Nucleophilic Substitution at Sulfur:  SN2 or Addition−Elimination?

The thiolate−disulfide exchange reaction was studied at the HF/6-31+G*, MP2/6-31+G*, MP4SDTQ/6-31+G*//MP2/6-31+G*, and CCSD(T)/6-31+G*//MP2/6-31+G* levels for three identity reactions: R 1 S - + R 2 SSR 3 → R 1 SSR 2 + R 3 S -, where (1) R 1 = R 2 = R 3 = H, (2) R 1 = R 3 = Me, R 2 = H, and (3) R 1 = R 3 = H, R 2 = Me. Results indicate that at the HF/6-31+G* level the reactions proceed via an S N 2 pathway. However, the potential energy surface at correlated levels has a triple-well structure, indicating an addition−elimination pathway. The HF TS becomes a stable intermediate upon inclusion of electron correlation, and an asymmetric transition state connects the ion−dipole complex with the intermediate. Structural and energetic results do suggest however that as R 2 becomes larger, the reaction may not be able to proceed via an addition−elimination pathway and the S N 2 mechanism operates.