Delocalization superexchange

CASE OUTER-ELECTRON CONFIGURATION CORRELATION SUPER EXCHAN6E DELOCALIZATION SUPEREXCHANGE SUM STRENGTH K... 

If the octahedral interstices of two neighboring cations share a common edge, then there is a direct overlap of the dxu (or dyz or dzx) orbitals of the two cations (see Fig. 43(a)). In this case the anion plays a less obvious role in the delocalization-superexchange process The transfer integral for t2g electrons varies as the overlap of the t2a orbitals of the two cations rather than as the product of their rcspec-... 

Delocalization superexchange between an eg orbital and a 90° cation can only occur via an intermediary anion orbital. Although considerably smaller, the magnitude of the transfer integral increases with the degree of covalence, as in the 180° case, and the signs of the interactions follow from equations 161-163. The 90° e0-e0 contribution is enhanced if strong 180° cation-anion-cation interactions are simultaneously present. 

If a pair of cations is separated by two anions, delocalization superexchange is still possible, though very much weakened, since the cation d orbitals spread out over the entire anion with which they form a partial covalent bond. Correlation superexchange is also weakened by the anion-anion correlation factor. Polarization super-exchange may be competitive in this case, but it is cooperative. It follows that the rules for the sign of 180° cation-anion-anion-cation interactions are the same as those for 180° cation-anion-cation interactions, but the magnitude is reduced, probably by an order of magnitude. 

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 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 ... 

Superexchange is another mechanism of electron transfer over relatively large distances in which the solvent or matrix acts as a bridge between the donor molecule D and the acceptor molecule A. It differs from electron hopping in that the electron is at no time actually localized on a molecule of the medium there is an interaction between the orbitals of the molecules A, B and D which form a sort of very loose supermolecule over which the electron is delocalized (Figure 4.10). This mechanism seems plausible when the relevant orbitals of A, B and D are rather close in energy. This is similar to the requirement for the interaction of atomic orbitals to form a molecule. 

There are three principal contributions to the superexchange a correlation effect, a delocalization effect, and a polarization effect. If orthogonal orbitals arc used, the last of these appears to be definitely the smallest (478). 

Figure 8. Proposed electron transfer pathway in blue copper proteins. The plastocyanin wave function contours have been superimposed on the blue copper (type 1) site in ascorbate oxidase (40). The contour shows the substantial electron delocalization onto the cysteine Spir orbital that activates electron transfer to the trinuclear copper cluster at 12.5 A from the blue copper site. This low-energy, intense Cys Sp - Cu charge-transfer transition provides an effective hole superexchange mechanism for rapid long-range electron transfer between these sites (2, 3, 28).

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