Empirical polarity scales

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. 

Empirical scales of solvent polarity based on solvatochromic shifts... 

It should be emphasized again that there are many parameters underlying the concept of polarity, and therefore the validity of empirical scales of polarity based on a single parameter is questionable. 

Quantitative determination of solvent polarity is difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property, due to the complexity of solute-solvent interactions, and for this reason empirical scales of solvent polarity based on chemical... 

The variations in the absorption energies of various dyes have been used to characterise the polarity of various media and create empirical scales. For this purpose, the most widely used dye is the highly, negatively solvatochromic betaine (1.98), known as Reichardt s dye, whose transition energy f .j,(30) in kcal mol, measured in a particular solvent, characterises the polarity of that solvent. 

A summary of the empirical scales of solvent polarity for the pure solvents frequently used for solvolytic studies is shown in Table 5. 

Since then, solvent-dependent absorptions of a great variety of compounds have been studied as potential reference processes for establishing empirical scales of solvent polarity. Most approaches include negatively or positively solvatochromic dyes because they are experimentally easy to handle. The use of solvatochromic dyes as solvent polarity indicators has been reviewed [1, 2, 293-296, 316], see, in particular, reference [293] only a few of these dyes can be mentioned in this Section. 

Several methods have been applied to determine the solvent power of ionic liquids determined by solvation polarity and nucleophilicity (donor power). A common approach for determining the solvent polarity and setting up an empirical scale is to evaluate the UV-Vis spectra of optical probes such as solvatochromic dyes or transition metal complexes in the solvents under investigation [53], The absorption or emission bands of the probe show a strong shift in their optical spectra according to the polarity of the solvent in which they are dissolved [54-57],... 

It should be noticed that, in many theoretical works, the term solvent polarity is defined by the values of the relative electric permittivity, also called dielectric constant. However, such a definition is by no means precise. The existence of hydrogen bonds (H-bonds) between solute and solvent molecules is one of the important limitations of the use of the continuum models based on the theory of dielectrics. In modern physical chemistry of solutions in order to quantitatively describe the solvatochromism phenomenon various empirical scales of the polarity are used. The exhaustive reviews on this topic have been presented by Reichardt [1, 2],... 

The above discussion neglects polarization effects. In part, these are taken into account by the empirical scaling procedures (e.g., the dipole moment of the water molecule in condensed-phase models is chosen to be larger than the gas-phase value).12 Internal charge rearrangements due to conformational... 

Empirical scales of solvent polarity based on solvatochromic shifts I 205 Tab. 7.1. Parameters of the n scale of polarity (data from Kamlet et aL, 1983)... 

Because of their sensitivities to environmental changes, wide applications for solvatochromic compounds were found in the study of solute-solvent interactions, mainly in the characterization of bulk or microenvironments. Various polarity scales employing solvatochromic dyes as solvent probes were proposed. Because these empirical scales may be used to characterize any solvent or solvent mixture, solvatochromism played an important role in the study of a wide variety of solvent-dependent processes. 

The large number of empirical polarity scales derived from solvatochromic sensors raises the question of the degree of correlation among them. In principle, there is no reason to expect a correlation between any pair of these empirical scales. The solvatochromic behavior of a given compound reflects a sum of specific and nonspecific solute-solvent interactions that vary from probe to probe. Good correlations are to be expected only between scales based on solvatochromic compounds that present a similar response to a range of solvents. As a result of this, the concept of solvent polarity is elusive and the claims of a universal polarity scale based on solvatochromic probes cannot be maintained. Different solvents may assume different polarity values, according to the nature of the scale employed to define them. 

Z values provide an empirical scale of solvent polarities, as defined by Kosower (J, Am. Chem. Soc. 80, 3253-3260 (1958)). 

Abstract - Empirical parameters of solvent polarity seem now-days as useful for the prediction of solvent effects as Hammett s substituent constants for the estimation of substituent effects on chemical reactions. The extreme sensitivity of the position of the long-wavelength UV/VIS absorption band of pyridinium-N-phenoxide betaine dyes not only changes in temperature (thermo-solvatochromism), external pressure (piezo-solvatochromism), and the introduction of substituents, but also to small changes in solvent polarity (negative solvatochromism) has been used to establish a comprehensive empirical scale of solvent polarity, called the E.p-scale. In addition, the influence of solvents and substituents on the chemical reactivity of the pyridinium-N-phenoxides has now also been studied. 

The polarity of ILs can be compared by the use of empirical scales. The most widely apvplied scale has been based on changes in the charge transfer n-n absorption band for the betaine dye, 2,6-diphenyl-4 (2,4,6-t tiiphenylpyridinium-l-yl) phenolate also known as Reichardt s dye (Dimroth et al., 1963). The Et(30) value describing solvent polarity is calculated on the basis of the following equation ... 

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