The structure and energetics for the reaction of OH + isoprene f adduct have been examined using ab initio molecular orbital methods. The structure of each HO-isoprene adduct was optimized using Becke's threeparameter hybrid method employing the LYP correction functional (B3LYP) with the 6-31G** basis set, and using Møller-Plesset correlation energy correction truncated at second-order (MP2) with both the 6-31G** and the 6-311G** basis sets. Single-point energy calculations using fourth-order Møller-Plesset perturbation theory including single, double, triple, and quadruple excitations, as well as spin projection (PMP4(SDTQ)) with the 6-311G** basis set, were carried at these optimized geometries. The single-point energy was further corrected with zero-point energy (ZPE) to assess the stability of the OH-isoprene adducts. At the PMP4-(SDTQ)/6-311G**//MP2/6-311G** + ∆ZPE level of theory addition of OH to the 1 and 4 carbons of isoprene produces adducts which are 37.9 and 35.4 kcal mol -1 (respectively) more stable than the OH and isoprene reactants, while addition of OH to the 2 and 3 carbons results in adducts which are 25.6 and 24.2 kcal mol -1 more stable than the reactants. Experimental detection of the products from the OH + isoprene reaction using a discharge-flow system coupled with a mass spectrometer shows evidence for the production of all four possible adducts. These results suggest that each adduct is formed with nonnegligible yields, allowing each to participate in subsequent steps in the OH-initiated oxidation of isoprene.

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The kinetics of the reaction of hydroxyl radical with 1,3-butadiene at 240-340 K and a total pressure of approximately 1 Torr has been studied using relative rate combined with the discharge flow and mass spectrometer technique. The reaction dynamics of the same reaction has also been investigated using ab initio molecular orbital theory. The rate constant for this reaction was found to be negatively dependent on temperature, with an Arrhenius expression of k1 = (1.58 +/- 0.07) x 10(-11) exp[(436 +/- 13)/T] cm3 molecule(-1) s(-1) (uncertainties taken as 2sigma), which was in good agreement with that reported by Atkinson et al. and Liu et al. at 299-424 K. Mass spectral evidences were found for the addition of OH to both the terminal and the internal carbons of 1,3-butadiene. Our computational results suggest that both addition of OH to 1,3-butadiene and the abstraction of hydrogen atom from 1,3-butadiene by the OH radical are exothermic processes and that the addition of OH to the terminal carbon of the 1,3-butadiene is predicted to have an activation energy of 0.7 kcal mol(-1), being the most energetically favored reaction pathway.