Ab initio investigation of the potential energy profiles for the gas phaseCH4+O-2(+)((2)Pi(g)) reaction system

High level ab initio and density functional calculations of the ground stat e potential energy profiles were carried out to study the mechanism of the ion-molecule reaction CH4+O-2(+)(X (2)Pi (g)) for four reaction channels: i nsertion of O-2(+) into the C-H bond of methane (INS), hydride abstraction from methane (HA), charge transfer (CT), and O-O cleavage path (OO) after I NS process to give CH2OH++OH. Common to these channels are initial encounte r 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 CH 3OOH+. This species can easily eliminate H to give CH2OOH+, a product obser ved experimentally. For the slightly endothermic HA channel to give CH3++OO H, 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 sm aller energy requirement consisting of elimination of OOH from the INS inte rmediate CH3OOH+. A transition state with a high energy requirement of 15 k cal/mol was found for O-O cleavage from CH3OOH+, consistent with the experi mental finding that O-O cleavage occurs at high energies. It was also found that the seam of crossing between two potential surfaces is facilitated an d therefore the CT channel is promoted by the O-O stretching and the methan e deformation vibrations, again consistent with the experimental results. ( C) 2001 American Institute of Physics.