Kinetics and Mechanism of the Acetylperoxy + HO₂ Reaction

The reaction of HO 2 with CH 3 C(O)O 2 is examined using flash photolysis and FTIR smog chamber techniques. Time-resolved UV spectroscopy is used to follow the transient peroxy species. It yields reasonable concentration versus time profiles for CH 3 C(O)O 2 and HO 2, but indicates anomalously high levels of secondary CH 3 O 2 radicals. Transient IR diode laser absorption confirms the HO 2 decay rates; however, the anticipated reaction model substantially underestimates the observed decay. The model is augmented by assuming that, in analogy with formaldehyde, there exists a reaction between HO 2 and acetaldehyde (the precursor for CH 3 C(O)O 2 ). Consistent with this, the fitted rate for the hypothesized reaction increases with increasing initial acetaldehyde level. Relative rate measurements reveal that chlorine atoms remove more CH 3 CHO relative to CH 3 OH in air as compared to nitrogen diluent. This supports the hypothesis since, in the presence of oxygen, HO 2 is formed and presents an additional acetaldehyde removal pathway. Employing the augmented model, analyses of HO 2 decay traces yield a CH 3 C(O)O 2 + HO 2 rate constant of k 1 = ( ) × 10 -13 e (1350±250)/ T cm 3 s -1 . Reasons are discussed for why the present rate constants are 2−3 times larger than previously reported. FTIR−smog chamber studies reveal the reaction to proceed via two channels to (a) peracetic acid and O 2 and to (b) acetic acid and O 3, with a branching fraction at 295 K that is less than half of the literature value. Time-resolved UV absorption measurements support this smaller fraction; averaged together the two methods give k 1b / k 1 = 0.12 ± 0.04. As part of this work, relative rate techniques are used to measure k (Cl+CH 3 C(O)OH) = (2.5 ± 0.3) × 10 -14 cm 3 s -1 and k (Cl+CH 3 C(O)OOH) = (4.5 ± 1.0) × 10 -15 cm 3 s -1 at 295 K.