THEORETICAL-STUDIES OF [N]PARACYCLOPHANES AND THEIR VALENCE ISOMERS .2. STUDY OF THE REACTIONS OF BENZENE, [6]-PARACYCLOPHANES AND [7]PARACYCLOPHANES TO THEIR DEWAR BENZENE AND PRISMANE ISOMERS IN THE GROUND-STATE

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 DELTAH(r)degrees = -68.9 kcal/mol and the activation enthalpy is DELTAH(degrees)double dagger = 27.9 kcal/mol (with C.I.). The reaction path has C2v 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 DELTAH(degrees)double dagger = 24.6 kcal/mol and the activation entropy is DELTAS0double dagger = 0.6 cal K-1 mol-1 calculated with C.I. The enthalpy of the reaction prismane --> Dewar benzene is DELTAH(r)double dagger almost-equal-to -32 kcal/mol and the activation enthalpy is DELTAH(degrees)double dagger almost-equal-to 19 kcal/mol. The highest molecular symmetry group common to both molecules is C2v, whereas the symmetry group of the reaction path is lowered to C(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.