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The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt= −kobs[CH2OO]; kobs= k0+ k1[water] + k2[water]2+ k3[water]3; k1= (4.22 ± 0.48) × 10–16cm3s–1, k2= (10.66 ± 0.83) × 10–33cm6s–1, k3= (1.48 ± 0.17) × 10–50cm9s–1at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1= 1.8 ± 1.1 kcal mol–1, Ea2= −11.1 ± 2.1 kcal mol–1, Ea3= −17.4 ± 3.9 kcal mol–1. The contribution of the k3[water]3term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2buffer gas) with those under 600 Torr, a weak pressure effect of k3was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2and CH2OO(H2O)2+ H2O pathways will dominate in the three water reaction.

This work investigated the reaction kinetics of HNO3with four Criegee intermediates (CIs): CH2OO, (CH3)2COO, methyl vinyl ketone oxide (MVKO), and methacrolein oxide (MACRO). Our results show that these reactions are extremely fast with rate coefficients of (1.51 ± 0.45) × 10–10, (3.54 ± 1.06) × 10–10, (3.93 ± 1.18) × 10–10, and (3.0 ± 1.0) × 10–10cm3s–1for reactions of HNO3with CH2OO, (CH3)2COO, syn-MVKO, and anti-MACRO, respectively. This is consistent with previous results for the reactions between CIs and carboxylic acids, but the rate coefficient of CH2OO + HNO3in the literature [ForemanAngew. Chem.2016, 128, 10575] was found to be overestimated by a factor of 3.6. In addition, we did not observe any significant pressure dependence in the HNO3reactions with CH2OO and (CH3)2COO under 100–400 Torr. Our results indicate that in a dry area with severe NOxpollution, the reactions of CIs with HNO3and their products may be worthy of attention, but these reactions may be insignificant under high-humidity conditions. However, CI reactions with HNO3may not play an important role in the atmospheric removal processes of HNO3because of the low concentrations of CIs.

Via intramolecular H atom transfer, 3-aminopropanol is more reactive toward Criegee intermediates, in comparison with amines or alcohols. Here we accessed the substituent effect of Criegee intermediates in their reactions with 3-aminopropanol. Through real-time monitoring the concentrations of two Criegee intermediates with their strong UV absorption at 340 nm, the experimental rate coefficients at 298 K (100–300 Torr) were determined to be (1.52 ± 0.08) × 10–11and (1.44 ± 0.22) × 10–13cm3s–1for the reactions of 3-aminopropanol with (CH3)2COO (acetone oxide) and CH2CHC(CH3)OO (methyl vinyl ketone oxide), respectively. Compared to our previous experimental value for the reaction with syn-CH3CHOO, (1.24 ± 0.13) × 10–11cm3s–1, we can see that the methyl substitution at the antiposition has little effect on the reactivity while the vinyl substitution causes a drastic decrease in the reactivity. Our theoretical calculations based on CCSD(T)-F12 energies reproduce this 2-order-of-magnitude decrease in the rate coefficient caused by the vinyl substitution. Using the activation strain model, we found that the interaction of Criegee intermediates with 3-aminopropanol is weaker for the case of vinyl substitution. This effect can be further rationalized by the delocalization of the lowest unoccupied molecular orbital for the vinyl-substituted Criegee intermediates. These results would help us better estimate the impact of similar reactions like the reactions of Criegee intermediates with water vapor, some of which could be difficult to measure experimentally but can be important in the atmosphere. We also found that the B2PLYP-D3BJ/aug-cc-pVTZ calculation can reproduce the CCSD(T)-F12 reaction barrier energies within ca. 1 kcal mol–1, indicating that the use of the B2PLYP-D3BJ method is promising for future predictions of the reactions of larger Criegee intermediates.

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO2, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278–319 K and 100–503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn-MVKO was measured to be kuni= 70 ± 15 s–1(1σ uncertainty) at 298 K with negligible pressure dependence. In addition, kuniincreases from ca. 30 s–1at 278 K to ca. 175 s–1at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol–1, kuni(T) = (9.3 × 107)exp(−4200/T) s–1. Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere. The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.

Criegee intermediates have substantial Zwitterionic character and interact strongly with hydrogen-bonding molecules like H2O, NH3, CH3OH, etc. Some of the observed reactions between Criegee intermediates and hydrogen-bonding molecules exhibit third-order kinetics. The experimental data indicate that these termolecular reactions involve one Criegee intermediate and two hydrogen-bonding molecules; quantum chemistry calculation shows that one of the hydrogen-bonding molecules acts as a catalytic bridge, which receives a hydrogen atom and donates another one. In this Feature Article, we will discuss the roles of the hydrogen-bonding molecules and the trend of the reactivity for the title reactions. To better predict the competition between a catalyzed reaction (a termolecular process) and its bare reaction (a bimolecular process), we analyzed the free energy landscape of the competing reaction paths under pseudo-first-order conditions. The results indicate that the entropy reduction in the translational degrees of freedom is the main cause to hinder a catalyzed termolecular process under typical experimental concentrations at near ambient temperatures. For such a termolecular process to be significant, its energy gain (barrier lowering) by adding the catalytic molecule has to be large enough to compensate the corresponding entropy cost. One great advantage of this analysis is that the translational entropy only depends on simple parameters like temperature, reactant masses, and concentrations and thus can be easily estimated.

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17)   ×  10–12cm3s–1. The results exhibit a weak negative temperature dependence: the activation energy Ea(k= Ae–Ea/RT) is −1.83 ± 0.05 kcal mol–1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6  ×  10–12cm3s–1at 298 K; Ea= – 2.80 kcal mol–1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH2OO or CH3SH.

We report a very significant cooperative effect of water–ammonia hydrogen bonding in their reactions with a Criegee intermediate, syn-CH3CHOO. Under near ambient conditions, we found that the reaction of syn-CH3CHOO with NH3becomes much faster (by up to 138 times) at high humidity. Intriguingly, merely adding NH3(or H2O) alone has almost no effect on the rate of syn-CH3CHOO decay. Quantum chemistry calculation shows that the main reaction involves a hydrogen-atom (or proton) relay in a hydrogen-bonded ring structure; on the product side, a C–N bond is formed and H2O is regenerated as a catalyst. This result demonstrates a new category of reaction in which having two types of hydrogen-bond players (NH3and H2O) is much more effective than having only one.

The ν4fundamental of the simplest Criegee intermediate, CH2OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4–94 Torr. This IR spectrum provides an unambiguous identification of CH2OO and is potentially useful to determine the number density of CH2OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH2OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm–1(covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH2OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10–19cm·molecule–1or 2.2 ± 0.4 km·mol–1. The whole rotational band (P, Q, and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν4band) to be 19.2 ± 3.5 km·mol–1. For a practical detection of CH2OO, this work also reports the peak cross section as a function of total pressure (4–94 Torr O2). At low pressure (≤4 Torr), where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2 ± 0.9) × 10–18cm2·molecule–1(at the system line width of 0.004 cm–1fwhm).