Solvents polarity/polarizability

The reverse situation, AH° < AH° and K solvent effect has been attributed [103] mainly to the existence of a strong dipole-(induced) dipole interaction between the very polar complex AB (for example = 4.95 D for Me3N-S02 and 6.02 D for McsN-BFs) [103, 104] and the polar (polarizable) solvent solute dipole/solvent forces are not so stabilizing for the weakly polar monomers A and B (for example /x = 0.86 D for McsN, 1.6 D for SO2 and 0 D for BF3). 

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...

Linear absorption and fluorescence spectra for the series of symmetrical cationic polymethines with 5-butyl-7,8-dihydrobenzo[ /]furo 2,3 /lindolium terminal groups are shown in Fig. 14 for solvents of different polarity. It is known that the polarity of solvents can be characterized by their orientational polarizability, which is given by Af = (e- l)/(2e + 1) — (n2 - l )/(2n2 +1), where e is the static dielectric constant and n is the refractive index of the solvent [41], Calculated A/values... 

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... 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

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. 

A most comprehensive discussion of the effect of solvent on spectra has been given by Bayliss and McRae.21 They point out that polarization or dispersion forces are the most general interactions involved in solution and that all solution spectra are subject to a generalized polarization red shift, relative to vapor spectra, due to solvent polarization by the transition dipole. However, these dispersion forces are relatively weak and are easily obscured by the effect of dipole-dipole and dipole-static charge forces in polar, but not highly polarizable, solvents. By applying the Franck-Condon principle, they showed... 

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. 

At the next level of complexity, the polarity of solvent models, as made manifest by their atomic partial charges, can be augmented with a polarizability. This allows the solvent molecule to respond to its surroundings in a fashion conceptually similar to the electronic component of die solvent polarization described in Section 11.1.1. Typically a polarizability tensor a is assigned either to the solvent molecule as a whole or to individual atoms. Then, die induced dipole moment at each polarizable position can be determined from... 

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. 

This is of course the major solvation energy of polar molecules in non-polar solvents, but it can be an important term in polar solvents as well, especially in highly polarizable solvents such as aromatic derivatives. 

Solvent polarity also affects the rate of peroxide decomposition. Most peroxides decompose faster in more polar or polarizable solvents. This is true even if the peroxide is not generally susceptible to higher order decomposition reactions. This phenomenon is illustrated by various half-life data for tert-butyl peroxypivalate [927-07-1]. The 10-h half-life temperature for tert-butyl peroxypivalate varies from 62°C in decane (nonpolar) to 55°C in benzene (polarizable) and 53°C in methanol (polar). 

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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