Pressure-Dependent I‑Atom Yield in the Reaction of CH₂I with O₂ Shows a Remarkable Apparent Third-Body Efficiency for O₂

The formation of I atom and Criegee intermediate (CH₂OO) in the reaction of CH₂I with O₂ has potential relevance for aerosol and organic acid production in the marine boundary layer. We report measurements of the absolute yield of I atom as a function of pressure for N₂, He, and O₂ buffer at 298 K. Although the overall rate coefficient is pressure-independent, the I-atom yield, correlated with CH₂OO, decreases with total pressure, presumably because of increased stabilization of CH₂IOO. The extrapolated yield of the I + Criegee channel under tropospheric conditions is small but nonzero, ∼0.04. The zero-pressure limiting I-atom yield is unity, within experimental error, implying negligible branching to IO + CH₂O. The apparent collision efficiency of O₂ in stabilizing CH₂IOO is a remarkable factor of 13 larger than that of N₂, which suggests unusually strong interaction or possible reaction between the chemically activated CH₂IOO^# and O₂

Kinetic (<i>T</i> = 201–298 K) and Equilibrium (<i>T</i> = 320–420 K) Measurements of the C₃H₅ + O₂ ⇆ C₃H₅O₂ Reaction

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (<i>T</i> = 201–298 K), association kinetics were observed, and the measured time-resolved C₃H₅ radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {<i>p</i> = 0.4–36 Torr, [He] = (1.7–118.0) × 10¹⁶ cm^–3}. The bimolecular rate coefficients obtained vary in the range (0.88–11.6) × 10^–13 cm³ s^–1. In higher-temperature experiments (<i>T</i> = 320–420 K), the C₃H₅ radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting ln <i>K</i>_p values versus 1/<i>T</i> in a modified van’t Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of Δ<i>H</i>₂₉₈^° = −78.3 ± 1.1 kJ mol^–1 and Δ<i>S</i>₂₉₈^° = −129.9 ± 3.1 J mol^–1 K^–1, with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as Δ<i>H</i>₂₉₈^° = −75.6 ± 2.3 kJ mol^–1

CH₂NH₂ + O₂ and CH₃CHNH₂ + O₂ Reaction Kinetics: Photoionization Mass Spectrometry Experiments and Master Equation Calculations

Two carbon centered amino radical (CH₂NH₂ and CH₃CHNH₂) reactions with O₂ were scrutinized by means of laboratory gas kinetics experiments together with quantum chemical computations and master equation modeling. In the experiments, laser photolysis of alkylamine compounds at 193 nm was used for the radical production and photoionization mass spectrometry was employed for the time-resolved detection of the reactants and products. The investigations were performed in a tubular, uncoated borosilicate glass flow reactor. The rate coefficients obtained were high, ranging from 2.4 × 10^–11 to 3.5 × 10^–11 cm³ molecule^–1 s^–1 in the CH₂NH₂ + O₂ reaction and from 5.5 × 10^–11 to 7.5 × 10^–11 cm³ molecule^–1 s^–1 in the CH₃CHNH₂ + O₂ reaction, showed negative temperature dependence with no dependence on the helium bath gas pressure (0.5 to 2.5 Torr He). The measured rate coefficients can be expressed as a function of temperature with: <i>k</i>(CH₂NH₂ + O₂) = (2.89 ± 0.13) × 10^–11 (<i>T</i>/300 K)^{−(1.10±0.47)} cm³ molecule^–1 s^–1 (267–363 K) and <i>k</i>(CH₃CHNH₂ + O₂) = (5.92 ± 0.23) × 10^–11 (<i>T</i>/300 K)^{−(0.50±0.42)} cm³ molecule^–1 s^–1 (241–363 K). The reaction paths and mechanisms were characterized using quantum chemical calculations and master equation modeling. Master equation computations, constrained by experimental kinetic results, were employed to model pressure-dependencies of the reactions. The constrained modeling results reproduce the experimentally observed negative temperature dependence and the dominant CH₂NH imine production in the CH₂NH₂ + O₂ reaction at the low pressures of the present laboratory investigation. In the CH₃CHNH₂ + O₂ reaction, similar qualitative behavior was observed both in the rate coefficients and in the product formation, although the fine details of the mechanism were observed to change according to the different energetics in this system. In conclusion, the constrained modeling results predict significant imine + HO₂ production for both reactions even at atmospheric pressure

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 <i>ab initio</i> 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), <i>cycl</i>-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 + ĊH₂OCH₂OOH reaction, resulting from the decomposition of the HOOCH₂OĊHOOH intermediate, which predominantly leads to the HPMF

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 <i>anti</i>-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, <i>k</i>_H/<i>k</i>_D = (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 <i>anti</i>-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