Effect of entrance channel topology on reaction dynamics: O(3P) + CH4 → CH3 + OH

The generalized semirigid vibrating rotor target (GSVRT) method is employed to investigate O(3P) + CH4 → CH3 + OH reaction on a global potential energy surface (PES): the C–T surface of Corchado et al. [J.C. Corchado, J. Espinosa-Garcia, O. Roberto-Neto, Y.-Y. Chuang, D.G. Truhlar, J. Phys. Chem. A 102, 4899, 1998]. The reaction probability and rate constant from the present 5D GSVRT dynamics calculation are consistent with those from the 4D SVRT calculation, indicating that the umbrella vibrational motion of CH4 is not strongly coupled to the reaction coordinate on the C–T surface. The calculated reaction threshold energy on the C–T surface is considerably larger than that from the experimental measurement. Our study indicates that the entrance channel topology of the C–T PES is responsible for the higher reaction threshold energy obtained from the dynamics calculation. This is the main reason for the large discrepancy in reaction rate constant between transition state-like results and the dynamics calculation which explores global potential energy surface beyond transition state region.