Robert G. Bergman Lecture: What is a transition state, and why should I care?
13th March 2018. University of California, Berkeley.
Since the early days of the development of Transition State Theory, there have been two descriptions of what a transition state (TS) is. The one that most chemists use identifies the TS with a saddle point on the potential energy surface (PES). The other is that it is a dividing surface (DS) in phase space, which reactive trajectories cross only once on their transit from reactant to product. Under limited circumstances, the two descriptors can be shown to be equivalent, but in most practical circumstances they are not. The DS description is the more rigorous, and this talk will focus on cases in which the location of the DS in configuration space is far from any PES saddle point. A common example occurs for reactions in solution that involve substantial changes in shape of the solute. If there are parallel paths to competing products after such an event, then incorrect identification of the true location of the TS can lead to a misunderstanding of what controls the product ratio. That, in turn, has obvious consequences for efforts to control product ratios through change of conditions or design of catalysts.