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:  R1S- + R2SSR3 → R1SSR2 + R3S-, where (1) R1 = R2 = R3 = H, (2) R1 = R3 = Me, R2 = H, and (3) R1 = R3 = H, R2 = Me. Results indicate that at the HF/6-31+G* level the reactions proceed via an SN2 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 R2 becomes larger, the reaction may not be able to proceed via an addition−elimination pathway and the SN2 mechanism operates.