Understanding the oxidation of the tricarbon radical C₃H: A reaction pathway survey

The very recent observation of molecular oxygen in interstellar space appeals for the great need of mechanistic understanding of the oxidation processes of various interstellar species. In this article, we report for the first time, the oxidation mechanism of the chainlike l-C₃H by molecular oxygen, which is known as one of the interesting carbon-chain hydrocarbon series C_nH detected in space. This reaction is also relevant to the combustion processes where various carbon hydrides are involved. The detailed reaction pathways were identified at the CCSD(T)/aug-cc-pVTZ// B3LYP/6-311++G(d,p)+ZPVE level including various fragmentation channels. Three types of fragmentation channels are identified as the C-transfer product P ₂ (CO₂+C₂H) (-129.2kcal/mol), the C,O-exchange product P₁ (CO+HC₂O) (-154.7kcal/mol), and the O-transfer product P₆ (³O+HC₃O) (-44.8kcal/mol). The initially entered unstable dioxygen isomer 1a HCCCOO (-26.6 kcal/mol) would either undergo the direct O-extrusion to give P₆ (the intrinsic barrier 7.5 kcal/mol) or take a 1,2-O-shift (0.8 kcal/mol barrier) to give a stable isomer 5 HCCC(O)O (-139.2kcal/mol) that can either dissociate to P ₁ or to P₂. The intrinsic barrier from 5 to P₁ and P₂ is 29.1 and 23.6 kcal/mol, respectively. Clearly, the entrance thermicity 26.6 kcal/mol of 1a can sufficiently initiate the subsequent formation of all the three products. To quantitatively evaluate the kinetic competition of the three products, we performed the master equation rate constant calculations. It was shown that at 298 K, the most favorable product is P₂ (64.8%) followed by P₆ (23.6%), and P₁ (11.6%). Interestingly, at elevated temperatures, the ratio of P₆ increases with the decrease of P₂, whereas that of P₁ is little changed. Notably, the thermodynamically most stable product P₁ is kinetically the least favorable, indicative of the importance of considering the kinetics. The dominant formation of P₂ (CO₂+C ₂H) shows that the important carbyne radical l-C₃H can be effectively degraded by O₂ via the chain-shortening step. The reactivity of the cyclic c-C₃H radical toward O₂ is also discussed. The results are expected to enrich our understanding of the chemistry of the simplest C₃-radical in both combustion and interstellar processes.