Marcus rate theory and the relationship between Brønsted exponents and energy of reaction

Abstract

An intersecting parabola model for the hydrogen atom transfer reaction AH + B→A + BH predicts a linear dependence of the Brønsted exponent α upon the energy of reaction ΔE, consistent with simple Marcus theory, as long as the force constants of the AH and BH bonds are equal and constant and the distance which the hydrogen atom travels is invariant; when these restrictions are removed, α becomes a sigmoid function of ΔE whose linear portion, if treated by simple Marcus theory, underestimates ‘intrinsic’ barriers by factors of the order of two.