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

   https://doi.org/10.3390/molecules26103058  

   Vansco, Michael F.; Zuraski, Kristen; Winiberg, Frank A.; Au, Kendrew; Trongsiriwat, Nisalak; Walsh, Patrick J.; Osborn, David L.; Percival, Carl J.; Klippenstein, Stephen J.; Taatjes, Craig A.; et al (May 2021, Molecules)

   null (Ed.)

   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. 

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2. Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

   https://doi.org/10.1073/pnas.1916711117  

   Caravan, Rebecca L.; Vansco, Michael F.; Au, Kendrew; Khan, M. Anwar H.; Li, Yu-Lin; Winiberg, Frank A. F.; Zuraski, Kristen; Lin, Yen-Hsiu; Chao, Wen; Trongsiriwat, Nisalak; et al (April 2020, Proceedings of the National Academy of Sciences)

   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 thesyn-conformers are observed;anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction ofsyn-MVK-oxide with SO₂and 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 SO₃and identifying organic hydroperoxide formation from reaction with SO₂and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model. 

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3. Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

   https://doi.org/10.1111/php.13665  

   Karsili, Tolga N. V.; Marchetti, Barbara; Lester, Marsha I.; Ashfold, Michael N. R. (July 2022, Photochemistry and Photobiology)

   ABSTRACT 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 CH₂OO, CH₃CHOO, (CH₃)₂COO 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 CH₂OO and (CH₃)₂COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss ratesviaunimolecular 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 absorbing¹ππ* 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. 

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4. The role of solar photolysis in the atmospheric removal of methacrolein oxide and the methacrolein oxide—water van‐der Waals complex in pristine environments

   https://doi.org/10.1111/php.14007  

   Guidry, Lily M; Bardash, Londyn A; Yigiter, Aylin; Ravi, Satyam; Marchetti, Barbara; Karsili, Tolga_N V (August 2024, Photochemistry and Photobiology)

   Abstract Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by‐products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation. In this manuscript, we argue that the extended conjugation of these Criegee intermediates enables electronic excitation of these compounds, on timescales that are competitive with their slow unimolecular decay and bimolecular chemistry. We show that the complexation of methacrolein oxide with water enhances the absorption cross section of the otherwise dark S₁state, potentially revealing a new avenue for forming lower volatility compounds via tropospherically relevant photochemistry. 

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5. Selective deuteration as a tool for resolving autoxidation mechanisms in α -pinene ozonolysis

   https://doi.org/10.5194/acp-23-4373-2023  

   Meder, Melissa; Peräkylä, Otso; Varelas, Jonathan G.; Luo, Jingyi; Cai, Runlong; Zhang, Yanjun; Kurtén, Theo; Riva, Matthieu; Rissanen, Matti; Geiger, Franz M.; et al (January 2023, Atmospheric Chemistry and Physics)

   Abstract. Highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy-radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second timescales in short-lived intermediates, making the process challenging to study. Similarly, while the end-products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C atoms have taken part in the isomerisation reactions. This relies on the fact that if a C−D bond is broken due to an abstraction by a peroxy group forming a −OOD hydroperoxide, the D atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D atom from the molecule. In this study, we test the applicability of this method using three differently deuterated versions of α-pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass-resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D atom (mD=2.0141) from those with two H atoms (mH2=2.0157). We found that the method worked well, and we could deduce that two of the three tested compounds had lost D atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level and likely is not limited to the system of α-pinene ozonolysis tested here. 

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