Solvent Effects for Nonadiabatic Proton Transfer in the Benzophenone/N,N-Dimethylaniline Contact Radical Ion Pair

Picosecond absorption spectroscopy is employed in determining the dynamics of proton transfer within a variety of substituted benzophenones/N,N-dimethylaniline in a series of alkanenitrile solvents. A correlation of the rate constants for proton transfer with driving force reveals a normal region and an inverted region for proton transfer. The kinetics are analyzed within two theoretical frameworks for nonadiabatic proton transfer. The Borgis−Hynes model, which is based upon a low-frequency promoting mode that modulates the tunneling frequency, is found to be only in qualitative accord with the experiment. The Lee−Hynes model, which not only includes a low-frequency promoting mode but also allows for contributions from vibrational excitation of the proton reaction coordinate in the reactant and product states, is found to give an excellent fit to the experimental data.