Atmospheric Chemistry of Criegee Intermediates: Unimolecular Reactions and Reactions with Water

Temperature-dependent kinetics of the atmospheric reaction between CH2OO and acetone

Criegee intermediates are important oxidants produced in the ozonolysis of alkenes in the atmosphere. Quantitative kinetics of the reactions of Criegee intermediates are required for atmospheric modeling. However, the experimental studies do not cover the full relevant range of temperature and pressure. Here we report the quantitative kinetics of CH₂OO + CH₃C(O)CH₃ by using our recently developed dual strategy that combines coupled cluster theory with high excitation levels for conventional transition state theory and well validated levels of density functional theory for direct dynamics calculations using canonical variational transition theory including tunneling. We find that the W3X-L//DF-CCSD(T)-F12b/jun-cc-pVDZ electronic structure method can be used to obtain quantitative kinetics of the CH₂OO + CH₃C(O)CH₃ reaction. Whereas previous investigations considered a one-step mechanistic pathway, we find that the CH₂OO + CH₃C(O)CH₃ reaction occurs in a stepwise manner. This has implications for the modeling of Criegee-intermediate reactions with other ketones and with aldehydes. In the kinetics calculations, we show that recrossing effects of the conventional transition state are negligible for determining the rate constant of CH₂OO + CH₃C(O)CH₃. The present findings reveal that the rate ratio between CH₂OO + CH₃C(O)CH₃ and OH + CH₃C(O)CH₃ has a significant negative dependence on temperature such that the CH₂OO + CH₃C(O)CH₃ reaction can contribute as a significant sink for atmospheric CH₃C(O)CH₃ at low temperature. The present findings should have broad implications in understanding the reactions of Criegee intermediates with carbonyl compounds and ketones in the atmosphere.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO₃ + HONO₂ reaction are pronounced below 1 bar. The SO₃ + HONO₂ reaction can be a potential removal reaction for SO₃ in the stratosphere and for HONO₂ in the troposphere, because the reaction can potentially compete well with the SO₃ + 2H₂O reaction between 25 and 35 km, as well as the OH + HONO₂ reaction. The present findings also suggest an unexpected new product from the SO₃ + HONO₂ reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO₃ reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm^-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate k_eff(E) on the order of 10⁸ s^-1 at ca. 3000 cm^-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 10⁶ s^-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Nitrous acid (HONO) as a sink of the simplest Criegee intermediate in the atmosphere

In the present work, we have investigated the reaction of nitrous acid with the simplest Criegee intermediate using chemical kinetics and quantum chemical calculations. It was found that reactions can occur through four different paths. Among them, one path involves hydrogen atom transfer and leads to the formation of hydroperoxymethyl nitrite, while two paths involve cycloaddition leading to the formation of ozonide and formic acid and the remaining path involves oxygen atom transfer leading to the formation of HNO₃ as a final product. Although there are various oxidation reactions of HONO present in the literature, which produce nitrogen dioxide as a final product, the possibility of in situ generation of HNO₃ and formic acid from HONO is reported for the first time. Nevertheless, the hydrogen atom transfer path, which leads to the formation of hydroperoxymethyl nitrite as a final product, was found to be the fastest, and hence dominating the Criegee reaction with HONO. By comparing the title reaction with other dominant Criegee reactions, it was found that although it will be more effective than Criegee oxidation by Cl˙ or ClO˙ and can compete with Criegee oxidation by OH˙ under special circumstances, it is negligible compared to the reaction of the Criegee intermediate with a water molecule.

Large Pressure Effects Caused by Internal Rotation in the s-cis-syn-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.

Pressure-Dependent Kinetics of o-Xylene Reaction with OH Radical

OH-initiated oxidation reactions of o-xylene are widely concerned both in combustion and atmospheric chemistry. In this work, the kinetics of o-xylene reaction with OH radical has been studied systematically in...

A computational study of the HO2 + SO3 → HOSO2 + 3O2 reaction catalyzed by water monomer, water dimer and small clusters of sulfuric acid: kinetics and atmospheric implications

Herein, the reaction mechanisms and kinetics for the HO2 + SO3 → HOSO2 + 3O2 reaction catalyzed by water monomer, water dimer and small clusters of sulfuric acid have been...

Unimolecular and water reactions of oxygenated and unsaturated Criegee intermediates under atmospheric conditions

Ozonolysis of unsaturated hydrocarbons (VOCs) is one of the main oxidation processes in the atmosphere. The stabilized Criegee intermediates (SCI) formed are highly reactive oxygenated species that potentially influence the...

Role of post-CCSD(T) corrections in predicting the energetics and kinetics of the OH • +O 3 reaction

The present work investigates the OH • +O 3 reaction by means of chemical kinetics and quantum chemical calculations. To predict the reaction barrier height and reaction energy, we have...

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

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

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

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

Experimental and Computational Studies of Criegee Intermediate syn-CH3CHOO Reaction with Hydrogen Chloride

Hydrogen chloride (HCl) contributes substantially to the atmospheric Cl; both species could affect the composition of Earth's atmosphere and the fate of pollutants. Here, we present the kinetics study for syn-CH₃CHOO reaction with HCl using experimental measurement and theoretical calculations. The experiment was conducted in a flow tube reactor at a pressure of 10 Torr and temperatures ranging from 283 to 318 K by using the OH laser-induced fluorescence (LIF) method. Transition-state theory and quantum chemistry calculations with QCISD(T) were used to calculate the rate coefficients. Weak negative temperature dependence was observed with a measured activation energy of -(2.98 ± 0.12) kcal mol^-1 and a calculated zero-point-corrected barrier energy of -3.29 kcal mol^-1. At 298 K, the rate coefficient was measured to be (4.77 ± 0.95) × 10^-11 cm³ s^-1, which was in reasonable agreement with 2.2 × 10^-11 cm³ s^-1 from the theoretical calculation.

Atmospheric ozonolysis of crotonaldehyde in the absence and presence of hydroxylated silica oligomer cluster adsorption

As one of the main components of combustion of tobacco products occurs (CARB), crotonaldehyde has an acute toxicity and widely exists in the atmosphere, which is harmful to human health. The removal efficiency of VOCs by ozonation can reach 80-90%. Based on the theory of quantum chemistry, the degradation mechanism, kinetics and toxicity of crotonaldehyde by ozonation in gas phase and heterogeneous phase were studied. Ozone was added to the olefins unsaturated double bond to form a five-membered ring primary ozonide, which was further fractured due to its unstable structure to form a Criegee intermediate and an aldehyde compound. The reaction rate constant of crotonaldehyde with ozone was 1.24 × 10^-17 cm³ molecule^-1 s^-1 at 298 K and 1 atm, which was an order of magnitude higher than the experimental value. From toxicity assessment, it was found that the ozonation of crotonaldehyde is beneficial to the removal of toxicity. Mineral dust aerosol exists in the atmosphere in large quantities, and SiO₂ is the most abundant component. VOCs are transformed into particle state on their surface through homogeneous nucleation and heterogeneous nucleation. Referring to the crystal structure of SiO₂, five hydroxylated silica oligomer cluster structures were simulated and the adsorption configurations of crotonaldehyde on their surface were simulated. The adsorption of crotonaldehyde on the surface of the clusters was achieved by forming hydrogen bonds and had good adsorption effects. The adsorption of hydroxylated silica oligomer clusters didn't change the ozonation mechanism of crotonldehyde, but had a certain effect on the reaction rate.

Dynamics of Reaction CH3CHI + O2 Investigated via Infrared Emission of Products CO, CO2, and OH

The reaction CH₃CHI + O₂ has been commonly employed in laboratories to produce a methyl-substituted Criegee intermediate CH₃CHOO, but the detailed dynamics of this reaction remain unexplored. We carried out this reaction by irradiating a flowing mixture of CH₃CHI₂ (∼70 mTorr) and O₂ (∼4 and 8 Torr) at 308 or 248 nm and observed infrared emission of the products with a step-scan Fourier-transform spectrometer. Upon irradiation at 248 nm with O₂ ∼4 Torr, a Boltzmann distribution of CO (v ≤ 4, J ≤ 25) with average vibrational energy (12 ± 2) kJ mol^-1 and of OH (v = 1, J ≤ 5.5) were observed and assigned to be produced from the decomposition of CH₃C(O)OH* to form CO + CH₃OH and OH + CH₃CO, respectively. The observed broadband emission of CO₂ was simulated with two vibrational distributions of average energies (42 ± 3) and (114 ± 6) kJ mol^-1 and assigned to be produced from the decomposition of CH₃C(O)OH* and (methyl dioxirane)*, respectively. The results upon irradiation of the sample at 308 nm are similar, likely indicating a small fraction of energy partition into these products and rapid thermalization of CH₃CHI*. Compared with reaction CH₂I + O₂, the title reaction yielded products with much less internal excitation, consistent with the expectation that these observed products receive much less fraction of available energy upon fragmentation when an additional methyl moiety was present in the parent. The large-v component of CO observed in experiments of CH₂I + O₂ at 248 nm, produced from secondary reaction HCO + O₂, was absent in this work because the corresponding secondary reaction CH₃CO + O₂ in decomposition of CH₃CHOO* produces α-lactone + OH or H₂CO + CO + OH.

Substituent Effect in the Reactions between Criegee Intermediates and 3-Aminopropanol

Via intramolecular H atom transfer, 3-aminopropanol is more reactive toward Criegee intermediates, in comparison with amines or alcohols. Here we accessed the substituent effect of Criegee intermediates in their reactions with 3-aminopropanol. Through real-time monitoring the concentrations of two Criegee intermediates with their strong UV absorption at 340 nm, the experimental rate coefficients at 298 K (100-300 Torr) were determined to be (1.52 ± 0.08) × 10^-11 and (1.44 ± 0.22) × 10^-13 cm³ s^-1 for the reactions of 3-aminopropanol with (CH₃)₂COO (acetone oxide) and CH₂CHC(CH₃)OO (methyl vinyl ketone oxide), respectively. Compared to our previous experimental value for the reaction with syn-CH₃CHOO, (1.24 ± 0.13) × 10^-11 cm³ s^-1, we can see that the methyl substitution at the anti position has little effect on the reactivity while the vinyl substitution causes a drastic decrease in the reactivity. Our theoretical calculations based on CCSD(T)-F12 energies reproduce this 2-order-of-magnitude decrease in the rate coefficient caused by the vinyl substitution. Using the activation strain model, we found that the interaction of Criegee intermediates with 3-aminopropanol is weaker for the case of vinyl substitution. This effect can be further rationalized by the delocalization of the lowest unoccupied molecular orbital for the vinyl-substituted Criegee intermediates. These results would help us better estimate the impact of similar reactions like the reactions of Criegee intermediates with water vapor, some of which could be difficult to measure experimentally but can be important in the atmosphere. We also found that the B2PLYP-D3BJ/aug-cc-pVTZ calculation can reproduce the CCSD(T)-F12 reaction barrier energies within ca. 1 kcal mol^-1, indicating that the use of the B2PLYP-D3BJ method is promising for future predictions of the reactions of larger Criegee intermediates.

Atmospheric Ring-Closure and Dehydration Reactions of 1,4-Hydroxycarbonyls in the Gas Phase: The Impact of Catalysts

1,4-Hydroxycarbonyls can potentially undergo sequential reactions involving cyclization followed by dehydration to form dihydrofurans. As dihydrofurans contain a double bond, they are highly reactive toward atmospheric oxidants such as OH, O₃, and NO₃. In the present study, we use ab initio calculations to examine the impact of various atmospheric catalysts on the energetics and kinetics of the gas-phase cyclization and dehydration reaction steps associated with 4-hydroxybutanal, a prototypical 1,4-hydroxycarbonyl molecule. The cyclization step transforms 4-hydroxybutanal into 2-hydroxytetrahydrofuran, which can subsequently undergo dehydration to form 2,3-dihydrofuran. As the barriers associated with the cyclization and dehydration steps for 4-hydroxybutanal are, respectively, 34.8 and 63.0 kcal/mol in the absence of a catalyst, both reaction steps are inaccessible under atmospheric conditions in the gas phase. However, the presence of a suitable catalyst can significantly reduce the reaction barriers, and we have examined the impact of a single molecule of H₂O, HO₂ radical, HC(O)OH, HNO₃, and H₂SO₄ on these reactions. We find that H₂SO₄ reduces the reaction barriers the greatest, with the barrier for the cyclization step being reduced to -13.1 kcal/mol and that for the dehydration step going down to 9.2 kcal/mol, measured relative to their respective separated starting reactants. Interestingly, our kinetic study shows that HNO₃ gives the fastest rate due to the combined effects of a larger atmospheric concentration and a reduced barrier. Thus, our study suggests that, with acid catalysis, the cyclization reaction step can readily occur for 1,4-hydroxycarbonyls in the gas phase. Because the dehydration step exhibits a significant barrier even with acid catalysis, the 2-hydroxytetrahydrofuran products, once formed, are likely lost through their reaction with OH radicals in the atmosphere. We have investigated the reaction pathways and the rate constant for this bimolecular reaction in the presence of excess molecular oxygen (³O₂), as it would occur under tropospheric conditions, using computational chemistry over the 200-300 K temperature range. We find that the main products from these OH-initiated oxidation reactions are succinaldehyde + HO₂ and 2,3-dihydro-2-furanol + HO₂.

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.

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.

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

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

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

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

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

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

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.

Atmospheric Chemistry of CH2OO: The Hydrolysis of CH2OO in Small Clusters of Sulfuric Acid

The hydrolysis of CH₂OO is not only a dominant sink for the CH₂OO intermediate in the atmosphere but also a key process in the formation of aerosols. Herein, the reaction mechanism and kinetics for the hydrolysis of CH₂OO catalyzed by the precursors of atmospheric aerosols, including H₂SO₄, H₂SO₄···H₂O, and (H₂SO₄)₂, have been studied theoretically at the CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2df,2pd) level. The calculated results show that the three catalysts decrease the energy barrier by over 10.3 kcal·mol^-1; at the same time, the product formation of HOCH₂OOH is more strongly bonded to the three catalysts than to the reactants CH₂OO and H₂O, revealing that small clusters of sulfuric acid promote the hydrolysis of CH₂OO both kinetically and thermodynamically. Kinetic simulations show that the H₂SO₄-assisted reaction is more favorable than the H₂SO₄···H₂O- (the pseudo-first-order rate constant being 27.9-11.5 times larger) and (H₂SO₄)₂- (between 2.8 × 10⁴ and 3.4 × 10⁵ times larger) catalyzed reactions. Additionally, due to relatively lower concentration of H₂SO₄, the hydrolysis of CH₂OO with H₂SO₄ cannot compete with the CH₂OO + H₂O or (H₂O)₂ reaction within the temperature range of 280-320 K, since its pseudo-first-order rate ratio is smaller by 4-7 or 6-8 orders of magnitude, respectively. However, the present results provide a good example of how small clusters of sulfuric acid catalyze the hydrolysis of an important atmospheric species.

The effect of ammonia and formic acid on the oxidation of CO via a simple Criegee intermediate

In the present work, we have investigated the effect of catalysts (ammonia, formic acid, ammonia dimer, and ammonia water complex) on the oxidation of CO via a simple Criegee intermediate by means of kinetics and quantum chemical calculations. Our finding suggests that, in the presence of ammonia and ammonia dimer the title reaction becomes a barrierless reaction with respect to the isolated reactants (energy barrier = ∼-0.53 and ∼-0.27 kcal mol-1, respectively), whereas in the presence of formic acid and ammonia-water complex the energy barrier of the CI + CO reaction becomes ∼2.84 and ∼0.82 kcal mol-1, respectively. However, among all the catalysts, due to the very low concentration of the ammonia dimer, its contribution towards the title reaction is insignificant as compared to that of the other catalysts. In addition, the relative rate of the other catalyzed channels against the uncatalyzed reaction suggests that the rate of the catalyzed CI + CO reaction is ∼8-10 orders of magnitude lower than the uncatalyzed reaction. However, the concentration of bimolecular complexes formed in the presence of catalysts (except the ammonia dimer) is ∼1-8 orders of magnitude higher than the concentration of bimolecular complexes formed in the uncatalyzed reaction.

Coupled Cluster Externally Corrected by Adaptive Configuration Interaction

An externally corrected coupled cluster (CC) method, where an adaptive configuration interaction (ACI) wave function provides the external cluster amplitudes, named ACI-CC, is presented. By exploiting the connection between configuration interaction and CC through cluster analysis, the higher-order T₃ and T₄ terms obtained from ACI are used to augment the T₁ and T₂ amplitude equations from traditional CC. These higher-order contributions are kept frozen during the CC iterations and do not contribute to an increased cost with respect to coupled cluster including the single and double excitations (CCSD). We have benchmarked this method on three closed-shell systems: beryllium dimer, carbonyl oxide, and cyclobutadiene, with good results compared to other corrected CC methods. In all cases, the inclusion of these external corrections improved upon the "gold standard" CCSD(T) results, indicating that ACI-CCSD(T) can be used to assess strong correlation effects in a system and as an inexpensive starting point for more complex external corrections.

Reactivity of aromatic contaminants towards nitrate radical in tropospheric gas and aqueous phase

Aromatic compounds (ACs) give a substantial contribution to the anthropogenic emissions of volatile organic compounds. Nitrate radicals (NO₃) are significant oxidants in the lower troposphere during nighttime, with concentrations of (2-20) × 10⁸ molecules cm^-3. In this study, the tropospheric gas and liquid phase reactions of ACs with nitrate radical are investigated using theoretical computational methods, which can give a deep insight into the reaction mechanisms and kinetics. Results show that the reactivity of ACs with nitrate radicals decreases as the electron donating characteristics of the functional group on the ACs decrease, as ΔG^≠ of the reaction with NO₃ increasing from -1.17 to 17.84 kcal mol^-1. The reaction of NO₃ towards ACs in the aqueous phase is more preferable, with the atmospheric lifetime 0.07-1281 min. An assessment of the aquatic toxicity of ACs and their degradation products indicated that the risk of their degradation products remains and should be given more attention.

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.

Effect of water on the oxidation of CO by a Criegee intermediate

The present work employs the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory to investigate the effect of a water monomer and dimer on the oxidation of carbon-monoxide by a Criegee intermediate (CH2OO). The present work suggests that in the presence of a water monomer the energy barrier of the title reaction reduced to ∼3.4 kcal mol-1 from the corresponding uncatalyzed barrier (∼12.4 kcal mol-1), whereas, in the presence of a water dimer it became as low as ∼-3.2 kcal mol-1. It has also been found that, in the presence of catalysts, additional channels become available from which the title reaction can proceed. The estimated values of rate constants suggest that within the temperature range of 210-320 K, the effective bimolecular rate constant for the water monomer catalyzed channel is 10 to 100 times lower than the bimolecular rate constant of the uncatalyzed channel, whereas in the case of the water dimer it is ∼5-10 times higher than that of the uncatalyzed channel.

Singlet (1Δg) O2 as an efficient tropospheric oxidizing agent: the gas phase reaction with the simplest Criegee intermediate

The reaction between CH₂OO and ¹Δ_g O₂ has been investigated by means of high level quantum chemical and chemical kinetic calculations. Post-CCSD(T) corrections in terms of full triplets and partial quadratic excitations, along with core corrections have been employed to estimate the reaction energetics. The title reaction was found to be effectively barrierless with the transition state lying -22.85 kcal mol^-1 below the isolated reactants. Rate coefficients under tropospheric conditions have been calculated using the master equation. The calculated rate coefficient was found to be marginally over the gas kinetic limit, implying that the reaction rate would be limited by the upper limit of bimolecular collision frequency. When compared against ˙OH and O₃, ¹O₂ was found to compete efficiently with the two well known tropospheric oxidants.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH₃ CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH₂ group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

Computational mechanistic study of the unimolecular dissociation of ethyl hydroperoxide and its bimolecular reactions with atmospheric species

A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH₂OO with methane has been performed using the density functional theory (DFT) method and high-level calculations. Solvation models were utilized to address the effect of water molecules on prominent reaction steps and their associated energies. The structures of all proposed mechanisms were optimized using B3LYP functional with several basis sets: 6-31G(d), 6-31G (2df,p), 6-311++G(3df,3pd) and at M06-2X/6-31G(d) and APFD/6-31G(d) levels of theory. Furthermore, all structures were optimized at the B3LYP/6-311++G(3df,3pd) level of theory. The intrinsic reaction coordinate (IRC) analysis was performed for characterizing the transition states on the potential energy surfaces. Fifteen different mechanistic pathways were studied for the reaction of Criegee intermediate with methane. Both thermodynamic functions (ΔH and ΔG), and activation parameters (activation energies E_a, enthalpies of activation ΔH^ǂ, and Gibbs energies of activation ΔG^ǂ) were calculated for all pathways investigated. The individual mechanisms for pathways A1, A2, B1, and B2, comprise two key steps: (i) the formation of ethyl hydroperoxide (EHP) accompanying with the hydrogen transfer from the alkanes to the terminal oxygen atom of CIs, and (ii) a following unimolecular dissociation of EHP. Pathways from C1 → H1 involve the bimolecular reaction of EHP with different atmospheric species. The photochemical reaction of methane with EHP (pathway E1) was found to be the most plausible reaction mechanism, exhibiting an overall activation energy of 7 kJ mol⁻¹, which was estimated in vacuum at the B3LYP/6-311++G(3df,3pd) level of theory. All of the reactions were found to be strongly exothermic, expect the case of the sulfur dioxide-involved pathway that is predicted to be endothermic. The solvent effect plays an important role in the reaction of EHP with ammonia (pathway F1). Compared with the gas phase reaction, the overall activation energy for the solution phase reaction is decreased by 162 and 140 kJ mol⁻¹ according to calculations done with the SMD and PCM solvation models, respectively.

Kinetic Studies for the Reaction of syn-CH3CHOO with CF3CH═CH2

Hydrofluoroolefins (HFOs, C_xF_2x+1CH═CH₂) have great potential to replace hydrofluorocarbons (HFCs) as refrigerants. Here the kinetics for the reaction of syn-CH₃CHOO with CF₃CH═CH₂ (HFO-1243zf), the simplest of HFOs, have been studied in a flash photolysis flow reactor at a total pressure of 50 Torr, by using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 283 to 318 K. A weak positive temperature dependence was observed, with an activation energy of 1.41 ± 0.12 kcal mol^-1. At 298 K, the measured rate coefficient was (2.42 ± 0.51) × 10^-14 cm³ s^-1, in the vicinity of the previously reported upper limit value for the reaction of CH₂OO with CF₃CH═CH₂.

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.

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.

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.