Exploring the methane combustion reaction: A theoretical contribution

This paper represents an attempt to extend the mechanisms of reactions and kinetics of a methane combustion reaction. Three saddle points (SPs) are identified in the reaction ${CH}_4+O\left({}^{3}P\right)\to OH+{CH}_3$ using the complete active space self-consistent field and the multireference configuration interaction methods with a proper active space. Our calculations give a fairly accurate description of the regions around the twin first-order SPs (³ A′ and ³ A″) along the direction of O(³ P) attacking a near-collinear H–CH₃. One second-order SP^2nd (³ E) between the above twin SPs is the result of the C _3v symmetry Jahn–Teller coupling within the branching space. Further kinetic calculations are performed with the canonical unified statistical theory method with the temperature ranging from 298 K to 1000 K. The rate constants are also reported. The present work reveals the reaction mechanism of hydrogen-abstraction by the O(³ P) from methane, and develops a better understanding for the role of SPs. In addition, a comparison of the reactions of O(³ P) with methane through different channels allows a molecule-level discussion of the reactivity and mechanism of the title reaction. (paper)