Synthesis of Cyclo[18]carbon via Debromination of C(18)Br(6)

[Image: see text] 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(18h) 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.