Kamlet-Abboud-Taft solvatochromic parameters

Here, n is the dipolarity and polarizability of the solvent (so-called Kamlet-Abboud-Taft solvatochromic parameter), a the hydrogen-bond donor (HBD) acidity, and P the hydrogen-bond acceptor (HBA) basicity of the solvent, whereas s, a, and h are the (so-called) susceptibilities of the solute with reference to tt, a, and / , respectively. In the absence of hydrogen bonds between the solvent and the... 

In most instances, the components of a reaction will be soluble in a large number of solvents, and the main concern is usually not in what solvent to dissolve the reactants, but rather what solvent will promote the reaction and provide the most product in the shortest period of time. Polarity parameters based on the UV absorbance of solvatochromic dyes are suitable for correlations with kinetics and equilibria. Because the wavelength of the dye absorption is determined by the relative energy difference between the ground state and the excited electronic state of the probe molecule, it mirrors the variable energy levels of reaction components and intermediates in solution (Figure 3.3). Although several polarity scales of this type exist, the most reliable and broadly applicable are the Kamlet-Abboud-Taft... 

R. W. Taft, J-L. M. Abboud, M. L. Kamlet, and M. H. Abraham,/ of Solution Chem. 14, 3, 153 (1985). An excellent review source including an extensive bst of properties correlated with solvatochromic parameters. 

Kamlet M. J., Abboud J.-L., Abraham M. H. and Taft R. W. (1983) Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, 7i, a, and ft, and Some Methods for Simplifying the Generalized Solvatochromic Equation, J. Org. Chem. 48, 2877-2887. 

Kamlet MJ, Abboud JLM, Abraham MH, Taft RW (1983) Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters, jr, a, and /3, and some methods for simplifying the generalized solvatochromic equation. J Org Chem 48 2877-2887. 

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... 

Both Kamlet, Abboud, and Taft et al. s [224, 226] and Swain et aVs [265] multiparameter solvent effect treatments have an inherent weakness in so far as the solvent parameters a, and n as well as and are averaged and statistically optimized parameters the former are derived from various types of solvatochromic indicator dyes. 

More recent solvatochromic studies [14-20] employ specialised dyes reflecting specific microscopic molecular interactions. For example, the Abboud-Kamlet-Taft solvent parameters [21-23] a, 3 and n give information on hydrogen bond donor and acceptor properties and the polarisability of a compound, respectively. For ionic liquids with [C4mim]-cations, the following order of [3- values was estab-... 

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)... 

Figure 19. Solvatochromic parameters of five supercritical fluids from measurements of Maiwald compared with literature data for some selected liquid solvents according to Kamlet, Abboud, and Taft. Filled symbols refer to the probe molecules 4-nitroaniline / N,N-dimethyl-4-nitroaniline (NH-hydrogen bonding), open symbols to 4-nitrophenol / 4-nitroanisole probes (OH-hydrogen bonding) adapted from [75].

The three scales devised by Kamlet, Abboud and Taft have been used many times to formulate relationships between reaction rate constants and solvent polarity. These are known as linear solvation energy relationships (LSERs). The rate of amide formation for example, the most common single reaction in medicinal chemistry, is inversely proportional to jS for entropic reasons (Figure 3.4). Limonene and its derivative p-cymene were thus justified as excellent options for a renewable amidation solvent, not only in terms of performance but also because they are produced from a renewable feedstock. Other solvents are less suitable according to their solvatochromic polarity parameters (Table 3.3). As hydrocarbons, some aquatic toxicity concerns surround the use of limonene and p-cymene, but ideally these would be minimised with recycling. 

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. 

Abraham, in collaboration with Taft, Kamlet, and Abboud, has made an interesting attempt to characterize the nature of a solvent in terms of its H-bond donor ability (HBD or), H-bond acceptor ability (HBA jS), and its specific dipolarity/polarizabiUty (jt ). He has used these parameters in multiparameter relationships to analyze medium effects on a variety of chemical processes. Briefly, the HBD propensities (a) were obtained from the enhanced sol-vatochromism of the t(30) probe relative to 4-nitroanisole, the HBA propensities (j8) from the enhanced solvatochromism of 4-nitroaniline relative to N,N-diethyl-4-nitroaniline in HBA solvents, and the rt values from solvent effects on the n n transition of nitro-substituted aromatic compounds. For a more detailed discussion of these parameters and their apphcations, the reader is referred to the original literature. ... 

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