The homotropenylium cation : a system with a pinched p ring current †

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 currentdensity 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 pp–pp interaction, from conventional p overlap around the conjugated seven-carbon perimeter to s overlap (ps–ps) 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.