We
explore solvent dynamics effects in interfacial bond breaking
electron transfer in terms of a multimode approach and make an attempt
to interpret challenging recent experimental results (the nonmonotonous
behavior of the rate constant of electroreduction of S2O82– from mixed water–EG solutions
when increasing the EG fraction; see Zagrebin, P.A. et al. J. Phys. Chem. B2010, 114, 311). The exact expansion of the solvent correlation function (calculated
using experimental dielectric spectra) in a series predicts the splitting
of solvent coordinate in three independent modes characterized by
different relaxation times. This makes it possible to construct a
5D free-energy surface along three solvent coordinates and one intramolecular
degree of freedom describing first electron transfer at the reduction
of a peroxodisulphate anion. Classical molecular dynamics simulations
were performed to study the solvation of a peroxodisulphate anion
(S2O82–) in oxidized and reduced
states in pure water and ethylene glycol (EG) as well as mixed H2O–EG solutions. The solvent reorganization energy of
the first electron-transfer step at the reduction of S2O82– was calculated for several compositions
of the mixed solution. This quantity was found to be significantly
asymmetric. (The reorganization energies of reduction and oxidation
differ from each other.) The averaged reorganization energy slightly
increases with increasing the EG content in solution. This finding
clearly indicates that for the reaction under study the static solvent
effect no longer competes with solvent dynamics. Brownian dynamics
simulations were performed to calculate the electron-transfer rate
constants as a function of the solvent composition. The results of
the simulations explain the experimental data, at least qualitatively.