Kamlet-Taft polarity parameter

Hildebrand solubility parameter for mobile and stationary phases TZ Kamlet-Taft polarity parameter... 

The properties of organic liquids relevant to their use as solvating agents have also been reviewed [76]. The ability of liquids to solvate a solute species depends mainly on their polarity and polarizability properties, ability to hydrogen bond, and cohesive electron density. These molecular properties are best measured by the Kamlet-Taft solvatochromic parameters, and the square of Hildebrand s solubility parameter. 

A number of studies have attempted to characterize ionic liquids through their dielectric constant, and all have observed inconsistencies between the measured dielectric constant and the solvation properties of the liquid. Recent experiments making use of dielectric reflectance spectroscopy [214] indicate dielectric constants in the range of 10-15 for a series of imidazolium-based ILs, substantially lower than those for molecular solvents observed to possess comparable polarities as estimated by solvatochromism. Weingartner [215] has recently published a series of static dielectric constants obtained from dielectric reflectance spectroscopy, and compared them with those of common molecular liquids. The analysis includes comparison with the Kamlet-Taft ji parameter for the liquids from Eq. (11) we have prepared a plot of n versus dielectric constant in Fig. 6. The relationship between n and e for molecular liquids... 

Kamlet-Taft solvent parameters Parameters of the Kamlet-Taft solvatochromic relationship that measure separately the hydrogen bond donor (a), hydrogen bond acceptor (P), and dipolarity/POLARiZABiLiTY (ji ) properties of solvents as contributing to overall solvent POLARITY. 

One such methodology is the Kamlet-Taft Solvatochromic parameter approach. In this methodology, a solvent can be characterized by three parameters, tt, a measure of the polarity and polarizability of the fluid, a, the acidity or hydrogen bond donor capability and P, the hydrogen bond acceptor capability or basicity. Each of these parameters is determined from the shift in UV-visible absorbance of a series of select indicator species dissolved in the solvent. Rather than depending on the bulk properties of the fluid, as is the case with the cohesive energy approaches, the solvatochromic parameters are derived from the interactions between the indicator solute and the immediate solvent shell, in effect they are a measure of how a solute sees the solvent. In each case, the scale of values has been normalized to between 0.0 for cyclohexane... 

From the values of the Kamlet-Taft solvatochromic parameters (Table IV), R-134a is seen to behave as a moderately polar, weakly polarizable fluid with little or no basicity and weak acidity (of the order of dichloromethane). The negative value for P has been commented on previously (J) and may be an artifact of the original scale definition, the regression for which did not include heavily fluorinated species. In any case, the value suggests that R-134a is a poorer hydrogen bond acceptor than, for example, hydrocarbons. 

Kamlet-Taft polarity/polarizability, hydrogen-bond donor (HBD), and hydrogen-bond acceptor (HBA) solvatochromic parameters taken from [138,148] 57 and p for TMP were obtained respectively from the correlations AN = 1.04 -I- 15.4(tr - 0.088) -I- 32.6a [149] and DN (kj/ mol) = — 3.8 -I- 163.9)3 [151], where 5 in this case only is a correction factor (not the Hildebrand solubility parameter) equal to zero forTMP. 

Here Rs is the excess molar refraction of the solute over that of an alkane with the same characteristic volume (Abraham et al. 1990) (not further specified). The Kamlet-Taft solvatochromic parameters of the solute (Kamlet et al. 1983) are n, the polarity/polarizibility, a the HB donation (electron pair acceptance) ability, and P the HB acceptance (electron pair donation) ability. The volume of the solute is represented by the Abraham-McGowan volumes Vx (Abraham and McGowan 1987), based on invariant atom and bond contributions. The parameters a and /9 pertain to the monomeric solutes (measured in dilute solutions thereof). The correlation coefficient for Eq. (1.21) for 408 solutes is 0.998 and the standard deviation is 0.15. 

Lu, J., Brown, J.S., Liotta, C.L. and Eckert, C.A., Polarity and hydrogen-bonding of ambient to near-critical water Kamlet-Taft solvent parameters, Chem. Commun., 2001, (7), 665-666. 

However, the quantities already include the interactions of the solutes with water, but properties foreign to such interactions are preferable for predictive purposes. Two such properties have been proposed and tested by Marcus [65] the Kamlet-Taft polarity/polarizability index a (Section 3.3.2.1) and the Hildebrand solubility parameter (Section 3.2.2). These as well as the intrinsic volumes of the solutes, expressed as their McGowan volumes are independent of interactions with water, hence true predictive parameters that are known or calculable from group contributions for a large number of solutes. The predictive expression (for ambient temperatures) is ... 

This behavior of X., or (E = hc/A , with Planck s constant h and velocity of light c) and the dependence on varying substituents or different solvents can be correlated with the Hammet Op parameter and the Kamlet-Taft tt parameter, respectively. With protic solvents and in aqueous solutions, the dependence of the 7T—band maxima on polarity and water content is less clear. The effect of water on the position of the low-energy band in two 4-(donor)-4 -nitroazobenzenes was studied in polyethyleneglycol/water mixtures. Although the polarity of the mixture obviously increased with increasing water content, the values showed a maximum. This could be due to an increased probability of... 

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] ... 

From a practical viewpoint, Ej values are quickly and easily obtained, giving a very useful and convenient scale. However, a general polarity scale based on a single probe molecule has its limitations because a single compound cannot experience the diversity of interactions that the whole range of solvents can offer. The Kamlet-Taft parameters a, /3 and n tackle this problem by using a series of seven dyes to produce a scale for specific and nonspecific polarity of liquids [23], Whilst it undoubtedly gives a more detailed description of the solvents properties,... 

Table 9.1 Ej and Kamlet-Taft 7r polarity parameters for n-hexane and oxidized...

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. 

Correlation analysis of solvent effects on the heterolysis of p-methoxyneophyl tosyl-ate has been performed by using the Koppel-Palm and Kamlet-Taft equations. The reaction rate is satisfactorily described by the electrophilicity and polarity parameters of solvents, but a possible role for polarizability or nucleophilicity parameters was also examined. 

As is shown below, the polarity measured by A T(30) for a protic solvent shows its ability to donate a hydrogen bond to a solute in addition to its polarity per se. A different solvatochromic polarity parameter, that is devoid of this complication (but has others), is Kamlet and Taft s 7t (Kamlet, Abboud and Taft 1977). This is based on the average of values of the 7C —> transition energies for several... 

Kamlet-Taft parameters are known to express three distinct measures of the solvent polarity such as dipolarity/polarizability hydrogen-bond acidity (a) and hydrogen-bond basicity (/S). These parameters have been determined by absorption measurements for individual or pairs of the following dye molecules N,N-diethyl-4-nitroaniline, 4-netroaniline and Reichardt s dye, as seen in Figure 3.4 [81-83]. 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

Kamlet-Taft parameters have also been obtained for some perfluorinated solvents.They are not hydrogen bond donors (a is typically 0.0) and are typically extremely poor hydrogen bond acceptors ( 3 is small and negative). They are extremely difficult to polarize, and this leads to a strongly negative tt parameter. This last property explains why they typically form biphasic systems with organic solvents. 

---