Delocalization process

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

The formation and transport properties of a large polaron in DNA are discussed in detail by Conwell in a separate chapter of this volume. Further information about the competition of quantum charge delocalization and their localization due to solvation forces can be found in Sect. 10.1. In Sect. 10.1 we also compare a theoretical description of localization/delocalization processes with an approach used to study large polaron formation. Here we focus on the theoretical framework appropriate for analysis of the influence of solvent polarization on charge transport. A convenient method to treat this effect is based on the combination of a tight-binding model for electronic motion and linear response theory for polarization of the water surroundings. To be more specific, let us consider a sequence... 

This delocalization process implies the four oxygens of an aluminum tetrahedron. Assume that a proton acceptor molecule, indicated in boldface, approaches an Oi oxygen at the same time as the proton. As depicted by either Reaction 11 or 12, there is some probability for this molecule to capture the proton. 

In heparin, a polysaccharide that contains sulfate groups in addition to the carboxylate ion, association with Ca is a delocalized process, as opposed to one involving specific binding-sites. ... 

We shall next show that a relatively simple mod O fedcalizatiotiH delocalization process can provide a quantitativ icoraftte thoexperi mental data of Table 10-3 [as well as numerous dtbeftiSfe illies e.gl. Refs. (27,23-25)]. This model is based on the Tofea i Yeasonable assumptions (cf. preceding discussion) "... 

Both discussed cases confirm that the Source Function picture nicely complies with that provided by more sophisticated instruments based on the pair density, like the delocalization indices. But the Source Function has, possibly, an advantage as it may also reveal chemically relevant asymmetries in the electron delocalization processes. It has been shown how these asymmetries disclose the non equivalence of the delocalization effects between two atoms direedy or indirectly connected along a sequence of bonds or the different magnitudes of such effects when moving perpendicularly to a bond and in opposite directions relative to the bond critical point. 

In Chapter 1, Equation (1.4.42), we have stressed that the gap width has two contributions, an ionic one which is a function of the energy separation between the anion and cation effective atomic levels, and a covalent contribution related to electron delocalization processes ... 

---