Ab initio study of the stability of [n]paracyclophanes and their Dewar benzene-type isomers

Abstract

Ab initio calculations are carried out for [n]paracyclophanes and their Dewar benzene isomers forn=5, 6, and 7 as well as for the benzene and Dewar benzene itself. The benzene isomers are studied by employing various AO basis sets ranging from 6–31G type to such of triple-zeta type including twod and onef function on the carbons and twop and oned on the hydrogens. The correlation energy contribution is computed by employing MP2, CAS-SCF, MRD-CI and MCPF procedures. Potential curves for the low-energy states in the isomerization from benzene to its Dewar form are also computed under certain geometrical assumptions. The energy difference between the two potential minima is calculated to be 3.35 eV; neglect of electron correlation increases the value by about 0.3 eV, deficiencies in the polarization description (6-31G basis) overestimates it by another 0.65 eV. The calculations suggest that the experimental Dewar benzene geometry determination needs refinement and that the isomerization energy of hexamethylbenzene is considerably smaller than that of benzene itself.