Digital Raman spectra of solutions of NH4NO3 (6.4, 8.7m), AgNO3 (1–4m), NaNO3 (1–5.5m), KNO3 (1–3m), and Ca(NO3)2 (1–2.6m) in liquid ammonia have been analyzed by least squares curve fitting. Perturbations of the nitrate ion spectra, particularly splitting of ν3, clearly indicate the presence of ion pairs in all of the solutions. The effects are similar to those observed for concentrated aqueous solutions, although the spectra generally are simpler, indicating only one environment for nitrate is important in all except Ca(NO3)2 solutions. The ion pairs appear to involve direct cation–anion contact, and the splitting of the nitrate ν4 correlates well with the ionic potential of the cations. The failure to observe a splitting of ν4 for potassium, ammonium, and silver cations is attributed to their low polarizing power, suggesting that the ν4-splitting criterion for inner-sphere complex formation fails with weakly polarizing cations. The splitting of nitrate ν3 by ion pairing in liquid ammonia is considerably less than that observed for aqueous solutions, and the same effect has been reported for alkali metal nitrates in glassy water and ammonia matrices at 12 °K. The ν3 splittings of univalent nitrates show an inverse correlation with the heats of ammoniation of the cations except in the case of the small lithium ion, suggesting that charge transfer from ammonia to metal ion decreases the cation polarizing power. Two weak, broad low frequency features (296, 329 cm−1) are assigned as silver–ammonia stretching modes, and this is the only system which exhibits vibrations for the first coordination sphere. This is compared with the behavior of aqueous solutions containing Ag(NH3)2+, and a fluxional first coordination sphere is suggested for Ag(I) in liquid ammonia. A band at 260 cm−1 is assigned to a calcium nitrate quasilattice mode, and this is the only system for which such a vibration is observed. The general features of the Raman spectra of electrolyte solutions in water and liquid ammonia are compared.
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Lundeen et al. (1975) studied this question.