Polarizability Polarization

H-bonding is an important, but not the sole, interatomic interaction. Thus, total energy is usually calculated as the sum of steric, electrostatic, H-bonding and other components of interatomic interactions. A similar situation holds with QSAR studies of any property (activity) where H-bond parameters are used in combination with other descriptors. For example, five molecular descriptors are applied in the solvation equation of Kamlet-Taft-Abraham excess of molecular refraction (Rj), which models dispersion force interactions arising from the polarizability of n- and n-electrons the solute polarity/polarizability (ir ) due to solute-solvent interactions between bond dipoles and induced dipoles overall or summation H-bond acidity (2a ) overall or summation H-bond basicity (2(3 ) and McGowan volume (VJ [53] ... 

Figure 6.5 Paraaeters Influencing the solvent polarity /polarizability (t ) of carbon dioxide and sose other supercritical...

Keywords Empirical force field, Electronic polarization, Polarizability, Force field, Inducible dipoles,... 

Pratt LR (1980) Effective field of a dipole in non-polar polarizable fluids. Mol Phys 40(2) 347-360... 

X0 is the value of the property in the gas phase. (In practice, X and X0 are often the logarithm of the property in question.) The parameters a and p are measures of a solvent s ability to donate and accept hydrogen bonds, respectively, and tt is an index of its polarity/polarizability. They were initially assigned on the basis of ultraviolet spectral shifts of certain dyes in a variety of solvents, and hence were labeled solvatochromic parameters.186"188... 

In Eq. (45), k is a constant. R2 is related to the solute s molar refraction198 and can be viewed as a correction to the polarity/polarizability term, analogous to one that has sometimes... 

Osterberg and Norinder [42] further analyzed a subset of the ATPase data published by Litman et al. [39]. The ATPase activity values were correlated by multivariate statistics with calculated descriptors (MolSurf descriptors) related to physicochemical properties suchaslipophilicity, polarity, polarizability and hydrogen bonding. After exclusion of one outlier and large molecules such as valinomycin, gramicidin S and so on, which were not handled by the MolSurf software, only 21 compounds were included in the study. Two models were derived model 1, based on... 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

Platts et al. [39] reported linear free energy relation (LEER) models of the equilibrium distribution of molecules between blood and brain, relating log BB values to fundamental molecular properties, such as hydrogen-bonding capability, polarity/polarizability, and size. They used the following modified form of Abraham s general Eq. 46 ... 

Platts [39] Hydrogen-bonding, polarity/polarizability, and size... 

Increase in polarity, polarizability, and hydrogen-bonding ability leads to a decrease in BBB permeation rate. Compound size as measured by the McGowan volume leads to an increase in the permeation rate. 

POLARITY POLARIZABILITY POLARIZATION Polarization of chemical bond,... 

In the present context, the solvation of a solvent molecule in its own liquid (i.e., condensation from the vapor, the opposite of evaporation) is of interest (Ben-Naim and Marcus, 1984). The solvation properties of solvents (solvent effects) depend mainly on their polarity/polarizability (accounting also for dispersion interactions), hydrogen-bond donation and acceptance abilities, and cohesive energy density (Marcus, 1993). 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

Despite the availability of numerous computer algorithms for logP0/w calculations, it is useful to consider qualitatively that the most significant molecular parameters that affect logPe/w have been noted by Tamlet et al. to be size, polarity/polarizability, and hydrogen bond acceptor strength [46]. 

Table 6.1 Measured Air-Solvent Partition Constants (KM) and Calculated Activity Coefficients (y,., Eq. 6-7) at 25°C of Five Model Compounds Exhibiting Different H-Donor, H-Acceptor, and Polarity/Polarizability Properties for Some Organic Solvents...

Step (v), conversion of [Fe(NO)2(SMe)2] to the product [Fe2(SMe)2(NO)4], occurs spontaneously under the appropriate conditions of solvent polarity/polarizability (23) [cf. Eq. (20) and Schemes 3 and 4], Step(vi) requires the methylation of [Fe2S2(NO)4]2 " while this has been demonstrated for methylation by methyl halides (3, 24, 29), methylation by biological methyl transfer, e.g., from methionine or S-adenosylmethionine, has yet to be investigated. Thus, although neither of the biosynthetic routes suggested in Scheme 5 has yet been fully investigated, several of the steps [(i), (ii), and (vi)], are established, while others [(iv) and (vi)] are already known to occur under nonbiological conditions. 

Sensitivity to Interesting Aspects of Environment. Of course the spectrum of the probe must be sensitive to the environmental property under investigation. Such properties could include polarity, polarizability, compressibility, stress, and ability to form a specific chemical complex with the probe. In order to simplify interpretation, an ideal probe should be much more sensitive to the property of interest than to other properties. 

Polarization, polarizability, perturbation theory, and stark effects... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

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