A theoretical study of the disulfide/dithione valence isomerism

Abstract

According to MNDO calculations on 1,2-dithiete (IIIa), 1,2-dithiin (IVa) and 3,4-dimethylene-1,2-dithiane (Va) the closed-ring compounds are thermodynamically more stable than the open-chain compounds whereas the reverse holds for 4-methylene-1,2-dithiole (VIa). Substitution of the exocyclic CH2 group of VIa by O, S, NH, and OH+ stabilizes its cyclic mesoionic structure. The low triplet state energy of VIa and of some derivatives relative to the lowest singlet state energy signalizes the biradicaloid nature of these electronic structures. Replacement of hydrogen of the methine groups adjacent to sulfur of III, IV, and VII by CH3, CF3, C6H5, CHO or CN results more or less in stabilization of the cyclic tautomer whereas donor substituents such as NH2, OCH3 or SCH3 act less uniformly but mostly in a destabilizing way. Contrary to former assumptions, there is no indication for a species intermediate between the disulfide and dithione structure that defines the molecular ground state. A relatively low ground state barrier separates the two isomeric structures although thermal isomerization of IIIa belongs to Woodward-Hoffmann forbidden concerted reactions (calculated activation energy for the ring opening of IIIa about 36 kcal/mol). The thermal and photochemical isomerization is discussed in terms of the change of the energy along the reaction path of the closed shell ground state (S0) and lowest energy excited states (S1, T1). The energies are obtained by MNDO, MNDO/2 x 2 CI, MNDO/HE, and MNDO/HE/3 x 3 CI calculations.