The Photoionization Spectrum of the Acetyl Radical and the Kinetics of Its Unimolecular and Bimolecular Reactions at Elevated Pressures

Abstract

The acetyl radical (CH₃CO) is a primary intermediate during the oxidation and combustion of CH₃CHO and represents a prototypical example of a weakly bound acyl radical. However, it is challenging to detect and has rarely been probed at elevated pressures, where CH₃CO decays by competing bimolecular and unimolecular reactions. The present study provides a combined modeling and experimental analysis of the comprehensive network of CH₃CO unimolecular and bimolecular kinetics at 500 kPa and 300–366 K. CH₃CO was generated by acetaldehyde (CH₃CHO) + Cl reactions, where the Cl is produced by pulsed laser photolysis of oxalyl chloride ((COCl)₂) at 248 nm. The progress of reactions was monitored via multiplexed photoionization mass spectrometry, providing concentration–time profiles of CH₃CHO, CH₃CO, methyl (CH₃), ketene (CH₂CO), acetone (CH₃COCH₃), and biacetyl ((CH₃CO)₂). The concentration of CH₃CO was quantified from time-resolved atom balance analysis. The absolute photoionization spectrum of CH₃CO was determined for the first time up to energies of 11 eV, enabling future quantitative measurements in more complex environments. The experimental data were simulated using a kinetic model consisting of unimolecular and bimolecular reactions driving CH₃CO formation and decay as well as relevant secondary reactions describing the evolution of the observed intermediates. The measured time profiles showed strong sensitivity to the following reactions involving CH₃CO: (R1) CH₃CO = CH₃ + CO, (R2a) CH₃CO + CH₃CO = (CH₃CO)₂, (R3a) CH₃CO + CH₃ = CH₃COCH₃, and (R5) CH₃CO + Cl = CH₂CO + HCl. The best-fit bimolecular reaction rate coefficients (k_2a, k_3a, and k₅) differ from the literature values by up to a factor of 2, most likely because the prior works studied each reaction separately, rather than in one unified experimental campaign, and were influenced by other competing reactions. Simulations using the assembled kinetic model enable a direct determination of the rate coefficient of (R1), which is fit to the Arrhenius expression, k₁ = 7.63 × 10¹⁰ exp(−6868/T) s^–1 (336–366 K, 500 kPa, He buffer, and uncertainty of ±40%). The present study extends the previously available data on CH₃CO dissociation to much higher pressures and provides improved constraints for collisional energy transfer parameters and activation energies from theoretical kinetics studies.