Thermal stability of carbonyl radicals Part II. Reactions of methylglyoxyl and methylglyoxylperoxy radicals at 1 bar in the temperature range 275–311 K

Abstract

Reactions of methylglyoxyl and methylglyoxylperoxy radicals were investigated at a total pressure of 1 bar in oxygen. Methylglyoxyl radicals were generated by stationary photolysis of Br₂–CH₃C(O)C(O)H–NO₂–O₂–N₂ mixtures at wavelengths ≥480 nm and of Cl₂–CH₃C(O)C(O)H–NO₂–O₂–N₂ mixtures in the wavelength range 315–460 nm. In the bromine system, rate constant ratios for the reactions CH₃C(O)CO → CH₃CO + CO (k_dis) and CH₃C(O)CO + O₂ → CH₃C(O)C(O)O₂ (k_O2) were measured as a function of temperature in the range 275–311 K. Assuming the constant value k_O2 = 5.1 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for our reaction conditions, k_dis = 1.2 × 10^10.0±0.7 × exp(−11.7 ± 3.8 kJ mol⁻¹/RT) s⁻¹ (2σ errors) was obtained for p_tot = 1 bar (M = O₂), in good agreement with the kinetic parameters calculated by Méreau et al. [R. Méreau, M.-T. Rayez, J.-C. Rayez, F. Caralp and R. Lesclaux, Phys. Chem. Chem. Phys., 2001, 3, 4712]. CH₃C(O)C(O)O₂ radicals oxidise NO₂, forming NO₃, CH₃CO and CO₂. This experimental result is supported by DFT and ab initio calculations. Possible mechanisms for the observed formation of several % of ketene and bromoacetyl peroxynitrate are discussed. Use of Cl rather than Br atoms to abstract the aldehydic H atom from methylglyoxal leads to chemically activated CH₃C(O)CO radicals, thus substantially increasing the fraction of CH₃C(O)CO radicals that decompose rather than add O₂.