Direct Observation of the Gas-Phase Criegee Intermediate (CH2OO)

Atmospheric Intermediates at the Air-Water Interface

The air-water interface (AWI) is a ubiquitous reaction field different from the bulk phase where unexpected reactions and physical processes often occur. The AWI is a region where air contacts cloud droplets, aerosol particles, the ocean surface, and biological surfaces such as fluids that line human epithelia. In Earth's atmosphere, short-lived intermediates are expected to be generated at the AWI during multiphase reactions. Recent experimental developments have enabled the direct detection of atmospherically relevant, short-lived intermediates at the AWI. For example, spray ionization mass spectrometric analysis of water microjets exposed to a gaseous mixture of ozone and water vapor combined with a 266 nm laser flash photolysis system (LFP-SIMS) has been used to directly probe organic peroxyl radicals (RO2·) produced by interfacial hydroxyl radicals (OH·) + organic compound reactions. OH· emitted immediately after the laser flash photolysis of carboxylic acid at the gas-liquid interface have been directly detected by time-resolved, laser-induced florescence techniques that can be used to study atmospheric multiphase photoreactions. In this Featured Article, we show some recent experimental advances in the detection of atmospherically important intermediates at the AWI and the associated reaction mechanisms. We also discuss current challenges and future prospects for atmospheric multiphase chemistry.

How Do the Position and Number of Methyl Substituents Affect the Photochemical Process of Criegee Intermediate? Trajectory Surface-Hopping Dynamics of Four-Carbon CIs

Electronic-structure calculations combined with nonadiabatic trajectory surface-hopping (TSH) dynamic simulations were carried out on two alkenyl-substituted Criegee intermediates (CIs), i.e., propenyl-substituted CI (PCI) and 1-methyl-propenyl substituted CI (MPCI), in order to investigate the influence of the position and number of substituents on the photochemical process of CI in S1 states. It is found that they play critical roles in the reactivity, dominant product channel, and mechanism of the CIs. More specifically, introducing a methyl group on either C1 (α-C) or C3 (γ-C) position of a vinyl-substituted CI (VCI) skeleton facilitates the rotation of the C1═O1 bond and leads to the formation of a three-membered dioxirane ring; meanwhile, it evidently enhances the reactively of the S1-state molecule. Meanwhile, methyl substitution on the vinyl moiety [i.e., C2 (β-C) and C3 (γ-C) positions] is beneficial for the rotation of the C2═C3 bond and thus facilitates the formation of the five-membered 1,2-dioxole ring, and the substitution on C2 site decreases the reactivity. The cosubstitution of C2 and C3 atoms by methyl groups well balances the features of VCI in the sense of high reactivity, consistently predominant channel, and possible dioxole side-product. The findings here not only deepen the knowledge on the photochemical processes of the CI but also inspire the rethinking of the "old" concept of substitution effect.

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

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.

Oxepin Derivatives Formation from Gas-Phase Catechol Ozonolysis

Quantum chemical calculations are performed to explore all of the possible pathways for primary ozonide (POZ) formation from gas-phase ozonolysis of catechol. Canonical transition state theory has been used to calculate the rate coefficients of individual steps for the formation of POZ. The calculated rate coefficients for 1,3-cycloaddition of ozone at the (i) unsaturated C(OH)═C(OH) bond and (ii) CH═C(OH) of catechol, respectively, are in good agreement with the experimental rate constant. In general, subsequent decomposition of POZ leads to well-known Criegee Intermediates. This work reveals a parallel pathway by which the endo-addition of ozone at CH═C(OH) of catechol proceeds through oxepin derivatives along with the paths leading to Criegee Intermediates and peroxy acids. The 7-membered heterocyclic oxepin derivatives have lower energies than Criegee Intermediates but similar relative energies with peroxy acids.

Interfacial Extraction to Trap and Characterize the Criegee Intermediates from Phospholipid Ozonolysis

Criegee intermediates (CIs) play a significant role in cell membrane peroxidation, but their identification remains elusive at the molecular level. Herein, we combined interfacial extraction and sonic spray ionization mass spectrometry to study the oxidation reaction of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) mediated by ozone (O₃) at/near the surface of a hung water droplet. On-line interfacial extraction and ionization provided a snapshot of the short-lived CIs. Experiments in which the content of water was varied provided evidence for the formation of CIs, which has not been previously observed. Capture experiments using 5,5-dimethyl-pyrroline N-oxide (DMPO) indicated that CIs could be selectively characterized, and the extracted ion current (EICs) of CIs vs DMPO-CI adducts further confirmed the successful observation of CIs. Theoretical calculation suggested that surface ozonolysis of POPG was mainly mediated by anti-CI. These results open a new route for aqueous surface reactive species identification, and benefit toward the understanding of disease development associated with cell oxidative stress mediated by CIs.

Semiclassical Dynamics on Machine-Learned Coupled Multireference Potential Energy Surfaces: Application to the Photodissociation of the Simplest Criegee Intermediate

Determination of high-dimensional potential energy surfaces (PESs) and nonadiabatic couplings have always been quite challenging. To this end, machine learning (ML) models, trained with a finite set of ab initio data, allow accurate prediction of such properties. To express the PESs in terms of atomic contributions is the cornerstone of any ML based technique because it can be easily scaled to large systems. In this work, we have constructed high fidelity PESs and nonadiabatic coupling terms at the CASSCF level of ab initio data using a machine learning technique, namely, kernel-ridge regression. Additional MRCI-level calculations were carried out to assess the quality of the PESs. We use these machine-learned PESs and nonadiabatic couplings to simulate excited-state molecular dynamics based on Tully's fewest-switches surface hopping method (FSSH). FSSH is a semiclassical method in which nuclei move on the PESs due to the electrons according to the laws of classical mechanics. Nonadiabatic effects are taken into account in terms of transitions between PESs. We apply this scheme to study the O-O photodissociation of the simplest Criegee intermediate (CH₂OO). The FSSH trajectories were initiated on the lowest optically bright singlet excited state (S₂) and propagated along the three most important internal coordinates, namely, O-O and C-O bond distances and the COO bond angle. Some of the trajectories end up on energetically lower PESs as a result of radiationless transfer through conical intersections. All of the trajectories lead to the dissociation of the O-O bond due to the dissociative nature of the excited PESs through one of the two dissociative channels. The simulation reveals that there is about 88.4% probability of dissociation through the lower channel leading to the H₂CO (X¹A₁) and O (¹D) products, whereas there is only 11.6% probability of dissociation through the upper channel leading to H₂CO (a³A″) and O (³P) products.

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.

Correlated quantum treatment of the photodissociation dynamics of formaldehyde oxide CH2OO

An extended theoretical analysis of the photodissociation of the smallest Criegee intermediate CH₂OO following excitation to the B state is presented. It relies on explicitly correlated multireference electronic wavefunctions combined with a quantum dynamical treatment for two interacting (B-C) electronic states and three coupled nuclear degrees of freedom. The 3D model relies on PESs along the O-O and C-O stretching as well as C-O-O bending modes for the two lowest excited states with A' symmetry, and is sufficiently accurate to reproduce the experimental B¹A'-X¹A' absorption spectrum, especially at the low-energy range to unprecedented accuracy. The existence of a deep well (∼0.4 eV) on the (diabatic) B state causes a considerable amount of the wavepacket to be reflected by the B state well, which can explain the vibronic structures appearing in the long wavelength range of 360-470 nm of the spectrum. The main progression appearing in the energy range from 360 to 470 nm is assigned to the O-O stretching mode while finer details are affected by couplings to the C-O stretching and C-O-O bending modes. The weakly avoided crossing between the B-state and C-state potential energy surfaces appearing near 3.1 eV excitation energy (for RS2-F12 method) causes considerable disturbance in the vibronic fine structure of the bands. The description of the latter is quite strongly affected by the type of electron correlation treatment adopted, either fully variational (MRCI type) or perturbation theoretic (RS2 type). The results give novel insight into the complex interactions governing that intriguing process.

Simultaneous Quantification of H2O2 and Organic Hydroperoxides by 1H NMR Spectroscopy

Due to similar reactivity of organic hydroperoxides (OHPs), an HPLC separation step is typically required for their indirect (chemical) quantification in mixtures. The high sensitivity of chemical shifts to chemical structure makes NMR an ideal tool for the simultaneous quantification of OHPs in mixtures, but the concentration of these analytes in the samples of interest is usually well below the sensitivity of standard NMR experiments. This sensitivity problem can be mitigated by taking advantage of the fact that the z magnetization of the H₂O₂ resonance recovers at the rate of hydrogen exchange with water, which is significantly faster than longitudinal relaxation, thus enabling very fast scanning for signal-to-noise enhancement. An adaptation of the E-BURP2 pulse is described that suppresses the water signal by more than 4 orders of magnitude, yielding uniform excitation of peroxide signals without interference of the ca. 10⁸-fold stronger H₂O resonance. We demonstrate the method for a mixture of OHPs and report the chemical shifts for multiple OHPs that are of interest in atmospheric chemistry. As shown for hydroxymethyl hydroperoxide, the chemical decay of OHPs can be tracked directly by NMR spectroscopy.

High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics

We present a new photoelectron photoion coincidence (PEPICO) spectrometer that combines high mass resolution of cations with independently adjustable velocity map imaging of both cations and electrons. We photoionize atoms and molecules using fixed-frequency vacuum ultraviolet radiation. Mass-resolved photoelectron spectra associated with each cation's mass-to-charge ratio can be obtained by inversion of the photoelectron image. The mass-resolved photoelectron spectra enable kinetic time-resolved probing of chemical reactions with isomeric resolution using fixed-frequency radiation sources amenable to small laboratory settings. The instrument accommodates a variety of sample delivery sources to explore a broad range of physical chemistry. To demonstrate the time-resolved capabilities of the instrument, we study the 193 nm photodissociation of SO₂ via the C̃(¹B₂) ← X̃(¹A₁) transition. In addition to the well-documented O(³P_j) + SO(³Σ^-) channel, we observe direct evidence for a small yield of S(³P_j) + O₂(³Σ_g^-) as a primary photodissociation product channel, which may impact sulfur mass-independent fractionation chemistry.

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet ³CH₂OO (the simplest Criegee intermediate) and ³CH₂O₂ (dioxirane) have mostly polar biradical character, while singlet ¹CH₂OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of ¹CH₂ + ³O₂ is more than ten times as fast as the reaction of ³CH₂ (X³B₁) + ³O₂ and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the ¹CH₂O + ³O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

Reactivity and internal dynamics in the Criegee intermediate CH2OOCO2 system: A rotational study

The reaction system between the simplest Criegee intermediate, CH₂OO, and the greenhouse gas CO₂ has been investigated by Fourier transform microwave spectroscopy. The CH₂OO-CO₂ weakly bound complex was identified in the rotational spectrum, where inversion doublets due to the tunnelling motion between two equivalent configurations of the complex, with CO₂ located at one side or the other side of the CH₂OO plane, were observed. Using a two-state torsion-rotation Hamiltonian, a complete set of rotational and centrifugal distortion constants for both tunneling states were derived. In addition, the torsional energy difference between both states could be accurately determined, being 23.9687 MHz. The non-observation of the cycloaddition reaction product is in agreement with our ab initio calculations and with previous results that concluded that the reactivity of CIs toward CO₂ is measured to be quite limited.

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.

Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

The accelerating chemical effect of ozone addition on the oxidation chemistry of methyl hexanoate [CH₃(CH₂)₄C(═O)OCH₃] was investigated over a temperature range from 460 to 940 K. Using an externally heated jet-stirred reactor at p = 700 Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5, 80% argon dilution), we explored the relevant chemical pathways by employing molecular-beam mass spectrometry with electron and single-photon ionization to trace the temperature dependencies of key intermediates, including many hydroperoxides. In the absence of ozone, reactivity is observed in the so-called low-temperature chemistry (LTC) regime between 550 and 700 K, which is governed by hydroperoxides formed from sequential O₂ addition and isomerization reactions. At temperatures above 700 K, we observed the negative temperature coefficient (NTC) regime, in which the reactivity decreases with increasing temperatures, until near 800 K, where the reactivity increases again. Upon addition of ozone (1000 ppm), the overall reactivity of the system is dramatically changed due to the time scale of ozone decomposition in comparison to fuel oxidation time scales of the mixtures at different temperatures. While the LTC regime seems to be only slightly affected by the addition of ozone with respect to the identity and quantity of the observed intermediates, we observed an increased reactivity in the intermediate NTC temperature range. Furthermore, we observed experimental evidence for an additional oxidation regime in the range near 500 K, herein referred to as the extreme low-temperature chemistry (ELTC) regime. Experimental evidence and theoretical rate constant calculations indicate that this ELTC regime is likely to be initiated by H abstraction from methyl hexanoate via O atoms, which originate from thermal O₃ decomposition. The theoretical calculations show that the rate constants for methyl ester initiation via abstraction by O atoms increase dramatically with the size of the methyl ester, suggesting that ELTC is likely not important for the smaller methyl esters. Experimental evidence is provided indicating that, similar to the LTC regime, the chemistry in the ELTC regime is dominated by hydroperoxide chemistry. However, mass spectra recorded at various reactor temperatures and at different photon energies provide experimental evidence of some differences in chemical species between the ELTC and the LTC temperature ranges.

Identification and Reactivity of s-cis,s-cis-Dihydroxycarbene, a New [CH2O2] Intermediate

We report the first preparation of the s-cis,s-cis conformer of dihydroxycarbene (1cc) by means of pyrolysis of oxalic acid, isolation of the lower-energy s-trans,s-trans (1tt) and s-cis,s-trans (1ct) product conformers at cryogenic temperatures in a N₂ matrix, and subsequent narrow-band near-infrared (NIR) laser excitation to give 1cc. Carbene 1cc converts quickly to 1ct via quantum-mechanical tunneling with an effective half-life of 22 min at 3 K. The potential energy surface features around 1 were pinpointed by convergent focal point analysis targeting the AE-CCSDT(Q)/CBS level of electronic structure theory. Computations of the tunneling kinetics confirm the time scale of the 1cc → 1ct rotamerization and suggest that direct 1cc → H₂ + CO₂ decomposition may also be a minor pathway. The intriguing latter possibility cannot be confirmed spectroscopically, but hints of it may be present in the measured kinetic profiles.

Molecular Mechanisms and Atmospheric Implications of Criegee Intermediate-Alcohol Chemistry in the Gas Phase and Aqueous Surface Environments

Criegee intermediates and alcohols are important species in the atmosphere. In this study, we use quantum chemistry and Born-Oppenheimer molecular dynamics (BOMD) simulations to investigate the reaction between methanol/ethanol and Criegee intermediates (anti- or syn-CH₃CHOO) in the gas phase and at the air-water interface. Reactions at the interface are found to be much faster than those in the gas phase. When water molecules are available, loop structures can be formed to facilitate the reaction. In addition, nonloop reaction pathways characterized by the formation of hydrated protons, although with a low possibility, are also identified at the air-water interface. Implications of our results on the fate of Criegee intermediates in the atmosphere are discussed, which deepen our understanding of Criegee intermediate-alcohol chemistry in humid environments.

Hydrogen bond, ring tension and π-conjugation effects: methyl and vinyl substitutions dramatically change the photodynamics of Criegee intermediates

The substitution effect in chemistry is a concept that is probably too common to mention, while for a molecule with an elusive electronic structure, substitution can introduce an unusual effect that dramatically tunes the chemical process. To reveal the substitution effects on the photodynamics of Criegee Intermediates (CIs), we carried out the multireference CASSCF trajectory surface-hopping (TSH) molecular dynamics and CASPT2 electronic-structure calculations for a methyl-substituted CI (MCI) and a vinyl-substituted CI (VCI). The results show that for different substituents, the hydrogen bond, ring tension and π-conjugation not only alter the relative stabilities of the conformers/configurations, but also dramatically change the photo-induced channel of CIs. For an anti-MCI, the dominant channel starting from the S₁ state is the ring-closure process leading to dioxirane, while in the syn configuration, the intramolecular CHO hydrogen bond hinders the rotation around the C-O bond and thus leads to a high yield of in-plane O-O dissociation towards acetaldehyde (X¹A') and the O(¹D) atom. In a VCI with an unsaturated substituent, the π-conjugation greatly strengthens the O-O bond and therefore no O-O dissociation is observed in all configurations. In addition, the CHO hydrogen bond in the syn_(CO)-VCI further stabilizes the S₁-state intermediates and makes them less reactive; in contrast, isomerization to dioxirane becomes the globally dominant channel in the anti_(CO)-VCI. The dramatic substitution effects by saturated and unsaturated substituents on CIs found here will deepen the understanding of Criegee-intermediate chemistry.

Theoretical Insight into the Reaction Mechanism and Kinetics for the Criegee Intermediate of anti-PhCHOO with SO2

In this study, the density functional theory (DFT) and CCSD(T) method have been performed to gain insight into the possible products and detailed reaction mechanism of the Criegee intermediate (CI) of anti-PhCHOO with SO2 for the first time. The potential energy surfaces (PESs) have been depicted at the UCCSD(T)/6-311++G(d,p)//UB3LYP/6-311++G(d,p) levels of theory with ZPE correction. Two different five-membered ring adducts, viz., endo PhCHOOS(O)O (IM1) and exo PhCHOOS(O)O (IM2) have been found in the entrance of reaction channels. Both direct and indirect reaction pathways from IM1 and IM2 have been considered for the title reaction. Our calculations show that the formation of PhCHO+SO3 (P1) via indirect reaction pathways from IM1 is predominant in all the pathways, and the production of P1 via direct dissociation pathway of IM1 and indirect reaction pathways of IM2 cannot be neglected. Moreover, PhCOOH+SO2 (P2) initiated from IM2 is identified as the minor product. According to the kinetic calculation, the total rate constant for the anti-PhCHOO+SO2 reaction is estimated to be 6.98 × 10−10 cm3·molecule−1·s−1 at 298 K.

Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor: unimolecular decomposition of CH2OO and its reaction with formic acid

We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CH₂OO) reactions. We also introduce and utilize a new photolytic precursor, bromoiodomethane (CH₂IBr), which photolysis at 213 nm in presence of O₂ produces CH₂OO. Importantly, this precursor appears to be free from secondary reactions that may regenerate CH₂OO in kinetic experiments. The unimolecular decomposition rate coefficient of CH₂OO has been measured over wide pressure (5-400 Torr) and temperature (296-600 K) ranges and master equation simulations of the decomposition kinetics have been performed using MESMER program. The MESMER simulations of the experimental data with the calculated zero-point energy corrected transition state energy 85.9 kJ mol^-1 for decomposition required no adjustment and returned 〈ΔE〉_down = 123.2 × (T/298 K)^0.74 cm^-1 for temperature-dependent exponential-down model of the collisional energy transfer in He. A very good agreement between results of simulations and experiments is obtained. The results are compared with the previously reported unimolecular decomposition study by Stone et al. (Phys. Chem. Chem. Phys., 2018, 20, 24940-24954). Current master equation simulations suggest about 61% decomposition yield for the predominant H₂ + CO₂ channel, whereas the yields of two other channels, H₂O + CO, and HCO + OH, are sensitive on the parameters involved in the simulations. The kinetics of CH₂OO reaction with formic acid has also been investigated as function of pressure (5-150 Torr) and temperature (296-458 K). The bimolecular rate coefficient for CH₂OO + HCOOH reaction shows a negative temperature dependency, decreasing from (1.0 ± 0.03) × 10^-10 cm³ molecule^-1 s^-1 at 296 K to (0.47 ± 0.05) × 10^-10 cm³ molecule^-1 s^-1 at 458 K with an Arrhenius activation energy of -4.9 ± 1.6 kJ mol^-1, where statistical uncertainties shown are 2σ. Estimated overall uncertainty in the measured rate coefficients is about ±20%. Current bimolecular rate coefficient at room temperature agrees with the previously reported rate coefficients from the direct kinetic experiments. The reaction is found to be pressure independent over the range between 5 and 150 Torr at 296 K in He.

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.

Gauging stability and reactivity of carbonyl O-oxide Criegee intermediates

In this study, we evaluated the effect of substitution on the stability and reactivity of carbonyl O-oxide Criegee intermediates (CIs). In this regard, we computed a set of more than 50 carbonyl oxides at the CBS-QB3 level of theory and assessed their stability by means of an isodesmic reaction equation defining a carbonyl oxide stabilization energy (COSE). Almost all substituents are stabilizing and amino groups in particular leading to COSE values of almost 60 kcal mol^-1. As opposed to π-donors, substituents with a strong σ-electron pull destabilize the C[double bond, length as m-dash]O-O group. Furthermore, we studied how the intrinsic stabilization of the Criegee intermediate is reflected in its C[double bond, length as m-dash]O and O-O bond lengths as well as the partial charges on the individual atoms of the carbonyl oxide moiety. As a potential measure for reactivity, we determined the adiabatic singlet-triplet energy gap of all carbonyl oxides. Amino substituted CIs exhibit high-lying triplet states and have relatively large barriers towards addition of water or the OH radical. However, the ΔE_S-T cannot serve as a rigorous measure for carbonyl oxide reactivity.

Rapid unimolecular reaction of stabilized Criegee intermediates and implications for atmospheric chemistry

Elucidating atmospheric oxidation mechanisms is necessary for estimating the lifetimes of atmospheric species and understanding secondary organic aerosol formation and atmospheric oxidation capacity. We report an unexpectedly fast mechanistic pathway for the unimolecular reactions of large stabilized Criegee intermediates, which involves the formation of bicyclic structures from large Criegee intermediates containing an aldehyde group. The barrier heights of the mechanistic pathways are unexpectedly low – about 2–3 kcal/mol – and are at least 10 kcal/mol lower than those of hydrogen shift processes in large syn Criegee intermediates; and the calculated rate constants show that the mechanistic pathways are 10⁵-10⁹ times faster than those of the corresponding hydrogen shift processes. The present findings indicate that analogous low-energy pathways can now also be expected in other large Criegee intermediates and that oxidative capacity of some Criegee intermediates is smaller than would be predicted by existing models.

Criegee intermediates have received much attention in atmospheric chemistry because of their importance in ozonolysis mechanisms. Here, using quantum mechanical kinetics, the authors reveal an unexpectedly fast mechanistic pathway for unimolecular reactions of large stabilized Criegee intermediates.

Calculation of the absolute photoionization cross-sections for C1-C4 Criegee intermediates and vinyl hydroperoxides

Criegee Intermediates (CIs) and their isomer Vinyl Hydroperoxides (VHPs) are crucial intermediates in the ozonolysis of alkenes. To better understand the underlying chemistry of CIs and VHPs, progress has been made to detect and identify them by photoionization mass spectrometric experiments. Further reliable quantitative information about these elusive intermediates requires their photoionization cross sections. The present work systematically investigated the near-threshold absolute photoionization cross-sections for ten C1-C4 CIs and VHPs, i.e., formaldehyde oxide (CH₂OO), acetaldehyde oxide (syn-/anti-CH₃CHOO), acetone oxide ((CH₃)₂COO), syn-CH₃-anti-(cis-CH=CH₂)COO, syn-CH₃-anti-(trans-CH=CH₂)COO and vinyl hydroperoxide (CH₂CHOOH), 2-hydroperoxypropene (CH₂=C(CH₃)OOH), syn-CH₂ = anti-(cis-CH=CH₂)-COOH, syn-CH₂ = anti-(trans-CH=CH₂)COOH. The adiabatic ionization energies (AIEs) were calculated at the DLPNO-CCSD(T)/CBS level with uncertainties of less than 0.05 eV. The calculated AIEs for C1-C4 CIs and VHPs vary from 8.75 to 10.0 eV with the AIEs decreasing as the substitutions increase. Franck-Condon factors were calculated with the double Duschinsky approximation and the ionization spectra were obtained based on the calculated ionization energies. Pure electronic photoionization cross sections are calculated by the frozen-core Hartree-Fock (FCHF) approximation. The final determined absolute cross sections are around 4.5-6 Mb for the first and second ionization of CIs and 15-25 Mb for VHPs. It is found that the addition of a methyl group or an unsaturated vinyl substitution for the CIs does not substantially change the absolute value of their cross sections.

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.

Threshold photoelectron spectrum of the CH2OO Criegee intermediate

We present the photoelectron spectroscopy of the simplest Criegee intermediate, CH₂OO, close to the first ionization energy.

The addition of methanol to Criegee intermediates

High level ab initio methods are employed to study the addition of methanol to the simplest Criegee intermediates and its methylated analogue. Kinetic rate constants over a range of temperatures are computed and compared to experimental results.

The Criegee intermediate-formic acid reaction explored by rotational spectroscopy

Products resulting from the reaction between the Criegee intermediate, CH₂OO, and formic acid are characterized by rotational spectroscopy.

Formation of Criegee intermediates and peroxy acids: a computational study of gas-phase 1,3-cycloaddition of ozone with catechol

A detailed theoretical investigation of gas-phase 1,3-cycloaddition of ozone with catechol is presented to explore the discrepancies in previous theoretical and experimental rate constants. DFT based PBE, TPSS, B3LYP, B3PW91, M06-2X, wB97XD, MN15 and high-level CCSD(T) methods are used for the calculation. Canonical transition state theory has been used to calculate the rate coefficients of individual steps. The calculated rate coefficients are compared with the experimental and previously calculated rate constant. The possible pathways for primary ozonide (POZ) formation and subsequent reactions to yield the Criegee Intermediates (CI) and peroxy acids (POA) are investigated. The endo-POZ may undergo conversion to exo-POZ or form the Creigee Intermediates. This work shows a novel pathway by which the exo-POZ can form more stable and chemically different species, peroxy acids, by abstracting an H atom from the OH group.

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.

High-resolution sub-Doppler infrared spectroscopy of atmospherically relevant Criegee precursor CH2I radicals: CH2 stretch vibrations and "charge-sloshing" dynamics

The combination of a pulsed supersonic slit-discharge source and single-mode difference frequency direct absorption infrared spectroscopy permit first high resolution infrared study of the iodomethyl (CH₂I) radical, with the CH₂I radical species generated in a slit jet Ne/He discharge and cooled to 16 K in the supersonic expansion. Dual laser beam detection and collisional collimation in the slit expansion yield sub-Doppler linewidths (60 MHz), an absolute frequency calibration of 13 MHz, and absorbance sensitivities within a factor of two of the shot-noise limit. Fully rovibrationally resolved direct absorption spectra of the CH₂ symmetric stretch mode (ν₂) are obtained and fitted to a Watson asymmetric top Hamiltonian with electron spin-rotation coupling, providing precision rotational constants and spin-rotation tensor elements for the vibrationally excited state. Analysis of the asymmetric top rotational constants confirms a vibrationally averaged planar geometry in both the ground- and first-excited vibrational levels. Sub-Doppler resolution permits additional nuclear spin hyperfine structures to be observed, with splittings in excellent agreement with microwave measurements on the ground state. Spectroscopic data on CH₂I facilitate systematic comparison with previous studies of halogen-substituted methyl radicals, with the periodic trends strongly correlated with the electronegativity of the halogen atom. Interestingly, we do not observe any asymmetric CH₂ stretch transitions, despite S/N ≈ 25:1 on strongest lines in the corresponding symmetric CH₂ stretch manifold. This dramatic reversal of the more typical 3:1 antisymmetric/symmetric CH₂ stretch intensity ratio signals a vibrational transition moment poorly described by simple "bond-dipole" models. Instead, the data suggest that this anomalous intensity ratio arises from "charge sloshing" dynamics in the highly polar carbon-iodine bond, as supported by ab initio electron differential density plots and indeed consistent with observations in other halomethyl radicals and protonated cluster ions.

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.