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Role of the α-hydroxyethylperoxy radical in the reactions of acetaldehyde and vinyl alcohol with HO2
Chemical Physics Letters 483 (2009) 25–29
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Role of the a-hydroxyethylperoxy radical in the reactions of acetaldehyde and vinyl alcohol with HO2 Gabriel da Silva a,*, Joseph W. Bozzelli b a b
Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Department of Chemistry and Environmental Science , New Jersey Institute of Technology, Newark, NJ 07102, USA
a r t i c l e
i n f o
Article history: Received 1 September 2009 In final form 13 October 2009 Available online 24 October 2009
a b s t r a c t The title reactions have been studied theoretically. At atmospheric conditions the acetaldehyde + HO2 reaction forms the a-hydroxyethylperoxy radical with rate constant of around 1011 cm3 mol1 s1. Further reaction of a-hydroxyethylperoxy with NO occurs for NO levels above around 10 ppb. Reaction with HO2 is therefore predicted to be a major sink of aldehydes in highly polluted areas, producing carboxylic acids and ozone. The reaction of vinyl alcohol with HO2 can be rapid at combustion temperatures, forming acetaldehyde + HO2 and a-hydroxyethyl + O2 products. This reaction achieves the isomerization of vinyl alcohol to acetaldehyde at a faster rate than keto-enol tautomerization, given suitably high levels of HO2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Aldehydes and ketones are key atmospheric species, produced in the oxidation of volatile organic compounds (VOCs) emitted from both anthropogenic and biogenic sources. Aldehydes are also direct products of incomplete combustion, particularly from oxygenated fuels like ethanol. Aldehyde pollutants in urban environments promote the formation of ozone; tropospheric ozone is a health concern, causing respiratory and other problems. Partial combustion of ethanol is known to form products that include formaldehyde (HCHO) and acetaldehyde (CH3CHO). Increased use of ethanol in gasoline may lead to higher levels of tropospheric ozone in urban environments [1], although other pollutants like hydrocarbons and PAHs are expected to decrease. The acetaldehyde isomer vinyl alcohol is another species of potential atmospheric importance. Enols such as vinyl alcohol (CH2CHOH) have been proposed as a source of carboxylic acids in the atmosphere [2]. Carboxylic acids are a significant contributor to acid rain, and are the main source of free acidity in water droplets at remote sites. Atmospheric chemistry models at present fail to completely account for carboxylic acid formation, in both urban and remote environments, implying a strong unknown source [3]. Enols have recently been identified as important intermediates in the combustion of common fuels [4], including ethanol, where the dominant source is thought to be alkene + OH addition–elimination reactions. Our understanding of enol formation in a wide variety of flames, however, remains incomplete [5].
* Corresponding author. E-mail addresses: [email protected] (G. da Silva), [email protected] (J.W. Bozzelli). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.045
This study investigates the reactions of both acetaldehyde and vinyl alcohol with HO2. The HO2 radical is a prominent intermediate in combustion processes, and is typically present in the troposphere at tens to hundreds of parts per trillion by volume (ppt). Because acetaldehyde and vinyl alcohol are present in combustion and atmospheric systems, their reactions with HO2 are potentially important. The HO2 radical is known to react with both acetaldehyde and vinyl alcohol to form the a-hydroxyethylperoxy radical [6,7], where a-hydroxyalkylperoxy radicals have been proposed as aldehyde sinks in the atmosphere [8,9].
2. Computational methods This study utilizes the C2H5O3 energy surface developed by da Silva et al. [6] in their study of the a-hydroxyethyl + O2 reaction system. All stationary points were located at the B3LYP/6-31G(d) level, with enthalpies calculated using the composite G3B3 theoretical method [10]. Where available, accurate experimental or theoretical heats of formation are utilized (for O2, CH3CHO [11], CH2CHOH [12], and HO2 [13]). Enthalpies of reaction and activation in the da Silva et al. [6] study are in generally good agreement with those of Zádor et al. [7], who used QCISD(T) and MRCI theory to develop a similar C2H5O3 energy surface. Elementary rate parameters, in the high-pressure limit, are also taken from the da Silva et al. study [6], and further details on these calculations are available there. The barrierless CH3CHO + HO2 and a-hydroxyethyl + O2 association reactions (and their reverse dissociations) are treated with canonical variational transition state theory, with canonical transition state theory for the remaining reactions. Rate constants involving H atom shifts are corrected
26
G. da Silva, J.W. Bozzelli / Chemical Physics Letters 483 (2009) 25–29
for quantum mechanical tunneling. Hindered and free rotor treatments are used in describing internal rotational motions. Apparent rate constants in the chemically activated CH3CHO + HO2 and CH2CHOH + HO2 reaction mechanisms are calculated here using RRKM theory, for temperatures between 300 and 2000 K (the CH3CHO + HO2 reaction is also extrapolated down to 200 K). Timedependent master equation analysis is used to describe collisional deactivation, with an exponential-down energy transfer model and hDEdowni = 500 cm1. All remaining parameters are as described in Ref. [6]. Calculations are performed using the CHEMRATE program [14]. Calculated rate constants are fit to the three-parameter Arrhenius equation k = A0 Tn exp(Ea/RT) in order to obtain the rate parameters Ea, A0 , and n. All reported rate constants are in units of cm3 mol1 s1 or s1, with activation energies in kcal mol1 and temperatures in K.
3. Results and discussion 3.1. Reaction mechanism and energy diagram The mechanism used to model reactions on the C2H5O3 energy surface is a reduced version of that reported by da Silva et al. [6], featuring only the important processes identified in that study. The CH3CHO + HO2 association proceeds without a barrier to form the weakly bound CH3CHO HO2 pre-reactive complex, which further reacts to the a-hydroxyethylperoxy radical. Direct addition of HO2 to CH3CHO, forming a hydroperoxy-oxyl type radical, is not considered due to the relatively large barrier for association. The a-hydroxyethylperoxy radical can dissociate to a-hydroxyethyl + O2, or eliminate HO2 to produce CH2CHOH + HO2. The CH2CHOH + HO2 addition reaction is assumed to produce only the a-hydroxyethylperoxy radical, where formation of a hydroperoxy-alkyl type radical is again discounted. Further reaction of the a-hydroxyethylperoxy radical in the CH2CHOH + HO2 system is as described above for CH3CHO + HO2. An energy diagram for the two C2H5O3 reaction processes under consideration here is illustrated in Fig. 1. The acetaldehyde + HO2 reaction forms the a-hydroxyethylperoxy radical with a barrier below the energy of the entrance channel. Accordingly, the formation of this peroxy radical is expected to be rapid even at low temperatures. Because of the small barriers for dissociation and isomerization of the CH3CHO HO2 complex, collisional stabilization of OH H 3C
this species is unlikely to be of major importance. Once formed, the a-hydroxyethylperoxy radical will predominantly dissociate back to CH3CHO + HO2, where the barrier is only 13.7 kcal mol1. This barrier is small enough to make dissociation rapid at even ambient temperatures, although forward reactions of chemically activated a-hydroxyethylperoxy radical adduct may still play a role, especially at elevated temperatures. Dissociation of ahydroxyethylperoxy to CH2CHOH + HO2 requires a barrier of 30.4 kcal mol1, which is only 16.7 kcal mol1 above the entrance channel energy. Dissociation to a-hydroxyethyl + O2 is a higherenergy process (37.4 kcal mol1, or 23.7 kcal mol1 above the entrance channel), but is favored by a relatively large pre-exponential factor. In the CH2CHOH + HO2 reaction a small but appreciable activation barrier is encountered (7.0 kcal mol1), and the forward reaction rate here will increase significantly with increasing temperature. In this mechanism the a-hydroxyethylperoxy radical is formed with more than enough energy to dissociate to CH3CHO + HO2 (16.7 kcal mol1 below the entrance channel). The formation of a-hydroxyethyl + O2 requires an additional 7 kcal mol1, and this process becomes important at higher temperatures (vide infra).
3.2. Acetaldehyde + HO2 Rate constants in the chemically activated CH3CHO + HO2 reaction, at 1 atm, are plotted in Fig. 2. The dominant process is dissoadducts back to ciation of the activated ½C2 H5 O3 acetaldehyde + HO2, which occurs at around 1013 cm3 mol1 s1. However, there remain other important channels that proceed at appreciable rates. The branching ratios to these forward reaction products are plotted in Fig. 3, with fitted Arrhenius parameters at 1 atm listed in Table 1 (rate constants for the formation of quenched a-hydroxyethylperoxy were fit for temperatures below 1000 K, above which it is negligible). At temperatures below around 800 K the only major product in this reaction mechanism is the a-hydroxyethylperoxy radical (CH3CH(OO)OH). The formation of this radical is therefore expected to dominate at atmospheric conditions. At temperatures relevant to combustion (ca. 1000 K and above) formation of the a-hydroxyethyl radical + O2 is the main forward reaction pathway, with a small contribution from CH2CHOH + HO2 (up to 3% of the total forward flux). Formation of the a-hydroxyethylperoxy radical may also play a role in stratospheric chemistry, where lower temperatures (200 K) and pressures (0.001 atm) are encountered. Rate constants in the
CH
+ O2 -13.1 -20.1 H 2C
OH CH
+ HO2 O H3C
-27.1
CH
+ HO2 H
-36.8 H3C
O
O
C
O
H
-45.2
-39.1 OH H3C
CH OO -50.5
Fig. 1. Energy diagram for reactions occurring on the C2H5O3 energy surface, with 298 K enthalpies of formation. Adapted from Ref. [6].
Fig. 2. Calculated rate constants in the CH3CHO + HO2 reaction, at 1 atm.
G. da Silva, J.W. Bozzelli / Chemical Physics Letters 483 (2009) 25–29
Fig. 3. Branching ratios to new products in the CH3CHO + HO2 reaction, at 1 atm.
Table 1 Fitted rate constant parameters for important reactions in the CH3CHO + HO2 and CH2CHOH + HO2 reactions.a
CH3CHO + HO2 ? a-hydroxyethylperoxy CH3CHO + HO2 ? a-hydroxyethyl + O2 CH2CHOH + HO2 ? CH3CHO + HO2 CH2CHOH + HO2 ? a-hydroxyethyl + O2 a-Hydroxyethylperoxy ? CH3CHO + HO2
Ea
A0
n
13.87 26.15 6.81 15.44 16.02
1.22 1072 7.17 1018 1.49 105 4.11 106 8.94 1031
21.04 1.80 1.67 1.62 6.81
a k = A0 Tn exp(Ea/RT), with Ea in kcal mol1 and A0 Tn in cm3 mol1 s1 or s1. From RRKM calculations at 1 atm and 300–2000 K.
CH3CHO + HO2 mechanism as a function of pressure (0.001–1 atm) are plotted in the Supplementary material, for 200 and 300 K. The 200 K results were obtained by extrapolating the high-pressure rate constant expression for the CH3CHO + HO2 addition reaction, and by assuming that this barrierless reaction proceeds via the same transition state structure as at 300 K. At these low temperature and pressure conditions the only important reaction pathways are for formation of the collision stabilized C2H5O3 radicals ahydroxyethylperoxy and CH3CHO HO2. Rate constants for the formation of both species are greater at 200 K than at 300 K, mainly due to reduced branching of the activated adducts back to CH3CHO + HO2. At stratospheric conditions the formation of the bound acetaldehyde–HO2 radical complex becomes more important than in the troposphere, accounting for close to 40% of the total C2H5O3 products. The effect of a bound HO2 radical on aldehyde reactivity may therefore require investigation. Conventionally, the acetaldehyde + HO2 reaction is assumed to proceed via an abstraction mechanism, producing the CH3CO radical + H2O2. This process has been studied extensively, with numerous determinations of the rate constant as a function of temperature. Critical review [15] suggests the rate expression k = 3.01 1012 exp(6000/T) cm3 mol1 s1 (from 900 to 1200 K), where the activation energy is 11.9 kcal mol1. Fig. 4 shows a comparison of the recommended CH3CHO + HO2 abstraction rate constant (extrapolated over 300–2000 K), and those calculated here for the a-hydroxyethylperoxy mechanism. The formation of collision stabilized a-hydroxyethylperoxy dominates for temperatures below around 600 K, and this reaction is therefore expected to be the main atmospheric pathway for CH3CHO + HO2. At combustion temperatures the abstraction reaction is the fastest process, with the formation of a-hydroxyethyl + O2 increasing in significance with increasing temperature. At 2000 K these products account for around 6% of the total reaction rate, and this reaction may
27
Fig. 4. Calculated rate constants for the formation of CH3CH(OO)OH and CH3CHOH + O2 in the CH3CHO + HO2 reaction, compared to recommended values for CH3CO + H2O2 formation [15].
therefore be of some minor importance in combustion chemistry (we note that there is considerable uncertainty in the abstraction rate constant at these extrapolated high temperatures). The ahydroxyethyl radical produced in the CH3CHO + HO2 reaction will predominantly re-associate with an O2 molecule, returning acetaldehyde + HO2. A minor fraction of the a-hydroxyethyl + O2 reaction (5%) does proceed to CH2CHOH + HO2 at high temperatures [6,7], and the CH3CHO + HO2 ? a-hydroxyethyl + O2 ? CH2CHOH + HO2 reaction sequence may provide a small source of vinyl alcohol in acetaldehyde flames. Isomerization of the a-hydroxyethyl radical to b-hydroxyethyl, with subsequent decomposition or reaction with O2 to species like CH2CHOH, HCHO, and HCO provides another mechanism to alternate products [7]. 3.3. Vinyl alcohol + HO2 Calculated RRKM rate constants for the CH2CHOH + HO2 reaction are plotted in Fig. 5. Branching ratios to new products are shown in Fig. 6, with fitted rate constant parameters in Table 1. We see that the main channel in this mechanism is for formation of CH3CHO + HO2, as expected due to the low barrier. At high temperatures, the entropically favored reaction to a-hydroxyethyl + O2 increases in importance, providing a similar contribution to the
Fig. 5. Calculated rate constants in the CH2CHOH + HO2 reaction, at 1 atm.
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Fig. 6. Branching ratios to new products in the CH2CHOH + HO2 reaction, at 1 atm.
total reaction rate. Because of the significant energy that the activated ½C2 H5 O3 adduct is formed at, little collisional stabilization of these isomers occurs (less than 1% of the total reaction flux). At ambient temperatures the total reaction rate is slow, with rate constant of around 105 cm3 mol1 s1. For even high HO2 levels this results in a very long vinyl alcohol lifetime towards HO2, and this will be an insignificant process compared to OH-initiated oxidation (where the vinyl alcohol lifetime should be on the order of hours). The CH2CHOH + HO2 reaction may provide a mechanism for rapid isomerization of vinyl alcohol to acetaldehyde, in a process that is effectively catalyzed by HO2. The vinyl alcohol to acetaldehyde keto-enol tautomerization requires a significant barrier (ca. 56 kcal mol1) [12,16], and is estimated to proceed with rate constant k = 8.59 1011exp(28 100/T) s1 [12]. This makes the reaction slow at even moderate to high temperatures. The HO2 catalyzed process identified here proceeds with a significantly reduced barrier (9.4 kcal mol1), but requires high levels of HO2 in order to proceed at a rapid rate. The lifetime of vinyl alcohol towards keto-enol tautomerization and towards conversion to acetaldehyde by reaction with HO2 (for a range of HO2 concentrations representative of combustion and atmospheric systems, assuming an ideal gas at 1 atm) is plotted in Fig. 7. At atmospherically relevant temperatures the vinyl alcohol + HO2 reaction
Fig. 7. Lifetime for vinyl alcohol considering conversion to acetaldehyde via reaction with HO2 (for different HO2 levels) and via unimolecular keto-enol tautomerization, at 1 atm.
is faster than keto-enol tautomerization even for low HO2 concentrations, but both reactions are relatively slow. At 300 K the unimolecular keto-enol tautomerization proceeds with lifetime of around 1028 s, and is clearly of no importance. At very high HO2 concentrations the bimolecular process can be relatively fast at 300 K (45 s lifetime with 1 ppm HO2), although it is unlikely that such concentrations would be encountered at these temperatures. For HO2 concentrations between 0.001 mol fraction and 1 ppm, the conversion of vinyl alcohol to acetaldehyde via reaction with HO2 is faster than keto-enol tautomerization for up to moderate to high temperatures. With a relatively high HO2 mole fraction of 0.001 we predict that more vinyl alcohol is converted to acetaldehyde via reaction with HO2 than by unimolecular isomerization at even high temperatures (>2000 K), and the vinyl alcohol lifetime towards HO2 becomes short (ca. ls or below) at around 1100 K. This result indicates that the acetaldehyde + HO2 reaction should play a role in some combustion systems, although relatively high levels of HO2 are required. Furthermore, the vinyl alcohol + HO2 ? a-hydroxyethyl + O2 reaction, which proceeds at a similar rate to vinyl alcohol + HO2 ? acetaldehyde + HO2 at these temperatures, is also expected to contribute. Because similar HO2 concentrations to the 0.001 mol fraction case can be achieved with lower mole fractions at the higher pressures encountered in an internal combustion engine (tens of atm), this mechanism may even play a role in ignition and post-combustion chemistry. Ultimately, integration of the chemistry and rate constant expressions developed here into detailed kinetic models will be required to assay the true importance of these new reaction pathways. 3.4. a-Hydroxyethylperoxy radical chemistry Above, we demonstrated that the a-hydroxyethylperoxy radical will form in the acetaldehyde + HO2 reaction at atmospheric temperatures and pressures. Reactions of this type may therefore constitute important aldehyde sinks in the polluted (urban) and/or unpolluted (remote) troposphere. The main removal path for peroxy radicals in polluted environments is via reaction with NO, where the NO radical can abstract an O atom to form an oxyl-type radical + NO2. The conversion of NO to NO2, followed by photolysis to NO + O with the subsequent reaction O + O2 + M ? O3 + M is a major source of tropospheric ozone in urban environments. The oxyl radicals that are produced in peroxy radical + NO reactions generally decompose or isomerize. Peroxy radicals will also associate with NO, and isomerize to form alkylnitrate compounds (RONO2), which act as NOx sinks and aerosol precursors, although these reactions typically proceed at significantly reduced rates to O atom abstraction. Generally, O atom abstraction reactions from alkylperoxy radicals by NO proceed at around 5 1012 cm3 mol1 s1 at 300 K. In remote environments, NO levels rival HO2 concentrations (typically on the order of tens of ppt). Under these conditions peroxy radicals can abstract an H atom from HO2 to form a hydroperoxide + O2, with rate constants often similar to those for reaction with NO. Alkylhydroperoxides find a variety of fates in the troposphere, for example: being photolyzed to OH + oxyl radicals, reacting with OH, or contributing to secondary organic aerosol formation. Previously, da Silva et al. [6] reported that the a-hydroxyethylperoxy radical would thermally decompose to CH3CHO + HO2 with very short lifetimes at even ambient temperatures. Here, we calculate the rate constant expression k = 8.94 1031T6.82 exp(8060/ T) s1 for a-hydroxyethylperoxy radical decomposition, from RRKM simulations at 1 atm and between 300 and 800 K. In the troposphere this decomposition reaction will compete with the bimolecular NO reaction, which will form the a-hydroxyethyloxyl radical (CH3CH(O)OH) + NO2 with estimated rate constant of 5 1012 cm3 mol1 s1 (whereas the first-order rate constant for
G. da Silva, J.W. Bozzelli / Chemical Physics Letters 483 (2009) 25–29
decomposition at 300 K is 2600 s1). Competition between the two channels will depend on the concentration of NO, and this will dictate the importance of a-hydroxyethylperoxy as an acetaldehyde sink. Branching ratios for the a-hydroxyethylperoxy radical decomposing to acetaldehyde + HO2 and reacting with NO to the ahydroxyethyloxyl radical + NO2 are plotted in Fig. 8, for NO levels from 100 ppt (i.e., 0.01 ppb) up to 1 ppm (i.e. 1000 ppb). We find that the NO reaction dominates when it is at levels of around 10 ppb and above, and that it is relatively unimportant below 1 ppb. In highly polluted environments, total NOx levels (NO + NO2) can reach tens of ppb, and here the CH3CHO + HO2 reaction should be an important aldehyde sink. This reaction may assume increased importance in colder climates, as the peroxy radical decomposition reaction slows down. For example, at 280 K the ahydroxyethylperoxy radical decomposition rate constant is 596 s1, and here reaction with NO is predicted to dominate at around 3 ppb and above. For the low temperatures encountered in the tropopause, the a-hydroxyethylperoxy lifetime will become long, and reaction with NO may also be important. Furthermore, in the troposphere we may achieve an equilibrium concentration of a-hydroxyethylperoxy at low NO levels, providing a constant supply of this peroxy radical for further reaction with species like HO2. The a-hydroxyethyloxyl radical (CH3CH(O)OH) that forms in the acetaldehyde + HO2 reaction under high NO levels will decompose in the troposphere. This oxyl radical can dissociate to either formic acid + CH3 or acetic acid + H, in processes that are exothermic by 4.8 and 0.3 kcal mol1, respectively (using literature enthalpies of formation) [6,17–19]. These reactions are also likely to possess only small activation barriers, and it is probable that the a-hydroxyethyloxyl radical decomposes mainly to formic acid + CH3, with perhaps some acetic acid + H. The a-hydroxyethyloxyl radical therefore provides a new mechanism for carboxylic acid formation in the polluted troposphere. The CH3 radical formed in the decomposition of this oxyl radical will react with O2 to form CH3O2, then lose an O atom to NO, producing a further ozone-promoting NO2 radical. The CH3O radical dissociates to HCHO + H, where H will associate with O2 to give HO2 (reaction of CH3O with O2 achieves the same end). The series of reactions that we see taking place following the reaction of acetaldehyde with HO2 in polluted environments are listed below. The overall result of this mechanism is the conversion of acetaldehyde into formic acid and formaldehyde, along with the production of two molecules of ozone.
Fig. 8. Branching ratios for the a-hydroxyethylperoxy radical between decomposition to CH3CHO + HO2 and reaction with NO to a-hydroxyethyloxyl (CH3CH(O)OH) + NO2.
29
CH3CHO + HO2 + M ? CH3CH(OO)OH + M CH3CH(OO)OH + NO ? CH3CH(O)OH + NO2 CH3CH(O)OH ? CH3 + CH(@O)OH CH3 + O2 ? CH3OO CH3OO + NO ? CH3O + NO2 CH3O ? HCHO + H H + O2 + M ? HO2 + M 2NO2 + hm ? 2NO + 2O 2O + 2O2 + M ? 2O3 + M CH3CHO + 4O2 ? CH(@O)OH + HCHO + 2O3
4. Conclusions The reactions of acetaldehyde and vinyl alcohol with HO2, proceeding via the a-hydroxyethylperoxy radical intermediate, are predicted to play a role in important atmospheric and combustion systems. The acetaldehyde + HO2 reaction produces quenched ahydroxyethylperoxy radicals at low temperatures. In the troposphere, this radical reacts with NO to form the a-hydroxyethyloxyl radical at a faster rate than decomposition back to the reactants, but only in highly polluted environments ([NO] 10 ppb). The overall process produces formic acid and formaldehyde, along with three ozone molecules. At around 2000 K the acetaldehyde + HO2 reaction forms the a-hydroxyethyl radical + O2, but is predicted to be slower than abstraction to CH3CO + H2O2. The vinyl alcohol + HO2 reaction predominantly forms acetaldehyde + HO2, along with the a-hydroxyethyl radical + O2. This reaction provides a new mechanism for the isomerization of vinyl alcohol to acetaldehyde, and given suitably high levels of HO2 this process is predicted to be faster than unimolecular keto-enol tautomerization. Appendix A. Supplementary material Plot of rate constants in the CH3CHO + HO2 reaction as a function of pressure, at 200 and 300 K, and rate constants for ahydroxyethylperoxy radical formation from acetaldehyde + HO2 as a function of temperature and pressure. Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2009.10.045. References [1] M.Z. Jacobson, Environ. Sci. Technol. 41 (2007) 4150. [2] A.T. Archibald, M.R. McGillen, C.A. Taatjes, C.J. Percival, Geophys. Res. Lett. 34 (2007) L21801. [3] D. Grosjean, Environ. Sci. Technol. 23 (1989) 1506. [4] C.A. Taatjes et al., Science 308 (2005) 1887. [5] C.A. Taatjes et al., J. Phys. Chem. A 110 (2006) 3254. [6] G. da Silva, J.W. Bozzelli, L. Liang, J.T. Farrell, J. Phys. Chem. A 113 (2009) 8923. [7] J. Zádor, R.X. Fernandes, Y. Georgievskii, G. Meloni, C.A. Taatjes, J.A. Miller, Proc. Comb. Inst. 32 (2009) 271. [8] I. Hermans, T.L. Nguyen, P.A. Jacobs, J. Peeters, J. Am. Chem. Soc. 126 (2004) 9908. [9] I. Hermans, J.-F. Müller, T.L. Nguyen, P.A. Jacobs, J. Peeters, J. Phys. Chem. A 109 (2005) 4303. [10] A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghavachari, J. Chem. Phys. 110 (1999) 7650. [11] G. da Silva, J.W. Bozzelli, J. Phys. Chem. A 110 (2006) 13058. [12] G. da Silva, C.-H. Kim, J.W. Bozzelli, J. Phys. Chem. A 110 (2006) 7925. [13] B. Ruscic, R.E. Pinzon, M.L. Morton, N.K. Srinivasan, M.-C. Su, J.W. Sutherland, J.V. Michael, J. Phys. Chem. A 110 (2006) 6592. [14] V. Mokrushin, V. Bedanov, W. Tsang, M. Zachariah, V. Knyazev, ChemRate, Version 1.5.2, National Institute of Standards and Testing, Gaithersburg, MD, 2006. [15] D.L. Bauch et al., J. Phys. Chem. Ref. Data 21 (1992) 411. [16] T. Yamada, J.W. Bozzelli, T.H. Lay, J. Phys. Chem. A 103 (1999) 7646. [17] J.P. Guthrie, J. Am. Chem. Soc. 96 (1974) 3608. [18] W.V. Steele, R.D. Chirico, A.B. Cowell, S.E. Knipmeyer, A. Nguyen, J. Chem. Eng. Data 42 (1997) 1052. [19] M.W. Chase Jr., J. Phys. Chem. Ref. Data, Monograph 9 (1998) 1.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Role of the a-hydroxyethylperoxy radical in the reactions of acetaldehyde and vinyl alcohol with HO2 Gabriel da Silva a,*, Joseph W. Bozzelli b a b
Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Department of Chemistry and Environmental Science , New Jersey Institute of Technology, Newark, NJ 07102, USA
a r t i c l e
i n f o
Article history: Received 1 September 2009 In final form 13 October 2009 Available online 24 October 2009
a b s t r a c t The title reactions have been studied theoretically. At atmospheric conditions the acetaldehyde + HO2 reaction forms the a-hydroxyethylperoxy radical with rate constant of around 1011 cm3 mol1 s1. Further reaction of a-hydroxyethylperoxy with NO occurs for NO levels above around 10 ppb. Reaction with HO2 is therefore predicted to be a major sink of aldehydes in highly polluted areas, producing carboxylic acids and ozone. The reaction of vinyl alcohol with HO2 can be rapid at combustion temperatures, forming acetaldehyde + HO2 and a-hydroxyethyl + O2 products. This reaction achieves the isomerization of vinyl alcohol to acetaldehyde at a faster rate than keto-enol tautomerization, given suitably high levels of HO2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Aldehydes and ketones are key atmospheric species, produced in the oxidation of volatile organic compounds (VOCs) emitted from both anthropogenic and biogenic sources. Aldehydes are also direct products of incomplete combustion, particularly from oxygenated fuels like ethanol. Aldehyde pollutants in urban environments promote the formation of ozone; tropospheric ozone is a health concern, causing respiratory and other problems. Partial combustion of ethanol is known to form products that include formaldehyde (HCHO) and acetaldehyde (CH3CHO). Increased use of ethanol in gasoline may lead to higher levels of tropospheric ozone in urban environments [1], although other pollutants like hydrocarbons and PAHs are expected to decrease. The acetaldehyde isomer vinyl alcohol is another species of potential atmospheric importance. Enols such as vinyl alcohol (CH2CHOH) have been proposed as a source of carboxylic acids in the atmosphere [2]. Carboxylic acids are a significant contributor to acid rain, and are the main source of free acidity in water droplets at remote sites. Atmospheric chemistry models at present fail to completely account for carboxylic acid formation, in both urban and remote environments, implying a strong unknown source [3]. Enols have recently been identified as important intermediates in the combustion of common fuels [4], including ethanol, where the dominant source is thought to be alkene + OH addition–elimination reactions. Our understanding of enol formation in a wide variety of flames, however, remains incomplete [5].
* Corresponding author. E-mail addresses: [email protected] (G. da Silva), [email protected] (J.W. Bozzelli). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.045
This study investigates the reactions of both acetaldehyde and vinyl alcohol with HO2. The HO2 radical is a prominent intermediate in combustion processes, and is typically present in the troposphere at tens to hundreds of parts per trillion by volume (ppt). Because acetaldehyde and vinyl alcohol are present in combustion and atmospheric systems, their reactions with HO2 are potentially important. The HO2 radical is known to react with both acetaldehyde and vinyl alcohol to form the a-hydroxyethylperoxy radical [6,7], where a-hydroxyalkylperoxy radicals have been proposed as aldehyde sinks in the atmosphere [8,9].
2. Computational methods This study utilizes the C2H5O3 energy surface developed by da Silva et al. [6] in their study of the a-hydroxyethyl + O2 reaction system. All stationary points were located at the B3LYP/6-31G(d) level, with enthalpies calculated using the composite G3B3 theoretical method [10]. Where available, accurate experimental or theoretical heats of formation are utilized (for O2, CH3CHO [11], CH2CHOH [12], and HO2 [13]). Enthalpies of reaction and activation in the da Silva et al. [6] study are in generally good agreement with those of Zádor et al. [7], who used QCISD(T) and MRCI theory to develop a similar C2H5O3 energy surface. Elementary rate parameters, in the high-pressure limit, are also taken from the da Silva et al. study [6], and further details on these calculations are available there. The barrierless CH3CHO + HO2 and a-hydroxyethyl + O2 association reactions (and their reverse dissociations) are treated with canonical variational transition state theory, with canonical transition state theory for the remaining reactions. Rate constants involving H atom shifts are corrected
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for quantum mechanical tunneling. Hindered and free rotor treatments are used in describing internal rotational motions. Apparent rate constants in the chemically activated CH3CHO + HO2 and CH2CHOH + HO2 reaction mechanisms are calculated here using RRKM theory, for temperatures between 300 and 2000 K (the CH3CHO + HO2 reaction is also extrapolated down to 200 K). Timedependent master equation analysis is used to describe collisional deactivation, with an exponential-down energy transfer model and hDEdowni = 500 cm1. All remaining parameters are as described in Ref. [6]. Calculations are performed using the CHEMRATE program [14]. Calculated rate constants are fit to the three-parameter Arrhenius equation k = A0 Tn exp(Ea/RT) in order to obtain the rate parameters Ea, A0 , and n. All reported rate constants are in units of cm3 mol1 s1 or s1, with activation energies in kcal mol1 and temperatures in K.
3. Results and discussion 3.1. Reaction mechanism and energy diagram The mechanism used to model reactions on the C2H5O3 energy surface is a reduced version of that reported by da Silva et al. [6], featuring only the important processes identified in that study. The CH3CHO + HO2 association proceeds without a barrier to form the weakly bound CH3CHO HO2 pre-reactive complex, which further reacts to the a-hydroxyethylperoxy radical. Direct addition of HO2 to CH3CHO, forming a hydroperoxy-oxyl type radical, is not considered due to the relatively large barrier for association. The a-hydroxyethylperoxy radical can dissociate to a-hydroxyethyl + O2, or eliminate HO2 to produce CH2CHOH + HO2. The CH2CHOH + HO2 addition reaction is assumed to produce only the a-hydroxyethylperoxy radical, where formation of a hydroperoxy-alkyl type radical is again discounted. Further reaction of the a-hydroxyethylperoxy radical in the CH2CHOH + HO2 system is as described above for CH3CHO + HO2. An energy diagram for the two C2H5O3 reaction processes under consideration here is illustrated in Fig. 1. The acetaldehyde + HO2 reaction forms the a-hydroxyethylperoxy radical with a barrier below the energy of the entrance channel. Accordingly, the formation of this peroxy radical is expected to be rapid even at low temperatures. Because of the small barriers for dissociation and isomerization of the CH3CHO HO2 complex, collisional stabilization of OH H 3C
this species is unlikely to be of major importance. Once formed, the a-hydroxyethylperoxy radical will predominantly dissociate back to CH3CHO + HO2, where the barrier is only 13.7 kcal mol1. This barrier is small enough to make dissociation rapid at even ambient temperatures, although forward reactions of chemically activated a-hydroxyethylperoxy radical adduct may still play a role, especially at elevated temperatures. Dissociation of ahydroxyethylperoxy to CH2CHOH + HO2 requires a barrier of 30.4 kcal mol1, which is only 16.7 kcal mol1 above the entrance channel energy. Dissociation to a-hydroxyethyl + O2 is a higherenergy process (37.4 kcal mol1, or 23.7 kcal mol1 above the entrance channel), but is favored by a relatively large pre-exponential factor. In the CH2CHOH + HO2 reaction a small but appreciable activation barrier is encountered (7.0 kcal mol1), and the forward reaction rate here will increase significantly with increasing temperature. In this mechanism the a-hydroxyethylperoxy radical is formed with more than enough energy to dissociate to CH3CHO + HO2 (16.7 kcal mol1 below the entrance channel). The formation of a-hydroxyethyl + O2 requires an additional 7 kcal mol1, and this process becomes important at higher temperatures (vide infra).
3.2. Acetaldehyde + HO2 Rate constants in the chemically activated CH3CHO + HO2 reaction, at 1 atm, are plotted in Fig. 2. The dominant process is dissoadducts back to ciation of the activated ½C2 H5 O3 acetaldehyde + HO2, which occurs at around 1013 cm3 mol1 s1. However, there remain other important channels that proceed at appreciable rates. The branching ratios to these forward reaction products are plotted in Fig. 3, with fitted Arrhenius parameters at 1 atm listed in Table 1 (rate constants for the formation of quenched a-hydroxyethylperoxy were fit for temperatures below 1000 K, above which it is negligible). At temperatures below around 800 K the only major product in this reaction mechanism is the a-hydroxyethylperoxy radical (CH3CH(OO)OH). The formation of this radical is therefore expected to dominate at atmospheric conditions. At temperatures relevant to combustion (ca. 1000 K and above) formation of the a-hydroxyethyl radical + O2 is the main forward reaction pathway, with a small contribution from CH2CHOH + HO2 (up to 3% of the total forward flux). Formation of the a-hydroxyethylperoxy radical may also play a role in stratospheric chemistry, where lower temperatures (200 K) and pressures (0.001 atm) are encountered. Rate constants in the
CH
+ O2 -13.1 -20.1 H 2C
OH CH
+ HO2 O H3C
-27.1
CH
+ HO2 H
-36.8 H3C
O
O
C
O
H
-45.2
-39.1 OH H3C
CH OO -50.5
Fig. 1. Energy diagram for reactions occurring on the C2H5O3 energy surface, with 298 K enthalpies of formation. Adapted from Ref. [6].
Fig. 2. Calculated rate constants in the CH3CHO + HO2 reaction, at 1 atm.
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Fig. 3. Branching ratios to new products in the CH3CHO + HO2 reaction, at 1 atm.
Table 1 Fitted rate constant parameters for important reactions in the CH3CHO + HO2 and CH2CHOH + HO2 reactions.a
CH3CHO + HO2 ? a-hydroxyethylperoxy CH3CHO + HO2 ? a-hydroxyethyl + O2 CH2CHOH + HO2 ? CH3CHO + HO2 CH2CHOH + HO2 ? a-hydroxyethyl + O2 a-Hydroxyethylperoxy ? CH3CHO + HO2
Ea
A0
n
13.87 26.15 6.81 15.44 16.02
1.22 1072 7.17 1018 1.49 105 4.11 106 8.94 1031
21.04 1.80 1.67 1.62 6.81
a k = A0 Tn exp(Ea/RT), with Ea in kcal mol1 and A0 Tn in cm3 mol1 s1 or s1. From RRKM calculations at 1 atm and 300–2000 K.
CH3CHO + HO2 mechanism as a function of pressure (0.001–1 atm) are plotted in the Supplementary material, for 200 and 300 K. The 200 K results were obtained by extrapolating the high-pressure rate constant expression for the CH3CHO + HO2 addition reaction, and by assuming that this barrierless reaction proceeds via the same transition state structure as at 300 K. At these low temperature and pressure conditions the only important reaction pathways are for formation of the collision stabilized C2H5O3 radicals ahydroxyethylperoxy and CH3CHO HO2. Rate constants for the formation of both species are greater at 200 K than at 300 K, mainly due to reduced branching of the activated adducts back to CH3CHO + HO2. At stratospheric conditions the formation of the bound acetaldehyde–HO2 radical complex becomes more important than in the troposphere, accounting for close to 40% of the total C2H5O3 products. The effect of a bound HO2 radical on aldehyde reactivity may therefore require investigation. Conventionally, the acetaldehyde + HO2 reaction is assumed to proceed via an abstraction mechanism, producing the CH3CO radical + H2O2. This process has been studied extensively, with numerous determinations of the rate constant as a function of temperature. Critical review [15] suggests the rate expression k = 3.01 1012 exp(6000/T) cm3 mol1 s1 (from 900 to 1200 K), where the activation energy is 11.9 kcal mol1. Fig. 4 shows a comparison of the recommended CH3CHO + HO2 abstraction rate constant (extrapolated over 300–2000 K), and those calculated here for the a-hydroxyethylperoxy mechanism. The formation of collision stabilized a-hydroxyethylperoxy dominates for temperatures below around 600 K, and this reaction is therefore expected to be the main atmospheric pathway for CH3CHO + HO2. At combustion temperatures the abstraction reaction is the fastest process, with the formation of a-hydroxyethyl + O2 increasing in significance with increasing temperature. At 2000 K these products account for around 6% of the total reaction rate, and this reaction may
27
Fig. 4. Calculated rate constants for the formation of CH3CH(OO)OH and CH3CHOH + O2 in the CH3CHO + HO2 reaction, compared to recommended values for CH3CO + H2O2 formation [15].
therefore be of some minor importance in combustion chemistry (we note that there is considerable uncertainty in the abstraction rate constant at these extrapolated high temperatures). The ahydroxyethyl radical produced in the CH3CHO + HO2 reaction will predominantly re-associate with an O2 molecule, returning acetaldehyde + HO2. A minor fraction of the a-hydroxyethyl + O2 reaction (5%) does proceed to CH2CHOH + HO2 at high temperatures [6,7], and the CH3CHO + HO2 ? a-hydroxyethyl + O2 ? CH2CHOH + HO2 reaction sequence may provide a small source of vinyl alcohol in acetaldehyde flames. Isomerization of the a-hydroxyethyl radical to b-hydroxyethyl, with subsequent decomposition or reaction with O2 to species like CH2CHOH, HCHO, and HCO provides another mechanism to alternate products [7]. 3.3. Vinyl alcohol + HO2 Calculated RRKM rate constants for the CH2CHOH + HO2 reaction are plotted in Fig. 5. Branching ratios to new products are shown in Fig. 6, with fitted rate constant parameters in Table 1. We see that the main channel in this mechanism is for formation of CH3CHO + HO2, as expected due to the low barrier. At high temperatures, the entropically favored reaction to a-hydroxyethyl + O2 increases in importance, providing a similar contribution to the
Fig. 5. Calculated rate constants in the CH2CHOH + HO2 reaction, at 1 atm.
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Fig. 6. Branching ratios to new products in the CH2CHOH + HO2 reaction, at 1 atm.
total reaction rate. Because of the significant energy that the activated ½C2 H5 O3 adduct is formed at, little collisional stabilization of these isomers occurs (less than 1% of the total reaction flux). At ambient temperatures the total reaction rate is slow, with rate constant of around 105 cm3 mol1 s1. For even high HO2 levels this results in a very long vinyl alcohol lifetime towards HO2, and this will be an insignificant process compared to OH-initiated oxidation (where the vinyl alcohol lifetime should be on the order of hours). The CH2CHOH + HO2 reaction may provide a mechanism for rapid isomerization of vinyl alcohol to acetaldehyde, in a process that is effectively catalyzed by HO2. The vinyl alcohol to acetaldehyde keto-enol tautomerization requires a significant barrier (ca. 56 kcal mol1) [12,16], and is estimated to proceed with rate constant k = 8.59 1011exp(28 100/T) s1 [12]. This makes the reaction slow at even moderate to high temperatures. The HO2 catalyzed process identified here proceeds with a significantly reduced barrier (9.4 kcal mol1), but requires high levels of HO2 in order to proceed at a rapid rate. The lifetime of vinyl alcohol towards keto-enol tautomerization and towards conversion to acetaldehyde by reaction with HO2 (for a range of HO2 concentrations representative of combustion and atmospheric systems, assuming an ideal gas at 1 atm) is plotted in Fig. 7. At atmospherically relevant temperatures the vinyl alcohol + HO2 reaction
Fig. 7. Lifetime for vinyl alcohol considering conversion to acetaldehyde via reaction with HO2 (for different HO2 levels) and via unimolecular keto-enol tautomerization, at 1 atm.
is faster than keto-enol tautomerization even for low HO2 concentrations, but both reactions are relatively slow. At 300 K the unimolecular keto-enol tautomerization proceeds with lifetime of around 1028 s, and is clearly of no importance. At very high HO2 concentrations the bimolecular process can be relatively fast at 300 K (45 s lifetime with 1 ppm HO2), although it is unlikely that such concentrations would be encountered at these temperatures. For HO2 concentrations between 0.001 mol fraction and 1 ppm, the conversion of vinyl alcohol to acetaldehyde via reaction with HO2 is faster than keto-enol tautomerization for up to moderate to high temperatures. With a relatively high HO2 mole fraction of 0.001 we predict that more vinyl alcohol is converted to acetaldehyde via reaction with HO2 than by unimolecular isomerization at even high temperatures (>2000 K), and the vinyl alcohol lifetime towards HO2 becomes short (ca. ls or below) at around 1100 K. This result indicates that the acetaldehyde + HO2 reaction should play a role in some combustion systems, although relatively high levels of HO2 are required. Furthermore, the vinyl alcohol + HO2 ? a-hydroxyethyl + O2 reaction, which proceeds at a similar rate to vinyl alcohol + HO2 ? acetaldehyde + HO2 at these temperatures, is also expected to contribute. Because similar HO2 concentrations to the 0.001 mol fraction case can be achieved with lower mole fractions at the higher pressures encountered in an internal combustion engine (tens of atm), this mechanism may even play a role in ignition and post-combustion chemistry. Ultimately, integration of the chemistry and rate constant expressions developed here into detailed kinetic models will be required to assay the true importance of these new reaction pathways. 3.4. a-Hydroxyethylperoxy radical chemistry Above, we demonstrated that the a-hydroxyethylperoxy radical will form in the acetaldehyde + HO2 reaction at atmospheric temperatures and pressures. Reactions of this type may therefore constitute important aldehyde sinks in the polluted (urban) and/or unpolluted (remote) troposphere. The main removal path for peroxy radicals in polluted environments is via reaction with NO, where the NO radical can abstract an O atom to form an oxyl-type radical + NO2. The conversion of NO to NO2, followed by photolysis to NO + O with the subsequent reaction O + O2 + M ? O3 + M is a major source of tropospheric ozone in urban environments. The oxyl radicals that are produced in peroxy radical + NO reactions generally decompose or isomerize. Peroxy radicals will also associate with NO, and isomerize to form alkylnitrate compounds (RONO2), which act as NOx sinks and aerosol precursors, although these reactions typically proceed at significantly reduced rates to O atom abstraction. Generally, O atom abstraction reactions from alkylperoxy radicals by NO proceed at around 5 1012 cm3 mol1 s1 at 300 K. In remote environments, NO levels rival HO2 concentrations (typically on the order of tens of ppt). Under these conditions peroxy radicals can abstract an H atom from HO2 to form a hydroperoxide + O2, with rate constants often similar to those for reaction with NO. Alkylhydroperoxides find a variety of fates in the troposphere, for example: being photolyzed to OH + oxyl radicals, reacting with OH, or contributing to secondary organic aerosol formation. Previously, da Silva et al. [6] reported that the a-hydroxyethylperoxy radical would thermally decompose to CH3CHO + HO2 with very short lifetimes at even ambient temperatures. Here, we calculate the rate constant expression k = 8.94 1031T6.82 exp(8060/ T) s1 for a-hydroxyethylperoxy radical decomposition, from RRKM simulations at 1 atm and between 300 and 800 K. In the troposphere this decomposition reaction will compete with the bimolecular NO reaction, which will form the a-hydroxyethyloxyl radical (CH3CH(O)OH) + NO2 with estimated rate constant of 5 1012 cm3 mol1 s1 (whereas the first-order rate constant for
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decomposition at 300 K is 2600 s1). Competition between the two channels will depend on the concentration of NO, and this will dictate the importance of a-hydroxyethylperoxy as an acetaldehyde sink. Branching ratios for the a-hydroxyethylperoxy radical decomposing to acetaldehyde + HO2 and reacting with NO to the ahydroxyethyloxyl radical + NO2 are plotted in Fig. 8, for NO levels from 100 ppt (i.e., 0.01 ppb) up to 1 ppm (i.e. 1000 ppb). We find that the NO reaction dominates when it is at levels of around 10 ppb and above, and that it is relatively unimportant below 1 ppb. In highly polluted environments, total NOx levels (NO + NO2) can reach tens of ppb, and here the CH3CHO + HO2 reaction should be an important aldehyde sink. This reaction may assume increased importance in colder climates, as the peroxy radical decomposition reaction slows down. For example, at 280 K the ahydroxyethylperoxy radical decomposition rate constant is 596 s1, and here reaction with NO is predicted to dominate at around 3 ppb and above. For the low temperatures encountered in the tropopause, the a-hydroxyethylperoxy lifetime will become long, and reaction with NO may also be important. Furthermore, in the troposphere we may achieve an equilibrium concentration of a-hydroxyethylperoxy at low NO levels, providing a constant supply of this peroxy radical for further reaction with species like HO2. The a-hydroxyethyloxyl radical (CH3CH(O)OH) that forms in the acetaldehyde + HO2 reaction under high NO levels will decompose in the troposphere. This oxyl radical can dissociate to either formic acid + CH3 or acetic acid + H, in processes that are exothermic by 4.8 and 0.3 kcal mol1, respectively (using literature enthalpies of formation) [6,17–19]. These reactions are also likely to possess only small activation barriers, and it is probable that the a-hydroxyethyloxyl radical decomposes mainly to formic acid + CH3, with perhaps some acetic acid + H. The a-hydroxyethyloxyl radical therefore provides a new mechanism for carboxylic acid formation in the polluted troposphere. The CH3 radical formed in the decomposition of this oxyl radical will react with O2 to form CH3O2, then lose an O atom to NO, producing a further ozone-promoting NO2 radical. The CH3O radical dissociates to HCHO + H, where H will associate with O2 to give HO2 (reaction of CH3O with O2 achieves the same end). The series of reactions that we see taking place following the reaction of acetaldehyde with HO2 in polluted environments are listed below. The overall result of this mechanism is the conversion of acetaldehyde into formic acid and formaldehyde, along with the production of two molecules of ozone.
Fig. 8. Branching ratios for the a-hydroxyethylperoxy radical between decomposition to CH3CHO + HO2 and reaction with NO to a-hydroxyethyloxyl (CH3CH(O)OH) + NO2.
29
CH3CHO + HO2 + M ? CH3CH(OO)OH + M CH3CH(OO)OH + NO ? CH3CH(O)OH + NO2 CH3CH(O)OH ? CH3 + CH(@O)OH CH3 + O2 ? CH3OO CH3OO + NO ? CH3O + NO2 CH3O ? HCHO + H H + O2 + M ? HO2 + M 2NO2 + hm ? 2NO + 2O 2O + 2O2 + M ? 2O3 + M CH3CHO + 4O2 ? CH(@O)OH + HCHO + 2O3
4. Conclusions The reactions of acetaldehyde and vinyl alcohol with HO2, proceeding via the a-hydroxyethylperoxy radical intermediate, are predicted to play a role in important atmospheric and combustion systems. The acetaldehyde + HO2 reaction produces quenched ahydroxyethylperoxy radicals at low temperatures. In the troposphere, this radical reacts with NO to form the a-hydroxyethyloxyl radical at a faster rate than decomposition back to the reactants, but only in highly polluted environments ([NO] 10 ppb). The overall process produces formic acid and formaldehyde, along with three ozone molecules. At around 2000 K the acetaldehyde + HO2 reaction forms the a-hydroxyethyl radical + O2, but is predicted to be slower than abstraction to CH3CO + H2O2. The vinyl alcohol + HO2 reaction predominantly forms acetaldehyde + HO2, along with the a-hydroxyethyl radical + O2. This reaction provides a new mechanism for the isomerization of vinyl alcohol to acetaldehyde, and given suitably high levels of HO2 this process is predicted to be faster than unimolecular keto-enol tautomerization. Appendix A. Supplementary material Plot of rate constants in the CH3CHO + HO2 reaction as a function of pressure, at 200 and 300 K, and rate constants for ahydroxyethylperoxy radical formation from acetaldehyde + HO2 as a function of temperature and pressure. Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2009.10.045. References [1] M.Z. Jacobson, Environ. Sci. Technol. 41 (2007) 4150. [2] A.T. Archibald, M.R. McGillen, C.A. Taatjes, C.J. Percival, Geophys. Res. Lett. 34 (2007) L21801. [3] D. Grosjean, Environ. Sci. Technol. 23 (1989) 1506. [4] C.A. Taatjes et al., Science 308 (2005) 1887. [5] C.A. Taatjes et al., J. Phys. Chem. A 110 (2006) 3254. [6] G. da Silva, J.W. Bozzelli, L. Liang, J.T. Farrell, J. Phys. Chem. A 113 (2009) 8923. [7] J. Zádor, R.X. Fernandes, Y. Georgievskii, G. Meloni, C.A. Taatjes, J.A. Miller, Proc. Comb. Inst. 32 (2009) 271. [8] I. Hermans, T.L. Nguyen, P.A. Jacobs, J. Peeters, J. Am. Chem. Soc. 126 (2004) 9908. [9] I. Hermans, J.-F. Müller, T.L. Nguyen, P.A. Jacobs, J. Peeters, J. Phys. Chem. A 109 (2005) 4303. [10] A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghavachari, J. Chem. Phys. 110 (1999) 7650. [11] G. da Silva, J.W. Bozzelli, J. Phys. Chem. A 110 (2006) 13058. [12] G. da Silva, C.-H. Kim, J.W. Bozzelli, J. Phys. Chem. A 110 (2006) 7925. [13] B. Ruscic, R.E. Pinzon, M.L. Morton, N.K. Srinivasan, M.-C. Su, J.W. Sutherland, J.V. Michael, J. Phys. Chem. A 110 (2006) 6592. [14] V. Mokrushin, V. Bedanov, W. Tsang, M. Zachariah, V. Knyazev, ChemRate, Version 1.5.2, National Institute of Standards and Testing, Gaithersburg, MD, 2006. [15] D.L. Bauch et al., J. Phys. Chem. Ref. Data 21 (1992) 411. [16] T. Yamada, J.W. Bozzelli, T.H. Lay, J. Phys. Chem. A 103 (1999) 7646. [17] J.P. Guthrie, J. Am. Chem. Soc. 96 (1974) 3608. [18] W.V. Steele, R.D. Chirico, A.B. Cowell, S.E. Knipmeyer, A. Nguyen, J. Chem. Eng. Data 42 (1997) 1052. [19] M.W. Chase Jr., J. Phys. Chem. Ref. Data, Monograph 9 (1998) 1.