Cyclo(18)carbon explained

Cyclooctadeca-1,3,5,7,9,11,13,15,17-nonayne or cyclo[18]carbon is an allotrope of carbon with molecular formula . The molecule is a ring of eighteen carbon atoms, connected by alternating triple and single bonds; thus, it is a polyyne and a cyclocarbon.

Cyclo[18]carbon is the smallest cyclo[''n'']carbon predicted to be thermodynamically stable, with a computed strain energy of 72 kilocalories per mole.^[1] Above 122 K, it explosively decomposes to amorphous graphite.^[2]

A collaboration of teams at IBM and the University of Oxford team claimed to synthesize it in solid state in 2019^[3] by electrochemical decarbonylation of several sites of a cyclobutanone structure: Later, researchers from Spain have used computational techniques to probe the structural and electronic properties of the molecule, and have discovered it to be an electron acceptor.^[4]

According to these IBM researchers, the electronic structure of their product consists of alternating triple bonds and single bonds, rather than a cumulene-type structure of consecutive double bonds. This supposedly makes this molecule a semiconductor.^[5]

Notes and References

1. George A. Adamson . Charles W. Rees . 1996. Towards the total synthesis of cyclo[''n'']carbons and the generation of cyclo[6]carbon. J. Chem. Soc., Perkin Trans. 1. 1535–1543. 10.1039/P19960001535 . 13.
2. François Diederich . Yves Rubin . Carolyn B. Knobler . Robert L. Whetten . Kenneth E. Schriver . Kendall N. Houk . Kendall Houk . Yi Li . All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor. Science. 8 September 1989. 245. 4922. 1088–1090. 10.1126/science.245.4922.1088. 17838807. 1989Sci...245.1088D . 23726682 .
3. Kaiser . Katharina . Lorel M. . Scriven . Fabian . Schulz . Przemyslaw . Gawel . Leo . Gross . Harry L. . Anderson . Harry Anderson (chemist) . An sp-hybridized molecular carbon allotrope, cyclo[18]carbon . Science . 365 . 6459 . 1299–1301 . 15 Aug 2019 . 10.1126/science.aay1914 . 31416933 . 1908.05904 . 201019470 .
4. Stasyuk . Anton J. . Stasyuk . Olga A. . Solà . Miquel . Voityuk . Alexander A.. Cyclo[18]carbon: the smallest all-carbon electron acceptor . Chemical Communications . 56 . 352–355 . 29 Nov 2019 . 3 . 10.1039/c9cc08399e . 31825030 . 10256/17613 . 209316603 . free .
5. Castelvecchi . Davide . Chemists make first-ever ring of pure carbon . Nature . 572 . 7770 . 426 . EN . 10.1038/d41586-019-02473-z . 31431741 . 15 August 2019. free .