N‑Oxide as a Proton Accepting Group in Multicomponent Crystals: X‑ray and Theoretical Studies on New p‑Nitropyridine‑N‑oxide Co-Crystals

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.