1 research outputs found
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 NO<sub>2</sub> 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 NO<sub>2</sub> 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 Ï• 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 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 π-system conjugation for the NO<sub>2</sub> 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