The molecular and crystal structure of acetylacetone (AA) was studied at 210 and 110 K in order to reveal the nature of the hydrogen bond and possible crystal disorder. The single crystal was grown by an in situ crystallization from the melt using an IR laser beam producing a molten zone in a Lindeman capillary. At both temperatures the crystals are orthorhombic (space group Pnma, Z = 4) with molecules having a crystallographic mirror plane perpendicular to the mean molecular plane. The intramolecular hydrogen bond is characterized by the O···O distances 2.541(2) and 2.547(1) Å at 210 and 110 K, with the central hydrogen atom equally distributed over the two positions near the oxygens. So, the hydrogen bond in AA has two distinct potential minima that are in agreement with previous NMR data and recent quantum-chemical calculations. The bond lengths C−O and C−C in the cis-enol fragment have averaged values (1.291(1) and 1.402(1) Å at 110 K) corresponding to the superposition of two enol isomers due to the crystallographic symmetry. The nature of the crystal disorder (static or dynamic) is discussed using analysis of the atomic anisotropic displacement parameters. Charge density analysis in AA has been performed using the high-order X-ray diffraction data at 110 K and a multipole model.

The molecular and crystal structure and electron density distribution were studied for a single crystal of pentaethyl-1,5-dicarba-closo-pentaborane C 2 B 3 (Et) 5 by the high-resolution X-ray diffraction method at 120 K (5696 observed reflections, R ) 0.056). The single crystal was grown by in situ crystallization with an IR-laser beam producing a molten zone in a Lindeman capillary. In crystal the molecule of the title compound has local C 1 -symmetry with different orientations of the Et-groups with respect to the central trigonal-bipyramidal C 2 B 3 fragment, mean B-C and B-B distances are 1.571 and 1.876 Å, respectively, the axial C...C distance is 2.277 Å. An ab initio calculations of the molecular structure indicated that rotation barriers of the Et groups with respect to the central frame are very small, that may be the reason of the low molecular symmetry and diffrerent orientation of the Et groups in molecule due to the crystal packing effects. Deformation electron density (DED) maps obtained via a conventional "X-X" method, and using a multipole refinement procedure, showed charge accumulation in the B-C bonds of the central frame, and these bonds were found to be essentially bent outward of the C 2 B 3 cage. On the contrary, no charge accumulation was detected in the B-B bonds, thus indicating the absence of the direct B-B interactions. Positive and delocalized DED was also found in all B-C-B triangle faces of the central cage, that indicates the contribution of the multicenter bonding in the electronic structure of this formally electron-precise molecule. Topological analysis of the experimental charge density distribution has been performed including an analysis of the Laplacian maps, and these data confirmed all important features of the DED maps.

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