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. Solution s for these systems obtained without destroying the alpha-beta spin-sy mmetry displayed Hartree-Fock instabilities. Optimized geometries of a ll structures are minima (even the cZc-isomer which according to previ ous work is a transition state). The biradical characters for all thes e structures were analyzed following the scheme of Yamaguchi. Accordin g to this scheme the 2-butenedithial isomers have significant singlet biradical character while 1,2-dithiin has only a small amount of birad ical character. Proposals regarding the nature of the intermediates in the photolysis of 1,2-dithiin are made. (C) 1998 Elsevier Science B.V . All rights reserved.