Spectroscopic Approaches to Obtaining Information on Structures near an Ion

SPECTROSCOPIC APPROACHES TO OBTAINING INFORMATION ON STRUCTURES NEAR AN ION 

One point should be noted here the importance of using a 10% D2O mixture with HjO in IR spectroscopic measurements because of the properties of HOD, which contributes a much more clearly resolved spectrum with respect to 0-D. Thus, greater clarity (hence information) results from a spectrum in the presence of HOD. However the chemical properties (e.g., dipole moment) of HOD are very similar to those of H O. 

Raman spectra have a special advantage in analyzing species in solution. This is because the integrated intensity of the spectral peaks for this type of spectroscopy is proportional to the concentration of the species that gives rise to them. From observations of the intensity of the Raman peaks, equilibrium constants A can be calculated and hence AG°s from the thermodynamic equation K = can be 

In obtaining information on solvation that can be deduced from IR spectra, the first thing that must be understood is that the raw data, the peaks and their frequencies, seldom speak directly but need to be decoded. Spectra in the IR region are mainly messages fed back from the solvent, and it is from the interpretation of evidence for changes in the solvent s libration and rotation when ions are introduced (rather than any new peaks) that information on solvation may sometimes be drawn. One has to take the spectrum of the solvent, then that of the solution, and subtract them to obtain the effect of the solute (Fig. 2.19). Vibration spectra have frequencies in the region of 10 s but it is usual to refer to the inverse of the wavelength, that is, the wavenumber, P= 1/A. Since Cq = vA, then /X = v/cq. It turns out then that the wavenumbers of most covalent bonds are numerically in the thousands.  

Intramolecular effects can be detected in the near infrared or high-frequency region (P-1000 cm ). Intermolecular effects are seen in the far infrared or low-frequency region (P down to 100 cm ). Early measurements showed that ions can cause new peaks to arise that are at distinctly higher wavenumbers than those in pure water. The explanation proposed is that some of the hydrogen bonds present in pure 

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