Abstract
Several intermediates for the CH~3~SH + OH^•^ → CH~3~S^•^ + H~2~O reaction were identified using MP2(full) 6‐311+g(2df,p) ab initio calculations. An adduct, CH~3~S(H)OH^•^, I, with electronic energy 13.63 kJ mol^−1^ lower than the reactants, and a transition state, II^‡^, located 5.14 kJ mol^−1^ above I, are identified as the entrance channel for an addition‐elimination reaction mechanism. After adding zero‐point and thermal energies, Δ__H__(reactants → I) = −4.85 kJ mol^−1^ and Δ__H__(I → II^‡^) = +0.10 kJ mol^−1^, which indicates that the potential energy surface is broad and flat near the transition state. The calculated imaginary vibrational frequency of the transition state, 62i cm^−1^, is also consistent with an addition–elimination mechanism. These calculations are consistent with experimental observations of the OH^•^ + CH~3~SH reaction that favored an addition–elimination mechanism rather than direct hydrogen atom abstraction. An alternative reaction, CH~3~SH + OH^•^ → CH~3~SOH + H^•^, with Δ__H__ = +56.94 kJ mol^−1^ was also studied, leading to a determination of Δ__H__(CH~3~SOH) = −149.8 kJ mol^−1^. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 612–618, 2005