A global full-dimensional potential energy surface and quasiclassical trajectory study of the O(1D) + CH4 multichannel reaction

Abstract

We report a new global, full-dimensional ground-state potential energy surface (PES) of the O(¹D) + CH₄ multichannel reaction, based on high-level ab initio calculations and fitting procedures. The PES is a permutationally invariant fit to roughly 340 000 electronic energies calculated by the MRCI + Q/aug-cc-pVTZ level of theory. Extensive quasiclassical trajectory calculations were carried out on the new PES at the collision energy of relevance to the previously universal crossed molecular beam experiments. The product branching ratios, translational energy distributions and angular distributions of OH + CH₃, H + CH₂OH/CH₃O and H₂ + HCOH/H₂CO product channels were calculated and compared with the available experimental results. Very good agreement between theory and experiment has been achieved. The O(¹D) + CH₄ reaction mainly proceeds through the CH₃OH intermediate via a trapped abstraction mechanism, starting with the abstraction of the hydrogen atom, rather than the direct insertion pathway with the O(¹D) atom directly inserting into the C–H bond of CH₄. The process with a very short lifetime behaves like an abstraction reaction, producing a pronounced forward scattering peak as found in the OH + CH₃ channel, while the process with a relatively long lifetime produces reaction products with nearly forward and backward scattering symmetry, similar to an insertion reaction, as found in other reaction channels.