Spontaneous Symmetry Breaking in Cyclo[18]Carbon

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Cyclo[18]carbon has been experimentally shown to exist in a stable polyynic form.
• Understanding the electronic structure and stability of carbon allotropes is crucial in materials science.

Purpose of the Study:

• To investigate the electronic structure of cyclo[18]carbon using ab initio calculations.
• To explain the observed stability of the polyynic form of cyclo[18]carbon.

Main Methods:

• Ab initio electronic structure calculations were performed.
• The study focused on the interactions between frontier molecular orbitals (ground and excited states).
• Analysis of symmetry breaking and the Jahn-Teller effect was central.

Main Results:

• The second-order Jahn-Teller effect was identified as the cause of symmetry breaking.
• Interactions between the ground state and excited states (LUMO and LUMO+1) were found to be significant.
• These interactions stabilize the less symmetric polyynic structure over the cumulenic form.

Conclusions:

• The polyynic form of cyclo[18]carbon is stabilized by frontier orbital interactions due to the Jahn-Teller effect.
• This electronic effect leads to distortions in symmetry and lowers the molecule's energy.
• The findings explain the preference for alternating single and triple bonds in cyclo[18]carbon.