Ab initio studies of parts of the potential surface for the system C3H6 + HO2

The addition reactions between HO₂ and propene leading to the radical intermediates CH₃CHCH₂OOH and CH₃CHOHCH₂ have been studied by ab initio molecular orbital calculations using a 6-31G * basis set and including electron correlation through fourth-order Møller–Plesset calculations. The intermediates are predicted to have energies of about 5 kcal/mol below the total reactant energies, the complex resulting from the HO₂ attack on the central carbon of propene being slightly preferred. The activation energies for the addition to the terminal carbon and the central carbon are predicted to be 8.5 and 8.0 kcal/mol, respectively, at the highest level of calculation MP 4(SDTQ )] with corrections for spin contamination. Spin contamination corrections are found to be very important in the calculation of these values. Referring to previous calculations at the same level for the addition of HO₂ to ethylene 12], we assume that the addition step is the rate-determining one in the reaction leading to HO and propene oxide. The observed activation energy for this reaction, 14.2 kcal/mol 2], is significantly higher than the predicted one for the addition step. The discrepancy found, 6.2 kcal/mol, is virtually the same as the one encountered in the ethylene case, 6.6 kcal/mol 12]. The barrier to intramolecular hydrogen migration leading to the intermediate radical CH₂CH₂CH₂OOH is found to be 42.6 kcal/mol at the highest level of calculation. Spin contaminiation corrections are not important for this energy.