COUNTERION EFFECTS IN INTRAMOLECULAR CHARGE-TRANSFER IN RADICAL-ANIONS

The influence of the presence of an electrolyte in a low-polarity solvent (tetrahydrofuran) on the mte of a weekly exoergic (almost-equal-to 50 meV) nonadiabatic intramolecular charge shift reaction and on the position of the absorption band in a degenerate system has been studied by pulse radiolysis. The investigation focused in particular on the dependence of the effect on the size of the cation of the chemically inert electrolyte. Electrolytes caused dramatic rate reductions of 2-3 orders of magnitude. Unexpectedly, the mte reduction was found to increase with an increasing size of the cation. The recorded spectral blue shifts were as large as 0.5 eV and exhibited the expected tendency: the smaller counterions produced larger spectral shifts, due to a stronger Coulombic interaction. In both cases the effects exceeded the limits predicted by the continuum approach and pointed to the ion-pairing mechanism. The discrepancy between the dependence of the thermal electron transfer and the optically assisted electron transfer on the radius of the counterion could not be easily explained on purely energetic grounds, and the dynamics of the counterion had to be considered. Unfortunately, no agreement between the experimental results and the Eigen model of dissociative diffusion of ion pairs in the zero ionic strength limit could be found. However, a qualitative correlation between the measured rates and the dissociation constants of the used salts, as well as the counterion mobilities was observed.