The biradical character of 1,2-dithiin and its valence isomers

1,2-Dithiin and its valence isomers were studied by ab initio methods at the UHF/6-31G* level and employing the guess=mix option. Solutions for these systems obtained without destroying the α–β spin-symmetry displayed Hartree–Fock instabilities. Optimized geometries of all structures are minima (even the cZc-isomer which according to previous work is a transition state). The biradical characters for all these structures were analyzed following the scheme of Yamaguchi. According to this scheme the 2-butenedithial isomers have significant singlet biradical character while 1,2-dithiin has only a small amount of biradical character. Proposals regarding the nature of the intermediates in the photolysis of 1,2-dithiin are made.