Anion—Cation Interactions in Molten Inorganic Nitrates: Vibrational Analysis

An assessment of the spectral data (solid state, solutions, and molten state) for LiNO3, NaNO3, KNO3, RbNO3, CsNO3, AgNO3, and TlNO3 reveals significant features not in harmony with the selection rules for NO3− ions of D3h point-group symmetry. Correlations with lower symmetries show that the selection rules for the Point Groups C2v and Cs are adequate for the observed spectra. A ``contact'' ion-pair model is advanced for nearest-neighbor pairwise association in which the metal ion interacts with the oxygen at a unidentate site in the ion pair. Force constants are reported, calculated from a normal-coordinate vibrational analysis for this model, and the Urey—Bradley force-field assumptions. Metal-ion—oxygen vibrational frequencies are predicted in the region of 100 to 350 cm−1; the magnitudes of the force constants for this mode imply definite ionic association interactions, in which the residence time is long compared to the time of a molecular vibration. The results are examined relative to spectroscopically based operational criteria for ion-pair formation for nitrates in solutions and as molten salts.