Mechanism and rate constants of the CH₂ + CH₂CO reactions in triplet and singlet states: A theoretical study

Abstract

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 CH 2 with ketene CH 2 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 + CH 3 and C 2 H 4 + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C 2 H 4 + 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 CH 2 COCH 2 intermediate or along the pathway of CO elimination from the initial CH 2 CH 2 CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C 2 H 4 + 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 CH 3 CHCO and cyclic CH 2 COCH 2 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.

Detailed Chemical Kinetic Study of Acetaldehyde Oxidation and Its Interaction with NOx

“…Konnov and co-workers , showed the importance of CH 2 CO chemistry during the modeling of diacetyl and acetic acid flames. In the light of their findings, we updated the CH 2 CO submechanism based on the results of recent theoretical works. − Again, we note that the available kinetic models in the literature could not accurately predict the laminar flame speed of CH 3 CHO–air mixtures. With the current updates, our model showed a significantly improved performance while predicting the available experimental literature data over a wide range of conditions.…”

Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4H-Pyran-4-one

“…The 4PN molecule is a prototypical cyclic enone and serves as a model for investigating the π* ← n chromophore in α,β-unsaturated carbonyls. Triplet excited states of unsaturated molecules can play key roles in photochemistry, 1 combustion, 2 and energy storage. 3 Triplet excited states are generally long-lived, enhancing their abiliity to participate in these processes.…”

Improving the Predictive Accuracy for Ketene in Diacetyl Laminar Premixed Flames: Experiment and Model Analysis

“…Ketene (CH 2 CO) is a key intermediate in the combustion of hydrocarbons and oxygenated fuels. Its formation and consumption pathways are an essential part of the C 0 –C 4 combustion core mechanisms. − One of the latest ketene mechanisms was developed by Konnov et al However, subsequent theoretical studies suggested that rate parameters of several important reactions in the mechanism need modifications. − Pyrolysis of ketene was investigated directly in early shock tube studies, ,, but due to its high reactivity, the preparation of a pure ketene sample is challenging. Therefore, indirect experiments in which ketene is produced as an intermediate were preferred in recent fundamental combustion experiments. ,,, Diacetyl ((CH 3 CO) 2 ) is one of the optional fuels that can be used as a precursor of ketene, and the reaction processes from diacetyl to ketene and corresponding mechanisms attracted interest. ,,− …”