Activation Energies for an Intramolecular Electron Transfer Reaction

Activation energies and solvent effects for the intramolecular electron transfer reaction from a ligated p-nitrobenzoate radical anion to a cobalt(III) center in the complexes formed by the reduction of I and II were measured. The rate of the intramolecular electron transfer reaction decreases with increasing solvent polarity. The activation energies range between 9.0 and 12.5 kcal/mol for complex I and between 13.3 and 19.5 kcal/mol for complex II. The rates always followed Arrhenius temperature dependence, even with 10⁴ changes in rate from room temperature to 203 K. Very large preexponential factors, 2 X 10¹¹-10¹⁸ s^-1, are calculated for these rate constants.