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

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 (kdis) and CH3C(O)CO + O2 → CH3C(O)C(O)O2 (kO2) were measured as a function of temperature in the range 275–311 K. Assuming the constant value kO2 = 5.1 × 10−12 cm3 molecule−1 s−1 for our reaction conditions, kdis = 1.2 × 1010.0±0.7 × exp(−11.7 ± 3.8 kJ mol−1/RT) s−1 (2σ errors) was obtained for ptot = 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.