Kamlet-Taft solvatochromic scale

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. 

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)... 

The standard molar Gibbs energy of solvation can also be derived from pure component data using spectroscopic information for determining solvatochromic parameters in respect of activity, basicity, polarity, etc. There exists a number of linear solvatochromic scales, the most widely used of which is the linear solvation energy relationship (LSER) devised by Kamlet and Taft [37, 38]. The Nernst distribution of solute i according to Kamlet is ... 

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. 

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... 

The Kamlet-Taft [Ka 76] scale of solvent basicity makes use of the magnitudes of the solvatochromic shifts of the longest wave band in the UV-visible spectrum of p-nitroaniline referred to that of N,N-diethyl-p-nitroaniline. The primary amino group of former compound interacts with Lewis bases, the latter one does not. So the difference between the solvatochrom shift in the spectra of these two compounds depends on the strength of the interaction between the Lewis base and p-nitroanilin. 

MJ Kamlet, J-LM Abbound, RW Taft. Solvatochromic comparison method. 6. Pi-Star scale of solvent polarities. J Am Chem Soc 98 6027, 1977. 

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... 

Kamlet MJ, Abboud J-L, Taft R (1977) The solvatochromic comparison method. 6. The it scale of solvent polarities. JACS 99 6027-6038... 

Kamlet MJ, Taft RW (1976) The solvatochromic comparison method. 1. The /6-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc 98 377-383. 

Further solvent polarity scales based on UV/Vis absorption as well as fluorescence spectra have been proposed by Brooker et al. [77], Dahne et al. [78], de Mayo et al. [217], Dubois et al. [79], Mukerjee et al. [218] and Wrona et al. [219], Walter et al. [220], Walther [81] and Lees et al. [82], Zelinskii et al. [80], Winnik et al. [222], Kamlet and Taft [84, 84a, 224, 226]. Buncel et al. [333], and Catalan et al. [296, 334-337]. In addition to these scales, a great variety of further positively and negatively solvato-chromic dyes have been recommended as solvent polarity indicators. A review describes about 60 organic and inorganic compounds, the solvatochromism of which is sufficiently large for their potential application as empirical solvent polarity probes [293]. 

Interestingly, a statistical principal component analysis of the solvatochromic shift data sets previously used by Kamlet and Taft in defining the n scale has shown that, rather than one [n ), two solvent parameters (0ik and 02k) are necessary to describe the solvent-induced band shifts of the studied solvatochromic indicators [236]. This is not unexpected since the n parameters are assumed to consist of a blend of dipolarity and polarizability contributions to the solute/solvent interactions. 

The SA scale was established using the solvatochromic comparison method of Kamlet and Taft [224, 226]. A good linear correlation (r = 0.961) has been found between the wavenumbers of the absorption maxima of dyes (52) and (53), measured in fifty non-acidic or non-HBD solvents, according to Eq. (7-40c) ... 

Taft, R.W. and Kamlet, MJ. (1976). TTie Solvatochromic Comparison Method. 2. The a-Scale of Solvent Hydrogen-Bond Donor (HBD) Acidities. J.Am.Chem.Soc., 98, 2886-2894. 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

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