How bent can a benzene be? The molecular structure, infrared spectrum and energetics of [6]paracyclophane

Abstract

Ab initio molecular quantum mechanical methods have been applied to the [6] paracyclophane molecule (C 12H 16), prepared by M. Jones and co-workers in 1974. The complete molecular structure has been predicted via the self-consistent-field (SCF) method, using both minimum and double-zeta [C(9s5p/4s2p), H(4s/2s)] basis sets. The critical geometrical parameter is the deviation φ of the benzene ring from planarity, and φ is predicted to be 17.2° and 18.6°, respectively, with the minimum and double-zeta (DZ) basis sets. The DZ angle agrees well with the experimental values, 19.4° and 20.7° , taken from crystal structures of substituted [6] paracyclophanes. These bending angles φ are 5.2° (minimum basis) and 5.1° (DZ basis) less than the analogous theoretical predictions for the less stable [5] paracyclophane. DZ SCF harmonic vibrational frequencies and infrared intensities are presented and compared with analogous predictions for benzene. The vertical excitation energies for the lowest triplet and first excited singlet states of [6] paracyclophane are also investigated. Finally the resonance energy is predicted from a suitable homodesmotic reaction and compared to the analogous result for benzene. The question of whether [6] paracyclophane is aromatic is discussed from several different perspectives.