Acta Crystallographica Section B

Structural Science

Volume 64, Part 2 (April 2008)

research papers

Acta Cryst. (2008). B64, 230-239 [ doi:10.1107/S0108768108000268 ]

Deuterated -malonic acid: its neutron crystal structure in relationship to other polymorphs of aliphatic dicarboxylic acids

R. K. McMullan, W. T. Klooster and H.-P. Weber

Abstract: Three complete neutron diffraction datasets have been collected for deuterated malonic acid single crystals, DOOC(CD₂)COOD, above (153 K), just below (56 K) and further below (50 K) the low-temperature phase transition (T_c = 57 K). The structural details obtained for this transition, studied previously solely by spectroscopic and calorimetric techniques, clearly establish its first-order nature. At 153 K, the space group is $P\bar 1$ , Z = 2, Z' = 1. The molecules are packed as linear chains linked end-to-end by asymmetric hydrogen bonds so that the carboxyl groups form cyclic dimers. The deuterons in the carboxyl links are ordered. Neighboring chains are cross-linked through C-DO hydrogen bonds. Upon cooling through the transition the cell doubles along the a axis. Molecules which are equivalent by symmetry above T_c become independent below T_c owing to conformational changes in alternate chains. At 50 K, the space group is $P\bar 1$ , Z = 4, Z' = 2. Thermal motion analysis, using for all three temperatures the same segmented rigid-body model, reveals a large torsional motion around the one COOD group associated with the conformational change. Refinements were carried out on all three datasets with an anharmonic structural model, including higher-order displacement tensors (Gram-Charlier expansion up to fourth order). Only atoms involved in torsional motion exhibit a significant anharmonic component which increases with temperature.

Keywords: polymorphism; neutron diffraction; low-temperature phase transition; hydrogen bonding; thermal motion analysis.


	Structure factor file (CIF format) (181.2 kbytes) [ doi:10.1107/S0108768108000268/bk506950Ksup2.hkl ] Contains datablock 50K

	Structure factor file (CIF format) (251.2 kbytes) [ doi:10.1107/S0108768108000268/bk506956Ksup3.hkl ] Contains datablock 56K

	Structure factor file (CIF format) (94.0 kbytes) [ doi:10.1107/S0108768108000268/bk5069153Ksup4.hkl ] Contains datablock 153K

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