Theoretical Investigation on the Relative Stability of XSSX and X₂SS Isomers (X = F, C1, CH₃, H).

The structures and relative stabilities of a series of disulfide (XSSX) and thiosulfoxide (X2SS) isomers have been studied for X = F, Cl, CH3, and H, using various levels of conventional ab initio and density functional theory (DFT). The XSSX isomers are more stable than the X2SS isomers for all substituents. The energy gap ΔE(X) between the two isomers increases (i.e., XSSX becomes more stable with respect to X2SS), and the S-S bond contracts in the series for X = F, Cl, CH3, H. The results are interpreted by means of natural population analysis (NPA) (e.g., the interaction between the disulfide moiety S2·· and the two substituents X·). The bonding in the hypervalent X2SS species is similar to the bonding in the nonhypervalent XSSX and does not involve a special role for sulfur‐3d 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.