Compared to the well-known conjugated (1,3-dipolar) and cross-conjugated (1,4-dipolar) heterocyclic mesomeric betaines (HMBs), semi-conjugated HMBs are unexplored and almost unknown. The three discrete classes of HMB are defined by the connectivity between their ring 2π heteroatoms and the odd-conjugated fragments that complete the ring. A single example of a stable, fully-characterized semi-conjugate HMB has been reported. This study employs the density functional theory (DFT) methodology to investigate the properties of a series of six-membered semi-conjugated HMBs. The electronic character of ring substituents is found to significantly influence the structure and electronic properties of the ring. The aromaticity measured by HOMA and NICS(1)_zz indices is increased by π-electron-donating substituents whereas π-electron-withdrawing substituents decrease the calculated aromatic character and ultimately lead to non-planar boat or chair structures. A notable property of all derivatives is the small energy gap between their frontier orbitals.

中文翻译：

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1,4-重氮-3,6-二醇酯的 DFT 研究：未开发的半共轭杂环介观甜菜碱的单环代表

与众所周知的共轭（1,3-偶极）和交叉共轭（1,4-偶极）杂环介观甜菜碱（HMB）相比，半共轭 HMB 尚未开发且几乎未知。HMB 的三个离散类别由其环 2π 杂原子与完成环的奇数共轭片段之间的连接性定义。已经报道了一个稳定的、完全表征的半共轭 HMB 的例子。本研究采用密度泛函理论 (DFT) 方法研究一系列六元半共轭 HMB 的性质。发现环取代基的电子特性显着影响环的结构和电子性质。_{HOMA和NICS(1) zz}测得的芳香度π-给电子取代基会增加指数，而π-吸电子取代基会降低计算出的芳香特征，并最终导致非平面的船或椅结构。所有衍生物的一个显着特性是它们的前沿轨道之间的小能隙。