The crystal structure of the monoclinic polymorph of the primary amino acid l-histidine has been determined for the first time by single-crystal neutron diffraction, while that of the orthorhombic polymorph has been reinvestigated with an untwinned crystal, improving the experimental precision and accuracy. For each polymorph, neutron diffraction data were collected at 5, 105 and 295 K. Single-crystal X-ray diffraction experiments were also performed at the same temperatures. The two polymorphs, whose crystal packing is interpreted by intermolecular interaction energies calculated using the Pixel method, show differences in the energy and geometry of the hydrogen bond formed along the c direction. Taking advantage of the X-ray diffraction data collected at 5 K, the precision and accuracy of the new Hirshfeld atom refinement method implemented in NoSpherA2 were probed choosing various settings of the functionals and basis sets, together with the use of explicit clusters of molecules and enhanced rigid-body restraints for H atoms. Equivalent atomic coordinates and anisotropic displacement parameters were compared and found to agree well with those obtained from the corresponding neutron structural models.

中文翻译：

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L-组氨酸两种多晶型物在 5、105 和 295 K 下的准确 H 原子参数

一级氨基酸的单斜晶型的晶体结构升组氨酸已被确定为在第一时间由单晶中子衍射，而斜方晶多晶型物已被重新调查与untwinned晶体，提高实验精度和准确度。对于每个多晶型物，在 5、105 和 295 K 下收集中子衍射数据。还在相同温度下进行单晶 X 射线衍射实验。这两种多晶型物的晶体堆积由使用像素方法计算的分子间相互作用能来解释，显示出沿c形成的氢键的能量和几何形状的差异方向。利用在 5 K 下收集的 X 射线衍射数据，探讨了在NoSpherA2中实施的新 Hirshfeld 原子精修方法的精度和准确性，选择了泛函和基组的各种设置，以及使用显式分子簇和增强了对 H 原子的刚体约束。比较了等效原子坐标和各向异性位移参数，发现与从相应的中子结构模型中获得的参数非常吻合。