Hydroxyacetone Production From C3 Criegee Intermediates - The

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Hydroxyacetone Production From C Criegee Intermediates Craig Allen Taatjes, Fang Liu, Brandon Rotavera, Manoj Kumar, Rebecca L. Caravan, David L. Osborn, Ward H. Thompson, and Marsha I. Lester J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07712 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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The Journal of Physical Chemistry

Hydroxyacetone Production From C3 Criegee Intermediates Craig A. Taatjes,1* Fang Liu,2 Brandon Rotavera,1 Manoj Kumar,3 Rebecca Caravan,1 David L. Osborn,1 Ward H. Thompson,4* Marsha I. Lester2* 1

Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969 2 Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 3 Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588-0304 4 Department of Chemistry, University of Kansas, Lawrence, KS 66045 Abstract

Hydroxyacetone (CH3C(O)CH2OH) is observed as a stable end product from reactions of the (CH3)2COO Criegee intermediate, acetone oxide, in a flow tube coupled with multiplexed photoionization mass spectrometer detection. In the experiment, the isomers at m/z = 74 are distinguished by their different photoionization spectra and reaction times. Hydroxyacetone is observed as a persistent signal at longer reaction times at a higher photoionization threshold of ca. 9.7 eV and definitively identified by comparison with known photoionization spectrum. Complementary electronic structure calculations reveal multiple possible reaction pathways for hydroxyacetone formation, including unimolecular isomerization via hydrogen atom transfer and –OH group migration as well as self-reaction of Criegee intermediates. Varying the concentration of Criegee intermediates suggests contributions from both unimolecular and selfreaction pathways to hydroxyacetone. The hydroxyacetone end product can provide an effective, stable marker for the production of transient Criegee intermediates in future studies of alkene ozonolysis.

*

Corresponding authors: [email protected], [email protected], [email protected] (215-898-4640)

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Introduction Alkenes are an abundant class of volatile organic compounds (VOCs) in the Earth’s troposphere, which have a substantial impact on the chemistry occurring in the lower atmosphere on a global and regional scale.1 A major degradation pathway for atmospheric alkenes is their reaction with ozone (ozonolysis), proceeding via cycloaddition of ozone across the C=C double bond to produce a primary ozonide, which subsequently decomposes into a carbonyl moiety and carbonyl oxide (R1R2COO), the latter known as the Criegee intermediate.2 Criegee intermediates are key branching points in ozonolysis reactions: their unimolecular and bimolecular reactions lead to many end products important in atmospheric chemistry, including hydroxyl radicals (OH), organic acids, hydroperoxides, and aerosols.3-4 Recently, several Criegee intermediates including CH2OO, CH3CHOO, and (CH3)2COO have been produced directly in the gas phase using an alternate synthetic pathway based on the reaction of O2 with α-iodoalkyl radicals generated by photolysis of gem-diiodoalkane precursors.5-7 The CH2OO,5 CH3CHOO,6 and (CH3)2COO8 Criegee intermediates have been identified through their characteristic photoionization spectra using a multiplexed photoionization mass spectrometer (MPIMS).9-10 The photoionization spectra provide benchmarks for experimental studies involving ionization detection of Criegee intermediates. The alternate synthetic pathway has also enabled direct kinetic measurements of the bimolecular reactions of Criegee intermediates with many atmospheric trace species, such as water vapor,11 SO2, NO2,5-6 and organic acids.12 The rates for many of these bimolecular reactions have been found to be significantly faster than previous measurements, and vary considerably with size and structure of the Criegee intermediates.13


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Acetone oxide (or dimethyl-substituted Criegee intermediate), (CH3)2COO, is a prototypical Criegee intermediate that has been broadly investigated in prior experimental and theoretical studies.7, 14-19 This Criegee intermediate is formed in the atmosphere from ozonolysis of alkenes with a (CH3)2C= subunit, such as 2,2-dimethyl-2-butene (or tetramethyl ethylene, TME) or terpenes such as myrcene,20 and subsequently undergoes unimolecular decay to OH radical products.3 The highly exothermic ozonolysis reactions leave a significant amount of internal energy in the newly formed Criegee intermediates, leading to rapid unimolecular decay. A major decay pathway for the (CH3)2COO Criegee intermediate is transfer of a methyl H atom to the terminal O to form the H2C=C(CH3)OOH species (2hydroperoxypropene, a methyl-substituted vinyl hydroperoxide, hereafter referred to as VHP), with a barrier recently computed to be 16.16 kcal mol-1,21 followed by subsequent dissociation to form OH radical and vinoxy products:22 (CH3)2COO → CH2=C(CH3)OOH → CH2=C(CH3)O + OH

(1)

Another pathway that is less favorable is the “ester channel”, which involves isomerization over a higher barrier to form dimethyl dioxirane and produces the CH3COOCH3 ester.23 In addition, a number of other final end products have also been reported from gas-phase ozonolysis of TME, including CO, CO2, (CH3)2CO, CH2O, CH3OH, and hydroxyacetone (CH3C(O)CH2OH).24-29 Evaluating the branching and yield of these end products is important in understanding the reaction pathways of alkene ozonolysis and improving atmospheric chemistry models in the future. Substantial uncertainty remains regarding the formation of many end products in ozonolysis reactions, one of which is hydroxyacetone. Hydroxyacetone has been identified in



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early IR/NMR,24 FTIR,25 GC-IR,28 and more recent HPLC/ESI-TOFMS20 studies of ozonolysis reactions, with reported yields ranging from 1% to 30%. However, knowledge about the mechanism for production of hydroxyacetone is very limited. In 1967, Story et al. proposed a pathway for the formation of hydroxyacetone through VHP:24 (CH3)2COO → CH2=C(CH3)OOH → CH3C(O)CH2OH

(2)

In 1987, an early theoretical study examined the relative stability of the Criegee intermediates, VHP, and hydroxyacetone.26 The isomerization from VHP to hydroxyacetone is expected to involve a shift of the OH group from the O to one of the β carbons. Only a few prior theoretical studies have characterized this OH shift process. Leonardo et al.30 calculated a free energy barrier of 23.3 kcal mol-1 (relative to the (CH3)2COO reactant) for the OH transfer from CH2=C(CH3)OOH at the B3LYP/6-31G(d,p) level of theory; Zhang et al.31 estimated an activation energy of 22.9 kcal mol-1 and 14.3 kcal mol-1 in an analogous OH transfer process for the VHP species produced in α- and β-pinene ozonolysis, respectively. The (CH3)2COO Criegee intermediate has been recently characterized by ultraviolet spectroscopy.7-8, 19 In a companion paper, Chhantyal-Pun et al.8 have obtained a photoionization spectrum for (CH3)2COO, identified by complementary ab initio calculations of photoionization energy and Franck-Condon factors, along with its bimolecular reactions. Kinetics studies have also examined the reactivity of (CH3)2COO with water,19 NO2, and SO2, along with self-reaction8 and unimolecular decay,8, 21, 32 with potential impact on the steady state concentration of (CH3)2COO in the atmosphere.19 This study identifies isomeric products of (CH3)2COO Criegee intermediates using the multiplexed photoionization mass spectrometry method. Measurements at various reaction


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times reveal the formation of hydroxyacetone as a stable end product from (CH3)2COO. The dependence of the product distribution on Criegee intermediate concentration indicates that hydroxyacetone is formed as a minor product by both first-order and second-order (i.e., selfreaction) removal processes. The formation mechanism is examined through ab initio potential energy surface calculations, which characterize reaction pathways leading to hydroxyacetone via both unimolecular isomerization and self-reaction. Methods The experiments are carried out in the Multiplexed Photoionization Mass Spectrometer (MPIMS)9-10 at the Chemical Dynamics Beamline (9.0.2) of the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratory.33 The (CH3)2COO Criegee intermediates are prepared from diiodo precursors in an analogous manner to many recent spectroscopy, kinetics, and dynamics measurements.5, 7, 19, 21 The synthesized and purified 2,2-diiodopropane precursor, (CH3)2CI2,7, 34 is held in a thermostated bubbler at 25°C. Gas-phase (CH3)2CI2 is mixed with high-purity O2 and He by calibrated mass flow controllers into a slow-flow quartz reactor maintained at 10 Torr and coated with halocarbon wax to minimize wall-loss reactions as described previously.5, 6 Concentrations of (CH3)2CI2 are ~ 1014 cm-3 and O2 concentrations are ~ 1016 cm-3. Pulsed photolysis using a XeF excimer laser at 351 nm dissociates (CH3)2CI2 to produce (CH3)2CI radicals, which undergo further reaction with O2 to yield (CH3)2COO Criegee intermediates.5-7 The photoionization cross-section of hydroxyacetone was measured in a separate experiment using a standard sample of >90% purity. The reactive gas mixture is continuously sampled through a ∼650 μm orifice on the reactor sidewall, and ionized using tunable vacuum-ultraviolet light from the synchrotron. The generated ions are pulse-extracted



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and recorded at 50 kHz repetition rate by an orthogonal acceleration time-of-flight mass spectrometer with mass resolution (m/Δm) of approximately 1500. The ion signals are collected as a function of mass (time-of-flight), reaction time, and photon energy. Possible mechanisms for formation of hydroxyacetone from the dimethyl-substituted Criegee Intermediate (CH3)2COO via unimolecular isomerization and self-reaction pathways have also been examined in detail using electronic structure calculations. The plausible pathways have been calculated assuming a singlet ground state. All calculations were performed with NWChem35 using an unrestricted level of theory to describe any biradical character of the species involved in the reaction pathways. All geometries were optimized using the density functional theory method M06-2X36 and the augmented correlation-consistent basis set, aug-cc-pVDZ.37 The energetics for the unimolecular Criegee-based reactions were further improved by performing single point calculations at the coupled cluster level with singles and doubles including triples contributions, CCSD(T),38 and the aug-cc-pVTZ basis set. This level of theory has been previously found to provide an accurate description of Criegee chemistries.39-40 All stationary points were characterized by frequency calculations and reported energies include zero-point energy corrections (unscaled) from the method used for geometry optimization. Results and Discussion A. Photoionization studies The tunable synchrotron photon energy enables the multiplexed measurement of timeresolved photoionization spectrum for each individual species, separated by their mass to charge ratio (m/z) in the mass spectrum. The time-dependent signal at m/z = 74 exhibits a fast


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rise in the first ∼0.5 ms following excimer photolysis of the precursor and reaction with O2, leading to formation of (CH3)2COO Criegee intermediates, which is illustrated in Figure 1. This is in accord with the production of (CH3)2COO Criegee intermediates discussed in the companion paper by Chhantyal-Pun et al.8 A subsequent decay occurs on a ∼2 ms timescale, corresponding to removal of Criegee intermediates via unimolecular decay, bimolecular processes, and heterogeneous loss at the wall. After a longer reaction time (> 5 ms), when the majority of Criegee intermediates are removed by these loss processes, a nonzero offset on the m/z = 74 channel persists, suggesting a stable end product formed at the same molecular mass. Selective integration of the photoionization spectra over different ranges of kinetic time, which in the present experiment spans 150 ms, can reveal information on the timescales of formation of isomers present in a spectrum. Figure 2 illustrates the m/z = 74 photoionization spectrum integrated over the initial 0-2.5 ms and later 5-150 ms time periods after photolysis, representing the photoionization characteristics of the m/z = 74 isomers at early and late times, respectively. VUV photon energies are scanned from 8.5 eV to 10.5 eV; the latter extreme is equivalent to the 118 nm VUV photon energy utilized in previous UV depletion measurements of (CH3)2COO under collision-free conditions.7 In the top panel of Figure 2 showing spectra taken at early reaction times, an appearance of the m/z = 74 photoionization signal is observed at ∼9 eV, in accord with the photoionization efficiency curves for (CH3)2COO presented in the companion paper.8 The computed photoionization efficiency curves for formation of the two cationic states of (CH3)2COO are shown in Figure 2, along with that for an isomer, H2C=C(CH3)OOH (VHP), which is predicted to have a similar ionization energy and may also be present. Bimolecular reaction 7 ACS Paragon Plus Environment


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studies have shown that the 9 eV signal disappears upon reaction with SO2 and NO2, and therefore the 9 eV signal is attributed to the more reactive (CH3)2COO intermediate rather than the less reactive VHP species.8 At higher photoionization energies of ∼9.7 eV, the photoionization signal exhibits a second rise. This feature is consistent with the characteristic photoionization spectrum obtained for a pure hydroxyacetone CH3C(O)CH2OH sample, indicating that the higher energy threshold on the same mass channel is due to the hydroxyacetone product. The presence of hydroxyacetone is more prominent after a longer reaction time (5-150 ms), as shown in the lower panel of Figure 2. At long times, the lower energy feature at ∼9 eV disappears, indicating the Criegee intermediates have been mostly eliminated. The experimental photoionization efficiency agrees favorably with the photoionization spectrum of the pure hydroxyacetone sample, identifying the long-time persistent offset on the m/z = 74 channel shown in Figure 1 to the hydroxyacetone end product. The intensity of this persistent offset signal is about 10% of the m/z = 74 signal at earlier reaction times. There are many other possible isomers at m/z = 74, most of which have adiabatic ionization energies (AIE) beyond the observed photoionization energy, including methyl acetate (AIE = 10.25 eV) and propanoic acid (AIE = 10.44 eV).41 Although the calculated adiabatic ionization energies for dimethyl dioxirane (AIE = 9.12 eV) and methyldioxetane (AIE = 9.9 eV) lie within the range investigated,8 their predicted ionization onsets and Franck-Condon envelopes are not consistent with the observed spectrum. Relative photoionization efficiency curves for possible isomers are compared in Figure S1. Among the m/z = 74 isomers, the Criegee


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intermediate (or VHP) and hydroxyacetone are in agreement with features observed in the experimental photoionization spectrum. B. Electronic structure calculations of unimolecular decay pathways Electronic structure calculations were undertaken to characterize relevant stationary points for the formation mechanism of hydroxyacetone from (CH3)2COO. The UCCSD(T)/aug-ccpVTZ//UM06-2X/aug-cc-pVDZ calculated potential energy profiles of the unimolecular pathways for the Criegee isomerization to hydroxyacetone are presented in Figure 3. Direct formation of hydroxyacetone from (CH3)2COO is not likely, because it is predicted to involve an O atom migration with an extremely high barrier height (77.7 kcal mol-1 estimated at the M062X/aug-cc-pVTZ level). The dominant unimolecular decay pathway found for the (CH3)2COO Criegee intermediate involves tautomerization via H-atom transfer from a CH3 group to the terminal O and results in the formation of VHP. This tautomerization has a calculated electronic barrier of 15.8 kcal mol-1. (This value is in good accord with a recent high-level CCSD(T)-f12/CBS calculation of 16.16 kcal mol-1, which includes zero-point energy and other corrections.21) Following the formation of VHP, a well-established reaction pathway is the fission of the OO bond, yielding the OH radical and a carbonyl co-product, which is responsible for formation of OH radicals from Criegee intermediates with an α-H in ozonolysis reactions.3, 21-22, 32 A transition state for this OO dissociation process (TSOH_Diss) is found to be 4.2 kcal mol-1 above the (CH3)2COO Criegee intermediate, which is consistent with the submerged barrier reported for the analogous OO dissociation from CH2=CHOOH.22, 42 In addition, a new pathway from VHP has been identified that yields hydroxyacetone. It proceeds via intramolecular OH migration to the terminal CH2 group, thereby breaking the OO 9 ACS Paragon Plus Environment


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bond in VHP and converting it to hydroxyacetone, CH3C(O)CH2OH. The corresponding barrier for this OH migration is predicted to be 36.1 kcal mol-1 relative to VHP and 20.1 kcal mol-1 relative to the (CH3)2COO Criegee intermediate. The overall hydroxyacetone-forming reaction is exothermic by 85.2 kcal mol-1. Among the two pathways from VHP, the isomerization to hydroxyacetone is expected to be a minor channel compared to the OO bond cleavage to form OH + carbonyl, consistent with a much higher (near unity) yield of OH43 compared to that for hydroxyacetone in ozonolysis of TME. However, given the significant amount of energy released in TME ozonolysis (∼50 kcal mol-1) as well as in 351 nm photolysis of the diiodo precursor (2-10 kcal mol-1)44 and subsequent reaction (14.8 kcal mol-1),8 the newly formed Criegee intermediate is expected to have sufficient energy to isomerize to VHP. These energetics suggest that it will also be possible to surmount the barrier TSOHshift to form hydroxyacetone. Another pathway for forming hydroxyacetone from (CH3)2COO is explored and depicted in Figure 3 (left side), starting from isomerization to a dioxirane species with a barrier of ∼20.2 kcal mol-1 relative to the Criegee intermediate. The formation of hydroxyacetone from dioxirane is predicted to involve multiple steps, including ring opening, H atom transfer, and OH migration. In general, this pathway is more complicated, with a higher barrier at TSHAT’ (25.2 kcal mol-1), and therefore less favorable than the OH shift pathway through VHP. However, it cannot be ruled out for Criegee intermediates formed with significant excess energy. In the companion paper, a kinetic study of (CH3)2COO revealed a very fast, collisionlimited self-reaction for (CH3)2COO.8 A rate coefficient has been determined to be (6.0 ± 1.1) x 10-10 cm3 molecule-1 s-1, which is significantly larger than the self-reaction rate for CH2OO.45-47 10 ACS Paragon Plus Environment

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Self-reaction of the Criegee intermediates is not expected to be significant in ozonolysis experiments or in the troposphere due to the low steady-state concentration of the Criegee intermediates. However, self-reaction can be important in laboratory studies such as the present work that achieves high concentrations of Criegee intermediates via the alternate synthetic route.45 Because of the possibility of self-reaction, we have explored whether hydroxyacetone can be produced from self-reaction of (CH3)2COO both theoretically and experimentally. C. Theoretical calculations of self-reaction pathways Self-reaction of the CH2OO Criegee intermediate is predicted to occur via formation of a dimer with a cyclic structure, which will undergo subsequent dissociation to form formaldehyde H2CO and 1O2.47 The self-reaction of the (CH3)2COO Criegee intermediate will likely undergo an analogous process through a strong attractive dipole-dipole interaction to dimerize and form diacetone diperoxide.8, 24 The computed reaction pathway for self-reaction of (CH3)2COO Criegee intermediates leading to the production of hydroxyacetone is shown in Figure 4. The self-reaction of the Criegee intermediates is a highly exothermic process, releasing ca. 90 kcal mol-1 and forming diacetone diperoxide with two bridging OO bonds. The diacetone diperoxide can undergo ring opening via homolysis of a bridging OO bond to form Intbirad, which has an open structure with two C-O·subunits. Two subsequent H atom transfer (HAT) processes from adjacent CH3 groups to the two C-O· subunits form two OH bonds at IntHAT2. The migration of one of the newly formed OH groups to the terminal CH2 group is accompanied by the dissociation of the second bridging OO bond and results in the formation of hydroxyacetone and its cyclic analogue, Intcyc. 11 ACS Paragon Plus Environment


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The Intcyc subsequently isomerizes into hydroxyacetone via TSOH_Mig2 that involves OH migration and ring opening. Although the pathway involves many steps, the significant amount of energy released in the self-reaction to form the diacetone diperoxide Criegee dimer is sufficient for the system to surmount the barriers that lead to hydroxyacetone. Thus, theory predicts an energetically viable self-reaction pathway for formation of hydroxyacetone from (CH3)2COO. The (CH3)2COO self-reaction may also produce acetone and oxygen, which is also reported to be a decomposition channel for diacetone diperoxide.48 The UM06-2X/aug-ccpVDZ calculations indicate that the bifurcation of diacetone diperoxide into the hydroxyacetone and acetone forming pathways occurs at Intbirad. As shown in Figure 5, the breakage of one of the C-OO bonds in Intbirad leads to the release of acetone and formation of another biradical species, Intbirad’. This acetone-forming step involves an effective barrier of 9.1 kcal mol-1 relative to Intbirad. The Intbirad’ subsequently loses singlet oxygen (1O2) to form another acetone molecule. Overall, the acetone forming reaction is 87.1 kcal mol-1 exothermic relative to separated (CH3)2COO monomers. The comparative analysis of the hydroxyacetone and acetone forming pathways indicate that the former pathway has larger barriers and larger exothermicity. Therefore, the acetone forming reaction, which involves a lower barrier, is expected to be favored; however, the excess energy stored in diacetone diperoxide suggests that a fraction of the self-reaction could lead to the formation of hydroxyacetone. D. Experimental study of first- and second-order processes Under the current experimental conditions, the (CH3)2COO concentration in the flow cell is estimated to be on the order of 1012 cm-3, for which self-reaction will occur on a ms time scale.8 On the other hand, the unimolecular decay rate for (CH3)2COO to OH products has been 12 ACS Paragon Plus Environment

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predicted to be 370 s-1 at 298 K,21 and recent experiments yield unimolecular decay rates for (CH3)2COO of 361 ± 49 s-1 and 305 ± 70 s-1.8, 32 Thus, both unimolecular decay and self-reaction may occur in the time frame of this study. In the absence of added reagents, the decay of (CH3)2COO may include a first-order term due to unimolecular decay or reaction on the walls, etc., and a second-order component arising from self-reaction. To assess the first-order and second-order components, the long-time signal intensities (integrated over times after the Criegee intermediate has been consumed) at m/z = 74 are evaluated at three photolysis laser power levels and two photoionization energies, 9.2 eV and 10.5 eV. The ratio of these integrated signals is shown in Figure 6. The signal at 9.2 eV includes Criegee intermediates and isomers with lower ionization energy, e.g., VHP, and the signal at 10.5 eV additionally includes contributions from hydroxyacetone. The ratio of the 10.5 eV signal to the 9.2 eV signal is found to increase with increasing photolysis laser power, which results in greater Criegee concentration and a larger contribution from self-reaction.8 The increase in the ratio of the 10.5 eV to the 9.2 eV signals with increasing laser power implies that the increasing contribution of self-reaction leads to a higher fraction of hydroxyacetone (relative to the other m/z = 74 isomers) than in the first-order reaction. A linear fit gives a pure first-order ratio (y-intercept) of ∼3 for the ratio of the 10.5 eV to 9.2 eV signals. This ratio of 10.5 eV to 9.2 eV signals is similar to that observed for the m/z = 74 product from acetone oxide in the presence of SO2 in the companion paper.8 That photoionization spectrum was attributed to a combination of hydroxyacetone, 2-hydroperoxypropene and methyldioxirane; therefore, although a full spectrum was not measured at low photolysis power, we conclude that the firstorder component likely also has a non-zero hydroxyacetone yield. 13 ACS Paragon Plus Environment


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Experimentally, both hydroxyacetone and acetone products are observed on their respective mass channels. As shown in Figure S2, the acetone/hydroxyacetone ratio can be estimated using the absolute photoionization cross sections for hydroxyacetone and acetone at 10.5 eV. At the highest photolysis laser power used, in which self-reaction is most prominent, the derived hydroxyacetone/acetone ratio is approximately 1/5. The acetone yield drops precipitously at low power, suggesting little or no acetone is formed in the low power limit (i.e., from the first-order reaction). The majority of acetone product is produced from the secondorder reaction, while both first- and second-order reactions form some hydroxyacetone. Although it is outside the scope of this paper, future work that aims at quantifying the yields of products in the first-order and second-order reactions could employ temperature-dependent measurements, as the positive activation energy of the unimolecular reaction should make it dominant at elevated temperature. Analogous photoionization spectroscopy for (CD3)2COO is described in a companion paper.8 The time-dependent measurements show a smaller amount of the persistent offset signal, indicating a smaller fraction of hydroxyacetone formed, suggesting a kinetic isotope effect in the formation of hydroxyacetone. The theoretical calculations indicate barriers involving hydrogen atom shifts in the unimolecular and self-reaction pathways that would likely slow considerably upon deuteration. Previously, hydroxyacetone has been detected from ozonolysis reactions,20, 24-25, 28 in which the concentration of Criegee intermediates is expected to be too low for self-reaction to be significant. Thus, in ozonolysis experiments, hydroxyacetone formation may arise from the unimolecular isomerization pathway shown in Figure 3.


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Evaluating the reaction mechanisms for alkyl-substituted Criegee intermediates, such as (CH3)2COO, is an important first step in understanding the reactions of larger Criegee intermediates originating from biogenic sources such as terpenes. One of the most atmospherically important bimolecular reactions is the reaction with water, its clusters, and/or water droplets49 due to the abundance of water vapor in the air. Recent kinetic studies revealed that the reaction with water dimer is a major loss channel for CH2OO,11, 50 however the reaction of (CH3)2COO with water vapor is found to be negligible.19 As a result, unimolecular decay of (CH3)2COO is expected to be the dominant loss channel8, 21, 32 and other bimolecular reactions (such as reaction with SO2)8, 19 may contribute to the overall loss. Due to the unstable nature of Criegee intermediates, their direct detection in alkene ozonolysis reactions is very challenging; for example, only trace amounts of CH2OO have been directly observed in the ozonolysis of ethene.51 The present study indicates alternative pathways for decay of (CH3)2COO Criegee intermediates, leading to production of hydroxyacetone. Although production of hydroxyacetone has a smaller yield than OH formation, it is a stable end product that can provide an effective marker for the production of Criegee intermediates in future studies of alkene ozonolysis in laboratory or natural environments. As a result, hydroxyacetone will be of importance in mapping out the complete picture of Criegee intermediate chemistry that is ubiquitous in the atmosphere. Conclusions Hydroxyacetone is identified as a stable product from the (CH3)2COO Criegee intermediate using tunable VUV photoionization mass spectrometry. Hydroxyacetone is separated from other isomeric species at m/z = 74, including (CH3)2COO, by its distinct reaction 15 ACS Paragon Plus Environment


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time and photoionization threshold. Compared to (CH3)2COO, hydroxyacetone is observed at longer (≥ 5 ms) reaction times and higher photoionization energies starting at ca. 9.7 eV. Electronic structure calculations identify possible formation pathways for the hydroxyacetone products, including unimolecular rearrangement and bimolecular self-reaction of the Criegee intermediates. The presence of contributions from both first- and second-order pathways for formation of hydroxyacetone is confirmed by varying the (CH3)2COO concentration through the photolysis laser power dependence. The unimolecular mechanism is more likely to occur in alkene ozonolysis reactions. The production of hydroxyacetone as a stable end product from Criegee intermediates can potentially be applied to monitor alkene ozonolysis reactions in field and laboratory studies. Supporting Information The Supporting Information is available on the ACS Publications website at DOI: The relative photoionization spectra of many possible isomers at m/z = 74 are shown in Figure S1, including the experimental spectrum for hydroxyacetone and computed spectra for acetone oxide, VHP, methyldioxetane, and dimethyl dioxirane adapted from Ref. 8. In addition, the ratio of the observed yield of hydroxyacetone to acetone is shown in Figure S2.

Acknowledgements This material is based upon work supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences (BES), U.S. Department of Energy (USDOE). Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the USDOE’s National Nuclear Security Administration under


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contract DEAC04-94AL85000. This research used resources of the Advanced Light Source of Lawrence Berkeley National Laboratory, which is a USDOE Office of Science User Facility. The Advanced Light Source is supported by the Director, Office of Science, BES/USDOE, under contract DE-AC02- 05CH11231 between Lawrence Berkeley National Laboratory and the USDOE. MIL and FL were supported by BES/USDOE under grant DE-FG02-87ER13792. WHT and MK acknowledge the support of NIFA/USDA under grant 2011-10006-30362.



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Figures

Figure 1. Time-dependent photoionization signal at m/z = 74 from reaction of (CH3)2CI + O2. Time t=0 corresponds to the 351 nm photolysis laser firing to initiate reaction, yielding a signal that rises and decays to a persistent nonzero offset at longer reaction times. The photoionization signal is integrated from 8.5 to 10.5 eV in photon energy and the nonzero offset is due to the formation of hydroxyacetone, which is formed on longer timescales than the Criegee intermediate (CH3)2COO.



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Figure 2. Photoionization spectrum on the m/z = 74 channel from reaction of (CH3)2CI + O2 integrated over the initial 2.5 ms (top) and later 5-150 ms (bottom) period after photolysis (timescales are post-photolysis). The photoionization spectra from the calculation for the Criegee intermediate (CH3)2COO to the two cationic states (blue) and vinyl hydroperoxide (black), and the experimental measurement for hydroxyacetone (red) are shown for comparison. Photoionization intensities are in arbitrary scale, and the calculated spectrum for species not observed are dashed in the bottom panel.


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Figure 3. Reaction schemes showing unimolecular decay pathways of (CH3)2COO Criegee intermediate that generate the OH radical (black) and hydroxyacetone (red). Energies (kcal mol-1) are zero-point corrected. The generation of hydroxyacetone from (CH3)2COO may proceed through VHP (right) or dioxirane (left) pathways involving hydrogen atom transfer and -OH group migration. Calculations are performed with UCCSD(T)/aug-cc-pVTZ//UM062X/augcc-pVDZ level of theory.



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Figure 4. Reaction scheme showing the self-reaction of Criegee intermediates (CH3)2COO to generate hydroxyacetone, calculated with UM062X/aug-cc-pVDZ level of theory. Energies (kcal mol-1) are zero-point corrected.


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Figure 5. Reaction scheme showing the self-reaction of Criegee intermediates (CH3)2COO to generate acetone and singlet oxygen (O2 1Σg+) calculated with UM062X/aug-cc-pVDZ level of theory. Energies (kcal mol-1) are zero-point corrected. aTSO2 could not be found at the UM062X/aug-cc-pVDZ level. However, we were able to find it at the UB3LYP/aug-cc-pVDZ level. The energy of the TSO2 has been calculated by taking the zero-point corrected electronic energy difference between the UB3LYP/aug-cc-pVDZ optimized Intbirad’ and TSO2.



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Figure 6. Ratio of persistent m/z = 74 photoionization signals at 10.5 eV (principally hydroxyacetone) to 9.2 eV (other isomers, including 2-hydroperoxypropene) as a function of photolysis laser power with linear fit. The (CH3)2COO Criegee intermediate concentration and self-reaction contribution increase with photolysis laser power. The intercept and slope indicate that hydroxyacetone is produced from first- and second-order reactions, respectively. Experimental uncertainty for the intensity measurements is ca. 15%.


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TOC figure:



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Figure 1. Time-dependent photoionization signal at m/z = 74 from reaction of (CH3)2CI + O2. Time t=0 corresponds to the 351 nm photolysis laser firing to initiate reaction, yielding a signal that rises and decays to a persistent nonzero offset at longer reaction times. The photoionization signal is integrated from 8.5 to 10.5 eV in photon energy and the nonzero offset is due to the formation of hydroxyacetone, which is formed on longer timescales than the Criegee intermediate (CH3)2COO. 68x55mm (300 x 300 DPI)

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Figure 2. Photoionization spectrum on the m/z = 74 channel from reaction of (CH3)2CI + O2 integrated over the initial 2.5 ms (top) and later 5-150 ms (bottom) period after photolysis (timescales are postphotolysis). The photoionization spectra from the calculation for the Criegee intermediate (CH3)2COO to the two cationic states (blue) and vinyl hydroperoxide (black), and the experimental measurement for hydroxyacetone (red) are shown for comparison. Photoionization intensities are in arbitrary scale, and the calculated spectrum for species not observed are dashed in the bottom panel. 93x104mm (300 x 300 DPI)



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Figure 3. Reaction schemes showing unimolecular decay pathways of (CH3)2COO Criegee intermediate that generate the OH radical (black) and hydroxyacetone (red). Energies (kcal mol-1) are zero-point corrected. The generation of hydroxyacetone from (CH3)2COO may proceed through VHP (right) or dioxirane (left) pathways involving hydrogen atom transfer and -OH group migration. Calculations are performed with UCCSD(T)/aug-cc-pVTZ//UM062X/aug-cc-pVDZ level of theory. 338x149mm (300 x 300 DPI)


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Figure 4. Reaction scheme showing the self-reaction of Criegee intermediates (CH3)2COO to generate hydroxyacetone, calculated with UM062X/aug-cc-pVDZ level of theory. Energies (kcal mol-1) are zero-point corrected. 266x187mm (300 x 300 DPI)



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Figure 5. Reaction scheme showing the self-reaction of Criegee intermediates (CH3)2COO to generate acetone and singlet oxygen, calculated with UM062X/aug-cc-pVDZ level of theory. Energies (kcal mol-1) are zero-point corrected. aTSO2 could not be found at the UM062X/aug-cc-pVDZ level. However, we were able to find it at the UB3LYP/aug-cc-pVDZ level. The energy of the TSO2 has been calculated by taking the zeropoint corrected electronic energy difference between the UB3LYP/aug-cc-pVDZ optimized Intbirad’ and TSO2. 121x81mm (300 x 300 DPI)


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Figure 6. Ratio of persistent m/z = 74 photoionization signals at 10.5 eV (principally hydroxyacetone) to 9.2 eV (other isomers, including 2-hydroperoxypropene) as a function of photolysis laser power with linear fit. The (CH3)2COO Criegee intermediate concentration and self-reaction contribution increase with photolysis laser power. The intercept and slope indicate that hydroxyacetone is produced from first- and second-order reactions, respectively. Experimental uncertainty for the intensity measurements is ca. 15%. 66x51mm (300 x 300 DPI)



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This is the TOC figure 82x44mm (300 x 300 DPI)


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Hydroxyacetone Production From C3 Criegee Intermediates - The

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