QCT dynamics study of the reaction of hydroxyl radical and methane using a new ab initio fitted full-dimensional analytical potential energy surface

Abstract

In the present work, we have developed a new analytical potential energy surface fitted to ab initio data for the seven atoms OH + CH 4 → H 2 O + CH 3 hydrogen abstraction reaction and its isotopomer analogues, named PES-2014. This surface provides analytically not only energy, but gradients, which is a computational advantage in dynamics calculations. It reproduces the topology of the reaction from reactants to products, with a barrier height of 6.4 kcal mol −1 , exothermicity of −13.3 kcal mol −1 , and the presence of intermediate complexes in the entrance and exit channels, in excellent agreement with recent accurate estimates and ab initio calculations. Using the PES-2014 surface, we performed quasi-classical trajectory calcula- tions, in the collision energy range 4.0-16.0 kcal mol −1 , to analyze the dynamics of this reaction and its isotopomer analogue, OH + CD4 → HOD + CD 3, comparing the results with experimental data. We analyze the excitation function, the product energy distribution, and the product angular distribution, obtaining reasonable agreement with the experimental information. Although the agreement is not yet quantitative, we conclude that the new PES-2014 surface and the QCT approach simulate reasonably well the dynamics of this reaction. Finally, by analyzing the role of the intermediate complexes in the dynamics, we find that the influence of the reactant complex is negligible while the product well permits randomization of the scattering angles for a small percentage (5-10 %) of the reactive trajectories that show a practically isotropic behavior, associated with an indirect mechanism.