OH radical initiated oxidation of 1,3-butadiene: isomeric selective study of the dominant addition channel.

We report the first isomeric selective kinetic study of the dominant isomeric pathway in the OH initiated oxidation of 1,3-butadiene in the presence of O(2) and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photodissociation of the precursor 2-iodo-but-3-en-1-ol results exclusively in the dominant OH-butadiene addition product, permitting important insight into the OH initiated oxidation mechanism. On the basis of analysis of the time dependent OH/OD signals, we have determined a rate constant for O2 addition to the hydroxyalkyl radical of 7.0(-3.0)(+7.0) x 10(-13) cm(3) s(-1), and we find a value of 1.5(-0.6)(+1.0) x 10(-11) cm(3) s(-1) for the overall reaction rate constant of the hydroxy peroxy radical with NO. We also report the first clear experimental evidence of the (E) form of the delta-hydroxyalkoxy channel through isotopic labeling experiments and provide an upper bound of 13 +/- 5% to its branching ratio. This species provides a mechanistic pathway for the formation of 4-hydroxy-2-butenal, which has been identified as a first generation end product. This isomeric selective kinetic study, together with a previous study on the minor channel of the 1,3-butadiene oxidation, yields a comprehensive picture of butadiene oxidation under high NOx conditions relevant to most regions in the continental US.