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**

Abstract

The bonding character, electron delocalization, and aromaticity of the cyclo [18]carbon (C 18 ) precursors, C 18 -(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 C 18 -(CO) n (n = 6, 4, and 2) molecules exhibit alternating long and short CÀ C bonds, and have out-of-plane and in-plane dual π systems (π out and π in ) perpendicular to each other, which are consistent with the relevant characteristics of C 18 . However, the presence of carbonyl (-CO) groups significantly reduced the global electron conjugation of C 18 -(CO) n (n = 6, 4, and 2) compared to C 18 . Specifically, the -CO group largely breaks the extensive delocalization of π in system, and the π out system is also affected by it but to a much lesser extent; as a consequence, C 18 -(CO) n (n = 6, 4, and 2) with larger n shows weaker overall aromaticity. Mostly because of the decreased but still apparent π out electron delocalization in the C 18 -(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 C 18 -(CO) n (n = 6, 4, and 2) and C 18 in terms of the gradual elimination of -CO from the precursors showed that the direct elimination of two CO molecules in C 18 -(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.

Cyclo[18]carbon Formation from C18Br6 and C18(CO)6 Precursors

“…This explains the five times higher reaction yield for debromination of C 18 Br 6 (64% in ref ( 19 )) compared to C 18 (CO) 6 (13% in ref ( 2 )) at similar conditions. It is also in agreement with the energetics of a synergistic mechanism or CO elimination from C 18 (CO) 6 demonstrated in the previous theoretical research, 31 showing that the reaction cannot happen in ambient conditions because of the high energy barrier of around 2.1 eV which thus is comparable to our NEB result (∼2.25 eV). The drastic decrease in energy for debromination of [C 18 Br 6 ] − can be associated with the population of the LUMO orbital of C 18 Br 6 by one electron.…”

“…The wB97XD range separated hybrid functional reproduces well the experimental structures of the neutral C 18 Br 6 and C 18 (CO) 6 species 2 , 19 ( Figure S1 ), which also are in agreement with previous theoretical results. 31 , 40 Some deviation in angles of C 18 Br 6 can be explained by the fact that the experimentally detected precursor is not flat, something that could be the result of surface effects. From Figure S3 it can be seen that the structures remain planar in all cases except for −2 charged C 18 Br 6 and +2 charged C 18 (CO) 6 which are deformed from their planar nature.…”

Incorporation of a Boron–Nitrogen Covalent Bond Improves the Charge-Transport and Charge-Transfer Characteristics of Organoboron Small-Molecule Acceptors for Organic Solar Cells

“…Molecular orbital correlation (MOC) analysis has been proven to be an effective method for analyzing the contribution of each molecular orbital fragment to the entire molecular orbital and further exploring the intramolecular orbital interactions [ 38 , 39 , 40 ]. To study the influence of different forms of boron and nitrogen substitution on the distribution and energy of frontier molecular orbitals, we carried out MOC analysis on these molecules.…”

Preparation and Aromaticity of Rhenacycloprop-2-ene-containing Polycyclic Compounds

“…To investigate the source of the σ-predominant aromatic property of the 3MR s, the charge decomposition analysis (CDA) − was carried out, considering as fragments [Re] and remnant. As shown in Figure , the molecular orbitals (MOs) of two fragments produced significant mixing.…”