Dynamical Effects of S N 2 Reactivity Suppression by Microsolvation: Dynamics Simulations of the F - (H 2 O) + CH 3 I Reaction on a 21-Dimensional Potential Energy Surface.

A comparison of atomistic dynamics between microsolvated and unsolvated reactions can expose the precise role of solvent molecules and thus provide deep insight into how solvation influences chemical reactions. Here we developed the first full-dimensional analytical potential energy surface of the F - (H 2 O) + CH 3 I reaction, which facilitates the efficient dynamics simulations on a quantitatively accurate level. The computed S N 2 reactivity suppression ratio of the monosolvated F - (H 2 O) + CH 3 I reaction relative to the unsolvated F - + CH 3 I reaction as a function of collision energy first increases and then decreases steadily, forming an inverted-V shape, due to the combined dynamical effects of interaction time, steric hindrance, and collision-induced dehydration. Moreover, further analysis reveals that the steric effect of the F - (H 2 O) + CH 3 I reaction resulting from the single water molecule is manifested mainly in dragging the F - anion away from the central C atom, rather than shielding F - from C. Our study shows there is great potential in rigorously studying the role of the solvent in more complicated reactions.