Molčanov Krešimir, Kojić-Prodić Biserka
Department of Physical Chemistry, Rudjer Bošković Institute, Bijenička 54, Zagreb 10000, Croatia.
IUCrJ. 2019 Feb 2;6(Pt 2):156-166. doi: 10.1107/S2052252519000186. eCollection 2019 Mar 1.
The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization ( quinones) than those involving delocalized systems ( aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding ('pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.
基于作者自身在量子化学计算辅助下的实验X射线电荷密度分析结果，首次对非芳香环之间的π相互作用进行了系统研究。具有里程碑意义的（非芳香）实例包括醌环、平面自由基和金属螯合环。结果可总结为：（i）非芳香平面多烯环可以堆叠，（ii）与涉及离域体系（芳烃）的体系或环相比，在几乎没有或没有π电子离域的体系（醌）之间的相互作用更为明显，以及（iii）相互作用的主要成分是闭壳层环之间的静电/多极相互作用，而（iv）自由基之间的相互作用涉及显著的共价贡献（多中心键合）。因此，堆积涵盖了广泛的相互作用和能量范围，从弱色散到非定域双电子多中心共价键合（“煎饼键合”），这使得在某些化学物种（醌阴离子）中能够实现面对面的堆积排列。特定堆积体系中的主要相互作用调节了物理性质，并为功能材料的晶体工程定义了一种策略。
