Indirect dynamics in SN2@N: insight into the influence of central atoms

Abstract

Central atoms have a significant influence on the reaction kinetics and dynamics of nucleophilic substitution (S_N2). Herein, atomistic dynamics of a prototype S_N2@N reaction F⁻ + NH₂Cl is uncovered employing direct dynamics simulations that show strikingly distinct features from those determined for a S_N2@C congener F⁻ + CH₃Cl. Indirect scattering is found to prevail, which proceeds predominantly through a hydrogen-bonded F⁻–HNHCl complex in the reactant entrance channel. This unexpected finding of a pronounced contribution of indirect reaction dynamics, even at a high collision energy, is in strong contrast to a general evolution from indirect to direct dynamics with enhanced energy that characterizes S_N2@C. This result suggests that the relative importance of different atomic-level mechanisms may depend essentially on the interaction potential of reactive encounters and the coupling between inter- and intramolecular modes of the pre-reaction complex. For F⁻ + NH₂Cl the proton transfer pathway is less competitive than S_N2. A remarkable finding is that the more favorable energetics for NH₂Cl proton transfer, as compared to that for CH₃Cl, does not manifest itself in the reaction dynamics. The present work sheds light on the underlying reaction dynamics of S_N2@N, which remain largely unclear compared to well-studied S_N2@C.