Combined X-ray diffraction structural and theoretical density functional theory research utilizing the Quantum Theory of Atom in Molecules (QTAIM) and natural bond orbital (NBO) approaches have been carried out to study the properties of the N-oxide group in hydrogen bonds. The N-oxide group may act as a proton acceptor of carboxyl, hydroxyl, amine groups, and water molecule donors. There are two types of N-oxide hydrogen bonding patterns: a single isolated dimer and much more common a double acceptor bifurcated complex. O−H•••ON hydrogen bonds of energies 10−20 kcal/mol, in contrast to weaker N−H•••ON (about 5 kcal/mol) ones, are more favored in the crystal state. The experimental evidence of proton transfer is found only for homonuclear O−H•••ON N-oxide hydrogen bridges. Hirshfeld surface fingerprint plots reveal areas useful for distinguishing N-oxide hydrogen bonds from other H•••O type interactions. It is also demonstrated that formation of hydrogen bonds significantly influence the NO bond length and its properties including π delocalization.

The crystal structures of new N-phenyl-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide derivatives are reported. The results of X-ray diffraction showed the existence of intramolecular hydrogen bonding between carboxamide nitrogen donors and N-oxide oxygen acceptors. The use of Quantum Theory of Atoms in Molecules allowed its classification as a strong interaction, with energy about 10 kcal/mol, and of intermediate character between closed shell and shared bonds. Comparison of experimental data and quantum theoretical calculations indicated that a substituent attached to the phenyl ring in the para position influences the strength and geometry of the title hydrogen bonding. Stronger π-electronwithdrawing properties of the higher energy substituent of the intramolecular hydrogen bond are observed. Among other intermolecular contacts in the studied crystals are C-H…O/ C-H…N hydrogen bonds of imidazole carbon atoms and some π…π stacking interactions between aromatic molecular fragments. Their importance in stabilization of the crystal structure was confirmed by the results of Hirshfeld surface analysis.

Novel solid-state structures of 2-hydroxypyridine-N-oxide and its co-crystal with 3,5-dinitrobenzoic acid have been determined. The crystallographic results indicate that this pyridine derivative exists in a neutral tautomeric form in a crystal state; these results are in contrast to the 2-hydroxy, 2-mercapto, and 2-nitramino counterparts of the derivative, which were in a zwitterionic form. The phenomenon of tautomerism in this study was subsequently analyzed with quantum chemistry. The results of theoretical DFT calculations were in agreement with experimental findings (apart from the 2-nitramino derivative), indicating that, in low energy difference between the tautomeric forms, a molecule adopts a more profitable structure in terms of crystal stability, even if it is less stable in the gas phase.