Half Reactions with Multiple Redox States Do Not Follow the Standard Theory: A Computational Study of Electrochemistry of C₆₀

The standard theory of electron transfer advanced by Marcus predicts that the solvent reorganization energy of electron transfer does not depend on the redox state of the reactant. For instance, it should be the same in the reduced and oxidized states of a half reaction. This theory prediction is verifiable by measuring activation barriers of electron transfer reactions involving multiple oxidation states. We use here the opportunity offered by electrochemistry of C₆₀, which allows charges from 0 to -4 in a sequence of reduction half reactions. We find that the activation barrier does change with altering redox state of a fullerene, which can be experimentally verified by measuring Arrhenius slopes of corresponding reaction rates. This outcome is connected to the alteration of the molecular polarizability caused by electronic transitions. Classical molecular dynamics simulations of a fullerene in water are combined here with the analytical Q-model of electron transfer involving polarizable molecules. The main outcome of the study is that altering molecular polarizability makes the reorganization energy and the reaction activation barrier depend on the redox state of the reactant.