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, C 18 Br 6 , 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 18 gives a higher yield than that reported previously from the cyclocarbon oxide C 24 O 6 . The experimental images of C 18 were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D 18 h cumulene, D 9 h polyyne, D 9 h cumulene, and C 9 h polyyne. Cumulenic structures, with ( D 9 h ) and without ( D 18 h ) bond-angle alternation, can be excluded. Polyynic structures, with ( C 9 h ) and without ( D 9 h ) bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.