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

Three complete neutron diffraction datasets have been collected for deuterated malonic acid single crystals, DOOC(CD(2))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-D...O 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.