Atmospheric Chemistry of Enols: A Theoretical Study of the Vinyl

Article pubs.acs.org/est

Atmospheric Chemistry of Enols: A Theoretical Study of the Vinyl Alcohol + OH + O2 Reaction Mechanism Sui So,† Uta Wille,‡ and Gabriel da Silva*,† †

Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia School of Chemistry and Bio21 Institute, The University of Melbourne, Victoria 3010, Australia

‡

S Supporting Information *

ABSTRACT: Enols are emerging as trace atmospheric components that may play a significant role in the formation of organic acids in the atmosphere. We have investigated the hydroxyl radical (•OH) initiated oxidation chemistry of the simplest enol, vinyl alcohol (ethenol, CH2CHOH), using quantum chemical calculations and energy-grained master equation simulations. A lifetime of around 4 h was determined for vinyl alcohol in the presence of tropospheric levels of •OH. The reaction proceeds by •OH addition at both the α (66%) and β (33%) carbons of the π-system, yielding the C-centered radicals •CH2CH(OH)2, and HOCH2C•HOH, respectively. Subsequent trapping by O2 leads to the respective peroxyl radicals. About 90% of the chemically activated population of the major peroxyl radical adduct • O2CH2CH(OH)2 is predicted to undergo fragmentation to produce formic acid and formaldehyde, with regeneration of •OH. The minor peroxyl radical HOCH2C(OO•)HOH is even less stable and undergoes almost exclusive HO2• elimination to form glycolaldehyde (HOCH2CHO). Formation of the latter has not been proposed before in the oxidation of vinyl alcohol. A kinetic mechanism for use in atmospheric modeling is provided, featuring phenomenological rate coefficients for formation of the three main product channels (•O2CH2CH(OH)2 [8%]; HC(O)OH + HCHO + •OH [56%]; HOCH2CHO + HO2• [37%]). Our study supports previous findings that vinyl alcohol should be rapidly removed from the atmosphere by reaction with •OH and O2 with glycolaldehyde being identified as a previously unconsidered product. Most importantly, it is shown that direct chemically activated reactions can lead to •OH and HO2• (HOx) recycling.

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pogenic and biogenic VOC oxidation.4 Organic acids are the primary source of acidity in water droplets seen in pristine environments, and they also play an important role in aerosol formation.13−15 Tropospheric carboxylic acid levels are, however, significantly underestimated,4 implying an incomplete mechanism describing their formation4,5,16,17 and/or important missing sources of these compounds in our current models of the atmosphere.12,18−20 Despite their potential significance for the chemistry of the troposphere, the photochemical oxidation of enols has barely been considered. A mechanism has been proposed for the oxidation of vinyl alcohol (VA),4 the simplest enol, where both the rate coefficient and products of the reaction with •OH were deduced from comparison with similar molecules. On the basis of this, it was suggested that this reaction would ultimately lead to formation of formic acid (FA). Because of the apparent lack of understanding of enol oxidation in the troposphere, we have decided to examine the reaction of vinyl alcohol with •OH, as well as the subsequent trapping of the vinyl alcohol−OH

INTRODUCTION Aldehydes and ketones are ubiquitous atmospheric components that are produced as intermediates in the photochemical oxidation of most volatile organic compounds (VOCs), and their atmospheric reactions have been exhaustively studied. However, little attention has so far been paid to enols, their higher-energy isomers. Enols may be emitted to the atmosphere as combustion byproducts,1−3 and it has been proposed that they undergo further oxidation to yield carboxylic acids.4 A number of enol isomers have also been implicated (but not detected) in the photochemical oxidation of isoprene.5 Although it has now been suggested that acid catalysis5 and photoisomerization6,7 mechanisms may be able to compete with other atmospheric processes for the destruction and/or formation of enols in the atmosphere, the oxidation of enols have been neglected in atmospheric chemical mechanisms, because keto−enol tautomerizations are associated with very large barriers in both directions.8 The photoisomerization of common aldehydes and ketones to their enol forms in the atmosphere is of particular significance, as it provides a new mechanism for organic acid formation. A variety of organic acids are routinely detected throughout the troposphere, and the major sources include biomass combustion,9,10 automotive emission,11,12 and anthro© 2014 American Chemical Society

Received: Revised: Accepted: Published: 6694

January 20, 2014 May 14, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es500319q | Environ. Sci. Technol. 2014, 48, 6694−6701

Environmental Science & Technology

Article

Figure 1. Optimized structures of the reaction intermediates in the vinyl alcohol + •OH + O2 reaction system, at M06-2X/6-31G(2df,p) level of theory.

CCSD(T) energy with a large basis set. G3X-K theory has been used, as it is specifically designed for thermochemical kinetics and can reproduce barrier heights in the DBH24/08 database with an average accuracy of 0.6 kcal mol−1.23 In comparison, M06-2X/aug-cc-pVTZ energy calculations achieve an average accuracy of 0.9 kcal mol−1 for these same barrier heights (albeit using higher-level QCISD geometries). The coordinates of the optimized structures, together with vibrational frequencies and moments of inertia, are provided as Supporting Information (SI). Kinetics Calculations. The MultiWell-2012.1 suite of programs24−27 was used for reaction rate simulations. Calculations were based on the optimized structures, vibrational frequencies, and moments of inertia at the M06-2X/631G(2df,p) level of theory, using both G3X-K and M06-2X/ aug-cc-pVTZ energies. Internal degrees of freedom are treated as harmonic oscillators and external rotations are described using an active 1D K-rotor and an inactive 2D J-rotor. The energy-grained master equation component was solved for energies up to 2000 cm−1 with grains of 10 cm−1. The continuum master equation component was solved up to 200 000 cm−1. The bath gas was approximated to be N2. Lennard− Jones theory was applied to predict the collision rate between reaction adducts and the bath gas. The Lennard−Jones

radical adducts by O2, using quantum chemical techniques and master equation based kinetic modeling. The computational studies confirmed that the oxidation of vinyl alcohol within the troposphere should indeed lead to formic acid production, as suggested previously. In addition, we were able to identify glycolaldehyde (GA) as a significant new product. Most importantly, the dominant processes in the vinyl alcohol + • OH + O2 reaction system involve chemically activated isomerization and decomposition of the peroxyl radical intermediates in mechanisms that regenerate •OH and HO2• (HOx).

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METHODS Electronic Structure Calculations. The vinyl alcohol + • OH + O2 reaction system has been investigated using Gaussian 09.21 Stationary points on the potential energy surface were located using the M06-2X/6-31G(2df,p) level of theory.22 Further single point energy calculations were then performed using the larger aug-cc-pVTZ basis set. The optimized structures and vibrational frequencies were also utilized in G3X-K theory calculations,23 which combine a series of single point energy calculations from Hartree−Fock, Møller−Plesset perturbation, and coupled cluster theory, along with empirical scaling corrections, to estimate the 6695

dx.doi.org/10.1021/es500319q | Environ. Sci. Technol. 2014, 48, 6694−6701


Article

Figure 2. Optimized structures of the transition states in the vinyl alcohol + •OH + O2 reaction system, at M06-2X/6-31G(2df,p) level of theory. The displacement vectors of the imaginary frequency are also shown.

(1). Vinyl Alcohol + •OH. The theoretical energy surface for the vinyl alcohol + •OH reaction is provided in Figure 3.

parameter σ for the C2H5O2• and C2H5O4• isomers was estimated to be 4.59 and 4.86 Å, respectively, using additivity calculations employing partial molar volumes.28 The corresponding ε/kb values are 519 and 544 K. Collisional energy transfer is modeled by the single exponential-down model with ΔEdown of 200 cm−1. Quantum mechanical tunneling has been incorporated for hydrogen shift reactions using unsymmetrical Eckart barriers. Reaction rate simulations were performed at 1 atm and 298 K to represent tropospheric conditions. Each simulation featured 109 trials to produce accurate statistics for reaction channels with low yields. The three barrierless reactions in the reaction system were treated using the restricted Gorin model,29 as described recently in detail.30,31 The high pressure limit rate coefficient between vinyl alcohol and •OH was approximated as the Lennard−Jones collision rate, which is 5.8 × 10−10 cm3 molecule−1 s−1 at 298 K. For the subsequent reactions with O2, there is no current kinetic data in the literature. As a consequence, high pressure limit rate coefficients for the • CH2CH(OH)2 and HOCH2C•HOH + O2 reactions were both approximated using a rate coefficient of 1.7 × 10−11 cm3 molecule−1 s−1 from the structurally similar α-hydroxyethyl and β-hydroxyethyl radical + O2 reactions.32,33

Figure 3. Ab initio potential energy surface of the vinyl alcohol + •OH reaction forming the C2H5O2• isomers. Energies at 0 K with zero point energy (E0 + ZPE) are shown using M06-2X/aug-cc-pVTZ (G3X-K) in the units of kcal mol−1.

The oxidation of vinyl alcohol begins with the addition of •OH. A weak prereaction complex (1) was identified with an energy of 3.9 kcal mol−1 below the entrance level. Formation of a C OH bond may then proceed exothermically on either side of the CC bond to give the α-substituted (2) or β-substituted (3) vinyl alcohol−OH radical adduct. The formation of • CH2CH(OH)2 (2) and HOCH2C•HOH (3) proceeds through transition states TS1 and TS2, with respective barrier heights of 0.7 kcal mol−1 and 1.3 kcal mol−1 relative to the prereaction complex, placing them around 2 to 3 kcal mol−1 below the reactant energies. Calculations using the G3X-K composite method are in relatively good agreement with those at the M06-2X/aug-cc-

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RESULTS AND DISCUSSION The proposed vinyl alcohol + •OH + O2 reaction mechanism is subdivided into three sections, namely the (1) vinyl alcohol + • OH reaction, (2) •CH2CH(OH)2 + O2 reaction, and (3) HOCH2C•HOH + O2 reaction. Optimized structures of all reaction intermediates and transition states in these mechanisms are illustrated in Figures 1 and 2, respectively. The reaction rate simulations reported here are based on energies at the G3X-K level of theory (M06-2X/aug-cc-pVTZ results are provided as SI), with the vinyl alcohol + •OH reaction modeled using the M06-2X/aug-cc-pVTZ energies as discussed below. 6696



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pVTZ level of theory, with an absolute mean deviation of 1.1 kcal mol−1. However, since G3X-K theory places the barrier for TS1 just below the energy of the prereaction complex, we have used the M06-2X energies in the kinetic modeling studies. The energy of TS1 leading to •CH2CH(OH)2 is consistently lower than TS2 leading to HOCH2C•HOH at both the M06-2X and G3X-K level of theory. It is also possible for the vinyl alcohol + •OH reaction to proceed via direct hydrogen abstraction, leading to the resonantly stabilized vinoxyl radical (CH2CHO•) and H2O. A theoretical potential energy surface for this reaction has also been developed, and is shown in Figure S1 of the SI. The transition state is located at 2.7 kcal mol−1 above the reactants at the G3X-K level of theory, or 0.7 kcal mol−1 below the reactants using the M06-2X/aug-cc-pVTZ energies. This indicates that direct abstraction may be a minor pathway in the vinyl alcohol + •OH reaction, although it is unlikely to be of any major significance, and is subsequently omitted from the kinetic analysis. Master equation simulations were performed to determine rate coefficients and to calculate product yields for the vinyl alcohol + •OH reaction. The computed time evolution of the wells and products using M06-2X barrier heights is shown in Figure 4. In this figure, the relative concentration (yield) of all

Figure 5. Ab initio potential energy surface of the •CH2CH(OH)2 + O2 reaction forming formic acid, •OH and HCHO. Energies at 0 K with zero point energy (E0 + ZPE) are shown using M06-2X/aug-ccpVTZ (G3X-K) in the units of kcal mol−1.

reaction is 18.8 kcal mol−1, placing it 59.5 kcal mol−1 below the energy of the reactants. The activation barrier of TS3 corresponds to the actual 1,5-hydrogen shift, which is subsequently followed by CC bond cleavage without transiting through a further stationary point, thus rendering the entire reaction mechanism as concerted. The yields of all wells and products of the •CH2CH(OH)2 + O2 reaction as a function of time from the master equation simulations are shown in Figure 6. About 12% of the chemically

Figure 4. Master equation simulation for the vinyl alcohol + •OH reaction, using M06-2X/aug-cc-pVTZ barrier heights at 298 K and 1 atm of N2. Results are presented as yields of wells and products as a function of number of collisions.

intermediate wells and product sets involved in the reaction are plotted as a function of time, and the sum of yields is 100% at any time. At steady state, about 88% of the excited adduct [CH2CH(OH)•OH]* dissociates back to vinyl alcohol and • OH, resulting in a total forward rate coefficient of 6.8 × 10−11 cm3 molecule−1 s−1. About 12% of the of the excited adduct [CH2CH(OH)•OH]* reacts onward to form the isomeric radical adducts, where attack at the α-carbon that leads to • CH2CH(OH)2 is more favorable (64%) than attack at the βcarbon that leads to HOCH2C•HOH (36%). (2). •CH2CH(OH)2 + O2. The potential energy surface for the •CH2CH(OH)2 + O2 reaction is shown in Figure 5. The addition of O2 to •CH2CH(OH)2 is a barrierless reaction and exothermic by 36.4 kcal mol−1. The resulting peroxyl radical • O2CH2CH(OH)2 can decompose into formic acid + •OH + HCHO via a 1,5-hydrogen shift (TS3). The barrier for this

Figure 6. Master equation simulation for the •CH2CH(OH)2 + O2 reaction, using G3X-K barrier heights at 298 K and 1 atm of N2. Results are presented as yields of wells and products as a function of number of collisions.

activated •O2CH2CH(OH)2 population undergoes collisional deactivation over the course of around 40 bath gas collisions. This process competes with both the reverse fragmentation to • CH2CH(OH)2 + O2 (2%) and the unimolecular dissociation to give formic acid + •OH + HCHO (86%). The rate coefficient of the •CH2CH(OH)2 + O2 reaction is 1.7 × 10−11 cm3 molecule−1 s−1. (3). HOCH2C•HOH + O2. The calculated potential energy surface for the HOCH2C•HOH + O2 reaction is shown in Figure 7. Similar to the •CH2CH(OH)2 radical, addition of O2 6697



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Figure 7. Ab initio potential energy surface of the HOCH2C•HOH + O2 reaction forming formic acid, •OH, HCHO, glycolaldehyde, and HO2•. Energies at 0 K with zero point energy (E0 + ZPE) are shown using M06-2X/aug-cc-pVTZ (G3X-K) in the units of kcal mol−1.

to HOCH2C•HOH is barrierless and leads to the peroxyl radical CH2(OH)CH(OH)O2• (5) with an exothermicity of 37.3 kcal mol−1. The latter subsequently undergoes isomerization via involvement of both the α-hydroxy and β-hydroxy substituents. A 1,5-hydrogen shift (TS4) can occur, followed by fragmentation (TS5) to formic acid + •OH + HCHO. There are two successive steps identified for the rearrangement/ fragmentation of the peroxyl radical CH2(OH)CH(OH)O2•. This is different from the concerted single-step process in • O2CH2CH(OH)2 previously discussed. The highest barrier for the rearrangement/fragmentation process is 23.8 kcal mol−1 relative to CH2(OH)CH(OH)O2•, which is significantly below the reactant energy (∼13 kcal mol−1). Alternatively, a lower energy pathway exists via the well-known concerted HO2• elimination mechanism (TS6).34 The barrier for the latter process is calculated to be only 10.9 kcal mol−1 relative to CH2(OH)CH(OH)O2•, or 26.4 kcal mol−1 below the initial reactants. This reaction leads to a HOCH2CHO-HO2• complex that undergoes barrierless dissociation to produce glycolaldehyde and HO2•. The free products are 22.8 kcal mol−1 below the reactants. Time dependent master equation simulations have been performed to quantify product formation in the chemically activated HOCH2C•HOH + O2 reaction, with the results reported in Figure 8. There is predicted to be no stabilization of the peroxyl radical intermediate (5) and its isomers (6 and 7), with these species rapidly decaying to negligible levels. A minor fraction of the peroxyl radical population undergoes reverse dissociation to HOCH2C•HOH + O2, with a slightly higher yield of formic acid + •OH + HCHO (about 0.2%). Therefore, the dominant reaction process is the chemically activated formation of glycolaldehyde + HO2•. Recommended Mechanism and Atmospheric Implications. Upon the basis of the calculations reported here, we are able to propose a revised model for the •OH initiated oxidation of vinyl alcohol. Important reactions in this mechanism along with suggested rate coefficients are provided in Table 1.

Figure 8. Master equation simulation for the HOCH2C•HOH + O2 reaction, using G3X-K barrier heights at 298 K and 1 atm of N2. Results are presented as yields of wells and products as a function of number of collisions.

Table 1. Proposed Mechanism for the Vinyl Alcohol + •OH + O2 Reaction, With Rate Coefficients (k) Determined from Theoretical Kinetic Simulations k (cm3 molecule−1 s−1) CH2CHOH + •OH + O2 → HC(O)OH + •OH + HCHO CH2CHOH + •OH + O2 → OCHCH2OH + HO2• CH2CHOH + •OH + O2 → •O2CH2CH(OH)2 • O2CH2CH(OH)2 + •NO + O2 → HC(O)OH + HCHO + NO2 + HO2•

3.8 × 10−11 2.4 × 10−11 5.2 × 10−12 1.0 × 10−11

The total phenomenological rate coefficient for the vinyl alcohol + •OH reaction is calculated as 6.8 × 10−11 cm3 molecule−1 s−1 at 298 K. Assuming a globally averaged •OH radical concentration of 106 molecules cm−3,35 we predict the tropospheric vinyl alcohol lifetime to be on the order of about 4 6698



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h, assuming that reaction with •OH is the only sink. This lifetime is significantly shorter than that previously reported by Archibald et al.4 Due to the lack of kinetic data available for vinyl alcohol, these authors used kinetic data for the structurally similar molecules ethene, vinyl bromide, and vinyl chloride to estimate a rate coefficient of 6.0 × 10−12 cm3 molecule−1 s−1 for the vinyl alcohol + •OH reaction, which would make the reaction nearly an order of magnitude slower than our calculations suggest. Vinyl alcohol has an electron-rich πsystem due to the electron-donating OH substituent. In contrast, vinyl chloride and vinyl bromide are electronwithdrawing due to the electronegativity of the halide substituent, and it is perhaps not surprising that the reaction of •OH with vinyl alcohol is considerably faster than that with the deactivated vinyl halides, or even the unactivated case of ethene. Note that our calculated rate coefficient is, however, in reasonable agreement with the value of 2.6 × 10−11 cm3 molecule−1 s−1 estimated by the AOPWIN program in the United States Environmental Protection Agency Estimation Program Interface (EPI) suite.36 This estimate basically equates vinyl alcohol with propene (i.e., methyl-substituted ethene). The calculated vinyl alcohol + •OH rate coefficient also coincides reasonably closely with measured values for the reaction of •OH with methyl vinyl ether (3.4 × 10−11 cm3 molecule−1 s−1) and ethyl vinyl ether (6.9 × 10−11 cm3 molecule−1 s−1),37,38 and these vinyl ether may prove to be better analogues for the enols. The short lifetime reported here for vinyl alcohol means that reaction with •OH can readily compete with formic acid catalyzed tautomerization to acetaldehyde, a process for which the tropospheric lifetime of vinyl alcohol has been estimated at around 1 h.5 Note also that it has recently been demonstrated that sulfuric acid is also an efficient catalyst of the vinyl alcohol to acetaldehyde tautomerization reaction,39 and this process may be an additional important sink of vinyl alcohol within the troposphere that needs to be incorporated into future modeling studies. Furthermore, vinyl alcohol may be removed from the troposphere through reactions with reactive species other than • OH, such as O3 and NO3•, and these processes may also need to be considered. Finally, little is known about heterogeneous processes such as the uptake of vinyl alcohol and other enols by aqueous particles, which may also warrant further study. The calculations reported here suggest that there will be three important product channels arising from the overall vinyl alcohol + •OH + O2 process. This is shown in Scheme 1, the product channels being (i) formic acid + •OH + HCHO (56%), (ii) glycolaldehyde + HO2• (36%), and (iii) the peroxyl radical •O2CH2CH(OH)2 (8%). Our findings support the previous assertion that formic acid will be the main product formed in the photochemical oxidation of vinyl alcohol, although we reveal that this will be accompanied by significant • OH radical regeneration which may have implications for tropospheric •OH concentration.40 This work shows for the first time that glycolaldehyde is expected to be a significant product in the tropospheric oxidation of vinyl alcohol. Glycolaldehyde is a commonly encountered oxygenated VOC formed as an intermediate, for instance, in the atmospheric degradation of isoprene, and its atmospheric chemistry has therefore been studied extensively. Interestingly, glycolaldehyde is a source of glyoxal, which is a contributor to secondary organic aerosol (SOA) formation.41−43

Scheme 1. Reaction Scheme of the Vinyl Alcohol + •OH + O2 Systema

a

There are three product channels identified, being (i) formic acid + OH + HCHO, (ii) glycolaldehyde + HO2•, and (iii) the peroxyl radical •O2CH2CH(OH)2. •

The final significant primary product of the vinyl alcohol + OH + O2 reaction sequence is collisional deactivation of the peroxyl radical •O2CH2CH(OH)2. Note that the yield of this peroxyl radical is much higher than that of the CH2(OH)CH(OH)O2• isomer, due to the very low barrier for HO2• radical elimination in the latter. The yield of this peroxyl radical is also sensitive to the collisional energy transfer parameters employed in our modeling, and experimental studies would be useful here to refine our predictions. Once formed, this peroxyl radical will likely react (in the polluted atmosphere) with •NO to form the alkoxyl radical •OCH2CH(OH)2. A potential energy surface for the further unimolecular reaction of this alkoxyl radical has been developed, and is provided as Figure S2 of the SI (for completeness, a similar diagram for the CH2(OH)CH(OH)O• isomer is also available as Figure S3). The lowest energy pathway available to this alkoxyl radical intermediate is for βscission into HC•(OH)2 and formaldehyde, where HC•(OH)2 should ultimately react with O2 to yield formic acid and HO2•. Accordingly, for the final products of •O2CH2CH(OH)2 decomposition we suggest HC(O)OH + HCHO + HO2• (see Table 1). Alternatively, reaction of •OCH2CH(OH)2 with O2 may be able to produce the aldehyde OCHCH(OH)2 (glyoxal hydrate) via a H atom abstraction mechanism, although no transition state could be located for this reaction using the M06-2X functional. Finally, it is of interest to extend our results from vinyl alcohol, the prototypical enol, to those formed from the tautomerization of more complex aldehydes and ketones. All of these larger enols would be predicted to react with •OH to form a hydroxyalkyl radical intermediate in an exothermic reaction. Moreover, in each case, subsequent HO2• radical elimination will be possible, which would lead to a new carbonyl compound with a higher O:C ratio than the initial enol and its tautomer. Taking glycolaldehyde as an example, the enol form HOCHCHOH will add OH to produce HOC•HCH(OH)2, which would then react with O2 to give OCHCH(OH)2. The ability for enols to undergo gas-phase processing to highly oxygenated and polar species such as this may therefore have implications for SOA formation. •

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(10) Goode, J. G.; Yokelson, R. J.; Ward, D. E.; Susott, R. A.; Babbitt, R. E.; Davies, M. A.; Hao, W. M. Measurements of excess O3, CO2, CO, CH4, C2H4, C2H2, HCN, NO, NH3, HCOOH, CH3COOH, HCHO, and CH3OH in 1997 Alaskan biomass burning plumes by airborne Fourier transform infrared spectroscopy (AFTIR). J. Geophys. Res.: Atmos. 2000, 105 (D17), 22147−22166. (11) Kawamura, K.; Ng, L. L.; Kaplan, I. R. Determination of organic acids (C1−C10) in the atmosphere, motor exhausts, and engine oils. Environ. Sci. Technol. 1985, 19 (11), 1082−1086. (12) Grosjean, D. Organic acids in southern California air: Ambient concentrations, mobile source emissions, in situ formation and removal processes. Environ. Sci. Technol. 1989, 23 (12), 1506−1514. (13) Kavouras, I. G.; Mihalopoulos, N.; Stephanou, E. G. Formation of atmospheric particles from organic acids produced by forests. Nature 1998, 395 (6703), 683−686. (14) Zhang, R.; Suh, I.; Zhao, J.; Zhang, D.; Fortner, E. C.; Tie, X.; Molina, L. T.; Molina, M. J. Atmospheric new particle formation enhanced by organic acids. Science 2004, 304 (5676), 1487−1490. (15) Blando, J. D.; Turpin, B. J. Secondary organic aerosol formation in cloud and fog droplets: A literature evaluation of plausibility. Atmos. Environ. 2000, 34 (10), 1623−1632. (16) Paulot, F.; Wunch, D.; Crounse, J.; Toon, G.; Millet, D.; DeCarlo, P.; Vigouroux, C.; Deutscher, N. M.; González Abad, G.; Notholt, J. Importance of secondary sources in the atmospheric budgets of formic and acetic acids. Atmos. Chem. Phys. 2011, 11 (5), 1989−2013. (17) Goldstein, A. H.; Galbally, I. E. Known and unexplored organic constituents in the earth’s atmosphere. Environ. Sci. Technol. 2007, 41 (5), 1514−1521. (18) Talbot, R. W.; Mosher, B. W.; Heikes, B. G.; Jacob, D. J.; Munger, J. W.; Daube, B. C.; Keene, W. C.; Maben, J. R.; Artz, R. S. Carboxylic acids in the rural continental atmosphere over the eastern United States during the Shenandoah Cloud and Photochemistry Experiment. J. Geophys. Res. 1995, 100 (D5), 9335−9343. (19) Rinsland, C. P.; Mahieu, E.; Zander, R.; Goldman, A.; Wood, S.; Chiou, L. Free tropospheric measurements of formic acid (HCOOH) from infrared ground-based solar absorption spectra: Retrieval approach, evidence for a seasonal cycle, and comparison with model calculations. J. Geophys. Res.: Atmos. 2004, 109, (D18308). doi: 10.1029/2004JD004917. (20) von Kuhlmann, R.; Lawrence, M. G.; Crutzen, P. J.; Rasch, P. J. A model for studies of tropospheric ozone and nonmethane hydrocarbons: Model evaluation of ozone-related species. J. Geophys. Res.: Atmos. 2003, 108, (D23). doi: 10.1029/2002JD003348. (21) R. B, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; , Jr., Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford CT, 2010. (22) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120 (1), 215−241. (23) da Silva, G. G3X-K theory: A composite theoretical method for thermochemical kinetics. Chem. Phys. Lett. 2013, 558 (0), 109−113.

ASSOCIATED CONTENT

S Supporting Information *

Energy diagram for the vinyl alcohol + •OH abstraction reaction. Energy diagram of the alkyl oxide + NO• reactions. Master equation simulations with M06-2X/aug-cc-pVTZ barrier heights at 298 K and 1 atm of N2. Optimized geometries, vibrational frequencies, and moments of inertia of all the stationary points and transition states. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +61 3 8344 6627; fax: +61 3 8344 4153; e-mail: [email protected]. Author Contributions

This 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 The authors are grateful to the Australian Research Council for funding through the Discovery Project (DP110103889, DP130100862) and Future Fellowship (FT130101304) schemes.

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REFERENCES

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Atmospheric Chemistry of Enols: A Theoretical Study of the Vinyl

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