Kinetic flash spectroscopic study of the CH3O2-CH3O2 and CH3O2-SO2 reactions

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Abstract
Methylperoxy radicals were generated by the flash photolysis of azomethane–oxygen mixtures. The observed broadband spectrum of the CH3O2 radical is similar, but not identical to those reported previously. The CH3O2 decay followed second‐order kinetics at high CH3O2 concentrations with k4' = (2.5 ± 0.3) × 108 liter/mol·sec (23 ± 2°C); 2CH3O2 → products (4). Because of the potential loss of CH3O2 through the reactions with HO2 and CH3O radicals subsequently formed in this system, simulations suggest that the true k4 is in the range: 2.5 × 108 ≥ k4 ≥ 2.3 × 108 liter/mol·sec. Deviations from linearity of the plot of the reciprocal of the CH3O2 absorbance versus time were seen at long times and were attributed to the reaction (5) with an apparent rate constant k5' ⋍ (1.6 ± 0.4) × 105 liter/mol·sec; CH3O2 + Me2N2 → product (5). The CH3O2–SO2 reaction, CH3O2 + SO2 → products (16), was studied by observing CH3O2 decay in flashed mixtures of Me2N2, O2, and SO2. The results gave the apparent second‐order rate constant k16' ⋍ (6.4 ± 1.4) × 106 liter/mol·sec. It appears likely that each occurrence of reaction (5) and (16) is followed by the loss of an additional CH3O2 radical and that k5 ⋍ k5'/2 and k16 ⋍ k16'/2. Our findings suggest that a significant fraction of the SO2 oxidation in a sunlight‐irradiated NOx−RH‐polluted atmosphere, may occur by reaction with CH3O2 as well as from the HO and HO2 reactions.
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