The conformational properties of the nitro group in nitroxoline (8-hydroxy5-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 phi = 8 +/- 3 degrees 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 pi-pi conjugation of the nitro group and aromatic system of 8-hydroxyoquinoline. The computation of equilibrium geometry using MP2/ccpVXZ (X = T, Q) shows a large overestimation of the pi value, while DFT with the cc-pVTZ basis set performs reasonably well. On the other hand, DFT computations with double-zeta basis sets yield a planar structure of NXN. The refined potential energy surface of the torsion vibration the of nitro group in the condensed phase derived from the THz-TDS data indicates the NXN molecule to be planar. This result stays in good agreement with the previous X-ray structure determination. The strength of the g-system conjugation for the NO2 group and 8-hydroxyoquinoline is discussed using NBO analysis, being further supported by comparison of the refined semiexperimental gas-phase structure of NXN from GED with other nitrocompounds.