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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Deuterated gamma-malonic acid: its neutron crystal structure in relationship to other polymorphs of aliphatic dicarboxylic acids
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Deuterated gamma-malonic acid: its neutron crystal structure in relationship to other polymorphs of aliphatic dicarboxylic acids

机译：氘代丙二酸：其中子晶体结构与脂肪族二羧酸其他多晶型的关系

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摘要

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 (1) over bar, 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 center dot center dot center dot O hydrogen bonds. Upon cooling through the transition the cell doubles along the a axis. Molecules which are equivalent by symmetry above Tc become independent below Tc owing to conformational changes in alternate chains. At 50 K, the space group is P (1) over bar, 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.
机译：已收集了氘代丙二酸单晶DOOC（CD2）COOD的三个完整的中子衍射数据集，低温相变（Tc = 57），高于（153 K），低于（56 K），再低于（50 K）。 K）。以前仅通过光谱学和量热技术研究过的这种转变获得的结构细节清楚地确定了其一级性质。在153 K处，该空间基团在bar上为P（1），Z = 2，Z'=1。这些分子堆积成通过不对称氢键首尾相连的线性链，因此羧基形成环状二聚体。羧基连接中的氘核是有序的。相邻的链通过C-D中心点中心点中心点O氢键交联。在通过过渡部冷却后，单元会沿a轴加倍。由于交替链中的构象变化，在Tc上方对称对称的分子在Tc下方变得独立。在50 K时，空间组在P上为P（1），Z = 4，Z'=2。热运动分析，针对所有三个温度，使用相同的分段刚体模型，揭示了围绕一个COOD的大扭转运动。与构象变化有关的组。使用非谐结构模型对所有三个数据集进行了细化，包括高阶位移张量（Gram-Charlier扩展到四阶）。只有参与扭转运动的原子才会表现出明显的非谐成分，该成分会随温度升高而增加。

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