ELECTRON-TUNNELING IN MOLECULAR SOLIDS - ORBITAL OVERLAP MODEL

The long-range transfer processes (range, R ∼ 20-50 Å) of excess electrons produced by radiolysis of molecular solids are analyzed in terms of an orbital overlap model. A united atom approximation makes possible the separation of angular and radial factors. The angular dependence is used to treat the effect of the relative orientation of donor and acceptor molecules on the transfer rate; variations of up to a factor of ∼ 1000 are predicted. A distinction is drawn between reactions of electrons with scavengers (or transfer between additives), where the significant overlap is localized around donor and acceptor sites, and recombination with a cation produced by the radiolysis, where the overlap is largely spread out over the intervening volume. In the former case, interference due to nodes in the wave functions is significant, while in the latter, various powers of R appear in the preexponential factor. The role of the Franck-Condon principle in determining the effective barrier height is discussed. Though Franck-Condon factors play a major role, electronic interaction is also important in determining transfer rates for different donors and acceptors. © 1979 American Chemical Society.