Hydronium ion complex of 18-crown-6:: Theory confirms three "normal" linear hydrogen bonds

The harmonic vibrational frequencies of the complex of H3O+ with 18-crown-6 are reported at a variety of ab initio (Hartree-Fock and MP2) and density functional levels (gradient-corrected and hybrid functionals). At all levels, two nu(as)(COC) vibrations with large IR intensities are obtained, which are in very good agreement, in terms of both position and relative height, with the bands observed experimentally. Effects of solvation and remote counterions on the appearance of these IR bands are indicated to be small. These results do not support the recent interpretation of the IR spectra in terms of an equivalence of all six O atoms of the crown ether, as well as the resulting claim of a "new type of low-barrier H-bonding involving rotational mobility of H3O+" (Stoyanov, E. S.; Reed, C. A. J. Phys. Chem. A 2004, 108, 907). A simple reassignment of one of the observed bands brings theory and experiment in concert and confirms theoretical predictions that the H3O+/18-crown-6 complex adopts a structure with three normal, linear hydrogen bonds.