Superexchange mechanism saturated hydrocarbon bridges

From these extensive studies, we may conclude that the distance dependence of ET rates, mediated by a superexchange mechanism involving saturated hydrocarbon bridges, is largely independent of the nature of the hydrocarbon and has an average /3 value of 0.99 0.2 bond-1. 

In contrast, in the superexchange (or TB) mechanism, A is very large—at least 2 eV—and so the electron cannot be thermally injected into the bridge. Instead, the electron moves coherently, in one sudden Jump , from donor to acceptor and it never becomes localized within the bridge. The distance dependence of the ET rate for this mechanism is exponential decay (Figure 40). The conduction band for saturated hydrocarbon bridges corresponds roughly to the empty a orbital levels. 

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