Kinetic flash spectroscopic study of the CH₃O₂CH₃O₂ and CH₃O₂SO₂ reactions

Abstract

Abstract Methylperoxy radicals were generated by the flash photolysis of azomethane–oxygen mixtures. The observed broadband spectrum of the CH 3 O 2 radical is similar, but not identical to those reported previously. The CH 3 O 2 decay followed second‐order kinetics at high CH 3 O 2 concentrations with k 4 ' = (2.5 ± 0.3) × 10 8 liter/mol·sec (23 ± 2°C); 2CH 3 O 2 → products (4). Because of the potential loss of CH 3 O 2 through the reactions with HO 2 and CH 3 O radicals subsequently formed in this system, simulations suggest that the true k 4 is in the range: 2.5 × 10 8 ≥ k 4 ≥ 2.3 × 10 8 liter/mol·sec. Deviations from linearity of the plot of the reciprocal of the CH 3 O 2 absorbance versus time were seen at long times and were attributed to the reaction (5) with an apparent rate constant k 5 ' ⋍ (1.6 ± 0.4) × 10 5 liter/mol·sec; CH 3 O 2 + Me 2 N 2 → product (5). The CH 3 O 2 –SO 2 reaction, CH 3 O 2 + SO 2 → products (16), was studied by observing CH 3 O 2 decay in flashed mixtures of Me 2 N 2 , O 2 , and SO 2 . The results gave the apparent second‐order rate constant k 16 ' ⋍ (6.4 ± 1.4) × 10 6 liter/mol·sec. It appears likely that each occurrence of reaction (5) and (16) is followed by the loss of an additional CH 3 O 2 radical and that k 5 ⋍ k 5 '/2 and k 16 ⋍ k 16 '/2. Our findings suggest that a significant fraction of the SO 2 oxidation in a sunlight‐irradiated NO x −RH‐polluted atmosphere, may occur by reaction with CH 3 O 2 as well as from the HO and HO 2 reactions.