Synthesis of Cyclo[18]carbon via Debromination of C₁₈Br₆

Cyclo[18]carbon (C₁₈, a molecular carbon allotrope) can be synthesized by dehalogenation of a bromocyclocarbon precursor, C₁₈Br₆, in 64% yield, by atomic manipulation on a sodium chloride bilayer on Cu(111) at 5 K, and imaged by high-resolution atomic force microscopy. This method of generating C₁₈ gives a higher yield than that reported previously from the cyclocarbon oxide C₂₄O₆. The experimental images of C₁₈ were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D₁₈ cumulene, D₉ polyyne, D₉ cumulene, and C₉ polyyne. Cumulenic structures, with (D₉) and without (D₁₈) bond-angle alternation, can be excluded. Polyynic structures, with (C₉) and without (D₉) bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.