A Theoretical Model for Orientation Effects in Electron Transfer Reactions

Citation

Cave, Robert J. (1986) A Theoretical Model for Orientation Effects in Electron Transfer Reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s9zp-mj05. https://resolver.caltech.edu/CaltechTHESIS:08302019-110545286

Abstract

In this thesis, the development and application of a model for the examination of orientation effects in electron transfer reactions are considered. The model is designed to describe broad features of the electronic interactions between large molecules, where the transferred electron is delocalized in both reactant and product.

The model employs spherical or oblate-spheroidal potentials of constant depth for the donor and acceptor sites. The Schrodinger equation is solved for the exact eigenfunctions of such a potential, and the electron transfer matrix element, T_BA, is calculated us1ng these wavefunctions. T_BA is the principal distance and orientation dependent quantity in current theories of nonadiabatic electron transfer. By comparison of results obtained using spherical and spheroidal wells, il was determined that both orbital shape and well shape (i.e., molecular shape) effects are important in determining the magnitude and orientation dependence of T_BA.

The model was used to examine orientation dependence in electron transfer reactions between porphyrins and porphyrin derivatives. T_BA was examined for a variety of mutual orientations, including: 1) face-to-face transfers, where it was found that T_BA for forward transfer from photoexcited reactants was considerably larger than Lhat for back transfer to yield ground state products, 2) edge-to-edge orientations, and 3) models of possible initial donor-acceptor pairs in a bacterial photosynthetic electron transfer system (Rhodopseudomonas viridis). It was observed that T_BA was a sensitive function of orbital shape and orientation.

In order to simplify the model, a semiclassical approximation was examined for the donor and acceptor wavefunctions, for both the spherical- and spheroidal-well states. The accuracy of the approximation supports the interpretations of the results obtained from the exact calculations. It also substantially reduced the calculational time involved.

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