On-Surface Synthesis of a Doubly Anti-Aromatic Carbon Allotrope: Cyclo[16]carbon

Cyclocarbons are rings of carbon atoms, often formed as gas-phase carbon clusters. The only cyclocarbons yet to be well characterized are C10 and C18, which are doubly aromatic with 4n+2 carbon atoms (where n is an integer), resulting in enhanced thermodynamic stability. Cyclocarbons with 4n atoms have been predicted to be less stable and doubly anti-aromatic. Here we report the first structural characterization of such a cyclocarbon, C16, generated from C16(CO)4Br2 on a NaCl surface. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) provide insight into the geometry and electronic structure, respectively, of neutral C16 and anionic C16–. We find that neutral C16 is circular, with significant bond-length alternation. This geometry confirms that it has an anti-aromatic ground state.