Electron transfer superexchange

Keywords. Singlet excitation, triplet excitation, energy transfer, electron transfer, superexchange, bridged systems, through-bond coupling. 

Fig. 2b. Both kcs and kcr are seen to decrease as the distance R between Sa and G C increases. In accord with a superexchange mechanism for photoin-duced electron transfer, the distance dependence can be described by Eq. (2) ... 

Stuchebrukhov AA (1996) Tunneling currents in electron transfer reaction in proteins. II. Calculation of electronic superexchange matrix element and tunneling currents using nonorthogonal basis sets. J Chem Phys 105(24) 10819-10829... 

Fig. 2 (a) Schematic representation of the energy levels diagrams for a DBA system and a MBM junction in which the electron transfer process is dominated (b) by superexchange or non-resonant tunnelling, (c) by resonant tunnelling or (d) by hopping ... 

Keywords Electron Tansfer m Energy Transfer m Through-Bond Coupling m Superexchange m Molecular Wires m Solvent-Mediated Electron Transfer m Electron Transfer in DNA m Charge Separation m Electron Transfer Through H-Bonds... 

It is useful to introduce a nomenclature for distinguishing between ET occurring by the conduction and superexchange mechanisms. The term electron transport is used in the context of molecular wire behavior, while electron transfer is used in the context of the superexchange mechanism. 

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 ... 

A Chapter of this volume is devoted to these techniques, which are merely illustrated in this section by one particular example. The electron transfer system that is the most intensively submitted to genetic manipulations is certainly the physiological complex between yeast cytochrome c and peroxide-oxidized cytochrome c peroxidase, which presents many advantages [143], Among the modifications performed on cytochrome c peroxidase, one may mention the substitution of Trp 191 which interacts directly with His 175 of the heme [144], and of His 181 [145] which was proposed as a bridging unit in a superexchange path involving Phe 87 of cytochrome c [136,146]. On the cytochrome c side, Phe 87 las been substituted [147], as well as other residues expected to play an important role in the stabilization of the noncovalent complex [143]. 

In this last interpretation, B could promote the electron transfer through a superexchange mechanism involving the (P B H) charge-transfer state. A variant of this model involves an internal charge-transfer state of the Bchl dimer [172]. 

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. 

Figure 29. Comparison of quantum path-integral results (thick tines) and ZPE-corrected mapping results (thin lines) for the diabatic electronic populations of a three-state electron transfer model describing (a) sequential and (b) superexchange electron transfer.

The fast electron transfer mentioned for the p-phenylene-linked system results in the transfer of the three-electron two-nitrogen unit from one-hydrazine ring to the other. Such superexchange transfer is a specific phenomenon. As found by Nelsen and Ismagilov (1999), this bis(hydrazine) cation-radical is significantly ion paired with PFg counterion in the methylene chloride solution. The counterion touches both hydrazine fragments, but the distance between the N.-.N linkage and PFg is permanently shorter than that between N—N and PFg . 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

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