Cooperativity Assisted Shortening of Hydrogen Bonds in Crystalline Oxalic Acid Dihydrate: DFT and NBO Model Studies

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

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.

Strong Hydrogen Bonds in Acetylenedicarboxylic Acid Dihydrate

Acetylenedicarboxylic acid dihydrate (ADAD) represents a complex with strong hydrogen bonding between the carboxylic OH and the water molecule. An X-ray re-examination of the ADAD crystal structure confirms the O^…O distance of the short hydrogen bonds, and clearly shows different bond lengths between the two oxygen atoms with respect to the carbon atom in the carboxyl group, indicating a neutral structure for the complex. The neutral structure was also confirmed by vibrational spectroscopy, as no proton transfer was observed. The diffraction studies also revealed two polymorph modifications: room temperature (α) and low temperature (β), with a phase transition at approximately 4.9 °C. The calculated vibrational spectra are in satisfactory agreement with the experimental spectra. A comparison of the structure and the vibrational spectra between the ADAD and the oxalic acid dihydrate reveals some interesting details. The crystal structures of both crystal hydrates are almost identical; only the O^…O distances of the strongest hydrogen bonds differ by 0.08 Å. Although it was expected that a larger O^…O spacing in the ADAD crystal may significantly change the infrared and Raman spectra, especially for the frequency and the shape of the acidic OH stretching vibration, both the shape and frequency are almost identical, with all subpeaks topped on the broad OH stretching vibration. The O^…O distance dependent are only in- and out-of-plane OH deformations modes. The presence of polarons due to the ionized defects was not observed in the vibrational spectra of ADAD. Therefore, the origin of the broad OH band shape was explained in a similar way to the acid dimers. The anharmonicity of a potential enhances the coupling of the OH stretching with the low-frequency hydrogen bond stretching, which, in addition to the Fermi resonance, structures the band shape of the OH stretching. The fine structure found as a superposition of a broad OH stretching is attributed to Davydov coupling.

Mechanical properties of anhydrous oxalic acid and oxalic acid dihydrate

The mechanical properties of oxalic acid dihydrate and anhydrous oxalic acid (α and β polymorphic forms) were obtained by using rigorous theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials.

Co-operativity in non-covalent interactions in ternary complexes: a comprehensive electronic structure theory based investigation

The structure and stability of various ternary complexes in which an extended aromatic system such as coronene interacts with ions/atoms/molecules on opposite faces of the π-electron cloud were investigated using ab initio calculations. By characterizing the nature of the intermolecular interactions using an energy decomposition analysis, it was shown that there is an interplay between various types of interactions and that there are co-operativity effects, particularly when different types of interactions coexist in the same system. Graphical abstract Weak OH-π, π-π and van der Waals-π ternary systems are stabilized through dispersion interactions. Cation-π ternary systems are stabilized by through-space electrostatic interactions.

Specific features of supramolecular organisation and hydrogen bonding in proline cocrystals: a case study of fenamates and diclofenac

New zwitterionic cocrystals of fenamate drugs and diclofenac with the naturally occurring amino acid l-proline have been obtained and thoroughly characterised by a variety of experimental and theoretical techniques.

Proton transfer aiding phase transitions in oxalic acid dihydrate under pressure

Oxalic acid dihydrate, an important molecular solid in crystal chemistry, ecology and physiology, has been studied for nearly 100 years now. The most debated issues regarding its proton dynamics have arisen due to an unusually short hydrogen bond between the acid and water molecules. Using combined in situ spectroscopic studies and first-principles simulations at high pressures, we show that the structural modification associated with this hydrogen bond is much more significant than ever assumed. Initially, under pressure, proton migration takes place along this strong hydrogen bond at a very low pressure of 2 GPa. This results in the protonation of water with systematic formation of dianionic oxalate and hydronium ion motifs, thus reversing the hydrogen bond hierarchy in the high pressure phase II. The resulting hydrogen bond between a hydronium ion and a carboxylic group shows remarkable strengthening under pressure, even in the pure ionic phase III. The loss of cooperativity of hydrogen bonds leads to another phase transition at ∼ 9 GPa through reorientation of other hydrogen bonds. The high pressure phase IV is stabilized by a strong hydrogen bond between the dominant CO2 and H2O groups of oxalate and hydronium ions, respectively. These findings suggest that oxalate systems may provide useful insights into proton transfer reactions and assembly of simple molecules under extreme conditions.

The applicability of the dimeric heterosynthon concept to molecules with equivalent binding sites. A DFT study of crystalline urea–H2O2

The limited applicability of the dimeric heterosynthon concept to a two-component urea–H₂O₂crystal is reported.