Global Potential Energy Contour Plots for Chemical Reactions. Muttiple Reaction Paths, Bifurcations, and Applicability of Transition-State Theory

A global reduced-dimensional potential energy contour plot is described for the simultaneous representation of reactions X-3Y ⇌ X_2 + XY, X_3Y ⇌ X + X_2Y, and X_2 + YX ⇌ X_2Y + X, where the X's may be different. The analysis provides some insight into the nature of the transition states, the role of bifurcations, and the applicability of transition-state theory (TST). Recent results of a quantum chemistry calculation for the X = H, Y = 0 system are discussed in these terms. More generally, some Topographical conditions for the inapplicability of TST are suggested. This limitation of TST arises from a preempting of the transition state for one reaction by another and appears to be relatively rare