Atmospheric Oxidation Mechanism of Toluene - ACS Publications

Jun 5, 2014 - Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol ... RRKM-ME calculations show that the reactions of R2/R4 with O2...
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Atmospheric Oxidation Mechanism of Toluene Runrun Wu,† Shanshan Pan,† Yun Li,† and Liming Wang*,†,‡ †

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: The atmospheric oxidation mechanism of toluene initiated by OH radical addition is investigated by quantum chemistry calculations at M06-2X, G3MP2-RAD, and ROCBS-QB3 levels and by kinetics calculation by using transition state theory and unimolecular reaction theory coupled with master equation (RRKM-ME). The predicted branching ratios are 0.15, 0.59, 0.05, and 0.14 for OH additions to ipso, ortho, meta, and para positions (forming R1−R4 adducts), respectively. The fate of R2, R4, and R1 is investigated in detail. In the atmosphere, R2 reacts with O2 either by irreversible H-abstraction to form o-cresol (36%), or by reversible recombination to R2-1OO-syn and R2-3OO-syn, which subsequently cyclize to bicyclic radical R2-13OOsyn (64%). Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol and 39% for R4-35OO-syn, while reaction of R1 and O2 leads to R1-26OO-syn. RRKM-ME calculations show that the reactions of R2/R4 with O2 have reached their highpressure limits at 760 Torr and the formation of R2-16O-3O-s is only important at low pressure, i.e., 5.4% at 100 Torr. The bicyclic radicals (R2-13OO-syn, R4-35OO-syn, and R1-26OO-syn) will recombine with O2 to produce bicyclic alkoxy radicals after reacting with NO. The bicyclic alkoxy radicals would break the ring to form products methylglyoxal/glyoxal (MGLY/GLY) and their corresponding coproducts butenedial/methyl-substituted butenedial as proposed in earlier studies. However, a new reaction pathway is found for the bicyclic alkoxy radicals, leading to products MGLY/GLY and 2,3-epoxybutandial/2-methyl-2,3epoxybutandial. A new mechanism is proposed for the atmospheric oxidation mechanism of toluene based on current theoretical and previous theoretical and experimental results. The new mechanism predicts much lower yield of GLY and much higher yield of butenedial than other atmospheric models and recent experimental measurements. The new mechanism calls for detection of proposed products 2,3-epoxybutandial and 2-methyl-2,3-epoxybutandial. from the −CH3 group (∼7%) to yield eventually benzaldehyde under the atmospheric conditions:

1. INTRODUCTION Aromatic hydrocarbons, emitted mainly by anthropogenic sources such as vehicle exhaust, industry use, and solvent evaporation, account for an important fraction (∼20%) of nonmethane hydrocarbons in the urban atmosphere.1 Atmospheric oxidation of alkylbenzenes contributes significantly to ozone and secondary organic aerosol (SOA) formation in the urban atmosphere.1−5 Toxic products like epoxide compounds6,7 and nitrated compounds8 were detected in the laboratory chamber studies of several alkylbenzenes at high NOx concentrations of 1013−1014 molecules cm−3. In the atmosphere, toluene is the most abundant alkylbenzenes. A large number of experimental studies6−37 have been performed to elucidate the oxidation mechanism of toluene by probing the possible radical intermediates, identifying products, and quantifying the product yields. However, only a few reactive radical intermediates have been detected experimentally. In the troposphere, the dominant removal of toluene is due to its reaction with OH radical, proceeding as additions to the aromatic rings to four adducts R1−R4 (∼93% in total) and as H-abstraction © 2014 American Chemical Society

The adducts have been detected by chemical ionization mass spectrometry (CIMS, m/e 109) by Molina et al.27 and by UVlaser absorption spectra at ∼308 nm (off-resonance to OH) by Bohn.29 In the atmosphere, R1−R4 would possibly react with O2, NOx, HOx, and other trace radicals. By fitting to the time profile of UV absorption of the OH radical and the adducts, Bohn29 Received: January 4, 2014 Revised: June 5, 2014 Published: June 5, 2014 4533

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of toluene is reflected in several atmospheric chemistry models such as the Master Chemical Mechanism (MCM),40−42 Carbon Bond (CB05-TU),43 and SAPRC-07/11.44−46 Scheme 1 shows the mechanism from MCM3.1/3.2, and similar ones can be abstracted from CB05-TU and SAPRC. Consistent yields on the ring-retaining products benzaldehyde (from H-abstraction channel) and cresols (o-, m-, and p-) are agreed, e.g., MCM 3.1/3.2 assigns yields of 7% for benzaldehyde and 18% for cresols, and SAPRC-11 assigns values of 6.5% and 19.4%. However, there exist much uncertainties and ambiguities on the ringfragmentation products, including epoxides, glyoxal (GLY) and coproducts (4-oxo-2-pentenal and/or 2-methylbutenedial), and methylglyoxal (MGYL) and coproduct butenedial. The detection and yields of these products vary significantly between different experiments using different techniques. Nonetheless, certain observations can still be perceived for the oxidation of toluene initiated by OH radicals: (1) yield of GYL is approximately equal to that of MGYL; (2) yields of GYL and MGYL are higher than those of their coproducts, e.g., 2-methylbutenedial was only assigned tentatively with a yield of ∼1%, being opposed to the high yield of 4−37% for GYL;33 (3) the carbon balance is about 70% even if the yields of 1,4-dicarbonyls (butenedial, 4-oxo-2-pentenal, and 2-methylbutenedial) are assumed to be the same as those for GYL and MGYL; and (4) the yield of cresols (∼20%) from toluene is lower than that of phenol (∼53%) from benzene.47 Theoretical studies have also been performed to elucidate the oxidation mechanism using quantum chemistry and kinetics calculations. The B3LYP/6-31G(d,p) study on the initial OH additions by Suh et al.48 found that OH additions would occur mainly at the ortho- and para-positions. For the further reactions of toluene-OH adducts with O2, Bartolotti and Edney suggested a few intermediate radicals such as49

found that the recombination of toluene-OH and O2 to tolueneOH-O2 was reversible at a rate constant of (3 ± 2) × 10−15 cm3 molecule−1 s−1 for the forward recombination and an equilibrium constant of (3.25 ± 0.33) × 10−19 cm3 molecule−1 at 299 K. Koch et al.38 further studied the reactions between the adducts and O2, NO, and NO2, obtaining rate constants of (5.6 ± 1.5) × 10−16 (299 K), 80 kJ/mol) of the reverse processes, e.g., the rate of 2.8 × 10−4 s−1 for reverse process from R2-13OO-s to R2-1OO-s (Table 3) is negligible if being compared to the bimolecular recombination of R2-13OO-s with O2 at rates of 102 to 104 s−1 (see section 3.3). The effective rate constants for R2 removal by O2 addition, i.e., the rate constants of R2-13OO formation, are kEff = k2Ak2RC/(k−2A + k2RC) if applying steadystate approximation to R2-1OO and R2-3OO. With the highpressure-limit values in Table 3, the effective bimolecular rate 4537

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Table 3. Predicted Reaction Energies, Barrier Heights (in kJ/mol), and High-Pressure-Limit Rate Constants at the ROCBS-QB3 Levela reactions

ΔE0K

ΔG298K

ΔE0K≠

ΔG298K≠

kforward

kreverse

R2 + O2 → R2-1OO-a R2 + O2 → R2-1OO-s R2 + O2 → R2-3OO-a R2 + O2 → R2-3OO-s R2 + O2 → R2-5OO-a R2 + O2 → R2-5OO-s R2 + O2 → o-cresol + HO2b R2-1OO-a → R2-13OO-a R2-1OO-a → R2-1OOH-a from −OHc R2-1OO-a → R2-1OOH-a from −C2Hc R2-1OO-s → R2-13OO-s R2-1OO-s → R2-1OOH-s from −OHc R2-3OO-a → R2-13OO-a R2-3OO-a → R2-3OOH-a from −OHc R2-3OO-a → R2-3OOH-a from −C2Hc R2-3OO-s → R2-13OO-s R2-3OO-s → R2-3OOH-s from −OHc R2-13OO-s → R2-34O-1O-s R2-13OO-s → R2-16O-3O-s R4 + O2 → R4-3OO-a R4 + O2 → R4-3OO-s R4 + O2 → R4-1OO-a R4 + O2 → R4-1OO-s R4 + O2 → p-cresol + HO2d R4-3OO-a → R4-35OO-a R4-3OO-a → R4-3OOH-a from −OHc R4-3OO-a → R4-3OOH-a from −C2Hc R4-3OO-s → R4-35OO-s R4-3OO-s → R4-3OOH-s from −OHc R1 + O2 → R1-2OO-a R1 + O2 → R1-2OO-s R1-2OO-a → R1-26OO-a R1-2OO-s → R1-26OO-s

−48.7 −39.5 −42.9 −33.5 −32.6 −36.7

−1.7 8.6 2.0 10.8 10.4 8.2 −35.4

−57.5

−54.8

−44.5

−39.0

−63.5

−57.1

−54.4 −59.2 −48.0 −43.9 −42.4 −45.1

−56.2 −61.6 −3.2 1.8 2.7 0.6

−32.3

−30.4

−51.1

−45.9

−40.9 −30.6 −41.3 −60.8

−0.5 8.6 −38.3 −56.8

47.8 45.5 54.8 50.9 41.9 36.6 49.2 64.7 104.6 88.1 38.5 103.0 62.2 89.2 92.4 38.8 101.1 83.2 71.8 48.8 45.0 33.5 28.1 47.7 71.0 94.9 95.0 48.3 109.9 47.7 43.2 69.0 43.1

1.40 × 10−15 3.79 × 10−15 8.27 × 10−17 3.79 × 10−16 1.46 × 10−14 1.23 × 10−13 9.35 × 10−16 4.44 × 101 2.9 × 10−6 to 2.9 × 10−4 2.3 × 10−3 to 2.3 × 10° 1.69 × 106 5.6 × 10−6 to 5.6 × 10−4 1.22 × 102 2.9 × 10−3 to 2.9 × 10−1 4.0× 10−4 to 2.9 × 10−1 2.03 × 106 1.2 × 10−5 to 1.2 × 10−3 1.64 × 10−2 1.64 1.88 × 10−15 1.33 × 10−14 4.16 × 10−13 3.61 × 10−12 1.72 × 10−15 3.45 1.5 × 10−4 to 1.5 × 10−2 1.4 × 10−4 to 1.4 × 10−1 3.20 × 104 3.5 × 10−7 to 3.5 × 10−5 2.85 × 10−15 1.75 × 10−15 7.49 2.64 × 105

1.75 × 104 3.01 × 106 4.48 × 103 7.35 × 105 2.42 × 107 8.22 × 107

−38.8

3.3 −0.9 11.8 5.3 2.0 −8.3 7.1 60.9 100.9 85.6 35.4 100.9 56.7 84.9 88.9 31.7 96.6 82.9 71.3 7.2 −1.4 −11.3 −7.2 6.1 66.5 91.6 90.6 44.4 107.0 9.1 4.3 66.0 38.2

1.82 × 10−5

2.78 × 10−4 1.16 × 10−5

1.37 × 10−4

6.32 × 103 3.46 × 105 3.03 × 107 1.12 × 108 1.09 × 10−5

1.97 × 10−4 2.84 × 104 6.86 × 106 9.85 × 10−7 2.00 × 105

Rate constants in cm3 molecule−1 s−1 for bimolecular reactions and in s−1 for unimolecular reactions. bTunneling correction factor = 1.54. Uncertainty from tunneling correction. dTunneling correction factor = 1.43.

a c

constants (in cm3 molecule−1 s−1) at 298 K via R2-1OO and R2-3OO are

value of (6.0 ± 0.5) × 10−16 cm3 molecule−1 s−1 by Bohn29 or (5.6 ± 1.5) × 10−16 cm3 molecule−1 s−1 by Koch et al.38 In studying benzene oxidation, Glowacki et al.53 argued that some unimolecular reactions might not reach their high-pressure limits under typical atmospheric conditions, though the resulting product yields from RRKM-ME and high-pressure limit agreed excellently. To check the validity of the high-pressure-limit approximation in toluene oxidation, RRKM-ME calculations are carried out here for the fate of R2 after O2 addition. The time profiles of R2 and other intermediates are shown in Figure 3 for reactions at 76, 760, and 7600 Torr. At [O2] of 5 × 1018 molecules cm−3, R2 disappears within 10−3 s, agreeing the experimental observation by Bohn.29 RRKM-ME simulations show that the “chemically significant eigenvalues” are not well separated from the “internal energy relaxation eigenvalues”; therefore, the phenomenological rate constants from RRKM-ME simulations are not welldefined, and only the pressure-dependent-product yields are presented (Figure 4). For o-cresol, R2-13OO-s, and R2-16O-3O-s, RRKM-ME predicts yields of 0.37, 0.56, and 0.07 at 76 Torr, 0.36, 0.63, and 0.01 at 760 Torr, and 0.36, 0.64, and 0.00 at 7600 Torr, comparing to the high-pressure-limit results of 0.36, 0.64, and 0.00, respectively. The yield of o-cresol remains almost constant, while the −O−O− breakage of R2-13OO-s to R2-16O-3O-s is quenched

kEff (R2 + O2 ⇌ R2‐1OO‐s → R2 − 13OO − s) = 1.36 × 10−15

kEff (R2 + O2 ⇌ R2‐3OO‐s → R2‐13OO‐s) = 2.79 × 10−16

kEff (R2 + O2 ⇌ R2‐1OO‐a → R2‐1OO‐a) = 3.73 × 10−18 kEff (R2 + O2 ⇌ R2‐3OO‐a → R2‐1OO‐a) = 2.04 × 10−18

Therefore, the addition to C1 and C3 occurs dominantly from the syn-direction, and the bicyclic intermediate formed is mainly R2-13OO-s. The total effective rate constant for addition is 16.4 × 10−16 cm3 molecule−1 s−1. In addition, the bimolecular rate constant for H-abstraction from C2 by O2 to form o-cresol is estimated to be 9.35 × 10−16 cm3 molecule−1 s−1 using ROCBSQB3 energies, resulting in yields of 0.36 and 0.64 for o-cresol and R2-13OO-s, respectively. For the most important route via R2-1OO-s, the recombination rate constant (k2A) of 3.79 × 10−15 cm3 molecule−1 s−1 (Table 3) agrees with the measured value of (3 ± 2) × 10−15 cm3 molecule−1 s−1 by Bohn,29 and the effective loss of 16.4 × 10−16 cm3 molecule−1 s−1 agrees with the measured 4538

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Figure 5 shows the potential energy diagram for the main reaction pathways of R4 and O2. The ROCBS-QB3 energetics and the high-pressure-limit rate constants at 298 K are given in Tables 3 and S3,Supporting Information. Again, the O2 additions to p-C1 are negligible even though the barriers for O2 additions to C1 (ΔG298K≠, 33.5 and 28.1 kJ/mol for anti- and syn-conformers) are ∼16 kJ/mol lower than those to C3 (48.8 and 45.0 kJ/mol). For O2 addition to C3, the syn-addition again dominates over the anti-addition. The peroxy radicals thus formed will cyclize to bicyclic intermediates R4-35OO, while the barriers for all other processes are too high. Therefore, the addition of O2 to R4 can be modeled as

and the effective bimolecular rate constants (in cm3 molecule−1 s−1) at 298 K can be obtained as kEff (R4 + O2 ⇌ R4‐3OO‐s → R4‐35OO‐s) = 1.13 × 10−15 kEff (R4 + O2 ⇌ R4‐3OO‐a → R4‐35OO‐a) = 9.22 × 10−19

The bicyclic intermediate thus formed from R4 and O2 is exclusively R4-35OO-s via R4-3OO-s. For direct H-abstraction by O2 to form p-cresol, a rate constant of 1.72 × 10−15 cm3 molecule−1 s−1 is estimated, rendering the branch ratios of 0.40 and 0.60 for p-cresol and R4-35OO-syn, respectively. Reaction of Toluene-ipso-OH Adduct (R1) with O2. Adduct R1 reacts with O2 by addition only. The potential energy diagram for the main reaction pathways of R1 and O2 is shown in Figure 6, and the ROCBS-QB3 energetics and rate constants in Tables 3 and S4, Supporting Information. Additions to C4 are ignored here; and to C2, the syn-addition again dominates over the antiaddition because of the lower barriers for syn-addition (43.2 kJ/mol for syn vs 47.7 kJ/mol for anti) and for the ensuing cyclization (43.1 kJ/mol vs 69.0 kJ/mol). The reaction between R1 and O2 can modeled as

Figure 3. Time-dependent species profile in atmospheric oxidation of R2 at 298 K and 1 atm pressure with 20% of O2 (ΔEdown = 250 cm−1).

and the effective bimolecular rate constants (in cm3 molecule−1 s−1) at 298 K can be obtained as kEff (R1 + O2 ⇌ R1‐2OO‐s → R1‐26OO‐s) = 6.47 × 10−16 Figure 4. Pressure-dependent yields of R2-16O-3O, R2-13OO-s, and o-cresol at 298 K.

kEff (R1 + O2 ⇌ R1‐2OO‐a → R1‐26OO‐a) = 7.51 × 10−19

with the increased pressure. Nevertheless, the reaction has already reached its high-pressure limits at 298 K at 760 Torr, even though the rates of certain unimolecular processes at 760 Torr are less than their high-pressure limits. The consistency between RRKM-ME at 760 Torr and high-pressure-limit calculation is understandable. In the reaction system, the highest high-pressure-limit unimolecular rates are ∼108 s−1, which is far slower than the typical collision frequency of 1011 s−1. The cyclization of R2-1OO and R2-3OO to R2-13OO is slow enough to ensure thermal equilibrium before being transformed. Therefore, high-pressure limit will be assumed for other processes discussed below. Reaction of Toluene-p-OH Adduct (R4) with O2. Adduct R4 would also react with O2 by H-abstraction and additions.

Therefore, reaction of R1 and O2 will form R1-26OO-s almost exclusively. The overall rate constants (298 K) for reactions of R2, R4, and R1 with O2, including H-abstraction and bicyclic radical formation, are 1.64 × 10−15, 2.86 × 10−15, and 6.47 × 10−16 cm3 molecule−1 s−1, respectively, being consistent with the experimental value of 5.6 × 10−16 cm3 molecule−1 s−1 (299 K).38 With the rate constants of 3.6 × 10−11 and 109 s−1, indicating pressure dependence. Therefore, RRKM-ME calculations are carried out to explore the pressure-dependence of the phenomenological rates and branching ratios. The phenomenological rates of R2-13OO-s-4O-a are displayed in Figure S3 (Supporting Information). All the Phenomenological rates are still in the falloff region at 760 Torr; however, the predicted branching ratios at 760 Torr are close to the high-pressure-limits (Table S6, Supporting Information). Ring-breakage is overestimated slightly, e.g., the branching ratio of 0.58 at highpressure limit versus 0.50 at 760 Torr from RRKM-ME calculations for ring-breakage of R2-13OO-s-4O-a. Scheme 3 shows the fate of eight bicyclic alkoxy radicals from RRKM-ME analysis.

C6H6-OH + O2 → C6H6-OH-2OO-syn → C6H6-OH-26OO-syn in benzene. Meanwhile, the H-abstraction barriers are less reduced, i.e., 49.2 kJ/mol for R2 and 47.7 kJ/mol for R4 in toluene and 50.9 kJ/mol in benzene. For R4, the −CH3 at the para-position accelerates both H-abstraction (from 5.6 × 10−16 to 17.3 × 10−16 cm3 molecule−1 s−1) and ring-closure (from 2.3 × 10−16 to 11.3 × 10−16 cm3 molecule−1 s−1). For benzene, the resulting branching ratio of 0.71 for phenol is comparable but slightly higher than the experimental value of 0.53 ± 0.05 from two different chamber studies by Volkamer et al.30 and of 0.61 ± 0.07 from a flow tube study by Berndt et al.72 Note that the MCM (Master Chemical Mechanism)41,42 and SAPRC-1145,46 adopted values are 0.53 and 0.57, respectively. For toluene, a yield of ∼0.21 can be estimated for o-cresol by multiplying the branching ratio of R2 formation (0.59) and the branching ratio of o-cresol formation in R2 + O2 (0.36), and yield of ∼0.08 for p-cresol from R4 with branching ratios of 0.14 and 0.60. The estimated yields for cresols are in reasonable agreement with the previous experimental values of 0.12−0.22 for o-cresol16,25,26 and 0.03−0.06 for p-cresol.25,26 The predicted total yield of ∼0.32 for cresols from toluene (assuming 0.03 of m-cresol from R3) is significantly lower than that of 0.71 for phenol from benzene, being consistent with the experimentally observed tendency from benzene to toluene. The o-CH3 in R2 of toluene reduces the barriers for O2 additions and ring-closures. Upon further alkyl substitution on benzene, the decreased yields of phenolic compounds and increased role of bicyclic intermediates are expected with more occurrence of o-CH3. This is in accordance with the recent experimental study and MCM modeling on the product yields in the oxidation of a few alkylbenzenes by Birdsall and Elrod,36 who found that the yields of phenolic compounds decreased gradually from benzene to trimethylbenzenes. Theoretical studies on the mechanisms of xylenes and trimethylbenzenes are ongoing to confirm such expectation. 3.4. Fates of Bicyclic Peroxy Radicals (BPRs). In the atmosphere, the bicyclic radical intermediates (R2-13OO-s, R4-35OO-s, and R1-26OO-s) react almost exclusively with O2 to form BPRs. Further ring-closure of these peroxy radicals to tricyclic intermediates can be excluded from the mechanism because of the extremely high barriers (ΔG298K≠ > 160 kJ/mol at M06-2X level, Tables S2 and S3, Supporting Information) and endothermicity (ΔG298K ≈ 40 kJ/mol). Therefore, the BPRs will react with NOx, HO2, or RO2 in the atmosphere to form bicyclic alkoxy radicals (BARs) and other products including nitrates, 4542

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Scheme 2. Fate of Bicyclic Alkoxy Radicals (BARs)

Scheme 3. Fate of Bicyclic Alkoxy Radicals

ROCBS-QB3 level and the barrier heights and corresponding high-pressure-limit rate constants are listed in Table S7, Supporting Information. For each BEAR, there are two possible −C−C− bond-breakage routes, of which the fastest ones at rates of ∼1010 s−1 diminish the role of H-abstraction by O2. The ultimate products would be MGYL + 2,3-epoxybutandial and GYL + 2-methyl-2,3-epoxybutandial. Reactions of toluene-m-OH adduct (R3) are not examined in the current study. The possible bicyclic alkoxy radical is R3-24OO-s-1O, which is assumed to break the C−C bond to form GLY and 4-oxo-2-pentenal.

All bicyclic alkoxy radicals break the ring dominantly, except that R4-35OO-s-2O-a would cyclize to R4-35OO-s-12O-a dominantly, and R2-13OO-s-4O-a undergoes ring-breakage and cyclization equally. The ring-breakage will lead to the formation of α-dicarbonyl (GLY and MGLY) and unsaturated dicarbonyl compounds; while the BEPR radicals would be transferred to bicyclic epoxyl alkoxy radicals (BEARs). The BEAR radicals are again subject to C−C bond breakages (Scheme 4) and to reaction with O2 (kb1[O2] ≈ 105 s−1). Transition states for the bond breakages have been identified at the M06-2X level, and energies are recalculated at the 4543

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Scheme 4. Fate of Bicyclic Epoxy Alkoxy Radicals (BEARs)

Scheme 5. Modified Mechanism for Toluene Oxidation in the Atmosphere

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Author Contributions

3.5. Effect of Interception of Vibrationally Excited Radicals by O2. In the present mechanism, the oxidation of toluene is well separated into a few types of isolated reactions, i.e., the addition of OH to toluene, the reaction of adducts with O2, and the addition of bicyclic intermediates with O2, etc. This is based on the fast energy exchange and randomization of the vibrational excitation in the toluene-OH adducts and the bicyclic intermediates. However, recent study by Glowacki et al.76 showed that about 20−30% of the vibrationally excited C2H2− OH adducts might be intercepted by the atmospheric O2 before their full relaxation. The fraction of adduct being intercepted depends on the reaction energy and the rate coefficients for recombination adduct with O2. According to the modeling (Figure 4 in Glowacki et al.76), the fractions being intercepted by O2 are less than 0.02 for toluene-OH adducts because of the low rate coefficients (∼10−13 cm3 molecule−1 s−1 or less, Table 3) and negligible for the bicyclic intermediates (k ≈ 10−15 cm3 molecule−1 s−1 or less, section 3.3). The assumption of complete relaxation, and therefore the use of Gibbs free energies in transition state theory calculation is valid in the atmospheric oxidation of toluene.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from National Natural Science Foundation of China (No. 21177041) and the service support from SCUTGrid from Information Network Engineering and Research Center of South China University of Technology.

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(1) Calvert, J. G.; Atkinson, R.; Becker, K. H.; Kamens, R. M.; Seinfeld, J. H.; Wallington, T. J.; Yarwood, G. The Mechanisms of Atmospheric Oxidation of the Aromatic Hydrocarbons; Oxford University Press: Oxford, UK, 2002. (2) Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; Seinfeld, J. H., Pandis, S. N., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. (3) Odum, J. R.; Jungkamp, T. P. W.; Griffin, R. J.; Flagan, R. C.; Seinfeld, J. H. The Atmospheric Aerosol-Forming Potential of Whole Gasoline Vapor. Science 1997, 276, 96−99. (4) Forstner, H. J. L.; Flagan, R. C.; Seinfeld, J. H. Secondary Organic Aerosol from the Photooxidation of Aromatic Hydrocarbons: Molecular Composition. Environ. Sci. Technol. 1997, 31, 1345−1358. (5) Offenberg, J. H.; Lewis, C. W.; Lewandowski, M.; Jaoui, M.; Kleindienst, T. E.; Edney, E. O. Contributions of Toluene and α-Pinene to SOA Formed in an Irradiated Toluene/α-Pinene/NOx/Air Mixture: Comparison of Results Using 14C Content and SOA Organic Tracer Methods. Environ. Sci. Technol. 2007, 41, 3972−3976. (6) Baltaretu, C. O.; Lichtman, E. I.; Hadler, A. B.; Elrod, M. J. Primary Atmospheric Oxidation Mechanism for Toluene. J. Phys. Chem. A 2009, 113, 221−230. (7) Yu, J.; Jeffries, H. E. Atmospheric Photooxidation of Alkylbenzenes - II. Evidence of Formation of Epoxide Intermediates. Atmos. Environ. 1997, 31, 2281−2287. (8) Nishino, N.; Atkinson, R.; Arey, J. Formation of Nitro Products from the Gas-Phase OH Radical-Initiated Reactions of Toluene, Naphthalene, and Biphenyl: Effect of NO2 Concentration. Environ. Sci. Technol. 2008, 42, 9203−9209. (9) Atkinson, R.; Carter, W. P. L.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr. A Smog Chamber and Modeling Study of the Gas Phase NOx-Air Photooxidation of Toluene and the Cresols. Int. J. Chem. Kinet. 1980, 12, 779−836. (10) Tully, F. P.; Ravishankara, A. R.; Thompson, R. L.; Nicovich, J. M.; Shah, R. C.; Kreutter, N. M.; Wine, P. H. Kinetics of the Reactions of Hydroxyl Radical with Benzene and Toluene. J. Phys. Chem. 1981, 85, 2262−2269. (11) Atkinson, R.; Carter, W. P. L.; Winer, A. M. Effects of Pressure on Product Yields in the NOx Photooxidations of Selected Aromatic Hydrocarbons. J. Phys. Chem. 1983, 87, 1605−1610. (12) Leone, J. A.; Seinfeld, J. H. Updated Chemical Mechanism for Atmospheric Photooxidation of Toluene. Int. J. Chem. Kinet. 1984, 16, 159−193. (13) Shepson, P. B.; Edney, E. O.; Corse, E. W. Ring Fragmentation Reactions on the Photooxidations of Toluene and o-Xylene. J. Phys. Chem. 1984, 88, 4122−4126. (14) Tuazon, E. C.; Atkinson, R.; Leod, H. M.; Biermann, H. W.; Winer, A. M.; Carter, W. P. L.; Pitts, J. N., Jr. Yields of Glyoxal and Methylglyoxal from the NOx-Air Photooxidations of Toluene and mand p-Xylene. Environ. Sci. Technol. 1984, 18, 981−984. (15) Bandow, H.; Washida, N.; Akimoto, H. Ring-Cleavage Reactions of Aromatic Hydrocarbons Studied by FT-IR Spectroscopy. I.

4. CONCLUSION AND MODIFIED ATMOSPHERIC OXIDATION MECHANISM OF TOLUENE Based on current theoretical study and the existing oxidation mechanism of toluene in MCM 3.2, a modified mechanism has been proposed and shown in Scheme 5, in which the peroxy radicals are converted to alkoxy radical by reacting with NO. The yields of first-generation products are given in Table 1. The current mechanism differs from the MCM mechanism in several aspects, including higher yields of cresols (32% vs 18%), MGLY (41% vs 22%), and butenedial (33% vs 11%), lower yields of GLY (13% vs 43%) and 2-methyl-butenedial (0.5% vs 11%), no yield of methyl hexadienedial or furanones, and newly proposed products 2,3-epoxybutanedial (7%) and 2-methyl-2,3-epoxybutanedial (4%). The current mechanism predicts a lower yield of GLY than the recent experimental measurements, and a higher yield of butenedial.30,32,33 One immediate reason is the reaction between butenedial and OH radical, which removes butenedial and produces GLY. The experimental studies have corrected the loss of butenedial and GLY by reaction and photolysis; however, no correction was made for other possible sources of GLY.

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving yields of products from OH-initiated oxidation of toluene in the literature (Table S1), calculated energy changes for reactions (Tables S2−S5), energy and Gibbs free energy changes at ROCBS-QB3 level (Table S6), barrier heights for C−C breakage in R2-13OO-s-45O-a-6O-a/s and R4-35OO-s12O-a-6O-a/s,and figures showing potential energy diagrams for the reactions of R2 (Figure S1) and R4 (Figure R4) and benzeneOH adduct with O2 at the ROCBS-QB3 level and (Figure S3) phenomenological rates as a function of pressure for ring-breakage, ring-closure and reverse in R2-13OO-s-4O-a. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-Mail: [email protected]. Tel: 0086 27 87112900. 4545

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