Dynamical Bifurcation in Gas‐Phase XH⁻ + CH₃Y S_N2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path

Abstract

Abstract The gas‐phase reactions of XH − (X=O, S) + CH 3 Y (Y=F, Cl, Br) span nearly the whole range of S N 2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH 3 XH⋅⋅⋅Y − (or CH 3 S − ⋅⋅⋅HF) hydrogen‐bonded postreaction complex. MP2 quasiclassical‐type direct dynamics starting at the [HX⋅⋅⋅CH 3 ⋅⋅⋅Y] − transition‐state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH 3 XH+Y − (or CH 3 S − +HF) products directly are non‐IRC, whereas those that sample the CH 3 XH⋅⋅⋅Y − (or CH 3 S − ⋅⋅⋅HF) complex are IRC. The IRCIRC/non‐IRC ratios of 90:10, 40:60, 25:75, 2:98, 0:100, and 0:100 are obtained for (X, Y)=(S, F), (O, F), (S, Cl), (S, Br), (O, Cl), and (O, Br), respectively. The properties of the energy profiles after the TS cannot provide a rationalization of these results. Analysis of the energy flow in dynamics shows that the trajectories cross a dynamical bifurcation, and that the inability to follow the minimum energy path arises from long vibration periods of the X−C⋅⋅⋅Y bending mode. The partition of the available energy to the products into vibrational, rotational, and translational energies reveals that if the vibrational contribution is more than 80 %, non‐IRC behavior dominates, unless the relative fraction of the rotational and translational components is similar, in which case a richer dynamical mechanism is shown, with an IRC/non‐IRC ratio that correlates to this relative fraction.