Molecule dipolar polarizability

Zhao and coworkers [53] also constructed a linear model using the Abraham descriptors. The MLR model possesses good correlation and predictability for external data sets. In this equation, E is an excess molar refraction (cm3/mol/ 10.0) and S the dipolarity/polarizability, A and B are the hydrogen bond acidity and basicity, respectively, and V is the McGowan characteristic volume (cm3/ mol/100). The large coefficients of A and B indicate too polar molecules having poor absorption. 

Kamlet-Taft parameters are known to express three distinct measures of the solvent polarity such as dipolarity/polarizability hydrogen-bond acidity (a) and hydrogen-bond basicity (/S). These parameters have been determined by absorption measurements for individual or pairs of the following dye molecules N,N-diethyl-4-nitroaniline, 4-netroaniline and Reichardt s dye, as seen in Figure 3.4 [81-83]. 

In Eq. [19], Vxi is the McGowan volume that models the energy needed to make a solute molecule-sized cavity in the solvent. Again, the subscript 2 denotes a solute molecule. The parameters ti and 82 account for dipolarity/ polarizability, and ai and pi model hydrogen bond (HB) acidity and basicity, respectively. This parameter set was used to correlate more than 250 biological, chemical, and physical properties successfully. ... 

In Eq. [20], Ri is the excess molar refraction (MR), which is the MR of the solute less the MR of the alkane with the same characteristic volume, V 2, as the solute. The 7t symbol is the dipolarity/polarizability, and X) P2 3re the so-called overall LIB acidity and basicity descriptors, respectively. The summation sign is used to emphasize that these are overall HB properties designed to be appropriate to situations where the solute molecule is surrounded by an excess of solvent molecules. These descriptors are in contrast to the HB descriptors 0.2 and p2 employed in Eq. [19], which are derived from 1 1 complexation constants. Equation [20] has also been used with the Vx2 term replaced by a log (L ) term, where is the equilibrium constant... 

The statistical results indicate that the five descriptors give a very good fit of the experimental data. The positive signs for Ri and 7I2 that suggest that aqueous solubility increases with an increase in the dipolarity/polarizability for each molecule. Similarly, aqueous solubility increases with an increase in HB parameters, while it decreases with an increase in size of the cavity that must be created in water. 

In the work of Famini and Wilson,a molecular volume, Vmc, (units of 100 A ) is used to model the cavity term that measures the energy required to create a solute-molecule sized cavity in the solvent. The dipolarity/polarizability term, which attempts to account for dispersion-type interactions, is modeled by the polarizability index, tij, (unitless). This index is defined as the average molecular polarizability divided by the molecular volume, a/Vmc, and helps account for the correlation between polarizability and molecular volume. 

The physical meaning of Eq. [42] is tied to the interpretation of some of the terms, some being convoluted. The A term can be associated with dispersion interactions an increase in surface area suggests an increase in dispersion interactions (attractions) and, thus, increased solubility in octanol that in turn results in enhanced Pq/w values. A similar interpretation holds if one associates p /V with dipolarity/polarizability effects. The positive sign on the HBA term ( Umin) fot the solute suggests that the HBD acidity of water is less important than the HBA basicity of water for those molecules partitioning between phases. This implies that increased solute HBD acidity would increase the solute-water interaction. 

The absorption of IR light by a vibrating molecule follows dipolar selection rules. This means that, to interact with the electric field of an incident photon so as to excite a normal mode of a molecule, absorb the photon, and be observed in the IR spectrum, a given normal mode must distort the molecule such that it alters the molecule s dipole moment. On the other hand Raman scattering, which is a two-photon process (the two being the incident and scattered photons), follows quadrupolar selection rules. This means that, to interact with an incident photon so as to result in the excitation of the normal mode of a molecule and the scattering of a Raman-shifted photon, the mode must distort the molecule such that it alters the molecule s polarizability. 

Non-linear Dipole Polarizabilities. In weak electric fields, the linear dipolar polarizability a appearing in equations (67) and (68) is a quantity specific to the atom or molecule (the volume polarizability Og corresponds to the spatial dimensions of the electron shell and is of the order and dimoision of 10 m ) in fact, it describes the linear distortion of the shell under the influence of the field strength E (in the case of atoms, the electrons are, classically, performing harmonic oscillations). In the general case, the polarizability can depend on the field strength, if the latter is sufficiently large. One thus has to re-write equation (67) as follows ... 

S. Kielich. Coherent light scattering by interacting anisotropic molecules with variable dipolar polarizability. J. Phys. (Paris), 43 1749-1757 (1982). 

Electric polarization, dipole moments and other related physical quantities, such as multipole moments and polarizabilities, constitute another group of both local and molecular descriptors, which can be defined either in terms of classical physics or quantum mechanics. They encode information about the charge distribution in molecules [Bbttcher et al, 1973]. They are particularly important in modelling solvation properties of compounds which depend on solute/solvent interactions and in fact are frequently used to represent the -> dipolarity/polarizability term in - linear solvation energy relationships. Moreover, they can be used to model the polar interactions which contribute to the determination of the -> lipophilicity of compounds. 

In order to extend the above treatment to the metal solution interface, one must consider the effect of the solvent molecules adsorbed on the metal on the electronic overspill. Because the solvent molecules are polarizable, an induced dipole moment is established in the solvent monolayer, which acts to reduce the extent of overspill. As a result, the dipolar potential due to the metal is reduced by a factor corresponding to the optical permittivity of the monolayer, Sop. Recalling that this dipole potential is designated as one has at the PZC... 

Many attempts have been made to develop empirical measures of solvent polarity that reflect the interaction of polar molecules with solutes and that correlate well with chemical reactivity, and Katritzky and co-workers discussed 184 such parameters. For example, Kamlet, Taft, and co-workers proposed a general dipolarity/polarizability index, n, to measure the ability of a solvent to stabilize an ionic or polar species by means of its dielectric... 

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