Molecular crystals exhibiting polar symmetry are important paradigms for developing new electrooptical materials. Though accessing bulk polarity still presents a significant challenge, in some cases it may be rationalized as being associated with the specific molecular shapes and symmetries and subtle features of supramolecular interactions. In the crystal structure of 3,5,7‐trinitro‐1‐azaadamantane, C₉H₁₂N₄O₆, the polar symmetry of the molecular arrangement is a result of complementary prerequisites, namely the C_3v symmetry of the molecules is suited to the generation of polar stacks and the inherent asymmetry of the principal supramolecular bonding, as is provided by NO₂(lone pair)…NO₂(π‐hole) interactions. These bonds arrange the molecules into a trigonal network. In spite of the apparent simplicity, the structure comprises three unique molecules (Z′ =  +  + ), two of which are donors and acceptors of three N…O interactions and the third being primarily important for weak C—H…O hydrogen bonding. These distinct structural roles agree with the results of Hirshfeld surface analysis. A set of weak C—H…O and C—H…N hydrogen bonds yields three kinds of stacks. The orientation of the stacks is identical and therefore the polarity of each molecule contributes additively to the net dipole moment of the crystal. This suggests a special potential of asymmetric NO₂(lone pair)…NO₂(π‐hole) interactions for the supramolecular synthesis of acentric materials.

中文翻译：

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不对称NO2（孤对）... NO2（π-孔）超分子键合介导的3,5,7-三硝基-1-氮杂金刚烷的本体极性

具有极性对称性的分子晶体是开发新型电光材料的重要范例。尽管获得体相极性仍然是一个巨大的挑战，但在某些情况下，它可能被合理化为与特定的分子形状和对称性以及超分子相互作用的细微特征有关。在3,5,7-三硝基-1-氮杂金刚烷C ₉ H ₁₂ N ₄ O ₆的晶体结构中，分子排列的极性对称是互补条件的结果，即分子的C _{3 v}对称性是适用于极性堆叠的生成和主要的超分子键的固有不对称性，如NO₂（孤对）…NO ₂（π孔）相互作用。这些键将分子排列成三角形网络。尽管表面上看起来很简单，但该结构仍包含三个独特的分子（Z '=  +   +  ），其中两个是三个N…O相互作用的供体和受体，第三个对于弱的C–H…O氢键至关重要。这些独特的结构作用与Hirshfeld表面分析的结果一致。一组弱的CHH和OCHN氢键产生三种堆叠。堆叠的方向是相同的，因此每个分子的极性对晶体的净偶极矩有附加作用。这表明非对称NO ₂（孤对）的特殊潜力…NO超分子合成无心材料的₂（π-hole）相互作用。