Towards understanding high-Z' organic molecular crystals through the superspace method.

通过超空间方法了解高Z有机分子晶体。

• 批准号: 389490692
• 负责人: Professor Dr. Sander van Smaalen
• 金额: --
• 依托单位: Laboratorium für Kristallographie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/389490692?language=en
• 关键词: Towards; understanding; organic; molecular; crystals

About 90% of the molecular crystals possess a single symmetry-independent molecule in the unit cell (in case of cocrystals, one molecule of each of the constituents). Z' is defined as the number of formula units in the unit cell, Z, divided by the multiplicity of the space group. Therefore, 90% of the crystal structures are characterized by Z' = 1 (or Z' < 1 in special cases). The abundance of Z' = 1 can be understood from the argument, that the crystalline state will contain the molecule in its optimal conformation in the optimal environment. Values Z' > 1 can occur, when the different interactions cannot be simultaneously optimized. A value of Z' > 4 is considered to be a high Z' value. About 0.1% of the known crystal structures of organic molecular crystals are high-Z' structures. Two recent review articles---by Steed & Steed (2015) and C. Brock (2016)---stress the importance of high-Z' structures for understanding crystal packing in general and polymorphism in particular. The crystal structure determines the solubility, stability and bioavailability of pharmaceuticals and agrochemicals. It is, therefore, of great industrial importance. High-Z' crystal structures possess large unit cells containing Z' copies of the molecule in different conformations and different environments. This property makes them ideal candidates for the superspace approach. Within the superspace approach, the crystal structure is described as a modulation of a basic structure, the latter being characterized by a small unit cell. The modulation provides a direct representation of the differences between the Z' copies of the molecule. Alternatively, the modulation can be incommensurate, with the consequence that a supercell does not exist and Z' is not defined. The present project is intended to demonstrate that the superspace approach to high-Z' crystal structures will lead to an understanding of these structures that goes beyond the classical approach with a large unit cell. For this purpose, the superspace description will be developed for the crystal structures of three substances of biological importance: Ciclopirox (Z' = 12; Pharmaceutical), Sodium saccharine dihydrate (Z' = 16; artificial sweetener) and cholesterol (Z' = 16; present in animal life). Superspace models will be obtained by x-ray diffraction for the crystal structures at several temperatures. The high-Z' structure model will be tested against a possible incommensurability of the crystal structure. Phase transitions will be investigated, as they may occur between incommensurate and high-Z' crystal structures. The superspace description of the crystal structures will elucidate pseudo-symmetries, and it will characterize the interactions that are responsible for the high value of Z'.

约90%的分子晶体在晶胞中具有单个非依赖于结晶度的分子（在共晶的情况下，每种组分的一个分子）。Z'定义为晶胞中的式单元的数目Z除以空间群的多重性。因此，90%的晶体结构的特征在于Z' = 1（或在特殊情况下Z' < 1）。Z' = 1的丰度可以从以下论点来理解，即结晶状态将包含在最佳环境中处于其最佳构象的分子。当不能同时优化不同的相互作用时，可能发生值Z' > 1。Z' > 4的值被认为是高Z'值。在已知的有机分子晶体结构中，约0.1%是高Z '结构。最近的两篇评论文章-Steed & Steed（2015）和C. Brock（2016）-强调高Z '结构对于理解一般晶体堆积和特别是多态性的重要性。晶体结构决定了药物和农用化学品的溶解度、稳定性和生物利用度。因此，它具有重要的工业意义。高Z '晶体结构具有在不同构象和不同环境中含有分子的Z'拷贝的大晶胞。这种性质使它们成为超空间方法的理想候选者。在超空间方法中，晶体结构被描述为基本结构的调制，后者的特征在于小晶胞。调节提供了分子的Z'拷贝之间的差异的直接表示。或者，调制可以是不相称的，结果是不存在超晶胞并且Z'未被定义。本项目旨在证明，超空间方法的高Z '晶体结构将导致这些结构的理解，超越了经典的方法与一个大的单位细胞。为此目的，将对三种具有生物重要性的物质的晶体结构进行超空间描述：环吡酮（Z' = 12;药物）、糖精钠二水合物（Z' = 16;人工甜味剂）和胆固醇（Z' = 16;存在于动物生命中）。超空间模型将通过在几个温度下的晶体结构的X射线衍射获得。高Z '结构模型将针对晶体结构的可能的不可逆性进行测试。将研究相变，因为它们可能发生在无公度和高Z '晶体结构之间。晶体结构的超空间描述将阐明伪对称性，并且它将表征负责高Z '值的相互作用。