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 k(OH + cyclohexa-1,3-diene) = (1.68 +/- 0.43) x 10(-10) cm(3) molecule(-1) s(-1). From the pseudo-first order decay of cyclohexa-1,3-diene in the presence of excess NO2, a value of k(NO2 + cyclohexa- 1,3- diene) = (1.75 +/- 0.15) x 10(-18) cm(3) molecule(-1) s(-1) was derived. An upper limit of k less than or equal to 7 x 10(-21) cm(3) 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 O-2 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 k(C6H7 + NO2)/ k(C6H7 + O-2) = ( 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 O-2, in the presence of O-2, and in the presence of O-2 and NO. Reaction mechanisms consistent with the observations are presented. In the presence of NO and O-2, 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 C-6 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.