Computational study of the reaction SH + O2.

The reaction of SH + O2 has been characterized using multireference methods, with geometries and vibrational frequencies determined at the CASSCF/cc-pVTZ level and single-point energies calculated at the MRCI/aug-cc-pV(Q+d)Z level. The dominant product channels are found to be SO + OH and HSO + O. Whereas the formation of SO + OH has a barrier of approximately 81 kJ mol-1, it is energetically more favorable than the formation of HSO + O, which is barrierless beyond the endothermicity of approximately 89 kJ mol-1 at 0 K. Thus, the reaction SH + O2 --> SO + OH is 2 orders of magnitude faster than the reaction SH + O2 --> HSO + O at room temperature, revealing that the atmospheric oxidation of SH leads directly to the formation of SO + OH with the rate coefficient of approximately 1.0 x 10(-2) cm3 mol-1 s-1. At temperatures above 1000 K, however, the rates of the two channels become comparable. This may be attributed to the entropy effects leading to the higher pre-exponential factor for the channel (forming HSO + O) via a more loose transition state than that (forming SO + OH) entailing a four-centered transition state. Whereas the hydrogen abstraction reaction producing S + HO2 is found to proceed on the quartet surface, the substantial barrier of approximately 165 kJ mol-1 means that it occurs as a minor product channel. Finally, the formation of possible products SO2 + H is prohibited due to the lack of a transition state for the direct sulfur insertion.