Distance Dependence of Electron Transfer Rates

Electron exchange processes between complex ions or aromatic radical ions in solution are typically close to electronically adiabatic. This will generally not be the case for electron transfers over large distances, particularly those in biological (e.g. photosynthetic) systems. The factors governing variation of rate constant with distance are discussed. An important result is that, in favourable cases, ‘through-σ-bond’ interaction can result in decrease of the electron coupling integral J according to a relatively low inverse power law, rather than the exponential fall-off with distance expected for ‘through-space’ interaction.1,2