Imaging nucleophilic substitution dynamics

Anion- molecule nucleophilic substitution ( SN2) reactions are known for their rich reaction dynamics, caused by a complex potential energy surface with a submerged barrier and by weak coupling of the relevant rotational- vibrational quantum states. The dynamics of the SN2 reaction of Cl- + CH3I were uncovered in detail by using crossed molecular beam imaging. As a function of the collision energy, the transition from a complex- mediated reaction mechanism to direct backward scattering of the I- product was observed experimentally. Chemical dynamics calculations were performed that explain the observed energy transfer and reveal an indirect roundabout reaction mechanism involving CH3 rotation.