The far-IR spectra for NH<sub>4</sub><sup>+</sup> and H<sub>3</sub>O<sup>+</sup> complexes with 18-crown-6 have been obtained and the hydrogen-bond vibrational mode <i>v</i><sub>σ</sub> has been assigned. These results illustrate the suitability of far-IR spectroscopy for exploring crown ether molecular ion interactions.
William P. McKenna and Edward M. Eyring, "Far-IR Spectroscopy of Ammonium and Oxonium Complexes of 18-Crown-6," Appl. Spectrosc. 40, 20-24 (1986)