title: Investigation of a putative mobius aromatic hydrocarbon. The effect of benzannelation on mobius [4n]annulene aromaticity.
abstract: The first experimental example of a [4n]annulene derivative with one Mobius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the Mobius [16]annulene core of 1 shows large bond alternation (Deltar up to 0.157 A). Delocalization in this core is inhibited by large dihedral angles, which hinders effective pi overlap. This conclusion is supported by computational results (B3LYP/6-311+G) on 1 and several less benzannelated derivatives, based on geometric (Deltar, Deltar(m), Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Mobius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE(II) values of just the polyene bridge portion of 1 and its Huckel counterpart suggests that stabilization of 1 relative to its Huckel isomer is confined to the polyene bridge and is not due to a delocalized pi circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE(II) value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic.
authors.affiliation: Department of Chemistry, University of San Francisco, 2130 Fulton Street, San Francisco, California 94117-1080, USA. castroc@usfca.edu
authors.forename: Claire
authors.initials: C
authors.lastname: Castro
authors.forename: Zhongfang
authors.initials: Z
authors.lastname: Chen
authors.forename: Chaitanya S
authors.initials: CS
authors.lastname: Wannere
authors.forename: Haijun
authors.initials: H
authors.lastname: Jiao
authors.forename: William L
authors.initials: WL
authors.lastname: Karney
authors.forename: Michael
authors.initials: M
authors.lastname: Mauksch
authors.forename: Ralph
authors.initials: R
authors.lastname: Puchta
authors.forename: Nico J R van Eikema
authors.initials: NJ
authors.lastname: Hommes
authors.forename: Paul von Ragué
authors.initials: Pv
authors.lastname: Schleyer