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

Insights into the catalytic effect of atmospheric organic trace species on the hydration of Criegee intermediates

The bimolecular reactions between Criegee intermediates (CIs) and atmospheric trace species have been extensively investigated, with a particular focus on the reaction with water, while the catalytic role of atmospheric organic compounds in hydration reactions was often neglected. In this study, we employed quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations to investigate the catalytic effects of atmospheric organic amines, organic acids, and alcohols on the hydration reactions of CIs in the gas phase and at the gas-liquid interface. The catalytic reactions were found to follow a cyclic catalytic structure and a stepwise reaction mechanism. Gas-phase studies revealed that organic acids exhibited stronger catalytic effects compared to amines and alcohols, and the catalytic efficiency of amines and alcohols was similar to those of single water molecule. In addition, the catalytic reaction barriers of organic acids and alcohols were positively correlated with their gas-phase acidity (R2 = 0.94 to 0.97). A negative correlation was observed between the catalytic reaction barrier of amines and their gas-phase basicity (R2 = 0.84 to 0.90) and proton affinity (R2 = 0.84 to 0.92). At the gas-liquid interface, organic acids promoted the formation of hydroxyethyl hydroperoxide (HEHP, CH3CH(OH)(OOH)), organic acid ions, and H3O+, whereas the catalytic hydration of CIs by organic amines resulted in the formation of CH3CH(OH)OO and amine ions. Both HEHP and CH3CH(OH)OO can be further decomposed to form OH and HO2, or participate in new particles formation as precursors. This study complements the research gap on the reaction of CIs with water, providing valuable insights into the atmospheric sources of HEHP and HOx as well as the formation of secondary organic aerosols (SOAs).

Effect of Carbon Chain Length on Nascent Yields of Stabilized Criegee Intermediates in Ozonolysis of a Series of Terminal Alkenes

The yields of stabilized Criegee intermediates (sCIs), CH2OO and RCHOO (C2H5CHOO, C3H7CHOO, C4H9CHOO, and C5H11CHOO), produced from ozonolysis of asymmetrical 1-alkenes (1-butene, 1-pentene, 1-hexene, and 1-heptene) were investigated at low pressures (5-16 Torr) using cavity ring-down spectroscopy and chemical titration with sulfur dioxide (SO2). By extrapolating the low-pressure measurements to zero-pressure limit, nascent sCI yields were obtained. Combined with our previous studies on ethene and propene ozonolysis, the nascent sCI yields demonstrated an intriguing trend of increasing with the addition of CH2 groups and eventually reached a plateau at around 31% for longer chain 1-alkenes. In particular, the fraction of nascent stabilized CH2OO reached the plateau from 1-butene, indicating that CH2OO was produced with nearly the same internal energy distribution from 1-butene to 1-heptene. The comparison between the experiments and RRKM calculations suggests that the dissociation of primary ozonide (POZ) of O3 + ethene and propene can be treated by statistical theory, while that of O3 + 1-butene to 1-heptene is nonstatistical and intramolecular vibrational redistribution of the initial energy on the 1,2,3-trioxolane of POZ throughout the entire molecule was incomplete on the dissociation time scale.

The role of solar photolysis in the atmospheric removal of methacrolein oxide and the methacrolein oxide-water van-der Waals complex in pristine environments

Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by-products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation. In this manuscript, we argue that the extended conjugation of these Criegee intermediates enables electronic excitation of these compounds, on timescales that are competitive with their slow unimolecular decay and bimolecular chemistry. We show that the complexation of methacrolein oxide with water enhances the absorption cross section of the otherwise dark S1 state, potentially revealing a new avenue for forming lower volatility compounds via tropospherically relevant photochemistry.

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.

Experimental and theoretical study of Criegee intermediate (CH2OO) reactions with n-butyraldehyde and isobutyraldehyde: kinetics, implications and atmospheric fate

The reactions of the simplest Criegee intermediate (CH2OO) with n-butyraldehyde (nBD) and isobutyraldehyde (iBD) were studied at 253-318 K and (50 ± 2) torr, using Cavity Ring-down spectroscopy (CRDS). The rate coefficients obtained at room temperature were (2.63 ± 0.14) × 10-12 and (2.20 ± 0.21) × 10-12 cm3 molecule-1 s-1 for nBD and iBD, respectively. Both the reactions show negative temperature-dependency, following equations, knBD(T = 253-318 K) = (11.51 ± 4.33) × 10-14 × exp{(918.1 ± 107.2)/T} and kiBD(T = 253-318 K) = (6.23 ± 2.29) × 10-14 × exp{(1051.4 ± 105.2)/T} cm3 molecule-1 s-1. High-pressure limit rate coefficients were determined from theoretical calculations at the CCSD(T)-F12/cc-pVTZ-F12//B3LYP/6-311+G(2df, 2p) level of theory, with <40% deviation from the experimental results at room temperature and above. The kinetic simulations were performed using a master equation solver to predict the temperature-dependency of the rate coefficients at the experimental pressure, as well as to predict the contribution of individual pathways. The major products predicted from the theoretical calculations were formaldehyde and formic acid, along with butyric acid from nBD and isobutyric acid from iBD reactions.

Significant influence of water molecules on the SO3 + HCl reaction in the gas phase and at the air-water interface

The products resulting from the reactions between atmospheric acids and SO3 have a catalytic effect on the formation of new particles in aerosols. However, the SO3 + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol-1. H2O and (H2O)2 play obvious catalytic roles in reducing the energy barrier of the SO3 + HCl reaction by over 18.2 kcal mol-1. The atmospheric lifetimes of SO3 show that the (H2O)2-assisted reaction dominates over the H2O-assisted reaction within the altitude range of 0-5 km, whereas the H2O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H2O-induced formation of the ClSO3-⋯H3O+ ion pair and HCl-assisted formation of the HSO4-⋯H3O+ ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSO3H in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO3- and H3O+ can attract H2SO4, NH3, and HNO3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO3 + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO3 and inorganic acids.

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.

Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants

Hydrofluoroolefins (HFO) are fourth-generation refrigerants designed to function as efficient refrigerants with no ozone depletion potential and zero global warming potential. Despite extensive studies on their chemical and physical properties, the ground- and excited-state chemistry of their atmospheric oxidation products is less well understood. This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), which is the simplest fluorinated CI formed from the ozonolysis of HFOs. HFCOO contains syn- and anti-conformers, which have Boltzmann populations of, respectively, 87 and 13% at 298 K. For both conformers, the calculated ground-state reaction energy profiles associated with cyclization to form fluorodioxirane is lower than the equivalent unimolecular decay path in the simplest CI, H2COO, with anti-HFCOO returning a barrier height more than half of that of H2COO. The excited-state dynamics reveal that photoexcitation to the bright S2 state of syn-HFCOO and anti-HFCOO is expected to undergo a prompt O-O fission─with the former conformer expected to dissociate with an almost unity quantum yield and to form both O (1D) + HFCO (S0) and O (3P) + HFCO (T1) products. In contrast, photoexcitation of anti-HFCOO is expected to undergo an O-O bond fission with a non-unity quantum yield. The fraction of photoexcited anti-HFCOO that dissociates is predicted to exclusively form O (1D) + HFCO (S0) products, which is in sharp contrast to H2COO. The wider implications of our results are discussed from both physical and atmospheric chemistry perspectives.

Direct gas-phase formation of formic acid through reaction of Criegee intermediates with formaldehyde

Ozonolysis of isoprene is considered to be an important source of formic acid (HCOOH), but its underlying reaction mechanisms related to HCOOH formation are poorly understood. Here, we report the kinetic and product studies of the reaction between the simplest Criegee intermediate (CH₂OO) and formaldehyde (HCHO), both of which are the primary products formed in ozonolysis of isoprene. By utilizing time-resolved infrared laser spectrometry with the multifunctional dual-comb spectrometers, the rate coefficient k_CH2OO+HCHO is determined to be (4.11 ± 0.25) × 10^-12 cm³ molecule^-1 s^-1 at 296 K and a negative temperature dependence of the rate coefficient is observed and described by an Arrhenius expression with an activation energy of (-1.81 ± 0.04) kcal mol^-1. Moreover, the branching ratios of the reaction products HCOOH + HCHO and CO + H₂O + HCHO are explored. The yield of HCOOH is obtained to be 37-54% over the pressure (15-60 Torr) and temperature (283-313 K) ranges. The atmospheric implications of the reaction CH₂OO + HCHO are also evaluated by incorporating these results into a global chemistry-transport model. In the upper troposphere, the percent loss of CH₂OO by HCHO is found by up to 6% which can subsequently increase HCOOH mixing ratios by up to 2% during December-January-February months.

Full-dimensional neural network potential energy surface and dynamics of the CH2OO + H2O reaction

An accurate global full-dimensional machine learning-based potential energy surface (PES) of the simplest Criegee intermediate (CH₂OO) reaction with water monomer was developed based on the high level of extensive CCSD(T)-F12a/aug-cc-pVTZ calculations. This analytical global PES not only covers the regions of reactants to hydroxymethyl hydroperoxide (HMHP) intermediates, but also different end product channels, which facilities both the reliable and efficient kinetics and dynamics calculations. The rate coefficients calculated by the transition state theory with the interface to the full-dimensional PES agree well with the experimental results, indicating the accuracy of the current PES. Extensive quasi-classical trajectory (QCT) calculations were performed both from the bimolecular reaction CH₂OO + H₂O and from HMHP intermediate on the new PES. The product branching ratios of hydroxymethoxy radical (HOCH₂O, HMO) + OH radical, formaldehyde (CH₂O) + H₂O₂ and formic acid (HCOOH) + H₂O were calculated. The reaction yields dominantly HMO + OH, because of the barrierless pathway from HMHP to this channel. The computed dynamical results for this product channel show the total available energy was deposited into the internal rovibrational excitation of HMO, and the energy release in OH and translational energy is limited. The large amount of OH radical found in the current study implies that the CH₂OO + H₂O reaction can provide crucially OH yield in Earth's atmosphere.

Water Charge Transfer Accelerates Criegee Intermediate Reaction with H2O- Radical Anion at the Aqueous Interface

Criegee intermediates (CIs) are important carbonyl oxides that may react with atmospheric trace chemicals and impact the global climate. The CI reaction with water has been widely studied and is a main channel for trapping CIs in the troposphere. Previous experimental and computational reports have largely focused on reaction kinetic processes in various CI-water reactions. The molecular-level origin of CI's interfacial reactivity at the water microdroplet surface (e.g., as found in aerosols and clouds) is unclear. In this study, by employing the quantum mechanical/molecular mechanical (QM/MM) Born-Oppenheimer molecular dynamics with the local second-order Møller-Plesset perturbation theory, our computational results reveal a substantial water charge transfer up to ∼20% per water, which creates the surface H₂O⁺/H₂O^- radical pairs to enhance the CH₂OO and anti-CH₃CHOO reactivity with water: the resulting strong CI-H₂O^- electrostatic attraction at the microdroplet surface facilitates the nucleophilic attack to the CI carbonyl by water, which may counteract the apolar hindrance of the substituent to accelerate the CI-water reaction. Our statistical analysis of the molecular dynamics trajectories further resolves a relatively long-lived bound CI(H₂O^-) intermediate state at the air/water interface, which has not been observed in gaseous CI reactions. This work provides insights into what may alter the oxidizing power of the troposphere by the next larger CIs than simple CH₂OO and implicates a new perspective on the role of interfacial water charge transfer in accelerating molecular reactions at aqueous interfaces.

Kinetics of the Simplest Criegee Intermediate CH2OO Reaction with tert-Butylamine

The kinetics of the simplest Criegee intermediate (CH₂OO) reaction with tert-butylamine ((CH₃)₃CNH₂) was studied under pseudo-first-order conditions with the OH laser-induced fluorescence (LIF) method at the temperature range of 283-318 K and the pressure range of 5-75 Torr. Our pressure-dependent measurement showed that at 5 Torr─the lowest pressure measured in the current experiment─this reaction was under the high-pressure limit condition. At 298 K, the reaction rate coefficient was measured to be (4.95 ± 0.64) × 10^-12 cm³ molecule^-1 s^-1. The title reaction was observed to be negative temperature-dependent; the activation energy of (-2.82 ± 0.37) kcal mol^-1 and the pre-exponential factor of (4.21 ± 0.55) × 10^-14 cm³ molecule^-1 s^-1 were derived from the Arrhenius equation. The rate coefficient of the title reaction is slightly larger than (4.3 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1 of the CH₂OO reaction with methylamine; the electron inductive effect and the steric hindrance effect might play a role in contributing to such difference.

Enthalpies of formation for Criegee intermediates: A correlation energy convergence study

Criegee intermediates, formed from the ozonolysis of alkenes, are known to have a role in atmospheric chemistry, including the modulation of the oxidizing capacity of the troposphere. Although studies have been conducted since their discovery, the synthesis of these species in the laboratory has ushered in a new wave of investigations of these structures, both theoretically and experimentally. In some of these theoretical studies, high-order corrections for correlation energy are included to account for the mid multi-reference character found in these systems. Many of these studies include a focus on kinetics; therefore, the calculated energies should be accurate (<1 kcal/mol in error). In this research, we compute the enthalpies of formation for a small set of Criegee intermediates, including higher-order coupled cluster corrections for correlation energy up to coupled cluster with perturbative quintuple excitations. The enthalpies of formation for formaldehyde oxide, anti-acetaldehyde oxide, syn-acetaldehyde oxide, and acetone oxide are presented at 0 K as 26.5, 15.6, 12.2, and 0.1 kcal mol^-1, respectively. Additionally, we do not recommend the coupled cluster with perturbative quadruple excitations [CCSDT(Q)] energy correction, as it is approximately twice as large as that of the coupled cluster with full quadruple excitations (CCSDTQ). Half of the CCSDT(Q) energy correction may be included as a reliable, cost-effective estimation of CCSDTQ energies for Criegee intermediates.

An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the CH₂OO + RCN reactions (R = H, CH₃, C₂H₅) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH₂ICl), whose photolysis at 193 nm in the presence of O₂ produces CH₂OO. Observed bimolecular rate coefficients for CH₂OO + HCN, CH₂OO + CH₃CN, and CH₂OO + C₂H₅CN reactions at 296 K are (2.22 ± 0.65) × 10^-14 cm³ molecule^-1 s^-1, (1.02 ± 0.10) × 10^-14 cm³ molecule^-1 s^-1, and (2.55 ± 0.13) × 10^-14 cm³ molecule^-1 s^-1, respectively, suggesting that reaction with CH₂OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH₂OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH₂OO + HCN and CH₂OO + CH₃CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH₂O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH₂O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.

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.

Organic acid formation in the gas-phase ozonolysis of α,β-unsaturated ketones

Organic acids are key species in determining the radiative properties of the atmosphere due to their contribution to particle formation. Reported discrepancies between field measurements and modelling suggest significant missing sources. Herein, we present a mechanistic investigation on the gas-phase ozonolysis of ethyl vinyl ketone (EVK, 1-penten-3-one), which we chose as a model compound for α,β-unsaturated ketones. Experiments were performed in a 1080 L quartz-glass reaction chamber (QUAREC) at 990 ± 15 mbar and 298 ± 2 K (r. h. ≪ 0.1%) and analysed via long-path FTIR spectrometry and PTR-ToF-MS. The experiments were performed in the presence of an excess of CO to suppress the chemistry of OH radicals. For a comprehensive picture, in selected experiments, SO₂ was also added to the reaction system to scavenge the stabilized Criegee intermediates (sCIs) and to investigate their formation yield. Combining the results of both set-ups allowed us to quantify 2-oxobutanal, for which we report vapour-phase FTIR spectra. In addition, we introduce the first-ever infrared spectra of perpropionic acid, which was also positively identified in the EVK + O₃ system. A detailed analysis of the experimental findings allowed us to link the identified reaction products (acetaldehyde, ethyl hydroperoxide, and perpropionic acid) to known bimolecular reactions of RO₂ radicals. Thereby, it is shown that the EVK + O₃ reaction yields formic acid, HC(O)OH, and propionic acid, C₂H₅C(O)OH, and their formation is not covered by mechanisms reported in the literature. Three different pathways accounting for their formation from chemically activated CIs are proposed and possible implications for the ozonolysis of α,β-unsaturated ketones in the atmosphere are discussed.

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation

Ab initio molecular dynamics studies of CH₂OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S₂ (¹ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S₀) state. Trajectories propagated for 100 fs from the minimum-energy region of the S₂ state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S₂ and S₁ (¹nπ*) states at geometries close to the S₂ minimum, which enables population transfer to the optically dark S₁ state. Greater than 80% of the excited population undergoes O-O bond fission on the S₂ or S₁ potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH₂ moiety is twisted relative to the COO plane. Trajectory analysis also reveals recurrences in the O-O stretch coordinate, consistent with the resonance structure observed at the red end of the parent S₂-S₀ absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S₁ PES that enables access to a conical intersection between the S₁ and S₀ states and cyclization to dioxirane products.

Reaction Kinetics of Criegee Intermediates with Nitric Acid

This work investigated the reaction kinetics of HNO₃ with 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^-10 cm³ s^-1 for reactions of HNO₃ with 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 + HNO₃ in the literature [Foreman Angew. 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 HNO₃ reactions with CH₂OO and (CH₃)₂COO under 100-400 Torr. Our results indicate that in a dry area with severe NO_x pollution, the reactions of CIs with HNO₃ and their products may be worthy of attention, but these reactions may be insignificant under high-humidity conditions. However, CI reactions with HNO₃ may not play an important role in the atmospheric removal processes of HNO₃ because of the low concentrations of CIs.

A direct dynamics study of the exotic photochemistry of the simplest Criegee intermediate, CH2OO

Criegee intermediates are amongst the most fascinating molecules in modern-day chemistry. They are highly reactive intermediates that find vital roles that range from atmospheric chemistry to organic synthesis. Their excited state chemistry is exotic and complicated, and a myriad of electronic states can contribute to their photodissociation dynamics. This article reports a multi-state direct dynamics (full-dimensional) study of the photoinduced fragmentation of the simplest Criegee intermediate, CH₂OO, using state-of-the-art MS-CASPT2 trajectory surface hopping. Following vertical excitation to the strongly absorbing S₂(¹ππ*) state, internal conversion, and thus changes in the electronic state character of the separating O + CH₂O fragments, is observed between parent electronic states at separations that, traditionally, might be viewed as the classically asymptotic region of the potential energy surface. We suggest that such long-range internal conversion may account for the unusual and non-intuitive total kinetic energy distribution in the O(¹D) + CH₂O(S₀) products observed following photoexcitation of CH₂OO. The present results also reveal the interplay between seven singlet electronic states and dissociation to yield the experimentally observed O(¹D) + CH₂O(S₀) and O(³P) + CH₂O(T₁) products. The former (singlet) products are favored, with a branching ratio of ca. 80%, quantifying the hitherto unknown product branching ratios observed in velocity map imaging experiments. To the best of our knowledge, such long-range internal conversions that lead to changes in the electronic state character of the fragment pairs originating from a common parent - at classically asymptotic separations - have not been recognized hitherto in the case of a molecular photodissociation.

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.

Gas-phase and aqueous-surface reaction mechanism of Criegee radicals with serine and nucleation of products: A theoretical study

Criegee intermediates (CIs) are short-lived carbonyl oxides, which can affect the budget of OH radicals, ozone, ammonia, organic/inorganic acids in the troposphere. This study investigated the reaction of CIs with serine (Ser) in the gas phase by using density functional theory (DFT) calculations and at the gas-liquid interface by using Born-Oppenheimer molecular dynamics (BOMD). The results reveal that the reactivity of the three functional groups of Ser can be ordered as follows: COOH > NH₂ > OH. Water-mediated reactions of CIs with NH₂ and OH groups of Ser on the droplet follow the proton exchange mechanism. The products, sulfuric acids, ammonia, and water molecules form stable clusters within 20 ns. This study shows that hydroperoxide products can contribute to new particle formation (NPF). The result deepens the understanding of the reaction of CIs with multifunctional pollutants and atmospheric behavior of CIs in polluted areas.

Reaction mechanism and kinetics of Criegee intermediate and hydroperoxymethyl formate

The reaction mechanism and kinetics of the simplest Criegee intermediate CH₂OO reaction with hydroperoxymethyl formate (HPMF) was investigated at high-level quantum chemistry calculations. HPMF has two reactive functional groups, -C(O)OH and -OOH. The calculated results of thermodynamic data and rate constants indicated that the insertion reactions of CH₂OO with -OOH group of HPMF were more favorable than the reactions of CH₂OO with -C(O)OH group. The calculated overall rate constant was 2.33 × 10^-13 cm³/(molecule⋅sec) at 298 K and the rate constants decreased as the temperature increased from 200 to 480 K. In addition, we also proved the polymerization reaction mechanism between CH₂OO and -OOH of HPMF. This theoretical study interpreted the previous experimental results, and supplied the structures of the intermediate products that couldn't be detected during the experiment.

Kinetics of CH2OO and syn-CH3CHOO reaction with acrolein

The kinetics for the reactions of CH2OO and syn-CH3CHOO with acrolein, a typical unsaturated aldehyde in the atmosphere, were studied in a flash photolysis flow reactor using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 281 to 318 K, and pressures ranging from 5 to 200 Torr. No obvious dependence of the rate coefficients on pressure was observed under the current experimental conditions. Both reactions exhibit negative temperature-dependence, with an activation energy of (-1.70 ± 0.19) and (-1.47 ± 0.24) kcal mol-1 for CH2OO and syn-CH3CHOO reacting with acrolein, derived from the Arrhenius equation. At 298 K, the measured rate coefficients for CH2OO/syn-CH3CHOO + acrolein reactions are (1.63 ± 0.19) × 10-12 cm3 s-1 and (1.17 ± 0.16) × 10-13 cm3 s-1, respectively. The rate coefficient of the former reaction is in reasonable agreement with a recent theoretical result.

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.

Directional Proton Transfer in the Reaction of the Simplest Criegee Intermediate with Water Involving the Formation of Transient H3O. J Phys Chem Lett 2021;12:3379-3386. [PMID: 33784110 DOI: 10.1021/acs.jpclett.1c00448]

The reaction of Criegee intermediates with water vapor has been widely known as a key Criegee reaction in the troposphere. Herein, we investigated the reaction of the smallest Criegee intermediate, CH₂OO, with a water cluster through fragment-based ab initio molecular dynamics simulations at the MP2/aug-cc-pVDZ level. Our results show that the CH₂OO-water reaction could occur not only at the air/water interface but also inside the water cluster. Moreover, more than one reactive water molecules are required for the CH₂OO-water reaction, which is always initiated from the Criegee carbon atom and ends at the terminal Criegee oxygen atom via a directional proton transfer process. The observed reaction pathways include the loop-structure-mediated and stepwise mechanisms, and the latter involves the formation of transient H₃O⁺. The lifetime of transient H₃O⁺ is on the order of a few picoseconds, which may impact the atmospheric budget of the other trace gases in the actual atmosphere.

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.

Kinetics of the Gas Phase Reactions of the Criegee Intermediate CH2OO with O3 and IO

The kinetics of the gas phase reactions of the Criegee intermediate CH₂OO with O₃ and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH₂OO, the CH₂OO precursor (CH₂I₂), O₃, and IO using flash photolysis of CH₂I₂/O₂/O₃/N₂ mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O₃, with the rate coefficients for reactions of CH₂OO with O₃ and IO obtained by fitting to the observed decays of CH₂OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O₃ and the average concentration of IO varying in the range 30-700, and gave k_CH2OO+O3 = (3.6 ± 0.8) × 10^-13 cm³ molecule^-1 s^-1 and k_CH2OO+IO = (7.6 ± 1.4) × 10^-11 cm³ molecule^-1 s^-1 (where the errors are at the 2σ level). The magnitude of k_CH2OO+O3 has a significant effect on the steady state concentration of CH₂OO in chamber studies. Atmospheric implications of the results are discussed.

A theoretical study of the addition of CH2OO to hydroxymethyl hydroperoxide and its implications on SO3 formation in the atmosphere

The reaction of hydroxymethyl hydroperoxide (HMHP, HOCH₂OOH) with the simplest Criegee intermediate, CH₂OO, has been examined using quantum chemical methods with transition state theory. Geometry optimization and IRC calculations were performed using the M06-2X, MN15-L, and B2PLYP-D3 functionals in conjunction with the aug-cc-pVTZ basis set. Single point energy calculations using QCISD(T) and BD(T) with the same basis set have been performed to determine the energy of reactants, reactive complexes, transition states, and products. Rate coefficients have been obtained using variational transition state theory. The addition of CH₂OO on the three different oxygen atoms in HMHP has been considered and the ether oxide forming channel, CH₂OO + HOCH₂OOH → HOCH₂O(O)CH₂OOH (channel 2), is the most favorable. The best computed standard enthalpy of reaction (ΔH) and zero-point corrected barrier height are -20.02 and -6.33 kcal mol^-1, respectively. The reaction barrier is negative and our results suggest that both the inner and outer transition states contribute to the corresponding overall reactive flux in the tropospheric temperature range (220 K to 320 K). A two-transition state model has been used to obtain reliable rate coefficients at the high-pressure limit. The pressure-dependent rate coefficient calculations using the SS-QRRK theory have shown that this channel is pressure-dependent. Moreover, our investigation has shown that the ether oxide formed may rapidly react with SO₂ at 298 K to form SO₃, which can, in turn, react with water to form atmospheric H₂SO₄. A similar calculation has been conducted for the reaction of HMHP with OH, suggesting that the titled reaction may be a significant sink of HMHP. Therefore, the reaction between CH₂OO and HOCH₂OOH could be an indirect source for generating atmospheric H₂SO₄, which is crucial to the formation of clouds, and it might relieve global warming.

Trends in stabilisation of Criegee intermediates from alkene ozonolysis

Criegee Intermediates (CI), formed in the ozonolysis of alkenes, play a central role in tropospheric chemistry as an important source of radicals, with stabilised CI (SCI) able to participate in bimolecular reactions, affecting climate through the formation of inorganic and organic aerosol. However, total SCI yields have only been determined for a few alkene systems, while speciated SCI yields from asymmetrical alkenes are almost entirely unknown. Here we report for the first time a systematic experimental exploration of the stabilisation of CH2OO and (CH3)2COO CI, formed from ten alkene-ozone systems with a range of different sizes and structures, under atmospherically relevant conditions in the EUPHORE chamber. Experiments in the presence of excess SO2 (an SCI scavenger) determined total SCI yields from each alkene-ozone system. Comparison of primary carbonyl yields in the presence/absence of SO2 determined the stabilisation fraction of a given CI. The results show that the stabilisation of a given CI increases as the size of the carbonyl co-product increases. This is interpreted in terms of the nascent population of CI formed following decomposition of the primary ozonide (POZ) having a lower mean energy distribution when formed with a larger carbonyl co-product, as more of the energy from the POZ is taken by the carbonyl. These findings have significant implications for atmospheric modelling of alkene ozonolysis. Higher stabilisation of small CI formed from large alkenes is expected to lead to lower radical yields from CI decomposition, and higher SCI concentrations, increasing the importance of SCI bimolecular reactions.

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.

CO2 as an auto-catalyst for the oxidation of CO by a Criegee intermediate (CH2OO)

The present work investigates the effect of CO₂ on the CH₂OO + CO reaction, employing the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory.

CH2OO Criegee intermediate UV absorption cross-sections and kinetics of CH2OO + CH2OO and CH2OO + I as a function of pressure

The UV absorption cross-sections of the Criegee intermediate CH₂OO, and kinetics of the CH₂OO self-reaction and the reaction of CH₂OO with I are reported as a function of pressure at 298 K.

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.

Observation of hydroperoxyethyl formate from the reaction between the methyl Criegee intermediate and formic acid

The hydroperoxide ester, hydroperoxyethyl formate, has been characterized as the nascent reaction product obtained from the reaction of the Criegee intermediate, CH₃CHOO, and formic acid.

The atmospheric importance of methylamine additions to Criegee intermediates

The methylamine addition to Criegee intermediates is investigated using high level ab initio methods.

Unexpected quenching effect on new particle formation from the atmospheric reaction of methanol with SO3

Despite the high abundance in the atmosphere, alcohols in general and methanol in particular are believed to play a small role in atmospheric new particle formation (NPF) largely due to the weak binding abilities of alcohols with the major nucleation precursors, e.g., sulfuric acid (SA) and dimethylamine (DMA). Herein, we identify a catalytic reaction that was previously overlooked, namely, the reaction between methanol and SO₃, catalyzed by SA, DMA, or water. We found that alcohols can have unexpected quenching effects on the NPF process, particularly in dry and highly polluted regions with high concentrations of alcohols. Specifically, the catalytic reaction between methanol and SO₃ can convert methanol into a less-volatile species--methyl hydrogen sulfate (MHS). The latter was initially thought to be a good nucleation agent for NPF. However, our simulation results suggest that the formation of MHS consumes an appreciable amount of atmospheric SO₃, disfavoring further reactions of SO₃ with H₂O. Indeed, we found that MHS formation can cause a reduction of SA concentration up to 87%, whereas the nucleation ability of MHS toward new particles is not as good as that of SA. Hence, a high abundance of methanol in the atmosphere can lower the particle nucleation rate by as much as two orders of magnitude. Such a quenching effect suggests that the recently identified catalytic reactions between alcohols and SO₃ need to be considered in atmospheric modeling in order to predict SA concentration from SO₂, while also account for their potentially negative effect on NPF.

Theoretical Study on Criegee Intermediate's Role in Ozonolysis of Acrylic Acid

Criegee intermediates have raised much attention in atmospheric chemistry because of their significance in ozonolysis mechanism. The simplest Criegee intermediate, CH₂OO, and its reactions with acrylic acid including cycloadditions and insertions as main entrance channels have been investigated at CCSD(T)/cc-pVTZ//M06-2X/6-31G(d,p) level. Temperature- and pressure-dependent kinetics were predicted by solving the time-dependent master equations based on Rice-Ramsperger-Kassel-Marcus theory using MESS program, with temperatures from 200 to 500 K and pressures from 0.001 to 1000 atm. Variational transition state theory (VTST) was used for barrierless pathways and conventional transition state theory (CTST) for pathways with distinct barriers. Results indicate that hydroperoxymethyl acrylate is the dominant product under atmospheric conditions. The combination of two reactants will reduce the volatility and makes a possible factor that induces formation of secondary organic aerosols, which suggests CH₂OO's entangled role in ever-increasing air pollution.