The chemical mechanism for electron transfer and bridge reducibility

One limiting mechanism proposed for bridged inner-sphere, electron transfer reactions, the chemical mechanism, involves rate-limiting reduction of the bridge to a radical by the reducing centre followed by rapid reduction of the oxidizing centre by the radical bridge. One predicted kinetic trait of such reactions is a correlation between the electron transfer rate constant and the "reducibility" of the bridge. This predicted trend is quantified here for the first time, with bridge reducibility expressed in terms of adiabatic gas-phase electron affinities. Also, a new kinetic test to distinguish outer-sphere electron transfers from those proceeding the chemical mechanism is shown to develop from the dependence of the rate constant on electron affinity.