Raman spectra and solution structure

One type of Raman study of solutions concentrates on water-water bonding as it is affected by the presence of ions. Hydrogen bonds give Raman intensities, and the variation of these with ionic concentration can be interpreted in terms of the degree and type of structure of water molecules around ions. The CIO4 ion has often been used in Raman studies to illustrate stmcture-breaking effects because it is a relatively large ion. 

In studies of the spectra of intramolecular water and how they are affected by ions, new Raman peaks can be interpreted in terns of the model of solvation suggested by 

Bockris in 1949 and by Frank and Evans in 1957. According to these workers, there are two regions. One is in the first (and for polyvalent cations, the second) layer near the ions, where water molecules are tightly bound and give rise to new frequencies. Such waters accompany the ion in its movements in the solution. 

There is also a broken-structure region outside the first one to two layers of water molecules around the ion. Here the solvating waters are no longer coordinated, as in the bulk, by other waters, because of the ion s effect, but they are outside the primary hydration shell, which moves with the ion. Such intermediate waters, though partly broken out of the bulk water structure, do not accompany an ion in its diffusional motion. 

Studies consistent with these ideas were first performed by Walfren in 1971 in H2O-D2O mixtures. Concentrations of 1 to 4 mol dm were employed to get measurable effects. Thus, at 4 mol dm , some 40-50% of the water present is at any moment in the primary hydration sheath  

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