Resolving the mystery of prompt CO 2: The HCCO+O 2 reaction

Abstract

The potential energy surface for the reaction of the ketenyl radical (HCCO) with O 2 is characterized with a combination of density functional. Moller Plesset perturbation and coupled cluster theories. Trajectory simulations, employing directly determined density functional force fields are used to explore the mechanism further. Variational transition state theory based master equation simulations, implementing the quantum, chemically determined molecular properties, provide estimated product state distributions and rate constants. For all temperatures considered here (300–2500 K), the dominant products are CO 2 +CO+H independent of pressure up to 100 atm, thereby explaining the coincident formation of CO 2 and CO in the oxidation of acetylene. These products arise primarily from the decomposition of the initial OOCHCO adduct via the formation of a four-membered OCCO ring, followed by, in succession, the splitting of the OO, the CC, and the CH bonds. At low temperatures, the modest barrier in the entrance channel, which arises from the need to break the, resonances in the reactants prior to bond formation, provides the rate-limiting transition state. A variational treatment of this transition state reduces the calculated rate coefficient relative to conventional transition state theory by ≈35%. At higher tempratures, the formation of the four-membered ring becomes a rate-limiting step, even though this process is essentially barrierless. An OCHCO+O channel becomes increasingly important with increasing temeprature, but still contributes only about 10% at 2500 K. The direct dynamics simulations indicate that various H transfers may occur during the final dissociation, steps, primarily to yield CO+CO+OH with an overall branching ratio of about 9%. A downward adjustment by 3.2 kcal/mol of the HCCO+O 2 entrance barrier results in total rate coefficients that are in good agreement with experiment. The predicted channel-specific rate coefficients are then 7.94×10 −12 T −0.142 exp(−1150/ RT )(CO 2 +CO+H), 3.62×10 −22 T 269 exp(−3541/ RT )(OCHCO+O) and 3.17×10 −13 T −0.020 exp(−1023/ RT )(CO+CO+OH) cm 3 molecule −1 s −1 , where R =1.987 cal mol −1 K −1 .