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Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO

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Science  12 Apr 2013:
Vol. 340, Issue 6129, pp. 177-180
DOI: 10.1126/science.1234689

More Criegee Sightings

The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H2CO2, was recently detected and monitored in the laboratory. Su et al. (p. 174; see the Perspective by Vereecken) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177; see the Perspective by Vereecken) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms.

Abstract

Although carbonyl oxides, “Criegee intermediates,” have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10−14 ± 0.4 × 10−14 centimeter3 second−1.

Ozonolysis of alkenes is generally understood to proceed via a 1,3-cycloaddition of ozone across the olefinic bond to produce a primary ozonide, the decomposition of which forms a carbonyl moiety and a Criegee intermediate (CI) (1). The fate of the CI determines the end products of the ozonolysis reaction and can have a substantial impact on the atmosphere (14). Recently, the simplest CI, CH2OO, has been directly produced in the gas phase with low internal energies from reaction of O2 with CH3SOCH2 (5, 6) or CH2I (4, 7, 8) and unambiguously detected by tunable synchrotron or laser photoionization mass spectrometry. These techniques allowed direct measurements of the reaction kinetics of CH2OO with several important tropospheric species, including SO2 and NO2, both of which react much faster with CH2OO than models had assumed (4). Since those direct measurements appeared, new high-level calculations (9), field measurements (10), and ozonolysis experiments (1012) continue to suggest that CI reactions are important in tropospheric sulfate chemistry. However, substantial uncertainty remains, partly because of the absence of direct kinetic measurements of any larger CI and partly because of uncertainty in the products of CI reactions. Moreover, the reactivity of larger CIs is predicted to be affected by the nature and location of the substituents (13), with a particularly large effect for the crucial reaction of CI with water (13, 14). Because of the large amount of water in Earth’s atmosphere, the rate of CI removal by water is a key determinant of the tropospheric impact of all CI reactions. Determining the conformer dependence of CI reactions is therefore not only a vital aspect of understanding their fundamental reactivity, it is also a key component for improving atmospheric chemistry models.

We have successfully extended our earlier technique of reacting α-iodoalkyl radicals with O2 to prepare CIs (4, 7, 15), and here we show that the reaction of the 1-iodoethyl radical (CH3CHI) with O2 forms both conformers of the CI acetaldehyde oxide (CH3CHOO) at 298 K and 4 torr. The conformers of acetaldehyde oxide, syn-CH3CHOO and anti-CH3CHOO, differ in the orientation of the C-O-O group (depicted in Fig. 1). By taking advantage of the difference in ionization energy of the two conformers, we demonstrate a dramatic conformer dependence of CH3CHOO reactivity toward SO2 and H2O.

Kinetic measurements were carried out in the Multiplexed Chemical Kinetics Reactor, which has been described in detail elsewhere (4, 16, 17). Reactions are initiated by pulsed laser photolysis in a slow-flow reactor. The contents are continuously sampled through a small orifice in the reactor and probed by photoionization time-of-flight mass spectrometry. Ionizing with tunable photon energy, from the Chemical Dynamics Beamline (9.0.2) of the Advanced Light Source, allows isomers to be distinguished on the basis of their photoionization spectra (1719). The reaction of CH3CHI with O2 shows similar behavior to the reaction of CH2I with O2, with the most prominent products being the stabilized CI. (CH3CHOO), an I atom, and secondary products IO and HOI. (A time-resolved mass spectrum of the reaction initiated by photolysis of CH3CHI2 in the presence of oxygen is displayed in fig. S2.)

The mass/charge (m/z) = 60 signal is identified as the CI, acetaldehyde oxide, on the basis of its exact mass and a comparison of its photoionization spectrum with a predicted spectrum derived from ab initio calculations (19) of both the adiabatic ionization energy (AIE) and Franck-Condon factors for photoionization of both conformers of CH3CHOO, as shown in Fig. 1. Detailed results of these calculations are given in the supplementary text.

Fig. 1 The photoionization spectrum for the m/z = 60 product from photolysis of CH3CHI2 in the presence of O2.

The best fit of the zero-water trace to the calculated photoionization spectra for syn- and anti-CH3CHOO is also shown. The fit allows the the energies for the excited (A′) cationic state of each conformer to vary within a range of ~50 meV about their calculated values. The black dashed line shows the calculated photoionization spectrum of vinyl hydroperoxide. Addition of water preferentially removes the anti conformer, as seen in the decreased signal between 9.3 and 9.4 eV for the high-H2O trace.

Calculations place the syn conformer ~15 kJ mol−1 lower in energy than anti-CH3CHOO (14). Reflecting the zwitterionic character of the C–O bond, the barrier to interconversion of these conformers is substantial (14), ~160 kJ mol−1. Therefore, syn- and anti-CH3CHOO act as distinct chemical species at atmospheric temperatures. The syn and anti conformers of CH3CHOO also have different photoionization spectra. In the threshold region, each conformer has ionization transitions to both A″ and A′ cationic states, and the calculated four photoionization bands overlap (fig. S4). Allowing a small adjustment to the computed ionization energies for each conformer gives a very close fit to the observed spectrum (Fig. 1). The low-energy edge of the spectrum is dominated by the anti conformer, which can be detected below the ionization energy of the syn conformer. Assuming that the electronic transition moments for the two conformers are similar, the fit parameters suggest a far larger overall production (90%) of the more stable syn conformer. The signals observed at higher ionizing photon energies will be dominated by photoionization of syn-CH3CHOO (supplementary text). The two conformers are independently detected, but their ratio is not varied in these experiments.

Acetaldehyde oxide has several other isomers (scheme S1), most of which have ionization energies well above the threshold observed for the m/z = 60 product of the reaction of CH3CHI with O2, including acetic acid, the AIE of which is 10.70 eV, and methyl formate, with an AIE of 10.85 eV (20). Only vinyl hydroperoxide has a low enough calculated AIE, 9.18 eV, to be considered as a carrier for the m/z = 60 product spectrum. However, the predicted ionization onset and Franck-Condon envelope of vinyl hydroperoxide (Fig. 1) is not consistent with the observed spectrum.

Vinyl hydroperoxide is calculated to be the dominant product of isomerization from syn-CH3CHOO, with a calculated barrier to isomerization of >80 kJ mol−1 (21). [Isomerization of anti-CH3CHOO is more likely to lead to methyl dioxirane (21), with a calculated AIE of 10.36 eV.] The O–O bond in vinyl hydroperoxide is weak (22), and fission of that bond is a pathway to formation of OH in ozonolysis reactions (23). In fact, recent multireference calculations predict extremely rapid dissociation of vinyl hydroperoxide after isomerization from CH3CHOO (24).

The first-order decay rate of CH3CHOO in the absence of additional reactants is the sum of unimolecular and heterogeneous loss reactions. The decay depends on the coating of the reactor, suggesting that heterogeneous reaction contributes substantially. The slowest decay observed in this context, ~250 s−1, is an upper limit to the thermal (298 K) rate coefficient for decomposition of CH3CHOO, consistent with the determination of 76 s−1 by Fenske et al. (25). Upon adding SO2 or NO2 as reagent, the decay of the CH3CHOO signal becomes more rapid (shown in Fig. 2 for SO2 reaction with CH3CHOO). A linear fit of the decay constant versus reactant concentration measured at photon energies where photoionization of the syn conformer dominates (Fig. 3 and fig. S5) returns as its slope k1,syn, the second-order rate coefficient for the reaction of syn-CH3CHOO with SO2 (uncertainty limits are 95%):

syn-CH3CHOO + SO2 → products k1,syn = 2.4 × 10−11 ± 0.3 × 10−11 cm3 s−1 (1)
Fig. 2 Representative time-dependent CH3CHOO signals.

The time behavior of the m/z = 60 signal (mainly from ionization of syn-CH3CHOO) from photolysis of CH3CHI2 in the presence of SO2 and [O2] = 1.2 × 1016 cm−3, measured with Lyman-α radiation at 10.2 eV.

Fig. 3 Rate coefficients for reaction of CH3CHOO conformers with SO2.

Fitted pseudo–first-order decay constants of syn-CH3CHOO (solid circles) and anti-CH3CHOO (open circles) are shown as a function of SO2 concentration. The lines are linear fits to the data, weighted by the statistical uncertainty in the exponential fit to the individual decay traces (±1σ error bars shown in the figure). Each fit included >20 points across the relevant decay. (Inset) The decay of CH3CHOO measured with 10.50 eV (solid circles, predominantly syn) and 9.37 eV (open circles, predominantly anti) in the presence of [SO2] = 7.2 × 1012 cm−3.

A similar plot measured with 9.37-eV photons, where photoionization of the anti-CH3CHOO conformer dominates (open symbols in Fig. 3), yields a rate coefficient more than twice as large,

anti-CH3CHOO + SO2 → products k1,anti = 6.7 × 10−11 ± 1.0 × 10−11 cm3 s−1 (2)

Similar measurements on the overall reaction of syn- and anti-CH3CHOO with NO2 (described in supplementary text) suffer from reduced signal-to-noise ratio but show a rate coefficient of 2 × 10−12 ± 1 × 10−12 cm3 s−1, with a slight (statistically significant at 1σ confidence interval) conformer dependence. This rate coefficient is substantially smaller than that for CH2OO reacting with NO2, k(CH2OO + NO2) = 7 × 10−12 cm3 s−1 (4).

The rapid reaction of CH3CHOO with SO2 supports predictions of barrierless addition of SO2 to CI (9, 26, 27) and indicates that the reactions of CI with SO2 should be generally facile. The substantial difference in reactivity between syn- and and anti-CH3CHOO may partly reflect increased steric hindrance for formation of the CI-SO2 secondary ozonide in the syn conformation or conformer-selective electron donating effects from the methyl group, as theoretically described, for example, in the CH3CHOO + H2O reaction (13). The rate coefficient for syn-CH3CHOO reaction with SO2 is slightly smaller than that measured earlier for CH2OO reacting with SO2 [k(CH2OO + SO2) = 3.9 × 10−11 ± 0.7 × 10−11 cm3 s−1] (4), but that for anti-CH3CHOO is larger. Both are orders of magnitude larger than estimates typically used in tropospheric models (28).

Measurements have also been carried out at higher ionizing photon energies (13 eV), where the SO3 co-product [AIE of 12.81 eV (29)] can be directly observed. To within the experimental uncertainty, SO3 is formed with a rise time that correlates with the decay time of the CI (figs. S6 to S9), showing that SO3 is a direct product of the reactions of both CH2OO and CH3CHOO with SO2. Vereecken et al. (9), improving on earlier calculations (26, 27), predicted that formation of SO3 and a carbonyl compound will be the principal product channel for small CIs and at low pressure but that stabilization of a CI-SO2 secondary ozonide becomes dominant for larger CIs under atmospheric conditions. Under the conditions of the present experiments, no stabilized CI-SO2 product is observed for either CH2OO or CH3CHOO reaction with SO2. The eventual fate of the CI-SO2 secondary ozonide may determine the relevance of CI reactions with SO2 to tropospheric sulfate chemistry and particulate formation, and will affect the interpretation and intercomparison of laboratory ozonolysis experiments that are sensitive to either CI consumption or H2SO4 formation (1012).

Because reaction with water dominates tropospheric CI removal (9), its rate constant is critical to modeling the CI concentration. The reaction of CH2OO with water is too slow to measure in the present apparatus (4), and only an upper limit could be obtained, 4 × 10−15 cm3 s−1. However, the reaction of water with CH3CHOO is predicted to favor the anti conformer by five orders of magnitude (13), and indeed (Fig. 1) the addition of H2O preferentially depletes the photoionization spectrum at the low-energy edge, where anti-CH3CHOO dominates. Measurements of CH3CHOO with 10.5-eV photons in the presence of H2O concentrations of 2.4 × 1016 cm−3 showed an identical decay to those with no added water, suggesting similar upper limits for the reaction of water with syn-CH3CHOO and with CH2OO: 4 × 10−15 cm3 s−1 (4). However, a significant and systematic increase in the decay rate with water addition was observed for measurements with 9.37-eV photons (Fig. 4), permitting direct measurement of the rate coefficient (uncertainty limits are 95%),

anti-CH3CHOO + H2O → products k3,anti = 1.0 × 10−14 ± 0.4 × 10−14 cm3 s−1 (3)
Fig. 4 Rate coefficient for reaction of anti-CH3CHOO with H2O.

Fitted pseudo–first-order decay constants of anti-CH3CHOO are shown as a function of H2O concentration. The dashed line is a fit to the data, weighted by the statistical uncertainty in the exponential fits to the individual decay traces (±1σ error bars shown in the figure). Each fit included >20 points across the relevant decay. (Inset) The decay of CH3CHOO measured with 10.50 eV (solid circles, predominantly syn) and 9.37 eV (open circles, predominantly anti) in the presence of [H2O] = 2.7 × 1016 cm−3.

This result is more than a factor of 10 below the high-pressure limiting rate coefficient from the transition state theory predictions of Anglada et al. (13), k3,anti = 1.68 × 10−13 cm3 s−1, but is a factor of 35 above the prediction of Kuwata et al. (14), k3,anti = 2.87 × 10−16 cm3 s−1. In any case, the rate coefficient is substantially larger than that calculated for other CI reactions with water and tends to validate the theoretical prediction of a dramatic lowering of the activation energy for the anti conformer (13, 14). Because the reaction is predicted to proceed by stabilization (supplementary text), it is possible that the rate coefficient is pressure dependent. Master-equation calculations, including accurate treatment of the association kinetics, would give the most rigorous comparison for the present experiments.

We emphasize that the low-pressure rate coefficients measured here for reactions of the CIs with SO2, NO2, and water are lower limits for the total removal of CI by these species at the higher pressures of the troposphere or laboratory ozonolysis experiments. Furthermore, even in cases where the CI removal is independent of pressure, the products may change substantially with pressure (9). Interpretation of indirect measurement of CI reactions often depends on product detection and on derived yields of stabilized CI in ozonolysis that are in turn based on inferences about CI scavenger reactions. Apparent discrepancies among indirect determinations, for example, for reactions of CI with SO2 (1012), may be related to that web of inference. The present measurements break the connection to ozonolysis, directly producing stabilized CIs in a nearly thermoneutral reaction, characterizing the Criegee reactant by its photoionization spectrum, and determining absolute rate constants by pseudo–first-order kinetics methods. Comparing indirect measurements to the present direct kinetics determinations may therefore provide a means to refine the detailed modeling of ozonolysis.

Moreover, computed rate coefficients for substituted CI calculated by state-of-the-art quantum chemistry and advanced theoretical kinetics (9, 30) will be of increasing importance in the development of chemical models for complex systems such as atmospheric chemistry. The present results are a benchmark for such calculations and will enable a more rigorous understanding of CI chemistry and a more accurate description of the role of CI in the troposphere.

Supplementary Materials

www.sciencemag.org/cgi/content/full/340/6129/177/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S19

Schemes S1 and S2

Tables S1 to S8

References (3149)

References and Notes

  1. Materials and methods are available as supporting materials on Science Online.
  2. Acknowledgments: Additional pseudo–first-order rate constants, details of the experiments and calculations, and spectroscopic data underpinning this work are presented in the supplementary materials. The participation of O.W., A.J.E., J.D.S., D.L.O., and C.A.T. and the development of the experimental kinetics apparatus were funded by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the U.S. Department of Energy. D.E.S., J.M.D., and C.J.P. thank the Natural Environment Research Council for funding, and J.M.D. thanks the Leverhulme Trust for a senior fellowship. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under contract DE-AC04-94-AL85000. D.K.W.M., J.M.D., C.J.P., and E.P.F.L. acknowledge support from the Research Grant Council of the Hong Kong Special Administrative Region (grant no. Polyu 5019/11P) and the National Service for Computational Chemistry Software (UK) for computational resources. The experiments were conceived by C.A.T., C.J.P., and D.E.S.; designed by C.A.T., D.L.O., C.J.P., O.W., and A.J.E.; and carried out by O.W., J.D.S., A.J.E., A.M.S., B.R., C.J.P., C.A.T., and D.L.O. E.P.F.L., D.K.W.M., and J.M.D. were responsible for the quantum chemistry and Franck-Condon calculations. All authors participated in the data analysis and interpretation and contributed to the manuscript.
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