State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China.
Chem Asian J. 2018 Dec 4;13(23):3691-3696. doi: 10.1002/asia.201801279. Epub 2018 Oct 31.
Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both σ- and π-aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, σ-aromaticity stabilizes fully saturated rings with σ-electron delocalization whereas the traditional π-aromaticity describes the π-conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus-independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the σ-aromaticity in an unsaturated three-membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is σ-aromatic. Our findings revolutionize the fundamental knowledge of the concept of σ-aromaticity, thus opening an avenue to design σ-aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of π-aromaticity.
芳香性是最基本和最迷人的化学主题之一，由于其多种表现形式，吸引了实验和理论化学家的关注。σ-和π-芳香性均可根据环状电子离域的性质进行分类。一般来说，σ-芳香性稳定具有σ-电子离域的完全饱和环，而传统的π-芳香性描述完全不饱和环中的π-共轭。在这里，我们展示了核独立化学位移（NICS）值与额外的环状共振能（ECRE）之间的强相关性，这些值用于评估环丙烯的不饱和三员环（3MR）中的σ-芳香性，这些值分别通过分子轨道（MO）理论和价键（VB）理论计算。进一步的研究表明，亚甲基环丙烯的完全不饱和环及其金属类似物是σ-芳香性的。我们的发现彻底改变了σ-芳香性概念的基本知识，从而为在其他完全不饱和系统中设计σ-芳香性开辟了道路，这些系统传统上被保留为π-芳香性的领域。
