Cyclocarbon

The smallest cyclo[n]carbon predicted to be relatively stable is C18, with a computed strain energy of 72 kilocalories per mole.

[1][3] An IBM/Oxford team claimed to synthesize its molecules in solid state in 2019:[4] According to these IBM researchers, the synthesized cyclocarbon has alternating triple and single bonds, rather than being made of entirely of double bonds.

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 of n. 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 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].

[9] On the basis of TAO-LDA results, the smaller c-CC[n] (up to  = 22, where m are positive integers) possess nonradical nature and sizable singlet-triplet energy gaps (e.g., larger than 20 kcal/mol).