Rotaxanes as well as catenanes are known as potential building blocks of molecular machines. The nanohoop [2]rotaxane investigated is composed of a macrocycle derived from a [6]cycloparaphenylene (CCP, designated as a carbon nanohoop), where one of the six para -linked phenyl rings is replaced by a 2,6-substituted pyridyl ring. This macrocycle is mechanically interlocked with a thread, a linear rod-shaped diyne fragment sitting in the cavity of the macrocycle. Two bulky 3,5-di- t -butyl-phenyl rests as end groups keep the thread fixed. The interplay between macrocycle and thread was examined by means of the electron density distribution (EDD) obtained by application of the invariom formalism, relying on X-ray diffraction data collected earlier. The so-obtained EDD was subjected to topological analysis using the QTAIM formalism. Moreover, molecular Hirshfeld and electrostatic potential (ESP) surfaces were calculated. The 73 C–C bonds were analysed in terms of bond topological properties. For the 46 single and the 22 aromatic bonds, the analysis gave average bond orders of 1.03 and 1.61. The five C–C bonds in the diyne fragment can clearly be distinguished into three types: formal triple bonds with bond orders above 3.0, arene bonds with bond orders of 1.6 and finally bond orders of 1.3 in the adjacent C–C bonds, which indicate a considerable electron delocalization in this fragment. Mapping the ED onto the Hirshfeld surfaces of the macrocycle and the thread does not show strong signals. This shows that in between the molecules only weak non-covalent interactions are present. The electrostatic potentials (ESPs) were mapped onto molecular EDD isosurfaces. For all phenyl rings, small regions of negative ESP are visible on the delocalized π systems. A potential gradient between the mostly positive ESP of the macrocycle and the diyne region of the thread exist, which can be considered the dominant force to hold this rotaxane together.

中文翻译：

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基于不变精细化的纳米环[2]轮烷的电子密度

轮烷和链烷被称为分子机器的潜在组成部分。研究的纳米环[2]轮烷是由衍生自[6]环对亚苯基（CCP，称为碳纳米环）的大环组成的，其中六个对位联苯环之一被2,6-取代的吡啶基环取代。该大环与一条线机械互锁，该线位于大环的腔中，是线性的杆状二炔碎片。两个大的3,5-二叔丁基苯基残基作为末端基团保持螺纹固定。大环与线之间的相互作用是通过依赖于先前收集的X射线衍射数据，通过应用不变式获得的电子密度分布（EDD）来检查的。使用QTAIM形式主义对如此获得的EDD进行拓扑分析。而且，计算了分子的Hirshfeld和静电势（ESP）表面。根据键的拓扑特性分析了73个C-C键。对于46个单键和22个芳族键，分析得出的平均键序为1.03和1.61。二炔片段中的五个C–C键可以清楚地分为三种类型：键序高于3.0的正式三键，芳烃键序为1.6的芳烃键以及最终的相邻CC键序中的1.3的键序，这表明在该片段中有相当大的电子离域。将ED映射到大循环的Hirshfeld表面和线程上不会显示强信号。这表明在分子之间仅存在弱的非共价相互作用。将静电势（ESP）映射到分子EDD等值面上。对于所有苯环，在离域π系统上可以看到负ESP的小区域。在大环的大部分为正的ESP与线的二炔区之间存在电势梯度，可以认为是将这种轮烷保持在一起的主导力。