Vibrational Spectra and Structures of Ionic Liquids. II. The Pure Alkali Metal Nitrates

Abstract

Attenuated total reflectioninfrared spectroscopy has been used together with thin‐film transmission measurements to obtain detailed spectra for the molten nitrates of lithium,sodium, and potassium in the 250–4000‐cm−1 range. The techniques, which yield spectra approaching the ultimate in resolution and precision possible for salt melts, permit the detection of several features which, when combined with Raman data, definitely rule out a melt structure dominated by either ion pairs or simple 1:1 complexes. Rather, systems wherein important factors include both cation distortion of the nitrate symmetry (site effect) and vibrational coupling of neighboring anions (correlation field effect) are definitely suggested. Thus, an assignment based on a perturbed orthorhombic lattice structure is presented which accounts very satisfactorily for all observed features. Order in the melts seems to be slightly greater in LiNO3 than for the other salts.