Solute-solvent interactions hydrogen bonding

The nature of solute-solute and solute-solvent interactions is dependent on the solvent environment. Solvent influences the hydrogen-bonding pattern, solute surface area, and hydrophilic and hydrophobic group exposures. 

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

The 7t scale of solvents (Kamlet-Taft scale) is intended to represent the solute-solvent interactions in the absence of strong forces such as hydrogen-bonding or dipole-dipole interactions. It is based on the n to n absorption of a series of seven solutes (six uitroaromatics, such as 4-uitroanisole, A, A -diethyl-3-nitroaniline, 1-ethyl-4-uitrobeuzeue, etc., aud 4-dimethylamiuobenzophenone) [51-53]. 

It is important to know the influence of the physicochemical parameters of the mobile phase (dipole moment, dielectric constant, and refractive index) on solvent strength and selectivity. The main interactions in planar chromatography between the molecules of the mobile phases and those of solutes are caused by dispersion forces related to the refractive index, dipole-dipole forces related to the dipole moment, induction forces related to a permanent dipole and an induced one, hydrogen bonding, and dielectric interactions related to the dielectric constant. Solvent strength depends mainly on the dipole moment of the mobile phase, whereas the solvent selectivity depends on the dielectric constant of the mobile phase. 

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

The parameters a and p indicate the capacity of a solvent to donate or accept a hydrogen bond from a solute, i.e., the solvent s hydrogen bond acidity or basicity. % is intended to reflect van der Waals-type solute-solvent interactions (dipolar, dispersion, exchange-repulsion, etc.). Equation (43) was subsequently expanded to include a term representing the need to create a cavity for the solute (and thus to interrupt solvent-solvent interactions).188 For this purpose was used the Hildebrand solubility parameter, 5, which is defined as the square root of the solvent s energy of vaporization per unit volume.189 Thus Eq. (43) becomes,190... 

After these preliminary remarks, the term polarity appears to be used loosely to express the complex interplay of all types of solute-solvent interactions, i.e. nonspecific dielectric solute-solvent interactions and possible specific interactions such as hydrogen bonding. Therefore, polarity cannot be characterized by a single parameter, although the polarity of a solvent (or a microenvironment) is often associated with the static dielectric constant e (macroscopic quantity) or the dipole moment p of the solvent molecules (microscopic quantity). Such an oversimplification is unsatisfactory. 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

As implied by the dipole orientation effect on the solvent dependence of VH-f e reaction field seems to be the major solute-solvent interaction mechanism. However, specific interactions, particularly hydrogen bonding, also are important. Neither finding is particularly surprising since the presence of a fluorine almost inevitably results in the solute molecule being polar. 

In the absence of charges and dipoles, short-range effects such as hydrogen bonding and hydrophobic hydration become important. The complex solute-solvent interactions in such cases, however, are not well understood. In addition there is the possibility of ion or ion-pair catalysis. Especially in cases with solvents of low dielectric constant, it has been observed that k3 rather than log k3 becomes linear in cs... 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

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