芳香性Aromaticity

休克尔规则：单环、平面、每个环原子都有 p 轨道参与共轭、π 电子数为 4n+2（2、6、10…）的体系具有芳香性；4n 个 π 电子的平面环状共轭体系则不稳定，称为反芳香性。现代判据包括：能量判据（芳香稳定化能）、几何判据（键长平均化，如 HOMA 指数）和磁性判据（¹H NMR 环外质子去屏蔽、NICS 为负值）。

Hückel's rule: a monocyclic, planar system in which every ring atom contributes a p orbital to conjugation and which has 4n+2 π electrons (2, 6, 10 …) is aromatic; a planar cyclic conjugated system with 4n π electrons is destabilised, or antiaromatic. Modern criteria are energetic (aromatic stabilisation energy), geometric (bond-length equalisation, such as the HOMA index) and magnetic (deshielding of outer ¹H protons and negative NICS values).

“芳香”最初指从安息香、苦杏仁油、冬青油等香料中得到的化合物，它们多含苯环。1855 年霍夫曼首次用 aromatic 指代这类结构。1865 年凯库勒提出苯的环状结构，1925 年阿米特和罗宾逊提出“芳香六隅体”（六个电子构成稳定的电子组）。1931 年休克尔用分子轨道法导出 4n+2 规则，给出理论解释。

Aromatic originally described compounds obtained from fragrant substances such as benzoin, bitter-almond oil and wintergreen oil, many of which contain a benzene ring. Hofmann first used the word for this class of structures in 1855. Kekulé proposed benzene's ring in 1865, and in 1925 Armit and Robinson proposed the 'aromatic sextet' of six electrons. In 1931 Hückel derived the 4n+2 rule by molecular orbital theory.

芳香性不限于苯：环戊二烯负离子、环庚三烯正离子、吡啶、吡咯、呋喃、噻吩等杂环以及萘、蒽等稠环也有芳香性。[18]轮烯是大环芳香体系的例子。后来又提出了莫比乌斯芳香性（4n 电子扭曲环）、球面芳香性（富勒烯，2(n+1)² 规则）和激发态芳香性（贝尔德规则）。

Aromaticity is not limited to benzene: the cyclopentadienyl anion, the tropylium cation, heterocycles such as pyridine, pyrrole, furan and thiophene, and fused rings like naphthalene and anthracene are aromatic too. [18]Annulene is a large-ring example. Later extensions include Möbius aromaticity (4n electrons on a twisted ring), spherical aromaticity in fullerenes (the 2(n+1)² rule) and excited-state aromaticity (Baird's rule).