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.