How do the Huckel and Baird Rules Fade away in Annulenes?
How do the Huckel and Baird Rules Fade away in Annulenes?
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DOI：
10.3390/molecules25030711
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发表时间：
2020-02-01
期刊：
影响因子：
4.6
通讯作者：
Matito, Eduard
中科院分区：
文献类型：
--
作者：
Casademont-Reig, Irene;Ramos-Cordoba, Eloy;Matito, Eduard
描述分子芳香性的两个最流行的规则是由Huckel和Baird提出的规则，它们支配着单重态和三重态的芳香性。在这项工作中，我们研究了这些规则是如何随着环结构的增加而消失的，并且由于几何限制，p轨道之间的最佳重叠不再可能。为此，我们研究了碳原子数在4到18之间的中性环烯的最低单重态和三重态。首先，我们从Huckel分子轨道方法分析了这些规律，然后用几种密度泛函近似对环烯进行几何优化，以分析平面性扭曲对环烯芳香性的影响。最后，我们分析了使用不同百分比的Hartree-Fock交换(B3LYP、CAM-B3LYP和M06-2X)和Hartree-Fock的三种密度泛函近似的性能。我们的结果表明，Hartree-Fock交换百分率较低的泛函在长程受到严重的离域误差的影响，这导致了错误的几何结构和对环烯芳香性的高估。
Two of the most popular rules to characterize the aromaticity of molecules are those due to Huckel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between p orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen. First of all, we analyze these rules from the Huckel molecular orbital method and, afterwards, we perform a geometry optimization of the annulenes with several density functional approximations in order to analyze the effect that the distortions from planarity produce on the aromaticity of annulenes. Finally, we analyze the performance of three density functional approximations that employ different percentages of Hartree-Fock exchange (B3LYP, CAM-B3LYP and M06-2X) and Hartree-Fock. Our results reveal that functionals with a low percentage of Hartree-Fock exchange at long ranges suffer from severe delocalization errors that result in wrong geometrical structures and the overestimation of the aromatic character of annulenes.