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 (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 S 2· 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. © 1995 by John Wiley & Sons, Inc.