Carbon Tunneling in the Automerization of Cyclo[18]carbon

Cyclo[18]carbon (C₁₈), a recently synthesized carbon allotrope, was found to have a polyynic ground-state structure with D_9h symmetry and formally alternating single and triple bonds. Yet, under less influencing experimental conditions this molecule might undergo an automerization reaction between its two degenerate geometries through a cumulenic (non-alternating, adjacent double bonds) D_18h transition state. Herein, we discuss the role of quantum mechanical tunneling (QMT) in this degenerate reaction. Our computations predict that at the experimental temperature (5 K) the reaction in the gas phase is completely driven by an extremely rapid heavy atom tunneling (k=2.1×10⁸ s⁻¹). Even when approaching room temperature, the QMT rate is still an order of magnitude faster than the semi-classical one. We propose an experimental test to support our prediction, by measuring a characteristic tunneling energy splitting within the radio wave region. Additionally, we examine the role of QMT in other hypothetical C_4n+2 carbon clusters.