The (Anti)aromatic Properties of Cyclo[n]Carbons: Myth or Reality?
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 o...
| Autores: | , , , , |
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| Tipo de recurso: | artículo |
| Estado: | Versión publicada |
| Fecha de publicación: | 2025 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/225924 |
| Acceso en línea: | https://hdl.handle.net/2445/225924 |
| Access Level: | acceso abierto |
| Palabra clave: | Compostos aromàtics Hidrocarburs aromàtics policíclics Estructura química Aromatic compounds Polycyclic aromatic hydrocarbons Chemical structure |
| Sumario: | 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 C<sub>16</sub> and C<sub>20</sub>, 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. |
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