Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

Daub, Christopher D.; Valiev, Rashid R.; Salo, Vili-Taneli; Zakai, Itai; Gerber, R. Benny; Kurtén, Theo

doi:10.1021/acsearthspacechem.2c00159

Cited by 6 publications

(15 citation statements)

References 34 publications

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“…59 The artifacts are presumably due to RO 2 reactivity being enhanced by reagent ions in the chemical ionization inlet, in a similar fashion to what has been observed for sulfuric acid ion cluster formation. 60 MD simulations based on a recent model for RO 2 + R′O 2 overall rates also supports the hypothesis that clustering with ions substantially increases the rate for the least oxidized RO 2 61,62 (see SI for more details).…”

Section: ■ Results and Discussionsupporting

confidence: 70%

“…The artifacts are presumably due to RO 2 reactivity being enhanced by reagent ions in the chemical ionization inlet, in a similar fashion to what has been observed for sulfuric acid ion cluster formation . MD simulations based on a recent model for RO 2 + R′O 2 overall rates also supports the hypothesis that clustering with ions substantially increases the rate for the least oxidized RO 2 , (see SI for more details). With this limitation in mind, and the dilution system in use, we observed even the C 18 H 26 O 4 accretion product to form more abundantly than the nonscissioned C 20 H 30 O 6 accretion product, supporting the assertion that such scissions are common and important.…”

Section: Resultssupporting

confidence: 64%

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Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

et al. 2023

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Dimeric accretion products have been observed both in atmospheric aerosol particles and in the gas phase. With their low volatilities, they are key contributors to the formation of new aerosol particles, acting as seeds for more volatile organic vapors to partition onto. Many particle-phase accretion products have been identified as esters. Various gas-and particle-phase formation pathways have been suggested for them, yet evidence remains inconclusive. In contrast, peroxide accretion products have been shown to form via gas-phase peroxy radical (RO 2 ) cross reactions. Here, we show that these reactions can also be a major source of esters and other types of accretion products. We studied α-pinene ozonolysis using state-ofthe-art chemical ionization mass spectrometry together with different isotopic labeling approaches and quantum chemical calculations, finding strong evidence for fast radical isomerization before accretion. Specifically, this isomerization seems to happen within the intermediate complex of two alkoxy (RO) radicals, which generally determines the branching of all RO 2 -RO 2 reactions. Accretion products are formed when the radicals in the complex recombine. We found that RO with suitable structures can undergo extremely rapid C−C β scissions before recombination, often resulting in ester products. We also found evidence of this previously overlooked RO 2 −RO 2 reaction pathway forming alkyl accretion products and speculate that some earlier peroxide identifications may in fact be hemiacetals or ethers. Our findings help answer several outstanding questions on the sources of accretion products in organic aerosol and bridge our knowledge of the gas phase formation and particle phase detection of accretion products. As esters are inherently more stable than peroxides, this also impacts their further reactivity in the aerosol.

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Section: ■ Results and Discussionsupporting

confidence: 70%

Section: Resultssupporting

confidence: 64%

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

et al. 2023

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“…The errors are largest for the fastest reactions, presumably due to the limitations in modeling barrierless formation of tetroxides from the RO 2 ···R′O 2 complexes. We demonstrated in a very recent study , that the experimental rates for RO 2 + R′O 2 reactions with submerged barriers can be predicted surprisingly well simply based on RO 2 ···R′O 2 complex lifetimes estimated from nonreactive classical molecular dynamics simulations. These two sets of results now provide a complete toolbox for estimating at least order-of-magnitude accuracy rates for overall RO 2 ···R′O 2 reactions in both the presence and the absence of substantial barriers for tetroxide formation.…”

Section: Results and Discussionmentioning

confidence: 86%

Gas-Phase Peroxyl Radical Recombination Reactions: A Computational Study of Formation and Decomposition of Tetroxides

et al. 2022

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The recombination (“dimerization”) of peroxyl radicals (RO 2 •) is one of the pathways suggested in the literature for the formation of peroxides (ROOR′, often referred to as dimers or accretion products in the literature) in the atmosphere. It is generally accepted that these dimers play a major role in the first steps of the formation of submicron aerosol particles. However, the precise reaction pathways and energetics of RO 2 • + R′O 2 • reactions are still unknown. In this work, we have studied the formation of tetroxide intermediates (RO 4 R′): their formation from two peroxyl radicals and their decomposition to triplet molecular oxygen ( 3 O 2 ) and a triplet pair of alkoxyl radicals (RO•). We demonstrate this mechanism for several atmospherically relevant primary and secondary peroxyl radicals. The potential energy surface corresponds to an overall singlet state. The subsequent reaction channels of the alkoxyl radicals include, but are not limited to, their dimerization into ROOR′. Our work considers the multiconfigurational character of the tetroxides and the intermediate phases of the reaction, leading to reliable mechanistic insights for the formation and decomposition of the tetroxides. Despite substantial uncertainties in the computed energetics, our results demonstrate that the barrier heights along the reaction path are invariably small for these systems. This suggests that the reaction mechanism, previously validated at a multireference level only for methyl peroxyl radicals, is a plausible pathway for the formation of aerosol-relevant larger peroxides in the atmosphere.

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“…Due to the difficulty in the experimental detection of tetroxides, computational studies of its reaction pathways are of significant interest. The reaction has been studied with various computational approaches, such as ab initio methods − and trajectory-based methods. , The standard single-reference (SR) electronic structure methods, such as density functional theory (DFT) or coupled cluster (CC), can be used to obtain accurate relative energies for separate, noninteracting species (RO, RO 2 , and O 2 ), as well as for the RO 4 R. The ground electronic states of these molecules can be qualitatively described with a single-determinant approximation, i.e., with the Hartree–Fock (HF) method.…”

Section: Introductionmentioning

confidence: 99%

Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO₄Me) Formation and Decomposition

Salo,

Chen,

Runeberg

et al. 2024

J. Phys. Chem. A

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Peroxyl radicals (RO 2 ) are important intermediates in the atmospheric oxidation processes. The RO 2 can react with other RO 2 to form reactive intermediates known as tetroxides, RO 4 R. The reaction mechanisms of RO 4 R formation and its various decomposition channels have been investigated in multiple computational studies, but previous approaches have not been able to provide a unified methodology that is able to connect all relevant reactions together. An apparent difficulty in modeling the RO 4 R formation and its decomposition is the involvement of openshell singlet electronic states along the reaction pathway. Modeling such electronic states requires multireference (MR) methods, which we use in the present study to investigate in detail a model reaction of MeO 2 + MeO 2 → MeO 4 Me, and its decomposition, MeO 4 Me → MeO + O 2 + MeO, as well as the two-body product complexes MeO•••O 2 + MeO and MeO•••MeO + O 2 . The used MR methods are benchmarked against relative energies of MeO 2 + MeO 2 , MeO 4 Me, and MeO + MeO + O 2 , calculated with CCSD(T)/ CBS, W2X, and W3X-L methods. We found that the calculated relative energies of the overall MeO 2 + MeO 2 → MeO 4 Me → MeO + O 2 + MeO reaction are very sensitive to the chosen MR method and that the CASPT2(22e,14o)-IPEA method is able to reproduce the relative energies obtained by the various coupled-cluster methods. Furthermore, CASPT2(22e,14o)-IPEA and W3X-L results show that the MeO•••O 2 product complex + MeO is lower in energy than the MeO•••MeO complex + O 2 . The formation of MeO 4 Me is exothermic, and its decomposition is endothermic, relative to the tetroxide. Both the formation and decomposition reactions appear to follow pathways with no saddle points. According to potential energy surface scans of the decomposition pathway, the concerted cleavage of both MeO•••O bonds in MeO 4 Me is energetically preferred over the corresponding sequential decomposition.

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Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

Cited by 6 publications

References 34 publications

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

Gas-Phase Peroxyl Radical Recombination Reactions: A Computational Study of Formation and Decomposition of Tetroxides

Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO₄Me) Formation and Decomposition

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Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

Cited by 6 publications

References 34 publications

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

Gas-Phase Peroxyl Radical Recombination Reactions: A Computational Study of Formation and Decomposition of Tetroxides

Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO4Me) Formation and Decomposition

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Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO₄Me) Formation and Decomposition