Cyclocarbon

Cyclo[3]carbon

Main article: Cyclopropatriene

The (hypothetical) three-carbon member of this family () is also called cyclopropatriene.

Cyclo[6]carbon

Main article: Cyclo(6)carbon

The six-carbon member of this family () is also called benzotriyne.

Cyclo[18]carbon

Main article: Cyclo(18)carbon

The smallest cyclo[n]carbon predicted to be relatively stable is C18, with a computed strain energy of 72 kilocalories per mole. An IBM/Oxford team claimed to synthesize its molecules in solid state in 2019:

According to these IBM researchers, the synthesized cyclocarbon has alternating triple and single bonds, rather than being made of entirely of double bonds. This supposedly makes this molecule a semiconductor.

Large cyclo[n]carbons

Larger cyclo[n]carbons, up to 100 carbon atoms, are predicted to exhibit polyradical character, like linear carbon chains. For all the cases investigated, n-atom linear and cyclic carbon chains (respectively l-CC[n] and c-CC[n]) are ground-state singlets, and energetically more stable as closed rings. The electronic properties of l-CC[n] and c-CC[n] display peculiar oscillation patterns for smaller values of n, followed by monotonic changes for larger values. For the smaller carbon chains, odd-numbered l-CC[n] are more stable than the adjacent even-numbered ones, and c-CC[4m+2]/c-CC[4m] (where m are positive integers) are respectively more/less stable than the adjacent odd-numbered ones. With the increase of n, l-CC[n] and c-CC[n] possess increasing polyradical nature in their ground states, with the active orbitals being delocalized over the entire length of l-CC[n] or the whole circumference of c-CC[n].

On the basis of TAO-LDA results, the smaller c-CC[n] (up to n = 22) possess nonradical nature and sizable singlet-triplet energy gaps (e.g., larger than 20 kcal/mol). In addition to the known c-CC[18], Seenithurai and Chai predicted that c-CC[10], c-CC[14], and c-CC[22] are likely to be synthesized in the near future.

c-CC[48] is not stable in solution at room temperature, but can be stabilized by threading it through bulky rings as a rotaxane.

References

References

1. (2023-10-05). "On-surface synthesis of anti-aromatic cyclo[12]carbon and aromatic cyclo[6]carbon".
2. (2023-10-05). "On-surface synthesis and characterization of anti-aromatic cyclo[20]carbon".
3. (2024-05-10). "The odd-number cyclo[13]carbon and its dimer, cyclo[26]carbon". Science.
4. (1996). "Towards the total synthesis of cyclo[''n'']carbons and the generation of cyclo[6]carbon". J. Chem. Soc., Perkin Trans. 1.
5. (15 Aug 2019). "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon". Science.
6. (15 August 2019). "Chemists make first-ever ring of pure carbon". Nature.
7. (4 August 2020). "TAO-DFT investigation of electronic properties of linear and cyclic carbon chains". Scientific Reports.
8. (14 Aug 2025). "Solution-phase stabilization of a cyclocarbon by catenane formation". Science.