Solvents 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... 

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

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... 

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. 

Table 2.3 Solvatochromic Parameters for Some Solvents and (Monomeric) Solutes (in Parentheses)... 

The standard molar Gibbs energy of transfer of CA is the sum v AG°(C) -i-v AtG°(A), where the charges of the cation C and anion A " and the designation of the direction of transfer, (aq org), have been omitted. The values for the cation and anion may be obtained from tables [5-7], which generally deal with solvents org that are miscible with water and not with those used in solvent extraction. However, AtG°(C) depends primarily on the (3 solvatochromic parameter of the solvent and AtG°(A) on its a parameter, and these can be estimated from family relationships also for the latter kind of solvents. 

Solvatochromic Approach Solvatochromic relationships are multivariate correlations between a property, usually solubility or partitioning property (see Sections 11.4 and 13.3), and solvatochromic parameters, parameters that account for the solutes interaction with the solvent. In the case of vapor pressure, the solvatochromic parameters only have to account for intermolecular interaction such as selfassociation between the solute (i.e., pure compound) molecules themselves. The following model has been reported for liquid and solid compounds, including hydrocarbons, halogenated hydrocarbons, alkanols, dialkyl ethers, and compounds such as dimethyl formamide, dimethylacetamide, pyridine, and dimethyl sulfoxide... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

To investigate the effect of alcohol hydrogen-bond donor (HBD) strength in a systematic way, a series of modifiers possessing a common 4-teri-octylphenoxy ethanol core element were prepared, in which the properties of the substituent attached to the alcohol carbon atom were varied. These modifiers were evaluated with respect to both the cesium extraction strength they afforded when used in combination with BC6 and the HBD strength of the solvent as assessed using the solvatochromic parameter E. ... 

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