Enthalpy, volume, and entropy changes associated with the electron transfer reaction between the (MLCT)-M-3 state of Ru(Bpy)(3)(2+) and methyl viologen cation in aqueous solutions

The electron-transfer reaction between the metal-to-ligand charge-transfer triplet ((MLCT)-M-3) state of Ru(bpy)(3)(2+) and the methyl viologen cation MV2+ was studied by laser-induced optoacoustic spectroscopy in the 8-35 de grees C temperature range in aqueous solutions in the absence and in the pr esence of various 0.1 M sodium salts. The enthalpy and the structural volum e changes for the formation of the (MLCT)-M-3 state, Delta H-MLCT = (196 +/ - 3) kJ/mol and Delta V-MLCT = (-3.6 +/- 0.2) cm(3)/mol, were independent o f the presence of quencher or salt. The values of Delta H-R and Delta V-R f or the production of the radical ion pair upon quenching of the 3MLCT state by MV2+ strongly depended on the added salt. In neat water the expansion D elta V-R (+10.1 +/- 1.2) cm(3)/mol is due to a decrease in solute-solvent i nteraction after electron transfer. This value can be calculated with the c lassical Drude-Nernst equation for electrostriction only if a semiempirical constant is employed instead of the theoretical one. The linear dependence between the relatively large changes in Delta H-R and Delta V-R along the series of added salts is explained in terms of enthalpy-entropy compensatio n effects due to the perturbation by the salts of the H-bond network in wat er. With the reported salt-independent reaction free energy a correlation b etween the reaction entropy and Delta V-R was found, i.e., Delta S-R = X/T Delta V-R, with X = (14.4 +/- 0.8) kJ/cm(3), similar to the (c(p)rho/beta)( T) value at 303 K (near the isokinetic temperature, ca. 300 K) in aqueous s olutions (c(p) = heat capacity; rho mass density; beta = volume expansion c oefficient). The large values of the entropy term are due to the reorganiza tion of the water network around the photoproduced radical ion pair, before recombination.