Synthesis of Cyclo[18]carbon via Debromination of CBr.

Cyclo[18]carbon (C, a molecular carbon allotrope) can be synthesized by dehalogenation of a bromocyclocarbon precursor, CBr, 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 C gives a higher yield than that reported previously from the cyclocarbon oxide CO. The experimental images of C were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: cumulene, polyyne, cumulene, and polyyne. Cumulenic structures, with () and without () bond-angle alternation, can be excluded. Polyynic structures, with () and without () bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.