title: Isomer-selective study of the OH initiated oxidation of isoprene in the presence of O(2) and NO. I. The minor inner OH-addition channel.
abstract: 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.
authors.affiliation: Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, USA.
authors.forename: Erin E
authors.initials: EE
authors.lastname: Greenwald
authors.forename: Buddhadeb
authors.initials: B
authors.lastname: Ghosh
authors.forename: Katie C
authors.initials: KC
authors.lastname: Anderson
authors.forename: Kristin S
authors.initials: KS
authors.lastname: Dooley
authors.forename: Peng
authors.initials: P
authors.lastname: Zou
authors.forename: Talitha
authors.initials: T
authors.lastname: Selby
authors.forename: David L
authors.initials: DL
authors.lastname: Osborn
authors.forename: Giovanni
authors.initials: G
authors.lastname: Meloni
authors.forename: Craig A
authors.initials: CA
authors.lastname: Taatjes
authors.forename: Fabien
authors.initials: F
authors.lastname: Goulay
authors.forename: Simon W
authors.initials: SW
authors.lastname: North