Comparative molecular orbital study of [6]-, [10]- and [18]- annulenes and the bridged-[10] annulenes.

In the present theoretical study we are able to show that the resonance energies of the higher annulenes are greater than previously recognized, particularly in the case of [10]annulene. The ring size for the onset of bond-length alternation is shown to be close to the 18-membered ring system [18]annulene. The synthetically accessible bridged annulenes are shown to provide good models for the free standing annulenes. The electronic structure of the annulenes and polyacetylene are compared and it is concluded that the annulenes remain more delocalized, at least up to [18]annulene.