Does Proton Transfer Always Take Place in Molecules of 2-Substituted Pyridine-N-oxides? The Case of 2-Aminopyridine-N-oxide Crystal Structure and Its 3,5-Dinitrobenzoic Acid Co-crystal

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.