Kekulene

Geometry and electronic structure

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.

File:Kekulene (kekule conjugation).png|"Kekulé" configuration: Two concentric aromatic rings File:Kekulene (linked benzenes).png|"Clar" configuration: Benzene rings alternating with non-aromatic linkers

The synthesis of the compound, first reported in 1978, allowed experimental determination of the electronic structure. In the late 1970s, 1H-NMR provided evidence of benzene rings and X-ray analysis determined that the structure had had alternating aromatic and non-aromatic rings, 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. 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.

References

References

1. (1983). "Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene". Chemische Berichte.
2. (October 1983). "Cycloarenes, a New Class of Aromatic Compounds, I. Synthesis of Kekulene". Chemische Berichte.
3. (September 1979). "Molecular Structure and Spectroscopic Properties of Kekulene". Angewandte Chemie International Edition in English.
4. (January 1992). "Is superaromaticity a fact or an artifact? The kekulene problem". Journal of the American Chemical Society.
5. (May 1978). "Benzenoidversus Annulenoid Aromaticity: Synthesis and Properties of Kekulene". Angewandte Chemie International Edition in English.
6. (1 November 1996). "Is Kekulene Really Superaromatic?". Angewandte Chemie International Edition in English.
7. (11 August 2006). "Electronic properties of kekulene". Molecular Physics.
8. (October 1983). "Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene". Chemische Berichte.
9. (February 1995). "Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices". Journal of Physical Organic Chemistry.
10. (17 September 2019). "Revisiting Kekulene: Synthesis and Single-Molecule Imaging". Journal of the American Chemical Society.