Deuterium NMR Kinetic Measurements of Solvent Effects on the Bimolecular Electron Transfer Self-Exchange Rates of Ruthenium Ammine Complexes. A Dominant Role for Solvent−Solute Hydrogen Bonding

Abstract

Electron transfer self-exchange rate constants for a series of three ruthenium ammine complexes have been determined in a range of solvents using deuterium NMR line-broadening measurements. It is found for all three complexes that the observed rates slow dramatically in solvents of strong Lewis basicity as measured by the Gutman donor number. The rates do not correlate significantly with either the dielectric continuum-based Pekar factor, (1/n2 − 1/Ds), or the solvent longitudinal relaxation time known to frequently characterize solvent dynamical effects on electron transfer rates. The nature of the self-exchange rate slow down is discussed within the context of Marcus−Hush theory and is attributed to redox-state-dependent rearrangements of hydrogen bonds between solvent and solute in the second coordination sphere. Quantitative comparisons between the observed donor-number effect on rate and relevant optical electron transfer data from chemically similar systems reveal evidence of an additional donor-number-dependent contribution to the work of reactant preassociation in the bimolecular process.