Majumdar Atreyee, Jindal Komal, Das Surajit, Ramakrishnan Raghunathan
Tata Institute of Fundamental Research, Hyderabad 500046, India.
Phys Chem Chem Phys. 2024 Oct 30;26(42):26723-26733. doi: 10.1039/d4cp02761b.
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) lower in energy than the triplet state (T). 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 or point group symmetries by vibronic coupling. Along two interatomic distances connecting these point groups to their subgroups or , 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 structure of cyclazine (1AP) is a saddle point, connecting two equivalent minima of symmetry undergoing rapid automerization. The combined effects of symmetry lowering and high-level corrections result in a nearly zero singlet-triplet gap for the 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, ωB97XD. Excited state calculations of these systems indicate that in their low-symmetry energy minima, T is indeed lower in energy than S, upholding the validity of Hund's rule. Jahn-Teller analysis predicts the symmetries of the in-plane distortion vibrational modes as or B: → agreeing with the vibrational frequencies of the saddle-points.
我们分析了在六种先前研究过的氮杂并苯中，由赝 Jahn-Teller 相互作用引起对称性降低的可能性，已知这些氮杂并苯的第一激发单重态（S）能量低于三重态（T）。本研究的主要目的是探究当这些分子的结构通过振动耦合从 或 点群对称性发生畸变时，是否会观察到洪德规则的违反。沿着连接这些点群与其子群 或 的两个原子间距离，我们放宽了其他内部自由度并计算了二维势能子表面。多体微扰理论（MP2）表明，高对称结构是所有六个体系的能量最小值。然而，使用耦合簇方法（CCSD(T)）进行的单点能量计算表明，在四种情况下存在对称性降低。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显示出变化。使用 cc-pVTZ 基组在 CCSD(T) 水平上进行的全几何优化表明，环嗪（1AP）的 结构是一个鞍点，连接着两个经历快速自动异构化的 对称性等效最小值。对称性降低和高级校正的综合作用导致环嗪的 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并苯——2AP、3AP 和 4AP——表现出更明显的面内结构畸变；这种效应通过长程交换相互作用校正的 DFT 方法 ωB97XD 得以捕捉。这些体系的激发态计算表明，在其低对称能量最小值处，T 的能量确实低于 S，这支持了洪德规则的有效性。Jahn-Teller 分析预测面内畸变振动模式的对称性为 或 B： → ，与鞍点的振动频率一致。
