Reaction mechanisms of cyclo[18]carbon and triplet oxygen

Recently, a new carbon allotrope, cyclo[18]carbon, of alternating short and long carbon-carbon bonds has been synthesized and characterized in the condensed phase. Inspired by experiments, a lot of theoretical studies involving adsorption, aromaticity, catalysis and spectra have been performed. Although cyclo[18]carbon is generally regarded as an unstable molecule, the theoretical explanation of its instability is still inadequate. In this work, we studied the intermediate process of reactions between cyclo[18]carbon and triplet oxygen by density functional theory calculations. The reaction is expected to happen easily because the maximal reaction energy barrier is less than 1 eV, and cyclo[16]carbon, cyclo[17]carbon and straight-chain C₁₈O₂ molecules have been considered as possible products. Infrared and Raman spectra were calculated to help in differentiating the final products. The thermal stability of cyclo[17]carbon is rather weak according to ab initio molecular dynamics simulations. This work sheds light on the synthesis of other cyclo[n]carbons and cumulenes by utilizing cyclo[18]carbon.