Ivanov Maxim V, Wang Denan, Wadumethridge Shriya H, Rathore Rajendra
Department of Chemistry, Marquette University , P.O. Box 1881, Milwaukee, Wisconsin 53201-1881, United States.
Faculty of Science, University of Ruhuna , Wellmadama, Matara 81000, Sri Lanka.
J Phys Chem Lett. 2017 Sep 7;8(17):4226-4230. doi: 10.1021/acs.jpclett.7b01971. Epub 2017 Aug 23.
Frontier molecular orbitals (FMOs) have played a critical role in predicting reactivity/selectivity of pericyclic reactions. Here we show that the structurally similar iptycene-based hydroquinone ether (HE), that is, Ipt and HE/HE, molecules have drastically different ordering of bisallylic and quinoidal FMOs. They are almost degenerate in HE/HE, while in Ipt, the bisallylic orbital lies far below the quinoidal HOMO. Oxidation of HE/HE induces coplanarization of the methoxy group and destabilizes the bisallylic HOMO, leading to a relatively low oxidation potential. In Ipt, considerable energy must be invested in coplanarization of the methoxy group to bring about orbital swapping, resulting in an oxidation potential higher than that in structurally similar HE/HE. As the quinoidal HOMO density does not extend to the substituent-bearing carbon in H-, alkyl-, and alkoxy-substituted iptycenes, their redox potentials remain invariant. This case study involving a simple visual inspection of the nodal arrangement as well as energetics of the FMOs and Walsh analysis could serve as a tool for the design of organic molecules with a desired redox potential.
前沿分子轨道（FMOs）在预测周环反应的反应性/选择性方面发挥了关键作用。在此我们表明，结构相似的基于异杯芳烃的对苯二酚醚（HE），即Ipt和HE/HE分子，其双烯丙基和醌型FMOs的排序截然不同。它们在HE/HE中几乎简并，而在Ipt中，双烯丙基轨道远低于醌型最高占据分子轨道（HOMO）。HE/HE的氧化会导致甲氧基共面化，并使双烯丙基HOMO不稳定，从而导致相对较低的氧化电位。在Ipt中，必须投入相当多的能量使甲氧基共面化以实现轨道交换，导致氧化电位高于结构相似的HE/HE中的氧化电位。由于醌型HOMO密度在氢、烷基和烷氧基取代的异杯芳烃中不会延伸到带有取代基的碳原子上，它们的氧化还原电位保持不变。这个涉及对FMOs的节点排列以及能量学进行简单目视检查和沃尔什分析的案例研究，可以作为设计具有所需氧化还原电位的有机分子的工具。
