Entropic Driving-Force Effects upon Preexponential Factors for Intramolecular Electron Transfer: Implications for the Assessment of Nonadiabaticity

Considerable effort has been directed recently toward measuring rate parameters for intramolecular redox reactions, typically involving electron transfer between a pair of transition metals linked by an organic bridge.lW3 Such reactions offer important advantages over bimolecular processes for investigating detailed aspects of electron transfer, since the uncertainties concerning the energetics of forming the precursor complex are absent and the geometry of the transition state is relatively well-defined.3 The study of intramolecular electron transfer should be particularly useful for examining electronic coupling effects; i.e. the factors influencing the occurrence and degree of nona d i a b a t i ~ i t y . ’ ~ , ~ ~ , ~ . ~ Since the rate constant, ket, for such processes should refer to an elementary electron-transfer step, it can be expressed as4