Solvatochromic parameter

The use of solvents for chemical reactions and separations depends on their abilities to solvate solutes and reaction intermediates to different extents, and these abilities concern RTILs too. Such abilities are commonly expressed in terms of linear solvation energy relationships (LSERs) in which a property of a solute in a solvent is expressed as the sum of terms that are products of suitable solute and solvent parameters and a coefficient. Properties such as solubility, toxicity, activity 

The general concept of solvent polarity is expressed by means of the normalized Reichardt polarity index [409]  

The Lewis basicity or HB A ability of solvents is described by the Kamlet-Taft parameter, measured by means of the couples of solvatochromic probes 4-nitrophenol compared to 4-nitroanisole or 4-nitroaniline compared to 4-nitro-N, N-diethyaniline, the second probe of the couple serving to eliminate the effects of the solvent polarity and polarizability on its first probe. The resulting values of from the compilation of Jessop et al. [410], supplemented with data from Spange etal. [416] are shown inTable 6.16. An alternative measure of this property that has been applied by Schade et al. [413] to RTILs is Catalan s SB parameter, using the solvatochromic probe N.N-dimethyl-4-aminobenzodifuranone, and the resulting values are shown in Table 6.16. The SB values are linear with the values SB = 0.126 +1.056/. The basicity decreases in a series of RTILs with a common anion phosphonium ammonium pyrrolidinium pyridinium imidazolium, but more moderately than the acidity increases in the opposite direction shown above as shown by Spange et al. [416]. 

The polarizability of solvents, combined with their polarity, are measured by the Kamlet-Taft n values, obtained most profitably with the solvatochromic probe 4-nitroanisole and shown, as compiled by Jessop et al. [410] and supplemented with values from Kochly et al. [414] and Spange et al. [416] in Table 6.16. The jt values are in a narrow range, 0.95 0.15, i.e., the RTILs are all very polar as expected for ionic liquids. The disadvantage of having both these properties in raie parameter is removed by the separate Catalan parameters SP for polarizability and SdP for dipolarity. These use results from two solvatochromic probes a thiazoline one and 2-(4-(N,N-dimethylamino)benzylidene)malononitrile according to Schade et al. [413], and the values of SP and SdP are also shown in Table 6.16. The polarizability of RTILs is not affected much by the acidity or basicity of its constituent ions, but their polarity does depend on these properties. 

Many dyes exhibit solvatochromism their colour depends on the solvent in which they are dissolved. This is because the ground state and excited states have different polarities, and the energy gap between the two states determines the colour. The scale of Reichardt is the most well used [8]. This scale measures the solvent induced shift in the absorption maximum of the indicator dye 1. Table 12.5 shows a selection of values [9]. 

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Owing to the original determination from uv—vis spectral solvatochromic shifts, 7T, B, and are called solvatochromic parameters. General rules for estimation of these variables have been proposed (258). Examples of individual parameter investigations are available (260,261). As previously mentioned, individual LEER—LSER studies are performed on related materials. A common method to link these individual studies to group contribution methods, and thereby expand the appHcabiUty, is by expansion of solvatochromic parameters to log—linear relationships, such as... 

A sampling of appHcations of Kamlet-Taft LSERs include the following. (/) The Solvatochromic Parameters for Activity Coefficient Estimation (SPACE) method for infinite dilution activity coefficients where improved predictions over UNIEAC for a database of 1879 critically evaluated experimental data points has been claimed (263). (2) Observation of inverse linear relationship between log 1-octanol—water partition coefficient and Hquid... 

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. 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

In Table 4.2 the solvatochromic parameters 7t, Ej(30), and its normalized value Ej of common solvents used in TLC are given. Two values for the 7t parameter are mentioned because in time the original solvatochromic values were revised. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

An example of an application is shown in Fig. 30.10. This concerns the classification of 42 solvents based on three solvatochromic parameters (parameters that describe the interaction of the solvents with solutes) [13]. Different methods were applied, among which was the average linkage method, the result of which is shown in the figure. According to the method applied, several clusterings can be found. For instance, the first cluster to split off from the majority of solvents consists of solvents 36, 37, 38, 39, 40, 41, 42 (t-butanol, isopropanol, n-butanol. 

A. de Juan, G. Fonrodona and E. Casassas, Solvent classification based on solvatochromic parameters a comparison with the Snyder approach. Trends Anal. Chem., 16(1) (1997) 52-62. T. Naes, The design of calibration in near infra-red reflectance analysis by clustering. J. Chemom., 1 (1987) 129-134. 

Taft et al. [28] proposed the solvatochromic parameters, tt, a, and p, which describe three solvent s abilities, respectively, to stabilize a charge or a dipole by virtue of its dielectric effect, to donate a proton (or accept an electron pair), and to accept a proton (or donate an electron pair). It was shown that the AN for nonprotonic solvents correlates... 

Otherwise, prediction of partition coefficient from the known physical properties of the solute and solvents have been developed by Taft, Abraham, Kamlet, and coworkers [207-210]. They have introduced solvatochromic parameters to assess the intermolecu-lar forces governing the partition mechanisms of neutral organic solutes nonspecific... 

TABLE 1 Solvatochromic Parameters Taken from the Literature for the Determination of log in Various Solvent Systems (oct Stands for Octanol, dee for 1,2-Dichloroethane, hep for Heptane, chf for Chloroform, and dbe for Dibutylether)... 

A number of models have been proposed to describe the solution formation process [505-509], some of which can be extended to Include chromatographic processes and other solvent-dependent phenomena. In terms of chromatographic aiqplications the most useful are the solubility parameter concept, solvatochromic parameters and Snyder s solvent strength and selectivity... 

Small molecule size-c Ecluslon chrmutography 441 Soap-film meter (GC) 4, 235 Solid-phase extraction 777 cartridges 777 membranes 780 optimization 777, 783 sorbents 778, 785 trace enrichment 777, 783 Solubility parameters 460 Solvatochromic parameters GC 191... 

Isohydric 385 localizing 378 mixed solvents strength 465 isoeluotropic 467 properties 460 solubility parameters 460 solvatochromic parameters 461 solvent triangle classification 463 types 458... 

Casassas, E. Fonrodona, G. de Juan, A., Solvatochromic parameters for binary mixtures and a correlation with equilibrium constants. 1. Dioxane-water mixtures, J. Solut. Chem. 

Solvatochromic parameters and intrinsic molar volume (Banerjee et al. 1990). 

Calculation with solvatochromic parameters (Hawker 1989, 1990b). 

Hawker, D. W. (1989) The relationship between octan-l-ol/water partition coefficient and aqueous solubility in terms of solvatochromic parameters. Chemosphere 19, 1586-1593. 

Hawker, D. W. (1990a) Vapor pressures and Henry s law constants of polychlorinated biphenyls. Environ. Sci. Technol. 23, 1250-1253. Hawker, D. W. (1990b) Description of fish bioconcentration factors in terms of solvatochromic parameters. Chemosphere 20, 267-477. Hawker, D. W., Connell, D. W. (1988) Octanol-water partition coefficients of polychlorinated biphenyl congeners. Environ. Sci. Technol. 22, 382-387. 

Hawker, D.W. (1990) Description of fish bioconcentration factors in terms of solvatochromic parameters. Chemosphere 20, 467 -77. 

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. 

X0 is the value of the property in the gas phase. (In practice, X and X0 are often the logarithm of the property in question.) The parameters a and p are measures of a solvent s ability to donate and accept hydrogen bonds, respectively, and tt is an index of its polarity/polarizability. They were initially assigned on the basis of ultraviolet spectral shifts of certain dyes in a variety of solvents, and hence were labeled solvatochromic parameters.186"188... 

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. 

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