Atmospheric Fate of Methacrolein. 1. Peroxy Radical Isomerization

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Atmospheric Fate of Methacrolein. 1. Peroxy Radical Isomerization Following Addition of OH and O2 John D. Crounse,*,† Hasse C. Knap,‡ Kristian B. Ørnsø,‡ Solvejg Jørgensen,‡ Fabien Paulot,§,∥ Henrik G. Kjaergaard,‡ and Paul O. Wennberg*,†,§ †

Division of Geological and Planetary Science, California Institute of Technology, Pasadena, California 91125, United States Department of Chemistry, DK-2100 Copenhagen Ø, University of Copenhagen, Copenhagen, Denmark § Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States ‡

S Supporting Information *

ABSTRACT: Peroxy radicals formed by addition of OH and O2 to the olefinic carbon atoms in methacrolein react with NO to form methacrolein hydroxy nitrate and hydroxyacetone. We observe that the ratio of these two compounds, however, unexpectedly decreases as the lifetime of the peroxy radical increases. We propose that this results from an isomerization involving the 1,4-H-shift of the aldehydic hydrogen atom to the peroxy group. The inferred rate (0.5 ± 0.3 s−1 at T = 296 K) is consistent with estimates obtained from the potential energy surface determined by high level quantum calculations. The product, a hydroxy hydroperoxy carbonyl radical, decomposes rapidly, producing hydroxyacetone and re-forming OH. Simulations using a global chemical transport model suggest that most of the methacrolein hydroxy peroxy radicals formed in the atmosphere undergo isomerization and decomposition.

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INTRODUCTION Methacrolein (2-methylpropenal, MACR) is a major product of gas-phase isoprene oxidation (∼8% globally averaged yield, or approximately 45 Tg yr−1)1−3 in the atmosphere. As with isoprene, Ravishankara and colleagues demonstrated that the atmospheric fate of methacrolein is dominated by its reaction with the hydroxyl radical, OH.4 The OH chemistry proceeds via two channels with approximately equal rates: (a) addition to the double bond, primarily at the external carbon,5,6 or (b) abstraction of the aldehydic hydrogen. The atmospheric chemistry of channel b (hydrogen abstraction) is the topic of our companion paper,7 whereas the chemistry of channel a (OH addition) is the focus of this work. The addition of OH to MACR is expected to be followed by reaction with oxygen (O2), leading to the formation of hydroxy peroxy radicals (Scheme 1). It is generally assumed that in the atmosphere, the subsequent fate of these hydroxy peroxy radicals is reaction with NO or HO2.8 Evidence for this chemistry is found in numerous laboratory studies of the fate of these and similar peroxy radicals. Most of the laboratory studies were performed, however, with NO or HO2 concentrations sufficiently large that the peroxy radical lifetime is relatively short (0.5 s). In low-NOx experiments using isotopically labeled H218O2 as a precursor to 18OH we observe significant levels of both labeled and unlabeled hydroxyacetone (HAC−18OH and HAC−16OH, respectively, Scheme 2). This is consistent with a significant level (>50%) of OH recycling (as unlabeled OH) in the initial step of methacrolein oxidation. This is substantially greater (>4 times) than the ratio of unlabeled to labeled 5759

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products are calculated to be ∼62 and ∼47 kcal/mol lower in energy, respectively, than the reactants (MACR + OH + O2). We have calculated the energetics and rates of the H-shift isomerizations of the major product, MACR-1-OH-2-OO (Scheme 3). From the thermalized MACR-1-OH-2-OO, the 1,4-H-shift and the 1,5-H-shift of the alcohol hydrogen to the peroxy group both have B3LYP/6-31+G(d,p) barriers of ∼20 kcal/mol, whereas the 1,4-H-shift of the α hydrogen to the OH group has a barrier of ∼30 kcal/mol, in agreement with previous calculations on vinoxy radical hydrogen shift reactions.43 We thus focus on the 1,4- (aldehydic H) and 1,5(alcohol H) H-shift reactions and the subsequent decomposition processes as sketched in Scheme 3. The F12 energies of the stationary points in the 1,4- and 1,5H-shift reactions are given in Tables 1 and 2, respectively. Figure 2. Relative energies for the 1,4- and 1,5-H-shift reactions of MACR-1-OH-2-OO. The F12//B3LYP/aVTZ energies are corrected for zero point vibrational energy with the B3LYP/aVTZ harmonic frequencies. The B3LYP/aVTZ geometries are shown for each stationary point.

Table 1. Energetics of the MACR + OH and 1,4-H-Shift Reactions (kcal/mol) species

B3LYP/6-31+G(d,p)a

B3LYP/aVTZb

F12b

MACR-1-OH-2-OO 1,4-TS1 1,4-P1 1,4-TS2 1,4-P2+CO+OH 1,4-TS1 ν̃IMAGc 1,4-TS2 ν̃IMAGc

0.0 18.2 5.6 13.0 −31.3 1720i 225i

0.0 17.8 4.8 11.1 −35.6 1718i 246i

0.0 19.0 2.1 10.5 −35.8

36 kcal/mol lower than the energy of MACR-1-OH-2-OO. The rate determining step in both routes is clearly the 1,4- and 1,5H-shift reactions. The reaction of 1,4-P1 over 1,4-TS2 (elimination of CO) will compete with collisional stabilization and possible reaction with O2. However, with a barrier of 10.5 kcal/mol for CO elimination, collisional stabilization is unlikely to occur with a high yield. Table 3 compares the energies of TS1 and products relative to MACR-1-OH-2-OO for these two H-shift reactions, along with results for similar reaction pathways of acrolein (ACR).45 The ZPVE corrected F12 barriers for the 1,4- and 1,5-H-shift reactions are 19.0 and 21.2 kcal/mol, respectively. The energies for the H-shift reactions in MACR compare well with the G3SX composite method calculations for the H-shift reactions of acrolein, as well as earlier CBS-QB3 results on similar reactions.43,46

a c

Including B3LYP/6-31+G(d,p) ZPVE. bB3LYP/aug-cc-pVTZ ZPVE. Imaginary frequency (cm−1).

Table 2. Energetics of the MACR + OH and 1,5-H-Shift Reactions (kcal/mol) species

B3LYP/6-31+G(d,p)a

B3LYP/aVTZb

F12b

MACR-1-OH-2-OO 1,5-TS1 1,5-P1 1,5-TS2 MGLX+HCHO+OH 1,5-TS1 ν̃IMAGc 1,5-TS2 ν̃IMAGc

0.0 17.1 18.3 19.8 −13.7 601i 351i

0.0 16.7 18.1 18.8 −16.6 661i 320i

0.0 21.2 19.7 22.7 −11.1

Table 3. Energetics of the 1,4- and 1,5-H-Shift Reactions for Methacrolein and Acrolein (kcal/mol) MACRa

a

Including B3LYP/6-31+G(d,p) ZPVE. bB3LYP/aug-cc-pVTZ ZPVE. c Imaginary frequency (cm−1). R TS1 P1

Figure 2 shows the relative energies along with the B3LYP/ aVTZ optimized structures of the associated stationary points. The 1,5-TS1, 1,5-P1, and 1,5-TS2 structures have similar energies, which are about 20 kcal/mol higher than MACR-1OH-2-OO. The 1,5-P1 structure decomposes with a low barrier of 2 kcal/mol, eliminating HCHO to form a vinoxy-like radical, which subsequently decomposes with no barrier to yield OH and methyl glyoxal (MGLX). The energy of the products (MGLX + OH + HCHO) is 11 kcal/mol lower than the energy of the reactant (MACR-1-OH-2-OO). This reaction will be slow with limited effect of tunneling. The energy of 1,4-TS1 is similar to that of 1,5-TS1 but the product, 1,4-P1, is significantly lower in energy (2 kcal/mol above MACR-1-OH-2-OO). This product eliminates CO via 1,4-TS2, lying 10.5 kcal/mol higher in energy than MACR-1OH-2-OO, forming an unstable alkyl radical, which undergoes a barrier-less decomposition to form OH and HAC, similar to what has been found previously for α-hydroperoxy-alkyl radicals.44 The energy of the products, HAC + OH + CO, is

ACRb

1,4-H-shift

1,5-H-shift

1,4-H-shift

1,5-H-shift

0.0 19.0 2.1

0.0 21.2 19.7

0.0 20.1 3.2

0.0 22.5 23.5

a

F12 energies on B3LYP/aVTZ geometry, including B3LYP/aVTZ ZPVE. bG3SX results from Asatryan et al.45

Tables 1 and 2 show the relative energies using three different methods, with the B3LYP/aVTZ IRC of the first step of the 1,4-H- and 1,5-H-shift reactions shown in Supporting Information (Figure S2). Peeters et al.46 have investigated the 1,5-H-shift reaction using B3LYP and the composite QB3 methods. They found the barrier to be 17.6 kcal/mol with the B3LYP/6-31+G(d,p) method (Table 2), in good agreement with our value obtained using the same method, and a barrier 19.7 kcal/mol with their preferred CBS/APNO method. We use the F12 energies and partition functions obtained with the B3LYP/aVTZ harmonic frequencies and moments of inertia to calculate the rate constants of the H-shift reactions according to eq 1. Without tunneling, the rate constants for the 1,4- and 1,5-H-shift reactions are 2.3 × 10−2 and 4.5 × 10−4 s−1, 5760

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hydroxy hydroperoxy radicals produced following OH addition to methacrolein proceed via isomerization and subsequent decomposition. Within the boundary layer, more than 95% of these hydroxy peroxy radicals are predicted to isomerize. This finding is not surprising given that to compete with the intermolecular chemistry, NO or HO2 mixing ratios need to be in excess of ∼2 or 1 ppbv, respectively (for equal reactivity at T = 296 K), conditions that are rarely encountered in biogenically rich environments. Combining the ∼8% global MACR yield from isoprene, the ∼50% MACR oxidation through OHaddition to form MACR-1-OH-2-OO (ignoring depositional, O3 and NO3 MACR losses), and the ∼90% of MACR-1-OH-2OO estimated here to undergo isomerization/decomposition in the atmosphere, we calculate this chemistry regenerates ∼4% OH per molecule of isoprene oxidized, globally averaged.

respectively, at 296 K. Visual inspection the IRC curves (see Supporting Information, Figure S2) along with the significant difference in the imaginary frequencies between 1,4-TS1 and 1,5-TS1 (Tables 1 and 2) indicate that tunneling is important for the 1,4-H-shift reaction and has little effect on the 1,5-Hshift reaction. The one-dimensional Eckart (Wigner) tunneling corrections are 99 (3.9) and 1.4 (1.4) for the 1,4- and 1,5-H-shift reactions at 296 K, respectively, indicating how important tunneling is for these H-shift reactions. Recent work by Zhang and Dibble on a similar reaction indicate that more sophisticated tunneling models result in tunneling factors approximately 1.4 times larger than the Eckart correction at 300 K.36,47 We have calculated the B3LYP/aVTZ energy at different positions of the peroxy radical group (COO) around the CO bond and find that there is no significantly preferred position of the OO radical that would favor either the 1,4- or 1,5-H-shift. The potential energy of rotation of OO bond around CO bond is given in the Supporting Information (Figure S1). It is clear from these considerations that the 1,4-H-shift reaction will dominate over the 1,5-H-shift reaction and therefore little if any MGLX will be produced via this route. The two largest uncertainties in the calculated H-shift rate are the tunneling correction and the barrier height. To bound the calculated 1,4-H-shift reaction rate, we assume an uncertainty in the Eckart tunneling correction of a factor of 1.5.36,47 In addition, we assume an uncertainty in the F12 calculated relative energy of 1,4-TS1 of ±1 kcal/mol, yielding an overall rate for the 1,4-H-shift in the range 0.3−17 s−1 at 296 K. This range, illustrated as the gray area in Figure 1, is consistent with the experimental data. Atmospheric Implications. Numerous field, laboratory, and computational/modeling studies have focused on improving our understanding of the impact of biogenic emissions on the oxidative chemistry of the troposphere. In particular, many of these studies have probed how elevated isoprene emissions impact oxidant (primarily OH) concentrations in the global atmosphere. The standard peroxy radical photochemical mechanism (leading to the formation of hydroperoxides) is highly oxidant consuming and, in simulations, this chemistry is predicted to greatly depress OH levels. In contrast, the OH concentrations measured in biogenically rich environments generally do not reflect this expected depression.9,48−51 We have previously demonstrated in the laboratory, novel HOx regeneration in several key channels of the isoprene oxidation mechanism. These include a HOx neutral channel in the initial OH oxidation of isoprene2 (originally proposed by Peeters et al.46) as well as the OH neutral oxidation of the primary hydroxy hydroperoxide formed from isoprene.14 Here, via the catalytic oxidation of methacrolein, we provide further evidence that the isoprene photochemical cascade is less oxidant consuming than expected. To evaluate what fraction of methacrolein oxidation proceeds by the channels described in Scheme 1, we use the GEOSChem global 3-D chemical transport model (v8.2.1).52 The model is driven by the GEOS-5 assimilated meteorology from the NASA Goddard Earth Observing System. Here the resolution of the model is 4° × 5° with 47 vertical layers. We have added the isomerization chemistry illustrated in Scheme 1 to the base mechanism described by Paulot et al.53 For these simulations we have used the calculated isomerization rate scaled to yield 0.5 s−1 at 296 K: k1,4‑H‑shift(T) = 2.9 × 107 exp(−5297/T). We find that, globally averaged, ∼90% of the

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ASSOCIATED CONTENT

S Supporting Information *

Further details (including supporting tables and figures) regarding calibrations, the yield and fate of MACR-2-OH-1OO, and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: J.D.C., [email protected]; P.O.W., wennberg@ caltech.edu. Phone: 626-395-8655. Present Address ∥

Now with the School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank N. C. Eddingsaas for the design and construction of the chamber used in this work, M. R. Beaver for assistance with experiments, J. M. St. Clair for instrumental assistance, and K. M. Spencer for providing CIMS calibrations of glycolaldehyde and hydroxyacetone. We thank J. Peeters for helpful initial discussions regarding the feasibility of the 1,4-H-shift of MACR-1-OH-2-OO. We thank NASA (NNX08AD29G) and NSF (ATM-0934408), the Danish Council for Independent Research - Natural Sciences, and the Danish Center for Scientific Computing (DCSC) for funding. F.P. acknowledges support from NASA earth and space science fellowship.

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