Kinetics and Mechanism of the Acetylperoxy + HO₂ Reaction

Abstract

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 ) (3.9 -2.3 +5.0) × 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.

Direct measurements of OH and other product yields from the HO<sub>2</sub> + CH<sub>3</sub>C(O)O<sub>2</sub> reaction

“…This work fitted k (R5) 0.38 ± 0.08 0.12 ± 0.02 0.50 ± 0.08 3.1 ± 0.3 2.1 b 0.37 ± 0.10 0.12 ± 0.04 0.51 ± 0.12 3.2 ± 0.2 2.4 ± 0.4 Groß et al (2014b) 0.23 ± 0.12 0.16 ± 0.08 0.61 ± 0.09 1.44 2.1 ± 0.4 Dillon and Crowley (2008) --0.50 ± 0.20 -1.4 ± 0.5 Jenkin et al (2007) 0.38 ± 0.13 0.12 ± 0.04 0.43 ± 0.10 3.16 ± 0.48 (1.4 c ) Le Crâne et al (2006) -0.20 ± 0.01 < 0.1 -1.50 ± 0.08 Hasson et al (2004) 0.40 ± 0.16 0.20 ± 0.08 0.40 ± 0.16 2.00 ± 0.57 2.2 Tomas et al (2001) -0.20 ± 0.02 --1.51 ± 0.07 Crawford et al (1999) (0.72) c 0.12 ± 0.04 -7.3 (2.6) d 4.4 ± 1.6 Horie and Moortgat (1992) ---2.7 - Moortgat et al (1989) -0.33 ± 0.07 --1.3 ± 0.3 Niki et al (1985) ∼ 0.75 ∼ 0.25 -∼ 3 -IUPAC (Atkinson et al, 2006) Crawford et al (1999) corrected for erroneous absorption cross section for CH 3 C(O)OOH by Orlando et al (2000).…”

“…k (R5a) /k (R5b) has been estimated as 3.2 ± 0.2 across the all experiments presented here, which is in agreement with the IUPAC recommendation and others all the way back to the first investigation of the reaction by Niki et al (1985), which was insensitive to Reaction (R5c). The high measurement of k (R5a) by Crawford et al (1999) was corrected for the CH 3 C(O)OOH absorption cross-section by Orlando et al (2000), calculating k (R5a) /k (R5b) = 2.6, in line with other reported values. The preservation of this ratio in the work presented here helps substantiate a higher rate coefficient for Reaction (R5c), although this does not correlate with the more recent study by Groß et al (2014b), where k (R5a) /k (R5b) = 1.44.…”

“…Previous measurements of k (R5) by Moortgat et al (1989), Crawford et al (1999), Tomas et al (2001) and Le Crâne et al (2006) required measurements of RO 2 by UV absorption spectroscopy. The convoluted UV signal was fit using predetermined absorption cross-sections and a numerical model simulation, which were likely to add uncertainty as no radical recycling channel was considered.…”

“…Hasson et al (2004) observed, using chamber studies and measuring stable products, that certain peroxy radical + hydroperoxy radical reactions such as the title reaction of acetylperoxy, CH 3 C(O)O 2 , can lead to the formation of OH radicals through a third channel (Reaction R5c). Previous studies had assumed radical termination through Reaction (R5a) (α (R5a) = k (R5a) /k (R5) = 0.8) and Reaction (R5b) (α (R5b) = k R5b /k R5 = 0.2) (Lightfoot et al, 1992;Moortgat et al, 1989;Crawford et al, 1999). Orlando and Tyndall (2003) were able to demonstrate that an underestimated IR cross-section for peracetic acid, CH 3 C(O)OOH (Reaction R5a), had led to the assignment of α (R5a) 3 times too high.…”

Direct detection of OH formation in the reactions of HO&lt;sub&gt;2&lt;/sub&gt; with CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt; and other substituted peroxy radicals

“…Recent experimental and theoretical work has, however, suggested that the reactions of HO 2 with some substituted RO 2 , do not exclusively terminate radical chemistry (Thornton et al, 2002). For the reaction (R1) of HO 2 with acetyl peroxyl, CH 3 C(O)O 2 , Hasson and coworkers reported a significant yield of OH radicals (R1c) (Hasson et al, 2004), in addition to the well-established radical terminating peracid (R1a) and O 3 (R1b) products (Niki et al, 1985;Moortgat et al, 1989;Horie and Moortgat, 1992;Crawford et al, 1999;Tomas et al, 2001). …”