Theoretical studies on isoprene ozonolysis under tropospheric conditions. 1. Reaction of substituted carbonyl oxides with water

The reactions between isoprene carbonyl oxides and water have been investigated using density functional theory and large scale ab initio methods. These reactions begin with the formation of a hydrogen-bond complex and may follow two different reaction paths. The main one corresponds to the water addition to carbonyl oxide and leads to the formation of (x-hydroxy hydroperoxides. This process is exothermic by about 38 kcal mol(-1) and, depending on the particular carbonyl oxide, has activation enthalpies in the 9-15 kcal mol(-1) range with respect to the corresponding H-bond complex. For carbonyl oxides having P-hydrogen substitutes in syn configuration with respect to the COO unit, a hydrogen-transfer reaction path leading to the formation of OH radicals is also possible. In this case the branching ratio has been calculated at different temperatures. This ratio is computed to be 13.5% at 298.14 K but rises up to 20.4% at 273 K because of a strong tunneling effect.