Kekulene

The nature of the π bonding within the molecule was long debated, as several distinctly different arrangements were possible. The two most significant proposals are the "Clar" configuration, consisting of six benzene-like (aromatic 6 π-electron) rings connected by bridging bonds and vinyl groups in non-aromatic rings, and the "Kekulé" configuration, consisting of two concentric aromatic rings (18 π-electron inner, 30 π-electron outer) linked by radial single bonds.^{[3][4][5][6][7][8][9]}

• Proposed electronic configurations
• 
  "Kekulé" configuration: Two concentric aromatic rings
• 
  "Clar" configuration: Benzene rings alternating with non-aromatic linkers

The synthesis of the compound, first reported in 1978,^[5] allowed experimental determination of the electronic structure. In the late 1970s, ¹H-NMR provided evidence of benzene rings^[5] and X-ray analysis determined that the structure had had alternating aromatic and non-aromatic rings,^[3] both consistent with the Clar configuration. In 2019, the configuration was determined to be one consisting of benzene-like rings alternating with non-aromatic linkages, by using single molecule atomic force microscopy to measure the carbon–carbon bond-lengths and bond orders.^[10] This configuration is in keeping with Clar's rule, as it has the largest number of disjoint aromatic π sextets.

Though the whole structure is essentially planar, it only has three-fold symmetry rather than six-fold. The carbon–hydrogen bonds in the center of the ring have a slight alternating tilt out of the plane to avoid steric hindrance among the hydrogen atoms.^[10]