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).