Solvation ability indices

Since these complications have prevented a quantitative estimation of the enthalpies and entropies of keto and enol forms in different solvents, a quantitative relationship between AG or Ki, and the solvating ability or polarity of the solvent (expressed in physical characteristics such as the relative permittivity e, the dipole moment fi, the refraction index n, etc.) has not been possible either. Such equations would have the form of Eq. (4-26) ... 

For oxides of the type MeO, there exists a linear dependence of the solubility product index of the oxide against the inverse-squared radius of the metal cation. The slopes of these plots in the melts based on alkali-metal halides and alkaline-earth metal halides are stated to be approximately the same. This can give evidence that, at high temperatures in the order of 1000 K, the changes in solvation ability of the ionic melts, proceeding from one melt to another, are close for different cations with radii in the order of 0.1 nm (from 0.74 nm for Mg2+ to 1.38 nm for Ba2+). An increase in the melt temperature... 

TABLE 3.9 Indexes of Solvent Solvation Ability Polarity and Ji, Electron Pair Donicity, DN and /, Hydrogen Bond Donicity, AA and a, and the Softness Parameter, /t... 

Especial points which emerge from these studies include (a) the almost complete absence of reactivity of the hydroxy-groups of simple carbohydrates in water, which is attributed to their powerful solvation by water preventing a close approach of any other solute and (b) the ability of ester groups to interact with proton-acceptors. The refractive index tests, examination of m.p. or b.p., and infrared spectra of certain mono- and poly-esters appear to be interpreted most simply by assuming the formation of weak CH bonds by ester groups under the activating influence of the adjacent (5—0 double bond. These bonds can account for certain properties of l 2-diesters and for the adsorption of proton-acceptor solutes by cellulose acetates. 

Solvent permittivity — is an index of the ability of a solvent to attenuate the transmission of an electrostatic force. This quantity is also called the -> dielectric constant. -> permittivity decreases with field frequency. Static (related to infinite frequency) and optical op (related to optical frequencies) permittivities are used in numerous models evaluating the solvation of ions in polar solvents under both static and dynamic conditions. Usually the refractive index n is used instead of op (n2 = eop), as these quantities are available for the majority of solvents. The theory of permittivity was first proposed by Debye [i]. Systematic description of further development can be found in the monograph of Frohlich [ii]. Various aspects of application to reactions in polar media and solution properties, as well as tabulated values can be found in Fawcetts textbook [iii]. 

Perfluorinated solvents exhibit extremely low polarities, which can be quantified in many ways. As analyzed elsewhere, one of the best scales in terms of modeling the ability of a solvent to solvate or complex a solute or transition state involves the shift of the absorption maximum of a perfluoroheptyl-substituted dye. This dye was optimized to be soluble in both fluorocarbons and very polar solvents such as DMSO (dimethyl sulfoxide). Over 100 solvents have been assayed, and some of the resulting Ps or Spectral Polarity Index values are given in Table 3.3. 

This behavior may be interpreted in terms of the operation of different solvation mechanisms snch as electronic polarizability, dipole density, and/or hydrogenbonding (HB) ability. For instance, the main physical difference between 71 and Ej(30), in the absence of HB interactions, is claimed to lie in different responses to solvent polarizability effects. Likewise, in the relationship between the 71 scale and the reaction field functions of the refractive index (whose square is called the optical relative permittivity ) and the relative permittivity, the aromatic and the halogenated solvents were found to constitute special cases.This feature is also reflected by the polarizability correction term ineqn. [12.1.1] below. For the selected solvents, the various polarity scales are more or less equivalent. An account of the various scales has been given by Marcus, and in particular of ti by Laurence et al. and of Ej by Reichardt. ... 

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