Are oxocarbon dianions aromatic?

Assessment of the cyclic electron delocalization of the oxocarbon dianions, CnOn2- and their neutral counterparts CnOn (n = 3-6), by means of structural, energetic, and magnetic criteria, shows that C3O32- is doubly aromatic (both a and Jr cyclic electron delocalization), C4O42- is moderately aromatic, but C5O52-, as well as C6O62-, are less so. Localized orbital contributions, computed by the individual gauge for localized orbitals method (IGLO), to the nucleus-independent chemical shifts (NICS) allow pi effects to be disected from the a single bonds and other influences. The C-C(n) contribution to (NICS(0,pi) (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 Angstrom above the ring. On the basis of the same criteria, C4O4 exhibits cyclic electron delocalization due to partial occupancy of the a CC bonds. However, the dissociation energies of all the neutral oxocarbons, CnOn, are highly exothermic.