Thomsen Maja K, Gatti Carlo, Overgaard Jacob
Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark.
CNR-ISTM Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133, Milan, Italy.
Chemistry. 2018 Apr 3;24(19):4973-4981. doi: 10.1002/chem.201800014. Epub 2018 Mar 2.
The extent of cyclic π-electron delocalization in the N-heterocyclic ring of an imidazol-2-ylidene (i.e., 1,3,4,5-tetramethylimidazol-2-ylidene) and its corresponding imidazolium salt (i.e., 1,3,4,5-tetramethylimidazolium chloride) has been investigated theoretically by using Bader's quantum theory of atoms in molecules (QTAIM) descriptors, delocalization indices, electron localizability indicators (ELI-Ds), and the source function tool. In addition, the experimental electron density distribution for the imidazolium salt has been obtained and analyzed from 100 K X-ray diffraction data. A significant drop is found in the ellipticity of the electron density along the C -N bond path in the imidazol-2-ylidene. This is shown to be a natural consequence of the σ lone pair of the C atom, which overwhelms the π-electron density, rather than a sign of a significantly diminished degree of π-electron delocalization in the imidazol-2-ylidene compared to its imidazolium salt. In fact, the source functions, the ELI-Ds, and the delocalization indices all probe a quite similar extent of cyclic π-electron delocalization in the N-heterocyclic rings of the two compounds.
通过使用巴德分子中原子的量子理论（QTAIM）描述符、离域指数、电子定域性指标（ELI-Ds）和源函数工具，从理论上研究了咪唑-2-亚基（即1,3,4,5-四甲基咪唑-2-亚基）及其相应的咪唑鎓盐（即1,3,4,5-四甲基咪唑鎓氯化物）的N-杂环中环状π电子离域的程度。此外，还从100K的X射线衍射数据中获得并分析了咪唑鎓盐的实验电子密度分布。在咪唑-2-亚基中，沿着C-N键路径的电子密度椭圆率有显著下降。这表明这是C原子的σ孤对电子压倒π电子密度的自然结果，而不是与咪唑鎓盐相比，咪唑-2-亚基中π电子离域程度显著降低的迹象。事实上，源函数、ELI-Ds和离域指数都探测到了这两种化合物的N-杂环中环状π电子离域的相当相似的程度。
