The far-IR spectra for NH4+ and H3O+ complexes with 18-crown-6 have been obtained and the hydrogen-bond vibrational mode vσ 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)