Linear solvation free energy relationships

B. Solvent Effects and Linear Solvation Free Energy Relationships.43... 

Usually, linear solvation free energy relationships adopt the form of Eq. 3, ... 

By the use of the multiple linear regression, Abraham and Rogers [16] determined the parameters of the Abraham linear solvation free energy relationship (LSFER) equation [17,18]... 

M. Kamlet and R.W. Taft, Linear Solvation Free Energy Relationships. Local Empirical Rules or Fundamental Laws of Chemistry Acta Chem. Scand., 1985,339,611. 

V. Ventosa, J. Llibre, J. Torras, C. Rovira and J. Veciana, Modeling of the solubilities of organic compounds in supercritical fluids using the linear solvation free energy relationship theory... 

Before analyzing concrete cases, a theoretical introduction with respect to some basic methodological issues will be provided in the next section. The methods to be applied are quantum mechanical calculations, which are different from the use of the linear solvation free energy relationships for predicting the equilibrium constant, as described in Chapter 12 of [1]. It is to be mentioned here that the forthcoming discussion of the theoretical results for the prototropic tautomerism always refers to ground-state processes. 

The second aspect is more fundamental. It is related to the very nature of chemistry (quantum chemistry is physics). Chemistry deals with fuzzy objects, like solvent or substituent effects, that are of paramount importance in tautomerism. These effects can be modeled using LFER (Linear Free Energy Relationships), like the famous Hammett and Taft equations, with considerable success. Quantum calculations apply to individual molecules and perturbations remain relatively difficult to consider (an exception is general solvation using an Onsager-type approach). However, preliminary attempts have been made to treat families of compounds in a variational way [81AQ(C)105]. 

Abraham, M.H. and Acree, Jr. W.E., Comparative analysis of solvation and selectivity in room temperature ionic liquids using the Abraham linear free energy relationship, Green. Chem., 8,906,2006. 

Part of the motivation behind so straightforward an approach derives from its ready application to certain simple systems, such as the solvation of alkanes in water. Figure 11.8 illustrates the remarkably good linear relationship between alkane solvation free energies and their exposed surface area. Insofar as the alkane data reflect cavitation, dispersion, and the hydrophobic effect, this seems to provide some support for the notion that these various terms, or at least their sum, can indeed be assumed to contribute in a manner proportional to solvent-accessible surface area (SASA). 

Figure 11.8 Approximately linear relationship between solvation free energy and solvent-accessible surface area for linear and branched alkanes. A best fit line passing through zero has a slope of...

The basic requirement for the development of a more generally applicable solvent concept is the need to try to separate the various factors responsible for the solvating power of a solvent. It is important to find criteria for the solvents character that can be correlated not only to salt solubility and apparent conductivity but also to the impact of the solvents on the thermodynamics and kinetics of the electrochemical reactions. There are several approaches to defining a typical solvent property that can represent its polarity and be correlated to the thermodynamics and kinetics of reactions conducted in its solutions (i.e., a linear free-energy relationship). A comprehensive review of such approaches by Reichardt [12] divides them into three categories ... 

A linear free-energy relationship has been suggested 113 by Abraham to describe various partition processes of molecules. Eq. (12.14) shows the so-called linear solvation equation ... 

The term classical QSAR is often used to denote the - Hansch analysis, -> Free-Wilson analysis, -> Linear Free Energy Relationships (LFER) and -> Linear Solvation Energy Relationships (LSER), i.e. those SRC approaches developed between 1960 and 1980 that can be considered the beginning of the modern QSAR/QSPR methods. 

Cohen, L.A. and Jones, W.M. (1963). A Study of Free Energy Relationships in Hindered Phenols. Linear Dependence for Solvation Effects in Ionization. J.Am.Chem.Soc., 85,3397-3402. 

Structure/Response Correlations, Hansch analysis, Hammett equation, Free-Wilson analysis. Linear Solvation Energy Relationships, Linear Free Energy Relationships, group contribution methods, substituent descriptors, extrathermodynamic approach, and biological activity indices. 

Moreover, there is an indication that the surface area coefficient may be different for small and large solutes [69]. Unfortunately, reliable solvation energies are known for small molecules only, where there is a linear relationship between the surface area and the volume. Consequently, it became common practice to include dependence on the surface area only. Furthermore, it is difficult to evaluate the van der Waals interactions between the solute and solvent in continuum models where there are no solvent molecules present. Thus it is generally assumed that the van der Waals energy is proportional to the surface area. Consequently, the nonelectrostatic solvation free energy is expressed as a simple linear relationship ... 

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