Experimental electron density of halogen bonds and interhalogen contacts

卤素键在晶体工程领域越来越受欢迎。卤素键发生在与共价键相对的重卤素的亲电区域（通常称为“σ空穴”）与作为亲核试剂的具有孤对电子的合适供体原子之间。在合适的几何形状中相邻卤素原子之间的接触可以被认为是卤素键的特殊情况。大多数讨论解决这种相互作用在固态是基于几何的考虑。我们将联合收割机结合我们在晶体工程和电荷密度研究方面的专业知识，以获得更深入的信息，并在电子密度水平上探测卤键。在我们的建议中，我们将研究重卤素和孤对施主原子对着分子或离子残基之间的短接触的晶体。这些目标特征可能出现在化学性质完全不同的固体中：我们设想研究中性配位化合物、适当取代的吡啶鎓卤化物或卤代金属酸盐以及中性有机分子的晶体。卤素键供体和受体之间的共晶提供了一种特别通用的方法。除了新化合物的合成和结构表征外，我们还将考虑对这些晶体的高分辨率研究，这些晶体只有标准分辨率的数据被报道。单晶的X射线衍射是研究分子间接触的一种简单方法，但我们不仅要研究目标固体的几何形状：为了评估电子密度和静电势等性质，我们将研究在低温和高分辨率下具有有趣堆积特性的化合物的高质量晶体。相变、孪生和无序是必须避免的潜在障碍。从一个成功的实验中，我们将推导出临界点的电子密度及其导出的量，如拉普拉斯，能量密度或少得多的利用源函数。我们希望这些电荷密度的结果与几何形状和卤素键的性质和卤素…我们的固体中遇到的卤素相互作用。我们有信心贡献可靠的实验数据，正在进行的讨论的相关性，具体的短接触与全球的结构特征，在较长的距离，如静电的贡献或偶极子的相对取向。

Halogen bonds rejoice growing popularity in the field of crystal engineering. A halogen bond occurs between the electrophilic region of a heavy halogen opposite to a covalent bond, often referred to as "sigma hole", and a suitable donor atom with a lone pair as nucleophile. Contacts between neighbouring halogen atoms in suitable geometry can be considered a special case of a halogen bond. Most of the discussions which address such interactions in the solid state are based on geometric considerations. We will combine our expertise in crystal engineering and charge density studies to obtain more in-depth information and probe the halogen bond at the level of electron density. In the context of our proposal, we will investigate crystals in which heavy halogens and lone pair donor atoms subtend short contacts between molecular or ionic residues. These target features may occur in chemically quite different solids: We envisage to study crystals of neutral coordination compounds, of suitably substituted pyridium halides or halogenometallates, and of neutral organic molecules. Co-crystals between halogen bond donors and acceptors offer a particularly versatile approach. In addition to the synthesis and structural chracterization of new compounds we will also consider high resolution studies of such crystals for which only data at standard resolution have been reported. X-ray diffraction on single crystals represents a straightforward method for studying intermolecular contacts, but we will not only investigate our target solids with respect to geometry: In order to assess the electron density and properties such as the electrostatic potential, we will study high quality crystals of compounds featuring interesting packing characteristics at low temperature and up to high resolution. Phase transitions, twinning and disorder represent potential obstacles which must be avoided. From a successful experiment we will deduce the electron density in critical points and its derived quantities such as the Laplacian, energy densities or the much less exploited source function. We expect to correlate these charge density results with the geometry and the nature of the halogen bonds and the halogen...halogen interactions encountered in our solids. We are confident to contribute reliable experimental data to the ongoing discussion about the relevance of specific short contacts versus global structural features acting at longer distances, such as electrostatic contributions or relative orientation of dipoles.