Electronic superexchange

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. 6.12 Virtual electron superexchange between two overlapping half-filled orbitals leading to antiferromagnetic coupling...

Figure C3.2.9. Both nearest neighbour and nonnearest neighbour coupling interactions mediate superexchange between tire temrinal pi-electron groups of rigid dienes witlr saturated bridging units. From [31],...

What determines the way in which the spins couple Parallel orientation always occurs when the corresponding atoms act directly on one another. This is the case in pure metals like iron or nickel, but also in EuO (NaCl type). Antiparallel orientation usually occurs when two paramagnetic particles interact indirectly by means of the electrons of an intermediate particle which itself is not paramagnetic this is called superexchange mechanism. That is the case in the commercially important spinels and garnets. 

In NiFe204, an inverse spinel Fe +[Ni2+Fe3+]004, the spins of the octahedral sites are parallel with one another the same applies to the tetrahedral sites (Fig. 19.8). The interaction between the two kinds of sites is mediated by superexchange via the oxygen atoms. High-spin states being involved, Fe3+ (d5) has five unpaired electrons, and Ni2+ (ds) has two unpaired electrons. The coupled parallel spins at the octahedral sites add up to a spin of S = + =. It is opposed to the spin of S = of the Fe3+ particles at the tetrahedral sites. A total spin of S = 1 remains which is equivalent to two unpaired electrons per formula unit. 

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) ... 

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. 

Figure 15. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the bridge states lie above the D level consequently the electron is transferred in one coherent jump and is never localized within the bridge. The distance dependence behavior is exponential decay.

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