Molecular crystal structure of acetylacetone at 210 and 110 K:: Is the crystal disorder static or dynamic?

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) Angstrom 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 1.403,(1) Angstrom 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.