Acceleration of electron transfer from organic molecular crystals to Fe(CN)63− and Mo(CN)83− through mono-, di-, and trivalent cations and protons

Abstract

For the same concentration of added electrolyte the rate constant of electron transfer from organic crystals to Fe(CN) 6 3− increases with the charge of the cation. The order of the effect k Li + < k Na + < k Cs + in the concentration range ≤1 mol l −1 can be explained by the different hydration of the cations yielding a reversal of the order k Cs + < k Li + at salt concentrations ≥3.5 mol l −1 . The dramatic difference in the strength of proton binding by the two redox ions Fe(CN) 6 4− and Mo(CN) 8 4− leads to a correspondingly large difference in the effect of protons on the rate constant of electron transfer to the oxidized species. There is a remarkable similarity between the relative effect of different cations on the rate constant of electron transfer to the anionic reactant Fe(CN) 6 3− and on the nuclear magnetic relaxation which they cause due to an electric field gradient in an anion such as I − . In the range of known values for the effective standard redox potential of the redox ions in the presence of added salt or acid the enhancement of the rate constant can be ascribed with nearly quantitative accuracy but in an empirical way to the change in only this parameter in the present system.