Climbing Jacob’s Ladder of Structural Refinement: Introduction of a Localized Molecular Orbital-Based Embedding for Accurate X‑ray Determinations of Hydrogen Atom Positions

The
positions of hydrogen atoms in molecules are fundamental in
many aspects of chemistry. Nevertheless, most molecular structures
are obtained from refinements of X-ray data exploiting the independent
atom model (IAM), which uses spherical atomic densities and provides
bond lengths involving hydrogen atoms that are too short compared
to the neutron reference values. To overcome the IAM shortcomings,
the wave function-based Hirshfeld atom refinement (HAR) method has
been recently proposed, emerging as a promising strategy able to give
element–hydrogen bond distances in excellent agreement with
the neutron ones in terms of accuracy and precision. In this Letter,
we propose a significant improvement of HAR based on the idea of describing
the crystal environment explicitly in the underlying wave function
calculation through a quantum mechanical embedding strategy that exploits
extremely localized molecular orbitals. Test-bed refinements on a
crystal structure characterized by strong intermolecular interactions
are also discussed.

分子中氢原子的位置在化学诸多领域中具有基础性意义。然而，绝大多数分子结构均通过X射线数据精修获得，其采用独立原子模型（Independent Atom Model, IAM）——该模型使用球形原子密度，所给出的含氢化学键键长相较于中子参考值普遍偏短。为弥补独立原子模型的缺陷，基于波函数的赫希菲尔德原子精修（Hirshfeld Atom Refinement, HAR）方法近年被提出，该方法展现出优异的应用潜力，其给出的元素-氢键键长在精度与准确度上均与中子参考值高度吻合。在本通讯中，我们提出了赫希菲尔德原子精修方法的一项重要改进：通过采用极局域分子轨道的量子力学嵌入策略，在底层波函数计算中显式描述晶体环境。本文还针对以强分子间相互作用为特征的晶体结构开展了精修测试，并对结果进行了讨论。