Mechanism of Thiolate-Disulfide Exchange: Addition–Elimination or Effectively S_N2? Effect of a Shallow Intermediate in Gas-Phase Direct Dynamics Simulations

Direct dynamics trajectory simulations were performed for two examples of the thiolate-disulfide exchange reaction, that is, HS^– + HSSH and CH₃S^– + CH₃SSCH₃. The trajectories were computed for the PBE0/6-31+G­(d) potential energy surface using both classical microcanonical sampling at the ion-dipole complex and quasi-classical Boltzmann sampling (<i>T</i> = 300 K) at the central transition state. The potential energy surface for these reactions involves a hypercoordinate sulfur intermediate. Despite the fact that the intermediate resides in a shallow well (less than 5 kcal/mol), very few trajectories follow a direct substitution path (the S_N2 pathway). Rather, the mechanism is addition–elimination, with several trajectories sampling the intermediate for long times, up to 15 ps or longer.