Slope of free energy plots in chemical kinetics

Abstract

In current electron-transfer theory, an activation free energy term, ΔF*, depends in a simple way on the “standard” free energy of reaction, ΔF^0’, in the prevailing medium. Namely, ΔF* = (1/4)λ[1 + (ΔF^0’/λ)]^2, apart from small work terms; λ/4 is ΔF_0*, the value of ΔF* at (ΔF^0’ = 0; and λ obeys a simple additivity property. As an exploratory step an equation of the same functional form is applied in the present paper to data on Brønsted slopes of 16 proton- and atom-transfer reaction series. Thereby, the instantaneous slope of a ΔF* vs. ΔF^0’ plot is calculated to be (1/2)[1 + (ΔF^0’/4 ΔF_0*)]. Thus far the experimental results are consistent with this equation, but more data are needed. Other applications of the ΔF* equation, including one to the kinetic isotope effect, are also discussed. A rather approximate derivation of the above equation for atom and other transfers was given in an earlier paper of this series.