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

3D model (JSmol)
Molar mass 216.198 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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][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:[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.[4]


  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 (13): 1535–1543. doi:10.1039/P19960001535.
  2. François Diederich; Yves Rubin; Carolyn B. Knobler; Robert L. Whetten; Kenneth E. Schriver; Kendall N. Houk; Yi Li (8 September 1989). "All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor". Science. 245 (4922): 1088–1090. Bibcode:1989Sci...245.1088D. doi:10.1126/science.245.4922.1088. PMID 17838807.
  3. Kaiser, Katharina; Scriven, Lorel M.; Schulz, Fabian; Gawel, Przemyslaw; Gross, Leo; Anderson, Harry L. (15 Aug 2019). "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon". Science. 365 (6459): 1299–1301. arXiv:1908.05904. doi:10.1126/science.aay1914.
  4. Castelvecchi, Davide (15 August 2019). "Chemists make first-ever ring of pure carbon". Nature. 572 (7770): 426. doi:10.1038/d41586-019-02473-z. PMID 31431741.
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