Perspective on Theoretical and Experimental Advances in Atmospheric Photochemistry

Research that explores the chemistry of Earth's atmosphere is central to the current understanding of global challenges such as climate change, stratospheric ozone depletion, and poor air quality in urban areas. This research is a synergistic combination of three established domains: earth observation, for example, using satellites, and in situ field measurements; computer modeling of the atmosphere and its chemistry; and laboratory measurements of the properties and reactivity of gas-phase molecules and aerosol particles. The complexity of the interconnected chemical and photochemical reactions which determine the composition of the atmosphere challenges the capacity of laboratory studies to provide the spectroscopic, photochemical, and kinetic data required for computer models. Here, we consider whether predictions from computational chemistry using modern electronic structure theory and nonadiabatic dynamics simulations are becoming sufficiently accurate to supplement quantitative laboratory data for wavelength-dependent absorption cross-sections, photochemical quantum yields, and reaction rate coefficients. Drawing on presentations and discussions from the CECAM workshop on Theoretical and Experimental Advances in Atmospheric Photochemistry held in March 2024, we describe key concepts in the theory of photochemistry, survey the state-of-the-art in computational photochemistry methods, and compare their capabilities with modern experimental laboratory techniques. From such considerations, we offer a perspective on the scope of computational (photo)chemistry methods based on rigorous electronic structure theory to become a fourth core domain of research in atmospheric chemistry.

A white cell based broadband transient UV-vis absorption spectroscopy with pulsed laser photolysis reactors for chemical kinetics under variable temperatures and pressures

UV-vis spectroscopy is widely used for kinetic studies in physical chemistry, as species' absolute cross-sections are usually less sensitive to experimental conditions (i.e., temperature and pressure). Here, we present the design and characterization of a multipass UV-vis absorption spectroscopy white cell coupled to a pulsed-laser photolysis flow reactor. The glass reactor was designed to facilitate studies of gas phase chemical reactions over a range of conditions (239-293 K and 10-550 Torr). Purged windows mitigate contamination from chemical precursors and photolysis products. We report the measured impact of this purging on temperature uniformity and the absorption length and present some supporting flow calculations. The combined optical setup is unique and enables the photolysis laser to be coaligned with a well-defined absorption pathlength probe beam. This alignment leverages the use of one long-pass filter to increase the spectrum flatness and increase the light intensity vs other systems that use two dichroic mirrors. The probe beam is analyzed with a dual exit spectrograph, customized to split the light between an intensified CCD and photomultiplier tube, enabling simultaneous spectrum and single wavelength detection. This multipass system yields a pathlength of ∼450 cm and minimum observable concentrations of ∼3.7 × 1011 molecule cm-3 (assuming cross-sections ∼1.2 × 10-17 cm2). The temperature profile across the reaction region is ±2 K, defined by the worst-case temperature of 239 K, validated by measurements of the N2O4 equilibrium constant. Finally, the system is implemented to study the simplest Criegee intermediate, demonstrating the instrument performance and advantages of simultaneous spectrum and temporal profile measurements.

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

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

Full-dimensional automated potential energy surface development and detailed dynamics for the CH2OO + NH3 reaction

With the help of the ROBOSURFER program package, a global full-dimensional potential energy surface (PES) for the reaction of the Criegee intermediate, CH2OO, with the NH3 molecule is developed iteratively using different ab initio methods and the monomial symmetrization fitting approach. The final permutationally-invariant analytical PES is constructed based on 23447 geometries and the corresponding ManyHF-based CCSD(T)-F12b/cc-pVTZ-F12 energies. The accuracy of the PES is confirmed by the excellent agreement of its stationary-point properties and one-dimensional potential energy curves compared with the corresponding ab initio data. The reaction probabilities and integral cross sections are calculated for the ground-state and several vibrationally excited-state reactions by quasi-classical trajectory simulations. Remarkable is that the maximum impact parameter b where reactivity vanishes is almost independent of collision energy ranging from 1 to 40 kcal mol-1, and the reaction probability increases with increasing collision energy for this negative-barrier reaction. At the same time, a slight mode-specificity effect is observed. In addition, the deuterium effect is investigated and the sudden vector projection is discussed.

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

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt = -kobs[CH2OO]; kobs = k0 + k1[water] + k2[water]2 + k3[water]3; k1 = (4.22 ± 0.48) × 10-16 cm3 s-1, k2 = (10.66 ± 0.83) × 10-33 cm6 s-1, k3 = (1.48 ± 0.17) × 10-50 cm9 s-1 at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1 = 1.8 ± 1.1 kcal mol-1, Ea2 = -11.1 ± 2.1 kcal mol-1, Ea3 = -17.4 ± 3.9 kcal mol-1. The contribution of the k3[water]3 term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction.

The simplest Criegee intermediate CH2OO reaction with dimethylamine and trimethylamine: kinetics and atmospheric implications

We have used the OH laser-induced fluorescence (LIF) method to measure the kinetics of the simplest Criegee intermediate (CH2OO) reacting with two abundant amines in the atmosphere: dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Our experiments were conducted under pseudo-first-order approximation conditions. The rate coefficients we report are (2.15 ± 0.28) × 10-11 cm3 molecule-1 s-1 for (CH3)2NH at 298 K and 10 Torr, and (1.56 ± 0.23) × 10-12 cm3 molecule-1 s-1 for (CH3)3N at 298 K and 25 Torr with Ar as the bath gas. Both reactions exhibit a negative temperature dependence. The activation energy and pre-exponential factors derived from the Arrhenius equation were (-2.03 ± 0.26) kcal mol-1 and (6.89 ± 0.90) × 10-13 cm3 molecule-1 s-1 for (CH3)2NH, and (-1.60 ± 0.24) kcal mol-1 and (1.06 ± 0.16) × 10-13 cm3 molecule-1 s-1 for (CH3)3N. We propose that the electronegativity of the atom in the co-reactant attached to the C atom of CH2OO, in addition to the dissociation energy of the fragile covalent bonds with H atoms (H-X bond), plays an important role in the 1,2-insertion reactions. Under certain circumstances, the title reactions can contribute to the sink of amines and Criegee intermediates and to the formation of secondary organic aerosol (SOA).

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

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

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

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

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

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

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

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

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.

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.

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

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

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.

Exploring The Sequence of Electron Density Along The Chemical Reactions Between Carbonyl Oxides And Ammonia/Water Using Bond Evolution Theory

The molecular mechanism of the reactions between four carbonyl oxides and ammonia/water are investigated using the M06-2X functional together with 6-311++G(d,p) basis set. The analysis of activation and reaction enthalpy shows that the exothermicity of each process increased with the substitution of electron donating substituents (methyl and ethenyl). Along each reaction pathway, two new chemical bonds C-N/C-O and O-H are expected to form. A detailed analysis of the flow of the electron density during their formation have been characterized from the perspective of bonding evolution theory (BET). For all reaction pathways, BET revealed that the process of C-N and O-H bond formation takes place within four structural stability domains (SSD), which can be summarized as follows: the depopulation of V(N) basin with the formation of first C-N bond (appearance of V(C,N) basin), cleavage of N-H bond with the creation of V(N) and V(H) monosynaptic basin, and finally the V(H,O) disynaptic basin related to O-H bond. On the other hand, in the case of water, the cleavage of O-H bond with the formation of V(O) and V(H) basins is the first stage, followed by the formation of the O-H bond as a second stage, and finally the creation of C-O bond.

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.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O₃, and Cl atom, HONO, NO₃, N₂O₅, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO₃, N₂O₅, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

Microcanonical Tunneling Rates from Density-of-States Instanton Theory

Semiclassical instanton theory is a form of quantum transition-state theory which can be applied to the computation of thermal reaction rates in complex molecular systems including quantum tunneling effects. There have been a number of attempts to extend the theory to treat microcanonical rates. However, the previous formulations are either computationally unfeasible for large systems due to an explicit sum over states or they involve extra approximations, which make them less reliable. We propose a robust and practical microcanonical formulation called density-of-states instanton theory, which avoids the sum over states altogether. In line with the semiclassical approximations inherent to the instanton approach, we employ the stationary-phase approximation to the inverse Laplace transform to obtain the densities of states. This can be evaluated using only post-processing of the data available from a small set of instanton calculations, such that our approach remains computationally efficient. We show that the new formulation predicts results that agree well with quantum scattering theory for an atom-diatom reaction and with experiments for a photoexcited unimolecular hydrogen transfer in a Criegee intermediate. When the thermal rate is evaluated from a Boltzmann average over our new microcanonical formalism, it can overcome some problems of conventional instanton theory. In particular, it predicts a smooth transition at the crossover temperature and is able to describe bimolecular reactions with pre-reactive complexes such as CH₃OH + OH.

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.

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.

Effect of multifunctional compound monoethanolamine on Criegee intermediates reactions and its atmospheric implications

The reactions of Criegee intermediates with trace gases (such as alcohols, amines, and acids) are primarily dependent on the trace gases' functional group activity. In this study, we used density functional theory calculations and ab initio dynamics simulation methods to explore the synergistic effect of NH₂ and OH groups, in the multifunctional compound monoethanolamine (MEA), on the Criegee reaction. The results showed that among the four evaluated MEA configurations, two functional groups in the g'Gg' and tGg' configurations, -NH₂ and -OH, have the synergistic effect on the C2 stabilized Criegee intermediates (sCIs). At the gas-liquid interface, sCIs react with NH₂ groups of MEA molecules directly or are mediated by water molecules, resulting in additional product formation. The rate calculation indicated that the reaction of sCIs with NH₂ groups of MEA molecules is prior to that with OH groups. In addition, OH groups promote the reactions between sCIs and NH₂ groups of MEA, while the presence of NH₂ groups weakens the reactions of sCIs and OH groups of MEA to some extent. At 298 K, the total rate constant of anti-CH₃CHOO with NH₂ group of MEA is 4.26 × 10^-11 cm³ molecule^-1 s^-1, which is four orders of magnitude higher than that of anti-CH3CHOO hydration. Under low humidity conditions, the reactions between sCIs and MEA could contribute to the removal of sCIs.

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.

Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

Reactions involving Criegee intermediates (CIs, R₁ R₂ COO) are important in atmospheric ozonolysis models. In recent years, density functional theory (DFT) and CCSD(T)-based ab initio methods are increasingly being used for modeling reaction profiles involving CIs. We obtain highly accurate CCSDT(Q)/CBS reaction energies and barrier heights for ring-closing reactions involving atmospherically important CIs (R₁ /R₂ = H, Me, OH, OMe, F, CN, cyclopropene, ethylene, acetaldehyde, and acrolein). We use this benchmark data to evaluate the performance of DFT, double-hybrid DFT (DHDFT), and ab initio methods for the kinetics and thermodynamics of these reactions. We find that reaction energies are more challenging for approximate theoretical procedures than barrier heights. Overall, taking both reaction energies and barrier heights into account, only one of the 58 considered DFT methods (the meta-GGA MN12-L) attains near chemical accuracy, with root-mean-square deviations (RMSDs) of 3.5 (barrier heights) and 4.7 (reaction energies) kJ mol^-1 . Therefore, MN12-L is recommended for investigations where CCSD(T)-based methods are not computationally feasible. For reaction barrier heights performance does not strictly follow Jacob's Ladder, for example, DHDFT methods do not perform better than conventional DFT methods. Of the ab initio methods, the cost-effective CCSD(T)/CBS(MP2) approach gives the best performance for both reaction energies and barrier heights, with RMSDs of 1.7 and 1.4 kJ mol^-1 , respectively. All the considered Gaussian-n methods show good performance with RMSDs below the threshold of chemical accuracy for both reaction energies and barrier heights, where G4(MP2) shows the best overall performance with RMSDs of 2.9 and 1.5 kJ mol^-1 , respectively. © 2019 Wiley Periodicals, Inc.

The atmospheric importance of methylamine additions to Criegee intermediates

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

Theoretical investigation on the reaction mechanism and kinetics of a Criegee intermediate with ethylene and acetylene

The competition among the possible pathways, the branching ratios of the adduct and the decomposition products at different temperatures and pressures have been evaluated.

Effects of water vapor on the reaction of CH2OO with NH3

A strong synergic effect of water and ammonia molecules may enhance the formation of H₂NCH₂OOH.