Superexchange theory

It is well known that superexchange theory provides useful relationships between one- and two-electron coupling elements (e.g., [28, 78, 138]). We consider the simple case of a monocation electron transfer system,... 

A density matrix approach makes it possible to examine more general models for systems like this [38,39]. For instance, unlike superexchange theory, this approach admits models that include strong couplings between the closely associated species in the intercalated DNA/donor-acceptor complex. More important, the density matrix description allows a proper treatment of quantum coherences, which is critical in bridging the tunneling and free-conduction regimes of transport. On the one hand,... 

Curtly J, Barbara B (2006) General Theory of Superexchange in Molecules. 122 207-250... 

As with conductivity measurements, methods and results of theoretical treatments of CT in DNA have varied significantly. Mechanisms invoking hopping, tunneling, superexchange, or even band delocalization have been proposed to explain CT processes in DNA (please refer to other reviews in this text). Significantly, many calculations predicted that the distance dependence of CT in DNA should be comparable to that observed in the a-systems of proteins [26]. This prediction has not been realized experimentally. The dichotomy between theory and experiment may be related to the fact that many early studies gave insufficient consideration to the unique properties of the DNA molecule. Consequently, CT models derived for typical conductors, or even those based on other biomolecules such as proteins, were not adequate for DNA. 

In the original Forster-Dexter theory [79-81] the superexchange terms in 7), have been ignored, that is, 7, - = //) . We shall present the derivation of their theory in our framework. Consider the energy transfer... 

In fact, since the early developments of electron transfer theories, it has been recognized that the magnitude of T b is greatly enhanced if /a and (j/b are delocalized through the intercenter medium [54, 55]. In the framework of one-electron models, this delocalization can be described by a mixing of (pu and (Pa with the medium orbitals which leads to the so-called superexchange contribution. The origin of this contribution may be introduced as follows. If only one medium orbital cPm interacts with both (po and (Pa, the initial and final states may be written ... 

The general framework of the quantum mechanical rate expression for long-range electron transfer processes in the very weak or non-adiabatic regime will be presented in Sect. 2 with an emphasis on the inclusion of superexchange interactions. The relation between the simplest case of direct donor-acceptor interactions, on the one hand, and long-range electronic interactions important in proteins, on the other, is considered in terms of the elements of electron transfer theory. 

The kinetic contribution to the exchange parameter J contains antiferromagnetic and ferromagnetic parts, / = /a + /f, which in the Anderson s theory of superexchange [29] have the form ... 

---