A Trade-off between Solvation and Collision Activation in Steering Competing E2 and S_N2 Dynamics

The prototypical E2 elimination and S_N2 substitution reactions between microsolvated fluoride and ethyl bromide show unexpected dynamic behaviors in mechanistic evolution driven by solvation and collision activation. Considering the steric effects, the gas-phase selectivity favors an E2 pathway barely dependent on collision energies. Remarkably, base solvation steers the reaction in an effective way toward substitution at a near-thermal energy, whereas the governing high-energy events retain elimination. Chemical dynamics simulations reproduce experimental findings and uncover a crucial solute-solvent coupling in determining such competing processes. Interestingly, collision activation can tune the underlying atomistic dynamics essentially in the reactant entrance channel and cause a mechanism shift. These features for the ubiquitous competing E2/S_N2 dynamics remain quite unknown, providing unique insight into reaction selectivity for complex chemical networks.