Kinetics and mechanism of the reaction of the hydroxyl radical with h8-isoprene and d8-isoprene:: Isoprene absorption cross sections, rate coefficients, and the mechanism of hydroperoxyl radical production

We have employed a pulsed laser photolysis-pulsed laser induced fluorescence technique to study the kinetics and mechanism of the reaction of OR with isoprene. Three isotopomeric variants of the reaction have been studied. A rate coefficient of (8.47 +/- 0.59) x 10(-11) cm(3) molecule(-1) s(-1) ( +/-2sigma) was obtained at room temperature and showed no kinetic isotope effect within the precision of the measurements. The rate coefficient was independent of pressure over the range of 60-600 Torr and showed no dependence on the nature of the buffer gas in nitrogen, air, and helium. A limited study of the temperature dependence indicated that the reaction displays a slight negative activation energy (E-a = -690 J/mol). The gas-phase ultraviolet absorption spectrum of both regular and deuterated isoprene was obtained at room temperature and showed a strong absorption feature in the far ultraviolet. The absolute absorption cross section at similar to215 nm, the absorption peak. is similar to7 x 10(-17) cm(2). The detailed oxidation mechanism was examined by experiments in which NO was added to the gas mixture in order to recycle product HO2 to OR. At least 20 OR temporal profiles were obtained for each of the 3 isotopomeric variants. The profiles were modeled using the current isoprene module of the master chemical mechanism (MCM) that has been developed at Leeds University. The MCM mechanism crave good fits to the experimental profiles for all three reactions. A sensitivity analysis was developed to examine the extent to which recycling experiments can constrain individual rate coefficients in the MCM reaction mechanism. We obtain a lower limit of 4 x 10(-12) cm(3) molecule(-1) s(-1) for the rate coefficient for the addition of O-2 to the hydroxyalkyl radical formed by OH addition to isoprene. Our results suggest that uncertainties in the database on NO radical termination steps are a major limitation in the ability of recycling experiments to constrain the MCM mechanism.