The OH-initiated atmospheric oxidation of pinonaldehyde: Detailed theoretical study and mechanism construction

The pinonaldehyde + OH reaction proceeds via H-atom abstraction by OH, which can occur on six different carbon atoms. For each of these OH-attack/H-abstraction sites in pinonaldehyde, a detailed mechanism of product formation in atmospheric conditions is developed, based solely on quantitative structure activity relationships (SARs) and on theoretical quantum chemistry methods, combined with transition state theory calculations or RRKM. Invoking only classical '' alkyl-, alkylperoxy- and alkoxy chemistries, the objective fate of some 30 organic radical intermediates was in this way predicted. Total primary product yields were obtained by propagating the product fractions of each step in the mechanism. Overall predicted product molar yields for high-NO conditions are: 22.9% 4-hydroxynorpinonaldehyde, 9.9% acetone, 12.9% CH2O, 30.3% organic nitrates, 73.8% CO2, 11.4% HC(O) OH, 16.6% norpinonaldehyde and CO, and 16.6% other (hydroxy)(poly) carbonyls. In more realistic atmospheric conditions, at lower NO levels, our theoretically predicted yields differ only regarding 4-hydroxynorpinonaldehyde (38.2%), HC(O) OH (0.3%), and nitrates (26.2%). The classical radical chemistries adopted in this work fail to reproduce the very high yield of CH2O (similar to150%) observed in a recent laboratory investigation.