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 (CH 3 C(O)CH 2 O 2 ) and hydro peroxy (HO 2 ) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO 2 and hydroxyl (OH), while UVA measurements monitored CH 3 C(O)CH 2 O 2 . For the CH 3 C(O)CH 2 O 2 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 CH 3 C(O)CH 2 O 2 and HO 2 ( 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 HO 2 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 r H 250K ° = -38.6 ± 3.3 kJ mol -1 and Δ r S 250K ° = -110.5 ± 13.2 J mol -1 K -1 , with the third-law analysis yielding Δ r H 250K ° = -37.5 ± 0.25 kJ mol -1 . The HO 2 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 term k 4,M ″ = (2.7 -1.7 +4.7 ) × 10 -36 exp [(4700 ± 255)/ T ]cm 6 molecule -2 s -1 , proportional to [CH 3 OH], 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 ] cm 6 molecule -2 s -1 , proportional to [CH 3 C(O)CH 3 ], over T = 270-296 K. From these parameters, the rate coefficients for the reactions between HO 2 and the respective complexes over the given temperature ranges can be estimated: for HO 2 ·CH 3 OH, k 12 = [(1.72 ± 0.050) × 10 -11 ] exp [(314 ± 7.2)/T] cm 3 molecule -1 s -1 and for HO 2 ·CH 3 C(O)CH 3 , k 15 = [(7.9 ± 0.72) × 10 -17 ] exp [(3881 ± 25)/ T ] cm 3 molecule -1 s -1 . Lastly, an estimate of the rate coefficient for the HO 2 ·CH 3 OH self-reaction was also determined to be k 13 = (1.3 ± 0.45) × 10 -10 cm 3 molecule -1 s -1 .