Bands in the far-IR spectra of 18-crown-6 complexes of nitromethane, acetronitrile, and malonodinitrile are attributed to hydrogen bonding. The vσ vibrational mode is assigned for these neutral complexes. Solution studies of the complexes in CHCl3 show that a substantial amount of both the nitromethane complex and the malonodinitrile complex remains intact. The acetonitrile complex, however, appears to dissociate in CHCl3.
William P. McKenna and Edward M. Eyring, "Far-IR Studies of Carbon Acid Complexes of 18-Crown-6," Appl. Spectrosc. 40, 16-20 (1986)