Solute-solvent interaction, specific

Specific solute-solvent interactions involving the first solvation shell only can be treated in detail by discrete solvent models. The various approaches like point charge models, siipennoleciilar calculations, quantum theories of reactions in solution, and their implementations in Monte Carlo methods and molecular dynamics simulations like the Car-Parrinello method are discussed elsewhere in this encyclopedia. Here only some points will be briefly mentioned that seem of relevance for later sections. 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

The QSAR technique, widely developed by Kamlet, Taft and coworkers38,98 for the prediction of specific solute-solvent interactions, has been used to predict the different solute-solvent contributions to property variations of compounds. The influence of solvent on the relative basicity of dipolar trimethylamines has been recently studied a descriptor was developed to describe a unique solute-solvent interaction involving dipolar amines99. 

This model assumes the absence of specific solute-solvent interactions and is based upon a linear relationship between the free energy of solution and solute surface area. It assumes that the overall solubility is simply the sum of the solubilities in the individual solvent components. This model treats the cosolvent and the water as distinct entities and neglects any interaction between them [19,145,226,253,261]. 

Table II shows the average end-to-end distance over 20 ps for mannitol and sorbitol in vacuuo and in solution of an argon-like (L-J) solvent and SPC/E water. The average lengths all indicate sickle shapes, except for mannitol in water which is fully extended. This points to a specific solute-solvent interaction between mannitol and water, not just an unspecific solvent effect that is not present in solvent other than water. The model non-aqueous solvent is very artificial, but it should represent the main features of the class of non-polar, spherically symmetric solvents.

Specific solute-solvent interactions, such as hydrogen bonds, undergo a significant change in the comse of SD. This was first observed by Fonseca and Ladanyi in the case of SD in methanoF and has since then been seen in a munber of other simulation studies. " ... 

Classification of Solvents in Terms of Specific Solute-Solvent Interactions Parker divided solvents into two groups according to their specific interactions with anions and cations, namely dipolar aprotic solvents and protic solvents (Parker, 1969). The distinction lies principally in the dipolarity of the solvent molecules and their ability to form hydrogen bonds. It appears appropriate to add to these two groups a third one, namely, the apolar aprotic solvents. 

This assumes that no significant solute-solute interactions or strong specific solute-solvent interaction occurs. The Kd value is constant when the distributing substance does not chemically react in either phase and temperature is kept constant. 

The effect of added water was also calculated for the five tautomeric forms. The results showed that the spectrum corresponded principally to the dithiol 8 (Figure 1) and monothiols 5-7, which overlap the best of the features enhanced on the spectrum measured at high water content solution. These results suggest that a specific solute-solvent interaction is present favoring the dithiols and monothiols isomers indicating that the hydrogen bond in solute-water is a 1 n complex. 

Resolution attempts in cholesteric phases. Hie body of data collected to date clearly Indicates that unless specific solute-solvent Interactions occur, the stereochemistry of reactions will be little affected by chiral solvents, whether they be macrosco-pically ordered or isotropic (48-50). In fact, the low optical activity In products from irradiations in cholesteric solvents may arise from the ability of a chiral mesophase to produce circularly polarized light from normal incident radiation (51). 

Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule may be obtained by the use of molecular mechanics simulations. At the semi-empirical MO level this approach has been successfully used to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

A weakness of this model is that the separation of the electrostatic and the so-called specific solute-solvent interactions is not defined. In practice, two main approaches are used to account for solvent effects on the spin-spin coupling constants the continuum and the supermolecular methods. The combined MD/QM approach is rarely used for the purpose, since calculations of the spin-spin coupling constants are much more time consuming than those of the shielding constants and the MD/QM approach is too expensive for the former. 

As shown in Figure 2.5, continuum solvent models (PCM) reproduce satisfactorily solvent effects on the aN parameter only for aprotic solvents (bulk effects), whereas there is a noticeable underestimation of solvent shifts for protic solvents (methanol and water). In these media also specific solute-solvent interactions have to be taken into account. 

The validity of the two approaches sketched above has been quite amply tested against the ability of reproducing various molecular properties of hydrogen-bonded systems (see elsewhere in this book) including vibrational spectroscopies [11,54-56], For example, in Figure 2.11 calculated versus experimental IR spectra of gallic acid in water solution are reported for different levels of treatment of the specific solute-solvent interaction [54],... 

The main advantage of the MFA is that it permits one to dramatically reduce the computational requisites associated with the study of solvent effects. This allows one to focus attention on the solute description, and it consequently becomes possible to use calculation levels similar to those usually employed in the study of systems and processes in the gas phase. Furthermore, in the case of ASEP/MD this high level description of the solute is combined with a detailed description of the solvent structure obtained from molecular dynamics simulations. Thanks to these features ASEP/MD [8] enables the study of systems and processes where it is necessary to have simultaneously a good description of the electron correlation of the solute and the explicit consideration of specific solute-solvent interactions, such as for VIS-UV spectra [9] or chemical reactivity [10]. 

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