S3O2--an unusual structure with pi*-pi* interaction.

The structures and relative stabilities of 15 S3O2 isomers have been investigated by G3X(MP2), CCSD(T)/aug-cc-pVTZ and MRCI/CASSCF calculations. The global energy minimum is a three-membered sulfur ring with two adjacent sulfoxide groups in a trans conformation, i.e. a vic-disulfoxide of C2 symmetry. The SS bond lengths are 2.136 (2x) and 2.354 angstroms at the CCSD(T)/cc-pVTZ level of theory. There is a strong interaction between the pi* orbitals of the two S=O moieties both in the trans and in the almost degenerate cis conformer. The corresponding chain-like singlet and triplet isomers of connectivity OSSSO lie close in energy (ca. 67 kJ mol(-1)) while five-membered and branched four-membered rings are significantly less stable. The structure of S3O2 is in contrast to that of the isoelectronic analogue S5, which exists as a five-membered twisted heterocycle.