Unexpected IR characteristics of hydrogen bonds in the 18-crown-6-ether complex of the H3O+hydronium ion. Can the location of the protons be specified?

The vas(COC) IR spectral signatures of polyethers in H-bonded hydronium ion complexes have been established in studies of H3O+and H5O2+complexes with 15-crown-5, 18-crown-6, dibenzo-18-crown-6, and their open chain poly(ethylene glycol) analogues in 1,2-dichoroethane solutions. When experiment meets theory in the structure of the H3O+·18-crown-6 complex, there is disagreement. While DFT calculations at the BLYP/II′// BLYP/6-31G* level indicate a C3vstructure with three normal, static linear H bonds to alternate O atoms of the crown ether, IR spectroscopy in the v(C-O-C) region indicates that all six O atoms are equivalent. A new type of low-barrier H-bonding involving rotational mobility of H3O+is proposed to rationalize the experimental observations.