Bond lengths in cyclic polyenes C2nH2n a re-examination from the valence-bond point of view

The valence-bond resonance method for predicting bond lengths in conjugated hydrocarbon molecules has been reconsidered. New values are obtained for the variation of the fundamental exchange and Coulomb integrals with bond length. Application to the cyclic polyenes C_2nH_2n shows that for large n there will be substantial bond alternation. Inclusion of Dewar structures diminishes this alternation, but does not destroy it.