Semiempirical AM 1 study of the structural properties in some large fused molecular systems

Abstract

A number of fused molecules exhibiting the Mills-Nixon (MN) effect are examined by the semiempirical AM1 method. The studied compounds include systems involving a naphthalene “aromatic” moiety fused to four- and five-membered rings, annelated biphenylene fragments consisting of alternating aromatic and anti-aromatic rings, and an eight-membered central ring (cyclooctatetraene or cyclooctatetradiyne) flanked by benzene or by benzo-cyclobutane and biphenylene fragments. The structural parameters are in good qualitative agreement with available experimental data. Most of the considered systems exhibit MN distortions which are discussed in terms of σ-rehybridization and π-bond orders. The importance of rehybridization is illustrated by the difference in C-C bond distances within the central ring in all- cis-tris(benzocyclobuta)cyclohexane (7)- a molecule with flat cyclohexane where π-electrons are absent in the central six-membered fragment. A difference in stability in angular and linear [3]phenylene is interpreted by synergistic and counteractive interaction of σ- and π-electrons, respectively. The concept of concertedness of σ- and π-electrons resolves the paradox that bent [3]phenylene has a lower total energy in spite of the fact that it is a more localized system than the linear isomer. The primitive graph-theoretical approach based on the “conjugate circuits” model fails completely in this respect.