Mechanism and rate constants of the CH2 + CH2 CO reactions in triplet and singlet states: A theoretical study

Temperature and Pressure Dependent Rate Coefficients for the Reaction of Ketene with Hydroxyl Radical

The reaction of ketene with hydroxyl radical is drawing growing attention, for it is found to constitute an important step during the combustion of hydrocarbon and oxygenated hydrocarbon fuels, e.g., acetylene, propyne, allene, acetone, gasoline, diesel, jet fuels, and biofuels. We studied the potential energy surface (PES) of this reaction using B2PLYP-D3/cc-PVTZ for geometry optimization and composite methods based on CCSD(T)-F12/cc-PVTZ-F12 for energy calculations. From this PES, temperature- and pressure-dependent rate coefficients and branching ratios at 200-3000 K and 0.01-100 atm were derived using the RRKM/ME approach. The reaction is dominated by four product channels: (i) OH addition on the olefinic carbon of ketene to form CH₂OH + CO, which is the most dominant under all conditions; (ii) H abstraction producing HCCO + H₂O, which is favored at high temperatures; (iii) OH addition on the carbonyl carbon to form CH₃ + CO₂, which is favored at low pressures and high temperatures; and (iv) collisional stabilization of CH₂COOH, which is favored at high pressures and low temperatures. With increasing temperatures, the overall rate constant k_overall exhibit first negative but then positive temperature dependency, with its switching point (also the minimum point) at ∼400 K. Both product channel CH₂OH + CO and HCCO + H₂O are independent of pressure, whereas formation of CH₃ + CO₂ and collisional stabilization of CH₂COOH are highly pressure dependent. Fitted modified Arrhenius expressions of the calculated rate constants are provided for the purpose of combustion modeling.