Neutron diffraction study of quinolinic acid recrystallized from D2O: evaluation of temperature and isotope effects in the structure

Abstract

The structure of quinolinic acid recrystallized from D2 0 (2,3-pyridinedicarboxylic acid; C7H3D2N04 ) has been refined based on neutron diffraction data measured at four temperatures: 35, 80, 100 and 298 K. The principal temperature dependence in cell constants is observed for the b axis, which is perpendicular to the molecular planes. The refined thermal parameters have been extrapolated by a least-squares procedure, to yield values for T = 0 K which provide estimates of the combined effects of static disorder and zero-point motion. The D atom shifts toward the midpoint of the short intramolecular 0 - - - 0 hydrogen bond when the crystal is cooled, just as was found in an earlier study to occur for the H atom in the undeuterated material. At 100 and 298 K, the D atom is displaced significantly further from the bond midpoint than is the H atom at the same temperature. The magnitude of this isotope effect appears to be independent of temperature. The exchangeable protons in the crystal have not been completely replaced by D; refinement of the D scattering lengths indicates the presence of approximately 2-7% H attached to N(l) and 4-4% H in the short hydrogen bond.