Semi-experimental equilibrium structure of pyrazinamide from gas-phase electron diffraction. How much experimental is it?

Tikhonov DS, Vishnevskiy Y, Rykov AN, Grikina OE, Khaikin LS. Semi-experimental equilibrium structure of pyrazinamide from gas-phase electron diffraction. How much experimental is it? JOURNAL OF MOLECULAR STRUCTURE. 2017;1132:20-27.A semi-experimental equilibrium structure of free molecules of pyrazinamide has been determined for the first time using gas electron diffraction method. The refinement was carried using regularization of geometry by calculated quantum chemical parameters. It is discussed to which extent is the final structure experimental. A numerical approach for estimation of the amount of experimental information in the refined parameters is suggested. The following values of selected internuclear distances were determined (values are in angstrom with 1 sigma in the parentheses): r(e)(C-pyrazine-C-pyrazine)(av) = 1.397(2), r(e)(N-pyrazine-C-pyrazine)(av) = 1.332(3), r(e)(C-pyrazine-C-amide) = 1.493(1), r(e)(N-amide-C-amide) = 1.335(2), r(e)(O-amide-C-amide) = 1.219(1). The given standard deviations represent pure experimental uncertainties without the influence of regularization. (C) 2016 Elsevier B.V. All rights reserved

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 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 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