伪Jahn-Teller活性对氮杂芘类单重态-三重态能隙的影响
Influence of Pseudo-Jahn-Teller Activity on the Singlet-Triplet Gap of Azaphenalenes
AI总结：
本研究通过多种量子化学方法分析6种氮杂芘类分子的伪Jahn-Teller对称性降低效应，发现高对称结构下的洪特规则违反现象在低对称畸变后消失，验证了洪特规则的有效性。
摘要：
我们分析了6种已被研究的氮杂芘类分子中由伪Jahn-Teller相互作用诱导的对称性降低可能性，已知这些分子的第一激发单重态（S₁）能量低于三重态（T₁）。本研究的主要目的是探究当这些分子的结构因振动耦合从C₂ᵥ或D₃ₕ点群对称性发生畸变时，是否会观测到洪特规则的违反。沿着连接这些点群与其子群Cₛ或C₃ₕ的两条原子间距坐标，我们弛豫了其他内自由度并计算了二维势能子面。多体微扰理论（MP2）表明，高对称结构是所有6个体系的能量极小值。然而，使用耦合簇方法（CCSD(T)）的单点能计算显示有4种情况存在对称性降低。绘制在势能面上的单重态-三重态能隙在偏离高对称结构时也呈现出变化。在CCSD(T)水平结合cc-pVTZ基组的全几何优化揭示，环嗪（1AP）的D₃ₕ结构是一个鞍点，连接着两个经历快速自异构化的等价C₃ₕ对称极小值。对称性降低与高级校正的共同作用使得环嗪的C₃ₕ结构的单重态-三重态能隙几乎为零。在缺电子位点含有氮原子的氮杂芘类——2AP、3AP和4AP——表现出更显著的面内结构畸变；该效应可被长程交换相互作用校正的DFT方法ωB97XD捕捉到。这些体系的激发态计算表明，在它们的低对称能量极小值处，T₁的能量确实低于S₁，从而支持了洪特规则的有效性。
英文摘要：
We analyze the possibility of symmetry-lowering induced by pseudo-Jahn--Teller interactions in six previously studied azaphenalenes that are known to have their first excited singlet state (S$_1$) lower in energy than the triplet state (T$_1$). The primary aim of this study is to explore whether Hund's rule violation is observed in these molecules when their structures are distorted from $C_{\rm 2v}$ or $D_{\rm 3h}$ point group symmetries by vibronic coupling. Along two interatomic distances connecting these point groups to their subgroups $C_{\rm s}$ or $C_{\rm 3h}$, we relaxed the other internal degrees of freedom and calculated two-dimensional potential energy subsurfaces. The many-body perturbation theory (MP2) suggests that the high-symmetry structures are the energy minima for all six systems. However, single-point energy calculations using the coupled-cluster method (CCSD(T)) indicate symmetry lowering in four cases. The singlet-triplet energy gap plotted on the potential energy surface also shows variations when deviating from high-symmetry structures. A full geometry optimization at the CCSD(T) level with the cc-pVTZ basis set reveals that the $D_{\rm 3h}$ structure of cyclazine (1AP) is a saddle point, connecting two equivalent minima of $C_{\rm 3h}$ symmetry undergoing rapid automerization. The combined effects of symmetry lowering and high-level corrections result in a nearly zero singlet-triplet gap for the $C_{\rm 3h}$ structure of cyclazine. Azaphenalenes containing nitrogen atoms at electron-deficient sites -- 2AP, 3AP, and 4AP -- exhibit more pronounced in-plane structural distortion; the effect is captured by the long-range exchange-interaction corrected DFT method, $ω$B97XD. Excited state calculations of these systems indicate that in their low-symmetry energy minima, T$_1$ is indeed lower in energy than S$_1$, upholding the validity of Hund's rule.
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