Nitroxoline Molecule: Planar or Not? A Story of Battle between π–π Conjugation and Interatomic Repulsion

The conformational properties of the nitro group in nitroxoline (8-hydroxy-5-nitroquinoline, NXN) were investigated in the gas phase by means of gas electron diffraction (GED) and quantum chemical calculations, and also with solid-state analysis performed using terahertz time-domain spectroscopy (THz-TDS). The results of the GED refinement show that in the equilibrium structure the NO2 group is twisted by angle ϕ = 8 ± 3° with respect to the 8-hydroxyoquinoline plane. This is the result of interatomic repulsion of oxygen in the NO2 group from the closest hydrogen, which overcomes the energy gain from the π–π conjugation of the nitro group and aromatic system of 8-hydroxyoquinoline. The computation of equilibrium geometry using MP2/cc-pVXZ (X = T, Q) shows a large overestimation of the ϕ value, while DFT with the cc-pVTZ basis set performs reasonably well. On the other hand, DFT computations with double-ζ basis sets yield a planar structure of NXN. The refined potential energy surface of the torsion vibrat...