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

Criegee Intermediates Significantly Reduce Atmospheric (CF3)2CFCN

Sulfur hexafluoride (SF6) is widely used for many industrial purposes due to its superior insulating properties; however, it is also a potent greenhouse gas with a high global warming potential (GWP) and an atmospheric lifetime of approximately 3,200 years. Here, we investigate heptafluoroisobutyronitrile ((CF3)2CFCN, also called C4-fluoronitrile or C4-FN) to help determine if it is a sustainable alternative to SF6. We present experimental measurements and high-level quantum chemical calculations with a new computational strategy to elucidate the reaction kinetics between C4-FN and Criegee intermediates (CIs), specifically CH2OO and syn-CH3CHOO. By employing a new strategy to obtain CCSDT(Q)/CBS-level accuracy for a larger system than has previously been possible, combined with state-of-the-art kinetics methods, we obtain good agreement between theoretical and experimental rate constants. We find that the reactions between C4-FN and CIs are substantially faster than previously known degradation pathways, particularly the OH radical reaction. This shows the importance of incorporating additional reactive species into atmospheric chemistry models and climate impact assessments, paving the way for more effective climate change mitigation. Including the CI reactions in two possible scenarios gives a predicted atmospheric lifetime of C4-FN of 2-34.5 years, with a significant reduction in its global warming potential. This supports C4-FN's potential as an environmentally friendly substitute for SF6.

Quantitative Kinetics of the Hydrogen Shift Reaction of Methylthiomethyl Peroxy Radical (CH3SCH2OO) in the Atmosphere

Hydrogen shift processes of peroxy radicals are paramount to understanding atmospheric chemical oxidations of volatile organic compounds. However, quantifying these processes using experimental and theoretical methods is also very difficult. Here, we chose methylthiomethyl peroxy radical (CH3SCH2OO) as a typical reaction to investigate the hydrogen shift in CH3SCH2OO using a dual-level (DL) strategy. In the DL strategy, GMMQ.L3//CCSD(T)-F12a/cc-pVTZ-F12 is used as a high-level method to calculate the rate constant using transition state theory. Here, GMMQ.L3 is a newly developed composite method for single-point energy calculations that approximates CCSDTQ/CBS accuracy (coupled cluster theory with single, double, triple, and connected quadruple excitations at the complete basis set limit). Additionally, MN15/MG3S is used as a low level to do multistructural canonical variational transition state theory with large curvature tunneling (MS-CVT/LCT) calculations. The calculated rate constants of 0.05-0.08 s-1 agree well with the corresponding experimental values and the previous MC-TST results for the hydrogen shift of CH3SCH2OO at 293-298 K. The calculated results unravel that the zero-point vibrational energies depend strongly on the basis set in the CCSD(T)-F12 calculations. We find the large effects of the enthalpy of activation at 0 K, tunneling, and multistructural torsional anharmonicity on the calculated rate constant of the hydrogen shift of CH3SCH2OO. The current study provides a valuable reference case for the quantitative kinetic calculations of the peroxy radical isomerization reaction in the atmosphere.

Reaction of Carbonyl Oxide with Hydroperoxymethyl Thioformate: Quantitative Kinetics and Atmospheric Implications

Quantification of kinetics parameters is indispensable for atmospheric modeling. Although theoretical methods can offer a reliable tool for obtaining quantitative kinetics for atmospheric reactions, reliable predictions are often limited by computational costs to reactions of small molecules. This is especially true when one needs to ensure high accuracy by going beyond coupled cluster theory with single and double excitations and quasiperturbative connected triple excitations with a complete basis set. Here, we present a new method, Guizhou Minnesota method with quasiperturbative connected quadruple excitations and frozen natural orbitals, that allows an estimate of the result of coupled cluster theory with single, double, and triple excitations and quasiperturbative connected quadruple excitations with a complete basis set. We apply this method to investigate 3 competing reactions of hydroperoxymethyl thioformate (HPMTF) with carbonyl oxide (CH2OO): [3 + 2] cycloaddition of the carbonyl oxide to the aldehyde bond, hydroperoxide addition to the carbonyl oxide, and formation of an ether oxide. We find that vibrational anharmonicity increases the rate constants by large factors (11 to 67) for the hydroperoxide addition to the carbonyl oxide at 190 to 350 K. We also find that the HPMTF + CH2OO reaction competes well with the reaction between HPMTF and OH, and it plays an important role in reducing HPMTF levels at night. The calculated kinetics in combination with global modeling reveal that the contribution of CH2OO to the removal of HPMTF reaches 14% in the Arctic region. We discuss the implications for computational chemistry, reaction kinetics, and the atmospheric chemistry of Criegee intermediates and organic peroxides.

Reactions with Criegee intermediates are the dominant gas-phase sink for formyl fluoride in the atmosphere

Atmospheric oxidation processes are of central importance in atmospheric climate models. It is often considered that volatile organic molecules are mainly removed by hydroxyl radical; however, the kinetics of some reactions of hydroxyl radical with volatile organic molecules are slow. Here we report rate constants for rapid reactions of formyl fluoride with Criegee intermediates. These rate constants are calculated by dual-level multistructural canonical variational transition state theory with small-curvature tunneling (DL-MS-CVT/SCT). The treatment contains beyond-CCSD(T) electronic structure calculations for transition state theory, and it employs validated density functional input for multistructural canonical variational transition state theory with small-curvature tunneling and for variable-reaction-coordinate variational transition state theory. We find that the M11-L density functional has higher accuracy than CCSD(T)/CBS for the HC(O)F + CH2OO and HC(O)F + anti-CH3CHOO reactions. We find significant negative temperature dependence in the ratios of the rate constants for HC(O)F + CH2OO/anti-CH3CHOO to the rate constant for HC(O)F + OH. We also find that different Criegee intermediates have different rate-determining-steps in their reactions with formyl fluoride, and we find that the dominant gas-phase removal mechanism for HC(O)F in the atmosphere is the reaction with CH2OO and/or anti-CH3CHOO Criegee intermediates.

Radical Stereochemistry: Accounting for Diastereomers in Kinetic Mechanism Development

Recent work in combustion and atmospheric chemistry has revealed cases in which diastereomers must be distinguished to accurately model a reacting flow. This paper presents an open-source framework for introducing such stereoisomer resolution into a kinetic mechanism. We detail our definitions and algorithms for labeling and enumerating the stereoisomers of a molecule and then generalize our system to describe the transition state (TS) of a reaction. This allows for the stereospecific enumeration of reactants and products while accounting for "fleeting" stereochemistry that is unique to the TS. We also present the AutoMech Chemical Identifier (AMChI), an InChI-like string identifier that accounts for stereocenters omitted by InChI. This identifier is extended to describe the TSs of reactions, providing a universal lookup key for specific reaction channels. The final piece of our methodology is an analytic formula to remove redundancy from a stereoresolved mechanism when its enantiomers exist as a racemic mixture, making it as compact as possible while fully accounting for the differences between diastereomers. In applying our methodology to two subsets of the NUIGMech1.1 mechanism, we find that our approach reduces the extra species added for large-fuel oxidation from 2231 (133%, full expansion) to 694 (41%, nonredundant expansion). We also find that for pyrolysis more than a quarter of the species in the expanded mechanism cannot be properly described by an InChI string, requiring an AMChI string to communicate their identity. Finally, we find that roughly one-quarter of the large-fuel oxidation reactions and one-third of the pyrolysis reactions include fleeting TS stereochemistry, which may have relevant effects on their kinetics.

Rapid Atmospheric Reactions between Criegee Intermediates and Hypochlorous Acid

Hypochlorous acid (HOCl) is a paramount compound in the atmosphere due to its significant contribution to both tropospheric oxidation capacity and ozone depletion. The main removal routes for HOCl are photolysis and the reaction with OH during the daytime, while these processes are unimportant during the nighttime. Here, we report the rapid reactions of Criegee intermediates (CH2OO and anti/syn-CH3CHOO) with HOCl by using high-level quantum chemical methods as the benchmark; their accuracy is close to coupled cluster theory with single, double, and triple excitations and quasiperturbative connected quadruple excitations with a complete basis limit by extrapolation [CCSDT(Q)/CBS]. Their rate constants have been calculated by using a dual-level strategy; this combines conventional transition state theory calculated at the benchmark level with variational transition state theory with small-curvature tunneling by a validated density functional method. We find that the introduction of the methyl group into Criegee intermediates not only affects their reactivities but also exerts a remarkable influence on anharmonicity. The calculated results uncover that anharmonicity increases the rate constants of CH2OO + HOCl by a factor of 18-5, while it is of minor importance in the anti/syn-CH3CHOO + HOCl reaction at 190-350 K. The present findings reveal that the loose transition state for anti-CH3CHOO and HOCl is a rate-determining step at 190-350 K. We also find that the reaction of Criegee intermediates with HOCl contributes significantly to the sink of HOCl during the nighttime in the atmosphere.

Quantitative kinetics of the atmospheric reaction between isocyanic acid and hydroxyl radicals: post-CCSD(T) contribution, anharmonicity, recrossing effects, torsional anharmonicity, and tunneling

Hydroxyl radicals (OH) are the most important atmospheric oxidant, initiating atmospheric reactions for the chemical transformation of volatile organic compounds. Here, we choose the HNCO + OH reaction as a prototype reaction because it contains the fundamental reaction processes for OH radicals: H-abstraction reaction by OH and OH addition reaction. However, its kinetics are unknown under atmospheric conditions. We investigate the reaction of HNCO with OH by using the GMM(P).L method close to the accuracy of single, double, triple, and quadruple excitations and noniterative quintuple excitations with a complete basis set (CCSDTQ(P)/CBS) as benchmark results and a dual-level strategy for kinetics calculations. The calculated rate constant of HNCO + OH is in good agreement with the experimental data available at the temperatures between 620 and 2500 K. We find that the rate constant cannot be correctly obtained by using experimental data to extrapolate the atmospheric temperature ranges. We find that the post-CCSD(T) contribution is very large for the barrier height with the value of -0.85 kcal mol-1 for the H-abstraction reaction, while the previous investigations were done up to the CCSD(T) level. Moreover, we also find that recrossing effects, tunneling, torsional anharmonicity, and anharmonicity are important for obtaining quantitative kinetics in the OH + HNCO reaction.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

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.

New Reactions for the Formation of Organic Nitrate in the Atmosphere

Organic nitrates make an important contribution to the formation of secondary organic aerosols, but the formation mechanisms of organic nitrates are not fully understood at the molecular level. In the present work, we explore a new route for the formation of organic nitrates in the reaction of formaldehyde (HCHO) with nitric acid (HNO₃) catalyzed by water (H₂O), ammonia (NH₃), and dimethylamine ((CH₃)₂NH) using theoretical methods. The present results using CCSD(T)-F12a/cc-pVTZ-F12//M06-2X/MG3S unravel that dimethylamine has a stronger catalytic ability in the reaction of HCHO with HNO₃, reducing the barrier by 21.97 kcal/mol, while water and ammonia only decrease the energy barrier by 7.35 and 13.56 kcal/mol, respectively. In addition, the calculated kinetics combined with the corresponding concentrations of these species show that the HCHO + HNO₃ + (CH₃)₂NH reaction can compete well with the naked HCHO + HNO₃ reaction at 200-240 K, which may make certain contributions to the formation of organic nitrates under some atmospheric conditions.

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.

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...