The geometries, vibrational frequencies of all stationary points involved in the title reaction were calculated at the MPW1PW91/6-311G(d,p) level. Relationships of reactants, intermediates, transition states and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. The single-point energy and the selected points along the minimum energy potential were calculated at the QCISD(T)/6-311G(d,p), as well as the singlet and triplet potential energy surfaces were investigated. The results show that there are 6 dominating reaction channels (7 pathways) for the CH₃SO＋HO₂ reaction system. The major channel (1) R→³IM→P1 (CH₃SOH＋³O₂) happened on the triplet potential energy surface included two pathways, in which apparent activation energies are 12.01 and －30.04 kJ•mol^－1 respectively, and the dominant path (2) R→³IM→³TS2→P1 (CH₃SOH＋³O₂) is a barrierless process. The rate constants of the path (2) are evaluated by means of the classical transition state theory (TST) and the canonical variational transition state theory (CVT) in which the small-curvature tunneling correction was included. The fitted three-parameter expression for the path (2) is in the temperature range of 200～2500 K, and the rate constant display a negative temperature dependence. In addition, the variational effect was almost negligible in the entire process, while the tunneling effect is considerable at the lower temperature.

Key words: CH₃SO, HO₂, singlet PES, triplet PES, reaction mechanism, rate constant