Wednesday, March 2, 2011

Chemical reactivity is more than transition states

The concept of a transition state is one of the key concepts in understanding chemical reactivity. It is the maximum on a potential energy surface (PES), for which the reactants and products are local minima. But there is more to the story...

This is nice paper Reaction Force and Its Link to Diabatic Analysis: A Unifying Approach to Analyzing Chemical Reactions, by Peter Politzer, Jeff Reimers, Jane Murray, and Alejandro Toro-Labb in JPC Letters.

It discusses the notion of the reaction force, the derivative of the potential energy, and how its sign and magnitude can be used to classify different parts of a chemical reaction. The blue dashed lines below define different regions: activation -> transition -> relaxation. 

It turns out that some reactions are dominated by activation (the weakening of bonds) rather than transition (the breaking of bonds). Hence, in seeking to speed up a specific reaction with a catalyst one should target that part of the reaction.

This leads to a variation of the Hammond-Leffler postulate that correlates activation energies with heats of reactions.

What is new in this paper? They connect the reaction force and this division of the reaction path with a diabatic state analysis: which divides a reaction into electronic factors (which dominate the transition state) and nuclear reorganisation.

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