Rate Constants for RO2 + HO2 Reactions Measured under a Large Excess of HO2

Rate constants for reactions of various peroxy radicals RO2 with HO2 have been measured at room temperature, using laser-flash photolysis and UV absorption spectrometry. The unique feature of those measurements is that they were performed under a large excess of HO2, so that the only significant reactions were those of HO2 with itself and with RO2. It results that the reaction mechanism was considerably simplified, because those reactions are terminating and the generally complex reaction sequence initiated by the RO2 self-reaction was of negligible importance. In addition, it was not necessary to know the RO2 concentration accurately, only the HO2 concentration. The measured rate constants agree with the literature values, when available. The results confirm the increase of rate constants with the size of RO2 radicals, from nearly 5 × 10-12 cm3 molecule-1 s-1 for CH3O2, up to a limit of about 2.2 × 10-11 cm3 molecule-1 s-1 for C10−C14 radicals. Interestingly, RO2 radicals originating from reactions of OH with terpenes react with rate constants close or equal to this upper limit.