The (Anti)aromatic Properties of Cyclo[n]Carbons: Myth or Reality?

Abstract

Recent advances in on-surface chemistry have enabled the synthesis and structural characterization of even-numbered cyclo[n]carbons, traditionally classified as either doubly aromatic (<em>n</em> = 4k + 2) or doubly antiaromatic (<em>n</em> = 4k) based on their in-plane and out-of-plane π-electron circuits. However, recent studies have increasingly questioned this classification, suggesting instead that these molecules are more accurately described as non-aromatic. In this work, we computationally examine the electron affinities and (anti)aromatic character of cyclo[n]carbons with <em>n</em> = 16–30 using energetic, structural, and electronic aromaticity descriptors. Adiabatic electron affinity (AEA) analysis reveals a high degree of uniformity across the series of both nominally aromatic and antiaromatic members. Aromatic stabilization energy (ASE) values, derived from homodesmotic and disproportionation reactions, indicate slight destabilization only for C₁₆ and C₂₀, and low stabilization for the remaining systems. In particular, ASE is less than 2 kcal/mol for cyclo[n]carbons with <em>n</em> ≥ 24. This suggests that neither aromatic nor antiaromatic character significantly contributes to the thermodynamic stability of larger cyclocarbons. EDDB analysis further supports this conclusion, with only about 22%–27% of π-electrons participating in delocalization. While delocalization is slightly greater in cyclo[n]carbons with <em>n</em> = 4k + 2, the difference diminishes with increasing size. Upon two-electron reduction to the dianionic state, all cyclo[n]carbons exhibit bond length equalization and increased delocalization. These results suggest that only small cyclo[n]carbons (<em>n</em> < 24) can be classified as weakly (anti)aromatic, while larger cyclo[n]carbons (<em>n</em> ≥ 24) are more appropriately classified as non-aromatic systems. The aromaticity of all considered cyclocarbons becomes more pronounced in corresponding dianionic forms due to cooperative structural and electronic effects. Thus, this work provides a unified framework for interpreting and predicting the electronic behavior of cyclocarbons.