Competiting bimolecular nucleophilic substitution(SN2)and Base-induced elimination(E2)reactions are of central importance in preparative organic synthesis.A recent report of microsolvated reactions between fluoride ions and alkyl bromide using the selected ion flow tube(SIFT)technique observed a stepwise SN2 to E2 transition as the alkyl group gets more hindered.However,the atomic-level dynamics of possible reaction intermediates and intrinsic mechanistic behaviors posed some puzzling issues.Here,we unravel how individual solvent molecules may affect underlying SN2/E2 atomistic dynamics.Results are presented for a prototype microsolvated case of fluoride anion reacting with ethyl bromide.Reaction dynamics simulations reproduce experimental findings at near thermal energies and show that the E2 mechanism dominates over SN2 for solvent-free reaction.Adding one solvating methanol molecule introduces strikingly distinct dynamical behaviors that largely promote the SN2 reaction,a feature which attributes to a differential solute-solvent interaction at the central barrier that more strongly stabilizes the transition state for substitution.Upon further solvation,this enhanced stabilization of the SN2 mechanism becomes more pronounced,concomitant with drastic suppression of the E2 route.This work highlights the interplay between energetics and dynamics in determining mechanistic selectivity and provides insight into the impact of solvent molecules on a general transition from elimination to substitution for chemical reactions proceeding from gas-to solution-phase environments.