Charge density index

The charge density index refers to the nature of the molecule before allowance is made for perturbing effects due to the approaching reactant. Such a method is often called a first-order method, a terminology that is discussed more fully in Chapter 12. For alternant molecules, it is necessary to proceed to a high-order method, one that reflects the ease with which molecular charge is drawn toward some atom, or pushed away from it, as approach by a charged chemical reactant makes that atom more or less attractive for electrons. An index which measures this is called atom self-polarizability, symbolized 7tr,r- The formulas for this and related polarizabilities are derived in Chapter 12. For now, we simply note that the formula is... 

One approach refers to local charge densities [188] the other one uses localization energies as a reactivity index [189]. In any event, the inductive effect of the methylene group, formed in the first protonation step, has to be taken into account if two sites provide comparable reactivity indices. 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

Important characteristics of chitosan are its MW, viscosity, DD (Bodek, 1994 Ferreira et al., 1994a,b), crystallinity index, number of monomeric units, water retention value, pKa, and energy of hydration (Kas, 1997). Chitosan has a high charge density, adheres to negatively charged surfaces, and chelates metal ions. 

Having resolved the molecular perception problem and achieved a unique representation of all atoms, bonds, and rings in the molecule, the second major step is the definition of the most useful measure for local similarity of atoms and atomic environment. For the purpose of COSMO/rag, we need to achieve the state that atoms are considered as most similar, if their partial molecular surfaces and surface polarities, i.e., polarization charge densities, are most similar. But since the latter is not known, at least for the new molecule under consideration, we have to ensure that the local geometries and the electronic effects of the surrounding atoms are most similar. Obviously, two similar atoms should at legist be identical with respect to their element and their hybridization. Turning this information into a unique real number, a similarity index of the lowest order (zeroth order) can be defined for each atom from the atom element numbers and... 

Figure 6. The photorefractive effect. Top in an idealized hole transport material, the net charge density is ti radians out of phase with the intensity pattern. Middle the electric field, E, due to this net charge density, p, is given by Gauss law, dEjdx = p/e, and is shifted in phase by njl radians relative to the charge density distribution. Bottom the refractive index will then follow the phase of the electric field. In real materials the charge distribution is not always n radians out of phase relative to the intensity pattern, as competition between drift and diffusion currents leads to a reduced phase shift. The refractive index contrast might therefore be shifted by only n/lO radians relative to the intensity pattern in some polymers.

Very similar to the electronic-topological descriptors, electron charge density connectivity index is defined for an - H-depleted molecular graph in which net atomic charges, calculated by computational chemistry, are used as weights for vertices [Estrada and Montero, 1993 Estrada, 1995d] ... 

Therefore a corrected electron charge density connectivity index is defined as ... 

However, when molecular descriptors are derived from molecular graphs, cis/trans isomerism is not usually recognized and some molecular descriptors were proposed in order to discriminate between cis/trans isomers, such as the - corrected electron charge density connectivity index, and - periphery codes. - Weighted matrices were also devised for obtaining the -> geometric modification number that is added to any topological index in order to discriminate cis/trans isomers. 

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