Solvatochromic scales

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

For phenol blue, the E, is zero for ethylene and CF3CI, but nonzero for the Lewis acids CF3H and CO2. One of the attractive features of solvatochromic scales is that the non-specific and specific interactions may be separated since the former can be calculated straightforwardly using Eq. 5. 

The standard molar Gibbs energy of solvation can also be derived from pure component data using spectroscopic information for determining solvatochromic parameters in respect of activity, basicity, polarity, etc. There exists a number of linear solvatochromic scales, the most widely used of which is the linear solvation energy relationship (LSER) devised by Kamlet and Taft [37, 38]. The Nernst distribution of solute i according to Kamlet is ... 

This equation is quite accurate in comparison with group contributing methods [40] or other predictive LSER methods [41]. For compounds where the solvatochromic parameters are known, the mean absolute error in log Dy is about 0.16. It is usually less than 0.3 if solvatochromic parameters of the solute and solvent must be estimated according to empirical rules [42], In contrast to the prediction of gas-liquid distribution coefficients, which is usually easier, the LSER method allows a robust estimation of liquid-liquid distribution coefficients. However, these equations always involve empirical terms, despite their being physico-chemically founded thermodynamic models. However, this is considered due to the fundamental character of the solvatochromic scales. 

This procedure was applied to numerous solvatochromic scales [Z, Xr. (30)], using 4-nitroanisole as the solute incapable of H bonding. As in the development of the TT scale, the a scale is averaged over several processes. A (3 scale of HBA ability (basicity) was established in a similar manner. Table 8-17 lists some it, a, and (3 values. ... 

Solvent strength of supercritical fluids based on solvatochromic scales... 

The purpose of these scales is to provide a guide for choosing SCF solvents and co-solvents to achieve a desired solvent strength, for example in a separation or reaction process. An example of a solvatochromic scale is presented in Figure 1 for the UV-vis absorption of phenol blue in ethylene as a function of density at two temperatures (S). The scale is defined as the transition energy, Ej = he/ Xmax where Xmax is the wavelength of maximum absorption. 

The final application of solvatochromic solvent strength scales is the correlation of reaction rate and equilibrium constants in SCF solvents. Solvatochromic scales are often quantitative indicators of the solvent effect on rate constants for a variety of reaction mechanismsU) In a SCF, this solvent effect may be achieved conveniently with a single solvent using pressure. Based on solvatochromic data, it was predicted that an activation volume can reach thousands of mL/mol in a SCF(8). This prediction was confirmed for various types of reactionsClSbZl). For example, the solvatochromic parameter Ex for phenol blue... 

VI. SOLVATOCHROMIC SCALES BASED ON ANILINES A. Neat Solvents... 

Among the approaches proposed so far, we recall here single-parameter models [102-111, 115, 118-120, 122, 123, 125, 126, 129], and multi-parametric correlation equations (either based on the combination of two or more existing scales or on the use of specific parameters to account for distinct types of effects) [112, 113, 116, 117, 121, 124]. Additional popular models are the Abraham s scales of solute hydrogen-bond acidity and solute hydrogen-bond basicity [127, 128], and the Catalan et al. solvatochromic scales [130,132, 133]. Methods based on quantitative stmcture-property relationships (QSPR) with solvent descriptors derived from the molecular structure [131, 134], and on principal component analysis (PCA) [135, 136] have been also proposed. An exhaustive review concerning the quantification of the solvent polarity has been recently published [138-140], including a detailed list of solvent scales, interrelations between parameters and statistical approaches. 

Laurence et al. [23, 24, 135] measured two solvatochromic scales for about 200 bases, each based on one reference hydrogen-bond donor, 4-nitrophenol or 4-nitroaniline, one standard state, the pure base, and one physical property, the enhanced bathochromic shift of the longest wavelength n n transition (with intramolecular charge-transfer character) upon hydrogen bonding. The solvatochromic comparison method was improved and inert solvents were carefully chosen [135] to fix the comparison lines of Equations 4.37 and 4.38 ... 

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