Solvent empirical differential

Figure S. Activation volume from empirical differential solvent pressures (16)...

Consequently there appears to be a sound empirical basis for the use of the OTs scale of solvent ionizing power. Its use should be restricted to sulphonates, however, because of the differential effects of electrophilic solvation in acidic solvents (see Section 4). The importance of these effects can be seen by comparing the Y and Iqxs values for carboxylic acids (Table 5) it appears that, relative to 80% ethanol/water, a carboxylic acid ionizes a tosylate about ten times more rapidly than a chloride. 

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

According to Eq. (16), the difference between the differential heats of solution of two polymorphs is a measure of the heat of transition AH between the two forms. Because enthalpy is a state function (Hess s law), this difference must necessarily be independent of the solvent system used. However, conducting calorimetric measurements of the heats of solution of the polymorphs in more than one solvent provides an empirical verification of the assumptions made. For instance, AH values of two losartan polymorphs were found to be 1.72 kcal/mol in water and 1.76 kcal/mol in dimethylformamide [53]. In a similar study with moricizine hydrochloride polymorphs, AH values of 1.0 kcal/mol and 0.9 kcal/mol were obtained from their dissolution in water and dimethylformamide, respectively [54]. These two systems, which show good agreement, can be contrasted with that of enalapril maleate, where was determined to be 0.51 kcal/mol in methanol and 0.69 kcal/mol in acetone [55]. Disagreements of this order (about 30%) suggest that some process, in addition to dissolution, is taking place in one or both solvents. 

As clearly shown by the examples in Chapters 2 and 3, the solvating power of a solvent is the resultant of a combination of several specific and non-specific interactions. It is diflRcult to differentiate these from one another. This is the reason why so many different types of empirical solvent scales have been proposed for the characterization of the solvating power. 

It is well known experimentally that the ability of a solvent to interact differently with the ground and excited states typically involves much more than just its dielectric constant it may also depend on the details of the solvent-solute interaction and the solvent structure. The solvent polarity scale is an empirically based approach to express quantitatively the differential solvation of the ground and excited states of a solute.It uses the electronic spectral shift as a convenient one-parameter characterization of the ability of the solvent to interact with the solute. Several solvent polarity scales have been developed on the basis of the spectral shift of several different dye molecules. For example, one of the most widely used scales, called the Ej-(30) scale, is equal to the spectral shift in kcal/mol for the Jt n transition in pyridinium N-phenolate betaine dye. The Et(30) values, as well as other polarity scales, have been tabulated for many solvents. ... 

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