ELECTRON DENSITY STUDIES ON 2,3-PYRIDINEDICARBOXYLIC ACID Márcia D. D. Costa, Nelson G. Fernandes, Department of Chemistry - Federal University of Minas Gerais CP 702, 31270-901 - Belo Horizonte - Brazil
The present work reports a deformation refinement from X-ray data for quinolinic acid, C7H5NO4. The compound adopts the zwitterion form in the crystalline state and has a short asymmetric hydrogen bond with O---O' distance of 2.401(4) Å. This is one of the shortest intramolecular hydrogen bonds known. In addition, neutron diffraction data at room and low temperatures are available. These are the reasons that motivated us to study 2,3-pyridinedicarboxylic acid.
Data collection was performed on a SIEMENS-P4 diffractometer, in q-2q scan mode, (sinq/l)max = 1.20 Å-1, MoKa. Data were corrected for absorption, Lorentz and polarization effects; no extintion correction was necessary. 6856 reflections collected, 4817 unique. Deformation density refinement was performed following the Hirshfeld procedure, that allows the charge-density redistribution accompanying molecule formation to be incorporated into the crystallographic model for least-squares refinement, together whith the usual positional and thermal parameters. This resulted in wR=0.0556 (F2) for 218 parameters refined, of which 109 were deformation parameters.
Deformation density map through the short hydrogen bond shows two maxima peak heights of approximately 0.3 e/Å3 localized in the bonds O-H and O'-H. One of them is at 0.6 Å from O and 0.7 Å from H. The other is located at 0.7 from O' and 0.5 Å from H. It is clear from the data that there are two different C-O bond orders with maxima of 0.3 and 0.6 e/Å3.