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Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects
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2023
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Source: The Journal of Physical Chemistry A, 127(37), 7772-7792
[PDF-3.94 MB]
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Journal Title:The Journal of Physical Chemistry A
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Description: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 Ea/R = −996 ± 334 K and the branching fraction to the alkoxy channel, k2b/k2, showed an inverse temperature dependence following the expression, k2b/k2 = (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 Ea/R = −547 ± 415 K for the hydroperoxide product channel and A = (6.23–4.4+15.3) × 10–17 and Ea/R = −3100 ± 870 K for the OH product channel. The branching fraction for the OH channel, k1b /k1, follows the temperature-dependent expression, k1b/k1 = (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 KP,M for the formation of the complex were found to be ΔrH250K° = −38.6 ± 3.3 kJ mol–1 and ΔrS250K° = −110.5 ± 13.2 J mol–1 K–1, with the third-law analysis yielding ΔrH250K° = −37.5 ± 0.25 kJ mol–1. The HO2 self-reaction rate coefficient was determined to be k4 = (3.34–0.80+1.04) × 10–13 exp [(507 ± 76)/T]cm3 molecule–1 s–1 with the enhancement term k4,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 k4,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, k12 = [(1.72 ± 0.050) × 10–11] exp [(314 ± 7.2)/T] cm3 molecule–1 s–1 and for HO2·CH3C(O)CH3, k15 = [(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 k13 = (1.3 ± 0.45) × 10–10 cm3 molecule–1 s–1.
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Source:The Journal of Physical Chemistry A, 127(37), 7772-7792
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ISSN:1089-5639;1520-5215;
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Rights Information:CC BY-NC-ND
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