A Coupled Cluster Study of the Electronic Structure, Double Aromaticity, and π–π Stacking Interactions of a C8 Carbon Ring

Recent experimental advances in the synthesis and characterization of carbon rings have stimulated renewed interest in their unusual structures and electronic properties. Here, we provide a detailed analysis of the electronic structures of three low-lying isomers of the C8 ring using high-level coupled-cluster methods. The ground-state minimum is a singlet polyynic structure with C4h symmetry, featuring alternating single and triple bonds. It possesses σ- and π-antiaromaticity, resulting in overall antiaromatic character. In contrast, the singlet D4d crown-shaped structure exhibits σ- aromaticity and π-antiaromaticity, leading to net antiaromaticity. The triplet D4h minimum adopts a cumulenic structure characterized by σ-antiaromaticity and π-aromaticity, giving rise to an overall aromatic character consistent with Baird’s rule. Furthermore, the C8 ring readily forms a stable π–π stacked structure, in which the interactions give rise to four nondegenerate excited states that correlate with a monomer excited state, forming two pairs of near-degenerate states. These results reveal the intricate interplay between σ- and π-aromaticity in carbon rings and provide further insights into the electronic structure and π–π stacking interactions of C8, highlighting the complexity of aromaticity in carbon-based molecular systems.