We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the presence of O 2 and NO using the LP/LIF technique. The photolysis of monodeuterated 1-iodo-3-buten-2-ol provides only one of the possible OD-butadiene adducts, the minor addition channel product, simplifying the oxidation mechanism. We find, based on analysis of OD time-dependent traces that prompt rearrangement of initial -hydroxyalkyl radicals to R-hydroxyalkyl radicals occurs in agreement with RRKM/ME theoretical predictions. We report a rate constant of (3.3(1.0) × 10 -11 cm 3 molecules -1 s -1 for deuterium abstraction from the R-hydroxyalkyl radical at 298 (2 K. Our approach demonstrates the feasibility of isomeric selective kinetic studies of the OH-initiated oxidation of unsaturated hydrocarbons.

We report isomer-selective kinetics and mechanistic details for the hydroxyl radical-initiated oxidation of isoprene, in the presence of O(2) and NO, employing complementary experimental and theoretical techniques. Using a recently demonstrated photolytic route to initiate isomer-selective kinetics in OH-initiated oxidation of unsaturated hydrocarbons via the UV photolysis of iodohydrins, the photolysis of 1-iodo-2-methyl-3-buten-2-ol results in a single isomer of the possible four OH-isoprene adducts, specifically the minor channel associated with OH addition to one of the inner carbon atoms. Employing both the laser-photolysis/laser-induced fluorescence (LP/LIF) technique and time-dependent multiplexed photoionization mass spectrometry, we find clear experimental evidence supporting the prompt rearrangement of the initially formed beta-hydroxyalkyl radicals to alpha-hydroxyalkyl radicals, in agreement with Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation predictions. We have determined a rate constant of (3.3 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1) for molecular oxygen to abstract a hydrogen atom from the alpha-hydroxyalkyl radical to form 4-penten-2-one and HO(2). This reaction provides a mechanistic route to C(5) carbonyl species as first-generation end products for the addition of hydroxyl radical to isoprene in the presence of O(2) and NO.

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