Are Oxocarbon Dianions Aromatic?

Assessment of the cyclic electron delocalization of the oxocarbon dianions, C n O n 2- and their neutral counterparts C n O n (n = 3−6), by means of structural, energetic, and magnetic criteria, shows that C3O3 2- is doubly aromatic (both σ and π cyclic electron delocalization), C4O4 2- is moderately aromatic, but C5O5 2-, as well as C6O6 2-, are less so. Localized orbital contributions, computed by the individual gauge for localized orbitals method (IGLO), to the nucleus-independent chemical shifts (NICS) allow π effects to be disected from the σ single bonds and other influences. The C−C(π) contribution to (NICS(0,π) (i.e., at the center of the ring) in oxocarbon dianions decreases with ring size but shows little ring size effect at points 1.0 Å above the ring. On the basis of the same criteria, C4O4 exhibits cyclic electron delocalization due to partial occupancy of the σ CC bonds. However, the dissociation energies of all the neutral oxocarbons, C n O n , are highly exothermic.