Perimeter effects on ring currents in polycyclic aromatic hydrocarbons: Circumcoronene and two hexabenzocoronenes

Current-density maps are calculated at an ab initio level for the three symmetrical polycyclic aromatic hydrocarbons, circumcoronene [1 (D6h)], hexabenzo[bc,ef,hi,kl,no, qr]coronene [2a (D6h) and 2b (D3d)], and hexabenzo[a,d,g,j,m,p]coronene [3a (D6h), 3b (D6) and 3c (D3d)], all of which can be formally derived by annelation of benzene rings to a coronene core. Whilst 1 is planar, 2 has a non-planar minimum that is effectively isoenergetic with its planar form, and 3 has a well defined non-planar structure. The shape of the molecular boundary rather than the planarity of the molecule plays the critical rôle in the character of the predicted currents. Formal deletion of outer hexagons from circumcoronene (1) in two different ways produces either hexabenzocoronene 2 with a prediction of disjoint local benzenoid diatropic currents linked by a global perimeter, or 3 with a giant diatropic perimeter current enclosing a weak paramagnetic circulation on the central hexagon. The current density map of 1 is effectively a superposition of those of 2 and 3. Its strong diatropic perimeter current subsumes the six weaker diatropic benzenoid circulations evident in 2, and bifurcates in the six outer benzenoid rings that form the corners of the giant hexagon; its benzene "hub" sustains a diatropic current, as would be expected from the partial cancellation of the strong diatropic hub current of 2 by the weaker paratropic hub current of 3. The relationship between the three molecules is rationalised by considering orbital contributions to their current density maps.