The interplay of angle strain and aromaticity:: Molecular and electronic structures of [0n]paracyclophanes

The belt-like polyphenylenes, [0(n)]paracyclophanes, (n = 5 and 6), have been investigated using semi-empirical, ab initio and DFT methods. The molecular structure, rotational barrier on twisting a single phenyl ring and the aromatic character within each ring as well as in the whole molecule have been evaluated. [0(5)]Paracyclophane is predicted to have a quinonoid structure. In contrast, the equatorial pentaphenyl fragment found in C-70 as well as the hexagons of the less strained [0(6)]paracyclophane have benzenoid character. Approximate band structures have been derived for larger cycles of [0(n)] paracyclophanes.