Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate

Nonstatistical Unimolecular Decay of the CH2OO Criegee Intermediate in the Tunneling Regime

Unimolecular decay of the formaldehyde oxide (CH2OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH2OO with two quanta of CH stretch (17-18 kcal mol-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.25 kcal mol-1, refined utilizing a high-level electronic structure method HEAT-345(Q)Λ. The observed unimolecular decay rate of 1.6 ± 0.4 × 106 s-1 is 2 orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states.

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2νOH) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products. A distinctive IR feature is observed at 7012.5 cm-1. Two conformers of CHHP are predicted to have similar stabilities (within 0.2 kcal mol-1) and overtone OH stretch transitions (2νOH), yet are separated by a significant interconversion barrier. The IR power dependence indicates that absorption of three or more IR photons is required for dissociation of CHHP to cyclohexoxy (RO) and OH radical products. Accompanying high-level single- and multi-reference electronic structure calculations quantitatively support the experimental results. Calculations are extended to a range of organic hydroperoxides to examine trends in bond dissociation energies associated with RO + OH formation and compared with prior theoretical results.

Rapid Atmospheric Reactions between Criegee Intermediates and Hypochlorous Acid

Hypochlorous acid (HOCl) is a paramount compound in the atmosphere due to its significant contribution to both tropospheric oxidation capacity and ozone depletion. The main removal routes for HOCl are photolysis and the reaction with OH during the daytime, while these processes are unimportant during the nighttime. Here, we report the rapid reactions of Criegee intermediates (CH2OO and anti/syn-CH3CHOO) with HOCl by using high-level quantum chemical methods as the benchmark; their accuracy is close to coupled cluster theory with single, double, and triple excitations and quasiperturbative connected quadruple excitations with a complete basis limit by extrapolation [CCSDT(Q)/CBS]. Their rate constants have been calculated by using a dual-level strategy; this combines conventional transition state theory calculated at the benchmark level with variational transition state theory with small-curvature tunneling by a validated density functional method. We find that the introduction of the methyl group into Criegee intermediates not only affects their reactivities but also exerts a remarkable influence on anharmonicity. The calculated results uncover that anharmonicity increases the rate constants of CH2OO + HOCl by a factor of 18-5, while it is of minor importance in the anti/syn-CH3CHOO + HOCl reaction at 190-350 K. The present findings reveal that the loose transition state for anti-CH3CHOO and HOCl is a rate-determining step at 190-350 K. We also find that the reaction of Criegee intermediates with HOCl contributes significantly to the sink of HOCl during the nighttime in the atmosphere.

A five-carbon unsaturated Criegee intermediate: synthesis, spectroscopic identification, and theoretical study of 3-penten-2-one oxide

Biogenic alkenes, such as isoprene and α-pinene, are the predominant source of volatile organic compounds (VOCs) emitted into the atmosphere. Atmospheric processing of alkenes via reaction with ozone leads to formation of zwitterionic reactive intermediates with a carbonyl oxide functional group, known as Criegee intermediates (CIs). CIs are known to exhibit a strong absorption (π* ← π) in the near ultraviolet and visible (UV-vis) region due to the carbonyl oxide moiety. This study focuses on the laboratory identification of a five-carbon CI with an unsaturated substituent, 3-penten-2-one oxide, which can be produced upon atmospheric ozonolysis of substituted isoprenes. 3-Penten-2-one oxide is generated in the laboratory by photolysis of a newly synthesized precursor, (Z)-2,4-diiodopent-2-ene, in the presence of oxygen. The electronic spectrum of 3-penten-2-one oxide was recorded by UV-vis induced depletion of the VUV photoionization signal on the parent m/z 100 mass channel using a time-of-flight mass spectrometer. The resultant electronic spectrum is broad and unstructured with peak absorption at ca. 375 nm. To complement the experimental findings, electronic structure calculations are performed at the CASPT2(12,10)/aug-cc-pVDZ level of theory. The experimental spectrum shows good agreement with the calculated electronic spectrum and vertical excitation energy obtained for the lowest energy conformer of 3-penten-2-one oxide. In addition, OH radical products resulting from unimolecular decay of energized 3-penten-2-oxide CIs are detected by UV laser-induced fluorescence. Finally, the experimental electronic spectrum is compared with that of a four-carbon, isoprene-derived CI, methyl vinyl ketone oxide, to understand the effects of an additional methyl group on the associated electronic properties.

Electronic Spectroscopy and Dissociation Dynamics of Vinyl-Substituted Criegee Intermediates: 2-Butenal Oxide and Comparison with Methyl Vinyl Ketone Oxide and Methacrolein Oxide Isomers

The 2-butenal oxide Criegee intermediate [(CH₃CH═CH)CHOO], an isomer of the four-carbon unsaturated Criegee intermediates derived from isoprene ozonolysis, is characterized on its first π* ← π electronic transition and by the resultant dissociation dynamics to O (¹D) + 2-butenal [(CH₃CH═CH)CHO] products. The electronic spectrum of 2-butenal oxide under jet-cooled conditions is observed to be broad and unstructured with peak absorption at 373 nm, spanning to half maxima at 320 and 420 nm, and in good accord with the computed vertical excitation energies and absorption spectra obtained for its lowest energy conformers. The distribution of total kinetic energy released to products is ascertained through velocity map imaging of the O (¹D) products. About half of the available energy, deduced from the theoretically computed asymptotic energy, is accommodated as internal excitation of the 2-butenal fragment. A reduced impulsive model is introduced to interpret the photodissociation dynamics, which accounts for the geometric changes between 2-butenal oxide and the 2-butenal fragment, and vibrational activation of associated modes in the 2-butenal product. Application of the reduced impulsive model to the photodissociation of isomeric methyl vinyl ketone oxide reveals greater internal activation of the methyl vinyl ketone product arising from methyl internal rotation and rock, which is distinctly different from the dissociation dynamics of 2-butenal oxide or methacrolein oxide.

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...