Structure Analysis of Molecular Compounds with <i>Z</i>′ = 2 Using Laboratory X-ray Powder Diffraction Data: 3-Phenylpropionic Acid and Its Derivatives

The structure determination of 3-phenylpropionic acid (1), 3-(3-methylphenyl) propionic acid (5), and 3-(3- methoxyphenyl) propionic acid (6), all three compounds crystallizing in the monoclinic space group P21/a (or P21/n) with Z′ = 2, has been accomplished from laboratory X-ray powder diffraction data following the direct space approach. The nature of intermolecular interactions in 1, 5, and 6 has been analyzed using the Hirshfeld surfaces and 2D fingerprint plots. While the two molecules (A and B) in the crystallographic asymmetric unit of 6 display almost identical conformation, those in 1 and 5 are significantly different due a rotation about the C–C bond linking the planar aromatic ring and propionic acid fragments. The presence of the COOH functional group in the compounds promotes the formation of cyclic R22(8) synthons via intermolecular O–H···O hydrogen bonds. Further linking of molecules through intermolecular C–H···O and C–H···π hydrogen bonds generates a three-dimensional supramolecular architecture in 1 and 5 and a two-dimensional molecular sheet in 6. Hirshfeld surface analyses of 1, 5, and 6 as well as a few substituted phenylpropionic acids retrieved from the Cambridge Structure Database (CSD) indicate that 94–99% of Hirshfeld surface areas in this class of compounds are due to H···H, O···H, and C···H contacts.本研究完成了3-苯基丙酸（1）、3-(3-甲基苯基)丙酸（5）以及3-(3-甲氧基苯基)丙酸（6）三种化合物的结构解析：三者均结晶于单斜晶系空间群P2₁/a（或P2₁/n），且Z′=2，结构解析工作基于实验室X射线粉末衍射数据，采用直接空间法完成。针对化合物1、5、6中的分子间相互作用特征，本研究借助赫希feld表面（Hirshfeld surfaces）与二维指纹图谱展开了分析。在化合物6的晶体学不对称单元中，两个分子（A与B）的构象几乎完全一致；而化合物1与5中的两个分子则因连接平面芳香环与丙酸片段的C-C键发生旋转，构象存在显著差异。化合物中所含的COOH官能团可通过分子间O-H···O氢键形成环状R₂²(8)合成子。进一步通过分子间C-H···O与C-H···π氢键实现分子间连接后，化合物1与5形成了三维超分子结构，而化合物6则形成二维分子片层。对化合物1、5、6以及从剑桥晶体结构数据库（Cambridge Structure Database, CSD）中检索得到的数种取代苯基丙酸开展赫希feld表面分析后发现，该类化合物94%~99%的赫希feld表面积均来自H···H、O···H及C···H接触作用。