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. 2013 Jul 25;117(29):8793-801.
doi: 10.1021/jp405097c. Epub 2013 Jul 15.

Interfacial bond-breaking electron transfer in mixed water-ethylene glycol solutions: reorganization energy and interplay between different solvent modes

Oksana Ismailova  1 , Alexander S Berezin, Michael Probst, Renat R Nazmutdinov
Affiliations

Interfacial bond-breaking electron transfer in mixed water-ethylene glycol solutions: reorganization energy and interplay between different solvent modes

Oksana Ismailova et al. J Phys Chem B. .

Abstract

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 S2O8(2-) from mixed water-EG solutions when increasing the EG fraction; see Zagrebin, P.A. et al. J. Phys. Chem. B 2010, 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 (S2O8(2-)) 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 S2O8(2-) 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.

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Figures

Figure 1
Figure 1
Three solvent relaxation times τi* (a) and their contributions to the reorganization energy δi. (b) Calculated for EG-water mixtures using experimental dielectric spectra (see eq 5).
Figure 2
Figure 2
Coordination number (dashed line) plotted up to first minimum of the radial distribution function (solid line, left) calculated for S2O82– (a) and S2O83– (b) in water (x(EG) = 0).
Figure 3
Figure 3
Coordination number (dashed line) plotted up to first minimum of the radial distribution function (solid line, left) calculated for S2O82– (a) and S2O83– (b) in ethylene glycol (x(EG) = 1).
Figure 4
Figure 4
Rate constant of the S2O82– reduction from content in water–EG mixtures versus EG calculated at different electrode overpotentials (2.8 to 3.1 V).
Figure 5
Figure 5
Symmetry coefficient describing the reduction of a peroxodisulphate anion at a mercury electrode from mixed water–EG solutions calculated as a function of x(EG).
Figure 6
Figure 6
Kramer’s transmission coefficient (κKr) describing the S2O82– reduction versus the EG content in water–EG mixtures calculated at different electrode overpotentials (η).
Figure 7
Figure 7
Saddle-point avoidance effect (see eq 23) as a function of the EG content presented at η = 3.1 V.
See this image and copyright information in PMC

References

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