The Question of Aromaticity in Open-Shell Cations and Anions as Ion-Radical Offsprings of Polycyclic Aromatic and Antiaromatic Hydrocarbons

Arene cation-radicals and anion-radicals result directly from the one-electron oxidation and reduction of
many aromatic hydrocarbons, yet virtually nothing is known of their intrinsic (thermodynamic) stability
and hence “aromatic character”. Since such paramagnetic ion radicals lie intermediate between aromatic
(Hückel) hydrocarbons with 4n+2-electrons and antiaromatic analogues with 4n-electrons, we can now
address the question of π-delocalization in these odd-electron counterparts. Application of the structure-based “harmonic oscillator model of aromaticity” or the HOMA method leads to the surprising conclusion
that the aromaticity of these rather reactive, kinetically unstable arene cation and anion radicals (as measured
by the HOMA index) is actually higher than that of their (diamagnetic) parentcontrary to conventional
expectations.

芳烃阳离子自由基与阴离子自由基，可直接由众多芳香烃经单电子氧化与还原反应生成，但目前学界对其本征（热力学）稳定性乃至固有芳香性几乎一无所知。由于这类顺磁性离子自由基介于拥有4n+2个电子的休克尔（Hückel）芳香烃与拥有4n个电子的反芳香性同类物种之间，我们如今得以探究这类奇电子对应物种的π离域问题。采用基于结构的芳香性谐振子模型（Harmonic Oscillator Model of Aromaticity，简称HOMA）进行分析后，我们得到了令人意外的结论：以HOMA指数为衡量标准，这些活性颇高、动力学稳定性较差的芳烃阴阳离子自由基，其芳香性实则高于其反磁性母体烃——这与传统认知相悖。