Mobius aromaticity in small [n] trans-annulenes?

It is shown that the approximations for the derivation of the Huckel-Mobius rules used in (semi-)empirical studies cannot be maintained at the ab initio level of theory. As the orbital degeneracy, obtained using the Huckel approximations, is not maintained at the ab initio level, the [4n + 2] and [4n] Huckel-Mobius rules do not apply exactly. The existence of possible Mobius character for cis, cis, trans-cyclohexatriene (traps-benzene, 2) and cis, cis, cis, traps-cyclooctatetraene (traps-4) is investigated; their structural and magnetic properties show that both compounds possess neither antiaromatic nor aromatic properties, respectively. The lack of cyclic pi -electron conjugation is due to the reduced overlap between the p(pi)-orbitals. This is demonstrated for traps-4 in which the trans double bond is forced into the geometry of that of the cyclononatetraenyl cation (6). In that case, traps-4 possesses aromatic properties (NICS-12.9 ppm). (C) 2001 John Wiley & Sons, Inc.