Theoretical studies on the C2H+O2 reaction: mechanism for HCO+CO, HCCO+O and CH+CO2 formation

The reaction of the ethynyl radical with molecular oxygen has been examined using density functional theory. Two major reaction routes are open to the chemically activated HCCOO adduct 1: dissociation to HCCO+O 16 and formation of the thermodynamically most stable products HCO + CO 12: HCCOO 1 → dioxirenyl 2 → oxyrenyloxy 3 → oxo-ketene 5 → HCO + CO 12 → H + 2CO 15. The CCSD(T)/6-311++G(d,p)// B3LYP/6-311++G(d,p) energies of the respective rate controlling transition states, 1/16 and 1/2, indicate that the route leading to H + CO + CO should dominate. Several other [C2,H,O2] isomers and other, minor pathways have also been characterised. The present study reveals this reaction to be a capture-limited association-elimination reaction with a high and pressure-independent rate coefficient.