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Description:
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Isoprene is the dominant non -methane organic compound emitted by vegetation
into the atmosphere , with a global emission rate of ~ 500 Tg yr -1 . Its oxidation serves as
a major source of ground level ozone in North America during the summer months .
Despite the significant impact on tropospheric chemistry , questions remain concerning
the detailed oxidation mechanism . The initial step in the mechanism is the addition of
OH to form four distinct isomers . The relative branching between these isomers
influences the distribution of the final products . I present a comprehensive investigation
into the mechanistic details of early steps in the oxidation mechanism of unsaturated
hydrocarbons in the troposphere and employ theoretical and experimental techniques .
To understand the detailed kinetics of the initial OH addition to unsaturated
hydrocarbons , I first present a model developed for the ethylene -OH system . I present
the details of a robust two -transition state model . I extend the developed two -transition state model to the case of OH addition to isoprene . Excellent agreement with observed
temperature and pressure dependent rate constants affords a high confidence level in
understanding of the kinetics and in the calculated branching ratio of the initial OH
addition step .
I then focus attention on the subsequent reactivity of the OH -isoprene adducts .
Until recently , all four of the OH -isoprene adducts were supposed to have reacted with
O2 via addition to form alkylperoxy radicals . Previous computational results suggest that
two of the OH -isoprene adducts undergo an intramolecular cyclic isomerization
followed by hydrogen abstraction by O2 to form stable carbonyl compounds . I have
synthesized photolytic precursors , presenting a novel approach to probe the subsequent
reactivity of individual hydroxyalkyl radicals .
Initial verification of the cyclic isomerization pathway involved synthesis of the
photolytic precursor corresponding to the 1 ,3 -butadiene -OH adduct . A culmination of
theoretical and experimental techniques allowed verification of the cyclic isomerization
pathway . I synthesized the photolytic precursor , which provided a single isoprene -OH
adduct . Employing laser photolysis /laser induced fluorescence , time -dependent
multiplexed mass spectrometry , velocity map ion imaging , and theoretical techniques ,
we present the full characterization of the reactivity of the single isoprene -OH adduct in
the presence of O2 . |