A kinetics and mechanistic study of the OH and NO2 initiated oxidation of cyclohexa-1,3-diene in the gas phase.

The kinetics and products of the OH and NO2-initiated oxidation of cyclohexa-1,3-diene have been investigated at 296 K and 700 Torr using long path FTIR spectroscopy. Relative rate methods were employed using the photolysis of cyclohexa-1,3-diene/CH3ONO/NO/air mixtures to measure kappa(OH + cyclohexa-1,3-diene) = (1.68 +/- 0.43) x 10(-10) cm3 molecule(-1) s(-1). From the pseudo-first order decay of cyclohexa-1,3-diene in the presence of excess NO2, a value of kappa(NO2 + cyclohexa-1,3-diene) = (1.75 +/- 0.15) x 10(-18) cm3 molecule)-1) s(-1) was derived. An upper limit of kappa < or = 7 x 10(-21) cm3 molecule(-1) s(-1) was established for the reaction of NO with cyclohexa-1,3-diene. Benzene was observed as a product of both the OH and NO2 initiated oxidation, providing evidence of H atom abstraction in both reactions. Assuming the reaction of cyclohexadienyl radicals (C6H7) with O2 produces benzene as the sole organic product, the results are consistent with abstraction channel branching ratios of (8.1 +/- 0.2)% and (1.5 +/- 0.4)%, respectively. The results also indicate that C6H7 reacts with NO2, with a relative rate coefficient kappa(C6H7 + NO2)/kappa(C6H7 +O2) = (1.8 +/- 0.5) x 10(5), and that this partially forms benzene, with a branching ratio of (27 +/- 7)%. The stoichiometry and products of the NO2 reaction were investigated in the absence of O2, in the presence of O2, and in the presence of O2 and NO. Reaction mechanisms consistent with the observations are presented. In the presence of NO and O2, the NO2-initiated chemistry leads to NO-to-NO2 conversion, and the formation of HOx radicals in significant yield, (0.79 +/- 0.05), such that cyclohexa-1,3-diene removal occurs by reaction with both NO2 and OH. HCOOH was detected as a product in this system, providing evidence for significant formation of stabilised C6 alpha-hydroxyperoxy radicals from the OH-initiated chemistry, and their subsequent reaction with NO. An estimate of ca. 500-1000 s(-1) is made for their decomposition rate, based on the [NO]-dependence of the HCOOH yields. The implications of the results are discussed within the context of the atmospheric chemistry of conjugated dienes.