Cyclo[18]carbon: Insight into Electronic Structure, Aromaticity, and Surface Coupling.

Area of Science:

• Theoretical Chemistry
• Computational Materials Science
• Physical Chemistry

Background:

• Cyclo[18]carbon (C18) is a fascinating allotrope of carbon with potential applications.
• Understanding its electronic structure and stability is crucial for its practical use.
• Previous computational studies have yielded conflicting results regarding its geometry.

Purpose of the Study:

• To computationally investigate the electronic structure, aromaticity, and adsorption properties of cyclo[18]carbon.
• To resolve discrepancies in previously reported molecular geometries.
• To assess the interaction of cyclo[18]carbon with a sodium chloride (NaCl) surface.

Main Methods:

• Density Functional Theory (DFT) calculations with varying Hartree-Fock exchange percentages.
• Ab initio Coupled Cluster Single-Double (CASSCF) calculations.
• Analysis of electronic structure, aromaticity (Hückel criteria), and adsorption energies.

Main Results:

• DFT functionals with low Hartree-Fock exchange incorrectly predict a cumulene structure.
• Functionals with high Hartree-Fock exchange and CASSCF calculations confirm the polyyne structure as the ground state.
• Both polyyne and cumulene structures exhibit double Hückel aromaticity.
• Adsorption energy on NaCl is low and similar for both geometries, indicating no specific stabilization.

Conclusions:

• The polyyne structure of cyclo[18]carbon is computationally confirmed as the ground state.
• Both polyyne and cumulene isomers of C18 are doubly Hückel aromatic.
• The NaCl surface does not preferentially stabilize either the polyyne or cumulene structure of C18.