Hydrogen abstractions by radicals. Different approaches to understanding factors controlling reactivity

This is a response to criticisms expressed by Roberts (B. P. Roberts and A. J. Steel, J. Chem. Soc., Perkin Trans. 2, 1994, 2155) and to reservations regarding the triplet repulsion term of our a priori method of calculating energies of activation for hydrogen abstractions by free radicals. It is shown here that this term is related to earlier approaches of London, Eyring and Polanyi, and that our calculation describes satisfactorily reactions for which the empirical approach of Roberts and Steel shows substantial discrepancies. We reply to criticisms and apply our calculation to the understanding of recent experimental results pertaining to the H18OH + ˙OH identity exchange. We attempt to place in perspective differing views on the factors controlling reactivity.