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

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/k₂, showed an inverse temperature dependence following the expression, k_2b/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 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 /k₁, follows the temperature-dependent expression, k_1b/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 K_P,M for the formation of the complex were found to be Δ_rH_250K^° = −38.6 ± 3.3 kJ mol^–1 and Δ_rS_250K^° = −110.5 ± 13.2 J mol^–1 K^–1, with the third-law analysis yielding Δ_rH_250K^° = −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 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, 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.