Solvation and Solvent Structure

Yamamoto, Y. Uwate, and Y. Yamamoto, Inorg. Nucl. Chem. Lett., 1976, 12, 713. 

Caminiti, G. Licheri, G. Piccaluga, and G. Pinna, Chem. Phys. Lett., 1979, 61, 45. 

Systems in which specific solvent structural effects do not seem to affect kinetic patterns include the reactions of Ni + with malonate in aqueous d-ftuc-tose, some substitutions at [Fe(CN)5L] anions, and aquation of the [Cr(NH3)sBr] + cation in several series of alcohol-water mixtures. Deviations from a newly proposed correlation of rates with solvent DN and AN values (c/. above, refs. 24 and 25) have been used as a probe for specific solvent structural effects. - Ethylene carbonate and propylene carbonate have no structure 

Lachmann, I. Wagner, D. H. Devia, and H. Strehlow, Ber. Bunsenges. Phys. Chem., 

In most of the investigations mentioned so far in this section involving solvent mixtures it is likely that the primary solvation shell does not vary with solvent composition. However, there are occasions when variation of reactivity with solvent mixture composition is attributed to changes in primary solvation shell composition, as in the case of reaction of Ni + with ammonia in aqueous methanol. In the reaction of Be + with sulphate in aqueous DMSO, there is strong n.m.r. evidence for variation in primary solvation shell composition. In the reaction of Co with tetraphenylporphine in acetic acid-water the kinetics also reflect the presence of varying amounts of mixed solvates. By way of contrast, in reactions of copper(ii) with polythiaethers in methanol-water mixtures only Cuaq + reacts mixed solvates are claimed to be of negligible reactivity. In the reaction of chromium(iii) with edta in methanol- and ethanol-water mixtures, variation in pK for the Cr + has an effect on the formation rates, but the individual rate constants for the reactions of Cr + and of CrOH + with the ligand seem to be practically independent of solvent composition.  

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It is curious that the striking deviations of electrochemical kinetic behavior from that expected conventionally, which are the subject of this review, have not been recognized or treated in the recent quantum-mechanical approaches, e.g., of Levich et al (e.g., see Refs. 66 and 105) to the interpretation of electrode reaction rates. The reasons for this may be traced to the emphasis which is placed in such treatments on (1) quantal effects in the energy of the system and (2) continuum modeling of the solution with consequent neglect of the specific solvational- and solvent-structure aspects that can lead, in aqueous media, to the important entropic factor in the kinetics and in other interactions in water solutions. However, the work of Hupp and Weaver, referred to on p. 153, showed that the results could be interpreted in terms of Marcus theory, with regard to potential dependence of AS, when there was a substantial net reaction entropy change in the process. 

The material of this chapter is arranged in three sections, on reactions in pure (that is single) solvents, in mixed solvents, and in salt solutions. In each section the discussion covers a variety of solvent properties, including fundamental properties such as dielectric constant and composition, empirical solvent parameters such as Grunwald-Winstein Y and Reichardt t values, and more qualitative, elusive, or nebulous properties such as solvation and solvent structure. The aim is to provide convenient cross-referencing rather than a detailed and critical treatment. In some cases a less cursory mention will be found in Chapters 1—5 of this Part, or in Part II of Volume 2. 

Earlier analyses making use of AH vs. AS plots generated many p values in the experimentally accessible range, and at least some of these are probably artifacts resulting from the error correlation in this type of plot. Exner s treatment yields p values that may be positive or negative and that are often experimentally inaccessible. Some authors have associated isokinetic relationships and p values with specific chemical phenomena, particularly solvation effects and solvent structure, but skepticism seems justified in view of the treatments of Exner and Krug et al. At the present time an isokinetic relationship should not be claimed solely on the basis of a plot of AH vs. A5, but should be examined by the Exner or Krug methods. 

Other energetic components associated widi the solvation process include non-electrostatic aspects of hydrogen bonding and solvent-structural rearrangements like the hydrophobic effect. Despite many years of study, the fundamental physics associated with both of these processes remains fairly controversial, and physically based models have not been applied with any regularity in the context of continuum solvation models. 

Krestov, G.A. Thermodynamics of Solvation. Solutions and Dissolution Ions and Solvents Structure and Energetics, Ellis Horwood, Chichester, 1991. 

The papers in the second section deal primarily with the liquid phase itself rather than with its equilibrium vapor. They cover effects of electrolytes on mixed solvents with respect to solubilities, solvation and liquid structure, distribution coefficients, chemical potentials, activity coefficients, work functions, heat capacities, heats of solution, volumes of transfer, free energies of transfer, electrical potentials, conductances, ionization constants, electrostatic theory, osmotic coefficients, acidity functions, viscosities, and related properties and behavior. 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

The results for the free energies of transfer of cations do not present as clear a picture. They suggest that solvents such as DMSO which contain a relatively basic oxygen atom solvate cations better than does water. To account for these solvation differences, cation-dipole interactions and solvent structure-making or -breaking effects could be invoked. Some cations (e.g., R3NH+) could also be stabilized in DMSO because of their ability to act as hydrogen bond donors. 

Solvation and Solvent Electronic Structure. III. Quantum Theory. 

The remaining eighteen chapters address a wide range of electrolyte effects on the liquid-phase properties of mixed-solvent systems, including such diverse but interrelated topics as solvation and liquid structure ... 

The SASA approach makes no attempt to separate the free energy of solvation into distinct components, such as the ENP and CDS terms, but simply assumes the net solvation energy to be proportional to the SASA. In later sections we will consider models that separate these effects. Even there, though, by grouping cavity and solvent structural effects into the same term, one will not distinguish solvent structural effects that occur upon creating a cavity from those over and above the change at a solvent—vacuum interface. 

Thus there is a direct link between free energy of solvation and the structure of the solvent surrounding the solute. 

G. A. Krestov, Thermodynamics of Solvation Solution and Dissolution, Ions and Solvents, Structure and Energetics, Ellis Norwood Ser. Phys. Chem., Ellis Horwood, New York, 1991, p. 106. 

This is the most sophisticated (and computationally demanding) approach and involves the explicit determination of the electronic wavefunctions for both the solvent and solute. At present serious approximations relating to the size of samples studied and/or the liquid structure, and/or the electronic wavefunctions are necessary. A very successful scheme is the local-density-functional molecular-dynamics approach of Car and Parrinello that treats the electronic wave functions and liquid structure in a rigorous and sophisticated manner but is at present limited to sample sizes of the order of 32 molecules per unit cell to represent liquid water, for example. Clusters at low temperatures are well suited to supermolecular approaches as they are intrinsically small in size and could be characterized on the basis of a relatively small number of cluster geometries. Often, however, liquids are approximated by low temperature clusters in supermolecular calculations with the aim of qualitatively describing the processes involved in a particular solvation process. Alternatively, semiempirical or empirical electronic structure methods can be used in supermolecular calculations, allowing for more realistic sample sizes and solvent structures. Care must be taken, however, to ensure that the method chosen is capable of adequately describing the intermolecular interactions. 

The analysis of recent measurements of the density dependence of has shown, however, that considering only the variation of solvent structure in the vicinity of the atom pair as a fiinction of density is entirely sufficient to understand tire observed changes in with pressure and also with size of the solvent molecules [38]. Assuming that iodine atoms colliding with a solvent molecule of the first solvation shell under an angle a less than (the value of is solvent dependent and has to be found by simulations) are reflected back onto each other in the solvent cage, is given by... 

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