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 States^1,2

Abstract

Ab initio MO calculations at the MP2/6-311G** level of theory show that the thiosulfoxide H2SS 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 Me2SS 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 H2SS and Me2SS should be kinetically stable toward unimolecular isomerization at low temperatures. Vibrational wavenumbers for H2SS and Me2SS 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 R2SS 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 S2 is exothermic but spin-forbidden in the case of triplet S2, and endothermic but spin-allowed when singlet S2 is formed. The reactions of Me2SS with sulfur molecules S x (x > 2) to give Me2S and S x +1 are exothermic.

Persulfidation (S-sulfhydration) and H2S

“…As mentioned above, the persulfides would react with electrophiles rather than with nucleophiles (Artaud and Galardon 2014;Bailey et al 2014), and when they do react with nucleophiles (such as other free thiols), this reaction leads to the release of H 2 S (which represents the basis of one of the methods for persulfide detection: reduction with DTT). The reaction with nucleophiles could only go as an attack to sulfane sulfur, but that leads to the formation of a mixed disulfide and elimination of H 2 S. Some literature data exist suggesting that persulfides could exist in the tautomeric thiosulfoxide form (Scheme 1), which would then act as a perfect donor of sulfane sulfur (Kutney and Turnbull 1982;Steudel et al 1997). However, it is worth noting that although this may be true for the extreme case of F 2 S 2 , neither experimental nor computational data support the existence of this tautomeric form (Steudel et al 1997).…”

“…The reaction with nucleophiles could only go as an attack to sulfane sulfur, but that leads to the formation of a mixed disulfide and elimination of H 2 S. Some literature data exist suggesting that persulfides could exist in the tautomeric thiosulfoxide form (Scheme 1), which would then act as a perfect donor of sulfane sulfur (Kutney and Turnbull 1982;Steudel et al 1997). However, it is worth noting that although this may be true for the extreme case of F 2 S 2 , neither experimental nor computational data support the existence of this tautomeric form (Steudel et al 1997). Therefore, the elucidation of this mechanism is to be done in some future studies.…”

Sulfur depletion in dense clouds and circumstellar regions

“…Nevertheless, H 2 SS has a lower stability, due to its energetic sizable barrier that prevents rearrangement back to the more stable H 2 S 2 form at cryogenic temperatures. Thermal isomerization of H 2 S 2 leading to H 2 SS can therefore be excluded, and H 2 SS should be kinetically stable toward unimolecular isomerization at low temperatures (Steudel et al 1997). For this reason, only a photochemical process can explain the formation of H 2 SS.…”

Synthesis, characterization and crystal structure of dicesium bis(1,2-dithiole-3,5-dithiolato-4-yl)disulfide

“…In general, longer SAS bond lengths correlate with more acute CASASAC torsion angles. MO calculations indicate that CASASAC torsion angles near 90°are stabilized by weak bonding interactions between the CAS r* orbital on one sulfur atom and the 3pz lone pair on the other [27][28][29]. Deviations from this 90°value would result in loss of this stabilization.…”