Computational studies of the ground and excited state potentials of DMSO and H2SO: Relevance to photostereomutation

A computational study on ground and excited states of dimethyl sulfoxide (D MSO) and the hypothetical molecule H2SO is reported. Full valence CASSCF/6- 311+G(3df,2p) with a multireference perturbation theory correction was used for the latter, while DMSO was examined with an active space that neglecte d only the CH bonds and an analogous basis set that neglected polarization functions on H. A realistic value of 41.5 kcal/mol was obtained for the gro und state pyramidal inversion of sulfur for DMSO, though no directly compar able experimental value is available. Calculations were also carried out on singlet and triplet excited state surfaces of both A' and A " symmetry. Re laxed excited state geometries fairly near the ground state geometry were f ound, but perhaps more importantly, excited state stationary points were al so found in C-2 nu symmetry. These were the lowest energy of any geometry o n their respective surfaces. This leads to the speculation that photochemic al stereomutation of alkyl sulfoxides may occur without C-S bond cleavage i n a mechanism formally analogous to the photochemical cis-trans isomerizati on of olefins.