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.

Reaction Kinetics of Criegee Intermediates with Nitric Acid.

This work investigated the reaction kinetics of HNO3 with four Criegee intermediates (CIs): CH2OO, (CH3)2COO, methyl vinyl ketone oxide (MVKO), and methacrolein oxide (MACRO). Our results show that these reactions are extremely fast with rate coefficients of (1.51 ± 0.45) × 10-10, (3.54 ± 1.06) × 10-10, (3.93 ± 1.18) × 10-10, and (3.0 ± 1.0) × 10-10 cm3 s-1 for reactions of HNO3 with CH2OO, (CH3)2COO, syn-MVKO, and anti-MACRO, respectively. This is consistent with previous results for the reactions between CIs and carboxylic acids, but the rate coefficient of CH2OO + HNO3 in the literature [Foreman Angew. Chem. 2016, 128, 10575] was found to be overestimated by a factor of 3.6. In addition, we did not observe any significant pressure dependence in the HNO3 reactions with CH2OO and (CH3)2COO under 100-400 Torr. Our results indicate that in a dry area with severe NOx pollution, the reactions of CIs with HNO3 and their products may be worthy of attention, but these reactions may be insignificant under high-humidity conditions. However, CI reactions with HNO3 may not play an important role in the atmospheric removal processes of HNO3 because of the low concentrations of CIs.

Absolute photodissociation cross sections of thermalized methyl vinyl ketone oxide and methacrolein oxide.

Methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO) are resonance-stabilized Criegee intermediates which are formed in the ozonolysis reaction of isoprene, the most abundant unsaturated hydrocarbon in the atmosphere. The absolute photodissociation cross sections of MVKO and MACRO were determined by measuring their laser depletion fraction at 352 nm, which was deduced from their time-resolved UV-visible absorption spectra. After calibrating the 352 nm laser fluence with the photodissociation of NO2, for which the absorption cross section and photodissociation quantum yield are well known, the photodissociation cross sections of thermalized (299 K) MVKO and MACRO at 352 nm were determined to be (3.02 ± 0.60) × 10-17 cm2 and (1.53 ± 0.29) × 10-17 cm2, respectively. Using their reported spectra and photodissociation quantum yields, their peak absorption cross sections were deduced to be (3.70 ± 0.74) × 10-17 cm2 (at 371 nm, MVKO) and (3.04 ± 0.58) × 10-17 cm2 (at 397 nm, MACRO). These values agree fairly with our theoretical predictions and are substantially larger than those of smaller, alkyl-substituted Criegee intermediates (CH2OO, syn-CH3CHOO, (CH3)2COO), revealing the effect of extended conjugation. With their cross sections, we also quantified the synthesis yields of MVKO and MACRO in the present experiment to be 0.22 ± 0.10 (at 299 K and 30-700 torr) and 0.043 ± 0.019 (at 299 K and 500 torr), respectively, relative to their photolyzed precursors. The lower yield of MACRO can be related to the high endothermicity of its formation channel.

Substituent Effect in the Reactions between Criegee Intermediates and 3-Aminopropanol.

Via intramolecular H atom transfer, 3-aminopropanol is more reactive toward Criegee intermediates, in comparison with amines or alcohols. Here we accessed the substituent effect of Criegee intermediates in their reactions with 3-aminopropanol. Through real-time monitoring the concentrations of two Criegee intermediates with their strong UV absorption at 340 nm, the experimental rate coefficients at 298 K (100-300 Torr) were determined to be (1.52 ± 0.08) × 10-11 and (1.44 ± 0.22) × 10-13 cm3 s-1 for the reactions of 3-aminopropanol with (CH3)2COO (acetone oxide) and CH2CHC(CH3)OO (methyl vinyl ketone oxide), respectively. Compared to our previous experimental value for the reaction with syn-CH3CHOO, (1.24 ± 0.13) × 10-11 cm3 s-1, we can see that the methyl substitution at the anti position has little effect on the reactivity while the vinyl substitution causes a drastic decrease in the reactivity. Our theoretical calculations based on CCSD(T)-F12 energies reproduce this 2-order-of-magnitude decrease in the rate coefficient caused by the vinyl substitution. Using the activation strain model, we found that the interaction of Criegee intermediates with 3-aminopropanol is weaker for the case of vinyl substitution. This effect can be further rationalized by the delocalization of the lowest unoccupied molecular orbital for the vinyl-substituted Criegee intermediates. These results would help us better estimate the impact of similar reactions like the reactions of Criegee intermediates with water vapor, some of which could be difficult to measure experimentally but can be important in the atmosphere. We also found that the B2PLYP-D3BJ/aug-cc-pVTZ calculation can reproduce the CCSD(T)-F12 reaction barrier energies within ca. 1 kcal mol-1, indicating that the use of the B2PLYP-D3BJ method is promising for future predictions of the reactions of larger Criegee intermediates.

Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions.

Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO2 by CIs is a potential source of H2SO4, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO2 has been found, kSO2 = (1.5 ± 0.4) × 10-10 cm3 s-1 (±1σ, σ is the standard deviation of the data) at 298 K (150 - 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10-17 cm3 s-1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO2 would be substantial, even more than that of syn-MVKO.

Kinetics of Unimolecular Decay of Methyl Vinyl Ketone Oxide, an Isoprene-Derived Criegee Intermediate, under Atmospherically Relevant Conditions.

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO2, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278-319 K and 100-503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn-MVKO was measured to be kuni = 70 ± 15 s-1 (1σ uncertainty) at 298 K with negligible pressure dependence. In addition, kuni increases from ca. 30 s-1 at 278 K to ca. 175 s-1 at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol-1, kuni(T) = (9.3 × 107)exp(-4200/T) s-1. Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere. The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design.

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH3 CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH2 group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

The role of the iodine-atom adduct in the synthesis and kinetics of methyl vinyl ketone oxide-a resonance-stabilized Criegee intermediate.

Isoprene is the most abundant alkene in the atmosphere. Ozonolysis of isoprene produces three very reactive carbonyl oxides (Criegee intermediates), including formaldehyde oxide, methyl vinyl ketone oxide (MVKO, CH3(C2H3)COO), and methacrolein oxide. The latter two Criegee intermediates are resonance-stabilized due to the vinyl group. Recently, the electronic spectrum of thermalized MVKO has been reported [Caravan, et al., Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 9733]. In this work, we utilized this strong UV/visible absorption to investigate the reaction kinetics of MVKO with SO2 under a wide pressure range of 4 to 700 Torr. We followed a new method [Barber, et al., J. Am. Chem. Soc., 2018, 140, 10866], in which MVKO is produced through the reaction of a resonance-stabilized iodoalkene radical with O2. The experimental data are consistent with a kinetic model that the reaction goes through an adduct of CH3(C2H3)CIOO, similar to the cases of H/alkyl substituted Criegee intermediates. However, different from the H/alkyl adducts, which are stable over the time scales of typical kinetic experiments, this vinyl adduct CH3(C2H3)CIOO is less stable and decomposes to MVKO + I at a time scale of 10-3 s (faster at higher temperature), consistent with the results of quantum chemistry calculations and the fact that the resonance stabilization is disrupted at the adduct structure. The adduct decomposition is the major pathway that forms MVKO for pressures higher than 50 Torr. In addition, temperature dependence has been investigated for 278-319 K. The experimental activation energy of the adduct decomposition was measured to be 12.7 ± 0.3 kcal mol-1, consistent with the calculated dissociation energy of the adduct to MVKO + I (14 kcal mol-1). Furthermore, the temperature dependent rate coefficient of MVKO + SO2 reaction has been measured to be kSO2 = (4.0 ± 0.6) × 10-11 cm3 s-1 at 4-700 Torr and 298 K with a negative activation energy of -3.7 ± 0.4 kcal mol-1.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate.

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO.

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278-318 K and 100-700 torr. We used water vapor as the scavenger of anti-CH3CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO (at 300 torr), which increases from (67 ± 15) s-1 at 278 K, (146 ± 31) s-1 at 298 K, to (288 ± 81) s-1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol-1 and a weak pressure dependence for 100-700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH3CHOO at higher pressures, which are more relevant to atmospheric conditions.

Effects of water vapor on the reaction of CH2OO with NH3.

The reaction of the simplest Criegee intermediate, CH2OO, with ammonia and water vapor has been investigated at 278-308 K and under 100-760 Torr by monitoring the strong UV absorption of CH2OO. We found that the observed decay rate of CH2OO becomes much larger when ammonia and water vapor are both present; the combinational effect of ammonia and water vapor is significantly greater than the sum of their individual contributions, revealing a strong synergic effect. The kinetic data are consistent with a termolecular process of CH2OO + NH3 + H2O reaction, of which the reaction rate coefficient was determined to be kNH3+H2O = (8.2 ± 1.2) × 10-31 cm6 s-1 at 298 K with a negative activation energy, Ea = -8.0 ± 0.8 kcal mol-1 [kNH3+H2O(T) = 1.04 × 10-36 exp(4047/T)]. Quantum chemistry calculation (at the QCISD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,2p) level) found a low-energy reaction pathway, on which water accepts a hydrogen atom (or proton) from ammonia and releases another hydrogen atom to the terminal oxygen of CH2OO. The predicted products are H2NCH2OOH and a new H2O molecule, indicating water catalysis. This reaction is very fast and probably barrierless, which poses a theoretical challenge to modeling the related kinetics.

Hydrogen-Bonding Mediated Reactions of Criegee Intermediates in the Gas Phase: Competition between Bimolecular and Termolecular Reactions and the Catalytic Role of Water.

Criegee intermediates have substantial Zwitterionic character and interact strongly with hydrogen-bonding molecules like H2O, NH3, CH3OH, etc. Some of the observed reactions between Criegee intermediates and hydrogen-bonding molecules exhibit third-order kinetics. The experimental data indicate that these termolecular reactions involve one Criegee intermediate and two hydrogen-bonding molecules; quantum chemistry calculation shows that one of the hydrogen-bonding molecules acts as a catalytic bridge, which receives a hydrogen atom and donates another one. In this Feature Article, we will discuss the roles of the hydrogen-bonding molecules and the trend of the reactivity for the title reactions. To better predict the competition between a catalyzed reaction (a termolecular process) and its bare reaction (a bimolecular process), we analyzed the free energy landscape of the competing reaction paths under pseudo-first-order conditions. The results indicate that the entropy reduction in the translational degrees of freedom is the main cause to hinder a catalyzed termolecular process under typical experimental concentrations at near ambient temperatures. For such a termolecular process to be significant, its energy gain (barrier lowering) by adding the catalytic molecule has to be large enough to compensate the corresponding entropy cost. One great advantage of this analysis is that the translational entropy only depends on simple parameters like temperature, reactant masses, and concentrations and thus can be easily estimated.

Temperature and isotope effects in the reaction of CH3CHOO with methanol.

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH3CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH3CHOO with methanol show a linear dependence on [CH3OH]. The bimolecular rate coefficient was determined to be k1Ha = (4.8 ± 0.5) × 10-12 cm3 s-1 at 298 K and 250 Torr with a negative activation energy Ea = -2.8 ± 0.3 kcal mol-1 for T = 288-315 K [k(T) = A exp(-Ea/RT)]. For the reaction of syn-CH3CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH3OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k2Hs = (8.0 ± 1.0) × 10-32 cm6 s-1 at 298 K and 250 Torr with a strong negative temperature dependence (Ea = -13.2 ± 0.3 kcal mol-1) at 273-323 K. No significant pressure effect was observed at 250-760 Torr. A kinetic isotope effect, k2Hs/k2Ds = 2.5, was observed by changing CH3OH to CH3OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH3CHOO with one CH3OH molecule was not observed in the studied concentration range.

Temperature-Dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee Intermediate.

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10-12 cm3 s-1. The results exhibit a weak negative temperature dependence: the activation energy Ea ( k = Ae- Ea/ RT) is -1.83 ± 0.05 kcal mol-1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6 × 10-12 cm3 s-1 at 298 K; Ea = - 2.80 kcal mol-1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH2OO or CH3SH.

Synergy of Water and Ammonia Hydrogen Bonding in a Gas-Phase Reaction.

We report a very significant cooperative effect of water-ammonia hydrogen bonding in their reactions with a Criegee intermediate, syn-CH3CHOO. Under near ambient conditions, we found that the reaction of syn-CH3CHOO with NH3 becomes much faster (by up to 138 times) at high humidity. Intriguingly, merely adding NH3 (or H2O) alone has almost no effect on the rate of syn-CH3CHOO decay. Quantum chemistry calculation shows that the main reaction involves a hydrogen-atom (or proton) relay in a hydrogen-bonded ring structure; on the product side, a C-N bond is formed and H2O is regenerated as a catalyst. This result demonstrates a new category of reaction in which having two types of hydrogen-bond players (NH3 and H2O) is much more effective than having only one.

Criegee Intermediate Reaction with Alcohol Is Enhanced by a Single Water Molecule.

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH3CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0-80%) under near-ambient conditions (298 K, 250-755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH3CHOO + CH3OH + H2O → products, has been determined to be (1.95 ± 0.11) × 10-32 cm6 s-1 at 298 K, with no observable pressure dependence for 250-755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H2O is reformed and acts as a catalyst.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory.

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of -0.53 kcal mol-1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10-14 cm3 molecule-1 s-1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10-14 cm3 molecule-1 s-1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time.

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate Near 1285.7 cm-1.

The ν4 fundamental of the simplest Criegee intermediate, CH2OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4-94 Torr. This IR spectrum provides an unambiguous identification of CH2OO and is potentially useful to determine the number density of CH2OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH2OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm-1 (covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH2OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10-19 cm·molecule-1 or 2.2 ± 0.4 km·mol-1. The whole rotational band (P, Q, and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν4 band) to be 19.2 ± 3.5 km·mol-1. For a practical detection of CH2OO, this work also reports the peak cross section as a function of total pressure (4-94 Torr O2). At low pressure (≤4 Torr), where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2 ± 0.9) × 10-18 cm2·molecule-1 (at the system line width of 0.004 cm-1 fwhm).

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 anti-CH3CHOO + CO2 reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed anti-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 (keff < 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).