Theoretical investigation of the relative stabilities of XSSX and X₂SS isomers (X = F, Cl, H, and CH₃)

The structures and relative stabilities of a series of disulfide ( X SS X ) and thiosulfoxide ( X 2 SS) isomers have been studied for X = F, Cl, CH 3 , and H, using various levels of conventional ab initio and density functional theory (DFT). The X SS X isomers are more stable than the X 2 SS isomers for all substituents. The energy gap Δ E ( X ) between the two isomers increases (i.e., X SS X becomes more stable with respect to X 2 SS), and the S  S bond contracts in the series for X = F, Cl, CH 3 , H. The results are interpreted by means of natural population analysis (NPA) (e.g., the interaction between the disulfide moiety S and the two substituents X ·). The bonding in the hypervalent X 2 SS species is similar to the bonding in the nonhypervalent X SS X and does not involve a special role for sulfur‐3 d orbitals. These orbitals acquire only minimal populations and are not to be conceived as valence orbitals. The DFT and conventional ab initio results, Xα/DZP and MP2/6‐31G** optimized structures and isomerization energies (at the highest levels of both methods), agree well.