AbstractThe α crystalline form of p-aminobenzoic acid is monoclinic and prone to twinning by pseudo-merohedry (a ≈ c). A structure was reported by Lai and Marsh (1967) on an almost untwinned specimen where (a-c)/σ is 36. In revisiting this structure, a sample showing twinning was obtained from the metabolites of a Streptomyces strain collected from the soil at the ancient Roman city of Pollentia on the Balearic island of Mallorca. A 106(2) K determination shows pseudo-merohedral twinning, with a = 18.5452(6), b = 3.73406(15), c = 18.5484(7) Å, β = 93.790(3)°, V = 1281.65(8) Å3, and (a−c)/σ 5. The refinement included twin contributions [major domain 0.7435(15)] and produced a higher resolution structure determination. The cell contains two neutral molecules per asymmetric unit. Both form complementary carboxylic acid dimers across inversions centers of P21/n. Nitrogens are pyramidal and donate Hydrogen-bonds through one amine hydrogen; a stronger two-center N–H⋯O in one case, and a weaker three-center N–H⋯(O, O’) in the other. The molecular structure shows a pronounced quinoid distortion, and a comparison is drawn with the previous studies, an ordered β monoclinic phase, and with other structural studies.Graphical AbstractThe α monoclinic phase of p-aminobenzoic acid crystallizes commonly as pseudo-merohedral twins, and a specimen studied at 106 K had a ≈ c within 5 esd’s, allowing an improved refinement and a more accurate molecular model.

中文翻译：

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孪晶α对氨基苯甲酸在 106 K

摘要 对氨基苯甲酸的 α 晶型为单斜晶型，易于通过拟多面体 (a ≈ c) 孪晶。Lai 和 Marsh (1967) 在一个几乎没有孪生的标本上报告了一个结构，其中 (ac)/σ 为 36。在重新审视这个结构时，从古代土壤中收集的链霉菌菌株的代谢物获得了一个显示孪生的样本。马略卡岛巴利阿里岛上的罗马城市波伦蒂亚。106(2) K 测定显示伪多面体孪晶，a = 18.5452(6), b = 3.73406(15), c = 18.5484(7) Å, β = 93.790(3)°, V = 1281.65(8) Å3 和 (a-c)/σ 5。改进包括双重贡献 [主要域 0.7435(15)] 并产生更高分辨率的结构测定。每个不对称单元包含两个中性分子。两者在 P21/n 的反转中心形成互补的羧酸二聚体。氮是金字塔形的，通过一个胺氢提供氢键；在一种情况下是更强的双中心 N–H⋯O，在另一种情况下是较弱的三中心 N–H⋯(O, O')。分子结构显示出明显的醌型畸变，并与之前的研究，有序的β单斜相和其他结构研究进行比较。 图文摘要对氨基苯甲酸的α单斜相通常以假多面体孪晶结晶，并且在 106 K 下研究的样本在 5 esd 内具有 ≈ c，允许改进改进和更准确的分子模型。