Theoretical Kinetics Study of the OH + CH3SH Reaction Based on an Analytical Full-Dimensional Potential Energy Surface

Abstract

Based on a recently developed full-dimensional analytical potential energy surface, named PES-2024, which was fitted to high-level ab initio calculations, three different kinetic theories were used for the computation of thermal rate constants: variational transition state theory (VTST), quasi-classical trajectory theory (QCT) and ring polymer molecular dynamics (RPMD) method. Temperature dependence of the thermal rate constants, branching ratios and kinetic isotope effects (KIEs) for the C1 (methyl-H-abstraction process) and C2 paths (thiol-H-abstraction process) of the OH + CH₃SH polyatomic gas-phase hydrogen abstraction reaction were theoretically determined within the 200–1000 K temperature range, except the RPMD values which were only reported at the highest temperature by computational limitations. We found that while the overall thermal rate constants obtained with the VTST theory show a V-shaped temperature dependence, with a pronounced minimum near 600 K, the QCT and RPMD dynamics theories question this abrupt change at high temperatures. At 1000 K, where the RPMD theory is exact, the VTST and QCT methods overestimate the RPMD results, which is associated with the consideration of recrossing effects. In general, the theoretical KIEs depicted a “normal” behavior for the C1 (values close to unity) and C2 paths in the OH+CH₃SH/OH+CH₃SD reactions, and an “inverse” behavior in the OH+CH₃SH/OD+CH₃SD reactions for both paths. Finally, the discrepancies between theory and experiment were analyzed as a function of several factors, such as limitations of the kinetics theories and the potential energy surface, as well as the uncertainties in the experimental measurements.