Direct Kinetic Measurements of a Cyclic Criegee Intermediate; Unimolecular Decomposition of c-(CH2)5COO.

We report the first direct kinetic measurements of a cyclic stabilized Criegee Intermediate. We have measured the unimolecular reaction rate coefficient of cyclohexanone oxide (c-(CH2)5COO) in the temperature 213-296 K and pressure 7-50 Torr ranges using absorption spectrometry. The c-(CH2)5COO was produced by the photolysis of c-(CH2)5CIBr at 213 nm in the presence of O2. We compare the measured fast c-(CH2)5COO unimolecular rate coefficient, 1998 ± 147 s-1 at 296 K, with the literature calculations for the structurally similar E-nopinone oxide formed in ß-pinene ozonolysis. The kuni(c-(CH2)5COO)/kuni(E-nopinone oxide) ratio calculated using transition-state theory and density functional theory agrees well with this comparison. We have also measured the bimolecular rate coefficient of the reaction between c-(CH2)5COO and trifluoroacetic acid at 253 K and 10 Torr and obtained the value (8.7 ± 1.0) × 10-10 cm3 molecule-1 s-1. This very large value agrees with previous kinetic measurements for reactions between stabilized Criegee intermediates and halogenated organic acids.

An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance.

We have performed direct kinetic measurements of the CH2OO + RCN reactions (R = H, CH3, C2H5) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH2ICl), whose photolysis at 193 nm in the presence of O2 produces CH2OO. Observed bimolecular rate coefficients for CH2OO + HCN, CH2OO + CH3CN, and CH2OO + C2H5CN reactions at 296 K are (2.22 ± 0.65) × 10-14 cm3 molecule-1 s-1, (1.02 ± 0.10) × 10-14 cm3 molecule-1 s-1, and (2.55 ± 0.13) × 10-14 cm3 molecule-1 s-1, respectively, suggesting that reaction with CH2OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH2OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH2OO + HCN and CH2OO + CH3CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH2O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH2O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.

Temperature and Pressure Dependence of the Reaction between Ethyl Radical and Molecular Oxygen: Experiments and Master Equation Simulations.

We have used laser-photolysis - photoionization mass-spectrometry to measure the rate coefficient for the reaction between ethyl radical and molecular oxygen as a function of temperature (190-801 K) and pressure (0.2-6 Torr) under pseudo-first-order conditions ([He] ≫ [O2] ≫ [C2H5•]). Multiple ethyl precursor, photolysis wavelength, reactor material, and coating combinations were used. We reinvestigated the temperature dependence of the title reaction's rate coefficient to resolve inconsistencies in existing data. The current results indicate that some literature values for the rate coefficient may indeed be slightly too large. The experimental work was complemented with master equation simulations. We used the current and some previous rate coefficient measurements to optimize the values of key parameters in the master equation model. After optimization, the model was able to reproduce experimental falloff curves and C2H4 + HO2• yields. We then used the model to perform simulations over wide temperature (200-1500 K) and pressure (10-4-102 bar) ranges and provide the results in PLOG format to facilitate their use in atmospheric and combustion models.

Formation of Organic Acids and Carbonyl Compounds in n-Butane Oxidation via γ-Ketohydroperoxide Decomposition.

A crucial chain-branching step in autoignition is the decomposition of ketohydroperoxides (KHP) to form an oxy radical and OH. Other pathways compete with chain-branching, such as "Korcek" dissociation of γ-KHP to a carbonyl and an acid. Here we characterize the formation of a γ-KHP and its decomposition to formic acid+acetone products from observations of n-butane oxidation in two complementary experiments. In jet-stirred reactor measurements, KHP is observed above 590 K. The KHP concentration decreases with increasing temperature, whereas formic acid and acetone products increase. Observation of characteristic isotopologs acetone-d3 and formic acid-d0 in the oxidation of CH3 CD2 CD2 CH3 is consistent with a Korcek mechanism. In laser-initiated oxidation experiments of n-butane, formic acid and acetone are produced on the timescale of KHP removal. Modelling the time-resolved production of formic acid provides an estimated upper limit of 2 s-1 for the rate coefficient of KHP decomposition to formic acid+acetone.

An experimental and computational study of the reaction between pent-3-en-2-yl radicals and oxygen molecules: switching from pure stabilisation to pure decomposition with increasing temperature.

We have used laser-photolysis-photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl a resonance-stabilised hydrocarbon radical, and molecular oxygen. The time-resolved experiments were performed over a wide temperature range (240-750 K) at relatively low pressures (0.4-7 Torr) under pseudo-first-order conditions (excess [O2]). Helium bath gas was used in most experiments, but nitrogen was employed in a few measurements to investigate the effect of a heavier collider on the kinetics of the studied reaction. The experimental traces were directly used to optimise parameters in the master equation model using the recently implemented trace fitting feature in the MESMER program. At low temperatures (T < 300 K), the reaction proceeds by barrierless recombination reactions to form peroxyl adducts, and the radical traces are single-exponential. Between 326 K and 376 K, equilibration between the reactants and the peroxyl adducts is observed, and the radical traces are multi-exponential. Interestingly, at temperatures above 500 K, single-exponential decays were again observed, although the reaction is much slower than at low temperatures. The master equation simulations revealed that at both low and high temperatures, the radical decay rate is governed by a single eigenvalue. At low temperatures, this eigenvalue corresponds to recombination reactions, and at high temperatures to the phenomenological formation of bimolecular products. Between low and high temperatures (the exact temperature thresholds depend on [O2]), there is a region of avoided crossing in which the rate coefficient "jumps" from one eigencurve to the other. Although chemically significant eigenvalues are not well separated from internal energy relaxation eigenvalues at elevated temperatures (600 K at 0.01 bar, 850 K at 100 bar), we observed that many of the Bartis-Widom rate coefficients produced by the master equation model were valid up to 1500 K. Our simulations predict that the most important reaction channel at high temperatures is the formation of (E/Z)-penta-1,3-diene and hydroperoxyl. The experimentally constrained master equation model was used to simulate the title reaction over a wide range of conditions. To facilitate the use of our results in autoignition and combustion models, modified Arrhenius representations are given for the most important reaction channels.

An experimental and computational study of the reaction between 2-methylallyl radicals and oxygen molecules: optimizing master equation parameters with trace fitting.

We have investigated the reaction between 2-methylallyl radicals and oxygen molecules with experimental and computational methods. Kinetic experiments were conducted in a tubular laminar flow reactor using laser photolysis for radical production and photoionization mass spectrometry for detection. The reaction was investigated as a function of temperature (203-730 K) and pressure (0.2-9 torr) in helium and nitrogen bath gases. At low temperatures (T < 410 K), the reaction proceeds by a barrierless reaction to form 2-methylallylperoxyl. Equilibration of the peroxyl adduct and the reactants was observed between 350-410 K. Measurements were extended to even higher temperatures, up to 730 K, but no reaction could be observed. Master equation simulations of the reaction system were performed with the MESMER program. Kinetic parameters in the master equation model were optimized by direct fitting to time-resolved experimental 2-methylallyl traces. Trace fitting is a recently implemented novel feature in MESMER. The trace approach was compared with the more traditional approach where one uses experimental rate coefficients for parameter optimization. The optimized parameters yielded by the two approaches are very similar and do an excellent job at reproducing the experimental data. The optimized master equation model was then used to simulate the reaction under study over a wide temperature and pressure range, from 200 K and 0.01 bar to 1500 K and 100 bar. The simulations predict a small phenomenological rate coefficient under autoignition conditions; about 1 × 10-18 cm3 s-1 at 400 K and 5 × 10-16 cm3 s-1 at 1000 K. We provide modified Arrhenius expressions in PLOG format for the most important product channels to facilitate the use of our results in combustion models.

Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl-substituted Criegee intermediate (CH3)2COO using a new photolytic precursor.

We have performed direct kinetic measurements of the thermal unimolecular reaction of (CH3)2COO in the temperature range 243-340 K and pressure range 5-350 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, 2-bromo-2-iodopropane ((CH3)2CIBr), which photolysis at 213 nm in the presence of O2 produces acetone oxide, (CH3)2COO. The results show that the thermal unimolecular reaction is even more important main loss process of (CH3)2COO in the atmosphere than direct kinetic studies have suggested hitherto. The current experiments show that the unimolecular reaction rate of (CH3)2COO at 296 K and atmospheric pressure is 899 ± 42 s-1. Probably more importantly, current measurements bring the direct and relative-rate measurements of thermal unimolecular reaction kinetics of (CH3)2COO into quantitative agreement.

An experimental and master-equation modeling study of the kinetics of the reaction between resonance-stabilized (CH3)2CCHCH2 radical and molecular oxygen.

The kinetics of the reaction between resonance-stabilized (CH3)2CCHCH2 radical (R) and O2 has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH3)2CCHCH2 + O2 reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O2 → ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O2 with (CH3)2CCHCH2 radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO(t) and ROO(nt) adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O2 reaction at high temperatures (T > 500 K) and pressure-independent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) × 10-15 cm3 molecule-1 s-1, were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at m/z = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO2 (yield > 91%) and (2R/S)-3-methyl-1,2-epoxybut-3-ene + OH (yield < 8%). After modest adjustments to ROO(t) and ROO(nt) well-depths (∼0.7 kcal mol-1 each) and barrier height for the transition state associated with the kinetically most dominant channel, R + O2 → isoprene + HO2 (∼2.2 kcal mol-1), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.

Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor: unimolecular decomposition of CH2OO and its reaction with formic acid.

We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CH2OO) reactions. We also introduce and utilize a new photolytic precursor, bromoiodomethane (CH2IBr), which photolysis at 213 nm in presence of O2 produces CH2OO. Importantly, this precursor appears to be free from secondary reactions that may regenerate CH2OO in kinetic experiments. The unimolecular decomposition rate coefficient of CH2OO has been measured over wide pressure (5-400 Torr) and temperature (296-600 K) ranges and master equation simulations of the decomposition kinetics have been performed using MESMER program. The MESMER simulations of the experimental data with the calculated zero-point energy corrected transition state energy 85.9 kJ mol-1 for decomposition required no adjustment and returned 〈ΔE〉down = 123.2 × (T/298 K)0.74 cm-1 for temperature-dependent exponential-down model of the collisional energy transfer in He. A very good agreement between results of simulations and experiments is obtained. The results are compared with the previously reported unimolecular decomposition study by Stone et al. (Phys. Chem. Chem. Phys., 2018, 20, 24940-24954). Current master equation simulations suggest about 61% decomposition yield for the predominant H2 + CO2 channel, whereas the yields of two other channels, H2O + CO, and HCO + OH, are sensitive on the parameters involved in the simulations. The kinetics of CH2OO reaction with formic acid has also been investigated as function of pressure (5-150 Torr) and temperature (296-458 K). The bimolecular rate coefficient for CH2OO + HCOOH reaction shows a negative temperature dependency, decreasing from (1.0 ± 0.03) × 10-10 cm3 molecule-1 s-1 at 296 K to (0.47 ± 0.05) × 10-10 cm3 molecule-1 s-1 at 458 K with an Arrhenius activation energy of -4.9 ± 1.6 kJ mol-1, where statistical uncertainties shown are 2σ. Estimated overall uncertainty in the measured rate coefficients is about ±20%. Current bimolecular rate coefficient at room temperature agrees with the previously reported rate coefficients from the direct kinetic experiments. The reaction is found to be pressure independent over the range between 5 and 150 Torr at 296 K in He.

Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O2 Reaction.

The kinetics of (CH3)2CCH + O2 (1) and (CH3)2CCCH3 + O2 (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k1(298 K) = (4.5 ± 0.5) × 10-12 cm3 s-1 and k2(298 K) = (8.9 ± 1.0) × 10-12 cm3 s-1. The observed products for reactions 1 and 2 were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both ß-hydrogens in the vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of the CH2CH + O2 reaction by about 50%. However, the rate coefficient of the triply substituted (CH3)2CCCH3 radical reaction with O2 is almost identical to the CH2CH + O2 rate coefficient under the covered temperature range.

Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals: The Pent-1-en-3-yl + O2 Reaction.

The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.

Kinetics of the Methyl-Vinyl Radical + O2 Reactions Associated with Propene Oxidation.

The bimolecular rate coefficients of reactions CH3CCH2 + O2 (1) and cis/ trans-CH3CHCH + O2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures ( T < 420 K) and appear to be independent of temperature at high temperatures ( T > 420 K). Observed products of the reaction CH3CCH2 + O2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O2. Methyl substitution of the vinyl radical changes its reactivity toward O2 upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the ß-positions, respectively.

Reaction of Perfluorooctanoic Acid with Criegee Intermediates and Implications for the Atmospheric Fate of Perfluorocarboxylic Acids.

The reaction of perfluorooctanoic acid with the smallest carbonyl oxide Criegee intermediate, CH2OO, has been measured and is very rapid, with a rate coefficient of (4.9 ± 0.8) × 10-10 cm3 s-1, similar to that for reactions of Criegee intermediates with other organic acids. Evidence is shown for the formation of hydroperoxymethyl perfluorooctanoate as a product. With such a large rate coefficient, reaction with Criegee intermediates can be a substantial contributor to atmospheric removal of perfluorocarboxylic acids. However, the atmospheric fates of the ester product largely regenerate the initial acid reactant. Wet deposition regenerates the perfluorocarboxylic acid via condensed-phase hydrolysis. Gas-phase reaction with OH is expected principally to result in formation of the acid anhydride, which also hydrolyzes to regenerate the acid, although a minor channel could lead to destruction of the perfluorinated backbone.

Direct kinetics study of CH2OO + methyl vinyl ketone and CH2OO + methacrolein reactions and an upper limit determination for CH2OO + CO reaction.

Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. 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 were 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. For CH2OO + CF3CHCH2, an upper limit of k(CH2OO + CF3CHCH2) < 2 × 10-14 cm3 s-1 is obtained.

The reaction of Criegee intermediate CH2OO with water dimer: primary products and atmospheric impact.

The rapid reaction of the smallest Criegee intermediate, CH2OO, with water dimers is the dominant removal mechanism for CH2OO in the Earth's atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10-90%, relative to previous modeling assumptions, which indicates that the reaction CH2OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.

Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism.

Product formation, in particular ketohydroperoxide formation and decomposition, were investigated in time-resolved, Cl-atom initiated neopentane oxidation experiments in the temperature range 550-675 K using a photoionization time-of-flight mass spectrometer. Ionization light was provided either by Advanced Light Source tunable synchrotron radiation or ∼10.2 eV fixed energy radiation from a H2-discharge lamp. Experiments were performed both at 1-2 atm pressure using a high-pressure reactor and also at ∼9 Torr pressure employing a low-pressure reactor for comparison. Because of the highly symmetric structure of neopentane, ketohydroperoxide signal can be attributed to a 3-hydroperoxy-2,2-dimethylpropanal isomer, i.e. from a γ-ketohydroperoxide (γ-KHP). The photoionization spectra of the γ-KHP measured at low- and high pressures and varying oxygen concentrations agree well with each other, further supporting they originate from the single isomer. Measurements performed in this work also suggest that the "Korcek" mechanism may play an important role in the decomposition of 3-hydroperoxy-2,2-dimethylpropanal, especially at lower temperatures. However, at higher temperatures where γ-KHP decomposition to hydroxyl radical and oxy-radical dominates, oxidation of the oxy-radical yields a new important channel leading to acetone, carbon monoxide, and OH radical. Starting from the initial neopentyl + O2 reaction, this channel releases altogether three OH radicals. A strongly temperature-dependent reaction product is observed at m/z = 100, likely attributable to 2,2-dimethylpropanedial.

Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH3)2COO.

The Criegee intermediate acetone oxide, (CH3)2COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O2 and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO2 was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10-11 cm3 s-1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10-10 cm3 s-1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti-CH3CHOO with SO2, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N2 from cavity ring-down decay of the ultraviolet absorption of (CH3)2COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10-10 to (2.29 ± 0.08) × 10-10 cm3 s-1. Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, kH/kD = (0.53 ± 0.06), for reactions with SO2, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD3)2COO with NO2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10-12 cm3 s-1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of anti-CH3CHOO with NO2. Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH3)2COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s-1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH3)2COO with SO2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH3)2COO with NO2 and with SO2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.

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-d4-DEK [d4-DEK; (CH3CD2)2C═O], and 1,1,1,5,5,5-d6-DEK [d6-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, RP) or secondary (3-pentan-on-2-yl, RS) 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 RS with O2. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (RP), 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 ß-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of d4-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products.

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether.

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + CH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OCHOOH intermediate, which predominantly leads to the HPMF.