α/β Conflicting Aromaticity Under the Microscope: Study of Pro-Aromatic Radicals

The aromaticity of a representative sample of pro-aromatic radicals and its nitro, amino, hydroxyl and imine substituted derivatives has been analysed by means of multicenter delocalization indices (MCI) and nuclear-independent chemical shifts (NICS). Because of their radical character, these compounds may exhibit conflicting α/β aromaticity, so that the contribution of α and β electrons to the MCI and NICS has been analysed separately and their values qualitatively interpreted in terms of the 2n+1/2n rule. All the monocyclic radicals investigated show conflicting α/β aromaticity. Thus, significant MCI values have been found for one of the spin components, whereas negligible values, even slightly negative, have been obtained for the other spin component. NICS indices confirm the aromatic character reflected by the MCIs and clearly show the antiaromatic or non-aromatic character of the other spin components. The same has been found for the donor rings of the bicyclic pro-aromatic radicals investigated, whereas all the fused benzene rings of these radicals show a clear aromatic character in both spin components. Substitutions at the donor carbon position by strong deactivating groups provoke a small increase of the aromatic character in the aromatic spin components and a significant decrease of the antiaromatic character in the antiaromatic spin components whereas activating groups have little effect.