Hydrolysis of Glyoxal in Water-Restricted Environments: Formation of

May 15, 2014 - The hydrolysis of glyoxal involving one to three water molecules and also in the presence of a water molecule and formic acid has been ...
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Hydrolysis of Glyoxal in Water-Restricted Environments: Formation of Organic Aerosol Precursors through Formic Acid Catalysis Montu K. Hazra,*,† Joseph S. Francisco,*,‡ and Amitabha Sinha*,§ †

Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States § Department of Chemistry and Biochemistry, University of CaliforniaSan Diego, La Jolla, California 92093-0314, United States ‡

S Supporting Information *

ABSTRACT: The hydrolysis of glyoxal involving one to three water molecules and also in the presence of a water molecule and formic acid has been investigated. Our results show that glyoxal-diol is the major product of the hydrolysis and that formic acid, through its ability to facilitate intermolecular hydrogen atom transfer, is considerably more efficient than water as a catalyst in the hydrolysis process. Additionally, once the glyoxal-diol is formed, the barrier for further hydrolysis to form the glyoxal-tetrol is effectively reduced to zero in the presence of a single water and formic acid molecule. There are two important implications arising from these findings. First, the results suggest that under the catalytic influence of formic acid, glyoxal hydrolysis can impact the growth of atmospheric aerosols. As a result of enhanced hydrogen bonding, mediated through their polar OH functional groups, the diol and tetrol products are expected to have significantly lower vapor pressure than the parent glyoxal molecule; hence they can more readily partition into the particle phase and contribute to the growth of secondary organic aerosols. In addition, our findings provide insight into how glyoxal-diol and glyoxal-tetrol might be formed under atmospheric conditions associated with water-restricted environments and strongly suggest that the formation of these precursors for secondary organic aerosol growth is not likely restricted solely to the bulk aqueous phase as is currently assumed.

I. INTRODUCTION Glyoxal (HCO)2, is the simplest α-dicarbonyl and is generated in the atmosphere through the oxidation of precursor organic molecules originating from biogenic and anthropogenic emission.1−4 The major removal processes for glyoxal are thought to be photolysis and reaction with OH radicals.5−7 Glyoxal also makes significant contribution to the formation and growth of secondary organic aerosols (SOA), which are believed to play an important role in climate change.8,9 Modeling the growth of organic aerosols represents a major challenge in atmospheric chemistry, with current models underestimating their formation from volatile precursors.10−16 The reasons for this discrepancy are not fully understood. It could be due to an incomplete knowledge of precursor molecules or insufficient understanding of the processes that convert organic gas phase precursors into low-volatility products that contribute to SOA.16 Bulk aqueous phase aldehyde chemistry is known to be important in the formation of SOA.16−18 In contrast, hydration of carbonyls in waterrestricted environments has not yet been considered in atmospheric models because it is commonly believed that there is an insufficient number of water molecules present to make the hydrolysis energetically feasible.18 However, recent experiments by Vaida and co-workers on methylglyoxal and ketene,18,19 for example, suggest that hydration of aldehydes can occur in the gas phase under conditions corresponding to a water-restricted environment far removed from the bulk aqueous phase. Further support for the potential contribution © 2014 American Chemical Society

from gas phase chemistry also comes from computational studies demonstrating that atmospheric acids can significantly lower the barrier of certain hydrolysis reactions and thus promote these reactions with fewer water molecules.20−23 Interfaces of aerosol particles coated with organic films represent another important environment where carbonyl hydrolysis can be suppressed due to the reduced availability of water molecules and the amorphous state of the SOA;24−28 hence acid catalysis may also be an important mechanism for promoting hydrolysis reactions in this scenario. Thus, with the help of organic acids, which are present in the atmosphere at significant trace levels,29−31 hydrolysis of atmospheric carbonyl compounds can potentially occur under conditions other than just the bulk aqueous phase. Because glyoxal is known to be an important contributor to SOA formation, the present work examines the hydrolysis of glyoxal to form glyoxal-diol and glyoxal-tetrol. The hydrolysis of an isolated glyoxal molecule to form the diol and tetrol species is hindered by the presence of a substantial barrier. However, as the present study shows, we find that with the aid of a single formic acid molecule this hydrolysis barrier is significantly reduced. For the purpose of direct comparison, we have also investigated the hydrolysis of glyoxal catalyzed by one, two, and three water molecules at the same level of theory. The present findings are significant as the Received: February 28, 2014 Revised: May 15, 2014 Published: May 15, 2014 4095

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Figure 1. (A) Three possible structures of the (HCO)2···H2O complex (Structures I, II, and III) with their zero point corrected relative energies computed at the MP2=Full/6-311++G(3df,3pd) level. (B) MP2=Full/6-311++G(3df,3pd) level optimized geometries of (HCO)2···H2O···H2O, CHOCH(OH)2···H2O, and TS for the (HCO)2···H2O···H2O → CHOCH(OH)2···H2O unimolecular isomerization reaction. (C) MP2=Full/6311++G(3df,3pd) level optimized geometries of (HCO)2···H2O···H2O···H2O, CHOCH(OH)2···H2O···H2O, and TS for the (HCO)2···H2O··· H2O···H2O → CHOCH(OH)2···H2O···H2O unimolecular isomerization reaction.

level in conjunction with the 6-311++G(3df,3pd) basis set to refine our energy estimates for the various stationary points. These single point calculations have been performed on the fully optimized geometries obtained at the MP2/6-311+ +G(3df,3pd) level. The computed total electronic energies (Etotal) along with the zero-point energy (ZPE) corrected electronic energies [Etotal(ZPE)] of the monomers, complexes, and the transition states obtained at the DFT, MP2, and CCSD(T) levels are given in Supplementary Table 1 (Supporting Information). Normal-mode vibrational-frequency calculations at both the B3LYP/6-311++G(3df,3pd) and MP2/ 6-311++G(3df,3pd) levels have also been performed to estimate the respective zero point energy (ZPE) corrections for the reactants, products, and TS. Furthermore, the frequency calculations established the nature of the stationary points by confirming that the stable minima have all positive vibrational frequencies and the transition states have only one imaginary frequency (Supplementary Table 2, Supporting Information).

results provide insight into a new mechanism for glyoxal-diol and glyoxal-tetrol formation in water-restricted environments and thus suggests the potential for additional pathways for atmospheric organic aerosol growth covering a wider range of ambient conditions.

II. COMPUTATIONAL METHODS The Gaussian-03/09 suite of programs32,33 has been used to carry out all the quantum chemistry calculations presented here. The calculations have been performed using both the second-order Møller−Plesset (MP2) perturbation theory and density functional theory (DFT) in conjunction with the 6311++G(3df,3pd) basis set. Full geometry optimizations were performed using Schlegel’s method34 with tolerances of better than 0.001 Å for bond lengths and 0.01° for angles and with a self-consistent field convergence of at least 10−9 of the density matrix. The residual root-mean-square (rms) forces were CO functional group of the glyoxal to form the target diol requires proper orientation of the water molecule with respect to >CO functional group in the (HCO)2···H2O reactant complex, as this constraint is expected to minimize the barrier height. An orientation is considered favorable if it facilitates the simultaneous transfer of a hydrogen atom from the water molecule to the oxygen atom of the carbonyl group while the remaining OH moiety from the water molecule binds to the carbon atom of the carbonyl through the formation of a carbon−oxygen single bond. Thus, on the basis of the above considerations, Structure III is seen to be more effective in promoting the hydrolysis reactions as it requires the least amount of reorientation compared to the other two structures. Having decided on the structure of the (HCO)2···H2O reactant complex, we next introduce the second water molecule, again keeping in mind that only orientations that will readily promote the formation of the diol through hydrogen atom transfer will be important. The preferred geometry of the trimeric (HCO)2···H2O···H2O entrance channel complex is shown in Figure 1B. For clarity, we point out that of the two nonequivalent >CO functional groups present in Structure III of the (HCO)2···H2O complex, in the diol formation step we are considering the hydrolysis of the >CO functional group that has the shortest intermolecular distance to the oxygen atom of the water subunit (Figure 1A). The binding energies of the various species involved in the hydrolysis of glyoxal through two water molecules are given in the Table 1. A schematic diagram of the potential energy profile for this reaction, computed at the MP2/6-311++G(3df,3pd) level, is shown in Figure 2. The optimized geometries of the entrance and exit channel complexes as well as the TS are given in Figure 1B. On the basis of the energetics summarized in Figure 2, it is clear that the introduction of a second water molecule does not reduce the barrier sufficiently to allow the hydrolysis of glyoxal to occur under typical atmospheric conditions. Indeed, at the MP2/6-311++G(3df,3pd) level we

(HCO)2 + H 2O + FA ⇌ (HCO)2 ··· H 2O + FA ⇌ (HCO)2 ··· H 2O ··· FA → CHOCH(OH)2 ··· FA ⇌ CH(OH)2 CHO + FA

(5)

(HCO)2 + H 2O + FA ⇌ (HCO)2 + FA ··· H 2O ⇌ (HCO)2 ··· H 2O ··· FA → CHOCH(OH)2 ··· FA ⇌ CH(OH)2 CHO + FA

(6)

Like before, there are two main pathways for forming the trimeric (HCO)2···H2O···FA entrance channel complex (see eqs 5 and 6), which then undergoes unimolecular isomerization via an eight-membered ring cyclic TS to form the diol-formic acid complex, CHOCH2(OH)2···FA, in the exit channel. To the best of our knowledge, there have been no previous studies reported in the literature investigating these reactions. Of the two pathways represented by eqs 5 and 6, the barrier for the path associated with the (HCO)2 + FA···H2O reactants (eq 6) is expected to be 4−5 kcal/mol higher than that for the (HCO)2···H2O + FA pathway (eq 5). This is due to the stronger binding energy of the FA···H2O complex compared to that of the (HCO)2···H2O dimer. Because the barrier for the (HCO)2···H2O + FA reaction pathway is lower, we restrict our focus to only this pathway. However, we point out that due to the stronger binding of the FA···H2O dimer complex, its concentration will be higher than that of the (HCO)2···H2O dimer, and hence the (HCO)2 + FA···H2O reaction pathway can also contribute.

Figure 2. Potential energy profile for the gas phase hydrolysis of glyoxal ((CHO)2) involving two water molecules. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level of theory with ZPE correction. 4098

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Table 2. Zero Point Vibrational Energy (ZPE) Corrected Barrier Heights (kcal/mol) for the Various Unimolecular Isomerization Reactions at the B3LYP/6-311++G(3df,3pd), MP2=Full/6-311++G(3df,3pd), and CCSD(T)/ 6-311+ +G(3df,3pd) Levels of Theorya unimolecular isomerization step (HCO)2···(H2O)2 → CHOCH(OH)2···H2O [(H2O)2H2O···H2O] (HCO)2···(H2O)3 → CHOCH(OH)2···(H2O)2 [(H2O)3H2O···H2O···H2O] (HCO)2···H2O···FA → CHOCH(OH)2···FA [Channel 1] (HCO)2···H2O···FA → CHOCH(OH)2···FA [Channel 2] CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA [Channel 1A] CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA [Channel 1B]

B3LYP/6-311++G(3df,3pd) b

21.52 (+15.97) [+16.48]

c

MP2=Full/6-311++G(3df,3pd) b

22.34 (+15.10) [+14.42]

c

CCSD(T)/6-311++G(3df,3pd) 24.51 (+17.57)b [17.07]c

19.36 (+6.76)d [+13.97]e

19.20 (+4.21)d [+11.52]e

22.04 (+7.65)d [14.88]e

10.97 (+1.70)f [+ 6.58]g 12.55 (+3.52)f [+ 8.4]g 12.41 (+3.03)

11.89 (+0.46)f [+ 4.77]g 12.91 (+1.52)f [+ 5.83]g 11.02 (−0.29)

13.65 (+2.64)f [+7.04]g 14.70 (+3.57)f [+ 8.16]g 12.78 (+1.98)

9.55 (+0.57)

8.51 (−3.10)

10.54 (−0.72)

a

The values in parentheses are the energies of the transition states (TS) relative to the reactants involved in the bimolecular encounters as discusses in the text. For an example, in the case of (HCO)2···H2O···FA → CHOCH(OH)2···FA unimolecular isomerization via Channel 1, the MP2=Full/6311++G(3df,3pd) level of calculations with ZPE corrections, predict that the TS is 0.46 kcal/mol higher in energy (indicated by positive sign) relative to the (HCO)2···H2O + FA reactants. bThe relative energy of TS with respect to the total energy of the (HCO)2···H2O + H2O reactants. c The relative energy of TS with respect to the total energy of the (HCO)2 + H2O···H2O reactants. dThe relative energy of TS with respect to the total energy of the (HCO)2···H2O + (H2O)2 reactants. eThe relative energy of TS with respect to the total energy of the (HCO)2 + (H2O)3 reactants. fThe relative energy of TS with respect to the total energy of the (HCO)2···H2O + FA reactants. gThe relative energy of TS with respect to the total energy of the (HCO)2 + FA···H2O reactants.

311++G(3df,3pd) levels respectively reveal that Structure IV is lower in energy by 0.24 and 0.04 kcal/mol. The two entrance channel complexes (Structures IV and V) in turn connect to two different transition states and two different CHOCH(OH)2··· FA exit channel complexes; the optimized geometries of these TS structures are shown respectively in Figure 4B and C. Finally, the zero point energy corrected potential energy profiles for the FA catalyzed hydrolysis of glyoxal to produce the diol are shown in Figures 5 and 6. These potentials are computed at the MP2/6-311++G(3df,3pd) level and correspond to the two possible (HCO)2···H2O···FA entrance channel complexes associated with the geometries of Structures IV and V. To keep the discussion simple, we designate the diol formation path occurring through Structure IV as Channel 1 and the path associated with Structure V as Channel 2. On the basis of the computed energetics displayed in the Figures 5 and 6, it is seen that the reaction path associated with Structure IV (Channel 1) is energetically more favorable than that associated with Structure V (Channel 2). Specifically, the calculations show that the barrier for the (HCO)2···H2O···FA → CHOCH(OH)2···FA unimolecular isomerization step for Structures IV and V are respectively 11.9 and 12.9 kcal/mol (Table 2). More importantly, it is also seen that the energy difference between the (HCO)2···H2O + FA reactants and the transition states (TSs) associated with Channels 1 and 2 are respectively only 0.46 and 1.52 kcal/mol at the MP2 level. At the CCSD(T) level these same barriers for Channels 1 and 2 are computed to be respectively +2.64 and +3.57 kcal/mol (Table 2). Given the relatively low barriers, these findings suggest that a single formic acid molecule can effectively catalyze the hydrolysis of gloxal to form the glyoxal-diol under atmospheric conditions. Finally, it is worth noting that even though the concentration of the dimeric (HCO)2···H2O complexes will likely be small,40 contribution from the above mechanism is expected to be particularly significant under water-restricted environments where the nominal aqueous phase hydrolysis mechanisms are absent or hindered (see discussion below). B. Tetrol Formation Mechanism. Next we investigate the formation of the glyoxal-tetrol from subsequent hydrolysis of the diol generated via eq 5. Because the reaction path associated

Figure 3. Potential energy profile for the gas phase hydrolysis of glyoxal ((CHO)2) involving three water molecules. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level of theory with ZPE correction.

Figure 4A shows the optimized geometries of the two glyoxal−water−FA trimeric (HCO)2···H2O···FA entrance channel complexes (Structures IV and V) that are produced from the (HCO)2···H2O + FA reactants. The geometries of these complexes have been obtained by first taking the optimized geometry of the (HCO)2···H2O complex (Structure III) and then bringing FA, at its isolated optimized geometry, toward the particular >CO functional group of the (HCO)2···H2O complex that has the shorter distance between it and the water subunit. As before, the criterion used in forming these entrance channel complexes was to consider only configurations in which formic acid is able to effectively facilitate the hydrolysis reaction in terms of simultaneously promoting carbon−oxygen bond formation and transfer a hydrogen atom between the water and the >CO functional group. This is analogous to the situation already discussed above for the (HCO)2 + 2H2O reaction. Calculation at both the B3LYP and MP2 levels reveal that Structure IV is slightly more stable than Structure V. Including zero-point energy, calculations at the B3LYP/6-311++G(3df,3pd) and MP2/64099

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Figure 4. (A) Two optimized geometries (Structures IV and VI) of the (HCO)2···H2O···FA entrance channel complex for the (HCO)2···H2O + FA pathways computed at the MP2=Full/6-311++G(3df,3pd) level of theory. Their zero point corrected relative energies are given in parentheses, and these two structures have been optimized using Structure III of the (HCO)2···H2O complex shown in Figure 1A. (B) MP2=Full/6-311++G(3df,3pd) level optimized geometries of (HCO)2···H2O···FA, CHOCH(OH)2···FA, and TS for the (HCO)2···H2O···FA → CHOCH(OH)2···FA unimolecular isomerization reaction associated with Structure IV of the (HCO)2···H2O···FA preassociation complex. (C) MP2=Full/6-311++G(3df,3pd) level optimized geometries of (HCO)2···H2O···FA, CHOCH(OH)2···FA, and TS for the (HCO)2···H2O···FA → CHOCH(OH)2···FA unimolecular isomerization reaction associated with Structure V of the (HCO)2···H2O···FA preassociation complex.

Figure 5. Potential energy profile for the (HCO)2 + H2O + FA → CH(OH)2CHO + FA reaction associated with Structure IV of the (HCO)2···H2O···FA preassociation complex. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level and includes ZPE corrections. This path is designated as Channel 1.

Figure 6. Potential energy profile for the (HCO)2 + H2O + FA → CH(OH)2CHO + FA reaction associated with Structure V of the (HCO)2···H2O···FA preassociation complex. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level and includes ZPE corrections. This path is designated as Channel 2.

In this reaction the glyoxal-diol produced via Channel 1 is assumed to bind to a H2O molecule and produce the dimeric CH(OH)2CHO···H2O reactant complex. In the presence of FA this dimer complex can generate the trimeric CH(OH)2CHO··· H2O···FA preassociation complex through a bimolecular collision. Finally, this preassociation complex undergoes unimolecular isomerization, CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA, via an eight-membered cyclic TS, to form the tetrol−FA exit channel complex. To explore the potential energy diagram for the above reaction leading to

with Channel 1 is energetically more favorable than that of Channel 2, we focus only on the hydrolysis of the glyoxal-diol connected with Channel 1. With FA as a catalyst the hydrolysis of the glyoxal-diol to produce the glyoxal-tetrol can be written as follows: CH(OH)2 CHO + H 2O + FA ⇌ CH(OH)2 CHO ··· H 2O + FA ⇌ CH(OH)2 CHO ··· H 2O ··· FA → CH(OH)2 CH(OH)2 ··· FA ⇌ CH(OH)2 CH(OH)2 + FA

(7) 4100

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Figure 7. MP2=Full/6-311++G(3df,3pd) level predicted five optimized geometries of the CH(OH)2CHO···H2O entrance channel complex associated with the glyoxal-diol (CH(OH)2CHO) produced via Channel 1 as shown in Figure 4. The zero point corrected relative energies are also given.

Figure 8. (A) Two optimized geometries (Structures XI and XII) of the CH(OH)2CHO···H2O···FA entrance channel complex for the CH(OH)2CHO···H2O + FA pathways computed at the MP2=Full/6-311++G(3df,3pd) level of theory. Their zero point corrected relative energies are given in parentheses, and these two structures have been optimized respectively from Structures IX and X of the CH(OH)2CHO···H2O complex, as shown in Figure 6. (B) MP2=Full/6-311++G(3df,3pd) level optimized geometries of CH(OH)2CHO···H2O···FA, CH(OH)2CH(OH)2···FA, and TS for the CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA unimolecular isomerization reaction associated with Structure IX of the CH(OH)2CHO···H2O preassociation complex. (C) MP2=Full/6-311++G(3df,3pd) level optimized geometries of CH(OH)2CHO···H2O···FA, CH(OH)2CH(OH)2···FA, and TS for the CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA unimolecular isomerization reaction associated with Structure X of the CH(OH)2CHO···H2O preassociation complex.

glyoxal-tetrol formation, we first investigate the various possible geometries of the dimeric CH(OH)2CHO···H2O reactant complex at both the B3LYP/6-311++G(3df,3pd) and MP2/6311++G(3df,3pd) levels. In enumerating the possible geo-

metries of the CH(OH)2CHO···H2O reactant complex, a search for structures whose relative orientation of the CH(OH)2CHO and H2O moieties that can potentially assist in the hydrolysis of the >CO functional group in the 4101

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CH(OH)2CHO diol subunit was conducted. Through this procedure a total of five possible geometries for the CH(OH)2CHO···H2O complexes were generated for further consideration. The optimized structures of these five configurations (Structures VI to X) are shown in Figure 7 along with their zero point corrected relative energies computed at the MP2/6-311++G(3df,3pd) level. From Figure 7, it is seen that Structures VI and VII are not expected to contribute effectively to the formation of the tetrol due to the orientation and location of the water subunit in these two complexes. Among the three remaining structures, Structures VIII and IX have similar energies (Table 1), as they differ from each other only with respect to the orientation of the free hydrogen atom of the water subunit in their respective complexes (Figure 7). Hence, Structures VIII and IX are expected to behave similarly toward the formation of the CH(OH)2CHO···H2O···FA preassociation complex. As a result of these considerations, only Structures IX and X are of relevance. Further, including ZPE, the MP2/6-311++G(3df,3pd) level calculations show that Structure IX is 2.2 kcal/ mol more stable than Structure X. In Figure 8A, we show the optimized geometries of the two trimeric CH(OH)2CHO···H2O···FA preassociation complexes (Structures XI and XII) resulting from adding FA to respectively Structures IX and X of the CH(OH)2CHO···H2O reactant complex. In forming these trimeric CH(OH)2CHO···H2O···FA entrance channel complexes we have again restricted our search to only those structures where FA acts as a effective catalyst to promote hydrolysis consistent with the basic criterion discussed earlier for diol formation. From the results of the MP2/6-311+ +G(3df,3pd) calculations, we find that the ZPE corrected binding energy of Structure XI is 1.9 kcal/mol larger than that for Structure XII. Finally, the MP2/6-311++G(3df,3pd) level predicted potential energy diagrams for tetrol formation via the pathways associated with Structures XI and XII are given respectively in Figures 9 and 10. We designate the CH(OH)2CHO···H2O + FA reaction paths, one involving the CH(OH)2CHO···H2O reactant dimer complex corresponding to Structure IX and proceeding through entrance channel complex Structure XI as

Figure 10. Potential energy profile for tetrol formation from the CHOCH(OH)2 + H2O + FA → CH(OH)2CH(OH)2 + FA reaction associated with Structure XII of the CH(OH)2CHO···H2O···FA preassociation complex. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level with ZPE corrections. This path is designated as Channel 1B.

Channel 1A (Figure 9) and the other path involving the reactant dimer complex corresponding to Structure X and the entrance channel complex Structure XII as Channel 1B (Figure 10). The optimized geometries of the transition states and the CH(OH)2CH(OH)2···FA exit channel complexes for these two channels are respectively shown in Figure 8B,C. Inspection of the potential energy diagrams in Figures 9 and 10 suggests that the formation of the glyoxal-tetrol from the hydrolysis of the diol is effectively a barrierless process. For the potential energy diagram associated with Channel 1A, the MP2 calculations show that the energy difference between the CH(OH)2CHO··· H2O + FA reactants and the TS, associated with the CH(OH)2CHO···H2O···FA → CH(OH)2CH(OH)2···FA unimolecular isomerization, is −0.3 kcal/mol, with the TS being at lower energy (Table 2). Similarly in Channel 1B, the energy difference between the CH(OH)2CHO···H2O + FA reactants and the unimolecular isomerization TS is −3.1 kcal/mol, with the TS being at lower energy. At the CCSD(T) level these same barriers for Channels 1A and 1B are computed to be respectively +2.0 and −0.7 kcal/mol (Table 2). Thus, between these two channels for the glyoxal-tetrol formation, Channel 1B is expected to be energetically more favorable than Channel 1A. However, as lower temperature favors the formation of preassociation complexes of higher binding energies, Channel 1A may become more favorable at colder temperatures.

IV. ATMOSPHERIC IMPLICATIONS The hydrolysis of glyoxal (HCO)2 to produce glyoxal-diol is of considerable interest because of its importance as an initiation reaction for organic aerosol production in the atmosphere. Previous studies suggest that glyoxal-diols are the key precursor and their formation (eq 8a) the primary step for initiating growth of larger molecules-glyoxal oligomers in the aqueous phase.43−46

Figure 9. Potential energy profile for tetrol formation from the CHOCH(OH)2 + H2O + FA → CH(OH)2CH(OH)2 + FA reaction associated with Structure XI of the CH(OH)2CHO···H2O···FA preassociation complex. The energy profile has been calculated at the MP2=Full/6-311++G(3df,3pd) level with ZPE corrections. This path is designated as Channel 1A. 4102

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matter, and depending on their exact composition and ambient conditions, SOA particles may be in a solid, liquid, or amorphous state.24−28 Measurements by Virtanen et al.,26 for example, show that biogenic SOA particles formed during new particle formation events over boreal forests are primarily in an amorphous semisolid state, which is consistent with the observed presence of oligomers and other organic compounds having high molecular weight and low volatility. The presence of SOA in such solid and/or amorphous state has important implications for their atmospheric processing and chemical reactivity because in the solid phase the reactants are essentially confined to the SOA surface, whereas for liquid particles they are able to rapidly mix throughout the bulk.26,27 As a result, chemical reactions can be impeded in highly viscous or glassy aerosol particles, with mass transport (diffusion) of reactants within the bulk of the aerosol becoming the rate-limiting step. Consequently, chemical reactions in highly viscous and/or solid particles are typically surface-limited, with the heterogeneous reactions occurring only on the particle surfaces.26,27 The importance of interfacial chemistry has also been highlighted in laboratory studies investigating the influence of various organic species at the air−aqueous interface on physical and chemical processes occurring there compared to those taking place at a pure water−air interface and in the bulk liquid. Donaldson and co-workers, for example, have examined the effect of organic coatings on the hydrolysis of nitric acid and ammonia at the air−water interface.28 They find that monolayer and submonolayer coatings of 1-octanol suppress hydrolysis in the interfacial region relative to that at the clean air−water interface. The suppression of interfacial hydrolysis in these experiments was attributed to the reduced availability of water molecules at the interface due to the presence of the organic coating. Although the above studies do not involve glyoxal, we expect an analogous behavior for glyoxal hydrolysis. Thus, if glyoxal is adsorbed onto the interface of an organic coated aerosol, the nominally hindered hydrolysis reactions can be facilitated on these water-restricted environments by the coadsorption of formic acid through the mechanism outlined above. The subsequent formation of glyoxal-diols and triols with their numerous OH groups and increased hydrogen bonding can then contribute toward the uptake of the glyoxal onto the aerosol even when they do not involve a liquid phase or bulk aqueous environment. Thus, the present mechanism provides a pathway for aerosol growth under a wider range of ambient conditions.

Once formed, either the glyoxal-diol can undergo further hydrolysis to form the glyoxal-tetrol (eq 8b) or two of the glyoxal-diols can combine together to form a structure involving a 1,3-dioxalane ring called the glyoxal dimer (eq 9). These glyoxal dimers in turn can react further with additional gyoxal-diol molecules to produce larger glyoxal trimers.43−46 Unlike the glyoxal-diols, the glyoxal-tetrols lack the carbonyl group required for participating in condensation reactions and their fate is believed to be controlled through oxidation by OH radicals to form oxalic acid. It has generally been assumed in the literature that the hydrolysis reaction to produce the glyoxaldiol can only occur in the bulk aqueous phase due to the high barrier associated with the reaction. Recent particle chamber experiments, for example, find that glyoxal condenses via particle phase reactions only when the relative humidity levels exceeded a threshold of ∼26%.46 The presence of a humidity threshold is consistent with a mechanism where the surface water layer of the aerosol becomes saturated with glyoxal-tetrol, triggering subsequent polymerization.46 The present study, which finds that four or more water molecules are required to make the hydrolysis energetically favorable when water is the catalyst, is certainly consistent with the observation of a humidity threshold for triggering glyoxal polymerization. However, we also find that in the presence of a single formic acid (FA) molecule the barrier for the (HCO)2···H2O···FA → CHOCH(OH)2···FA unimolecular isomerization, which is the rate limiting step for glyoxal-diol formation, is substantially lowered. Thus, formic acid, which is present in the atmosphere at fairly significant levels,29−31 can act as an effective catalyst for glyoxal hydrolysis even in the presence of a single water molecule. We point out that this acid catalysis mechanism differs from that normally associated with bulk aqueous phase acid−base chemistry and already known to be important in the atmosphere,47 in that no ions are involved in the reaction. The present study finds that, for catalysis with formic acid, the reaction barrier, corresponding to the energy difference between the (HCO)2···H2O + FA reactants and the TS (Channel 1), is only about +2.6 kcal/mol. The relatively low barrier associated with the (HCO)2···H2O + FA bimolecular encounter suggests that hydrolysis of glyoxal, and hence glyoxal-diol formation, can occur under water-restricted conditions with the aid of formic acid. Further, we find that once the glyoxal-diol is formed, the subsequent hydrolysis of the glyoxal-diol to form the glyoxal-tetrol occurs through a reaction that is effectively barrierless (Channel 1B) with the aid of a formic acid molecule. Examples of water-restricted environments where the present mechanism may be applicable include low humidity gas phase reaction conditions such as that observed for the case of methyl glyoxal and ketene hydrolysis,18,19 as well as reactions occurring on certain aerosol interfaces.24−28 Aerosols often contain significant organic

V. CONCLUSION In summary, our results suggest that hydrolysis of glyoxal to form the glyoxal-diol when catalyzed by formic acid is considerably more efficient compared to that involving catalysis by an equal number of water molecules. Further, once the glyoxal-diol is formed, the barrier for subsequent hydrolysis of the diol to form the glyoxal-tetrol is effectively reduced to zero in the presence of a single water and formic acid molecule. The present findings are of atmospheric importance as the results provide insight into how glyoxal-diol and glyoxal-tetrol might be formed in the atmosphere in water-restricted environments; hence permitting organic aerosol growth under a wider range of ambient conditions. An important example of such waterrestricted environment involves interfaces of organic film coated aerosols. A reduction in the available number of water molecules can make it difficult for a glyoxal−water cluster to 4103

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reach the “critical size” needed for initiating hydrolysis and subsequent growth using just the water molecules accessible at these interfaces. The presence of a single adsorbed formic acid molecule can facilitate glyoxal hydrolysis under these waterrestricted conditions. Once formed, the enhanced hydrogen bonding associated with the polar OH groups in the glyoxaldiol and glyoxal-tetrol species are expected to result in significantly lower vapor pressure compared to the case for the parent glyoxal molecule; hence they are expected to partition more easily onto the particle phase and contribute to the growth of SOA. A more general inference from the present study is that organic acids can play an important role in the hydrolysis of various carbonyl compounds in water-restricted environments and suggest a paradigm shift from the current view that these reactions are restricted solely to the bulk aqueous phase.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometries of reactants, products, and transition states in terms of their Z-matrices, their calculated total electronic energies including zero point energy corrections, imaginary frequencies of various TS as discussed in the text at different levels of theories, and the potential energy profile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M. K. Hazra: e-mail, [email protected]. *J. S. Francisco: e-mail, [email protected]. *A. Sinha: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S. thanks the U.C. San Diego Academic Senate for support of this research. M.K.H. acknowledges the financial support received from the Biomolecular Assembly, Recognition and Dynamics (BARD) project (PIC No. 12-R&D-SIN-5.04-0103), Department of Atomic Energy (DAE), Government of India.

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REFERENCES

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