Revival of an old structure problem: Trithiapentalene - real or time-averaged C2v symmetry?

The structures of 1, 6,6a\Lambda4-trithiapentalene la and the related compounds 1, 6-dioxa-6a\Lambda44-thiapentalenel b and I,6-diaza-6ah4-thiapentaleneI C were examined on the basis of ab initio MO theory employing the 3-21G(*) and 6-31G* basis sets. In case of the trithia and dioxathia molecules, the bridged C, structures appear as most stable arrangements when the correlation energy is considered, whereas the corresponding open C, forms are preferred at the SCF level. The same conclusions can be drawn for the 1,6-disubstituted diazathiapentalenes. Contrary to this, structure 4 with an aromatic isothiazole ring is the most stable form for the unsubstituted compounds.