Bonding in the monohomocyclooctatetraene anion radical

Abstract

We have carried out a series of INDO calculations on the monohomocyclooctatetraene anion radical (MHCOT ▪ ). Based upon both the calculated molecular binding energy and comparison of calculated proton hyperfine coupling constants with previously published data, we conclude that the 8-membered ring of MHCOT ▪ is planar. Good agreement was obtained with the experimental hyperfine splittings when the angle between the plane of the 8-membered ring and the plane of the fused cyclopropyl ring was 83·5°. A high π bond order between the carbon atoms at the base of the fused cyclopropyl group strongly supports Winstein's concept of homoconjugation. However, the σ bond orders across the interruption are only reduced by 10–20% relative to the other CC bonds in the ring, so this bond cannot be considered opened in the anion radical. This is in agreement with the prediction, based on molecular-orbital symmetries, that both disrotatory and conrotatory ring openings are disallowed processes for this species.