An sp-hybridized all-carboatomic ring, cyclo[18]carbon: Bonding character, electron delocalization, and aromaticity

cyclo[18]carbon is an all-carboatomic ring observed recently in experiment. Inspired by the distinctive feature in structure and bonding nature, we explored comprehensively its bonding character, electron delocalization, and aromaticity by quantum chemistry calculation and wavefunction analysis. Bond order analysis reveals that this sp-hybridized all-carboatomic ring exhibits a structure of alternating short and long C–C bonds with D9h symmetry, rather than a polyyne with strict triple and single bonds as declared in experiment. Our in-depth investigations in terms of localized orbital and real space function analyses not only confirm this conclusion, but also provide a lot of useful information about electronic structure. Further electron delocalization analysis shows that the cyclo[18]carbon possesses two perpendicular π systems, and the 4n + 2 π electrons of each one exhibit strong delocalization behavior. Under the action of external magnetic field, the cyclo[18]carbon shows a significant induced ring current, which reveals its remarkable aromatic characteristic.