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Revision of the Crystal Structure of the First Molecular Polymorph in History

机译:修订历史上第一个分子多晶型物的晶体结构

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Despite being the earliest reported case of polymorphism in a molecular crystal, the unstable form II of benzamide has thus far evaded thorough structural characterization because collection of experimental data at atomic resolution has proven extremely challenging. Using a highly validated computational crystal structure prediction (CSP) method based on dispersion-corrected density functional theory, we correctly predict the stable form I with the lowest energy among all sampled structures and its polytypic form III with slightly higher energy. From Rietveld refinement of selected CSP models against synchrotron X-ray powder diffraction data of the historical polymorph, we are able to identify a subtle weakness in the available experimental data and arrive at a revised structure of form II. The revised crystal structure is the first benzamide structure to form catemers rather than dimers and possesses the rare space-group symmetry Fdd2 with two molecules in the asymmetric unit, which is necessary to support the new hydrogen bonding network. This rare space group is only found in the CSP by a complete structural search in all 230 space groups with one or two molecules in the asymmetric unit.
机译:尽管是最早报道的在分子晶体中发生多态性的案例,但苯甲酰胺的不稳定形式II至今仍未进行彻底的结构表征,因为事实证明以原子分辨率收集实验数据极具挑战性。使用基于色散校正密度泛函理论的高度验证的计算晶体结构预测(CSP)方法,我们可以正确预测所有采样结构中能量最低的稳定晶型I及其能量稍高的多型晶型III。通过针对历史多晶型物的同步加速器X射线粉末衍射数据对选定的CSP模型进行Rietveld精修,我们能够确定可用实验数据中的细微弱点,并得出II型的修订结构。修改后的晶体结构是第一个形成分类异构体而不是二聚体的苯甲酰胺结构,并具有罕见的空间对称性Fdd2,在不对称单元中有两个分子,这对于支持新的氢键网络是必需的。仅在CSP中,通过在不对称单元中具有一个或两个分子的所有230个空间组中进行完整的结构搜索,才能发现这种稀少的空间组。

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