The multi-channel reaction of CH3S+3O2: Thermochemistry and kinetic barriers

We review the experimental and computational chemistry literature on the CH3S·+O2 reaction system and use computational chemistry to further evaluate a number of reaction paths and barriers along with the enthalpies of formation for stable intermediates. Enthalpies of formation for products and radical intermediates are computed by isodesmic work reactions at the CBS-QB3 level of theory. Barriers for important reaction paths from B3LYP/6-311++G(d,p), B3LYP/6-311++G(3df,2p), CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p), B3P86/6-311G(2d,2p)//B3P86/6-31G(d), B3PW91/6-311++G(3df,2p), G3MP2, and CBS-QB3 are compared. Results are shown to depend on the calculation levels and basis set. Enthalpy values from the G3MP2 and CBS-QB3 composite methods are recommended for these carbon-sulfur-oxygen systems, when feasible. The well depth for the CH3S·+3O2 reaction to CH3SOO· adduct is found to be 9.7 kcal/mol. One low barrier outlet channel, CH3SOO·→CH2S+HO2, is found to have 9.9 (or 13.1) kcal/mol activation energy, relative to the stabilized CH3SOO adduct at G3MP2 (or CBS-QB3) levels; but has a tight transition state structure relative to reverse reaction.