Ab initio investigation of the potential energy profiles for the gas phase CH4+O2+(2Πg) reaction system

Abstract

High level ab initio and density functional calculations of the ground state potential energy profiles were carried out to study the mechanism of the ion–molecule reaction CH4+O2+(X 2Πg) for four reaction channels: insertion of O2+ into the C–H bond of methane (INS), hydride abstraction from methane (HA), charge transfer (CT), and O–O cleavage path (OO) after INS process to give CH2OH++OH. Common to these channels are initial encounter complexes, and our calculations match very closely experimental estimates for binding energies. The INS channel proceeds through CH4OO+ and gives a deep minimum corresponding to the exothermic and metastable intermediate CH3OOH+. This species can easily eliminate H to give CH2OOH+, a product observed experimentally. For the slightly endothermic HA channel to give CH3++OOH, two pathways were found: a direct pathway (likely to dominate at higher collision energy) from the encounter complex via a HA transition state at 5.8 kcal/mol above the reactants, and an indirect pathway with a slightly smaller energy requirement consisting of elimination of OOH from the INS intermediate CH3OOH+. A transition state with a high energy requirement of 15 kcal/mol was found for O–O cleavage from CH3OOH+, consistent with the experimental finding that O–O cleavage occurs at high energies. It was also found that the seam of crossing between two potential surfaces is facilitated and therefore the CT channel is promoted by the O–O stretching and the methane deformation vibrations, again consistent with the experimental results.