Whence the Energy Term of the Rate Constant?

Insights into the causes of energy barriers to reactions are obtained from our E* model for hydrogen abstractions. Results of the model are in good agreement with all eight experimentally studied symmetrical reactions of the type X-H + (center dot)X -> X(center dot) + H-X. The E* energy is broken down into three components: energy needed to overcome triplet repulsion between the terminal Xs, energy needed to bring the H-X bonds to their transition state distances, and energy gain by the resonance stabilization of delocalization of the odd electron over three atoms. The strength of the X-H bond is a minor factor. The conclusion that triplet repulsion is a major factor is supported by the London equation and by results of recent high level theoretical calculations. The E* model requires inputs of bond dissociation energies, bond lengths, and infrared stretching frequencies of X-H and X-X. Some reported failures of E* are shown to have been caused by use of input values subsequently found to be incorrect.