Decomposition of β-Hydroxypropoxy Radicals in the OH-Initiated Oxidation of Propene. A Theoretical and Experimental Study

Environmental chamber studies of the OH-initiated oxidation of propene have been carried out in the presence of nitrogen oxides under conditions relevant to the atmosphere. The major products observed at all temperatures studied (220−300 K) are CH₂O and CH₃CHO, indicating that the β-hydroxypropoxy radicals formed in the oxidation process (from reaction of the corresponding β-hydroxypropylperoxy radicals with NO) predominantly undergo unimolecular decomposition. A full theoretical study of the chemistry of the dominant β-hydroxypropylperoxy, β-hydroxypropylperoxynitrite, and β-hydroxypropoxy species has been carried out. On the basis of B3LYP-DFT/6-31G** quantum chemical characterizations, the most stable conformations of the oxy radicals are found to contain intramolecular hydrogen bonds, which provide stabilizations of about 2 kcal/mol. The internal hydrogen bond in the lowest-energy oxy species is found to persist in the transition states for C−C bond rupture, which keeps the barrier to their decomposition down to 7.2 kcal/mol. By use of SSE theory, the internal energy distribution of the nascent HOCH₂CH(O)CH₃ oxy radicals has been determined; it is found that most radicals are born with internal energies well above the calculated barrier for their decomposition. Thus, as determined by master equation analysis, the majority of these oxy radicals (80% at 300 K and 1 atm, 75% at 220 K and 0.2 atm) will decompose promptly before collisional stabilization, yielding CH₂OH and CH₃CHO, while the remainder are thermalized. The rate coefficient of the thermal dissociation of HOCH₂CH(O)CH₃ was also theoretically evaluated; the results at 1 atm in the 220−300 K range can be expressed as k_∞ = 3.5 × 10¹³ exp(−7.91 kcal mol^-1/(RT)) s^-1 and k_1atm = 3.6 × 10¹² exp(−7.05 kcal mol^-1/(RT)) s^-1. Thus, dissociation is also found to be the dominant fate of the thermalized oxy radicals.