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 Carbonyl (-CO) Groups.

The bonding character, electron delocalization, and aromaticity of the cyclo[18]carbon (C18 ) precursors, C18 -(CO)n (n=6, 4, and 2), have been studied by combining quantum chemical calculations and various electronic wavefunction analyses with different physical bases. It was found that C18 -(CO)n (n=6, 4, and 2) molecules exhibit alternating long and short C-C bonds, and have out-of-plane and in-plane dual π systems (πout and πin ) perpendicular to each other, which are consistent with the relevant characteristics of C18 . However, the presence of carbonyl (-CO) groups significantly reduced the global electron conjugation of C18 -(CO)n (n=6, 4, and 2) compared to C18 . Specifically, the -CO group largely breaks the extensive delocalization of πin system, and the πout system is also affected by it but to a much lesser extent; as a consequence, C18 -(CO)n (n=6, 4, and 2) with larger n shows weaker overall aromaticity. Mostly because of the decreased but still apparent πout electron delocalization in the C18 -(CO)n (n=6, 4, and 2), a notable diatropic induced ring current under the action of external magnetic field is observed, demonstrating the clear aromatic characteristic in the molecules. The correlation between C18 -(CO)n (n=6, 4, and 2) and C18 in terms of the gradual elimination of -CO from the precursors showed that the direct elimination of two CO molecules in C18 -(CO)n (n=6, 4, and 2) has a synergistic mechanism, but it is kinetically infeasible under normal conditions due to the high energy barrier.