Antiaromatic ions and their value in quantifying aromaticity, as probes of delocalization, and potential as stable diradicals

Abstract

Antiaromatic dications and dianions are available through oxidation or reduction of unsaturated precursors as well as through ionization of diols. Their experimental accessibility allows them to be used to evaluate the effectiveness of theoretical treatments of antiaromaticity such as the nucleus-independent chemical shift (NICS). Because measures of antiaromaticity are generally larger in magnitude than corresponding measures of aromaticity, they demonstrate relationships between different measures of aromaticity–antiaromaticity, such as those based on magnetic and energetic properties, more effectively than do aromatic measures. Antiaromatic species are more sensitive probes of delocalization than are aromatic species, giving the possibility of observation of subtle effects. Antiaromatic species possess a small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap, giving rise to species with diradical character.