Dynamical Bifurcation in Gas-Phase XH + CH Y S 2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path.

The gas-phase reactions of XH (X=O, S) + CH Y (Y=F, Cl, Br) span nearly the whole range of S 2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH XH⋅⋅⋅Y (or CH S ⋅⋅⋅HF) hydrogen-bonded postreaction complex. MP2 quasiclassical-type direct dynamics starting at the [HX⋅⋅⋅CH ⋅⋅⋅Y] transition-state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH XH+Y (or CH S +HF) products directly are non-IRC, whereas those that sample the CH XH⋅⋅⋅Y (or CH 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.