Investigation of very fast motions in electrolyte solutions by far infrared spectroscopy

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Abstract
The far infrared spectra of solutions of a number of alkali halide salts in protic (methanol) and dipolar aprotic solvents (acetonitrile, acetone, dimethyl sulfoxide) have been investigated in the frequency range 25–650 cm−1. Special attention has been paid to detect weak bands in making the difference between the absorption spectrum of the solution and that of the pure solvent. These spectra are characterized by several absorption peaks spread all over the FIR spectral range. Introducing a chemical model of the electrolyte solution, it is shown that the time correlation function at the origin of the absorption phenomenon is governed essentially by three processes, namely, (i) the ion-induced dipole mechanism, (ii) the ionic association, and (iii) the relaxation of the solvent molecules within the ionic solvation shell. A band shape analysis, based upon the Mori theory, permits us to identify the molecular motions which give rise to specific frequency modes on the absorption spectrum. In particular, the role played by the time scale separation between the fast dynamics occuring within the first solvation shell of ionic species and the time spent to interconvert inner and outer solvation shells, is emphasized.