Progressive Shortening of sp-Hybridized Carbon Chains through Oxygen-Induced Cleavage

Linear sp-hybridized carbon chains have recently been proposed as one-dimensional nanostructures for electronic applications and as intermediate products in many nanoscale processes. However, their synthesis and observation are affected by their degradation upon oxygen exposure. Carbon chains consisting of alternating single and triple bonds (polyynic) are reported to be more stable than those consisting of double bonds (cumulenic), but the details of the degradation mechanism are still unknown. We use density functional theory to show that adsorption of O-2 on carbon chains anchored to carbon substrates can cause their cleavage through the collective scission of O-O and C-C bonds, yielding separate CO-terminated chains. Further O-2 attack progressively shortens them, causing CO2 formation. While the shortening process has general validity, the cleavage step of the reaction is exothermic for cumulenic chains only. Perturbations of the ideal structure, such as bending of the chains or modifications to the structure of their termination, affect the oxidation mechanism and can make the cleavage reaction exothermic for polyynic chains as well. These results contribute to the interpretation of the available experimental results and reveal atomic-scale details of the oxidation process,. thus allowing predictions on the chemistry of nanostructured carbon surfaces and suggesting possible new directions for the study of the chemical stability of carbynoid structures.