OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions

Abstract

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C–O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10–11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).

OH Roaming during the Ozonolysis of α-Pinene: A New Route to Highly Oxygenated Molecules?

“…A recent study of Lester and co-workers detected and quantified the roaming-induced formation of hydroxybutanone in the decomposition of methyl ethyl CI (MECI). 22 A companion ab initio kinetic analysis provided predictions that agreed well with the experimental observations. The combination of multireference electronic structure methods with variable reaction coordinate transition-state theory analyses within a master equation framework provided a firm foundation for the postulated roaming mechanism.…”

“…55 The present analysis employs multireference wave function-based ab initio calculations as in our collaborative studies with Lester and co-workers on related VHP dissociations. 21,22 Particular attention is paid to the possible role of roaming-induced isomerization/ additions and abstractions, which have not been considered in prior studies. These ab initio predictions provide the basis for transition-state theory-based master equation predictions for the temperature dependence of the branching fractions.…”

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO2

“…Prior work indicates that photolytic production will lead to the formation of both conformers, and thus, our experiment probes a combined rate of syn - and anti -MECI + SO 2 . Kinetic measurements of MECI loss in the presence and absence of SO 2 were performed using a photoionization energy of 8.9 eV to minimize interference from stable isomers formed from MECI unimolecular decay that appear on m / z 88 at ionization energies above 9.3 eV, such as 1-hydroxy-2-butanone or 3-hydroxy-2-butanone, as presented in a companion paper . Additional measurements at various other photoionization energies were performed, including at 13.0 eV to probe the kinetics of SO 3 and 9.7 eV to probe the kinetics of 2-butanone (see the SI).…”

Carbonyl Oxide Stabilization from Trans Alkene and Terpene Ozonolysis

“…When the terminal oxygen of the C–O–O moiety faces an α carbon, they are known as syn-intermediates; when the terminal oxygen faces an α hydrogen, they are known as anti-intermediates. ,, Because of the conjugation associated with the zwitterionic character of the planar C–O–O moiety, the barrier for interconversion between these conformers is high and effectively insurmountable under atmospheric conditions . Under most circumstances, the syn-intermediates can abstract an H atom from the α-carbon in the R group to form a closed-shell (but weakly bound) vinyl hydroperoxide. , This then decomposes to produce OH radicals with nearly 100% yield, although for more complex vinyl hydroperoxides, OH roaming is an interesting but relatively minor (<10%) pathway . The low-energy pathway for anti-intermediates is ring closure in the C–O–O moiety to form a dioxirane, which subsequently reopens to form a bisoxy O–C–O biradical.…”