AB-INITIO STUDY OF THE ISOMERIZATION OF SUBSTITUTED BENZENES AND [6]PARACYCLOPHANES TO THE DEWAR BENZENE ISOMERS

The energy differences (Delta E) between the benzene and the correspon ding Dewar benzene forms of seven substituted benzenes and three [6]pa racyclophanes were calculated by ab initio methods with basis sets ran ging from split valence type to triple-zeta plus polarization type. Co rrelation effects were taken into account using second-order Moller-Pl esset perturbation theory (MP2). The results show that among all the i nvestigated molecules the isomerization energy is largest for benzene itself; that is, benzene is 79 kcal/mol more stable than its Dewar for m. Both, electron-accepting substituents and electron-donating substit uents lower Delta E. Steric repulsion between substituents in the orth o position additionally destabilizes the benzene form relative to the Dewar form. [6]Paracyclophanes are influenced by substitution effects in the same way as the unbridged compounds. This finding can be interp reted as a further criterion for the aromaticity of the strongly defor med benzene ring in [6]paracyclophanes. Due to the deformation of the benzene ring, the Delta E values for the investigated [6]paracyclophan es are found to be considerably lower than those of the corresponding benzenes (about 25 kcal/mol). Our results are found to agree with the experimental isomerization enthalpies of hexamethylbenzene and perfluo rohexamethylbenzene to within 5 kcal/mol, while a large discrepancy be tween theory and experiment is obtained for 8,9-dicarbethoxy[6]paracyc lophane.