Cyclo[6]carbon-connected isomers of cyclo[18]carbon: exploring the electronic structure, aromaticity and...

Recently, molecular all-carbon rings (cyclo[n]carbons) attracted widespread interest due to their unique geometric features and electronic structures. From cyclo[18]carbon, several cyclocarbons with both even and odd numbers of atoms have been generated and structurally characterized in condensed phases, as the field of carbon chemistry flourishes once again. Inspired by sp²-hybridized fullerene, carbon nanotubes, graphene and sp-hybridized cyclo[n]carbons, we theoretically designed nine allotropes of C₁₈, all of which contain both sp²- and sp-hybridized carbon atoms. The electronic structures, aromaticity and antiaromaticity of these isomers in their ground states were comprehensively investigated using quantum-chemical calculations and wave function analyses. Four of the isomers are formed from three directly connected cyclo[6]carbons: three of which possess planar structures and are named as ortho[3-R6]carbon, meta[3-R6]carbon and para[3-R6]carbon according to their chemical structures, while the fourth isomer is connected by a ring and labeled as cyclo[3-R6]carbon. The other five isomers are derived from cyclo[3-R6]carbon, and all of which have global ring structures. The analyses of nucleus-independent chemical shift and the anisotropy of induced current density reveal that seven of the designed molecules display aromatic character, whereas the other two exhibit antiaromaticity. The reasons for these phenomena are discussed in detail in combination with electron delocalization analysis. The findings in this work will enrich the field of C₁₈ chemistry.