Are Oxocarbon Dianions Aromatic?
- 1 January 2000
- journal article
- Published by American Chemical Society (ACS) in The Journal of Organic Chemistry
- Vol. 65 (2) , 426-431
- https://doi.org/10.1021/jo991267n
Abstract
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 σ and π 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 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, CnOn, are highly exothermic.Keywords
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