An extended form of the Evans–Polanyi equation: a simple empirical relationship for the prediction of activation energies for hydrogen-atom transfer reactions

Abstract

An empirical approach has been used to devise a simple relationship [eqn. (B)] between the activation energy for an elementary hydrogen-atom transfer reaction (A) and ground state properties A˙+ H–B → A–H + B˙(A), Ea=Eof+αΔH°(1–d)+βΔχAB2+γ(sA+sB)(B) of the reactants and products. The role of polar effects, which operate in the transition state, is emphasised and described quantitatively in terms of the difference in Mulliken electronegativities (ΔχAB) of the radicals A˙ and B˙. Eqn. (B) reproduces the activation energies for 65 reactions, taken from the literature, within a standard error of ±2.0 kJ mol –1 and with a correlation coefficient of 0.988. Reactions of widely differing types are included and no distinction is made between gas-phase reactions and those which take place in non-polar solvents. Examples of hydrogen-atom transfer reactions which are not treated satisfactorily by eqn. (B) are discussed.