High-level ab initio calculations on the 1,2-dithioglyoxal/1,2-dithiete isomerism

High-level ab initio calculations have been carried out to study the relative stabilities of 1,2-dithioglyoxal and its isomer 1,2-dithiete. Particular attention was paid to the effects of including g polarization functions in the basis. Several levels of theory were used, namely, MP2, MP4, CASMP2, QCISD(T) and CCSD(T). The performance of three different density functional methods, namely, B-LYP, B3-LYP and B3-P86 was also investigated. Our best estimate of the relative stabilities of these two species was obtained in the framework of G2 theory, modified to include explicitly the effects of high angular momentum polarization functions at the QCI and MP2 levels, so that total energies of effective QCISD(T)/6-311+ G(3dfg,2p) quality are obtained. These results predict 1,2-dithiete to be 4.1 kcal/mol more stable than the non-cyclic isomer, 1,2-dithioglyoxal.