Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH3C(O)CH2O2) self-reaction and the reaction with hydro peroxy (HO2) at a temperature and pressure of 298 K and 100 Torr, respectively. Near-IR and mid-IR lasers simultaneously monitored HO2 and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH3C(O)CH2O2 concentrations. The overall rate constant for the reaction between CH3C(O)CH2O2 and HO2 was found to be (5.5 ± 0.5) × 10-12 cm3 molecule-1 s-1 and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH3C(O)CH2O2 self-reaction rate constant was measured to be (4.8 ± 0.8) × 10-12 cm3 molecule-1 s-1 and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO2 self-reaction was also observed as a function of acetone (CH3C(O)CH3) concentration which suggests a chaperone effect resulting from hydrogen-bond complexation between HO2 and CH3C(O)CH3. The chaperone enhancement factor for CH3C(O)CH3 was determined to be kA = (4.0 ± 0.2)  10-29 cm6 molecule¬-2 s-1 and the equilibrium constant for HO2• CH3C(O)CH3 complex formation was found to be Kc(R15) = (2.0 ± 0.89) × 10-18 cm3 molecule-1; from these values the rate constant for the HO2 + HO2• CH3C(O)CH3 reaction was estimated to be (2 ± 1) × 10-11 cm3 molecule-1 s-1. Results from UV cross-section measurements of CH3C(O)CH2O2 and prompt OH radical yields arising from possible oxidation of the CH3C(O)CH3-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and this remains unexplained.