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.