Acetonyl peroxy and hydro peroxy 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, the rate parameters were determined to be A = (〖1.5〗_(-0.3)^(+0.4))  10-13 and Ea/R = 996  334 K-1 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). The rate parameters for the reaction between CH3C(O)CH2O2 and HO2 were determined to be A = (〖3.4〗_(-1.5)^(+2.5))  10-13 and Ea/R = 547  415 K-1 for the hydroperoxide product channel and A = (6.23 〖6.23〗_(-4.4)^(+15.3))  10-17 and Ea/R = 3100  870 K-1 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 2nd Law thermodynamic parameters for KP,M for the formation of the complex were found to be ∆rH_(250 K)^° = −38.6 ± 3.3 kJ mol-1 and ∆rS_(250 K)^° = −110.5 ± 13.2 J mol-1 K-1, with 3rd Law analysis yielding ∆rH_(250 K)^° = −37.5 ± 0.25 kJ mol-. The HO2 self-reaction rate coefficient was determined to be k_4=(〖3.34〗_(-0.80)^(+1.04) )×〖10〗^(-13) exp⁡[(507±76)/T]〖 cm〗^3 〖 molecule〗^(-1) 〖 s〗^(-1) with the enhancement factor k_(4,M)^''=(〖2.7〗_(-1.7)^(+4.7) )×〖10〗^(-36) exp⁡[(4700 ± 255)/T] 〖 cm〗^6 〖 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.