Nucleophilic substitution at sulfur: S(N)2 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) 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 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 R2 becomes larger, the reaction may not be able to proceed via an addition-elimination pathway and the S(N)2 mechanism operates.