SOLVENT REORGANIZATION ENERGIES MEASURED BY AN ELECTRON-TRANSFER REACTION IN SUPERCRITICAL ETHANE

The intermolecular electron transfer reaction between a biphenyl anion and pyrene in supercritical ethane was studied using pulse radiolysis . Second-order electron transfer rates were found to be of the order o f 10(11) M-1 s(-1) The rate constants appear to be approximately const ant over the pressure range 55-133 bar and slower than the predicted d iffusion-controlled rate constants. Two possibilities are discussed th at could explain the present results: diffusion constants that are not well predicted by the hydrodynamic equation; or a solvent reorganizat ion energy of approximately 0.35 eV and dependent on pressure. The reo rganization energy E-r of non-polar supercritical ethane was estimated from the observed rate constant using the modified Marcus equation. E -r may be larger than normally expected for non-polar solvents because of density fluctuations. (C) 1998 Elsevier Science B.V.