Impact of the water dimer on the atmospheric reactivity of carbonyl oxides

Reactivity of syn-CH3CHOO with H2O enhanced through a roaming mechanism in the entrance channel

Criegee intermediates are highly reactive species that play a pivotal role in the chemistry of the atmosphere, substantially impacting global climate and air quality. They are formed through the reaction of ozone with alkenes and considerably influence the formation of hydroxyl radicals and aerosols through their unimolecular decomposition and their reaction with key atmospheric components, respectively. However, their interaction with water vapour, a major atmospheric component, remains inadequately characterized. Here, using both time-dependent laser-induced fluorescence experiments and full-dimensional dynamics calculations, we investigate the reaction of syn-CH3CHOO, a prevalent Criegee intermediate, with water vapour. Our results reveal a much higher reaction rate than previously estimated, challenging the conventional notion that unimolecular decomposition dominates syn-CH3CHOO removal. Notably, we uncover a complex mechanism involving a roaming process that enhances reactivity. Our findings necessitate a revised assessment of reactions involving syn-mono- and di-substituted Criegee intermediates with water, which are crucial for accurately estimating the OH budget derived from these intermediates.

Reaction Kinetics of CH2OO and syn-CH3CHOO Criegee Intermediates with Acetaldehyde

Criegee intermediates exert a crucial influence on atmospheric chemistry, functioning as powerful oxidants that facilitate the degradation of pollutants, and understanding their reaction kinetics is essential for accurate atmospheric modeling. In this study, the kinetics of CH2OO and syn-CH3CHOO reactions with acetaldehyde (CH3CHO) were investigated using a flash photolysis reaction tube coupled with the OH laser-induced fluorescence (LIF) method. The experimental results indicate that the reaction of syn-CH3CHOO with CH3CHO is independent of pressure in the range of 5-50 Torr when using Ar as the bath gas. However, the rate coefficient for the reaction between CH2OO and CH3CHO at 5.5 Torr was found to be lower compared to the near-constant values observed between 10 and 100 Torr. Furthermore, the reaction of syn-CH3CHOO with CH3CHO demonstrated positive temperature dependence from 283 to 330 K, with a rate coefficient of (2.11 ± 0.45) × 10-13 cm3 molecule-1 s-1 at 298 K. The activation energy and pre-exponential factor derived from the Arrhenius plot for this reaction were determined to be 2.32 ± 0.49 kcal mol-1 and (1.66 ± 0.61) × 10-11 cm3 molecule-1 s-1, respectively. In comparison, the reaction of CH2OO with CH3CHO exhibited negative temperature dependence, with a rate coefficient of (2.16 ± 0.39) × 10-12 cm3 molecule-1 s-1 at 100 Torr and 298 K and an activation energy and a pre-exponential factor of -1.73 ± 0.31 kcal mol-1 and (1.15 ± 0.21) × 10-13 cm3 molecule-1 s-1, respectively, over the temperature range of 280-333 K.

Composition of the Water Dimer and the Heterodimers of Water with N2 and O2 in Earth's Atmosphere

The mole fractions χ and number concentrations n of the water dimer and the heterodimers H2O-N2 and H2O-O2 in Earth's atmosphere are reported up to 20 km. The water dimer data is obtained from published values of the equilibrium constant based on the water equation of state. The mixed equilibrium constants for the heterodimers are obtained from the respective second virial coefficients using an approach introduced by Stogryn and Hirschfelder that extracts the components pertaining to pairwise interactions producing bound and metastable dimers. From these calculations, χ and n for the water dimer and the (H2O)(N2) and (H2O)(O2) heterodimers at standard sea level are 1.79(6) × 10-5, 4.77(12) × 10-5 and 9.90(5) × 10-6 and 4.55(15) × 1014, 1.23(3) × 1016 and 2.56(1) × 1015, respectively. Analytical expressions are provided for these quantities for altitudes between 0-20 km and temperatures from 200-300 K. Sea level values of χ and n are given for two specific locations.

Methanetriol─Formation of an Impossible Molecule

Orthocarboxylic acids─organic molecules carrying three hydroxyl groups at the same carbon atom─have been distinguished as vital reactive intermediates by the atmospheric science and physical (organic) chemistry communities as transients in the atmospheric aerosol cycle. Predicted short lifetimes and their tendency to dehydrate to a carboxylic acid, free orthocarboxylic acids, signify one of the most elusive classes of organic reactive intermediates, with even the simplest representative methanetriol (CH(OH)3)─historically known as orthoformic acid─not previously been detected experimentally. Here, we report the first synthesis of the previously elusive methanetriol molecule in low-temperature mixed methanol (CH3OH) and molecular oxygen (O2) ices subjected to energetic irradiation. Supported by electronic structure calculations, methanetriol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies and the detection of photoionization fragments. The first synthesis and detection of methanetriol (CH(OH)3) reveals its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition. These findings progress our fundamental understanding of the chemistry and chemical bonding of methanetriol, hydroxyperoxymethane (CH3OOOH), and hydroxyperoxymethanol (CH2(OH)OOH), which are all prototype molecules in the oxidation chemistry of the atmosphere.

Reaction Pathways of Water Dimer Following Single Ionization

Water dimer (H2O)2─a vital component of the earth's atmosphere─is an important prototypical hydrogen-bonded system. It provides direct insights into fundamental chemical and biochemical processes, e.g., proton transfer and ionic supramolecular dynamics, occurring in astro- and atmospheric chemistry. Exploiting a purified molecular beam of water dimer and multimass ion imaging, we report the simultaneous detection of all generated ion products of (H2O)2+ fragmentation following single ionization. Detailed information about ion yields and reaction energetics of 13 ion-radical pathways, 6 of which are new, of (H2O)2+ are presented, including strong 18O-isotope effects.

Thermodynamics of the van der Waals Dimers of O2, N2 and the Heterodimer (N2)(O2) and Their Presence in Earth's Atmosphere

The dimerization thermodynamics of N2 and O2, the principal components of Earth's atmosphere, have been determined from the respective second virial coefficients of the bound and metastable dimers calculated using the method of Stogryn and Hirschfelder that utilizes the Lennard-Jones (LJ) potential to account for intermolecular interactions. In addition, the thermodynamic properties of the heterodimer (N2)(O2) have been obtained using the same approach, employing combining rules to construct the LJ potential. Thus, Keq, ΔH, and ΔS for the three dimers are reported between 80-120 K. Over this temperature range, the ranking of Keq is (N2)(O2) > (O2)(O2) > (N2)(N2). The same trend is found for the exoethalpicity of dimer formation. For example, at 100 K, the Keq values are, respectively, 0.0406(14), 0.0215(5), and 0.0181(10), and the corresponding ΔH values are -2401(5), -2344(7), and -2279(1) J/mol. The mole fraction composition of the dimers in the atmosphere was calculated for altitudes up to 20 km. These calculations show that in the troposphere and the lower stratosphere (up to 20 km), the three dimers rank fifth to seventh in abundance, between CO2 and Ne. In this region, the average mole fractions of (N2)(N2), (O2)(O2), and (N2)(O2) are calculated to be 3.4(2) × 10-4, 2.80(9) × 10-5, and 1.95(7) × 10-4, respectively.

Criegee Intermediate-Mediated Oxidation of Dimethyl Disulfide: Effect of Formic Acid and Its Atmospheric Relevance

The oxidation-reduction reactions of disulfides are important in both chemistry and biology. Dimethyl disulfide (DMDS), the smallest reduced sulfur species with a disulfide bond, is emitted in significant quantities from natural sources and contributes to the formation of aerosols and hazardous haze. Although atmospheric removal of DMDS via the reactions with OH or NO3 radicals and photolysis is known, the reactions of DMDS with other atmospheric oxidants are yet to be explored. Herein, using quantum chemical calculations, we explored the reactions of DMDS with CH2OO (formaldehyde oxide) and other methyl-substituted Criegee intermediates. The various reaction pathways evaluated were found to have positive energy barriers. However, in the presence of formic acid, a direct oxygen-transfer pathway leading to the corresponding sulfoxide (CH3SS(O)CH3) was found to proceed through a submerged transition state below the separated reactants. Calculations for the methyl-substituted Criegee intermediates, particularly for anti-CH3CHOO, show a significant increase in the rate of the direct oxygen-transfer reaction when catalyzed by formic acid. The presence of formic acid also alters the mechanism and reduces the enthalpic barrier of a second pathway, forming thioformaldehyde and hydroperoxide without any rate enhancement. Our data indicated that, although Criegee intermediates are unlikely to be an important atmospheric sink of DMDS under normal conditions, in regions rich in DMDS and formic acid, the formic acid-catalyzed Criegee intermediate-mediated oxidation of DMDS via the direct oxygen-transfer pathway could lead to organic sulfur compounds contributing to atmospheric aerosol.

Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect

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) × 10-16 cm3 s-1, k2 = (10.66 ± 0.83) × 10-33 cm6 s-1, k3 = (1.48 ± 0.17) × 10-50 cm9 s-1 at 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]3 term 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 (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was 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)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction.

Substituent Dependence on the Reactions of Criegee Intermediates with Carbon Dioxide and Carbon Monoxide

Criegee intermediates (CIs), R1 R2 COO, are active molecules produced in the atmosphere from the ozonolysis of alkenes. Here, we systematically evaluated the reactivity of ten CIs with carbon monoxide and carbon dioxide using CCSD(T)-F12/cc-pVTZ-F12//B3LYP/6-311+G(2d,2p) energies and transition state theory. Many previous studies focused on alkyl substitution, but here we evaluated both alkyl and vinyl substitution toward the reactivity by studying five anti-type CIs: CH2 OO, anti-CH3 CHOO, anti-cis-C2 H5 CHOO, anti-trans-MACRO, anti-cis-MACRO; and five syn-type CIs: syn-CH3 CHOO, (CH3 )2 COO, syn-trans-C2 H5 CHOO, syn-trans-MVKO, and syn-cis-MVKO. Our study showed that reactions involving CO2 have a large substituent dependence varying nearly five orders of magnitude, while those involving CO have a much smaller two orders of magnitude difference. Analysis based on the strain interaction model showed that deformation of the CI is an important feature in determining the reactivity with CO2 . On the other hand, we used the OO and CO bond ratios to analyze the zwitterionic character of the CIs. We found that vinyl substitution with π-conjugation results in smaller zwitterionic character and lower reactivity with CO. Lastly, the reactivity of CIs with CO as well as CO2 were found to be not fast enough to be important in an atmospheric context.

Unimolecular Reactions of E-Glycolaldehyde Oxide and Its Reactions with One and Two Water Molecules

The kinetics of Criegee intermediates are important for atmospheric modeling. However, the quantitative kinetics of Criegee intermediates are still very limited, especially for those with hydroxy groups. Here, we calculate rate constants for the unimolecular reaction of E-glycolaldehyde oxide [E-hydroxyethanal oxide, E-(CH₂OH)CHOO], for its reactions with H₂O and (H₂O)₂, and for the reaction of the E-(CH₂OH)CHOO…H₂O complex with H₂O. For the highest level of electronic structure, we use W3X-L//CCSD(T)-F12a/cc-pVDZ-F12 for the unimolecular reaction and the reaction with water and W3X-L//DF-CCSD(T)-F12b/jun-cc-pVDZ for the reaction with 2 water molecules. For the dynamics, we use a dual-level strategy that combines conventional transition state theory with the highest level of electronic structure and multistructural canonical variational transition state theory with small-curvature tunneling with a validated density functional for the electronic structure. This dynamical treatment includes high-frequency anharmonicity, torsional anharmonicity, recrossing effects, and tunneling. We find that the unimolecular reaction of E-(CH₂OH)CHOO depends on both temperature and pressure. The calculated results show that E-(CH₂OH)CHOO…H₂O + H₂O is the dominant entrance channel, while previous investigations only considered Criegee intermediates + (H₂O)₂. In addition, we find that the atmospheric lifetime of E-(CH₂OH)CHOO with respect to 2 water molecules is particularly short with a value of 1.71 × 10^-6 s at 0 km, which is about 2 orders of magnitude shorter than those usually assumed for Criegee intermediate reactions with water dimer. We also find that the OH group in E-(CH₂OH)CHOO enhances its reactivity.

Master Equation Studies of the Unimolecular Decay of Thermalized Methacrolein Oxide: The Impact of Atmospheric Conditions

Master equation simulations of the unimolecular reaction dynamics of the Criegee intermediate methacrolein oxide (MACR oxide) have been performed under a variety of temperature and pressure conditions. These simulations provide insight into how the unimolecular kinetics vary across temperatures spanning the range 288-320 K. This work has incorporated a new potential energy surface and includes the anti-to-syn and cis-to-trans conformational dynamics of MACR oxide, as well as the unimolecular reactions to form dioxirane and dioxole species. The competition between the unimolecular reactivity of MACR oxide and previously documented bimolecular reactivity of MACR oxide with water vapor is explored, focusing on how this competition is affected by changes in atmospheric conditions. The impact on the role of MACR oxide as an atmospheric oxidant of SO₂ is noted.

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH₂ OO, CH₃ CHOO, (CH₃ )₂ COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH₂ OO and (CH₃ )₂ COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing ¹ ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O-O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

Mechanistic Insights into Criegee Intermediate-Hydroperoxyl Radical Chemistry

The reaction between a Criegee intermediate and the hydroperoxyl radical (HO₂) is believed to play a role in the formation of new particles in the troposphere. Although the reaction has been previously studied in the gas phase, there are several knowledge gaps that still need to be filled. We simulated the reaction of anti-CH₃CHOO with HO₂ and HO₂-H₂O radical complexes in the gas phase at 0 K, which exhibited a low-barrier profile for water-containing systems and a barrierless profile for water-free systems. Moreover, the reaction was found to follow a proton-transfer mechanism, which challenges previous assumptions that the gas-phase reaction involves a hydrogen atom transfer. The HO₂ radical was found to mediate the Criegee hydration reaction in the gas phase. Metadynamics simulations further confirmed that the expected radical adduct formation between anti-CH₃CHOO and the HO₂ radical, as well as the HO₂- and H₂O-mediated reactions in the gas phase, followed a concerted mechanism. By combining constrained ab initio molecular dynamics simulations with thermodynamic integration, we quantitively evaluated the free-energy barriers at high temperatures. The barriers obtained for all three Criegee-HO₂ reaction systems were found to be temperature-dependent. We also compared the free-energy barriers of water-free and water-containing systems; the results revealed that water could hinder the reaction between the Criegee and HO₂ radical. These results suggest that HO₂ radicals may be involved in the formation of tropospheric radical adducts, and water molecules may also play important roles in the reactions of Criegee intermediates.

Dramatic Conformer-Dependent Reactivity of the Acetaldehyde Oxide Criegee Intermediate with Dimethylamine Via a 1,2-Insertion Mechanism

The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron-multiplexed photoionization mass spectrometry experiments (298 K and 4 Torr) and high-level theory [CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction], we explore the conformer dependence of the reaction of acetaldehyde oxide (CH₃CHOO) with dimethylamine (DMA). The experimental data support the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product. Tunable-vacuum ultraviolet photoionization probing of anti- or anti- + syn-CH₃CHOO reveals a strong conformer dependence of the title reaction. The rate coefficient of DMA with anti-CH₃CHOO is predicted to exceed that for the reaction with syn-CH₃CHOO by a factor of ∼34,000, which is attributed to submerged barrier (syn) versus barrierless (anti) mechanisms for energetically downhill reactions.

Photodissociation dynamics of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (¹D) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first π* ← π transition associated primarily with the carbonyl oxide group at λ > 300 nm results in a prompt dissociation and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (¹D) + methyl vinyl ketone products. By contrast, electronic excitation at λ ≤ 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (¹D) products. One pathway is analogous to that at λ > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.

Criegee Intermediates Beyond Ozonolysis: Synthetic and Mechanistic Insights

After more than 70 years since their discovery, Criegee intermediates (CIs) are back at the forefront of modern chemistry of short-lived reactive intermediates. They play an important role in the mechanistic context of chemical synthesis, total synthesis, pharmaceuticals, and, most importantly, climate-controlling aerosol formation as well as atmospheric chemistry. This Minireview summarizes key aspects of CIs (from the mechanism of formation, for example, by ozonolysis of alkenes and photolysis methods employing diiodo and diazo compounds, to their electronic structures and chemical reactivity), highlights the most recent findings and some landmark results of gas-phase kinetics, and detection/measurements. The recent progress in synthetic and mechanistic studies in the chemistry of CIs provides a guide to illustrate the possibilities for further investigations in this exciting field.

The favorable routes for the hydrolysis of CH2OO with (H2O)n (n = 1-4) investigated by global minimum searching combined with quantum chemical methods

The hydrolysis reaction of CH₂OO with water and water clusters is believed to be a dominant sink for the CH₂OO intermediate in the atmosphere. However, the favorable route for the hydrolysis of CH₂OO with water clusters is still unclear. Here global minimum searching using the Tsinghua Global Minimum program has been introduced to find the most stable geometry of the CH₂OO(H₂O)_n (n = 1-4) complex firstly. Then, based on these stable complexes, favorable hydrolysis of CH₂OO with (H₂O)_n (n = 1-4) has been investigated using the quantum chemical method of CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2d,2p) and canonical variational transition state theory with small curvature tunneling. The calculated results have revealed that, although the contribution of CH₂OO + (H₂O)₂ is the most obvious in the hydrolysis of CH₂OO with (H₂O)_n (n = 1-4), the hydrolysis of CH₂OO with (H₂O)₃ is not negligible in atmospheric gas-phase chemistry as its rate is close to the rate of the CH₂OO + H₂O reaction. The calculated results also show that, in a clean atmosphere, the CH₂OO + (H₂O)_n (n = 1-2) reaction competes well with the CH₂OO + SO₂ reaction at 298 K when the concentrations of (H₂O)_n (n = 1-2) range from 20% relative humidity (RH) to 100% RH, and SO₂ is 2.46 × 10¹¹ molecules per cm³. Meanwhile, when the RH is higher than 40%, it is a new prediction that the CH₂OO + (H₂O)₃ reaction can also compete well with the CH₂OO + SO₂ reaction at 298 K. Besides, Born-Oppenheimer molecular dynamics simulation results show that all the favorable channels of the CH₂OO + (H₂O)_n (n = 1-3) reaction cannot react on a time scale of 100 ps in the NVT simulation. However, the NVE simulation results show that the CH₂OO + (H₂O)₃ reaction can be finished well at 8.5 ps, indicating that the gas phase reaction of CH₂OO + (H₂O)₃ is not negligible in the atmosphere. Overall, the present results have provided a definitive example of how the favorable hydrolysis of important atmospheric species with (H₂O)_n (n = 1-4) takes place, which will stimulate one to consider the favorable hydrolysis of water and water clusters with other Criegee intermediates and other important atmospheric species.

Atmospheric Kinetics: Bimolecular Reactions of Carbonyl Oxide by a Triple-Level Strategy

Criegee intermediates in the atmosphere serve as oxidizing agents to initiate aerosol formation, which are particularly important for atmospheric modeling, and understanding their kinetics is one of the current outstanding challenges in climate change modeling. Because experimental kinetics are still limited, we must rely on theory for the complete picture, but obtaining absolute rates from theory is a formidable task. Here, we report the bimolecular reaction kinetics of carbonyl oxide with ammonia, hydrogen sulfide, formaldehyde, and water dimer by designing a triple-level strategy that combines (i) benchmark results close to the complete-basis limit of coupled-cluster theory with the single, double, triple, and quadruple excitations (CCSDTQ/CBS), (ii) a new hybrid meta density functional (M06CR) specifically optimized for reactions of Criegee intermediates, and (iii) variational transition-state theory with both variable rection coordinates and optimized reaction paths, with multidimensional tunneling, and with pressure effects. For (i) we have found that quadruple excitations are required to obtain quantitative reaction barriers, and we designed new composite methods and strategies to reach CCSDTQ/CBS accuracy. The present findings show that (i) the CH₂OO + HCHO reaction can make an important contribution to the sink of HCHO under wide atmospheric conditions in the gas phase and that (ii) CH₂OO + (H₂O)₂ dominates over the CH₂OO + H₂O reaction below 10 km.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

The oxidation mechanism of gas-phase ozonolysis of limonene in the atmosphere

Limonene with endo- and exo-double bonds is a significant monoterpene in the atmosphere and has high reactivity towards O3. We investigated the atmospheric oxidation mechanism of limonene ozonolysis using a high level quantum chemistry calculation coupled with RRKM-ME kinetic simulation. The additions of O3 can take place at both the endo- and exo-double bonds with a branching ratio of 0.87 : 0.13, forming four major highly energized CIs* (named Syn-2a*, Syn-2b*, Anti-2b* and Anti-2c*) with the relative higher fractions of 0.21 : 0.35 : 0.27 : 0.11. A yield of 4% for Limona-ketone was obtained as well. For the unimolecular isomerization pathways of limonene + O3 → POZs → CIs* → SOZ, VHP, or dioxirane, five, one, or none of the internal rotations are treated as hindered internal rotors for CIs*. We obtained percentages of 0.59 : 0.18 : 0.14 in total for separate isomerization routes in the formation of VHPs, dioxirane and SOZs from CIs* using the fourth-order Runge-Kutta method. Additionally, a yield of ∼5% was acquired for stabilized CIs compiling the fractions of different addition routes. About ∼10% of stabilized Anti-2b would isomerize to VHP and 90% would isomerize to SOZs. Isomerization to VHPs dominates the fate of stabilized Syn-2a, Syn-2b and Anti-2c. The overall yield of OH radicals was 0.61. Our study suggested a yield of 0.17 for stabilized SOZs and 0.18 for dioxirane, although both their fates are ambiguous.

Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions

Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH₂OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO₂ by CIs is a potential source of H₂SO₄, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO₂ has been found, k_SO2 = (1.5 ± 0.4) × 10^-10 cm³ s^-1 (±1σ, σ is the standard deviation of the data) at 298 K (150 - 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10^-17 cm³ s^-1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO₂ would be substantial, even more than that of syn-MVKO.

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

Unimolecular decomposition of acetyl peroxy radical: a potential source of tropospheric ketene

The unimolecular decomposition of acetyl peroxy radicals followed by subsequent nitration is known to lead to the formation of peroxy acetyl nitrate (PAN) in the troposphere. Using high level quantum chemical calculations, we show that the acetyl peroxy radical is a precursor in the formation of tropospheric ketene. The results show that the presence of a single or double water molecule(s) as a catalyst does not influence the decomposition reaction directly to form ketene and hydroperoxy radicals. The electronic excitation of the reactive and product complexes occurs in the wavelength range of ∼1400 nm, suggesting that the complexes undergo photoexcitation in the near IR region. The results ascertain that the dissociation of acetyl peroxy radicals into ketene and hydroperoxy radicals occurs more likely through the excitation route and the corresponding excitation wavelength reveals that the reactions are red-light driven. Three different product complexes, ketene·HO2, ketene·H2O·HO2 and ketene·(H2O)2·HO2, are formed from the reaction. The direct dynamics simulations show that the product complexes are more stable and possess a long lifetime. The calculated temperature dependent equilibrium constant of the product complexes reveals that their atmospheric abundances decrease with increasing altitudes.

Kinetics of Unimolecular Decay of Methyl Vinyl Ketone Oxide, an Isoprene-Derived Criegee Intermediate, under Atmospherically Relevant Conditions

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 k_uni = 70 ± 15 s^-1 (1σ uncertainty) at 298 K with negligible pressure dependence. In addition, k_uni increases from ca. 30 s^-1 at 278 K to ca. 175 s^-1 at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol^-1, k_uni(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.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O₂, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O₂ are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O₂ are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO₂ radical coproducts.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO₂ and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO₃ and identifying organic hydroperoxide formation from reaction with SO₂ and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Probing Criegee intermediate reactions with methanol by FTMW spectroscopy

Methoxymethyl hydroperoxide (HOOCH₂OCH₃) and methoxyethyl hydroperoxide (HOOC(CH₃)HOCH₃) have been characterized as the nascent reaction products from the reaction of methanol with CH₂OO and CH₃CHOO, respectively.

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the earth's atmosphere, generates the four-carbon unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate. The first laboratory synthesis and direct detection of MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MACR-oxide is characterized on its first π* ← π electronic transition using a ground-state depletion method. MACR-oxide exhibits a broad UV-visible spectrum peaked at 380 nm with weak oscillatory structure at long wavelengths ascribed to vibrational resonances. Complementary theory predicts two strong π* ← π transitions arising from extended conjugation across MACR-oxide with overlapping contributions from its four conformers. Electronic promotion to the 1¹ππ* state agrees well with experiment, and results in nonadiabatic coupling and prompt release of O ¹D products observed as anisotropic velocity-map images. This UV-visible detection scheme will enable study of its unimolecular and bimolecular reactions under thermal conditions of relevance to the atmosphere.

Ozonolysis of 3-carene in the atmosphere. Formation mechanism of hydroxyl radical and secondary ozonides

The gas-phase ozonolysis mechanism of 3-carene is investigated using high level quantum chemistry and kinetic calculations. The reaction follows the Criegee mechanism with an initial addition of O₃ to the [double bond splayed left]C[double bond, length as m-dash]C[double bond splayed right] bond, followed by a chain of unimolecular isomerizations, as 3-carene + O₃→ POZs (primary ozonides) → CIs (Criegee intermediates, 4 conformers) → Ps (products). In the course of the reaction, a large excess of energy retained in the POZs* lead to the prompt unimolecular processes in POZs*, CIs*, and Ps*, and only ∼4% of CIs* could be stabilized by collision at 298 K and 760 Torr. From RRKM-ME calculations, the VHPs* could further dissociate to vinoxy-type radical and OH radical, the SOZs* could isomerize to 3-caronic acid, and DIOs* could be stabilized via collision. The fractional yield of OH radical, in the range of 0.56 to 0.59, agrees reasonably well with the previously measured value of 1.06 (with an uncertainty factor of 1.5).

An Extended Computational Study of Criegee Intermediate-Alcohol Reactions

High-level ab initio calculations (DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ) are performed on a range of stabilized Criegee intermediate (sCI)-alcohol reactions, computing reaction coordinate energies, leading to the formation of α-alkoxyalkyl hydroperoxides (AAAHs). These potential energy surfaces are used to model bimolecular reaction kinetics over a range of temperatures. The calculations performed in this work reproduce the complicated temperature-dependent reaction rates of CH₂OO and (CH₃)₂COO with methanol, which have previously been experimentally determined. This methodology is then extended to compute reaction rates of 22 different Criegee intermediates with methanol, including several intermediates derived from isoprene ozonolysis. In some cases, sCI-alcohol reaction rates approach those of sCI-(H₂O)₂. This suggests that in regions with elevated alcohol concentrations, such as urban Brazil, these reactions may generate significant quantities of AAAHs and may begin to compete with sCI reactions with other trace tropospheric pollutants such as SO₂. This work also demonstrates the ability of alcohols to catalyze the 1,4-H transfer unimolecular decomposition of α-methyl substituted sCIs.

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH₂OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH₃, CH═CH₂) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O₂. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm^-1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol^-1) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s^-1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

Effect of unsaturated substituents in the reaction of Criegee intermediates with water vapor

Criegee intermediates (CIs), formed in the reactions of unsaturated hydrocarbons with ozone, are very reactive carbonyl oxides and have recently been suggested as important oxidants in the atmosphere. In this work, we studied the substituent effect on the water monomer and dimer reaction with CIs which include up to three carbon atoms at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. Our calculation showed that for saturated CIs with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor would likely dominate the removal processes of these CIs in the atmosphere. On the other hand, for unsaturated CIs, the reactivity toward water vapor decreases compared to the saturated species allowing them to survive in humid atmospheric environments. We also evaluated the kinetic isotope effect in the reaction between CI and water vapor by performing calculations with deuterated water. We found that tunneling is not important and the kinetic isotope effect mainly comes from the difference in the zero point energy between water and deuterated water.

The reactivity of the Criegee intermediate CH3CHOO with water probed by FTMW spectroscopy

The reaction of Criegee intermediates with water is one of the dominant removal mechanisms for these species in the atmosphere. The reactivity of alkyl substituted Criegee intermediates has been shown to be affected by the nature and location of the substituents. CH₃CHOO, acetaldehyde oxide, can be considered as a prototypical Criegee intermediate with a single alkyl substituent to examine the conformer specific reactivity for Criegee intermediates. Pulsed Fourier-transform microwave spectroscopy has been used to probe the products resulting from the reaction between CH₃CHOO and water. The hydrogen-bonded complex between CH₃CHOO and water together with the reaction product, hydroxyethyl hydroperoxide, were observed in the discharged plasma of a CH₃CHI₂/O₂/water gas mixture. The experimentally determined rotational parameters support the identification of the complex between water and the syn-CH₃CHOO conformer and two conformers of hydroxyethyl hydroperoxide, produced from the anti-CH₃CHOO conformer and water.

The influences of ammonia on aerosol formation in the ozonolysis of styrene: roles of Criegee intermediate reactions

The influences of ammonia (NH₃) on secondary organic aerosol (SOA) formation from ozonolysis of styrene have been investigated using chamber experiments and quantum chemical calculations. With the value of [O₃]₀/[styrene]₀ ratios between 2 and 4, chamber experiments were carried out without NH₃ or under different [NH₃]/[styrene]₀ ratios. The chamber experiments reveal that the addition of NH₃ led to significant decrease of SOA yield. The overall SOA yield decreased with the [NH₃]₀/[styrene]₀ increasing. In addition, the addition of NH₃ at the beginning of the reaction or several hours after the reaction occurs had obviously different influence on the yield of SOA. Gas phase reactions of Criegee intermediates (CIs) with aldehydes and NH₃ were studied in detail by theoretical methods to probe into the mechanisms behind these phenomena. The calculated results showed that 3,5-diphenyl-1,2,4-trioxolane, a secondary ozonide formed through the reactions of C₆H₅ĊHOO· with C₆H₅CHO, could make important contribution to the aerosol composition. The addition of excess NH₃ may compete with aldehydes, decreasing the secondary ozonide yield to some extent and thus affect the SOA formation.

Reaction of Criegee Intermediate with Nitric Acid at the Air-Water Interface

The role of aqueous surfaces in promoting atmospheric chemistry is increasingly being recognized. However, the bimolecular chemistries of Criegee intermediates, which influence the tropospheric budget of OH radicals, organic acids, hydroperoxides, nitrates, sulfates, and particulate material, remain less explored on an aqueous surface. Herein we have employed Born-Oppenheimer molecular dynamics simulations and two-layer ONIOM (QM:MM) in an electronic embedding scheme to study the reaction and the spectroscopic signal of anti-CH₃CHOO with nitric acid (HNO₃) at the air-water interface, which is expected to be an important reaction in polluted urban environments. The results reveal that on the water surface, the HNO₃-mediated hydration of anti-CH₃CHOO is the most dominant pathway, whereas the traditionally believed direct reaction between anti-CH₃CHOO and HNO₃, which results in the formation of nitrooxyethyl hydroperoxide, is only the minor channel. Both reaction pathways follow a stepwise mechanism at the air-water interface and occur on the picosecond time scale. These new reactions are expected to be relevant in the hazy environments of globally polluted urban regions where nitrates and sulfates are abundantly present. During the hazy period, the high relative humidity and the presence of fog droplets may favor the HNO₃-mediated Criegee hydration over the nitrooxyethyl hydroperoxide forming reaction. A similar reaction mechanism with Criegee intermediates could be expected on the water surface for organic acids, which possess HNO₃-like functionalities, and may play a role in improving our knowledge of the organic acid budget in the terrestrial equatorial regions and high northern latitudes. The ONIOM calculations suggest that the N-O stretching bands around 1600-1200 cm^-1 and NO₂ bending band around 750 cm^-1 in nitrooxyethyl hydroperoxide could be used as spectroscopic markers for distinguishing it from hydrooxyethyl hydroperoxide on the water surface.

Unimolecular decay strongly limits the atmospheric impact of Criegee intermediates

Stabilized Criegee intermediates (SCI) are reactive oxygenated species formed in the ozonolysis of hydrocarbons. Their chemistry could influence the oxidative capacity of the atmosphere by affecting the HO_x and NO_x cycles, or by the formation of low-volatility oxygenates enhancing atmospheric aerosols known to have an important impact on climate. The concentration of SCI in the atmosphere has hitherto not been determined reliably, and very little is known about their speciation. Here we show that the concentration of biogenic SCI is strongly limited by their unimolecular decay, based on extensive theory-based structure-activity relationships (SARs) for the reaction rates for decomposition. Reaction with water vapor, H₂O and (H₂O)₂ molecules, is the second most important loss process; SARs are also proposed for these reactions. For SCI derived from the most common biogenic VOCs, we find that unimolecular decay is responsible for just over half of the loss, with reaction with water vapor the main remaining loss process. Reactions with SO₂, NO₂, or acids have negligible impact on the atmospheric SCI concentration. The ambient SCI concentrations are further characterized by analysis of field data with speciated hydrocarbon information, and by implementation of the chemistry in a global chemistry model. The results show a highly complex SCI speciation, with an atmospheric peak SCI concentrations below 1 × 10⁵ molecule cm^-3, and annual average SCI concentrations less than 7 × 10³ molecule cm^-3. We find that SCI have only a negligible impact on the global gas phase H₂SO₄ formation or removal of oxygenates, though some contribution around the equatorial belt, and in select regions, cannot be excluded.

The reaction of Criegee intermediate CH2OO with water dimer: primary products and atmospheric impact

The rapid reaction of the smallest Criegee intermediate, CH₂OO, with water dimers is the dominant removal mechanism for CH₂OO in the Earth's atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10-90%, relative to previous modeling assumptions, which indicates that the reaction CH₂OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.

Reactivity of Criegee Intermediates toward Carbon Dioxide

Recent theoretical work by Kumar and Francisco suggested that the high reactivity of Criegee intermediates (CIs) could be utilized for designing efficient carbon capture technologies. Because the anti-CH₃CHOO + CO₂ reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed anti-CH₃CHOO with its strong UV absorption at 365 nm and measured the rate coefficient to be ≤2 × 10^-17 cm³ molecule^-1 s^-1 at 298 K, which is consistent with our theoretical value of 2.1 × 10^-17 cm³ molecule^-1 s^-1 at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level but inconsistent with their results obtained at the M06-2X/aug-cc-pVTZ level, which tends to underestimate the barrier heights. The experimental result indicates that the reaction of a Criegee intermediate with atmospheric CO₂ (400 ppmv) would be inefficient (k_eff < 0.2 s^-1) and cannot compete with other decay processes of Criegee intermediates like reactions with water vapor (∼10³ s^-1) or thermal decomposition (∼10² s^-1).

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory

The negative temperature dependence of the rate coefficient for CH₂OO + NH₃ reaction was observed using an OH laser-induced fluorescence method.