Vibrational Spectra and Structures of Ionic Liquids. IV. Isotopic Dilution of the Alkali Metal and Ammonium Nitrates

The isotopic isolation of the ¹⁴NO₃⁻ ion in molten Li¹⁵NO₃ has been found to produce a 3‐cm⁻¹ increase in the ${\nu }_2$ (out‐of‐plane) frequency. This is interpreted as the result of the decoupling of ¹⁴NO₃⁻ ions in a perturbed lattice structure. The shift direction and magnitude resembles that observed upon isotopic decoupling of ions in solid LiNO₃ and NaNO₃. This comparison, plus the recently reported similarity between the far‐infrared phonon spectrum of solid and molten NaNO₃, is contrasted with other spectroscopic observations which suggest a substantial structural modification upon melting. For example, a 50% ¹⁵N lithium nitrate sample displayed an infrared ${é}_2$ intensity anomaly which is thought to be characteristic of a stacked (orthorhombic) structure. It is suggested that most spectral features which seem to indicate substantial structural change upon melting may merely reflect the inevitable reduction in the symmetry of the cation field at the anion site as a consequence of cation vacancies. Several new Raman sightings are also reported, certain of which support the validity of earlier ATR infrared results in the ${\nu }_3$ range. The infrared and Raman spectra of NH₄NO₃ and ND₄NO₃ have also been determined guided by the assumption that ammonium nitrate would represent the limiting case of weak cation–anion interaction in ionic melts. However, the data indicate some distortion of both polyatomic ions, but, in contrast to the alkali metal nitrates, no evidence for anion–anion or cation–cation dynamical coupling has been detected.