Solvents electrostatic factor

Dack (4) has suggested that solvents be elassified on the basis of the product ep., the electrostatic factor. There seems to be no theoretical basis for this function [indeed, the dipole moment appears as ix in physical theory, as in Eqs. (8-10) and (8-6)], but ep. does contain more information than either of the quantities alone,... 

These changes in regio- and stereochemistry are likely due to conformation changes and electrostatic factors within the cavity. The intrazeolite oxidations can be improved by use of fluorocarbon solvents, owing to an enhanced lifetime of 102 and to improved occupancy of the cavity by hydrocarbons in this solvent.176... 

To see how the data can be used to provide insights into the spin trapping process, PBN would correspond to A with Ea = 1.5 V, and acetate ion to A with E° = 1.5 V (Table 5 gives 1.6 V in acetonitrile, and 1.5 V is therefore somewhat too low, but then it is presumably adequate for dichloromethane). In dichloromethane, the OsvCl6-PBN reaction is estimated to be very fast, more than 6 powers of ten faster than the OsvClg-acetate ion reaction, whereas in acetonitrile the absolute rates are still high but the ratio is only about 50. This difference resides only in the difference between electrostatic factors and illustrates the problems of understanding ET reactions in solvents of even lower dielectric constant such as benzene. 

The dielectric constants play a particular role in the characterization of solvents. Their importance over other criteria is due to the simplicity of electrostatic models of solvation and they have become a useful measure of solvent polarity. Since both the dielectric constant r and the dipole moment p are important complementary solvent properties, it has been recommended that organic solvents should be classified according to their electrostatic factor EF (defined as the product of 8 and p). 

Since both the relative permittivity and the dipole moment p are important complementary solvent properties, it has been recommended that organic solvents should be classified according to their electrostatic factor EE (defined as the product of r... 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

Coates and Jordan, 1960 Herskovits et cU., 1961) suggest that the phosphate groups of the DNA molecule are extensively paired to counterions in methanol solution, such that a /D (Section IV,B,1) is only roughly one-sixth of its value in water. This applies to the disrupted conformation of the DNA molecule as it exists in methanol solutions, but it may be assumed that a /D is not greatly different for the hypothetical native helical conformation in methanol. It can be concluded, therefore, that there is a net reduction in electrostatic repidsive interactions, and in electrostatic free energy, for the native conformation of DNA in methanol compared to water. Similar considerations apply in ethanol, whose dielectric constant is still smaller than that of methanol. Therefore, electrostatic factors alone would tend to stabilize the helical conformation in these nonaqueous solvents, and in spite of this, the structure is disrupted. 

DIP-chymotrypsin in 0.14 M KCl and of enzyme electrostatic factor high in denaturing solvent (1962)... 

Dack (1976, p. 98) has listed a classification of solvents on the basis of electrostatic factor (EF) values. These values separate solvents into four classes, as outlined in Table 2. EF values represent the product of the relative permittivity and the dipolar moment, and thus they take account of the influence of both of these properties on the electrostatic solvation of solutes. It can be predicted that solvents in classes 1, II, and 111 will have little influence in solvating humic macromolecules. Dack placed water in Class III on the basis of its structure rather than of its EF value, but on the basis of its EF value it is regarded as a Class IV solvent for the purposes of the present discussion. 

TABLE 2. Classification of Solvents on the Basis of Electrostatic Factor (EF) Values... 

The product of dipole moment and dielectric constant is called the electrostatic factor and it is a means of classifying solvents according to their polarity. 

Dielectric constant A simple measure of solvent polarity (the electrostatic factor is a uoduct of dielectric constant and dipole moment). The electrical conductivity of solvent indicates if there is a need to earth (or ground) the equipment which handles solvent to prevent static spark ignition. Admixtures affect solvent conductivity. These are most important in electronics industry. 

The interactions between the solvent and solute, as discussed above, are the result of a number of different specific (coordination, hydrogen bonding) and non-specific (electrostatic) factors therefore, it is not possible to find a single physical parameter characterizing the solvent, which in itself could rationalize the solvation process. Accordingly, it was necessary to introduce empirical parameters serving to characterize the solvent effect. 

Solvent potential. The averaged solvent electrostatic field, , is important for inhomogeneous media, such as enzymes, membranes, miscelles and crystalline environments systems. Due to the existence of strong correlations, such a field does not cancel out. This factor becomes an important contribution to solvent effects at a microscopic level. In a study of non-rigid molecules in solution, Sese et al. [25] constructed a by using the solute-solvent atom-atom radial distribution function. Electrostatic interactions in three-dimensional solids were treated by Angyan and Silvi [26] in their self-consistent Madelung potential approach such a procedure can be traced back to a calculation of . An earlier application of the ISCRF theory to the study of proton mechanisms in crystals of hydronium perchlorate both [Pg.441]

In the other cases, it is essential to examine if the electrostatic factor is important, for instance in weakly polar solvents that have small dielectric constants (e low) and when the reacting species are small (d low). 

When dealing with a reaction preferring one isomer over the other, it is important to remember that only small activation energy differences are needed to change a reaction from nonstereospecific (1 1) to highly stereospecific.However, given the number of factors affecting the steric selectivity such as catalysts, solvents, electrostatic field, it is difficult to predict a priori the steric outcome of a reaction. Furthermore, the nature of the diene may often alter the course of the reaction. ... 

A.A. Kossiakoff, J. Shpungin and M.D. Sintchak, Hydroxyl hydrogen conformations in trypsin determined by the neutron diffraction solvent difference map method Relative importance of steric and electrostatic factors in defining hydrogen-bonding geometries, Proc. Nat. Acad. Sci., USA, 87 (1990) 4468. 

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