4‑Methoxypicolinic\nAcid <i>N</i>‑Oxide:\nOne of the Shortest Hydrogen Bonds Known Characterized by Neutron\nDiffraction, Inelastic Neutron Scattering, Infrared Spectroscopy,\nand Periodic DFT Calculations

The present work focuses on the case of an extremely\nshort intramolecular\nO–H···O hydrogen bond (H-bond) found in 4-methoxypicolinic\nacid <i>N</i>-oxide (MPANO). The donor···acceptor\nseparation of 2.403 Å makes the H-bond in MPANO one of the shortest\nH-bonds known. We elucidated the structure and dynamics of the H-bond\nby two neutron-based techniques, namely, single-crystal diffraction\nand inelastic scattering (INS) vibrational spectroscopy. We also utilized\nconventional infrared (IR) spectroscopy as well as quantum chemical\ncomputations on isolated and periodic models. Both the protiated and\ndeuterated variants of MPANO were investigated by INS and IR. All\nthe methods used unequivocally confirm the existence of an extremely\nshort, asymmetric H-bond, with the proton located near yet off the\nmidpoint. The main relevant feature of the IR spectrum is an extremely\nbroad, complex, and red-shifted OH (OD) stretching band spanning between\n1800 and 500 cm<sup>–1</sup> and centered at about 1360 cm<sup>–1</sup>, which indicates the presence of extensive anharmonicity\nand coupling with other H-bond modes. Of the modes characteristic\nof H-bond dynamics, only the out-of-plane OH (OD) bending can clearly\nbe detected in the INS spectra; it has a relatively high frequency\nindicative of the strength of the H-bond. The computed structure is\nin excellent agreement with the diffraction measurement when periodicity\nis taken into account. The calculated harmonic frequencies show a\nreasonable match with the observed spectral features, whereby the\nassignment of the IR and INS spectra is facilitated. The hydrogen\nstretching frequency, however, appears to be significantly overestimated,\non account of the limitations of the harmonic approximation and the\ncomplex nature of the short H-bond.