A Trade-off between Solvation and Collision Activation in Steering Competing E2 and S2 Dynamics.

The prototypical E2 elimination and S2 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/S2 dynamics remain quite unknown, providing unique insight into reaction selectivity for complex chemical networks.