HOW UNSTABLE ARE THIOSULFOXIDES - AN AB-INITIO MO STUDY OF VARIOUS DISULFANES RSSR (R=H, ME, PR, ALL), THEIR BRANCHED ISOMERS R(2)SS, AND THE RELATED TRANSITION-STATES

Ab initio MO calculations at the MP2/6-311G* level of theory show tha t the thiosulfoxide H2SS is 143 kJ mol(-1) less stable than the disulf ane HSSH, separated by an activation barrier of 210 kJ mol(-1). Using much higher levels of theory, these results are only slightly 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 stab le than MeSSMe, with the transition state lying 340 kJ mol(-1) above M eSSMe. Thermal isomerization of HSSH or MeSSMe can therefore be exclud ed, and H2SS and Me(2)SS should be kinetically stable toward unimolecu lar isomerization at low temperatures, Vibrational wavenumbers for H2S S and Me(2)SS are given. The allyl methyl thiosulfoxide MeAllSS is 83 kJ mol(-1) less stable than the disulfane MeSSAll, but isomerization o f 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.