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

Electron transfer self-exchange rate constants for a series of three r uthenium 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 r ates do not correlate significantly with either the dielectric continu um-based Pekar factor, (1/n(2) - 1/D-s), or the solvent longitudinal r elaxation time known to frequently characterize solvent dynamical effe cts on electron transfer rates. The nature of the self-exchange rate s low down is discussed within the context of Marcus-Hush theory and is attributed to redox-state-dependent rearrangements of hydrogen bonds b etween solvent and solute in the second coordination sphere. Quantitat ive comparisons between the observed donor-number effect on rate and r elevant 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.