Structural, Aromatic, Optical and Reactive Features of C12-(CO)n (n = 2, 4, 6): A DFT Study.

The geometric, electronic, spectroscopic, adsorption, and reactive properties of C12-(CO)n (n = 2, 4, 6) were systematically investigated using density functional theory at the ωB97XD/def2-TZVP level. All three molecules adopt planar structures with pronounced bond length alternation along the C12 skeleton. Multi-index bonding analyses reveal significant π-electron localization at short C-C bonds rather than full-ring delocalization. As the number of OCCO fusion units increases, molecular symmetry evolves from C2v to D2h and further to D3h. Aromaticity assessments using AV1245, HOMA, FLU-π, and ELF-π consistently indicate the absence of global aromaticity; OCCO fusion primarily disrupts πin delocalization while leaving πout conjugation comparatively less affected. Simulated UV-Vis spectra show that increasing OCCO units shifts the dominant absorption band toward the ultraviolet region, with low-energy excitations confirmed as locally excited states dominated by charge redistribution within the C12 ring. vdW potential calculations disclose an anomalous adsorption strength sequence for light noble gases (He > Ne), and it is contrary to the counterpoise-corrected interaction energy results. Local electron attachment energy (LEAE) and orbital-weighted dual descriptor (OWDD) jointly demonstrate that the intrinsic electron-accepting reactivity follows C12-(CO)6 > C12-(CO)4 > C12-(CO)2, qualitatively consistent with LUMO energy lowering yet modulated by electron-attachment relaxation effects. This work provides theoretical insights into the structure-bonding-reactivity relationships of OCCO-fused carbon macrocyclic systems.