Spatial decomposition schemes for infrared spectra reveal the importance of both dipolar couplings and correlations in particle motion in aqueous solutions of Na ⁺ and Cl ⁻ .

Vibrational spectroscopy is an ideally suited tool for the study of solute hydration. Nevertheless, water is commonly considered by spectroscopists a difficult solvent to work with. However, by using the isotopic dilution technique, in which a small amount of D₂O is introduced into H₂O or vice versa with formation of semiheavy water (HDO), many technical and interpretative problems connected with measurement of infrared spectra of water may be circumvented. Particularly, the isotopic decoupling of stretching vibrational modes greatly simplifies interpretation of the spectra. Systematic studies conducted in several laboratories since the 1980s up to the present day have provided a vast amount of data, concerning mainly ionic hydration. Many of these experiments have been performed in our laboratory. The analysis method we applied is based on the quantitative version of the difference spectra technique and allows separation of the spectrum of solute-affected HDO from the bulk solvent. This review illustrates the development of vibrational spectroscopy of HDO and spectral analysis methods over the years, as well as summarizes the results obtained for ionic and nonionic solutes, including some general hydration models formulated on their basis.