Reactivity of Criegee Intermediates toward Carbon Dioxide

Recent theoretical work by Kumar and Francisco suggested that the high reactivity of Criegee intermediates (CIs) could be utilized for designing efficient carbon capture technologies. Because the <i>anti</i>-CH₃CHOO + CO₂ reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed <i>anti</i>-CH₃CHOO with its strong UV absorption at 365 nm and measured the rate coefficient to be ≤2 × 10^–17 cm³ molecule^–1 s^–1 at 298 K, which is consistent with our theoretical value of 2.1 × 10^–17 cm³ molecule^–1 s^–1 at the QCISD­(T)/CBS//B3LYP/6-311+G­(2d,2p) level but inconsistent with their results obtained at the M06-2X/aug-cc-pVTZ level, which tends to underestimate the barrier heights. The experimental result indicates that the reaction of a Criegee intermediate with atmospheric CO₂ (400 ppmv) would be inefficient (<i>k</i>_eff < 0.2 s^–1) and cannot compete with other decay processes of Criegee intermediates like reactions with water vapor (∼10³ s^–1) or thermal decomposition (∼10² s^–1)

Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt = −kobs[CH2OO]; kobs = k0 + k1[water] + k2[water]2 + k3[water]3; k1 = (4.22 ± 0.48) × 10–16 cm3 s–1, k2 = (10.66 ± 0.83) × 10–33 cm6 s–1, k3 = (1.48 ± 0.17) × 10–50 cm9 s–1 at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1 = 1.8 ± 1.1 kcal mol–1, Ea2 = −11.1 ± 2.1 kcal mol–1, Ea3 = −17.4 ± 3.9 kcal mol–1. The contribution of the k3[water]3 term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction

Does Ozone–Water Complex Produce Additional OH Radicals in the Atmosphere?

Ozone–water complex has been thought to play a role in producing atmospheric OH radicals through its photolysis. Here, we re-examined the absorption cross-section of the ozone–water complex with a new method to tell whether the above speculation is valid. With argon solvation and photoionization by tunable vacuum ultraviolet light, we were able to selectively probe the ozone–water 1:1 complex. The measured cross-section of the complex is only similar to the sum of the cross-sections of ozone and water monomers at 157.6, 248.4, and 308.4 nm. In addition, we did not observe any absorption of the complex at 351.8 nm. The results indicate that the OH production through the photolysis of the ozone–water complex is much slower than previously thought

Unimolecular Decomposition Rate of the Criegee Intermediate (CH₃)₂COO Measured Directly with UV Absorption Spectroscopy

The unimolecular decomposition of (CH₃)₂COO and (CD₃)₂COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient <i>k</i>_d for (CH₃)₂COO shows a strong temperature dependence, increasing from 269 ± 82 s^–1 at 283 K to 916 ± 56 s^–1 at 323 K with an Arrhenius activation energy of ∼6 kcal mol^–1. The bimolecular rate coefficient for the reaction of (CH₃)₂COO with SO₂, <i>k</i>_SO₂, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for <i>k</i>_d and <i>k</i>_SO₂ are consistent with previously reported relative rate coefficients <i>k</i>_d/<i>k</i>_SO₂ of (CH₃)₂COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of <i>k</i>_d for (CH₃)₂COO are consistent with the experiment, and the combination of experiment and theory for (CD₃)₂COO indicates that tunneling plays a significant role in (CH₃)₂COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH₃)₂COO measured here, in light of the relatively slow rate for the reaction of (CH₃)₂COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO₂ for atmospheric removal of the dimethyl-substituted Criegee intermediate

Photoproduct Channels from BrCD₂CD₂OH at 193 nm and the HDO + Vinyl Products from the CD₂CD₂OH Radical Intermediate

We present the results of our product branching studies of the OH + C₂D₄ reaction, beginning at the CD₂CD₂OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD₂CD₂OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD₂CD₂OH), and of the secondary dissociation of vibrationally excited CD₂CD₂OH radicals (OH, C₂D₄/CD₂O, C₂D₃, CD₂H, and CD₂CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD₂CD₂O and DBr + CD₂CDOH, and measure the branching ratio between the C–Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at <i>m</i>/<i>e</i> = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C₂D₃) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C₂D₃ product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals

Temperature-Dependent Rate Coefficients for the Reaction of CH₂OO with Hydrogen Sulfide

The reaction of the simplest Criegee intermediate CH₂OO with hydrogen sulfide was measured with transient UV absorption spectroscopy in a temperature-controlled flow reactor, and bimolecular rate coefficients were obtained from 278 to 318 K and from 100 to 500 Torr. The average rate coefficient at 298 K and 100 Torr was (1.7 ± 0.2) × 10^–13 cm³ s^–1. The reaction was found to be independent of pressure and exhibited a weak negative temperature dependence. <i>Ab initio</i> quantum chemistry calculations of the temperature-dependent reaction rate coefficient at the QCISD­(T)/CBS level are in reasonable agreement with the experiment. The reaction of CH₂OO with H₂S is 2–3 orders of magnitude faster than the reaction with H₂O monomer. Though rates of CH₂OO scavenging by water vapor under atmospheric conditions are primarily controlled by the reaction with water dimer, the H₂S loss pathway will be dominated by the reaction with monomer. The agreement between experiment and theory for the CH₂OO + H₂S reaction lends credence to theoretical descriptions of other Criegee intermediate reactions that cannot easily be probed experimentally

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere