Dickens Timothy K, Mallion Roger B
Peterhouse , Cambridge CB2 1RD , England, United Kingdom.
J Phys Chem A. 2018 Nov 15;122(45):8858-8864. doi: 10.1021/acs.jpca.8b08722. Epub 2018 Nov 6.
It is well-known that the very special family of conjugated systems described as "fully benzenoid hydrocarbons" have a unique Clar structure that causes all of their rings to be classified as either "full" or "empty". In this work, π-electron ring-currents in more than two dozen such structures are calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. In the case of benzenoid hydrocarbons, the calculated values of such "topological" ring-currents depend on only the carbon-carbon connectivity of the system in question. It is found that, until a certain fully benzenoid hydrocarbon comprising 17 rings is examined, all ring currents in full rings are seen to be larger in intensity than the ring current in benzene, while such currents in empty rings are smaller than the benzene value. There is an indication that this distinction might break down when "giant" fully benzenoid structures are considered. These conclusions are consistent with chemical intuition, with earlier studies examining Clar aromatic sextets by means of NICS calculations, and with previous qualitative ab initio predictions of the patterns of π-electron current-flow in the fully benzenoid hydrocarbons.
众所周知，被描述为“全苯型烃”的共轭体系这一特殊家族具有独特的克拉尔结构，该结构使得它们所有的环被归类为“满”或“空”。在这项工作中，通过准图形理论的、无参数的休克尔 - 伦敦 - 波普尔 - 麦克韦尼（HLPM）方法计算了二十多种此类结构中的π电子环流。对于苯型烃，这种“拓扑”环流的计算值仅取决于所讨论体系的碳 - 碳连接性。研究发现，在研究包含17个环的某个全苯型烃之前，满环中的所有环流强度都比苯中的环流强度大，而空环中的此类环流则比苯的值小。有迹象表明，当考虑“巨型”全苯型结构时，这种区别可能会失效。这些结论与化学直觉、早期通过核独立化学位移（NICS）计算研究克拉尔芳香六隅体的研究以及之前对全苯型烃中π电子电流流动模式的定性从头算预测一致。
