LINEAR VS ANGULAR PHENYLENES - AN INTERPLAY OF AROMATICITY, ANTIAROMATICITY, AND BAEYER STRAIN IN FUSED MOLECULAR-SYSTEMS

Geometric structure and electronic properties of some first [N]phenyle nes, where N stands for the number of benzene fragments, are examined by the HF/6-31G and MP2(fc)6-31G*//HF/6-31G* theoretical methods. It is found that angular isomers are slightly more stable than their line ar counterparts. The difference increases with N. The fundamental impo rtance of the biphenylene bonding pattern for the stability of higher [N]phenylenes is established. It appears, somewhat paradoxically, that the lower total energies of the angular [N]phenylenes are a consequen ce of the fact that the electronic structure is more localized in the angular than in the linear [N]phenylenes. A plausible explanation is g iven in terms of decreased antiaromatic character of the planar four-m embered rings and by the synergism between sigma and pi electrons, in contrast to some antagonism in the linear [N]phenylenes. Simple indice s of localization based on the CC bond distances and/or pi-bond orders are introduced, which offer an interesting insight into aromaticity d efects in particular benzene rings. Intrinsic destabilization energies E(d) (generalized strain) are estimated by homodesmic chemical reacti ons. It is found that E(d) values follow an approximate but simple add itivity rule being determined by the number of cyclobutadiene moieties . The calculated structural parameters are in good agreement with the available experimental data. Both are in accordance with the significa nt Mills-Nixon effect in angular phenylenes.