Theoretical studies of [n]paracyclophanes and their valence isomers

Abstract

The valence isomerisations of benzene, [6]- and [7]paracyclophane to their Dewar benzene and prismane isomers are studied with the MNDO method using the unrestricted Hartree-Fock (UHF) and the configuration interaction (C.I.) approximations. The enthalpy of the reaction Dewar benzene → benzene is ΔH° r =−68.9 kcal/mol and the activation enthalpy is ΔH°‡=27.9 kcal/mol (with C.I.). The reaction path hasC 2v symmetry. The determination of several points of the lowest potential energy surface of [6]- and [7]paracyclophanes leads to a minimum reaction path having the same topology as for the potential energy surface of the nonbridged benzene. The only difference is a quantitative change in the energy values of the aromatic isomers due to the deformation introduced by the alkyl chain. For [6]paracyclophane, the activation enthalpy is ΔH°‡=24.6 kcal/mol and the activation entropy is ΔS 0‡=0.6 cal K−1 mol−1 calculated with C.I. The enthalpy of the reaction prismane → Dewar benzene is ΔH° r ≈−32 kcal/mol and the activation enthalpy is ΔH°‡≈19 kcal/mol. The highest molecular symmetry group common to both molecules isC 2v , whereas the symmetry group of the reaction path is lowered toC s . Along this reaction path is located a biradicaloid intermediate, separated by low activation barriers from the products. No significant changes of the potential energy surfaces are found for the bridged [n]prismanes and the [n]Dewar benzenes. All the calculated values, reaction enthalpies, activation enthalpies and entropies, are in a good agreement with literature experimental data.