title: The homotropenylium cation: a system with a pinched π ring current.
abstract: The homotropenylium cation (1, C8H9(+)) is a key species in the discussion of homoaromaticity. Constrained optimisations around the minimum structure have been performed, varying the size of the gap spanned by the CH2-bridge and optimising all other geometrical parameters. At each bridging distance, ab initio current-density maps have been calculated and plotted using the ipsocentric approach. Analysis of the maps, including decomposition into localised orbital contributions, gives a clear indication of a global diatropic ring current passing through the gap. The change in pπ-pπ interaction, from conventional π overlap around the conjugated seven-carbon perimeter to σ overlap (pσ-pσ) in the gap, results in a distinctive pinched topology, with two streams of current pinched down into one for part of the circuit. This ring current is diatropic and therefore the species 1 is aromatic on the magnetic criterion.
authors.affiliation: Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK. P.W.Fowler@sheffield.ac.uk.
authors.forename: Mark
authors.initials: M
authors.lastname: Lillington
authors.forename: Remco W A
authors.initials: RW
authors.lastname: Havenith
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Fowler
authors.forename: Jon
authors.initials: J
authors.lastname: Baker
authors.forename: Leonardus W
authors.initials: LW
authors.lastname: Jenneskens