Significance: Ozonolysis is an important sink of alkenes in Earth’s troposphere, leading to the formation of Criegee intermediates (CIs), whose reactions significantly impact tropospheric composition. The reactivity of the four-carbon unsaturated CIs derived from isoprene, the most abundant alkene, has remained unknown until now. Direct measurements of one such CI, methyl vinyl ketone oxide (MVK-oxide), with water vapor, SO ₂ , and formic acid are reported herein, substantiated by high-level theory, revealing the long lifetime of syn- MVK-oxide with respect to unimolecular decay and reaction with water vapor. Through a combination of direct experiment, high-level theory, and global modelling, syn -MVK-oxide is shown to survive high-humidity tropospheric environments and play a role in sulfuric acid formation and formic acid removal.

Spectral fingerprint of stabilized •QOOH: Carbon-centered radicals containing the hydroperoxy group, commonly denoted as •QOOH, are elusive but are among the most critical intermediate species for kinetic modeling of hydrocarbon oxidation in various atmospheric and combustion processes. Their direct experimental observation is a long-standing challenge, with only one successful previous attempt. Using a combination of infrared activation spectroscopy and an ultraviolet laser–induced fluorescence detection method, Hansen et al . directly characterized the vibrational structure of a •QOOH intermediate in isobutane oxidation, collisionally stabilized and isolated, and followed its dissociative evolution under infrared activation with time and energy resolution. High-level electronic structure calculations revealed an important role of heavy-atom tunneling in this process. —YS

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the atmosphere, proceeds through methyl vinyl ketone oxide (MVK-oxide), methacrolein oxide, and formaldehyde oxide (CH₂OO) Criegee intermediates. The present study focuses on MVK-oxide, a four-carbon unsaturated carbonyl oxide intermediate, using vacuum ultraviolet photoionization at 118 nm and UV-visible induced depletion of the m/z = 86 mass channel to characterize its first π^* ← π electronic transition. The electronic spectrum is broad and unstructured with its peak at 388 nm (3.2 eV). The MVK-oxide spectrum is shifted to a significantly longer wavelength than CH₂OO and alkyl-substituted Criegee intermediates studied previously due to extended conjugation across the vinyl and carbonyl oxide groups. Electronic excitation results in rapid dissociation at λ ≤ 430 nm to methyl vinyl ketone and O ¹D products, the latter detected by 2 + 1 resonance enhanced multiphoton ionization using velocity map imaging. Complementary electronic structure calculations (CASPT2(12,10)/AVDZ) predict two π^* ← π transitions with significant oscillator strength for each of the four conformers of MVK-oxide with vertical excitation energies (and corresponding wavelengths) in the 3.1-3.6 eV (350-400 nm) and 4.5-5.5 eV (220-280 nm) regions. The computed electronic absorption profile of MVK-oxide, based on a Wigner distribution of ground state configurations and summed over the four conformers, is predicted to peak at 397 nm. UV-visible spectroscopy on the first π^* ← π transition is shown by a combination of experiment and theory to provide a sensitive method for detection of the MVK-oxide Criegee intermediate that will enable further studies of its photochemistry and unimolecular and bimolecular reaction dynamics.

Criegee intermediates are reactive intermediates formed in Earth’s atmosphere through ozonolysis of alkenes. Here the authors outline the fundamental chemistry that influences their highly conformer- and substituent-dependent unimolecular and bimolecular reactivity, and discuss open questions of fundamental and atmospheric interest.