Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects

Abstract

The temperature-dependent kinetic parameters, branching fractions, and chaperone effects of the self- and cross-reactions between acetonyl peroxy (CH3C(O)CH2O2) and hydro peroxy (HO2) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO2 and hydroxyl (OH), while UVA measurements monitored CH3C(O)CH2O2. For the CH3C(O)CH2O2 self-reaction (T = 270–330 K), the rate parameters were determined to be A = (1.5–0.3 +0.4) × 10–13 and E a/R = −996 ± 334 K and the branching fraction to the alkoxy channel, k 2b/k 2, showed an inverse temperature dependence following the expression, k 2b/k 2 = (2.27 ± 0.62) – [(6.35 ± 2.06) × 10–3] T(K). For the reaction between CH3C(O)CH2O2 and HO2 (T = 270–330 K), the rate parameters were determined to be A = (3.4–1.5 +2.5) × 10–13 and E a/R = −547 ± 415 K for the hydroperoxide product channel and A = (6.23–4.4 +15.3) × 10–17 and E a/R = −3100 ± 870 K for the OH product channel. The branching fraction for the OH channel, k 1b /k 1, follows the temperature-dependent expression, k 1b/k 1 = (3.27 ± 0.51) – [(9.6 ± 1.7) × 10–3] T(K). Determination of these parameters required an extensive reaction kinetics model which included a re-evaluation of the temperature dependence of the HO2 self-reaction chaperone enhancement parameters due to the methanol–hydroperoxy complex. The second-law thermodynamic parameters for K P,M for the formation of the complex were found to be Δr H 250K ° = −38.6 ± 3.3 kJ mol–1 and Δr S 250K ° = −110.5 ± 13.2 J mol–1 K–1, with the third-law analysis yielding Δr H 250K ° = −37.5 ± 0.25 kJ mol–1. The HO2 self-reaction rate coefficient was determined to be k 4 = (3.34–0.80 +1.04) × 10–13 exp [(507 ± 76)/T]cm3 molecule–1 s–1 with the enhancement term k 4,M ″ = (2.7–1.7 +4.7) × 10–36 exp [(4700 ± 255)/T]cm6 molecule–2 s–1, proportional to [CH3OH], over T = 220–280 K. The equivalent chaperone enhancement parameter for the acetone-hydroperoxy complex was also required and determined to be k 4,A ″ = (5.0 × 10–38 – 1.4 × 10–41) exp[(7396 ± 1172)/T] cm6 molecule–2 s–1, proportional to [CH3C(O)CH3], over T = 270–296 K. From these parameters, the rate coefficients for the reactions between HO2 and the respective complexes over the given temperature ranges can be estimated: for HO2·CH3OH, k 12 = [(1.72 ± 0.050) × 10–11] exp [(314 ± 7.2)/T] cm3 molecule–1 s–1 and for HO2·CH3C(O)CH3, k 15 = [(7.9 ± 0.72) × 10–17] exp [(3881 ± 25)/T] cm3 molecule–1 s–1. Lastly, an estimate of the rate coefficient for the HO2·CH3OH self-reaction was also determined to be k 13 = (1.3 ± 0.45) × 10–10 cm3 molecule–1 s–1.

Lifetimes of pre-reactive complexes of peroxy radicals revisited: thermostat effects, temperature dependence and highly oxygenated molecules

“…8–16 In particular, we have been attempting to extend our studies to include the dynamics of the reactions. 13,14 In many cases, experimental results 5,17–21 show a decrease in overall reactivity as temperature is increased, consistent with a negative activation energy E a in the framework of an Arrhenius relation, k = A e − E a / RT .…”

“…In one case (AceOO), there is a very recent experimental result which we can directly compare with. 18 Some other older experimental estimates, albeit with high uncertainties, are also available in the review by Orlando and Tyndall 5 and/or the website of the IUPAC Task Group on Atmospheric Chemical Data Evaluation, 21 or in other publications. We show comparisons between our results for the activation energy E a / R determined from the Arrhenius equation fits and the available experimental data in Table 4.…”