Bicyclic 1,6-methano[10]annulene (or homonaphthalene)

The carbon atoms at the bridgeheads are each involved in two benzenoid interactions with perimeter carbons and in a sigma bond of the CH2 bridge. We confirm that there is a significant transannular interaction in this molecule, so that the bridgehead carbons exhibit some form of four coordination without being 'tetrahedral' or sp3 hybridised.

The spin-coupled description of how this may be achieved turns out to be particularly simple, being based on an adaptation of sp2 hybridisation to the nonplanarity. The p[[pi]]-like functions on the bridgehead carbon atoms point at an angle to the C2 rotation axis, so as to reduce their proximity to the 'bridge' orbitals above the ring while increasing the overlap with one another under the ring. The calculations provide also a description of the resonance between Kekulé structures associated with the aromaticity of this molecule, and allow a comparison between the bonding in the methano bridge and that in the methane molecule. The study confirms the validity of denoting this molecule homonaphthalene.