Isomerization of OH-Isoprene Adducts and Hydroxyalkoxy Isoprene Radicals

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.