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

Rate constants for reactions of various peroxy radicals RO 2 with HO 2 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 HO 2, so that the only significant reactions were those of HO 2 with itself and with RO 2 . It results that the reaction mechanism was considerably simplified, because those reactions are terminating and the generally complex reaction sequence initiated by the RO 2 self-reaction was of negligible importance. In addition, it was not necessary to know the RO 2 concentration accurately, only the HO 2 concentration. The measured rate constants agree with the literature values, when available. The results confirm the increase of rate constants with the size of RO 2 radicals, from nearly 5 × 10 -12 cm 3 molecule -1 s -1 for CH 3 O 2, up to a limit of about 2.2 × 10 -11 cm 3 molecule -1 s -1 for C 10 −C 14 radicals. Interestingly, RO 2 radicals originating from reactions of OH with terpenes react with rate constants close or equal to this upper limit.