Oxidation mechanism of the butadiynyl radical, C₄H: Analogue of C₂H or not

Reactions of the carbon-chain radicals are of great importance in the combustion and astrophysical processes. The kinetics of the butadiynyl radical, C₄H, has received recent attention. While there has been sufficient knowledge concerning the oxidation of the ethynyl radical, C₂H, oxidation of the higher even-numbered members C_2nH (n > 1) is hardly known. In this paper, to enrich the C₄H-chemistry, we report the first study of the oxidation mechanism of C₄H. At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energy surface (PES) survey is presented covering various product channels P ₁(CO + HC₃O) (-152.7 kcal/mol), P₂(C ₃H + CO₂) (-117.9), P₃(HCO + C₃O) (-108.5), P₄(HC₄O+³O) (-45.2), and P ₅(OH + C₄O) (-33.2) accompanied by the master equation rate constant calculations. Despite the similarity in the PES, the kinetics of C₄H+³O₂ differs dramatically from that of the analogous C₂H+³O₂ reaction. For the C ₄H+³O₂ reaction, the O-abstraction product P₄(HC₄O+³O) is almost the exclusive product, whereas the lowest C,O-exchange product P₁(CO + HC₃O) and other products have little importance. By contrast, the C₂H+ ³O₂ reaction favors the C,O-exchange product HCO + CO. Being overall barrierless and mainly associated with the molecular → atomic oxygen conversion, the C₄H+³O₂ reaction should play an important role in the soot formation and interstellar chemistry where C₄H is involved.