[4] Metacyclophane: STO‐3G molecular structure, strain energy and CNDO/SCl electronic transition energies

The molecular structure of [4] metacyclophane is optimized at the ab initio STO‐3G SCF level and a genuine minimum is located. In the STO‐3G structure the benzene ring is less bent and geometrically distorted than in the previously reported MNDO structure. Although some bond alternation is calculated for the carbon–carbon bond lengths of the bent benzene ring, their values still fall in the range of highly delocalized compounds. Despite the fact that the STO‐3G strain energy [SE(tot.)] is larger than the corresponding MNDO strain energy, the distribution of SE(tot.) over the bent benzene ring [SE(bb.)] and the oligomethylene bridge [SE(br.)] is similar. To facilitate the hitherto unsuccesful identification of [4] metacyclophane, its electronic transition energies were calculated with CNDO/S–CI using the STO‐3G geometry as input geometry. The reliability of the CNDO/S–CI results was assessed by a comparison of calculated and experimental electronic transition energies for p ‐ and, m ‐xylene, [5] para‐ and [5] metacyclophane and [4] paracyclophane following the same approach.