Theoretical investigations on mechanisms and kinetics of methylketene with O(3P) reaction in the atmosphere

The O(³P)-initiated conversion mechanism and dynamics of CH₃CHCO were researched in atmosphere by executing density functional theory (DFT) computations. Optimizations of all the species and single-point energy computations were implemented at the B3LYP/6-311++G(d,p) and CCSD(T)/cc-pVTZ level, respectively. The explicit oxidation mechanism was introduced and discussed. The results state clearly that the O(³P) association was more energetically beneficial than the abstraction of H. The rate coefficients over the probable temperature range of 200-3000 K were forecasted by implementing Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Specifically, the total rate coefficient of O(³P) association reactions is 1.19 × 10^-11 cm³ molecule^-1 s^-1 at 298 K, which is consistent with the experimental results (1.16 × 10^-11 cm³ molecule^-1 s^-1). The rate coefficients for the O(³P) with CH₂CO, CH₃CHCO, and (CH₃)₂CCO suggest that rate coefficient of ketene derivatives increase with the increase of methylation degree. Graphical abstract.

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.

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

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂ CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice-Ramsperger-Kassel-Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂ H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂ H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂ COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂ CH₂ CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂ H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃ CHCO and cyclic CH₂ COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300-3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.