Solvent dipolarity/polarizability index

Taft, Table 4.23 [507,508,514,515]. The T value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide - 1, which measures the ability of a solvent to... 

The solvatochromic parameters are derived from spectroscopic and other measurements specifically designed to measure only a single interaction. In addition, the values are averages of the results from several solutes for each parameter and somewhat independent of solute identity. The most comprehensive solvatochromic treatment of solvent selectivity are the Tti, ai and Pi parameters of Kamlet and Taft, Table 4.15 [568-570, 578]. The rti value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide = 1. The i scale of hydrogen-bond acidity measures the... 

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... 

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. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

The three parameters that are central to the method are tt, a, and The tv parameter provides a comprehensive measure of the ability of a solvent to stabilize a solute molecule based on the dielectric effects. It is a quantitative index of solvent dipolarity and polarizability. The acidity a for a solvent is a measure of its strength as a hydrogen-bond donor HBD, its ability to donate a proton in a soIvent-to-solute hydrogen bond. The estimation of a is based on the experimental determination of n and t(30). The t(30) scale, developed by Reichardt et al., indicates a solvent strength by combining polarity and HBD acidity, which itself serves as a useful solvent parameter for physicochemical correlations in a wide range of solvents. ° The basicity parameter denotes the solvent s hydrogen-bond acidity HBA or an index of the solvent s ability to accept a proton in a... 

In the original treatment of Debye and Hiickel these constants were determined under the assumption that the ion had a point charge at r = 0 and that the interior of the ion had the same dielectric constant D as the solvent. In the On-sager (5) theory of dipolar liquids it is assumed that the molecule is represented by a spherical cavity in the liquid with a singularity at its center. The characteristics of the molecule are its electric moment in vacuum po and its polarizability a. This is to be related to an internal refractive index n by... 

An analysis of the t(30) values, using multivariate statistical methods, has been carried out by Chastrette et al. [193]. According to this analysis, the x(30) values of non-HBD solvents are measures of the dipolarity and polarizability as well as the cohesion of the solvents. Another analysis of x(30) values in terms of functions of the dielectric constant sf and refractive index ( d) of forty non-HBD solvents has been given by Bekarek et al he emphasizes the predominant influence of the f(fir) term on the iix(30) parameter of those solvents [194]. For further correlations of the x(30) values with other empirical parameters of solvent polarity, see Section 7.6. 

Concerning solvent reorganization, it is important to remember that polarizability corrections, included via the refractive index, is a part of the electronic energy. The dependence on the refractive index can be expressed as in the Weller model or in other ways. Kuriyama, Ogilby, and Mikkelsen have shown that the dependence alternatively can be written in a dipolar form. 

The polarity of the molecules is usually considered to be measured on a gross scale by the relative permittivity and on a molecular scale by the electrical dipole and higher moments. Molecules lacking a dipole moment (carbon dioxide, for example) may still exert short-range effects due to quadrupole, and so on, moments. Dipolar bonds that are well separated in a molecule may act almost independently on neighboring molecules Hildebrand and Carter (1930) showed that the three isomeric dinitrobenzenes, in their binary solutions in benzene, exhibit nearly identical deviations from Raoult s law, though their dipole moments are different. The part of the electrical influence of a solvent on solute molecules that arises from the polarizability of the solvent molecules may be represented by the refractive index, n, or by functions of n such as the volume polarization, R, given by ... 

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