Atmospheric Chemistry of Enols: Vinyl Alcohol + OH + O2 Reaction

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry 2

Atmospheric Chemistry of Enols: Vinyl Alcohol + OH + O Reaction Revisited Xiaoyang Lei, Weina Wang, Jie Cai, Changwei Wang, Fengyi Liu, and Wenliang Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12240 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

Atmospheric Chemistry of Enols: Vinyl Alcohol + OH + O2 Reaction Revisited Xiaoyang Lei, Weina Wang, Jie Cai, Changwei Wang, Fengyi Liu, Wenliang Wang* Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, China

Abstract The OH-initiated oxidation of vinyl alcohol (VA) produced by photo-tautomerization of acetaldehyde is thought to be a source of formic acid (FA) in the atmosphere. A recent theoretical study predicted that VA + OH + O2 reaction (1) proceeds by OH addition at the α-C (66%) and β-C (33%) of VA, and FA is a main product of reaction 1. However, the metastable reactant (anti-VA, ~18% at 298 K, 1.42 kcal/mol higher than syn in energy) used in that study inspired us to reinvestigate reaction 1. By using the state-of-the-art quantum-chemical and kinetic calculations, we first found that conformer of VA has a significant influence on the rate coefficient and branching ratio of reaction 1. Deriving ~84% of reaction 1 takes place through β-C-addition channel, and ~16% of reaction 1 happens by α-C-addition channel. The calculated total initial rate coefficient at 298 K is 1.48 × 10−11 cm3•molecule−1•s−1, which is in reasonable agreement with the experimental values of similar systems (vinyl ethers + OH reactions). The predicted main products of reaction 1 are glycolaldehyde and HO2 radical, while FA is just a by-product.

1. Introduction Enols, as the less stable isomers of carbonyl compounds, have been increasingly recognized as important reactive intermediates in the atmospheric chemistry and have recently been implicated as precursors for the formation of organic acids (key species in the formation of secondary organic aerosol) in the atmosphere.1-3 A recent study4 suggested that the concentration of formic acid (FA) in Earth’s boundary layer is of several parts per billion, which is 2–3 times larger than that predicted from current atmospheric models. The mechanism responsible for the production of the missing organic acids is unknown because of the large number of volatile organic compounds

*Corresponding

authors. Tel: +86-029-81530815; E-mail: [email protected](W. L.Wang). 1

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(VOCs) in the troposphere.5 One potential source of FA is thought to be the photoisomerization of acetaldehyde (AC) to its enol form vinyl alcohol (VA). Archibald et al.6 first suggested that enols emitted directly from combustion sources including motor vehicles would oxidize to produce organic acids in the atmosphere. Andrews et al.2 subsequently stated that even larger sources of enols could arise from sunlight-driven isomerization of carbonyl compounds in the atmosphere. Clubb et al.7 provided the first experimental evidence for the AC → VA photo-tautomerization reaction. Under natural conditions, the simplest enol VA has two conformers syn-VA and anti-VA (depicted in Scheme 1), both of which possessing Cs symmetry. The syn-VA is energetically favored by 1.27 kcal/mol at the CCSD(T)/∞ level of theory or 1.39 kcal/mol at B3LYP/6-311++G(d,p) level of theory with the adiabatic rotation barrier (syn → anti) 5.03 kcal/mol.8 In syn orientation, there is a weak interaction between the hydroxyl hydrogen and the electron-rich C=C double bond, which is impossible in the anti geometry. Recently, Shaw et al.9 reported the absolute infrared absorption cross sections of propenols and assigned vibrational modes for syn- and anti-VA, the calculated ratio of syn/anti is 85:15 (at 300 K, computed at CCSD(T)//B3LYP/6-31+G(d,p) level of theory), which agrees well with their experimental results. While it should be pointed out that this estimation was based on approximating the free energy difference at 300 K by the enthalpy difference at 0 K, which is inconsistent. Reaction with hydroxyl radical (OH) is one of the key removal processes for VA in the atmosphere.10,11 To the best of our knowledge, there’s no direct experimental data associated with the reaction of VA with OH radical. In the early time, Truhlar et al.12 theoretically investigated the barrier heights (the difference between energy of the transition state and the summation of the reactants (isolated VA with OH radical)) for the various possible channels of the VA + OH reaction at MRMP2/nom-CPO+π/aug-cc-pVTZ theoretical level. They found that six barrier heights for abstracting H from a C–H bond range from 3.1 to 7.7 kcal/mol for syn- and anti-VA, two barrier heights for abstracting H from an O–H bond are both 6.0 kcal/mol for syn- and anti-VA, and two barrier heights for OH-addition to the C=C double bond are -1.8 (α-C) and -2.8 (β-C) kcal/mol just for syn-VA. Consequently, they concluded that H-abstraction dominates the reaction at high temperature, while OH-addition takes over the governing role at low temperature. 2


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Recently, da Silva et al.13 investigated the mechanism of VA + OH + O2 reaction, using quantum chemical techniques. They found that the OH-initiated atmospheric oxidation of VA primarily through OH addition to the C=C double bond of VA, and the predicted barrier heights are -3.2 (-3.3)

and

-2.6

(-2.3)

kcal/mol

for

α-C

and

β-C-addition

respectively,

at

the

M06-2X/aug-cc-pVTZ(G3X-K)//M06-2X/6-31G(2df,p) level of theory. Notably, by using metastable conformer (anti-VA), da Silva et al.13 estimated the barrier heights. By comparing results from Truhlar et al.12 and da Silva et al.13, it can be concluded that the favorable OH-addition sites for syn-VA and anti-VA are different (for syn-VA: β-C favored, for anti-VA: α-C favored). Therefore, it is necessary to revisit the reaction theoretically using both stable and metastable conformers to represent the overall VA in the VA + OH + O2 reaction because the rate coefficient and branching ratio are strongly dependent on the initial reaction channels. Importantly, theoretical model of da Silva et al.13 was adopted by Kable et al.3, and the reaction mechanism was incorporated into the atmospheric chemistry box model and the global chemical transport model (GEOS-Chem 3D) to estimate the contribution of worldwide FA production arising from photo-tautomerization of AC, which makes theoretical reinvestigation of the reaction essential. In this work, we reinvestigate the VA + OH + O2 reaction theoretically, by performing high level quantum-chemical calculations to evaluate whether it’s reasonable to use the metastable anti-VA conformer to represent the overall VA in the VA + OH + O2 reaction.

2. Computational methods All of the quantum chemical calculations were carried out using the Gaussian 09 program.14 The geometries were optimized at the M06-2X/aug-cc-pVTZ theoretical level. It has been proven that the M06-2X functional performs well in calculating the interaction energy and stability of nonbonding interaction involved in the main group thermochemistry.15-18 Harmonic frequency calculations were also performed at the same level of theory to characterize each stationary point and to obtain the zero-point energies (ZPE). The local minima has all real frequencies whereas transition state (TS) possesses only one imaginary frequency. For each TS, the intrinsic reaction coordinate (IRC) calculation was performed to verify it’s connection between reactant and product. To improve the accuracy of energy, single point calculations using CCSD(T) method19,20 along 3



with the aug-cc-pVTZ basis set were also performed. For the doublets, the unrestricted UCCSD(T) method was used, and that there is little spin contamination, as appears from the = 0.75~0.76 after annihilation in Table S5. The enthalpy of each species at 0 K was determined by adding the total electronic energy (including nuclear repulsion) to the ZPE, where the latter was calculated with a scaling factor (0.971).21 Meanwhile, the Gibbs free energies were calculated as the sum of the thermal contributions to Gibbs free energy (TCG) and the single-point CCSD(T) energies. Moreover, the T1 diagnostic values were taken into account to evaluate the possible multireference character of CCSD(T) results, which were all less than that the critical value 0.04422 for the doublet states of the species. In addition, to confirm the potential energy surface (PES) of the initial VA + OH reaction obtained at the CCSD(T)//M06-2X/aug-cc-pVTZ level, we also constructed its corresponding PES (provided in the Supporting Information) at the CCSD(T)//BH&HLYP/aug-cc-pVTZ level of theory. The BH&HLYP functional23,24 was formerly uesd by Zhou et al.25 to investigate the propenols + OH reactions, and it was also widely applied to study the reactions between alkenes and OH radical.26-29 The quantum theory of atoms in molecules (QTAIM)30 has been employed using the Multiwfn program,31 in order to provide an intuitive understanding of the breaking and forming of chemical bonds in the reaction. The rate coefficients for the initial reaction pathways incorporated in the title reaction were computed using the canonical variational transition state theory (CVT)32 with an one-dimensional asymmetric Eckart tunneling correction in the KiSThelP program33 coupled with the steady state approximation (SSA)34,35. As shown in Figure 1, each reaction takes place by an interaction between each of the two conformers (syn- and anti-) of VA and the OH radical. Accordingly, the kinetic study has been completed considering that the formation of reactant complex (RC) occurs before the TS that leads to the release of corresponding products.

k1  k2  syn  (anti )VA + OH   RC  Products k 1 Following this process, the kinetic model employed in the computation of rate coefficient of each elementary reaction follows eqn (1).

4


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k

k1 k1 k 2  k 2  Keqk 2 k -1  k 2 k -1

(1)

where, we have assumed SSA and k2 ≪ k-1, because all of the TSs lie higher than the reactants in terms of Gibbs free energy (as shown in Figure 1). Keq is the equilibrium coefficient for the formation of the RC, which was calculated according to eqn (2).

Keq  

QRC(T )  ER  ERC  exp   QR1(T )QR2(T )  RT 

(2)

where σ is the reaction symmetry number; the various Q values correspond to the partition functions obtained at the M06-2X/aug-cc-pVTZ level; R is the ideal gas constant; T is the temperature in Kelvin; ERC and ER are the energies of RC and reactants, respectively. The unimolecular rate coefficient (k2) between the RC and the reaction product was calculated using CVT with the Eckart tunneling correction. Finally, the total rate coefficients were obtained by eqn (3).

ktot (T )  Pop (syn,T )k (syn,T )  Pop (anti,T )k (anti,T )

(3)

where Pop(syn,T) and Pop(anti,T) are the populations of syn- and anti-VA at different temperatures, and k(syn,T), k(anti,T) are the corresponding rate coefficients.

3. Results and discussion 3.1 Which carbon of VA is the preferred site for OH-addition reaction? 3.1.1 The PES of VA + OH reaction. According to the Boltzmann distribution rule, 82% of syn-VA and 18% of anti-VA at 298.15 K were predicted using the realtive Gibbs free energies (0.89 kcal/mol). Therefore, the syn-VA should be treated as reactant, but for making a full consideration, the anti-VA + OH reaction was also investigated. Based on the works of Truhlar et al.12 and da Silva et al.13 for the current system, it can be concluded that OH-addition channels contribute the most to the total reaction under atmospheric condition. Therefore in this study we focus on the OH-addition reactions. The PESs for the syn-VA + OH and anti-VA + OH reactions are shown in Figure 1. All cartesian coordinates and energies of the stationary points characterized 5



on the PESs are presented in the Supporting Information. Owing to the contributions of entropy effects, the values show large differences between relative Gibbs free energy barriers (∆G≠) and its corresponding energy barriers. In this work, the ∆G≠ is used to discuss the barrier height. As shown in Figure 1, the β-C-addition channel is favored by 0.89 kcal/mol for syn-VA, while the α-C-addition channel is favored by 0.97 kcal/mol for anti-VA. As mentioned above, we have also constructed the corresponding PES of VA + OH reaction (in Figure S4) at the CCSD(T)//BH&HLYP/aug-cc-pVTZ level of theory, which confirms the preference of β-carbon in the OH-addition reaction for syn-VA, while α-carbon is the favored site for anti-VA. 3.1.2 Regioselectivity Analysis. The regioselectivity of free radical transfer and addition reactions has been found by many studies.36-41 In fact, free radical reactions are more complicated than heterolytic processes in solution. Sekušak et al.37 investigated the reactivity and regioselectivity of OH radical addition to ethene and halogenated ethenes (fluoroethene and chloroethene) theoretically, and proposed two factors for regioselectivity. The first factor is the spin density in the attacked molecules, which directs the addition toward the carbon atom with larger spin density, and the other is the strength of the formed C–O bond, which favors the addition to the carbon atom that forms the stronger C–O bond. Notably, regioselectivity in this study is more complicated because the syn- and anti-VA have the same OH substituent, but different binding sites are preferred by the two conformers. It’s a challenge to analyze the origin of regioselectivity for the current system based on the two factors proposed by Sekušak et al.37. To better understand the different regioselectivity for syn- and anti-VA, the QTAIM was applied to analyse the intermolecular interactions of RC and TS involved in the VA + OH reaction. Herein, we highlight the important role of RC, one of factors proposed by Tedder36 that determines the regioselectivity of free radical addition reactions. Because in current systems, the reactant complexes differ in nature: for RCsyn it is clearly a hydrogen-bonded complex (as shown in Figure S3), whereas for RCanti it is a so-called π-complex. In addition, the bonding site in RC is consistent with TS structure which is energetically preferred. In Table 1, both values of the CM5 charge42,43 and the NPA charge indicate that the β-C is more negative than α-C in syn- and anti-VA, due to the electron-donating effect of OH substituent. 6


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Electrostatic interaction could provide a simple understanding for the preference of OH-addition site: α-C is preferred for anti-VA due to its electron deficiency and the stronger electrostatic attraction with the OH radical, which is electron-rich; while for syn-VA, a bond critical point (BCP) was observed between the hydrogen of OH substituent and the oxygen of OH radical in RCsyn (as shown in Figure 2), suggesting a stabilizing intramolecular hydrogen bonding, which is absent in RCanti and alters the OH-addition site predicted using the electrostatic model. The bonding site preferred in TS is consistent with the site in RC, which has a shorter C–O bond length and a higher electron density at the BCP. From Table 2, it can be seen that the distance of α-C to the oxygen of OH radical (2.499, in RCanti) is shorter than that of β-C to the oxygen of OH radical (2.741, in RCsyn). Moreover, all of the bond length (C2–O7) of the hydroxy oxygen to α-C are shorter than that to β-C (C1–O7) in the TS structures of each reaction system. These indicate that α-C in fact is the more favorable OH-addition site for both syn- and anti-VA, since it is more positively charged than β-C. While due to the formation of an additional intramolecular hydrogen bonding in RCsyn, which compensates energies to the losed electrostatic interaction between α-C and the oxygen of OH radical, the β-C becomes the preferred site for syn-VA. 3.1.3 Rate coefficients and branching ratios. The calculated rate coefficients for the initial reaction pathways are listed in Table 3, and the branching ratios are shown in Figure 3. A σ value of 2 was incorporated in the calculations of Keq for both syn- and anti-VA, and the values of Keq and k2 are presented in Table S7. From Figure 3, one can see that the β-C-addition channel contributes the most to the syn-VA + OH reaction, while the α-C-addition channel is the main pathway for the anti-VA + OH reaction. In addition, Figure 3 shows that the branching ratios of anti-VA + OH reaction are quite different from that of the total VA + OH reaction. Therefore, it’s inadequate to use the metastable anti-VA conformer to represent the overall VA in the VA + OH + O2 reaction. The computed total phenomenological rate coefficient at 298 K is 1.48 × 10−11 cm3•molecule−1•s−1, which is in reasonable agreement with the previous investigations.6,44,45 Experimentally, Archibald et al.6 using kinetic data from the analogous molecules (ethene, vinyl chloride, and vinyl bromide) estimated a rate coefficient of 6.0 × 10−12 cm3•molecule−1•s−1 for the 7



VA + OH reaction. It can be seen that the rate coefficient was underestimated by them. The calculated rate coefficient by da Silva et al.13 is 6.8 × 10−11 cm3•molecule−1•s−1, which is 4~5 times higher than our theoretical result. Note that da Silva et al.13 in their theoretical work on the anti-VA + OH reaction used a much more involved master equation simulation in which they consider all three steps involved, with k(E,J) allowing for rotational effects. Our computed total rate coefficient also coincides reasonably with the experimental values for the reactions of OH radical with methyl vinyl ether (6.4 × 10−11 cm3•molecule−1•s−1)44 and ethyl vinyl ether ((6.8 ± 0.7) × 10−11 cm3•molecule−1•s−1)45. According to Table 3, the total rate coefficients show a slightly negative T-dependence, which has been reported for OH-alkene46,47 and OH-unsaturated alcohol48,49 addition reactions. For the present VA + OH system, the calculated PESs are similar to those for OH-alkene and OH-unsaturated alcohol addition reactions, therefore we conclude that the rate coefficients should also negatively depend on temperature. To provide theoretical reference for further laboratory investigations, the calculated rate coefficients were fitted by the following Arrhenius expressions in the temperature range of 200−350 K (in units of cm3•molecule-1•s-1):

ksyn(T )  8.27 1013 exp[(847.27K) / T ] kanti (T )  7.34 1013 exp[(938.85K) / T ] ktot (T )  8.73 1013 exp[(842.71K) / T ] From above discussions, it’s clear that the β-C-addition channel (~84%, at 298 K) is the dominant pathway for the total VA + OH reaction in the atmosphere, which differs from the theoretical results of da Silva et al.13 that the α-C-addition channel is the main pathway of VA + OH reaction. Therefore, we will mainly focus on the subsequent reactions of β-C-addition intermediates (IM-bsyn/anti) in the next section. 3.2 What are the main products of OH-initiated oxidation of VA in the presence of O2? 3.2.1 IM-bsyn/anti + O2 Reaction. As pointed out in the previous section, the adduct IM-bsyn 8


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(CH2OH–•CHOH) is the predominant intermediate on the doublet energy surface of VA + OH reaction. Under atmospheric condition, it is necessary to investigate the possible chemical fate of the newly formed hydroxyalkyl radicals (IM-asyn/anti and IM-bsyn/anti), which as some other C-centered radicals will react rapidly with triplet O2 molecule to form α or β-hydroxyl-substituted alkylperoxy radicals (α or β-HOQOO) via a barrierless process.50,51 The α or β-HOQOO radicals are different from the general peroxy radicals, unimolecular decomposition could be dominant process for them in the atmosphere and removal by NO or other species may be a minor proportion.52-58 Here, due to the two OH substituents in the newly formed CH2OH–C(OO•)HOH radical, which may have a distinct effect on the decomposition mechanism, we investigate the various product pathways of the unimolecular reaction of the CH2OH–C(OO•)HOH radical in the present study. Figure 4 shows that the recombination reaction of β-C-addition intermediate with triplet O2 is exothermic by 35.33 kcal/mol for IM-bsyn, and 36.73 kcal/mol for IM-banti on the doublet PES. For spin conservation, the association of IM-bsyn/anti with triplet O2 may also take place on the quartet PES. Based on the structures on the doublet PES, single-point energies on the quartet state at CCSD(T)/aug-cc-pVTZ level were computed and the species cannot be stabilized owing to their substantial high energies. Therefore, we mainly focused on the doublet PES. In addition, there are large amount of rotamers for the intermediates in the PESs of the subsequent IM-bsyn/anti + O2 system. The structures that can form hydrogen bonding interactions and possess lowest energies in all obtained conformers were treated as intermediates, because these equilibrium geometries were found to be stabilized by an intramolecular interaction between the hydroxyl hydrogen and the peroxy function. For the α- and β-HOQOO radical IMbOO, four different pathways were found and represented in Figure 4. The terminal oxygen in IMbOO can either abstract a hydroxy hydrogen from the adjacent OH group to produce glycolaldehyde (P1) and HO2 radical by passing the transition state TS-P1 or abstract an alkyl hydrogen in CH2(OH) group to yield CH(OH)=CH(OH) (P4) and HO2 radical by overcoming the transition state TS-P4. The corresponding ∆G≠ for this two steps were predicted to be 14.69 and 36.51 kcal/mol, respectively. Analysing this two direct HO2 elimination 9



channels, it can be found that the formation of P4 needs more energies than that P1. The reason may lie in that the C=O double bond has larger bond energy than that C=C double bond, and the formation process of P1 will release more energies than that P4. Furthermore, the hydroxyl H can migrate from the CH2(OH) group to the terminal O atom to form CH2(O•)CH(OOH)OH (IM1), which is associated with the ∆G≠ of 23.96 kcal/mol. Eventually, IM1 dissociates via C–C bond breaking to form HCOOH + HCHO + OH. The corresponding transition state is TS-IM1-P2, which sits 9.88 kcal/mol above IM1 in free energy. In the last pathway, IMbOO rearranges to •CH(OH)CH(OH)(OOH)

(IM2) by 1,4-H migration (the alkyl H in CH2(OH) group migrates to

the terminal O atom) via TS-IM2 with a ∆G≠ of 32.03 kcal/mol. Two possible dissociation pathways exist for IM2: direct HO−OCH(OH) bond cleavage and cyclization result in the epoxide P3 and OH radical via TS-IM2-P3 with a ∆G≠ of 6.28 kcal/mol; direct HO2−CH(OH) bond cleavage leads to the formation of HO2 + CH(OH)=CH(OH) (P4) by TS-IM2-P4 with a ∆G≠ of 18.41 kcal/mol. Overall, the theoretical results obtained for the unimolecular decomposition channels of IMbOO indicate that the most facile channel is the IMbOO→TS-P1→CH2OHCHO (P1) + HO2 pathway (red line in Figure 4). While other channels owing to the larger ∆G≠ involved are expected to be minor product channels. Therefore, the CH2OHCHO and HO2 radical are anticipated to be the major products for the IM-bsyn/anti + O2 reaction. 3.2.2 IM-asyn/anti + O2 Reaction. The α-C-addition intermediate IM-asyn/anti (•CH2–CH(OH)2) is the second intermediate on the doublet energy surface of VA + OH reaction. Figure 5 clearly shows that there are three possible pathways starting from α-HOQOO radical IMaOO formed via IM-asyn/anti + O2 reaction. The dissociation modes of IMaOO are similar with IMbOO, except that IMaOO can facilely convert to HCOOH + HCHO + OH via a concerted process. The hydroxyl H atom of CH(OH)2 group migrates to the terminal O atom and the C–C bond splits simultaneously by overcoming cyclic six-membered transition state TS-P2 with a ∆G≠ of 19.60 kcal/mol. According to Figure 5, the dissociation route (IMaOO→TS-P2→HCOOH + HCHO + OH) has the lowest ∆G≠ (blue line in Figure 5). However, the other isomerization pathways don’t contribute much to the total reaction because of the higher isomerization ∆G≠ involved. Thus, the HCOOH, 10


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HCHO and OH radical are predicted to be the predominant dissociation products of IMaOO. Summarizing the present section, it can be concluded that CH2OHCHO (P1) + HO2 are the major products of the OH-initiated oxidation of VA in the presence of O2. The main dissociation products (HCOOH + HCHO + OH) of IMaOO derived from IM-asyn/anti + O2 reaction are just by-products for the final VA + OH + O2 reaction because of the less proportion of α-C-addition intermediates IM-asyn/anti on the doublet PES of the initial VA + OH reaction. Additionally, the results obtained for the decompositions of the current two HOQOO radicals can be usefully compared with these for the analogous β-hydroxy-ethylperoxy reactions and even the ethylperoxy reactions.59,60 Notably, for ethylperoxy radical the lowest-barrier channel and experimentally dominant reaction pathway is concerted elimination of HO2 radical to form CH2=CH2.60 3.3 Atmospheric implications. According to the average OH radical concentration of 1 × 106 molecules•cm−3 in the Earth’s atmosphere,61 the atmospheric lifetime (τ) of VA can be calculated by the following equation:



1 kOH [OH]

The lifetime of VA at 298 K is estimated to be 19.4 and 16.1 h for syn-VA and anti-VA, respectively, and the average lifetime is calculated to be 18.8 h. The concentration of OH radical is relevant to season, location and time of day. Therefore, different atmospheric lifetimes can be deduced from different OH radical concentrations.62 Nevertheless, the computed lifetime in this study can serve as a reference for estimating the potential atmospheric effect of VA. In the atmosphere, another important VA sink is the tautomerization to AC that may compete with its oxidation by OH radical.10 Whereas both catalyzed and uncatalyzed reverse tautomerization of gas-phase VA back into AC is believed to be negligible. A feasible pathway is the aerosol-mediated tautomerization to AC.11 Summarizing the theoretical results of our present work in Scheme 2, it can be seen that α-C-addition channel is the main FA production pathway. 66% of the title reaction takes place through α-C-addition channel from the prediction of da Silva et al.,13 while ~16% of the reaction 11



happens by α-C-addition pathway from our results. It’s clear that the production of FA will be overestimated if one consider the metastable anti-VA conformer as the initial reactant of VA in the VA + OH + O2 reaction. At last, as can be seen in Figure 4 and 5, the OH-initiated oxidation of VA involves an O-exchange mechanism, that is, all of the oxygen of the regenerated OH and HO2 radicals are from O2 molecule. Such findings here need an experimental demonstration, e.g., using the isotopic labeling method, to probe the reaction channels in the lab. The ability for VA to regenerate OH and HO2 radicals in the OH-initiated oxidation process may have significant implications for atmospheric chemistry, because there are large amount of aldehydes and ketones under sunlight exposure may convert to their enol forms2,63 for this type of reaction in the atmosphere. In each case, the production of OH and HO2 radicals are possible, which may greatly influence their concentrations in the atmosphere.

4. Conclusions The VA + OH + O2 reaction has been reinvestigated theoretically. The calculations clearly show that β-C-addition channel (~97%) contributes the most to the syn-VA + OH reaction, while anti-VA + OH reaction mainly happens through α-C-addition pathway (~65%). Due to the abundance of syn-VA (~82%) in the total VA, we conclude that the β-C-addition channel (~84%) is the dominant pathway for the total VA + OH reaction in the atmosphere. The kinetic data indicate that both rate coefficients of the two initial reactions should exhibit a slightly negative T-dependence. At tropospheric OH radical concentration, the lifetime of VA toward reaction with OH radical is calculated to be 19.4 and 16.1 h for syn-VA and anti-VA, respectively, and the average lifetime is estimated to be 18.8 h. In the subsequent reactions of OH-addition intermediates with O2, the most likely oxidation products for VA are glycolaldehyde and HO2 radical, while FA is just a by-product, which is different from an earlier theoretical prediction as one of the main oxidation products. Our theoretical results are helpful to better understand the atmospheric fate of VA due to the OH-initiated oxidation reactions.

Supporting Information 12


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Geometrical parameters, the PES for the conversion of syn-VA with anti-VA and the BH&HLYP computed PES for the initial reactions. Cartesian coordinates, values of the T1 diagnostic and absolute energies for all of the stationary points. The Boltzmann populations of syn- and anti-VA at different temperatures.

Acknowledgment This work was supported by the National Natural Science Foundation of China (No: 21473108, 21873060, 21636006) and Fundamental Research Funds for the Central Universities (Grant No. GK201901007). References (1) Bunn, H.; Hudson, R. J.; Gentleman, A. S.; Raston, P. L. Far-Infrared Synchrotron Spectroscopy and Torsional Analysis of the Important Interstellar Molecule, Vinyl Alcohol. ACS Earth Space Chem. 2017, 1, 70–79. (2) Andrews, D. U.; Heazlewood, B. R.; Maccarone, A. T.; Conroy, T.; Payne, R. J.; Jordan, M. J. T.; Kable, S. H. Photo-Tautomerization of Acetaldehyde to Vinyl Alcohol: A Potential Route to Tropospheric Acids. Science 2012, 337, 1203–1206. (3) Shaw, M. F.; Sztáray, B.; Whalley, L. K.; Heard, D. E.; Millet, D. B.; Jordan, M. J. T.; Osborn, D. L.; Kable, S. H. Photo-tautomerization of acetaldehyde as a photochemical source of formic acid in the troposphere. Nat. Commun. 2018, 9, 2584. (4) Millet, D. B.; Baasandorj, M.; Farmer, D. K.; Thornton, J. A.; Baumann, K.; Brophy, P.; Chaliyakunnel, S.; de Gouw, J. A.; Graus, M.; Hu, L.; et al.. A large and ubiquitous source of atmospheric formic acid. Atmos. Chem. Phys. 2015, 15, 6283–6304. (5) Goldstein, A. H.; Galbally, I. E. Known and Unexplored Organic Constituents in the Earth's atmosphere. Environ. Sci. Technol. 2007, 41, 1514–1521. (6) Archibald, A. T.; McGillen, M. R.; Taatjes, C. A.; Percival, C. J.; Shallcross, D. E. Atmospheric transformation of enols: A potential secondary source of carboxylic acids in the urban troposphere. Geophys. Res. Lett. 2007, 34. L21801. (7) Clubb, A. E.; Jordan, M. J.; Kable, S. H.; Osborn, D. L. Phototautomerization of Acetaldehyde to Vinyl Alcohol: A Primary Process in UV-Irradiated Acetaldehyde from 295 to 335 nm. J. Phys. Chem. Lett. 2012, 3, 3522–3526. (8) Rao, H. B.; Zeng, X. Y.; He, H.; Li, Z. R. Theoretical investigations on removal reactions of ethenol by H atom. J. Phys. Chem. A 2011, 115, 1602–1608. (9) Shaw, M. F.; Osborn, D. L.; Jordan, M. J. T.; Kable, S. H. Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers. J. Phys. Chem. A 2017, 121, 3679–3688. (10) da Silva, G. Carboxylic acid catalyzed keto-enol tautomerizations in the gas phase. Angew. Chem. Int. Ed. 2010, 49, 7523–7525. (11) Peeters, J.; Nguyen, V. S.; Müller, J. F. Atmospheric Vinyl Alcohol to Acetaldehyde Tautomerization Revisited. J. Phys. Chem. Lett. 2015, 6, 4005–4011. (12) Tishchenko, O.; Ilieva, S.; Truhlar, D. G. Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: the importance of internal hydrogen bonding at the transition state. J. Chem. Phys. 2010, 133, 021102. (13) So, S.; Wille, U.; da Silva, G. Atmospheric Chemistry of Enols: A Theoretical Study of the Vinyl Alcohol + OH + O2 Reaction Mechanism. Environ. Sci. Technol. 2014, 48, 6694–6701. (14) 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., et al. Gaussian 09, Revision D.01, Gaussian, Inc.: Wallingford, CT, 2009. 13



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Scheme 1. Conformers of syn-VA and anti-VA.

Scheme 2. Proposed Reaction Scheme for the VA + OH + O2 Reaction at 298 K.

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Table 1. The CM5 Charge and NPA Charge (in bracket) of the Selected Atoms for the Reactants syn- and anti-VA. Species

α-C

β-C

syn-VA anti-VA

0.003 (0.176) -0.005 (0.151)

-0.241 (-0.536) -0.228 (-0.500)

Table 2. Calculated Topological Parameters at the BCPs of the Selected Bonds for the Reactant Complexes and Transition States in the syn-VA + OH and anti-VA + OH Reactions. a Species

bond

ρbcp

∇2ρbcp

bond length

H6–O7 0.017 0.071 2.147 RCsyn C1–O7 0.017 0.067 2.741 TS-asyn C2–O7 0.055 0.144 2.128 TS-bsyn C1–O7 0.043 0.120 2.248 RCanti C2–O7 0.023 0.082 2.499 TS-aanti C2–O7 0.044 0.125 2.210 TS-banti C1–O7 0.043 0.121 2.242 aAll the results are obtained at the M06-2X/aug-cc-pVTZ level of theory. The ρ 2 bcp and ∇ ρbcp are electron density and the Laplacian of the electron density (in a.u.) at the BCP, respectively. Bond lengths are in angstroms.

Table 3. Individual and Total Rate Coefficients (in cm3•molecule-1•s-1) for the VA + OH Reaction over the Temperature Range 200-350 K. T/K

k(TS-asyn)

k(TS-bsyn)

k(tot-syn)

k(TS-aanti)

k(TS-banti)

k(tot-anti)

k(tot-α-C)

k(tot-β-C)

k(tot)

200 210 220 230 240 250 260 270 280 290 298 310 320 330 340 350

3.98 × 10-13 3.93 × 10-13 3.89 × 10-13 3.86 × 10-13 3.85 × 10-13 3.85 × 10-13 3.88 × 10-13 3.89 × 10-13 3.91 × 10-13 3.95 × 10-13 3.97 × 10-13 4.02 × 10-13 4.07 × 10-13 4.11 × 10-13 4.17 × 10-13 4.23 × 10-13

5.74 × 10-11 4.63 × 10-11 3.82 × 10-11 3.22 × 10-11 2.74 × 10-11 2.37 × 10-11 2.08 × 10-11 1.84 × 10-11 1.65 × 10-11 1.50 × 10-11 1.39 × 10-11 1.25 × 10-11 1.16 × 10-11 1.08 × 10-11 1.01 × 10-11 9.56 × 10-12

5.78 × 10-11 4.67 × 10-11 3.86 × 10-11 3.25 × 10-11 2.78 × 10-11 2.41 × 10-11 2.12 × 10-11 1.88 × 10-11 1.69 × 10-11 1.54 × 10-11 1.43 × 10-11 1.29 × 10-11 1.20 × 10-11 1.12 × 10-11 1.06 × 10-11 9.98 × 10-12

6.12 × 10-11 4.73 × 10-11 3.74 × 10-11 3.05 × 10-11 2.52 × 10-11 2.12 × 10-11 1.81 × 10-11 1.57 × 10-11 1.38 × 10-11 1.23 × 10-11 1.12 × 10-11 9.95 × 10-12 9.08 × 10-12 8.34 × 10-12 7.70 × 10-12 7.17 × 10-12

2.20 × 10-11 1.67 × 10-11 1.43 × 10-11 1.24 × 10-11 1.08 × 10-11 9.57 × 10-12 8.57 × 10-12 7.77 × 10-12 7.10 × 10-12 6.53 × 10-12 6.14 × 10-12 5.66 × 10-12 5.32 × 10-12 5.03 × 10-12 4.73 × 10-12 4.48 × 10-12

8.12 × 10-11 6.40 × 10-11 5.17 × 10-11 4.29 × 10-11 3.60 × 10-11 3.08 × 10-11 2.67 × 10-11 2.35 × 10-11 2.09 × 10-11 1.88 × 10-11 1.73 × 10-11 1.56 × 10-11 1.44 × 10-11 1.34 × 10-11 1.24 × 10-11 1.17 × 10-11

5.14 × 10-12 4.52 × 10-12 4.05 × 10-12 3.70 × 10-12 3.39 × 10-12 3.13 × 10-12 2.90 × 10-12 2.73 × 10-12 2.58 × 10-12 2.47 × 10-12 2.38 × 10-12 2.25 × 10-12 2.18 × 10-12 2.10 × 10-12 2.04 × 10-12 1.99 × 10-12

5.45 × 10-11 4.37 × 10-11 3.58 × 10-11 3.00 × 10-11 2.54 × 10-11 2.18 × 10-11 1.91 × 10-11 1.68 × 10-11 1.50 × 10-11 1.35 × 10-11 1.25 × 10-11 1.12 × 10-11 1.03 × 10-11 9.57 × 10-12 8.90 × 10-12 8.38 × 10-12

5.96 × 10-11 4.83 × 10-11 3.99 × 10-11 3.37 × 10-11 2.88 × 10-11 2.50 × 10-11 2.20 × 10-11 1.95 × 10-11 1.76 × 10-11 1.60 × 10-11 1.48 × 10-11 1.35 × 10-11 1.25 × 10-11 1.17 × 10-11 1.10 × 10-11 1.04 × 10-11

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Figure Captions: Figure 1. PESs for the anti-VA + OH (in the left) and syn-VA + OH (in the right) reactions, the ∆G obtained at the CCSD(T)//M06-2X/aug-cc-pVTZ + TCG level of theory, the ∆E were calculated at the CCSD(T)//M06-2X/aug-cc-pVTZ + ZPE level. Figure 2. Selected geometrical structures with BCPs for the partial stationary points in the syn-VA + OH and anti-VA + OH reactions. Figure 3. The branching ratios for the VA + OH reaction as a function of temperetures. Figure 4. PESs (∆G and ∆E (in parenthese)) for the IM-bsyn/anti + O2 reactions, the ∆G obtained at the CCSD(T)//M06-2X/aug-cc-pVTZ + TCG level of theory, the ∆E were calculated at the CCSD(T)//M06-2X/aug-cc-pVTZ + ZPE level of theory. Figure 5. PESs (∆G and ∆E (in parenthese)) for the IM-asyn/anti + O2 reactions, the ∆G obtained at the CCSD(T)//M06-2X/aug-cc-pVTZ + TCG level of theory, the ∆E were calculated at the CCSD(T)//M06-2X/aug-cc-pVTZ + ZPE level of theory.

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ChemDraw Professional 15.0

Figure 1.

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Multiwfn 3.6

Figure 2.

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Originpro 8.5

Figure 3.

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Figure 4.

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Figure 5.

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TOC Graphic:

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Atmospheric Chemistry of Enols: Vinyl Alcohol + OH + O2 Reaction

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