Mechanism of thiolate-disulfide exchange: Addition-elimination or effectively SN2? 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 (T = 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 SN2 pathway). Rather, the mechanism is addition-elimination, with several trajectories sampling the intermediate for long times, up to 15 ps or longer.
Paranjothy, Manikandan, Matthew R. Siebert, William L. Hase, and Steven M. Bachrach. "Mechanism of Thiolate-Disulfide Exchange: Addition–Elimination or Effectively SN2? Effect of a Shallow Intermediate in Gas-Phase Direct Dynamics Simulations." The Journal of Physical Chemistry A 116, no. 47 (2012): 11492-11499.
Journal of Physical Chemistry A