Faculty of Pharmacy, University of Sydney, NSW 2006, Australia.
J Phys Chem A. 2012 Jun 14;116(23):5618-28. doi: 10.1021/jp210803m. Epub 2012 May 30.
The experimental charge density distributions in a host-guest complex have been determined. The host, 1,4-bis[[(6-methylpyrid-2-yl)amino]carbonyl]benzene (1) and guest, adipic acid (2). The molecular geometries of 1 and 2 are controlled by the presence in the complex of intermolecular hydrogen bonding interactions and the presence in the host 1 of intramolecular hydrogen bonding motifs. This system therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density, and the transferability of electron distribution properties between closely related molecules. For the complex, high resolution X-ray diffraction data created the basis for a charge density refinement using a pseudoatomic multipolar expansion (Hansen-Coppens formalism) against extensive low-temperature (T = 100 K) single-crystal X-ray diffraction data and compared with a selection of theoretical DFT calculations on the same complex. The molecules crystallize in the noncentrosymmetric space group P2(1)2(1)2(1) with two independent molecules in the asymmetric unit. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with multipole populations, showing that the host and guest structures are relatively unaltered by the geometry changes on complexation. Three separate refinement protocols were adopted to determine the effects of the inclusion of calculated hydrogen atom anisotropic displacement parameters on hydrogen bond strengths. For the isotropic model, the total hydrogen bond energy differs from the DFT calculated value by ca. 70 kJ mol(-1), whereas the inclusion of higher multipole expansion levels on anisotropic hydrogen atoms this difference is reduced to ca. 20 kJ mol(-l), highlighting the usefulness of this protocol when describing H-bond energetics.
实验测定了主体-客体配合物中的电荷密度分布。主体是 1,4-双[[(6-甲基吡啶-2-基)氨基]羰基]苯（1），客体是己二酸（2）。1 和 2 的分子几何形状受到配合物中分子间氢键相互作用的存在以及主体 1 中分子内氢键模式的存在的控制。因此，该体系是研究非共价相互作用及其对结构和电子密度的影响以及在密切相关的分子之间转移电子分布性质的能力的理想模型。对于该配合物，高分辨率 X 射线衍射数据为使用赝原子多极扩展（Hansen-Coppens 形式主义）进行电荷密度精修提供了基础，该精修是基于大量低温（T = 100 K）单晶 X 射线衍射数据，并与同一配合物的一系列理论 DFT 计算进行了比较。分子在非中心对称空间群 P2(1)2(1)2(1)中结晶，每个不对称单位中有两个独立的分子。使用分子中的原子方法对所得密度分布进行拓扑分析，并给出了多极子分布，表明主体和客体结构在配合物形成时的几何变化相对没有改变。采用了三种不同的精修方案来确定包含计算的氢原子各向异性位移参数对氢键强度的影响。对于各向同性模型，总氢键能量与 DFT 计算值相差约 70 kJ mol(-1)，而在各向异性氢原子上包含更高的多极扩展水平时，这种差异减小到约 20 kJ mol(-l)，突出了该方案在描述氢键能量学时的有用性。
