Absolute Ultraviolet Absorption Spectrum of a Criegee Intermediate CH₂OO

We present the time-resolved UV absorption spectrum of the B̃ (¹A′) ← X̃ (¹A′) electronic transition of formaldehyde oxide, CH₂OO, produced by the reaction of CH₂I radicals with O₂. In contrast to its UV photodissociation action spectrum, the absorption spectrum of formaldehyde oxide extends to longer wavelengths and exhibits resolved vibrational structure on its low-energy side. Chemical kinetics measurements of its reactivity establish the identity of the absorbing species as CH₂OO. Separate measurements of the initial CH₂I radical concentration allow a determination of the absolute absorption cross section of CH₂OO, with the value at the peak of the absorption band, 355 nm, of σ_abs = (3.6 ± 0.9) × 10^–17 cm². The difference between the absorption and action spectra likely arises from excitation to long-lived B̃ (¹A′) vibrational states that relax to lower electronic states by fluorescence or nonradiative processes, rather than by photodissociation

A Combined Experimental and Theoretical Study of the Reaction OH + 2‑Butene in the 400–800 K Temperature Range

We report a combined experimental and theoretical study of the OH + cis-2-butene and OH + trans-2-butene reactions at combustion-relevant conditions: pressures of 1–20 bar and temperatures of 400–800 K. We probe the OH radical time histories by laser-induced fluorescence and analyze these experimental measurements with aid from time-dependent master-equation calculations. Importantly, our investigation covers a temperature range where experimental data on OH + alkene chemistry in general are lacking, and interpretation of such data is challenging due to the complexity of the competing reaction pathways. Guided by theory, we unravel this complex behavior and determine the temperature- and pressure-dependent rate coefficients for the three most important OH + 2-butene reaction channels at our conditions: H abstraction, OH addition to the double bond, and back-dissociation of the OH–butene adduct

Resonance Stabilization Effects on Ketone Autoxidation: Isomer-Specific Cyclic Ether and Ketohydroperoxide Formation in the Low-Temperature (400–625 K) Oxidation of Diethyl Ketone

The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CH₃CH₂)₂CO], 2,2,4,4-<i>d</i>₄-DEK [<i>d</i>₄-DEK; (CH₃CD₂)₂CO], and 1,1,1,5,5,5-<i>d</i>₆-DEK [<i>d</i>₆-DEK; (CD₃CH₂)₂CO] is studied at 8 torr and 1–2 atm and from 400–625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, R_P) or secondary (3-pentan-on-2-yl, R_S) radicals, which in turn react with O₂. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary R_S with O₂. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (R_P), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O₂ concentrations and higher pressures (1–2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of <i>d</i>₄-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products

Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether

Dimethyl ether (DME) oxidation is a model chemical system with a small number of prototypical reaction intermediates that also has practical importance for low-carbon transportation. Although it has been studied experimentally and theoretically, ambiguity remains in the relative importance of competing DME oxidation pathways in the low-temperature autoignition regime. To focus on the primary reactions in DME autoignition, we measured the time-resolved concentration of five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl formate, HPMF), CH2O, and CH3OCHO (methyl formate, MF), from photolytically initiated experiments. We performed these studies at P = 10 bar and T = 450–575 K, using a high-pressure photolysis reactor coupled to a time-of-flight mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization at the Advanced Light Source. Our measurements reveal that the timescale of ROO decay and product formation is much shorter than predicted by current DME combustion models. The models also strongly underpredict the observed yields of CH2O and MF and do not capture the temperature dependence of OOQOOH and HPMF yields. Adding the ROO + OH → RO + HO2 reaction to the chemical mechanism (with a rate coefficient approximated from similar reactions) improves the prediction of MF. Increasing the rate coefficients of ROO ↔ QOOH and QOOH + O2 ↔ OOQOOH reactions brings the model predictions closer to experimental observations for OOQOOH and HPMF, while increasing the rate coefficient for the QOOH → 2CH2O + OH reaction is needed to improve the predictions of formaldehyde. To aid future quantification of DME oxidation intermediates by photoionization mass spectrometry, we report experimentally determined ionization cross-sections for ROO, OOQOOH, and HPMF

Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether

Dimethyl ether (DME) oxidation is a model chemical system with a small number of prototypical reaction intermediates that also has practical importance for low-carbon transportation. Although it has been studied experimentally and theoretically, ambiguity remains in the relative importance of competing DME oxidation pathways in the low-temperature autoignition regime. To focus on the primary reactions in DME autoignition, we measured the time-resolved concentration of five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl formate, HPMF), CH2O, and CH3OCHO (methyl formate, MF), from photolytically initiated experiments. We performed these studies at P = 10 bar and T = 450–575 K, using a high-pressure photolysis reactor coupled to a time-of-flight mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization at the Advanced Light Source. Our measurements reveal that the timescale of ROO decay and product formation is much shorter than predicted by current DME combustion models. The models also strongly underpredict the observed yields of CH2O and MF and do not capture the temperature dependence of OOQOOH and HPMF yields. Adding the ROO + OH → RO + HO2 reaction to the chemical mechanism (with a rate coefficient approximated from similar reactions) improves the prediction of MF. Increasing the rate coefficients of ROO ↔ QOOH and QOOH + O2 ↔ OOQOOH reactions brings the model predictions closer to experimental observations for OOQOOH and HPMF, while increasing the rate coefficient for the QOOH → 2CH2O + OH reaction is needed to improve the predictions of formaldehyde. To aid future quantification of DME oxidation intermediates by photoionization mass spectrometry, we report experimentally determined ionization cross-sections for ROO, OOQOOH, and HPMF

Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether

Dimethyl ether (DME) oxidation is a model chemical system with a small number of prototypical reaction intermediates that also has practical importance for low-carbon transportation. Although it has been studied experimentally and theoretically, ambiguity remains in the relative importance of competing DME oxidation pathways in the low-temperature autoignition regime. To focus on the primary reactions in DME autoignition, we measured the time-resolved concentration of five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl formate, HPMF), CH2O, and CH3OCHO (methyl formate, MF), from photolytically initiated experiments. We performed these studies at P = 10 bar and T = 450–575 K, using a high-pressure photolysis reactor coupled to a time-of-flight mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization at the Advanced Light Source. Our measurements reveal that the timescale of ROO decay and product formation is much shorter than predicted by current DME combustion models. The models also strongly underpredict the observed yields of CH2O and MF and do not capture the temperature dependence of OOQOOH and HPMF yields. Adding the ROO + OH → RO + HO2 reaction to the chemical mechanism (with a rate coefficient approximated from similar reactions) improves the prediction of MF. Increasing the rate coefficients of ROO ↔ QOOH and QOOH + O2 ↔ OOQOOH reactions brings the model predictions closer to experimental observations for OOQOOH and HPMF, while increasing the rate coefficient for the QOOH → 2CH2O + OH reaction is needed to improve the predictions of formaldehyde. To aid future quantification of DME oxidation intermediates by photoionization mass spectrometry, we report experimentally determined ionization cross-sections for ROO, OOQOOH, and HPMF

Low-Temperature Combustion Chemistry of <i>n-</i>Butanol: Principal Oxidation Pathways of Hydroxybutyl Radicals

Reactions of hydroxybutyl radicals with O₂ were investigated by a combination of quantum-chemical calculations and experimental measurements of product formation. In pulsed-photolytic Cl-initiated oxidation of <i>n</i>-butanol, the time-resolved and isomer-specific product concentrations were probed using multiplexed tunable synchrotron photoionization mass spectrometry (MPIMS). The interpretation of the experimental data is underpinned by potential energy surfaces for the reactions of O₂ with the four hydroxybutyl isomers (1-hydroxybut-1-yl, 1-hydroxybut-2-yl, 4-hydroxybut-2-yl, and 4-hydroxybut-1-yl) calculated at the CBS-QB3 and RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G­(d,p) levels of theory. The observed product yields display substantial temperature dependence, arising from a competition among three fundamental pathways: (1) stabilization of hydroxybutylperoxy radicals, (2) bimolecular product formation in the hydroxybutyl + O₂ reactions, and (3) decomposition of hydroxybutyl radicals. The 1-hydroxybut-1-yl + O₂ reaction is dominated by direct HO₂ elimination from the corresponding peroxy radical forming butanal as the stable coproduct. The chemistry of the other three hydroxybutylperoxy radical isomers mainly proceeds via alcohol-specific internal H-atom abstractions involving the H atom from either the −OH group or from the carbon attached to the −OH group. We observe evidence of the recently reported water elimination pathway (Welz et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i> (3), 350–354) from the 4-hydroxybut-2-yl + O₂ reaction, supporting its importance in γ-hydroxyalkyl + O₂ reactions. Experiments using the 1,1-<i>d</i>₂ and 4,4,4-<i>d</i>₃ isotopologues of <i>n</i>-butanol suggest the presence of yet unexplored pathways to acetaldehyde

Pressure-Dependent Competition among Reaction Pathways from First- and Second‑O₂ Additions in the Low-Temperature Oxidation of Tetrahydrofuran

We report a combined experimental and quantum chemistry study of the initial reactions in low-temperature oxidation of tetrahydrofuran (THF). Using synchrotron-based time-resolved VUV photoionization mass spectrometry, we probe numerous transient intermediates and products at <i>P</i> = 10–2000 Torr and <i>T</i> = 400–700 K. A key reaction sequence, revealed by our experiments, is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals (QOOH), followed by a second O₂ addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO₂ or to γ-butyrolactone hydroperoxide + OH. The competition between these two pathways affects the degree of radical chain-branching and is likely of central importance in modeling the autoignition of THF. We interpret our data with the aid of quantum chemical calculations of the THF-yl + O₂ and QOOH + O₂ potential energy surfaces. On the basis of our results, we propose a simplified THF oxidation mechanism below 700 K, which involves the competition among unimolecular decomposition and oxidation pathways of QOOH

C–H Bond Strengths and Acidities in Aromatic Systems: Effects of Nitrogen Incorporation in Mono-, Di-, and Triazines

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ_acid<i>H</i>₂₉₈). The measured Δ_acid<i>H</i>₂₉₈ for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C–H bond dissociation energy (BDE). The site-specific C–H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol^–1, respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C–H bonds in six-membered rings

Unimolecular Kinetics of Stabilized CH₃CHOO Criegee Intermediates: <i>syn</i>-CH₃CHOO Decomposition and <i>anti</i>-CH₃CHOO Isomerization

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH3CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH3CHI2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol–1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150–81+176 s–1 when tunneling effects are included but only 5–2+3 s–1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH3CHOO to methyldioxirane, indicating a rate coefficient of 54–21+34 s–1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH3CHOO are provided for use in atmospheric models, and atmospheric implications are discussed