Polarity-polarizability scale

The Kamlet-Taft u polarity/polarizability scale is based on a linear solvation energy relationship between the n it transition energy of the solute and the solvent polarity ( 1). The Onsager reaction field theory (11) is applicable to this type of relationship for nonpolar solvents, and successful correlations have previously been demonstrated using conventional liquid solvents ( 7 ). The Onsager theory attempts to describe the interactions between a polar solute molecule and the polarizable solvent in the cybotatic region. The theory predicts that the stabilization of the solute should be proportional to the polarizability of the solvent, which can be estimated from the index of refraction. Since carbon dioxide is a nonpolar fluid it would be expected that a linear relationship... 

Molecular spectroscopy methods have also been applied to the study of the entrainer effect in supercritical fluid-cosolvent mixtures. Again, the molecular probes employed for absorption and fluorescence measurements include the Kamlet-Taft u polarity/polarizability scale probes (13,14), pyrene (15,16), and TICT molecules (17). 

There is very little problem in calculating an acceptable measure of solute size. Simple calculations of either molecular volume or area based on either Bondi s (Bondi, 1964) or McGowan s (Abraham, 1987) methods work almost as well as those derived from molecular mechanics and quantum chemistry (Leo, 1993). When volume in cubic Angstroms is used, V is normally scaled by 0.01 to produce a coefficient comparable to the others in the equation polarity/polarizability. 

The effect of pressure on the dissolving power of various supercritical fluids has been examined spectroscopically, through comparisons on the tt polarity and polarizability scale of Taft et al. [8], Thus, the polarizability of CO, has been found to be lower than those of all hydrocarbons except methane. Also, for its polarizability per unit volume to be comparable to that of liquid cyclohexane at 45°C, CO, must be subjected to a pressure of 2700 bar [9]. 

The probe 2-(dimethylamino)-7-nitrofluorene (54) (see Scheme 2) was proposed by Catalan and coworkers59 as the basis of the SPP scale of solvent polarity/polarizability (in conjunction with 2-fluoro-7-nitrofluorene (55)). On the assumption that the band shapes are the same for these two probes, they cancel out in the difference AvSpp = v(54)-v(55). The normalized SPP scale was then defined in equation 13 ... 

Correlations of Infrared Spectral Data with the ir -Scale of Solvent Polarity-Polarizabilities... 

To adapt the solvatochromic equations and the v scale to properties and phenomena involving different relative contributions of polarity and polarizability, we have modified Equations 42,42 and 89 by the addition of a d5 term. The d coefficient is intended to serve as a measure of the difference between the polarity- polarizability blend for XYZ and the rr blend. The 5 parameter is taken to be 0.00 for all select solvents, 0.50 for polychlorinated aliphatics, and 1.00 for all aromatic solvents. 

Taft and coworkers described the formulation of three scales of solvent properties which were used to unravel and rationalize solvent effects on many types of physico-chemical properties. A tt scale of polarity/polarizabilities describes the solvent s ability to stabilize a charge or a dipole by virtue of its dielectric effect. The n values have been shown to be generally proportional to molecular dipole moments. The a scale of hydrogen bond donor acidities provides a measure of the solvent s ability to donate a proton. The jS scale of hydrogen bond acceptor basicities quantifies the solvent s ability to donate an electron pair (accept a proton). 

A brief review has been presented of the correlation analysis of solvolysis rates 50 years later, i.e. since Grunwald and Winstein proposed their eponymous equation in 1948.111 -pije authors then propose a method of correlation analysis involving multiple regression on solvent scales SPP (polarity-polarizability), SA (acidity) and SB (basicity). These scales are based on the solvatochromism of suitable probes and were initially for pure (i.e. one-component) solvents, but have now been extended to binary solvent mixtures. This enabled the authors to present a correlation for the solvolysis rate constants of r-butyl chloride in 27 pure solvents and 147 binary solvent mixtures, having a correlation coefficient r = 0.990 and a standard error of the estimate s = 0.40. The most important term in the equation is that involving SPP next comes... 

Numerous experimental studies have been conducted on solute-solvent interactions in supercritical fluid solutions. In particular, issues such as the role of characteristic supercritical solvent properties in solvation and the dependence of solute-solvent interactions on the bulk supercritical solvent density have been extensively investigated. Results from earlier experiments showed that the partial molar volumes 02 became very large and negative near the critical point of the solvent (4-12). The results were interpreted in terms of a collapse of the solvent about the solute under near-critical solvent conditions, which served as a precursor for the solute-solvent clustering concept. Molecular spectroscopic techniques, especially ultraviolet-visible (UV-vis) absorption and fluorescence emission, have since been applied to the investigation of solute-solvent interactions in supercritical fluid solutions. Widely used solvent environment-sensitive molecular probes include Kamlet-Taft jt scale probes for polarity/polarizability... 

The K scale of solvent polarity/polarizability is based on the correlation between the experimentally observed absorption or emission shifts (vmax values) of various nitroaromatic probe molecules and the ability of the solvent to stabilize the probe s excited state via dielectric solute-solvent interactions (18). Since tt values are known for many commonly used liquid solvents, the k scale allows comparison of the solvation strength of supercritical fluids and normal liquid solvents. Several research groups have utilized the k probes to investigate solvent characteristics for a series of supercritical fluids (19-34). For example, Hyatt (19) employed two nitroaromatic dyes and the penta-ferf-butyl variation of the Riechardt dye (18) to determine the k values in liquid and supercritical CO2 (0.7 reduced density at 41°C). The experimental results were also used to calculate the t(30) solvent polarity scale (19), which is similar to the n scale. ... 

Solvent effects based on pure solvent scales parameters of solvents Polarity/Polarizability SPP, Basicity... 

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