Specific Solute-Solvent Associations

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]. 

Thus picric acid reacts with n alcohol molecules forming a complex (Equation 13) which rearranges to form an ion pair (Equation 14) which then dissociates (Equation 15). The processes involved in Equations 13 and 14 are conditioned by specific solute-solvent interactions, while the process depicted by Equation 15 is controlled by electrostatics. K3 is equal to 1/KA, where KA is the theoretical association constant given by Equation 2. It can be easily shown that the experimental association constant is given by the equation ... 

As a part of the continuing studies of the effect of dipolar aprotic solvent plus water mixtures on these specific solute-solvent interactions, we have examined the first and second dissociation steps of glycine and computed their associated thermodynamic quantities in 10 mass % tetrahydrofuran-water (THF-H20) solvents (dielectric constant, c = 71.8 at 298.15 K), 30 mass % THF-H20 (c — 56.6 at 298.15 K), and 50 mass % THF-H20 (c — 40.0 at 298.15 K) from 278.15 to 328.15 K. 

The term has previously been applied to the effect on chemical shift of specific solute—solvent interactions in solution, and it has been pointed out that these should cause a small paramagnetic solvent shift in this usage, shifts due to anisotropy effects associated with specific interactions are included in the term. In this discussion, however, we shall take to represent the effect of solvent anisotropy in a geometrically specific solute—solvent orientation which has been brought... 

Proper inclusion of the solvent into the calculations is unfortunately quite difficult [46]. One can use classical molecular dynamics or Monte Carlo simulations, classical continuum models based on the Poisson-Boltzman equation, and quantum-chemical studies using various variants of the Self Consistent Reaction Field (SCRF) approach at the semiempirical or ab initio level. There are serious approximations associated with these methods. Continuous models neglect the specific solute-solvent interactions which are very important for polar solvent. Classical methods neglect the changes in the electronic structure of the solute due to the solvent effects. These uncertainties can be illustrated using the predicted solvation energy of adenine treated by various modem approaches. The calculated values vary from -8 to -20 kcal/mol [68]. 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

The swollen specific volume n p (cm /g) is defined when an anhydrous biomacromolecules essentially expand in suspended or dissolved in solution because of solvent association, and... 

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. 

Typically, solute-solvent interactions are divided into two broad categories Specific and nonspecific interactions. Specific interactions include phenomena such as hydrogen bonding and ji-ji interactions, which depend on the presence of particular functional groups or steric structures. They are short ranged, and are specific in the sense that they involve individual solvent species within the first solvation shell of the liquid. In contrast, nonspecific interactions represent interactions that are not associated with the presence of individual functional groups. In molecular liquids, these include dispersion and electrostatic interactions, such as dipole-dipole forces. We will discuss the nature of each type of interaction in ionic liquids in the sections that follow. 

There is also direct IR spectroscopic evidence that addition of a basic co-solvent to aqueous solutions reduces the percentage of non-associated free OH groups which are necessary for hydrogen-bonding to Sn reactants [585], When basic co-solvents are added, they scavenge the free OH groups in water, thus lessening the specific protic solvent effect [585]. 

There are several dilute solution viscosity quantities used in the determination of the intrinsic viscosity. The size of macromolecules in solution is associated with an increase in viscosity of the solvent brought about by the presence of these molecules. Relative viscosity is a dimensionless quantity representing a solution/solvent viscosity ratio, r], = 7j/i o. where 17 and 170 are the solution and solvent viscosities, respectively. The specific viscosity yj p = 77 1 is the fractional increase in vis-... 

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