Density functional theory study of the dimerization of the sulfine H2CSO

The dimerization mechanism of the sulfine H2CSO (1) is studied using a hybrid DFT method (B3LYP) with a large basis set on the sulfur atom. Single-points post-HF calculations up to the CCSD(T) level were performed for some representative stationary points of the potential energy surface. The (2 + 3) mechanism was predicted to be the lowest energy pathway, with an activation barrier of 12.3 kcal/mol; the bond formation process is very asynchronous tin the first stage of the reaction, the C-C bond formation is more advanced than the S-O one) and probably a stepwise mechanism occurs involving a biradical intermediate. The alternative pathway involving an anti transition structure occurs with an activation energy equal to 13.7 kcal/mol and is slightly less favorable. In both cases, the obtained cycloadduct is the five-membered ring 2. Concerted (2, + 2,) or (3 + 3) pathways necessitate larger activation energies (congruent to 33 kcal/mol). Despite an extensive search, we were unable to find a reaction path for the rearrangement of 2 to the more stable four-membered adduct 11, which is the expected final term of the dimerization reaction.