Electronic, deformation polarization

Structural modifications brought about by an electric field can be of three kinds (a) orientation polarization, which can only take place for molecules having a permanent dipole moment, (b) atomic deformation polarization, (c) electronic deformation polarization. 

As A4> can amount to several volts, the electron deformation of the adsorbed molecules can be expected to influence their chemical behavior substantially. Therefore, when reactions are catalyzed via the intermediate formation of boundary layers on a catalyst, we may assume that the activation of the reacting molecules is frequently correlated to their polarization on the catalyst surface. There are two effects of polarization either it causes a strong but reversible adsorption, or the deformation of the electron shell of the adsorbed molecule is so thorough that the system—provided that it possesses sufficient activation energy— switches over irreversibly into a new quantized equilibrium position, forming a chemical bond (1) under liberation of energy. Intermediate states exist between these two extremes. 

In this discussion another type of interaction between the hydrogen atom and ion has been neglected to wit, the deformation (polarization) of the atom in the electric field of the ion. This has been considered by Dickinson,22 who has shown that it contributes an additional 10 kcal /mole to the energy of the bond. We may accordingly say that of the total energy of the one-electron bond in (61 kcal/mole) about 80 percent (50 kcal/mole) is due to the resonance of the electron between the two nuclei, and the remainder is due to deformation. 

Deformation polarization It can be divided into two independent types Electron polarization—the displacement of nuclei and electrons in the atom under the influence of an external electric field. Because electrons are very light, they have a rapid response to the field changes they may even follow the field at optical frequencies. 

Before we continue with the Mott insulators, we will introduce the concept of polar-ons. It is clear from earlier chapters that a localized electron deforms the structure around it. This deformation is a change of solvent structure and also bond length. If the electron jumps to another site, the same polarization occurs in the new site of course, while the old site goes back to the structure of all the unoccupied sites. 

The natural orbitals %2v and %3p are, in contrast to the hydrogenlike functions, localized within approximately the same region around the nucleus as the Is orbital. This means that the polarization caused by the long-range interaction is associated mainly with an angular deformation of the electronic cloud on each atom. If %2p and %3p are expanded in the standard hydrogen-like functions, an appreciable contribution will again come from the continuum. 

Our work described in this section clearly illustrates the importance of the nature of the cations (size, charges, electronegativities), electronegativity differences, electronic factors, and matrix effects in the structural preferences of polar intermetallics. Interplay of these crucial factors lead to important structural adaptations and deformations. We anticipate exploratory synthesis studies along the ZintI border will further result in the discovery of novel crystal structures and unique chemical bonding descriptions. 

In the Kohn-Sham Hamiltonian, the SVWN exchange-correlation functional was used. Equation 4.12 was applied to calculate the electron density of folate, dihydrofolate, and NADPH (reduced nicotinamide adenine dinucleotide phosphate) bound to the enzyme— dihydrofolate reductase. For each investigated molecule, the electron density was compared with that of the isolated molecule (i.e., with VcKt = 0). A very strong polarizing effect of the enzyme electric field was seen. The largest deformations of the bound molecule s electron density were localized. The calculations for folate and dihydrofolate helped to rationalize the role of some ionizable groups in the catalytic activity of this enzyme. The results are,... 

As mentioned in [Section 24.1], and as already demonstrated in Equation 24.39, the Fukui functions as well as the chemical hardness of an isolated system can be properly defined without invoking any change in its electron number. We define a new Fukui function called polarization Fukui function, which very much resembles the original formulation of the Fukui function but with a different physical interpretation. Because of space limitation, only a brief presentation is given here. More details will appear in a forthcoming work [33]. One assumes a potential variation <5wext(r), which induces a deformation of the density 8p(r). A normalized polarization Fukui function is defined by... 

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