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

Area of Science:

• Organic Chemistry
• Theoretical Chemistry
• Materials Science

Background:

• Even-numbered cyclo[n]carbons were traditionally classified as aromatic or antiaromatic based on π-electron circuits.
• Recent studies suggest these classifications may be inaccurate, proposing a non-aromatic nature for many cyclocarbons.

Purpose of the Study:

• To computationally investigate the aromaticity and electron affinities of cyclo[n]carbons (n=16-30).
• To re-evaluate the traditional aromaticity/antiaromaticity classifications for larger cyclocarbons.

Main Methods:

• Computational examination of electron affinities using adiabatic electron affinity (AEA).
• Assessment of aromatic stabilization energy (ASE) via homodesmotic and disproportionation reactions.
• Analysis of π-electron delocalization using electron delocalization difference (EDDB) calculations.

Main Results:

• AEA values showed uniformity across the series.
• ASE values indicated minimal stabilization for larger cyclocarbons (n≥24), with slight destabilization for C16 and C20.
• EDDB analysis revealed limited π-electron delocalization (22%-27%), diminishing with increasing size.
• Two-electron reduction to dianionic states enhanced bond equalization and delocalization for all cyclocarbons.

Conclusions:

• Cyclocarbons with n<24 exhibit weak (anti)aromaticity, while those with n≥24 are better described as non-aromatic.
• The aromatic character of all studied cyclocarbons is enhanced in their dianionic forms.
• This study offers a unified framework for understanding cyclocarbon electronic properties.