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]
Synthesis of cyclocarbon
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.[8]
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.[9] 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].[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). 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.[9] Among them, c-CC[18] (i.e. cyclo[18]carbon) has been recently synthesized by an IBM/Oxford team in 2019.[4]
^ abGeorge 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.
^
Yves Rubin; Carolyn B. Knobler; Francois Diederich (1990). "Precursors to the cyclo[n]carbons: from 3,4-dialkynyl-3-cyclobutene-1,2-diones and 3,4-dialkynyl-3-cyclobutene-1,2-diols to cyclobutenodehydroannulenes and higher oxides of carbon". J. Am. Chem. Soc. 112 (4): 1607–1617. doi:10.1021/ja00160a047.
^ abFrançois 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. PMID17838807. S2CID23726682.