Kinetics of the reactions between propargylic radicals and oxygen molecules: Experiments, master-equation simulations, and reanalysis of literature data

We use the recently implemented trace-fitting feature in MESMER (a master-equation code) to reanalyze published kinetic data for the propargyl + O2, 1-methylpropargyl + O2, and 3-methylpropargyl + O2 reactions by directly comparing experimental and simulated kinetic traces (concentration–time profiles). New kinetic measurements are also reported for the 3-ethylpropargyl + O2 reaction. The current work identifies a flaw in the way which rate coefficients were extracted from multi-exponential traces in the previous works, and
highlights the benefits of using trace fitting to analyze traces exhibiting complicated time behavior. Quantum-chemistry calculations are performed to find the kinetically important reaction channels that need to be included in the master-equation models. Key parameters in the models are optimized by minimizing differences between experimental and simulated traces with automated routines. The optimized models are then used to simulate the studied reactions over a wide range of temperatures and pressures. We provide the results of the master-equation simulations in modified Arrhenius form to facilitate their use in combustion modeling. Shortcomings in the previous works are discussed and corrected. The reaction mechanism of the studied systems is examined in some detail, and we find there is only a single bimolecular-product channel that is kinetically significant. This channel forms formyl and ketene for the propargyl + O2 reaction and ketenic and formylic products for the methyl- and ethyl-substituted systems. Under combustion conditions, only a single, pressure-independent rate coefficient is needed for each propargylic radical + O2 reaction to describe the conversion of bimolecular reactants to bimolecular products.
Novelty and Significance Statement
In this work, we present several new findings relating to propargylic radical + O2 reactions. Firstly, we report first-ever kinetic measurements for the 3-ethylpropargyl + O2 reaction. We also report some new kinetic measurements for the 3-methylpropargyl + O2 reaction. Secondly, we identified a shortcoming in the way which kinetic traces (time-concentration profiles) were analyzed in previous publications that investigated propargylic radical + O2 reactions. Thus, we reanalyzed the published data with master-equation based trace
fitting to correct for the shortcoming. Thirdly, we performed a systematic comparison of the propargyl + O2, 1-methylpropargyl + O2, 3-methylpropargyl + O2, and 3-ethylpropargyl + O2 reactions to probe how small differences in the molecular structures affect reactivity and collisional energy transfer.