# 4-Methoxypicolinic Acid N-Oxide: One of the Shortest Hydrogen Bonds Known Characterized by Neutron Diffraction, Inelastic Neutron Scattering, Infrared Spectroscopy, and Periodic DFT Calculations

## TL;DR

This study identifies one of the shortest hydrogen bonds in a compound called 4-methoxypicolinic acid N-oxide using advanced techniques like neutron diffraction and infrared spectroscopy.

## Contribution

The paper presents the characterization of an exceptionally short and asymmetric hydrogen bond using multiple experimental and computational methods.

## Key findings

- The H-bond in MPANO has a donor-acceptor separation of 2.403 Å, making it one of the shortest known.
- The IR spectrum shows a broad and red-shifted OH stretching band, indicating anharmonicity and coupling.
- Periodic DFT calculations align well with diffraction data but overestimate the hydrogen stretching frequency.

## Abstract

The present work focuses on the case of an extremely
short intramolecular
O–H···O hydrogen bond (H-bond) found in 4-methoxypicolinic
acid N-oxide (MPANO). The donor···acceptor
separation of 2.403 Å makes the H-bond in MPANO one of the shortest
H-bonds known. We elucidated the structure and dynamics of the H-bond
by two neutron-based techniques, namely, single-crystal diffraction
and inelastic scattering (INS) vibrational spectroscopy. We also utilized
conventional infrared (IR) spectroscopy as well as quantum chemical
computations on isolated and periodic models. Both the protiated and
deuterated variants of MPANO were investigated by INS and IR. All
the methods used unequivocally confirm the existence of an extremely
short, asymmetric H-bond, with the proton located near yet off the
midpoint. The main relevant feature of the IR spectrum is an extremely
broad, complex, and red-shifted OH (OD) stretching band spanning between
1800 and 500 cm–1 and centered at about 1360 cm–1, which indicates the presence of extensive anharmonicity
and coupling with other H-bond modes. Of the modes characteristic
of H-bond dynamics, only the out-of-plane OH (OD) bending can clearly
be detected in the INS spectra; it has a relatively high frequency
indicative of the strength of the H-bond. The computed structure is
in excellent agreement with the diffraction measurement when periodicity
is taken into account. The calculated harmonic frequencies show a
reasonable match with the observed spectral features, whereby the
assignment of the IR and INS spectra is facilitated. The hydrogen
stretching frequency, however, appears to be significantly overestimated,
on account of the limitations of the harmonic approximation and the
complex nature of the short H-bond.