Ab initio ground potential energy surface and quasiclassical trajectory study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction dynamics

An ab initio study of the ground potential energy surface (PES) of the O(1D)+CH4→OH+CH3 reaction has been performed using the second and fourth order Mo/ller–Plesset methods with a large basis set. From the ab initio data a triatomic analytical ground PES with the methyl group treated as an atom of 15.0 amu has been derived. This PES has been employed to study the dynamics of the reaction by means of the quasiclassical trajectory (QCT) method. A good agreement between the experimental and QCT OH rovibrational distributions at a collision energy of 0.212 eV with the methane molecule at 298 K has been obtained. The analysis of the microscopic reaction mechanism shows that the reaction takes place almost exclusively through the insertion of the O(1D) atom into a C–H bond, due to the presence of the deep (CH3)OH minimum, and the resulting trajectories may be direct or nondirect (short-lived collision complexes mainly) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect mechanism leads to a noninverted one. There is some tendency to give broader OH rotational distributions peaking at higher N′ values, particularly for the vibrational levels v′=0–1, in the case of the nondirect trajectories. The PES derived here may be used in dynamics studies under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction.