Fusao Takusagawa

A neutron diffraction study of the crystal structure of ferrocene

The structure of ferrocene has been refined with singlecrystal neutron diffraction data measured at temperatures of 173 and 298 K [Fe(^ 5-C 5H 5) 2, C 1 0 H 1 0 Fe, space group PI J a, Z = 2]. The positions of all H atoms have been varied in the refinements. The cyclopentadienyl ligands are found to be significantly non-planar, with the H atoms displaced toward the Fe atom, in agreement with the results of earlier electron diffraction studies [Bohn & Haaland (1966). J. Organomet. Chem. 5, 470-476; Haaland & Nilsson (1968). Acta Chem. Scand. 22, 2653-2670]. The crystal structure is dis* Research carried out at Brookhaven National Laboratory under contract with the US Department of Energy and supported by its Office of Basic Energy Sciences. t To whom correspondence should be addressed. ordered, as indicated by Williss original neutron diffraction measurements [Willis (1960\Acta Cryst. 13,1088; (1961). AERE Report R3708, Harwell, Oxfordshire, England]. In the present work, refinements were carried out by three procedures: {A) conventional refinement with anisotropic thermal parameters introduced for all atoms, (.B) extension of (A) to include thirdand fourthorder thermal tensors for C and H atoms, and (C) refinement with the cyclopentadienyl ligands assumed to occupy two disordered positions. Based upon the results of these refinements and rigid-body-motion analyses of the thermal parameters, it appears that the disorder of the cyclopentadienyl rings results from the presence of molecules in different orientations randomly distributed throughout the crystal. A similar conclusion has been reached from X-ray diffraction data [Seiler & Dunitz (1979). Acta Cryst. B35, 10681074]. FUSAO TAKUSAGAWA AND THOMAS F. KOETZLE 1075

Hydrogen bond studies. 85. A very short, asymmetrical, intramolecular hydrogen bond: A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (C7H5NO4)

A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (quinolinic acid) has been carried out. The intensities for 3780 reflexions were measured at the Brookhaven High Flux Beam Reactor. The structure was refined to an R(F2) of 0.054 using starting parameters from a previous x‐ray study. The short intramolecular hydrogen bond [O ··· O: 2.398(3) A] has no symmetry restriction operating on the hydrogen atom position and is asymmetric. The two O ··· H distances are 1.163(5) and 1.238(5) A, respectively, and the O–H–O angle is 174.4(4)°. The asymmetry of the bond can be explained by the intramolecular environment. The strain in the molecule caused by the short hydrogen bond results in long carbon‐carbon bonds to the carboxyl groups [1.516(2) and 1.541(2) A]. The C–O bond lengths [1.219(3), 1.263(2), 1.222(3), and 1.270(2) A] vary depending on the hydrogen‐bond involvement of the oxygen atoms. The mean C–H distance is 1.087 A and the N–H distance is 1.036(4) A. The nitrogen atom is involved in an interm...

A refinement of the crystal structure of quinolinic acid at 100 K with neutron diffraction data

The crystal structure of quinolinic acid (pyridine-2,3-dicarboxylic acid: CTHsNO4) has been refined, based upon neutron diffraction data measured at 100 K. Crystal data: space group P2ffc; a = 7.415 (5), b -12.396 (9), c = 7.826 (6) .A, fl = 117.05 (4) °, Z = 4. The final unweighted R value based on F 2 is 0.048 for all 1643 unique reflections, and bond distances have been determined with precision better than 0-003 ,/~. The major temperature dependence in the cell constants is observed for the b axis, which is perpendicular to the molecular planes. Rigid-body analyses of the thermal parameters determined here at 100 K and those found in a prior study at 297 K indicate that the translational motion changes with temperature more along the b axis than in other directions. Significant differences between the structure at 100 and 297 K are observed in the intramolecular O . . . H . . . O hydrogen bond, where the H atom shifts towards the midpoint of the bond when the crystal is cooled. This temperature effect is discussed in the light of results of the rigid-body thermalmotion analyses. 1149