Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, China.
CAEP Software Center for High Performance Numerical Simulation, Beijing 100088, China.
Molecules. 2020 Jul 15;25(14):3232. doi: 10.3390/molecules25143232.
Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.
理解含氮杂环的稳定性在高能材料领域至关重要，需要对氮芳烃有创新的认识。在此，我们首次报道，在五元含氮杂环中，氮孤对电子（NLPE）离域除了产生典型的π-芳香性外，还会产生第二种σ-芳香性。随着环中更多的碳原子被不饱和氮原子取代，NLPE 离域和伴随的双重芳香性增强。环中相邻氮原子的存在可以增强氮杂环的芳香性，并提高晶体间的分子间结合强度，但会降低单个分子结构的牢固性。值得注意的是，这种σ-芳香性不存在于六元含氮杂环中，可能是由于其键更长，环的区域更宽；因此，六元杂环的整体芳香性低于五元杂环。这项工作为氮芳烃带来了新知识，预计将激发化学界的广泛兴趣。
