Kinetics and Mechanism of the Glyoxal + HO2 Reaction: Conversion

Theoretical studies of atmospheric reaction mechanisms in the troposphere. Luc Vereecken , Joseph S. Francisco. Chemical Society Reviews 2012 41 (19),...
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J. Phys. Chem. A 2011, 115, 291–297

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Kinetics and Mechanism of the Glyoxal + HO2 Reaction: Conversion of HO2 to OH by Carbonyls Gabriel da Silva* Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, ParkVille 3010, Victoria, Australia ReceiVed: September 2, 2010; ReVised Manuscript ReceiVed: December 1, 2010

The kinetics of the glyoxal + HO2 reaction have been investigated using computational chemistry and statistical reaction rate theory techniques, with consideration of a novel pathway that results in the conversion of HO2 to OH. Glyoxal is shown to react with HO2 to form an R-hydroxyperoxy radical with additional R-carbonyl functionality. Intramolecular H atom abstraction from the carbonyl moiety proceeds with a relatively low barrier, facilitating decomposition to OH + CO + HC(O)OH (formic acid). Time-dependent master equation simulations demonstrate that direct reaction to form OH is relatively slow at ambient temperature. The major reaction product is predicted to be collisionally deactivated HC(OH)(OO)CHO, which predominantly dissociates to reform the reactants under low-NOx conditions. The mechanism described here for the conversion of OH to HO2 is available to a diverse range of carbonyls, including methylglyoxal, glycolaldehyde, hydroxyacetone, and glyoxylic acid, and energy surfaces are reported for the reaction of these species with HO2. Introduction To model the transformation and destruction of volatile organic compounds (VOCs) and key pollutants in the atmosphere, we need a complete and fundamental understanding of the sources and sinks of HOx (OH + HO2) radicals. The hydroxyl radical (OH) is the atmosphere’s primary cleansing agent, as it initiates the oxidation of most compounds. The hydroperoxyl radical (HO2) is a product of OH-initiated VOC oxidation and carbonyl photolysis and can also initiate the oxidation of carbonyls. In polluted environments with high levels of NOx (NO + NO2), the HO2 radical plays a key role in OHproducing cycles, through its reactions with NO and O3. In the daytime urban atmosphere, where there are large anthropogenic VOC and NOx sources, this chemistry results in relatively low ratios of HO2 to OH (on the order of 10:1) along with relatively high OH mixing ratios (ca. 107 molecules cm-3).1 As such, urban environments tend to maintain high oxidative capacities, efficiently removing VOCs. However, cycling of NO to NO2 by ozone and organic peroxy radicals, followed by NO2 photolysis to NO + O, leads to the formation of unwanted tropospheric ozone (O3). Air quality in urban areas is significantly degraded by elevated levels of tropospheric ozone and by VOC oxidation byproduct like acids and aerosols. Oxidation chemistry in the pristine and nocturnal planetary boundary layer is considerably different from that taking place in well-studied polluted urban environments. This is particularly true for plumes above tropical forests, where there are large fluxes of biogenic VOCs (BVOCs) such as isoprene. In the relative absence of NOx (hundreds of ppt and below), known cycles between HO2 and OH are far less significant, and HO2 to OH ratios are found to be around 100:1 or more.1 Similar results are found in the nighttime planetary boundary layer,2 where NOx is present almost exclusively as NO2 due to a lack of photolysis back to NO. Here, HO2 levels (ca. 107 molecules cm-3) can generally be explained by known radical chemistry.2 * To whom correspondence should be addressed. E-mail: gdasilva@ unimelb.edu.au.

Again, the lack of HO2 to OH cycling results in low OH levels and high ratios of HO2 to OH.2 Fundamental aspects of chemistry in the pristine forest boundary layer and nighttime planetary boundary layer appear to be missing. It is relatively well-accepted that there are large sources of nighttime OH currently absent from atmospheric chemistry models.2 While these models describe HO2 levels quite accurately, they predict OH mixing ratios that are around an order of magnitude too low, resulting in HO2:OH ratios that are commensurately high. It is also emerging that models of the pristine forest boundary layer are beset by similar problems, where elevated OH levels appear to be somehow sustained without the involvement of NO and O3.3 It has been suggested that large-scale recycling of OH consumed in isoprene oxidation under low-NOx conditions leads to elevated oxidative capacities;3f several novel pieces of chemistry that can achieve this end have been identified,4-8 but they cannot simultaneously account for observed levels of HOx radicals, isoprene, and key oxygenated VOCs (OVOCs).9-11 As in the nighttime troposphere, HO2:OH ratios for these pristine forested environments are often overpredicted by atmospheric chemistry models, which can perhaps be attributed to the presence of some unknown chemistry that converts HO2 to OH, but which bypasses NO and O3. This study reveals a new class of reaction that can convert HO2 to OH, via the HO2 radical initiated oxidation of certain functionalized carbonyls. Computational chemistry and statistical reaction rate theory are used to develop the kinetics of this process for glyoxal, an important trace component of the atmosphere with both primary (biomass burning, anthropogenic emissions) and secondary (VOC photooxidation) sources.12,13 This new reaction mechanism is based on recent discoveries that organic peroxy radicals with common oxygenated functional groups can isomerize and decompose at ambient temperatures, producing OH.6,7,14-16 New routes to these peroxy radicals are described here in the HO2-initiated oxidation of a range of OVOCs. The mechanism described here also affords a new secondary source of carboxylic acids. Carboxylic acids are found

10.1021/jp108358y  2011 American Chemical Society Published on Web 12/21/2010

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Figure 1. Energy surface for the glyoxal + HO2 reaction, at the G3SX level of theory.

at high levels throughout the troposphere and are important contributors to aerosol formation and acid rain in both urban and remote environments.17 Levels of key carboxylic acids are at present under-predicted by atmospheric chemistry models,18 with new sources actively being sought.19 Methods The glyoxal + HO2 reaction is studied using the G3SX composite theoretical method.20 Optimized geometries and vibrational frequencies for all minima and transition states are obtained at the B3LYP/6-31G(2df,p) level of theory. These B3LYP structures are then used in a range of single-point wave function theory energy calculations from HF through QCISD(T) theory, with basis sets of incrementally decreasing size. The wave function theory energies are combined with empirical scaling corrections to provide the final G3SX energy, which essentially approximates the QCISD(T) energy at the complete basis set limit. The G3SX method reproduces enthalpies of formation in the G3/99 test set with a mean error of 0.95 kcal mol-1 20 and less when nonhydrogens are excluded. The G3SX method also reproduces barrier heights in the DBH24/08 test set with a mean error of 0.57 kcal mol-1 21 and is thus wellsuited to studying the thermodynamics and kinetics of organic reactions. Thermochemical properties (∆fH°298, S°298, Cp(T)) are reported for all minima and transition states involved in the glyoxal + HO2 reaction. Enthalpies of formation (∆fH°298, kcal mol-1) are determined from atomization work reactions, as described recently.22 Entropy and heat capacity values (S°298 and Cp(T), kcal mol-1) are obtained according to standard principles of statistical mechanics, using B3LYP/6-31G(2df,p) structures and unscaled vibrational frequencies. Rate constants for elementary reactions, in the high-pressure limit, are calculated via canonical transition state theory. Intramolecular hydrogen shift reactions feature Eckart tunnelling corrections, where barrier widths are estimated from the transition state’s imaginary vibrational frequency and the forward and reverse critical reaction energy.23 Rate constants in the barrierless glyoxal + HO2 association reaction are treated using variational transition state theory (VTST), on the basis of a G3SX level energy surface for HC(O)CHO · · · O(H)O dissociation in the glyoxal · · · HO2 complex. Rate constants are calculated for the forward and reverse processes using structures at 0.1 Å intervals along the energy surface, which are then minimized as a function of temperature and O · · · O distance. Calculated rate constants in all elementary reactions are fit to a three-parameter form of the Arrhenius equation, k ) A′Tn exp(-Ea/RT), to obtain the rate constant parameters Ea, A′, and n.

Phenomenological rate constants and branching ratios in the chemically activated glyoxal + HO2 reaction are calculated as a function of temperature and pressure by solving the timedependent one-dimensional (1D) master equation, based upon thermochemical and kinetic parameters obtained as described above. RRKM theory is used to determine k(E), with collisional energy transfer treated using an exponential-down model with ∆Edown ) 1000 cm-1. Lennard-Jones parameters for the C2H3O4 isomers are 5.5 Å and 350 K, where N2 is the bath gas. The master equation simulations are performed for energy levels up to 200 kcal mol-1 above the lowest-lying well, with an energy grain of 0.2 kcal mol-1. Gaussian 0924 is used for all electronic structure theory calculations reported here, with ChemRate 1.5.825 employed for all statistical mechanics, transition state theory, and master equation/RRKM calculations. Optimized structures, vibrational frequencies, and G3SX energies are reported in the Supporting Information for all stationary points used in the rate constant calculations. Results and Discussion Glyoxal + HO2. An energy surface for the glyoxal + HO2 reaction is shown in Figure 1. Optimized structures for all C2H3O4 minima are depicted in Figure 2, with transition state structures illustrated in Figure 3. Thermochemical properties for the stationary points in this mechanism are listed in Table 1, with rate constant parameters for elementary reactions provided in Table 2. Figure 1 demonstrates that HO2 associates with glyoxal to form a weakly bound glyoxal · · · HO2 complex. This complex then isomerizes to an R-hydroxyperoxy type radical, HC(OH)(OO•)CHO, with a barrier that is below the glyoxal + HO2 energy. This process is well-known for simple carbonyls,27,28 although in the troposphere the resultant peroxy radicals are so unstable toward reverse reaction that they predominantly decompose without further reaction, unless highly elevated levels of NO are present.28 However, here we identify that the peroxy radical formed from HO2 addition to glyoxal also has a reactive R-formylperoxy moiety. It has been recently discovered that certain β-hydroxyperoxy,6,7 R-carboxyperoxy,15 and R-formylperoxy14 radicals are relatively unstable at ambient temperatures, decomposing to produce OH along with other oxygenated products. Similarly, low-energy isomerization reactions are also available to Z conformer δ-hydroxyperoxy radicals.6 As we see in Figure 1, the HC(OH)(OO•)CHO radical can abstract a H atom from the formyl group with a barrier of around 20 kcal mol-1, resulting in the production of CO and a formic acid · · · OH radical complex. This complex will then further dissociate to formic acid and OH,29 with the possibility for some formation

Kinetics and Mechanism of Glyoxal + HO2

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Figure 2. Optimized structures for C2H3O4 minima in the glyoxal + HO2 reaction, at the B3LYP/6-31G(2df,p) level of theory.

of HC(O)O• + H2O via a roaming radical mechanism. The barrier for HC(OH)(OO•)C(O)H radical decomposition to OH + CO + formic acid is consistent with that for other proposed OH-reforming peroxy radical decompositions, which are in the range of ca. 15-22 kcal mol-1.6,7,14,15 While this barrier is above the glyoxal + HO2 reactants by 3.6 kcal mol-1, it may still amount to a non-negligible product channel in the chemically activated glyoxal + HO2 reaction and in HC(OH)(OO•)CHO radical decomposition. To model the kinetics of the glyoxal + HO2 reaction and HC(OH)(OO•)CHO radical decomposition, we require rate constants for barrierless formation of the glyoxal · · · HO2 complex. A G3SX level minimum energy potential (MEP) for this barrierless dissociation reaction, along the HC(O)CHO · · · O(H)O coordinate, is depicted in Figure 4. The complex energy approaches that of dissociated glyoxal + HO2 at an O · · · O separation distance of around 5 Å, consistent with results for the acetaldehyde + HO2 reaction.28,30 Rate constants have been calculated in the glyoxal + HO2 reaction at 0.1 Å intervals along the MEP, with the variational transition state located at 4.5 Å at 200 K and 3.3 Å at 2000 K, with the 4.4 Å, 4.2 Å, and 3.4 Å structures controlling at intermediate temperatures. Rate constants for the glyoxal + HO2 association reaction with each of these controlling structures are plotted in Figure 5; here we

Figure 3. Optimized structures for transition states (TS1-TS3, from top to bottom) in the glyoxal + HO2 reaction, at the B3LYP/631G(2df,p) level of theory.

find that this reaction proceeds at around 10-11 cm3 molecule-1 s-1 and that it is relatively insensitive to temperature. Rate constants and branching ratios in the chemically activated glyoxal + HO2 mechanism have been calculated as a function of temperature and pressure by solving the time dependent 1D master equation. Rate constants to the different product sets, which comprise the formation of OH + CO + HC(O)OH, reverse dissociation to glyoxal + HO2, and collisional deactivation of glyoxal · · · HO2 and HC(OH)(OO•)C(O)H, are provided in Figure 6 for 1 atm pressure. Rate constant parameters for each overall reaction are provided in Table 3. Figure 6 indicates that at all considered temperatures the main process in the chemically activated glyoxal + HO2 reaction is for dissociation back to these reactants. However, at 300 K and 1 atm collisional stabilization of both C2H3O4 intermediates is relatively fast (ca. 10-12-10-14 cm3 molecule-1 s-1), with slow formation of the dissociated products OH + CO + HC(O)OH (2.05 × 10-18 cm3 molecule-1 s-1). The glyoxal · · · HO2 complex will predominantly isomerize to the peroxy radical, rather than dissociate back to glyoxal + HO2. The HC(OH)(OO•)C(O)H

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TABLE 1: Thermochemical Properties for Minima and Transition States Used to Model the Glyoxal + HO2 Reactiona HC(O)CHO HO2 HC(O)CHO · · · HO2 HC(OH)(OO)CHO HC(OH)(OOH)CO TS1 TS2 TS3 a

∆fH°298

S°298

Cp300

Cp400

Cp500

Cp600

Cp800

Cp1000

Cp1500

Cp2000

-51.6 3.3 -54.7 -64.3 -61.1 -52.4 -44.7 -54.1

64.820 54.692 90.730 82.384 82.661 79.334 77.651 83.446

14.172 8.262 25.056 22.292 24.135 21.482 21.439 23.732

16.400 8.770 28.231 26.476 28.160 25.496 25.938 27.400

18.452 9.323 31.083 30.000 31.387 28.954 29.548 30.328

20.243 9.823 33.531 32.850 33.932 31.818 32.383 32.646

23.071 10.622 37.348 37.026 37.606 36.081 36.412 36.037

25.105 11.227 40.097 39.886 40.140 38.965 39.075 38.424

28.067 12.239 44.197 44.054 43.982 42.945 42.815 42.121

29.488 12.813 46.236 46.126 46.002 44.782 44.627 44.091

Enthalpies of formation (∆fH°298) in kcal mol-1. Entropies (S°298) and heat capacities (Cp) in cal mol-1 K-1.

TABLE 2: High-Pressure Limit Rate Constant Parameters for Elementary Reactions in the Glyoxal + HO2 Mechanisma Ea

HC(O)CHO + HO2 f HC(O)CHO · · · HO2 HC(O)CHO · · · HO2 f HC(O)CHO + HO2 HC(O)CHO · · · HO2 f HC(OH)(OO)CHO HC(OH)(OO)CHO f HC(O)CHO · · · HO2 HC(OH)(OO)CHO f HC(OH)(OOH)CO HC(OH)(OOH)CO f OH + CO + HC(O)OH HC(OH)(OOH)CO f HC(OH)(OO)CHO a

-0.90

A′ 1.17 × 10

n -15

1.320

6.52

1.66 × 1018

2.11

3.13 × 109

0.351

11.44

9.34 × 109

0.827

12.13 7.39 9.25

-4

1.26 × 10 2.24 × 1012 3.41 × 10-3

-1.394

4.782 0.369 4.262

k∞ ) A′Tn exp(-Ea/RT). Ea in kcal mol-1, A′Tn in cm3 molecule-1 s-1 or s-1. Fit between 200 and 2000 K.

Figure 4. Minimum energy potential for the dissociation of the glyoxal · · · HO2 complex, at the G3SX level of theory.

radical can dissociate to glyoxal + HO2 or to OH + CO + HC(O)OH; again, while dissociation back to the reactants should dominate (a null reaction overall), a forward reaction may be important. Master equation calculations have been performed for HC(OH)(OO•)C(O)H radical decomposition at 1 atm, with the resultant rate constant expressions included in Table 3. At 300 K decomposition to OH + CO + HC(O)OH proceeds with a lifetime of 25 s, where the branching ratio to this channel is 1.4 × 10-5. Given that the rate of HC(OH)(OO•)C(O)H radical formation from glyoxal + HO2 is predicted to be orders of magnitude faster than direct (chemically activated) formation of OH + CO + HC(O)OH, the two-step mechanism to these products accounts for a significant fraction of the overall reaction rate. For example, assuming that all of the quenched glyoxal · · · HO2 complex isomerizes to HC(OH)(OO•)C(O)H and that HC(OH)(OO•)C(O)H only dissociates to either glyoxal + HO2 or OH + CO + HC(O)OH, then the indirect rate constant for the glyoxal + HO2 (+ M) f HC(OH)(OO•)C(O)H f OH + CO + HC(O)OH reaction sequence at 300 K and 1 atm is 6.4 × 10-18 cm3 molecule-1 s-1. This process, however, relies on the HC(OH)(OO•)C(O)H peroxy radical being relatively

Figure 5. Rate constants (k) as a function of O · · · O separation distance and temperature in the barrierless glyoxal + HO2 association reaction. Solid line indicates a three-parameter fit of the minimum (variational) rate constant.

long-lived toward other radicals such as NO, HO2, and RO2, which should be the case for concentrations below several hundred parts per trillion. Included in Figure 6 is the experimentally determined rate constant for the glyoxal + HO2 reaction (5.1 × 10-16 cm3 molecule-1 s-1 at 298 K). Interestingly, this value falls somewhere between the predicted rate constants for stabilization of the C2H3O4 intermediates (predominantly HC(OH)(OO•)C(O)H) and for direct formation of new dissociated products. This is interpreted as indicating that only a fraction of the HC(OH)(OO•)C(O)H peroxy radicals formed in the experiments of Niki et al.26 underwent further reaction to products other than glyoxal + HO2 (unimolecular or bimolecular), with the predominant fate being dissociation back to these reactants. The apparent rate constant for the glyoxal + HO2 reaction at radical concentrations relevant to the troposphere is expected to differ significantly from that measured by Niki et al. It is clear that the glyoxal + HO2 reaction is slower than glyoxal + OH, but under conditions where [OH] is much smaller than [HO2] it may begin to play some role.

Kinetics and Mechanism of Glyoxal + HO2

J. Phys. Chem. A, Vol. 115, No. 3, 2011 295 above the methylglyoxal + HO2 energy (versus 3.6 kcal mol-1 for glyoxal), and this process may therefore be of increased significance relative to glyoxal + HO2. The ultimate products of this reaction would be OH, CO, and acetic acid (CH3C(O)OH). Other dicarbonyls such as 2-methylpropanedial (MPDL, HC(O)CH(CH3)CHO), another isoprene photoxidation product,31 may undergo similar reactions involving β-formylperoxy radical intermediates.16 The MPDL + HO2 reaction would be expected to proceed via HC(OH)(OO•)CH(CH3)CHO, where abstraction of the weak acyl H atom, followed by CO loss, could lead to the product HC(OH)(OOH)C•HCH3. This radical would be expected to eliminate OH to form the epoxide c-CH(OH)OC(CH3)8,16 but could also lose HO2 to form the enol CH(CH3))CHOH. Another group of unstable peroxy radicals for which this new mechanism may need to be considered is the β-hydroxyperoxy radicals.6,7,32 These radicals are expected as products of RC(O)CH2OH + HO2 reactions, where RC(O)OH + OH + HCHO would be the products. The archetypal case here is the HO2-initiated oxidation of glycolaldehyde, for which an energy surface is depicted in Figure 8. The highest barrier to the products HC(O)OH + OH + HCHO is 6.3 kcal mol-1 above the reactants, and this process is unlikely to be of any significance in the atmosphere. The introduction of a methyl substituent, as in hydroxyacetone (CH3C(O)CH2OH), lowers the overall reaction barrier considerably (to 4.6 kcal mol-1), although it is still relatively large (see Figure S1 of the Supporting Information). The presence of a vinyl substituent, which results in a conjugated unsaturated aldehyde product, appears sufficient to reduce the overall reaction barrier to around the energy of the reactants. This is demonstrated in Figure 9 for the 2-hydroxy-3-butenal + HO2 reaction, where the overall barrier to produce OH is reduced to only 0.5 kcal mol-1 above the reactants. It has also been identified that R-carboxyperoxy radicals can undergo facile decomposition to the OH radical plus other products.15 This process is analogous to that of the

Figure 6. Apparent rate constants in the chemically activated glyoxal + HO2 reaction at 1 atm. Experimental result is from ref 26.

Other Carbonyl + HO2 Reactions. The mechanism revealed above for HO2-initiated oxidation of glyoxal should be available to a range of atmospherically important OVOCs, particularly those carbonyls that also contain hydroxy, carboxy, and formyl functional groups. While a detailed treatment of the kinetics of all relevant carbonyls with HO2 is outside the scope of this work, this section does present energy surfaces for a number of them. Other OVOCs that will react with HO2 to form peroxy radicals with R-formyl moieties have the general structure RC(O)CHO, and the main candidate other than glyoxal is methylglyoxal. Methylglyoxal is formed in the photochemical oxidation of VOCs, particularly isoprene, and is present at relatively high levels throughout the planetary boundary layer.13 An energy surface for the methylglyoxal + HO2 reaction is depicted in Figure 7, and we find that it qualitatively resembles that for glyoxal + HO2. The barrier for isomerization of the CH3C(OH)(OO)CHO radical, however, lies 3.0 kcal mol-1

TABLE 3: Apparent Rate Constant Parameters for the Chemically Activated Glyoxal + HO2 Reaction and for HC(OH)(OO)CHO Radical Decompositiona Ea

HC(O)CHO + HO2 f HC(O)CHO · · · HO2 HC(O)CHO + HO2 f HC(OH)(OO)CHO HC(O)CHO + HO2 f OH + CO + HC(O)OH HC(OH)(OO)CHO f HC(O)CHO + HO2 HC(OH)(OO)CHO f OH + CO + HC(O)OH a

-1

k ) A′T exp(-Ea/RT). Ea in kcal mol , A′T in cm molecule n

n

3

-1

s

-1

A′

n

2.13

1.19 × 10

1.44

2.20 × 107

0.30

5.52 × 10-28

15.79

1.93 × 1029

-5.781

15.19

1.57 × 1010

0.051

-1

20

or s . Fit is between 200 and 2000 K at 1 atm.

Figure 7. Energy surface for the methylglyoxal + HO2 reaction, at the G3SX level of theory.

-13.005 -7.532 3.995

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Figure 8. Energy surface for the glycolaldehyde + HO2 reaction, at the G3SX level of theory.

Figure 9. Energy surface for the 2-hydroxy-3-butenal + HO2 reaction, at the G3SX level of theory.

Figure 10. Energy surface for the CH2(OOH)C(CH3)CHC•HOH + O2 reaction, at the G3SXMP3 level of theory.

β-hydroxyperoxy radicals, where the aldehyde leaving group is replaced by CO2, inferring additional thermodynamic stability on the products and lowering the barrier to the reaction.15 The reaction of glyoxylic acid (CH(O)C(O)OH) with HO2 can produce such a radical, as depicted in Figure S2 of the Supporting Information. A barrier of 4.8 kcal mol-1 above the reactants is required to produce formic acid + OH + CO2. Finally, we pay some attention to the related reactions of Z-conformation δ-hydroxyperoxy radicals. Peroxy radicals of this form, which are products of the isoprene + OH + O2 reaction sequence, are capable of abstracting weak allylic H atoms with relatively low barriers to form hydroperoxidefunctionalized R-hydroxyalkyl radicals.6 A further reaction of these radicals with O2 is then assumed to produce a hydroper-

oxide + HO2. Figure 10 illustrates an energy surface for the reaction of one of these isoprene-OH-O2 isomerization products (CH2(OOH)C(CH3)CHC•HOH) with O2, at the G3SXMP3 level of theory. The addition of O2 to this radical produces a peroxy radical with an R-hydroxyperoxy moiety, which can eliminate HO2 with overall barrier 12.0 kcal mol-1 below the entrance channel energy. In competition with this process, a further intramolecular 1,6-hydrogen shift can occur, with barrier 7.6 kcal mol-1 below the energy of the reactants. This reaction leads to concerted O-OH bond dissociation, ultimately producing the OH radical and a complex hydroperoxide. Given the relatively similar reaction barriers, some branching to this product set would be expected in the chemically activated reaction of these isoprene oxidation products with O2.

Kinetics and Mechanism of Glyoxal + HO2 Conclusions Detailed quantum chemistry and statistical reaction rate theory calculations have been performed on the glyoxal + HO2 reaction. This modeling work considers a novel reaction channel, which achieves the conversion of HO2 to OH, producing a carboxylic acid. At atmospheric temperature and pressure the glyoxal + HO2 reaction predominantly forms a collisionally deactivated peroxy radical, HC(OH)(OO)CHO, with the slow formation of formic acid + CO + OH. The peroxy radical intermediate is expected to dissociate to reform glyoxal + HO2 in the atmosphere, except in the presence of very high levels of NO. Preliminary results are also presented for the reaction of HO2 with a range of oxygenated organics that can undergo the same mechanism, such as methylglyoxal, glycolaldehyde, hydroxyacetone, and glyoxylic acid. Acknowledgment. Computational resources provided by the Victorian Partnership for Advanced Computing (VPAC). Supporting Information Available: B3LYP/6-31G(2df,p) structures and vibrational frequencies, G3SX energies. Energy surfaces for the hydroxyacetone + HO2 and glyoxylic acid + HO2 reactions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Heard, D. E.; Pilling, M. J. Chem. ReV. 2003, 103, 5163. (2) (a) Faloona, I.; Tan, D.; Brune, W.; Hurst, J.; Barket, D., Jr.; Couch, T. L.; Shepson, P.; Apel, E.; Riemer, D.; Thornberry, T.; Carroll, M. A.; Sillman, S.; Keeler, G. J.; Sagady, J.; Hooper, D.; Paterson, K. J. Geophys. Res. 2001, 106, 24315. (b) Sillman, S.; Carroll, M. A.; Thornberry, T.; Lamb, B. K.; Westberg, H.; Brune, W. H.; Faloona, I.; Tan, D.; Shepson, P. B.; Sumner, A. L.; Hastie, D. R.; Mihele, C. M.; Apel, E. C.; Riemer, D. D.; Zika, R. G. J. Geophys. Res. 2002, 107, 4043. (c) Ren, X.; Harder, H.; Martinez, M.; Lesher, R. L.; Oliger, A.; Simpas, J. B.; Brune, W. H.; Schwab, J. J.; Demerjian, K. L.; He, Y.; Zhou, X.; Gao, H. Atmos. EnViron. 2003, 37, 3639. (d) Martinez, M.; Harder, H.; Kovacs, T. A.; Simpas, J. B.; Bassis, J.; Lesher, R.; Brune, W. H.; Frost, G. J.; Williams, E. J.; Stroud, C. A.; Jobson, B. T.; Roberts, J. M.; Hall, S. R.; Shetter, R. E.; Wert, B.; Fried, A.; Alicke, B.; Stutz, J.; Young, V. L.; White, A. B.; Zamora, R. J. J. Geophys. Res. 2003, 108, 4617. (3) (a) Tan, D.; Faloona, I.; Simpas, J. B.; Brune, W.; Shepson, P. B.; Couch, T. L.; Sumner, A. L.; Carroll, M. A.; Thornberry, T.; Apel, E.; Riemer, D.; Stockwell, W. J. Geophys. Res. 2001, D20, 24407–24427. (b) Carslaw, N.; Creasey, D. J.; Harrison, D.; Heard, D. E.; Hunter, M. C.; Jacobs, P. J.; Jenkin, M. E.; Lee, J. D.; Lewis, A. C.; Pilling, M. J.; Saunders, S. M.; Seakins, P. W. Atmos. EnViron. 2001, 35, 4725–4737. (c) Thornton, J. A.; Woolridge, P. J.; Cohen, R. C.; Martinez, M.; Harder, H.; Brune, W. H.; Williams, E. J.; Roberts, J. M.; Fehsenfeld, F. C.; Hall, S. R.; Shetter, R. E.; Wert, B. P.; Fried, A. J. Geophys. Res. 2002, 107, doi: 10.1029/ 2001JD000932. (d) Kuhn, U.; Andreae, M. O.; Ammann, C.; Araujo, A. C.; Brancaleoni, E.; Ciccioli, P.; Dindorf, T.; Frattoni, M.; Gatti, L. V.; Ganzeveld, L.; Kruijt, B.; Lelieveld, J.; Lloyd, J.; Meixner, F. X.; Nobre, A. D.; Poschl, U.; Spirig, C.; Stefani, P.; Thielmann, A.; Valentini, R.; Kesselmeier, J. Atmos. Chem. Phys. 2007, 7, 2855–2879. (e) Ren, X.; Olson, J. R.; Crawford, J. H.; Brune, W. H.; Mao, J.; Long, R. B.; Chen, Z.; Chen, G.; Avery, M. A.; Sachse, G. W.; Barrick, J. D.; Diskin, G. S.; Huey, L. G.; Fried, A.; Cohen, R. C.; Heikes, B.; Wennberg, P. O.; Singh, H. B.; Blake, D. R.; Shetter, R. E. J. Geophys. Res. 2008, 113, D05310. (f) Lelieveld, J.; Butler, T. M.; Crowley, J. N.; Dillon, T. J.; Fischer, H.; Ganzeveld, L.; Harder, H.; Lawrence, M. G.; Martinez, M.; Taraborrelli, D.; Williams, J. Nature 2008, 452, 737–740. (g) Hofzumahaus, A.; Rohrer, F.; Lu, K.; Bohn, B.; Brauers, T.; Chang, C.-C.; Fuchs, H.; Holland, F.; Kita, K.; Kondo, Y.; Li, X.; Lou, S.; Shao, M.; Zeng, L.; Wahner, A.; Zhang, Y. Science 2009, 324, 1702–1704. (4) Hasson, A. S.; Tyndall, G. S.; Orlando, J. J. J. Phys. Chem. A 2004, 108, 5979–5989. (5) Dillon, T. J.; Crowley, J. N. Atmos. Chem. Phys. 2008, 8, 4877– 4889.

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