considering all atoms of the system. The best VTST methods used lead to a good agreement with the experimental rate constant for 1000-2500 K, but QCT rate constant values are about one-third the experimental ones for 1500-2500 K. The cold QCT OH( nu=0) rotational distribution arising from the simulation of the reaction with O(P-3) atoms produced in the photodissociation of NO2 at 248 nm is in good agreement with experiment, while the very small QCT OH(nu=1) population obtained is consistent with measurements. The triatomic PES model derived in this work may be used in studies of the kinetics and dynamics under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction. (C) 1999 American Institute of Physics. [S0021-9606(99)00215-9].