Mobius aromaticity in small (n) trans-annulenes?

It is shown that the approximations for the derivation of the Hückel–Möbius rules used in (semi-)empirical studies cannot be maintained at the ab initio level of theory. As the orbital degeneracy, obtained using the Hückel approximations, is not maintained at the ab initio level, the [4n+2] and [4n] Hückel–Möbius rules do not apply exactly. The existence of possible Möbius character for cis, cis, trans-cyclohexatriene (trans-benzene, 2) and cis, cis, cis, trans-cyclooctatetraene (trans-4) is investigated; their structural and magnetic properties show that both compounds possess neither antiaromatic nor aromatic properties, respectively. The lack of cyclic π-electron conjugation is due to the reduced overlap between the pπ-orbitals. This is demonstrated for trans-4 in which the trans double bond is forced into the geometry of that of the cyclononatetraenyl cation (6). In that case, trans-4 possesses aromatic properties (NICS −12.9 ppm). © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 85: 52–60, 2001