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1 Polish Journal of Chemistry

1 1998

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Molecular structure of pyridine N-oxide complex with 2,6-dichloro-4-nitrophenol

Tykarska E., Dega-Szafran Z., Grunwald-Wyspiańska M., Szafran M.

Polish Journal of Chemistry

1998

Vol. 72, nr 2

470-479

A complex of pyridine N-oxide (PyO) with 2,6-dichloro-4-nitrophenol (DCNP) was studied by X-ray diffraction, FT-IR spectroscopy and quantum-mechanical calculatios with the DFT and semiempirical methods. The crystal of the PyOxDCNP are triclinic, space group P1, a=6.833(1)A, b=8.717(2)A, c=c=11.482(2)A, alpha=98.32(2)degree, beta= 93.63(1)degree, gamma=109.12(2) degree, V=633.6(3)A(3), Z=2. The molecules of the complex are joined by the N-O...H-O hydrogen bond with the O...O distance of 2.476(2)A and the O(4)...H(1)-O(1) angle of 165.1degree. The dihedral angle between the planes of the bridged pyridine and phenyl rings is 71.8degree. The weak C-H...O, C-H...Cl interactions and stacking forces stabilize three dimensional packing pattern. The SAM1 and DFT methods predict one minimum for B...H-A form, while the PM3 method predict two minima, the deeper one for B...H-A complex and the shallower one for B(+)-H...A(-) form. For the most stable complexes the predicted O...O distances are longer than the experimental value by 0.141, 0.067 and 0.231 A, respectively for the DFT, SAM1 and PM3 methods. The calculatedbond lengths, except N(1)-O(4), are longer than those from the X-ray as results of intermolecular interactions in the crystal. The SAM1 geometry of PyOxDCNP is slightly better than this obtained by the PM3 method and it is recommended as input in ab initio calculations. The protonic broad absorption in the 1500-250 cm-1 region is typical for such a short hydrogen bond and the proton motion may be described by a potential curve with an asymmetric double minimum. Proton motion in the bridges is faster than the time range of IR spectroscopy.