What is the topology of a molecule made of fused benzene rings? Hopf and co-workers have examined the case where the benzene rings are fused in an ortho arrangement to complete a circle, the so-called [n]circulenes 1n.1 They computed the series of [3]- to [20]circulene at B3LYP/6-31G(d).


The most common examples of this class are corannulene 15 and coronene 16. Hopf finds that the small circulenes, [3]- through [6]circulene, are bowls, consistent with many previous studies.

15, corannulene

16, coronene

The larger circulenes fall into two distinct topological categories. [7]circulene through [16]circulene are saddles, as shown in Figure 1a. When the compounds are even larger, namely [17]- through [20]circulene, they adopt a helical topology, as seen in Figure 1b. Unfortunately, Hopf does not supply the optimized geometries; there is no supporting material at all. So I have reoptimized [12]circulene at B3LYP/6-31G(d) and [18]circulene at AM1. It is a real shame that authors do not routinely deposit their structures, that referees do not call out the authors on this, and that editors of journals do not demand full geometrical descriptions of all reported computed structures.



112: [12]circulene

118: [18]circulene

Figure 1. Optimized structures of (a) [12]circulene (B3LYP/6-31G(d)) and (b) [20]circulene (AM1).
Note the hydrogens have been omitted for clarity.

Hopf does not provide a comparison of structures and their energies. For example, what is the energy difference between the bowl and saddle topologies of [7]circulene or the energy difference between the saddle and helical topologies of [17]circulene?

The change in topology of the circulenes is fascinating. One wonders if this change is strictly a function of a stringing fused hexagons in a circle and minimizing the surface. Or is their some π-π stacking that leads to the saddle and helical topologies? Further details would be interesting – as would be examining other types of ciculenes as hinted by the authors at the end of the paper regarding isomeric kekulenes 2.

Scheme 1 – examples of kekulenes 2


(1) Christoph, H.; Grunenberg, J.; Hopf, H.; Dix, I.; Jones, P. G.; Scholtissek, M.; Maier, G., "MP2 and DFT Calculations on Circulenes and an Attempt to Prepare the Second Lowest Benzolog, [4]Circulene," Chem. Eur. J. 2008, 14, 5604-5616, DOI: 10.1002/chem.200701837


15: InChI=1/C20H10/c1-2-12-5-6-14-9-10-15-8-7-13-4-3-11(1)16-17(12)19(14)20(15)18(13)16/h1-10H

16: InChI=1/C24H12/c1-2-14-5-6-16-9-11-18-12-10-17-8-7-15-4-3-13(1)19-20(14)22(16)24(18)23(17)21(15)19/h1-12H