A quantum chemical study of the C-C bond fission pathways of alkoxy radicals formed following OH addition to isoprene

Isoprene is one of the most important non-methane organic compounds in tropospheric chemistry, yet its atmospheric degradation pathways remain incompletely understood. The uncertain fates of alkoxy radicals formed in the first stages of the OH-initiated degradation of isoprene contribute significantly to our ignorance. This paper examines the C-C bond fission pathways of these radicals at the B3LYP/6-311G(2df,2p) level of theory. For the four beta-hydroxyalkoxy radicals that are expected to be formed, C-C bond fission (decomposition) pathways exist with very low barriers (0.7-2.1 kcal/mol) that are likely to dominate the chemistry. These radicals appear to possess intramolecular hydrogen bonds which typically persist in the transition states. For the two delta-hydroxyalkoxy radicals formed, C-C bond fission is endothermic (16-20 kcal/mol) and this pathway is unlikely to be important.