Odd-Number Cyclo[n]Carbons Sustaining Alternating Aromaticity

Area of Science:

• Computational chemistry
• Theoretical organic chemistry
• Aromaticity studies

Background:

• Cyclo[n]carbons with odd numbers of carbon atoms are novel carbon allotropes.
• Understanding their electronic structure and aromaticity is key to predicting their properties.

Purpose of the Study:

• To investigate the electronic structure and aromaticity of odd-numbered cyclo[n]carbons (n=5-29).
• To compare results from density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) methods.

Main Methods:

• Density functional theory (DFT) calculations.
• Ab initio complete active space self-consistent field (CASSCF) calculations.
• Current-density calculations.

Main Results:

• DFT predicts delocalized carbene structures and aromaticity for all studied cyclo[n]carbons.
• CASSCF calculations reveal bent, localized carbene structures with alternating double aromaticity.
• CASSCF indicates singlet ground states for most, except C25; DFT results vary with functional, predicting triplet ground states for larger systems (n>=13) with BHandHLYP.
• Current-density calculations show through-space delocalization, supporting alternating aromaticity and adherence to [4k+1] and [4k+3] rules.

Conclusions:

• Odd-numbered cyclo[n]carbons exhibit complex electronic structures with alternating aromaticity, differing between DFT and CASSCF.
• C11, C15, and C19 are identified as aromatic and potential targets for future synthesis.
• A bond-shift phenomenon is predicted for triplet states, influencing reactivity.