SOLVENT EFFECTS ON INTERMOLECULAR ELECTRON-TRANSFER PROCESSES

The solvent effects on the rate constants and activation parameters of several intermolecular electron transfer reactions were investigated. The studies were carried out in protic (methanol, ethanol) and aproti c (acetonitrile, propionitrile) solvents. It was found that rate const ants are generally higher in the protic solvents and that activation e nthalpies are generally small and in some cases negative. The negative values were found in acetonitrile, while for the same reaction positi ve values are observed in alcoholic solvents. Large negative values of activation entropies were observed. They are more negative in the apr otic solvents. These observations can be explained by considering the entropy changes caused by the solvent reorganization on forming the tr ansition state. The solvent effect on the quantum yield of the charge separation process was also investigated. For the system pyrene-indole the free-ions quantum yield changes in accordance with the trend of t he cage escape rate constant. On the other hand, in the triplet quench ing of dibenz-anthracene by nitrobenzenes the charge separation proces s is much less efficient in acetonitrile than in methanol. These resul ts cannot be explained if the solvent is considered as a dielectric co ntinuum.