The smallest cyclo[n]carbon predicted to be thermodynamically 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. This supposedly makes this molecule a semiconductor.[5]
Large cyclo[n]carbons
Seenithurai & Chai 2020 found that larger cyclo[n]carbons [up to 100 carbon atoms] exhibit polyradical character and report linear carbon chains (l-CC[n]) as well as cyclic carbon chain or cyclo[n] carbon (c-CC[n]), where n=10-100.[6] For all the cases investigated, l-CC[n] and c-CC[n] are ground-state singlets, and c-CC[n] are energetically more stable than l-CC[n]. 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].[6]
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). In view of their high stability, it can be anticipated that these relatively stable cyclic carbon chains, such as c-CC[10], c-CC[14], c-CC[18], and c-CC[22], are likely to be synthesized in the near future.[6] Among them, c-CC[18] (i.e. cyclo[18]carbon) has been recently synthesized by an IBM/Oxford team in 2019.[4]
