The three possible production channels for the reaction CH~3+OClO are studied by ab initio calculations. Geometries of the reactants, products, intermediates and transition states are optimized at the UMP2(full)/6-31G (d, p) level. The corresponding vibrational frequencies are calculated at the same level. Based on the total energies calculated at the G2MP2 level, the reaction heats and activation barriers are obtained. The computation results show that the production channel HOCl+H~2CO is the most feasible pathway, which is exothermic by 443.80 kJ·mol^-^1. It suggests that an activated intermediate is first formed through a barrierless process in this reaction, and the intermediate will dissociate to HOCl+H~2CO via a fivecenter transition state.

Key words: AB INITIO CALCULATION, METHANE, FREE RADICALS, REACTION MECHANISM, TRANSITION STATE THEORY