Cyclopent[b,c]acenaphthylene: An elusive isomer of pyracylene with the ring currents of an annelated pentalene

Abstract

Ab initio ipsocentric current–density mapping techniques are used to visualise the current density induced by a perpendicular magnetic field in the still unknown cyclopent[b,c]acenaphthylene (1). The π system divides into distinct, but contiguous, paratropic and diatropic regions, both supporting extended ring currents, the two currents running together along the pentagon–hexagon border. On the usual magnetic definition, cyclopent[b,c]acenaphthylene (1) is therefore part aromatic and part antiaromatic. Orbital analysis attributes the ring–current magnetic response of 1 to its eight most energetic π electrons. It is concluded that cyclopent[b,c]acenaphthylene (1) constitutes an ‘annelated pentalene’. This is supported by the computed ¹H NMR chemical shifts of 1; they are all positioned upfield with respect to naphthalene (3).