Solvatochromic descriptors

Solvatochromic descriptors described by Abraham are summarized in Table 1, including the overall or summation hydrogen-bond acidity scale basicity scale (XIP2 )... 

Table 1. Test substances and related solvatochromic descriptors.

The so-called solvatochromic or linear solvation energy relationship (LSER) descriptors developed by Abraham and coworkers (Kamlet et al., 1983) have proved valuable in correlating a wide variety of biological endpoints and physicochemical properties, and two studies have utilized them to model BCF. Park and Lee (1993) found the following QSAR for the fish BCF values of a set of diverse chemicals ... 

Of particular interest is the analysis of other, experimentally determined empirical solvent polarity scales by means of the theoretically derived TLSER equation (7-67). For example, correlation of the solvatochromic Ej scale [see Eq. (7-29) in Section 7.4] with the TLSER descriptors of Eq. (7-67) leads to Eq. (7-68) for n = 29 solvents, with r = 0.937 and S = 0.075 [351] ... 

A rational strategy in identifying structural parameters appropriate for QSRR analysis should start from the accepted theories of chromatographic separations. These structural parameters obtained. should quantify the abilities of analytes to take part in the postulated intermolecular interactions which determine chromatographic. separations. Empirical or semi-empirical structural parameters of analytes based on the solvatochromic comparison method and on the linear solvation energy relationships (LSER) belong to that categoiy of structural descriptors. 19,40). 

In analogy with solvatochromic parameters but based on quantum theoretical chemistry, a set of - quantum-chemical descriptors intended to describe the hydrogen bonding effects of molecules by theroretical linear solvation energy relationships (TLSER) were proposed. 

Usually hydrophobicity is encoded by - steric descriptors such as molar or - molecular volume which account satisfactorily for nonpolar interactions polarity can be described by polar terms which are negatively related to lipophilicity. An important factorization of lipophilicity is provided by the - solvatochromic parameters. Moreover, a measure of the global polarity of a given solute was proposed by Testa and coworkers [Testa and Seiler, 1981 El Tayar and Testa, 1993 Vallat et al., 1995] and called the interactive polar parameter A (or the Testa lipophobic constant). It is defined as the difference between the experimentally measured lipophilicity and that estimated for a hypothetical n-alkane of the same molecular volume V as ... 

Three major approaches to the prediction of aqueous solubility of organic chemicals using Quantitative Structure Activity Relationship (QSAR) techniques arc reviewed. The rationale behind six QSAR models derived from these three approaches, and the quality of their fit to the experimental data are summarized. Their utility and predictive ability are examined and compared on a common basis. Three of the models employed octanol-water partition coefficient as the primary descriptor, while two others used the solvatochromic parameters. The sixth model utilized a combination of connectivity indexes and a modified polarizability parameter. Considering the case of usage, predictive ability, and the range of applicability, the model derived from the connectivity- polarizability approach appears to have greater utility value. 

Further theoretical and computational studies of betaine(30) of the Et(30) scale are reviewed by Mente and Maroncelli. Despite several differences in opinion obvious in these papers, an adequate treatment of at least the nonspecific components of solvatochromism would seem to be just aroimd the comer . Finally, a suggestion should be mentioned on using the calculated it values taken from ref 55 as a descriptor of nonspecific solvent effects. However, this is not meaningful since these values are just a particular blend of inductive, dispersive, and dipole-dipole forces. 

The PSP approach [39-43] is a novel predictive thermodynamic framework, which combines elements from the solubility parameter approach [112,114,116,126] detailed earlier, the solvatochromic/ LSER approach [127-133], and the COSMO-RS theory of solutions [134-136]. It retains the simplicity of the solubility parameter approach, it uses molecular descriptors that can be mapped one to one to the Abraham/LSER descriptors, and these descriptors are derived from the moments of the a-profiles of the quantum mechanics-based COSMO-RS model. Because of this combination, the PSP approach has a broader range of applications compared to each of the earlier three... 

The solute descriptors such as Jt, a, and P can be extracted from the hterature [2-5] and are in most cases empirically determined by spectroscopic measurements. Their calculation is based on the shift of absorption bands of the given solutes due to varying solvatochromic effects when the dipolar or hydrogen-bond donor /acceptor properties of the solvent mixture used for spectroscopy are altered. Hence, they are not true thermodynamic data. Abraham and co-workers also determined descriptors from GC retention data and octanol/water partition coefficients [6-9]. With the help of such descriptors, the so-called solvation equation (Eq. 5) can be set out [10,11] ... 

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. 

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