How Unstable are Thiosulfoxides? An ab Initio MO Study of Various Disulfanes RSSR (R = H, Me, Pr, All), Their Branched Isomers R2SS, and the Related Transition States1,2

Ab initio MO calculations at the MP2/6-311G** level of theory show that the thiosulfoxide H 2 SS is 143 kJ mol - 1 less stable than the disulfane HSSH, separated by an activation barrier of 210 kJ mol - 1 . Using much higher levels of theory, these results are only slighty changed, proving the suitability of the economical MP2/6-311G** scheme for the present purpose. At the same level Me 2 SS is 84 kJ mol - 1 less stable than MeSSMe, with the transition state lying 340 kJ mol - 1 above MeSSMe. Thermal isomerization of HSSH or MeSSMe can therefore be excluded, and H 2 SS and Me 2 SS should be kinetically stable toward unimolecular isomerization at low temperatures. Vibrational wavenumbers for H 2 SS and Me 2 SS are given. The allyl methyl thiosulfoxide MeAllSS is 83 kJ mol - 1 less stable than the disulfane MeSSAll, but isomerization of the latter requires only 110 kJ mol - 1 . The corresponding RSSR and R 2 SS structures for R = Pr and All have been calculated at the HF/6-31G* level. The bimolecular decomposition of dimethyl thiosulfoxide to dimethyl sulfide and S 2 is exothermic but spin-forbidden in the case of triplet S 2, and endothermic but spin-allowed when singlet S 2 is formed. The reactions of Me 2 SS with sulfur molecules S x ( x > 2) to give Me 2 S and S x +1 are exothermic.