title: Super-delocalized valence isomer of coronene.
abstract: Coronene (1) has been proposed to be "superaromatic", but energetic, geometric, and magnetic criteria of global and local aromaticity fail to support this proposal, and indeed, the calculated current-density map shows opposition of currents: diatropic on the 18-carbon rim and paratropic on the 6-carbon hub. However, [7,5,7,5,7,5]-isocoronene (2) ([7,5,7,5,7,5:6]-circulene, or isocoronene, for short), which is a valence isomer in which alternate pentagons and heptagons replace the hexagons surrounding the central ring, is predicted to have a single, unopposed, intense diatropic perimeter current arising from its four pi HOMO electrons, such as in the ipsocentric description of classically aromatic [4n + 2]-annulenes, hence, qualifying 2 as superaromatic on the magnetic criterion. This conclusion is in excellent agreement with anisotropy of magnetic susceptibility (359 cgs-ppm for isocoronene vs 247 cgs-ppm for coronene) and exaltation of magnetic susceptibility (isocoronene exceeds coronene by 51.4 cgs-ppm). Central and perimeter bond lengths suggest an increased aromaticity of isocoronene. In contrast, the energetic criterion shows that isocoronene is destabilized with respect to coronene by ca. 105 kcal/mol of which only ca. 30 kcal/mol can be attributed to differential strain.
authors.affiliation: Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. arcad@ibb.waw.pl
authors.forename: Arkadiusz
authors.initials: A
authors.lastname: Ciesielski
authors.forename: Michał K
authors.initials: MK
authors.lastname: Cyrański
authors.forename: Tadeusz M
authors.initials: TM
authors.lastname: Krygowski
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Fowler
authors.forename: Mark
authors.initials: M
authors.lastname: Lillington
mesh_terms: D016880:Anisotropy; D007536:Isomerism; D008956:Models, Chemical; D011083:Polycyclic Compounds / Q000737:chemistry*
chemical_list: D011083:Polycyclic Compounds; C012256:coronene