A 14-person strong team from IBM and oxford University report the on-surface synthesis of a C16 carbon allotrope in the journal Nature this week (Y. Gao et al. doi). The hydrocarbon goes by the full name cyclohexadeca-1,3,5,7,9,11,13,15-octyne and is a member of the cyclocarbon family. Three members of this team have prior experience with this particular class of compounds: in 2019 they reported on the C18 homologue (as featured in an edition of the too infrequent segment "Elements in the News" on this blog).

As the article explains, the molecule has a very interesting orbital layout with one pi-system in the molecular plane and one orthogonal to it. When both orbital systems obey the Huckel rule for aromaticity with respectively 18 and 14 electrons (recall the 4n+2 rule), the molecule has a potential for double-aromaticity.
As before, a cyclic precursor was deposited on a copper / sodium chloride surface and the formation of the allotrope was deduced from atomic force microscopy at 5 degrees Kelvin. A specific voltage at the AFM tip was required for the debromination and another for the carbon monoxide extrusion in a estimated yield of 30%. Perseverance counts in this line of work: many cyclic precursors, each a challenge to synthesise, were tried and found failing.
C16 is camera-shy! It hops around on the surface and is difficult to track. Only when trapped in an NaCl well (visualize a car tire stuck in a pothole?) the researchers were able to get a clear view. In it, the article explains, the molecule is flat but triple bonds are clearly distinguishably from single bonds with the solid implication that the molecule is not doubly aromatic but rather doubly anti-aromatic with 16 electrons in each pi-system. Authors dissapointed? They keep a brave face and hint at future work.

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