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

Abstract

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) × 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 k(NO₂ + cyclohexa-1,3-diene) = (1.75 ± 0.15) × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹ was derived. An upper limit of k ≤ 7 × 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 k(C₆H₇ + NO₂)/k(C₆H₇ + O₂) = (1.8 ± 0.5) × 10⁵, 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 HO_x 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 α-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.