Dipolarity/polarizability

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

Sinee ir, a, and (3 are approximately normalized seales, the coeffieients s, a. and b are measures of the relative weights of the dipolarity/polarizability, HBD ability, and HBA ability of the solvent. Equation (8-77) has been extended to take aeeount of the eavity effect and certain anomalies, as we will see later in this seetion. 

IT is the dipolarity/polarizability parameter. a. is the solvent HBD (aeidity) parameter. 

Attempts have also been made to separate non-specific effects of the local electrical field from hydrogen-bonding effects for a small group of ionic liquids through the use of the k scale of dipolarity/polarizability, the a scale of hydrogen bond donor acidity, and the (i scale of hydrogen bond basicity (see Table 3.5-1) [13, 16]. 

Another different 7t -scale which indicates solvent dipolarity/polarizability and which is a measure of the ability of the solvent to stabilize a charge or a dipole by virtue of the dielectric effect, has been proposed for numerous Lewis bases including sulphoxides89. 

Valko et al. [37] developed a fast-gradient RP-HPLC method for the determination of a chromatographic hydrophobicity index (CHI). An octadecylsilane (ODS) column and 50 mM aqueous ammonium acetate (pH 7.4) mobile phase with acetonitrile as an organic modifier (0-100%) were used. The system calibration and quality control were performed periodically by measuring retention for 10 standards unionized at pH 7.4. The CHI could then be used as an independent measure of hydrophobicity. In addition, its correlation with linear free-energy parameters explained some molecular descriptors, including H-bond basicity/ acidity and dipolarity/polarizability. It is noted [27] that there are significant differences between CHI values and octanol-water log D values. 

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 retention depends on the nature of both the stationary phase and the organic modifier in the mobile phase. Therefore CHI values obtained using different systems show different sensitivities towards solute characteristics. This has been studied systematically and used for the quantitative calculation of solute molecular descriptors (H-bond donor capacity, H-bond acceptor capacity and dipolarity/polarizability) for application in a general solvation equation [21]. 

Polymers can be characterized via the Kamlet-Taft approach which describes the ability of a species to act as a hydrogen bond acid (ai), the ability to act as a hydrogen bond base (Pj), the dipolarity/polarizability (7ti ), and the size of a species. These parameters are obtained by dissolving solvatochromic indicator dyes in the respective polymer and by measuring the shift of their absorbance maxima18. 

Water absorption can also cause significant changes in the permittivity and must be considered when describing dielectric behavior. Water, with a dielectric constant of 78 at 25°C, can easily impact the dielectric properties at relatively low absorptions owing to the dipolar polarizability contribution. However, the electronic polarizability is actually lower than solid state polymers. The index of refraction at 25°C for pure water is 1.33, which, applying Maxwell s relationship, yields a dielectric constant of 1.76. Therefore, water absorption may actually act to decrease the dielectric constant at optical frequencies. This is an area that will be explored with future experiments involving water absorption and index measurements. 

Abraham s solute descriptors to yield a predictive regression equation. Further, the solute dipolarity/polarizability, hydrogen bond acidity, and hydrogen bond basicity were found to favor blood and solute size favor brain. 

FIGURE 9.7 Variation of Kamlet-Taft solvatochromic parameters for methanol/C02 mixtures as a function of added CO2 at 298 K and 17.2 MPa (A)n (dipolarity/polarizability, ( ) (H-bond acidity), and (T)/8 (H-bond basicity). (Adapted from Y. Cui, S. V. Olesik, Anal. Chem., 63 1812(1991).)... 

Parameters of the Kamlet-Taft solvatochromic relationship. These parameters measure the contributions to overall solvent polarity of the hydrogen bond donor, the hydrogen bond acceptor, and the dipolarity/polarizability properties of solvents. 

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. 

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