Understanding the oxidation of the tricarbon radical C₃ H: A reaction pathway survey

Abstract

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-C 3 H by molecular oxygen, which is known as one of the interesting carbon-chain hydrocarbon series C n H 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=6-31111G(d,p)1ZPVE level including various fragmentation channels. Three types of fragmentation channels are identified as the C-transfer product P 2 (CO 2 1C 2 H) (2129.2kcal=mol), the C,O-exchange product P 1 (CO1HC 2 O) (2154.7kcal=mol), and the O-transfer product P 6 ( 3 O1HC 3 O) (244.8kcal=mol). The initially entered unstable dioxygen isomer 1a HCCCOO (226.6 kcal/mol) would either undergo the direct O-extrusion to give P 6 (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 (2139.2kcal=mol) that can either dissociate to P 1 or to P 2 . The intrinsic barrier from 5 to P 1 and P 2 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 P 2 (64.8%) followed by P 6 (23.6%), and P 1 (11.6%). Interestingly, at elevated temperatures, the ratio of P 6 increases with the decrease of P 2 , whereas that of P 1 is little changed. Notably, the thermodynamically most stable product P 1 is kinetically the least favorable, indicative of the importance of considering the kinetics. The dominant formation of P 2 (CO 2 1C 2 H) shows that the important carbyne radical l-C 3 H can be effectively degraded by O 2 via the chain-shortening step. The reactivity of the cyclic c-C 3 H radical toward O 2 is also discussed. The results are expected to enrich our understanding of the chemistry of the simplest C 3 -radical in both combustion and interstellar processes.