Coupled-electron pair formalism

There are other techniques for introducing electron-correlation effects such as the coupled-cluster method (Bartlett 1989, Paldus and Cizek 1989). The coupled-electron pair formalism (CPF) (Arhlichs 1985), modified coupled-pair formalism (MCPF) and more recently averaged coupled pair formalism (ACPF) have been used (see, e.g., the papers of Bauschlicher and Langhoff 1989, 1991). The coupled-cluster methods have been mostly used for lighter molecules and have not yet been used for molecules containing lanthanides or actinides. However, the MCPF method has been used for La-containing species by Bauschlicher and co-workers. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

GVB Generalized valence bond. A theory that employs CF orbitals to calculate electronic structure with wave functions in which the electrons are formally coupled in a covalent manner. The simplest level of the theory is GVB PP (PP-perfect pairing), in which all the electrons are paired into bonds, as in the Lewis structure of the molecule. 

Hammond, Wamser, Chang, and Baylor [54] studied the photochemistry of the BP/MK pair formally using flash photolysis techniques and showed that electron transfer occurs via triplet exciplexes that can be formed by excitation of either partner, though under normal circumstances MK absorbs the vast majority of the light. Benzophenone ketyl radical was observed as a transient, and a second species present was identified as either the cation radical of MK or its corresponding deprotonated radical. The coupling product of the latter two radicals (16) was observed as a product of steady-state irradiation, and it was formed with a quantum... 

It must be stressed, however, that this is a purely formal device and that (disregarding the small relativistic effects, which are taken up in Chapter 11) there is no physical coupling between the electron spins. There are obvious attractions in a theory that so neatly introduces a formal model , complete with empirical parameters, and apparently reduces the intricacies of chemical binding to the coupling of pairs of electron spins but there are many pitfalls in the interpretation of this result. 

The predominant interaction for a 2H spin system is the quadrupolar interaction, which couples the electric quadrupole moment of the 2H nucleus to its electronic surrounding. This interaction is a second-rank tensor Hq which lies approximately along the C-2H bond in organic molecules. Thus, in practice, 2H nuclei may be considered to be isolated. It shows that the 2H NMR formalism is similar to that of an isolated proton pair [8] ... 

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