Theory of Outer-Sphere Electron-Transfer Reactions

Citation

Siders, Paul David (1983) Theory of Outer-Sphere Electron-Transfer Reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/mgjj-rj90. https://resolver.caltech.edu/CaltechTHESIS:08012019-155724801

Abstract

Classical, semiclassical and quantum theories of outer-sphere electron-transfer reactions in polar media are discussed. For each, the Franck-Condon overlap factors for the hexaamminecobalt, hexaaquoiron and hexaammineruthenium self-exchange rates and for the cross-reaction of hexaaquoiron(II) with tris(2,2’-bipyridine)ruthenium(III) are evaluated and compared. The quantum effect on the rates is small in the region of moderate driving force; the "normal" ΔG^o region. Direct-sum and saddle-point evaluations of the quantum Franck-Condon factors are made and compared. The saddle-point approximation is shown to be an excellent approximation in the cases considered.

Quantum effects in homogeneous outer-sphere electron transfer reactions in the region of large negative ΔG^o (the "inverted" region) are considered. The results of quantum, semiclassical and classical calculations on model systems are presented. A sequence of highly exothermic photoinduced reactions of tris(2,2'-bipyridyl) complexes is discussed with regard to the possible importance of quantum effects and of alternate reaction pathways in understanding the failure of the sequence of reactions to exhibit pronounced "inverted" behavior. A mechanism leading to electronically excited products provides a possible explanation for the large discrepancy.

The theory of highly exothermic homogeneous outersphere electron-transfer reactions is discussed for transfers occurring over a range of distances. A finite rate of diffusion of reactants and their long-range force are treated by solving the reaction-diffusion equation numerically for the reactant pair distribution function. Steady-state solutions are compared with experimental data. On the basis of short-time solutions it is proposed that experiments which measure electron-transfer rates at short times following the onset of reaction improve the possibility of observing the inverted effect in bimolecular systems.

The effect of the reactants' relative orientation on the electron-transfer rate is considered. Reactants are modeled as oblate-spheroidal potential wells of constant, finite depth. Energy levels and wavefunctions are obtained for an electron localized in such a well. The electronic matrix elements that govern electron transfer within a nonadiabatic quantum theory are evaluated. Significant orientational preferences are predicted for electron transfer between nonspherical donor and acceptor sites.

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