Controlling the Stereodynamics of the F^- + NH₂Cl S_N2 Reaction by Vibrational Excitation: A Quasi-Classical Study

Here we report how specific reactant vibrational excitations impact the dynamics of the F^- + NH₂Cl reaction, with a special emphasis on the stereoselectivity of its S_N2 channel, by performing quasi-classical trajectory simulations on a high-quality ab initio analytical potential energy surface. Exciting the N-Cl stretching is found to enhance the S_N2 reactivity the most, while N-H stretching excitation is the most efficient in promoting proton transfer. The stereospecificity of the title reaction can be drastically manipulated by exciting (1) the umbrella vibration, which facilitates repeated inversion through a H-bonded entrance channel minimum and transition state, thereby making multi-inversion the dominant mechanism over a wide collision energy range, accompanied by the complete loss of stereoselectivity; (2) the N-H stretching motion, which helps the H-bonded complex to form and thus lowers the inversion barrier around the N atom, thereby also promoting multi-inversion and racemization; and (3) the N-Cl stretching mode, which favors direct Walden inversion over complex-forming multi-inversion, resulting in reinforced stereoselectivity. The excess vibrational energy mainly flows into the internal degrees of freedom of the products, and while making the S_N2 reaction more direct, it has an overall moderate impact on proton-transfer dynamics.