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Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. Here, this work investigates the reactions of the smallest Criegee intermediate, CH₂OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-α (10.2 eV) radiation for photoionization. CH₂OO was produced via pulsed laser photolysis of CH₂I₂ in the presence of excess O₂. Time-resolved measurements of reactant disappearance and of product formation were performed to monitor reaction progress; first order rate coefficients weremore » obtained from exponential fits to the CH₂OO decays. The bimolecular reaction rate coefficients at 300 K and 4 Torr are k(CH₂OO + MVK) = (5.0 ± 0.4) × 10^-13 cm³ s^-1 and k(CH₂OO + MACR) = (4.4 ± 1.0) × 10^-13 cm³ s^-1, where the stated ±2σ uncertainties are statistical uncertainties. Adduct formation is observed for both reactions and is attributed to the formation of a secondary ozonides (1,2,4-trioxolanes), supported by master equation calculations of the kinetics and the agreement between measured and calculated adiabatic ionization energies. Kinetics measurements were also performed for a possible bimolecular CH₂OO + CO reaction and for the reaction of CH₂OO with CF₃CHCH₂ at 300 K and 4 Torr. For CH₂OO + CO, no reaction is observed and an upper limit is determined: k(CH₂OO + CO) < 2 × 10^-16 cm³ s^-1. Lastly, for CH₂OO + CF₃CHCH₂, an upper limit of k(CH₂OO + CF₃CHCH₂) < 2 × 10^-14 cm³ s^-1 is obtained.« less

The chlorine atom-initiated oxidation of two unsaturated primary C5 alcohols, prenol (3-methyl-2-buten-1-ol, (CH₃)₂CCHCH₂OH) and isoprenol (3-methyl-3-buten-1-ol, CH₂C(CH₃)CH₂CH₂OH), is studied at 550 K and low pressure (8 Torr). The time- and isomer-resolved formation of products is probed with multiplexed photoionization mass spectrometry (MPIMS) using tunable vacuum ultraviolet ionizing synchrotron radiation. The peroxy radical chemistry of the unsaturated alcohols appears much less rich than that of saturated C4 and C5 alcohols. The main products observed are the corresponding unsaturated aldehydes – prenal (3-methyl-2-butenal) from prenol oxidation and isoprenal (3-methyl-3-butenal) from isoprenol oxidation. No significant products arising from QOOH chemistry are observed. Thesemore » results can be qualitatively explained by the formation of resonance stabilized allylic radicals via H-abstraction in the Cl + prenol and Cl + isoprenol initiation reactions. The loss of resonance stabilization upon O₂ addition causes the energies of the intermediate wells, saddle points, and products to increase relative to the energy of the initial radicals and O₂. These energetic shifts make most product channels observed in the peroxy radical chemistry of saturated alcohols inaccessible for these unsaturated alcohols. The experimental findings are underpinned by quantum-chemical calculations for stationary points on the potential energy surfaces for the reactions of the initial radicals with O₂. Under our conditions, the dominant channels in prenol and isoprenol oxidation are the chain-terminating HO₂-forming channels arising from radicals, in which the unpaired electron and the –OH group are on the same carbon atom, with stable prenal and isoprenal co-products, respectively. These results suggest that the presence of C=C double bonds in alcohols will reduce low-temperature reactivity during autoignition.« less