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 C_2v to D_2h and further to D_3h. Aromaticity assessments using AV1245, HOMA, FLU-π, and ELF-π consistently indicate the absence of global aromaticity; OCCO fusion primarily disrupts π_in delocalization while leaving π_out 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.