Synthesis of Cyclo[18]carbon via Debromination of C18Br6

Cyclo[18]carbon (C-18, 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 highresolution atomic force microscopy. This method of generating C-18 gives a higher yield than that reported previously from the cyclocarbon oxide C24O6. The experimental images of C-18 were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D18h cumulene, D-9h polyyne, D-9h cumulene, and C-9h polyyne. Cumulenic structures, with (D-9h) and without (D-18h) bond-angle alternation, can be excluded. Polyynic structures, with (C-9h) and without (D-9h) bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.