Jirásek Michael, Rickhaus Michel, Tejerina Lara, Anderson Harry L
Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, U.K.
J Am Chem Soc. 2021 Feb 10;143(5):2403-2412. doi: 10.1021/jacs.0c12845. Epub 2021 Jan 26.
Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol. Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol. The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of ∼11 kJ mol, deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of ∼37 kJ mol in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.
增强的热力学稳定性是芳香族分子的一个基本特征，但以往大多数关于芳香稳定化能（ASE）的研究都局限于具有多达18个π电子的小环。在此，我们证明了在具有76 - 108个π电子的Hückel环的π共轭卟啉纳米环中，可以通过实验检测到ASE。这一结论通过分析氧化还原电位以计算将芳香环转化为反芳香环或反之的等键反应的能量变化得到支持。它也通过分析在过渡态破坏芳香性的构象平衡的能垒得到支持。这两种类型的实验均表明阳离子卟啉纳米环显示出1 - 5 kJ/mol的ASE。密度泛函理论计算重现了这两种类型实验的结果，并预测ASE在1 - 16 kJ/mol范围内。将卟啉纳米环中的实验ASE与通过分析[16]、[18]和[20]轮烯中构象平衡的能垒推导得出的[18]轮烯约11 kJ/mol的实验ASE进行了比较。等键反应的计算能量给出[18]轮烯中约37 kJ/mol的ASE。这项工作有助于对大型大环中芳香性的基本理解。
