Kamlet-Taft scale

The 7t scale of solvents (Kamlet-Taft scale) is intended to represent the solute-solvent interactions in the absence of strong forces such as hydrogen-bonding or dipole-dipole interactions. It is based on the n to n absorption of a series of seven solutes (six uitroaromatics, such as 4-uitroanisole, A, A -diethyl-3-nitroaniline, 1-ethyl-4-uitrobeuzeue, etc., aud 4-dimethylamiuobenzophenone) [51-53]. 

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,... 

Baker, S.N., Baker, G.A., Bright, F.V., Temperature-dependent microscopic solvent properties of dry and wet l-butyl-3-methylimidazolium hexafluo-rophosphate Correlation with ET(30) and Kamlet-Taft polarity scales. Green Chem., 4,165-169, 2002. 

For reviews, sec Abraham Doherty Kamlet Taft Chem. Br. 1986, 551-554 Kamlet Abboud Taft Prog. Phys. Org. Chem. 1981,13, 485-630. For a comprehensive table and a and p values, see Kamlet Abboud Abraham Taft J. Org. Chem. 1983, 48, 2877. For a criticism of the p scale, see Laurence Nicolet Helbert J. Chem. Soc., Perkin Trans. 2 1986, 1081. See also Nicolet Laurence Lugon J. Chem. Soc., Perkin Trans. 2 1987, 483 Abboud Roussel Gcntric Sraidi Lauransan Guihgncuf Kamlet Taft./. Org. Chem. 1988,53, 1545 Abraham Grcllier Prior Morris Taylor J. Chem. Soc., Perkin Trans. 2 1990, 521. 

The method of linear solvation energy (LSER), based on the Kamlet-Taft multiparameter scale (10) has been successfully exploited to study retention in LC. The LSER approach, when applied to phase-transfer processes, correlates a general solute property (SP), such as logarithmic capacity factor, with parameters of the solute and both the mobile and stationary phases ... 

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Of the existing solvent scales, the Kamlet and Taft n scale ( D appears appropriate for use with supercritical fluid solvents. The basis of the Kamlet and Taft scale is the effect of solvent polarity and polarizability on the energy of the n - it electronic transition of a solute probe molecule. Solvent molecules in proximity to a solute (the cybotatic region) differentially affect the electronic energy levels of solutes (the solvatochromic effect)... 

Since its introduction the Kamlet-Taft it scale has been applied to a large number of diverse liquid solvents ( 5). Several studies have shown that the it solvent scale is consistent with other popular measures of solvent strength (6J. It has also been shown that there are good correlations between the empirical measurements of the Kamlet and Taft scale and several theoretical models of solvent behavior (J). Use of the solutes chosen by Kamlet and... 

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... 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

Values of I /Is have been correlated for liquid organics with the Kamlet-Taft n scale by the following relationship (32) ... 

The Kamlet-Taft n scale Is based on a linear free energy relationship and measures the strength of the polarity and polarizability of the solvent, n values In SC fluids have been measured by Yonker and coworkers (19,21) with values for COj ranging from -0.5 to -0.1. This would predict I1/I3 values of 0.4 to 0.5, which are much lower than those observed. Therefore, we observe I1/I3 values Indicative of much stronger solute/ solvent Interactions than those predicted, suggesting a serious difference between the Interactions In SCF s and those observed In normal liquid organics. 

The averaged Kamlet-Taft solvent basicity scale / kt was established by averaging similar values of /J obtained within narrow limits for several homomorphic HBD and non-HBD... 

FIGURE 1. Correlation of scales for some common EPD/HBA solvents with solvatochromic comparison probe pairs with the Kamlet-Taft /3kt parameters. Circles pair 8/7, squares pair 1/2, and triangles pair 17/18 (empty symbols for alkylamines), all normalized with 0.00 for cyclohexane and 1.00 for HMPA... 

FIGURE 26. Correlation of the peaks of Pekarian functions (energy scale) used to approximate the UV-vis absorption spectra of 2-naphthol (a) blue band and (b) red band with Kamlet-Taft parameters . See Figure 24 for details of the absorption spectra... 

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. 

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