Mechanism superexchange

What is common to all the sequences is that they all start with the transfer of a band electron k to one of the ions, which produces a hole in the band but leads to different intermediate states in the perturbation Hamiltonian matrix depending on the sequence. The intermediate state after the first virtual transition should correspond to one magnetic ion with N and the second with N -F 1 electrons. The virtual state after the second transition may be of two different types, either N -F 1 electrons on both ions and two holes in the valence band (ABCD, ABDC, BACD, or BADC) or one ion with N + 1 and the other with N — 1 electrons and no holes in the valence band (ACBD or CADB). The unperturbed valence band states, which must be summed up over the entire Brillouin zone, are typically described within the empirical tight-binding model [64]. 

---

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. 

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) ... 

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.

A number of distance dependence studies have been carried out using synthetic DNA duplexes. In most of these investigations, the data were found to be consistent with the superexchange mechanism since the ET and HT rates followed exponential decays with increasing distance, with /J ranging from 0.1 A 1 to 1.4 A A1461... 

For 28(1)-28(4),[4S1 in which one to four A-T base pairs are inserted between G23 and GGG, the relative HT rate from G23 to GGG follows an exponential decay with increasing G23 -GGG separation, with an associated / value of 0.7 A 1. The operation of the superexchange mechanism for HT in this... 

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]. 

Such a stabilization is precisely provided by a superexchange mechanism implying the charge-transfer excited state (P B H). When this mechanism prevails, Tlj (Q) is obtained from Eq. (20) or the equivalent expression derived in Sect. 2.2.3 ... 

---