17.8 : 不同共轭系统的 Frost 圆

According to Huckel's rule, benzene is stable and aromatic, while cyclobutadiene and cyclooctatetraene are unstable and not aromatic.

The aromaticity of these compounds can be understood from their Frost circle diagrams.

The Frost circle for benzene has six π molecular orbitals. All the low-energy bonding orbitals are fully occupied with six π electrons, while the high-energy antibonding orbitals are empty. Therefore, benzene is stable and aromatic.

The Frost circles for cyclobutadiene and cyclooctatetraene have four and eight π  molecular orbitals, respectively.

The bonding orbitals are fully occupied for both of the compounds, while the nonbonding orbitals are singly occupied. The antibonding orbitals are empty.

As is evident, the presence of π electrons in high-energy nonbonding orbitals makes the two compounds unstable and not aromatic.

Overall, a given molecule is considered aromatic when all of the π electrons are paired in bonding molecular orbitals. Compounds with electrons in orbitals other than bonding are not aromatic.