Rate constants for the reactions between the simplest Criegee intermediate, CH₂OO, with acetone, the α-diketones biacetyl and acetylpropionyl, and the β-diketones acetylacetone and 3,3-dimethyl-2,4-pentanedione have been measured at 295 K. CH₂OO was produced photochemically in a flow reactor by 355 nm laser flash photolysis of diiodomethane in the presence of excess oxygen. Time-dependent concentrations were measured using broadband transient absorption spectroscopy, and the reaction kinetics was characterized under pseudo-first-order conditions. The bimolecular rate constant for the CH₂OO + acetone reaction is measured to be (4.1 ± 0.4) × 10^-13 cm³ s^-1, consistent with previous measurements. The reactions of CH₂OO with the β-diketones acetylacetone and 3,3-dimethyl-2,5-pentanedione are found to have broadly similar rate constants of (6.6 ± 0.7) × 10^-13 and (3.5 ± 0.8) × 10^-13 cm³ s^-1, respectively; these values may be cautiously considered as upper limits. In contrast, α-diketones react significantly faster, with rate constants of (1.45 ± 0.18) × 10^-11 and (1.29 ± 0.15) × 10^-11 cm³ s^-1 measured for biacetyl and acetylpropionyl. The potential energy surfaces for these 1,3-dipolar cycloaddition reactions are characterized at the M06-2X/aug-cc-pVTZ and CBS-QB3 levels of theory and provide additional support to the observed experimental trends. The reactivity of carbonyl compounds with CH₂OO is also interpreted by application of frontier molecular orbital theory and predicted using Hammett substituent constants. Finally, the results are compared with other kinetic studies of Criegee intermediate reactions with carbonyl compounds and discussed within the context of their atmospheric relevance.

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: k_AcOH(T) = (4.3 ± 1.7) × 10^-15 exp[(1630 ± 120)/T] and k_4H2B(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.

Temperature-dependent rate constants for the reactions of CH2OO with acetone (Ac), biacetyl (BiAc), and acetylacetone (AcAc) have been measured over the range 275–335 K using a flash photolysis, transient absorption spectroscopy technique. The measurements were performed at a total pressure of ∼80 Torr in N2 bath gas, which corresponds to the high-pressure limit for these reactions. All three reactions show linear Arrhenius plots with negative temperature dependences. Rate constants increase in the order Ac < AcAc « BiAc across the temperature range; at 295 K the rate constants are kAc = (4.8 ± 0.4) × 10–13 cm3 s–1, kAcAc = (8.0 ± 0.7) × 10–13 cm3 s–1, and kBiAc = (1.10 ± 0.09) × 10–11 cm3 s–1. Sensitivity to temperature, characterized by the magnitude of the negative activation energy, increases in the order AcAc < BiAc < Ac (Ea/R values of –1830 ± 170 K, –1260 ± 170 K, and –460 ± 180 K, respectively). CBS-QB3 calculations show that the Ac and BiAc reactions proceed via formation of an entrance channel complex followed by 1,3-dipolar cycloaddition to form secondary ozonide products via a submerged transition state. For the BiAc reaction, the rate limiting step appears to be rearrangement of a long-range van der Waals complex into the short-range complex that subsequently leads directly to the cycloaddition transition state with a very low energy barrier. The calculations show that two reaction pathways are competitive for AcAc with nearly identical transition state free energies (ΔG° = +10.1 kcal mol–1 at 298 K) found for cycloaddition at the C=O and at the C=C site of the dominant enolone tautomer. The weak temperature dependence observed is likely due to competition between these pathways.

Criegee intermediates, formed by alkene ozonolysis in the troposphere, can react with volatile organic compounds (VOCs). The temperature-dependent kinetics of the reactions between the Criegee intermediate CH₂OO and three aliphatic aldehydes, RCHO where R = H, CH₃, and C₂H₅ (formaldehyde, acetaldehyde, and propionaldehyde, respectively), have been studied using a laser flash-photolysis transient absorption spectroscopy technique. The experimental measurements are supported by ab initio calculations at various composite levels of theory that characterize stationary points on the reaction potential and free energy surfaces. As with other reactions of CH₂OO with organic carbonyls, the mechanisms involve 1,3-dipolar cycloaddition at the C=O group, over submerged barriers, leading to the formation of 1,2,4-trioxolane secondary ozonides. The bimolecular rate constants of all three reactions decrease with increasing temperature over the range 275-335 K and are characterized by equations of Arrhenius form: k(T) = (7.1 ± 1.5) × 10^-14exp((1160 ± 60)/T), (8.9 ± 1.7) × 10^-15exp((1530 ± 60)/T), and (5.3 ± 1.3) × 10^-14exp((1210 ± 70)/T) cm³ s^-1 for HCHO, CH₃CHO, and C₂H₅CHO, respectively. Based on estimated concentrations of CH₂OO, the reactions with aldehydes are unlikely to play a significant role in the atmosphere.

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