Isomerization of OH-Isoprene Adducts and Hydroxyalkoxy Isoprene Radicals
journal contributionposted on 20.06.2002 by Theodore S. Dibble
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This paper employs quantum chemical methods to investigate gaps in our understanding of the fates of radical intermediates in the OH-initiated degradation of isoprene. We employ two density functional theory (DFT) approaches: the well-known B3LYP functional and the recently constructed MPW1K functional. The Complete Basis Set method CBS-QB3 is used selectively to verify certain DFT results. The paper focuses on the configuration of the isoprene-OH adducts with the hydroxyl radical bound to carbons 1 or 4 of isoprene and the fate of the δ-hydroxyalkoxy radicals produced from these adducts. The chemically activated isoprene-OH adducts undergo prompt E/Z isomerization in competition with quenching. This reaction allows formation of the δ-hydroxyalkoxy radicals possessing the (Z) configuration, enabling a fast 1,5 H-shift reaction to dominate the fate of these radicals. The (E) isomer of the δ-hydroxyalkoxy radical that cannot undergo a 1,5 H-shift is predicted to react exclusively with O2. The (E) isomer of the δ-hydroxyalkoxy radical appears likely to undergo a 1,5 H-shift reaction, but that conclusion depends more sensitively than the other conclusions on the assumed rate of the O2 reaction. The effect of tunneling, which has been ignored in most previous calculations of the rate constants of 1,5 H-shift reactions, is estimated using an asymmetric Eckart potential.