Rate coefficients of the H-migration in aldehydic peroxy radicals, and the reaction of the hydroxyl radical (OH) with unsaturated aldehydes.

The reaction of selected aldehydes with the hydroxyl radical (OH), the most important atmospheric oxidant, was studied in a flow tube at 1 atm pressure and a temperature range between 285 and 340 K. OH radicals were produced by flash photolysis of ozone in the flow tube and the time resolved OH concentration was observed by laser-induced fluorescence in a low-pressure measurement cell which sampled air from the flow tube. OH was consumed in the reaction with the aldehyde and could also be regenerated from the 1,4-H-migration of one isomer of the organic peroxy radicals (RO2) formed in the OH reaction with the aldehyde. By simultaneous optimisation of the OH rate coefficient and of the rate coefficient of the 1,4-H-migration, temperature dependent rate coefficients of both reactions could be derived from the observed OH concentration. In addition, high-level theoretical kinetic calculations were carried out to characterize the rate coefficients of the H-migration in aldehydic RO2 radicals, covering OH-substituted and alkylic RO2. A structure-activity relationship (SAR) is presented covering migration spans from 1,4 to 1,20-H-migration for various substitution patterns, showing good agreement with the experimental data.