Bonding character, electron delocalization, and aromaticity of cyclo[18]carbon (C18) precursors, C18-(CO)n (n = 6, 4, and 2): Focusing on the effect of -CO groups

Although the unique cyclo[18]carbon (C18) realized by recent experiments has been greatly concerned, it has so far remained elusive. In contrast, its precursors C18-(CO)n (n = 6, 4, and 2), which can be separated stably, are of more practical significance. In this paper, the bonding character, electron delocalization, and aromaticity of the C18-(CO)n (n = 6, 4, and 2) with out-of-plane and in-plane dual π systems (πout and πin) perpendicular to each other are studied by combining quantum chemical calculations and wavefunction analyses. These cyclocarbon oxides exhibit alternating long and short C-C bonds and extensive electron delocalization, and a significant diatropic induced ring current under the action of external magnetic field is therefore observed, which reveals the aromatic characteristic in the molecules. The global electron delocalization and significant influence of the number of intramolecular carbonyl (-CO) on the two sets of π conjugated systems have been focused on, and the essential reason for the distinct difference in the overall aromaticity of the molecules was also clarified. It seems that the substituent -CO groups hinders the electron delocalization of the πin system but has relatively small effect on the πout system, resulting in the molecules with less -CO group showing greater aromaticity.