Solvent dynamical effects in electron transfer

George Edward McManis, Purdue University

Abstract

Measurements of exchange kinetics for some simple electrochemical and homogeneous-phase electron-transfer reactions have been utilized together with optical electron-transfer energies and detailed theoretical analyses to elucidate the role of solvent polarization dynamics in electron-transfer processes. Solvent-dependent self-exchange kinetics were evaluated for a series of cobaltocenium-cobaltocene and ferrocenium-ferrocene redox couples. In a given solvent, large (up to 100 fold) rate variations were observed which are shown to be due chiefly to differences in the degree of donor-acceptor orbital overlap, i.e. the extent of reaction adiabaticity. Analyses were undertaken for the rate-solvent dependence for each couple in a range of Debye-like media. For the most facile cobaltocene couples, the rate constants (corrected for solvent-dependent barrier heights), k$\sbsp {\rm ex}{\prime}$, varied approximately with $\tau\sbsp {\rm L}{-1}$, where $\tau\sb {\rm L}$ is the solvent longitudinal relaxation time, as anticipated for adiabatic processes. In contrast, for the least facile ferrocene systems, k$\sbsp {\rm ex}{\prime}$ is virtually independent of $\tau\sbsp {\rm L}{-1}$, indicating nonadiabatic behavior. Comparisons of these data with corresponding theoretical predictions enabled estimates of the electronic matrix coupling element, H$\sb{12}$, for each redox couple to be extracted. The larger values of H$\sb{12}$, ca. 0.5-1 kcal mol$\sp{-1}$, for the cobaltocene couples relative to those, 0.075-0.25 kcal mol$\sp{-1}$, for the ferrocene systems are consistent with optical electron-transfer data for related mixed-valence compounds. The influence of high-frequency solvent relaxations upon the adiabatic reaction kinetics in non-Debye media has also been explored and compared with contemporary theoretical predictions. By utilizing solvent relaxation data extracted from dielectric loss spectra in a theoretical formalism due to Hynes, the relative importance of high-frequency components to be barrier-crossing dynamics has been identified, especially in alcohol solvents. Theoretical modeling indicates that the polarization dynamics controls the degree of nuclear tunneling and that solvent molecularity exerts relatively little influence on the electron transfer rate--at least in the mean spherical approximation. A theoretical examination of the transition state rate expression is examined. It is found that quantum effects, specifically nuclear tunneling, can serve to mask the onset of the transition-state limit.

Degree

Ph.D.

Advisors

Weaver, Purdue University.

Subject Area

Chemistry

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