Determination of Equilibrium Constants for the Reaction between Acetone and HO₂ Using Infrared Kinetic Spectroscopy

Abstract

The reaction between the hydroperoxy radical, HO(2), and acetone may play an important role in acetone removal and the budget of HO(x) radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215-272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO(2). The HO(2) concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO(2)] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO(2)] followed by subsequent decay. These curves are interpreted as a rapid (<100 μs) equilibrium reaction between acetone and the HO(2) radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 × 10(-16) to 7.7 × 10(-18) cm(3) molecule(-1) for T = 215-272 K. Thermodynamic parameters for the reaction determined from a second-law fit of our van't Hoff plot were Δ(r)H°(245) = -35.4 ± 2.0 kJ mol(-1) and Δ(r)S°(245) = -88.2 ± 8.5 J mol(-1) K(-1). Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO(2)-acetone) with subsequent isomerization to a hydroxy-peroxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 μs). Formation of the hydrogen-bonded complex will have a negligible impact on the atmosphere. However, the complex could subsequently isomerize to form the 2-HIPP radical on longer time scales. Further experimental studies are needed to assess the ultimate impact of the reaction of HO(2) and acetone on the atmosphere.

The atmospheric oxidation of hydroxyacetone: Chemistry of activated and stabilized CH3C(O)CH(OH)OO• radicals between 252 and 298 K

“…Numerous experimental and theoretical studies of reversible addition of HO 2 to various carbonyl‐containing compounds have been conducted . Perusal of the available data shows that our derived values for K 3 , k 3 , and k −3 are reasonable (ie, of similar magnitude to those previously reported).…”

“…The mechanism described here, involving an alcohol (RCH 2 OH), the corresponding carbonyl compound (RCH=O), and the interactions of these species with OH and/or HO 2 , is not unique to the HYAC/MGLY pair, but in fact is generally applicable: Numerous experimental and theoretical studies of reversible addition of HO 2 to various carbonyl-containing compounds have been conducted. 28,33,40,47,52,53 Perusal of the available data shows that our derived values for K 3 , k 3 , and k −3 are reasonable (ie, of similar magnitude to those previously reported). Our derived equilibrium constants for the HO 2 /MGLY system fall intermediate between those previously reported for HCHO and CH 3 CHO, 28,37-39 and thus represent one of the stronger HO 2 /carbonyl interactions identified to date.…”

“…Results are compared with those obtained previously and are used to establish the behavior of HYAC under tropospheric conditions. The results are also discussed in the more general context of the behavior of radicals of the type R‐C•HOH in atmospheric chemistry, as these radicals are formed in the atmospheric oxidation of alcohols and unsaturated VOCs (including aromatics and biogenics), as well as in the interactions of HO 2 with carbonyl compounds …”

“…The results are also discussed in the more general context of the behavior of radicals of the type R-C•HOH in atmospheric chemistry, as these radicals are formed in the atmospheric oxidation of alcohols 20,32 and unsaturated VOCs (including aromatics and biogenics), 33,34 as well as in the interactions of HO 2 with carbonyl compounds. 30,[33][34][35][36][37][38][39][40]…”

Direct Measurement of the Equilibrium Constants of the Reaction of Formaldehyde and Acetaldehyde with HO2 Radicals

“…From such experiments, i.e., the observation of the overall loss of HO 2 radicals through establishing the equilibrium (R1), it is not possible to distinguish between (R1a) and (R1b). In a recent work on the possible corresponding equilibrium reaction between acetone and HO 2 radicals , experimental evidence has been found that only the corresponding complex formation (R1a) takes place and that the subsequent rearrangement (R1b) to formation of the peroxy radical is too slow. In the case of CH 2 O, however, the fast establishment of the equilibrium (R1b) is supported from earlier experiments by end‐product studies , ESR detection of the HOCH 2 O 2 radicals as well as UV‐absorption studies .…”