Enthalpy, Volume, and Entropy Changes Associated with the Electron Transfer Reaction between the ³MLCT State of Ru(Bpy)₃²⁺ and Methyl Viologen Cation in Aqueous Solutions

The electron-transfer reaction between the metal-to-ligand charge-transfer triplet ( 3 MLCT) state of Ru(bpy) 3 2+ and the methyl viologen cation MV 2+ was studied by laser-induced optoacoustic spectroscopy in the 8−35 °C temperature range in aqueous solutions in the absence and in the presence of various 0.1 M sodium salts. The enthalpy and the structural volume changes for the formation of the 3 MLCT state, Δ H MLCT = (196 ± 3) kJ/mol and Δ V MLCT = (−3.6 ± 0.2) cm 3 /mol, were independent of the presence of quencher or salt. The values of Δ H R and Δ V R for the production of the radical ion pair upon quenching of the 3 MLCT state by MV 2+ strongly depended on the added salt. In neat water the expansion Δ V R = (+10.1 ± 1.2) cm 3 /mol is due to a decrease in solute−solvent interaction after electron transfer. This value can be calculated with the classical 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 Δ H R and Δ V R along the series of added salts is explained in terms of enthalpy−entropy compensation effects due to the perturbation by the salts of the H-bond network in water. With the reported salt-independent reaction free energy a correlation between the reaction entropy and Δ V R was found, i.e., Δ S R = X / T Δ V R, with X = (14.4 ± 0.8) kJ/cm 3, similar to the ( c p ρ/β) T value at 303 K (near the isokinetic temperature, ca. 300 K) in aqueous solutions ( c p = heat capacity; ρ = mass density; β = volume expansion coefficient). The large values of the entropy term are due to the reorganization of the water network around the photoproduced radical ion pair, before recombination.