Spectroscopic studies of solvents and solvation

Early work was based on the ultraviolet spectra of halide (especially iodide) ions [1,2]. It was already known that intense p s type spectra were exhibited by these ions in alkali-halide crystals and in aqueous solutions. We found that these bands were remarkably sensitive to changes in solvent, and hence could be used as a method for studying primary solvation. 

Another early development was the discovery that ion-pair equilibria and rates could be precisely studied by electron spin resonance (ESR or EPR) spectroscopy. These results, together with others using this specialised technique, are discussed in section 3.4. Similarly, it was found that the NMR chemical shifts for nuclei of ions such as or Na varied markedly with solvent changes, as did the H resonances of protic solvents. However, as shown in section 3.5, structural information was difficult to obtain and there has been much speculation regarding the correct interpretation of the observed shifts (see section 3.8). 

More recently, vibrational spectroscopy has been used systematically, especially for neutral solutes. Certain polar groups in solutes (such as 

In section 3.8 an attempt is made to link results from this wide range of studies, and to show how they overlap with results from other molecular level studies. Finally, in section 3.9, a brief introduction to the applications of these techniques in biological systems is given. 

Throughout, attention is focused primarily on structural parameters rather than on relaxation or rate processes. Spectroscopic studies are, of course, of very great importance in the latter field, but it seemed better to concentrate here on structure, since a good understanding of structure must precede a proper interpretation of rates. 

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A UV-visible spectroscopic study of 3 and related substances revealed a strong solvatochromic effect, which served as the basis of the establishment of a solvent polarity scale (Buncel and Rajagopal, 1989, 1990,1991). The theoretical study of Rauhut et al. (1993) was based on AMI methodology (Dewar and Storch, 1985,1989) but used a double electrostatic reaction field in a cavity, dependent on both the relative permittivity and the refractive index. Nuclear motions interact with the medium through the relative permittivity, but electronic motions are too fast only the extreme high-frequency part of the dielectric constant is relevant. These authors were able to evaluate solvent-specific dispersion contributions to the solvation energy. The calculations reproduced satisfactorily the experimental solvatochromic results for 3 in 29 different solvents. The method has also been successfully applied to other solvatochromic dyes, including Reichardt s .j,(30) betaine. 

ILs is in the range of 230-250 nm) which make them suitable to be used as solvents for spectroscopic measurements especially in the visible region. Because of their ionic origin, ILs allow the coordination of a complex compound in a liquid sfafe to be sfudied. An additional advanfage of ILs is that their solvating properties can be designed in such a way that differently coordinating solvents are obtained. A lot of examples can be presented on spectroscopic studies with lanthanides and actinides. 

In addition, recent spectroscopic studies of Column et al., dealing with the etheral solvation of LiHMDS, revealed no support for the often-cited correlation of reduced aggregation state with increasing strength of the lithium-solvent interaction. [19] Moreover, exact spectroscopic data could be obtained for the structures of complexes of lithium bases and salts and their dependence on the concentra-... 

The elimination of these secondary effects and hence a clearer indication of the correlation between the Mossbauer parameters and the donicity of the solvent can be expected from the investigation of systems in which the relative permittivity is kept constant or nearly so. For this reason, Vertes and Burger [Ve 72] continued their Mossbauer spectroscopic studies of solvation by comparing the Mossbauer parameters measured in mixtures of a given inert solvent with the solvents under examination. 

The ability of vibrational spectroscopy (infrared and Raman) to probe the different interactions which take place in a solution is well known. From the classic reviews by Irish and Brooker [1] and Gardiner [2], both published in 1977, which cover the Raman spectroscopy of ionic interactions in aqueous and nonaqueous solutions, a number of works have appeared reviewing vibrational spectroscopic studies [3-13]. However, many of these embrace only partial aspects or they are exclusively devoted to one specific type of solution (aqueous or nonaqueous) and do not include topics that will be discussed in the present chapter, such as solutions at high pressures and temperatures, electrolyte polymers, or solutions in the glassy state. The aims of this review are, as its title indicates, the ion-ion interactions whose theoretical aspects have been recently approached in a comprehensive monograph by Barthel and co-workers [14]. This means that aspects related to the Raman spectroscopic studies of the solvent s structure or the interactions between the solute and the solvent (ion hydration or, in general, solvation) will be treated briefly. 

R. H. Erlich and A. 1. Popov. 1971. Spectroscopic studies of ionic solvation. X. Study of the solvation of sodium ions in nonaqueous solvents by sodium-23 nuclear magnetic resonance. Am Chem Soc 93(22) 5620-5623. 

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

Although continuum solvation models do appear to reproduce the structural and spectroscopic properties of many molecules in solution, parameterization remains an issue in studies involving solvents other than water. In addition, the extension of these approaches to study proteins embedded in anisotropic environments, such as cell membranes, is clearly a difficult undertaking96. As a result, several theoretical studies have been undertaken to develop semi-empirical methods that can calculate the electronic properties of very large systems, such as proteins28,97 98. The principal problem in describing systems comprised of many basis functions is the method for solving the semi-empirical SCF equations ... 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

Table VII collects the results for all monovalent ion systems for which spectroscopic data are available. Studies of preferential solvation are still at a stage comparable to the establishment of Raoult s and Henry s laws for binary nonelectrolyte solutions. Correlation with thermodynamic data is encouraging for isodielectric solvent systems, but further consideration of the electrostatic terms necessary in the discussion of other systems is required. It is hoped that this present work, which coordinates, correlates, and advances progress made by other workers (7, 18,19, 20, 45, 46, 61, 62, 66, 67, 68), will stimulate systematic experimental investigations of suitable systems by both spectroscopic and thermodynamic methods.

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