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首页> 外文期刊>Crystal growth & design >Polymorph-Selective Role of Hydrogen Bonding and π–π Stacking in p-Aminobenzoic Acid Solutions
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Polymorph-Selective Role of Hydrogen Bonding and π–π Stacking in p-Aminobenzoic Acid Solutions

机译：多晶型选择性作用的氢键和π-π堆叠在对氨基苯甲酸溶液中的堆叠

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Understanding molecular self-association in solution is vital for uncovering polymorph-selective crystal nucleation pathways. In this paper, we combine solution NMR spectroscopy and molecular dynamics simulations to shed light on the structural and dynamical features of p -aminobenzoic acid (pABA) in solution, and on their role in pABA crystals nucleation. pABA is known to yield different crystal forms (α, and β) depending on solvent choice and supersaturation conditions. NMR reveals that dominant interactions stabilizing pABA oligomers are markedly solvent-dependent: in organic solvents, hydrogen bonds dominate, while water promotes π–π stacking. Despite this clear preference, both types of interactions contribute to the variety of self-associated species in all solvents considered. MD simulations support this observation and show that pABA oligomers are short-lived and display a fluxional character, therefore indicating that the growth unit involved in pABA crystallization is likely to be a single molecule. Nevertheless, we note that the interactions dominating in pABA oligomers are indicative of the polymorph obtained from precipitation. In water, at low pABA concentrations—conditions that are known to yield crystals of the β form—carboxylic–carboxylic hydrogen bonds are exclusively asymmetric. At higher pABA concentration conditions in which the crystallization is known to yield the α form—a small but statistically significant fraction of symmetric carboxylic–carboxylic hydrogen-bonded dimers is present. We interpret the presence of these interactions in solvated pABA oligomers as indicative of the fact that a simultaneous and complete desolvation of two carboxylic groups, necessary to form the symmetric hydrogen-bonded dimer typical of the α crystal form, is accessible, therefore directing the nucleation pathway toward the nucleation of α-pABA.
机译：了解溶液中的分子自缔合对揭示多晶型选择性晶体成核途径至关重要。在本文中，我们结合溶液核磁共振波谱和分子动力学模拟来阐明溶液中对氨基苯甲酸（pABA）的结构和动力学特征，以及它们在pABA晶体成核中的作用。根据溶剂选择和过饱和条件，pABA可以产生不同的晶型（α和β）。核磁共振表明，稳定pABA低聚物的主要相互作用明显依赖于溶剂：在有机溶剂中，氢键占主导地位，而水促进π-π堆积。尽管有这种明显的偏好，但这两种类型的相互作用都有助于在所考虑的所有溶剂中形成各种自缔合物种。MD模拟结果支持这一观察结果，并表明pABA低聚物的寿命较短，且具有流动性，因此表明参与pABA结晶的生长单元可能是单个分子。然而，我们注意到，在pABA低聚物中占主导地位的相互作用表明了从沉淀中获得的多晶型。在水中，在pABA浓度较低的情况下，已知产生β-羧基-羧基氢键晶体的条件完全不对称。在pABA浓度较高的条件下，已知结晶产生α形式——存在少量但在统计上显著的对称羧基-羧基氢键二聚体。我们将溶剂化pABA低聚物中存在的这些相互作用解释为两个羧基同时完全去溶剂化的事实，这是形成α晶型典型对称氢键二聚体所必需的，因此将成核途径指向α-pABA的成核。

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