Photo-Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2-butanone with Cl Atoms and OH Radicals in the Gas Phase

Mechanisms of isomerization and hydration reactions of typical β-diketone at the air-droplet interface

Acetylacetone (AcAc) is a typical class of β-diketones with broad industrial applications due to the property of the keto-enol isomers, but its isomerization and chemical reactions at the air-droplet interface are still unclear. Hence, using combined molecular dynamics and quantum chemistry methods, the heterogeneous chemistry of AcAc at the air-droplet interface was investigated, including the attraction of AcAc isomers by the droplets, the distribution of isomers at the air-droplet interface, and the hydration reactions of isomers at the air-droplet interface. The results reveal that the preferential orientation of two AcAc isomers (keto- and enol-AcAc) to accumulate and accommodate at the acidic air-droplet interface. The isomerization of two AcAc isomers at the acidic air-droplet interface is more favorable than that at the neutral air-droplet interface because the "water bridge" structure is destroyed by H3O+, especially for the isomerization from keto-AcAc to enol-AcAc. At the acidic air-droplet interface, the carbonyl or hydroxyl O-atoms of two AcAc isomers display an energetical preference to hydration. Keto-diol is the dominant products to accumulate at the air-droplet interface, and excessive keto-diol can enter the droplet interior to engage in the oligomerization. The photooxidation reaction of AcAc will increase the acidity of the air-droplet interface, which indirectly facilitate the uptake and formation of more keto-diol. Our results provide an insight into the heterogeneous chemistry of β-diketones and their influence on the environment.

Temperature-Dependent Kinetics of the Reactions of the Criegee Intermediate CH2OO with Hydroxyketones

Though there is a growing body of literature on the kinetics of CIs with simple carbonyls, CI reactions with functionalized carbonyls such as hydroxyketones remain unexplored. In this work, the temperature-dependent kinetics of the reactions of CH2OO with two hydroxyketones, hydroxyacetone (AcOH) and 4-hydroxy-2-butanone (4H2B), have been studied using a laser flash photolysis transient absorption spectroscopy technique and complementary quantum chemistry calculations. Bimolecular rate constants were determined from CH2OO loss rates observed under pseudo-first-order conditions across the temperature range 275-335 K. Arrhenius plots were linear and yielded T-dependent bimolecular rate constants: kAcOH(T) = (4.3 ± 1.7) × 10-15 exp[(1630 ± 120)/T] and k4H2B(T) = (3.5 ± 2.6) × 10-15 exp[(1700 ± 200)/T]. Both reactions show negative temperature dependences and overall very similar rate constants. Stationary points on the reaction energy surfaces were characterized using the composite CBS-QB3 method. Transition states were identified for both 1,3-dipolar cycloaddition reactions across the carbonyl and 1,2-insertion/addition at the hydroxyl group. The free-energy barriers for the latter reaction pathways are higher by ∼4-5 kcal mol-1, and their contributions are presumed to be negligible for both AcOH and 4H2B. The cycloaddition reactions are highly exothermic and form cyclic secondary ozonides that are the typical primary products of Criegee intermediate reactions with carbonyl compounds. The reactivity of the hydroxyketones toward CH2OO appears to be similar to that of acetaldehyde, which can be rationalized by consideration of the energies of the frontier molecular orbitals involved in the cycloaddition. The CH2OO + hydroxyketone reactions are likely too slow to be of significance in the atmosphere, except at very low temperatures.

Kinetic and Mechanistic Investigations of OH-Initiated Atmospheric Degradation of Methyl Butyl Ketone

Methyl butyl ketone (MBK, 2-hexanone) is a common atmospheric oxygenated volatile organic compound (OVOC) owing to broad industrial applications, but its atmospheric oxidation mechanism remains poorly understood. Herein, the detailed mechanisms and kinetic properties of MBK oxidation initiated by OH radicals and subsequent transformation of the resulting intermediates are performed by employing quantum chemical and kinetic modeling methods. The calculations show that H-abstraction at the C4 position of MBK is more favorable than those at the other positions, with the total rate coefficient of k(T) = 4.13 × 10^-14 exp(1576/T) cm³ molecule^-1 s^-1 at 273-400 K. The dominant pathway of unimolecular degradation of the C-centered alkyl radical is 1,2-acyl group migration. For the isomerization of the peroxy radical RO₂, 1,5- and 1,6-H shifts are more favorable than 1,3- and 1,4-H shifts. The multiconformer rate coefficient k_MC-TST of the first H-shift of the RO₂ radical is estimated to be 1.40 × 10^-3 s^-1 at room temperature. Compared to the H-shifts of analogous aliphatic RO₂ radicals, it can be concluded that the carbonyl group enhances the H-shift rates by as much as 2-4 orders of magnitude. The rate coefficients of the RO₂ radical reaction with the HO₂ radical exhibit a weakly negative temperature dependence, and the pseudo-first-order rate constant k'_HO2 = k_HO2[HO₂] is calculated to be 3.32-22.10 × 10^-3 s^-1 at ambient temperature. The bimolecular reaction of the RO₂ radical with NO leads to the formation of 3-oxo-butanal as the main product with the formation concentration of 2.2-7.4 μg/m³ in urban areas. The predicted pseudo-first-order rate constant k'_NO = k_NO[NO] is 2.20-9.98 s^-1 at room temperature. By comparing the k_MC-TST, k'_HO2, and k'_NO, it can be concluded that reaction with NO is the dominant removal pathway for the RO₂ radical formed from the OH-initiated oxidation of MBK. These findings are expected to deepen our understanding of the photochemical oxidation of ketones under realistic atmospheric conditions.