Analytical potential energy surface for the CH4 + O(3P)→CH3 + OH reaction. Thermal rate constants and kinetic isotope effects

Abstract

We present a modified and recalibrated potential energy surface (PES) for the gas-phase CH₄ + O(³P)→CH₃ + OH reaction and its deuterated analogue, which is completely symmetric in all the terms pertaining to the four methane hydrogen atoms. This surface is then used to analyze dynamical features. Thus, from the analysis of the reaction path curvature, we qualitatively find that excitation of the CH₄ stretch and umbrella modes enhances the forward rate constants, while only the CH₃ umbrella mode could appear vibrationally excited. The forward thermal rate constants were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200–2500 K, finding good agreement with the available experimental data. We also calculated kinetic isotope effects for the, deuterated analogue CD₄, although, unfortunately, no comparison with experimental data was possible.