The structure of the bitetrahedryl molecule—A major shift due to electron correlation: Effects of carbonyl substituents, implications for the structure of coupled tricyclo[3.1.0.0^2,6]hexyl, and extension to cubylcubane

Ab initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ+d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central CC bond distance while increasing the adjacent CC distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.0^2,6]hexane that carries over to the coupled bicyclo[3.1.0.0^2,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree–Fock theory and experiment occurs for the central CC distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed‐shell hydrocarbon molecules for which Hartree–Fock theory encounters structural difficulties.