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 init io level of theory. As the orbital degeneracy, obtained using the Huckel ap proximations, is not maintained at the ab initio level, the [4n + 2] and [4 n] Huckel-Mobius rules do not apply exactly. The existence of possible Mobi us character for cis, cis, trans-cyclohexatriene (traps-benzene, 2) and cis , cis, cis, traps-cyclooctatetraene (traps-4) is investigated; their struct ural and magnetic properties show that both compounds possess neither antia romatic nor aromatic properties, respectively. The lack of cyclic pi -elect ron conjugation is due to the reduced overlap between the p(pi)-orbitals. T his is demonstrated for traps-4 in which the trans double bond is forced in to 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.