Organic Acid Formation from the Atmospheric Oxidation of Gem Diols

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

Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates Parandaman Arathala, Manoj Kumar, Joseph S. Francisco, and Amitabha Sinha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01773 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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The Journal of Physical Chemistry

Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates Arathala Parandaman,1 Manoj Kumar,2 Joseph S. Francisco,2 and Amitabha Sinha*1 1

Department of Chemistry and Biochemistry, University of California─San Diego, La Jolla, California 92093 2 Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588

Abstract Computational chemistry is used to investigate the gas phase reaction of several gem diols in the presence of OH radicals and molecular oxygen (3O2) as would occur in the earth’s troposphere. Four gem diols, represented generically as R-HC(OH)2, with R being either -H, -CH3, -HC(O), and CH3C(O) are investigated. We find that after the abstraction of the hydrogen atom from the C-H moiety of the diol by atmospheric OH, molecular oxygen quickly adds onto the resulting radicals leading to the formation of a geminal diol peroxy adduct (R-C(OO)(OH)2), which is the key intermediate in the oxidation process. Unimolecular reaction of this R-C(OO)(OH)2 radical adduct, occurs via a proton coupled electron transfer (PCET) mechanism and leads to the formation of an organic acid and a HO2 radical. Further, the barrier for the unimolecular reaction step decreases along the R substitution series: -H, -CH3, -HC(O), -CH3C(O); this trend most likely arises from increased internal hydrogen bonding along the series. The reaction where the R group is CH3C(O), associated with methylglyoxal diol, has the lowest barrier with its transition state being ~4.3 kcal/mol above the potential energy well of the corresponding CH3C(O)-C(OO)(OH)2 peroxy adduct. The rate constants for the four diol oxidation reactions were investigated using the MESMER master equation solver kinetics code over the temperature range between 200-300 K. The calculations suggest that once formed, gem diol radicals react rapidly with O2 in the atmosphere to produce organic acids and HO2 with an effective gas phase bimolecular rate constant of ~1x10-11 cm3/molecule s at 300 K.

* Address correspondence to: [email protected]

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1. Introduction: Organic acids are important trace constituents of the atmosphere which contribute to the acidity of cloud water,1,2 catalyze atmospheric reactions,3-9 and promote the formation of aerosols.9-11 Formic acid and acetic acid, for example, are ubiquitous and have been detected in polluted urban atmospheres in ppbv (parts per billion by volume) level concentrations.12 The main source of atmospheric organic acids derives from the oxidation of volatile organic compounds arising from vehicle emissions and biomass burning.13-19 The reaction of OH radicals with hydrated formaldehyde, present in clouds in the form of methane diol, is a major source of tropospheric formic acid.20 Several laboratory studies have also observed organic acid formation from the oxidation of glyoxal and methylglyoxal.21-24 These diverse studies suggest that carbonyl compounds are an important precursor for atmospheric organic acids potentially through their ability to form gem diols which are subsequently oxidized to form the organic acid. It is well known that the reaction of carbonyl compounds (aldehydes and to a lesser extent ketones) with liquid water produces gem diols (R-CH(OH)2).25-30 Interestingly, both theoretical31,32 and recent experimental33-35 studies suggest that these diol forming hydration reactions may also occur in the gas phase. Once formed, the gem diols can undergo oxidation in the presence of atmospheric oxidants such as OH and O2 to form organic acids. The first step in these acid formation reactions is believed to involve the abstraction of the hydrogen atom from the C-H moiety (the α-hydrogen) of the gem diol (R-CH(OH)2 by atmospheric OH to form the corresponding geminal diol radical (R-C(OH)2) as shown in Eq.1.25 R-CH(OH)2 + OH → R-C(OH)2 + H2O

(1)

We note that although the OH radical in Eq. 1 can also abstract hydrogen atoms from other sites of the diol, such as from the R group or the OH group, here we restrict our study to the chemistry 2 ACS Paragon Plus Environment

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occurring at the α-carbon site as abstraction of this hydrogen provides the most direct route to organic acid formation and, as we show below, typically corresponds to the lowest energy hydrogen abstraction pathway from these diols. The reaction step involving the abstraction of the diol C-H hydrogen (Eq.1) is expected to be fast and under atmospheric conditions is expected to be followed by the addition of a ground state oxygen (3O2) molecule onto the resulting R-C(OH)2 radical to form the corresponding R-C(OO)(OH)2 peroxy radical adduct (Eq. 2). Once formed, this radical adduct can undergo unimolecular reaction to form the organic acid RC(O)OH and HO2 radical as shown in Eqs. 3.25 R-C(OH)2 + 3O2 → (R-C(OO)(OH)2)

(2)

R-C(OO)(OH)2 → R-C(O)OH + HO2

(3)

Although earlier experiments have reported the formation of organic acids from the oxidation of gem diols,21-25 to the best of our knowledge there have been no prior studies investigating the detailed mechanism, energetics, or kinetics associated with these reactions. In the present work, we examine the details of these diol oxidation reactions using high level ab initio electronic structure methods for four different gem diols involving methane diol (CH2(OH)2), ethane diol (CH3CH(OH)2), glyoxal diol (HCOCH(OH)2) and methylglyoxal diol (CH3COCH(OH)2). These four gem-diols are generically represented as R-CH(OH)2 with the R group being either -H, CH3, -HC(O), or -CH3C(O). We show that the concerted elimination of the organic acid and HO2 radicals from the unimolecular reaction of the geminal diol peroxy adduct (R-C(OO)(OH)2), Eq.3, occurs via a proton-coupled electron-transfer (PCET) mechanism.36-43 In addition, we also examine the kinetics of these acid forming reactions using the ab-initio determined potential energy surface in conjunction with the MESMER master equation solver kinetics code over the



temperature range between 200-300 K and 100-7600 Torr pressure. Our calculations not only provide insight into the mechanism and energetics of these diol oxidation reactions, but also confirm that under atmospheric conditions, gem diols can rapidly react in the gas phase to generate organic acids. We note that present atmospheric models significantly under predict the concentrations of tropospheric organic acids,6,14,15 thus implying that our current understanding of reactions that generate organic acids is incomplete. The present study partially addresses this issue by providing new insight into one important organic acid formation pathway-that involving gem diols.

2. Computational Methods: The electronic structure calculations were performed using the Gaussian 09 program.44 The geometries of all relevant stationary points on the potential energy surface (PES) for the RCH(OH)2 oxidation reaction (where R is either -H, -CH3, -HC(O), -CH3C(O)) were optimized using density functional theory (DFT) as well as second order Møller-Plesset perturbation theory (MP2).45 The M06-2X hybrid generalized gradient approximation (GGA) functional was used to carry out the DFT calculations as this functional has been shown to be reliable for computing energies, barrier heights, and handling noncovalent interactions.46,47 We used the Pople 6311++G(3df, 3pd) basis set for both the M06-2X and MP2 calculations. The keyword OPT=TS was used to locate the transition states (TSs) and the nature of the various stationary points were confirmed by determining whether they had one or no imaginary vibrational frequencies. Intrinsic reaction coordinate (IRC) calculations were carried out to verify that the transition state correlated with the correct entrance and product channel complexes. The energies of the fully optimized M06-2X and MP2 geometries were further refined by calculating their single point energies at the CCSD(T) (coupled-cluster single double including perturbative triple excitation) 4 ACS Paragon Plus Environment

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level using the 6-311++G(3df,3pd) basis set.48 The values of the calculated total electronic energies (Etotal) and zero-point energy (ZPE) corrected electronic energies [Etotal(ZPE)] for all the stationary points obtained at the various levels of theory are given in Tables S1 and S2 of the Supporting Information; Table S3 summarizes the barrier heights for the unimolecular reaction of the R-C(OO)(OH)2 radical adducts to form their respective organic acids. The optimized geometries in terms of the Z-matrices, vibrational frequencies, and rotational constants for the various species are provided in Tables S4-S8 of the Supporting Information.

3. Theoretical Kinetic Analysis: To explore the atmospheric fate of the gem diols, we have carried out a kinetics analysis of the RCH(OH)2 + OH and RC(OH)2 + 3O2 reactions using the MESMER 5.0 code (MESMER: master equation solver for multi-potential energy well reactions).49 Below we outline the use of the code using the RC(OH)2 + 3O2 reaction as a reference; the procedure used for the RCH(OH)2 + OH reactions are similar. The input parameters required for the MESMER calculation, such as the energies, vibrational frequencies, and rotational constants of the various chemical species, were obtained from the electronic structure calculations presented in the previous section. Several other additional input parameters that were required for the rate calculations are described below along with a brief description of MESMER. We note that several studies have successfully used this kinetics software to investigate the rates of O2 association reactions with various other compounds.50-52 In the MESMER program a master equation is constructed to describe the time dependent ro-vibrational population density pm(E) for each isomeric chemical species m having an energy E on the PES. The total ro-vibrational energy (E) is an independent variable and is included because a specie’s energy can change due to collisions with the bath gas as well as due to 5 ACS Paragon Plus Environment


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chemical reactions that absorb or release energy. The expression for the master equation with the energy treated as a continuous variable is given by the following expression.49,50 ∞ 𝑑𝑑𝑑𝑑𝑚𝑚 (𝐸𝐸) = 𝜔𝜔 � 𝑃𝑃(𝐸𝐸|𝐸𝐸 ′ )𝑝𝑝𝑚𝑚 (𝐸𝐸 ′ )𝑑𝑑𝐸𝐸 ′ − 𝜔𝜔𝑝𝑝𝑚𝑚 (𝐸𝐸) 𝑑𝑑𝑑𝑑 𝐸𝐸0𝑚𝑚 𝑀𝑀

+ � 𝑘𝑘𝑚𝑚𝑚𝑚 (𝐸𝐸)𝑝𝑝𝑛𝑛 (𝐸𝐸) 𝑛𝑛≠𝑚𝑚 𝑀𝑀

𝑒𝑒𝑒𝑒

− � 𝑘𝑘𝑛𝑛𝑛𝑛 (𝐸𝐸)𝑝𝑝𝑚𝑚 (𝐸𝐸) − 𝑘𝑘𝑆𝑆𝑆𝑆 (𝐸𝐸)𝑝𝑝𝑚𝑚 (𝐸𝐸) + 𝐾𝐾𝑅𝑅𝑅𝑅 𝑘𝑘𝑅𝑅𝑅𝑅 (𝐸𝐸) 𝑛𝑛≠𝑚𝑚

− 𝑘𝑘𝑅𝑅𝑅𝑅 (𝐸𝐸)𝑝𝑝𝑚𝑚 (𝐸𝐸)

𝜌𝜌𝑚𝑚 (𝐸𝐸)𝑒𝑒 −𝛽𝛽𝛽𝛽 𝑛𝑛𝐴𝐴 𝑝𝑝𝐵𝐵 𝑄𝑄𝑚𝑚 (𝛽𝛽)

(4)

The three positive and four negative terms on the right-hand side of the Eq. 4 respectively account for the population flux into and out of the energy grain E associated with isomer m (i.e. pm(E)). The first term of Eq. 4 represents the population increase into pm(E) arising from collisional energy transfer. These collisions occur at the Lennard-Jones collision frequency (ω) and P(E|E′) gives the probability that a collision with the bath gas (in our case N2) will result in a transition from the energy grain E′ to the energy grain E. The population loss of isomer m from the energy grain E due to collisions is described by the second term. The population gain and loss for pm(E) at constant energy E (microcanonical rates), due to reactions transferring population from isomer n to isomer m, and vice versa, are represented respectively by the third and fourth terms in Eq. 4. Here the kmn(E) and knm(E) terms denote the microcanonical rate coefficients for population transfer between isomers n and m respectively. The fifth term is associated with the irreversible loss from pm(E) due to the formation of the products S. The final two terms in Eq. 4 describe the so-called bimolecular source terms and apply only to those isomers on the PES that are populated via bimolecular association reactions. It is assumed that


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the bimolecular reactants, A and B, (in our case A and B correspond respectively to the O2 molecules and the geminal diol radicals) are maintained in a Boltzmann distribution and that reactant A is in significant excess compared to reactant B (i.e., [A]≫[B]), so that a pseudo-firstorder kinetics treatment is applicable. Under these conditions, the sixth and seventh terms in Eq. 4, describe respectively the population gain by pm(E) from the association of reactants A and B (together denoted as R) and the population loss from pm(E) via re-dissociation to the reactants. In these last two terms kRm(E) represents the rate constant at which pm(E) re-dissociates to give the 𝑒𝑒𝑒𝑒

bimolecular reactants, R, and 𝐾𝐾𝑅𝑅𝑅𝑅 is the equilibrium constant between isomer m and the

reactants. The rovibrational partition function for the molecular species m is given by Qm(β)=∫dEρm(E)e−βE, while nA is the number density of reactant A, and pB the population of reactant B. Because the number of states in polyatomic molecules is typically large, it is necessary to simplify the phase space by bundling similar energies into grains. In the present kinetic calculations, we used a energy grain size of 20 cm−1. By discretizing Eq. 4 with the application of energy grains, it can be cast in matrix form which can then be diagonalized to yield a set of chemically significant eigenvalues (CSEs) and a set of internal energy relaxation eigenvalues (IEREs).49,50 In MESMER the phenomenological rate coefficients are obtained from the eigenvalues and eigenvectors of the system using a Bartis-Widom method.53-56 The potential energy surface for the present RC(OH)2 + 3O2 reactions basically consists of three parts involving reactants that proceed down a barrierless entrance channel to form an entrance complex, followed by the entrance complex undergoing unimolecular reaction over a well-defined barrier to form the product complex, and then the product complex going on to form the products. As the association reaction steps connected with the entrance and exit

channels are barrierless, the transition states for these reaction steps are not fixed and vary along



the reaction coordinate with energy.57 Hence conventional transition state theory is not suitable for handling these features of the RC(OH)2 + 3O2 reaction. For these reaction steps, MESMER uses the inverse Laplace transform (ILT) approach to calculate rate coefficients of the corresponding association reactions. The rate for the unimolecular reaction step involving the RC(OO)(OH)2 adduct, which occurs over a well-defined barrier, is handled using RRKM theory. The Arrhenius pre-exponential factor used in the MESMER ILT method was set to 1.0 × 10−11 cm3molecule-1 s-1 for the entrance and exit channels of the RC(OH)2 + O2 reaction. This was done so that the rate coefficient calculations using MESMER would closely match the experimental values obtained for the analogous CH3CH(OH) + O2 reaction at 298 K.58 The activation energy and modified Arrhenius parameters were set to 0 kcal/mol, and 0.1 respectively. These values along with ground level atmospheric O2 concentrations, suggest that the addition of O2 on to the gem diol radical to form the corresponding gem diol peroxy adduct, occurs on the time scale of some tens of nanoseconds. MESMER also needs Lennard-Jones (L-J) parameters for the geminal diol peroxy adduct, product complexes, and bath gas. Different Lennard-Jones parameters were used for each diol reactions (see Table S9 of supporting information) and the values used were that for the nearest size alkane.59 For example, the L-J parameters for both the entrance and exit complexes associated with methylglyoxal diol + 3O2 reaction were kept the same, as they are roughly of similar size, and these L-J values were set to that of pentane (σ = 4.23 Å and ε = 276.2 K),59 which is roughly the same size as the methylglyoxal diol adducts. Also, in the present calculations we used nitrogen (N2) as the bath gas and its L-J parameter were set to σ = 3.919 Å and ε = 91.85 K.52 The collisional energy transfer process was modeled using the exponential down model with the average energy transfer in a downward direction set to ⟨ΔE⟩d = 200 cm-1 8 ACS Paragon Plus Environment

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for the O2 + gem diol radical reaction and 100 cm-1 for the OH + gem diol abstraction reactions. These values were based on studies of similar reactions appearing in the literature.52,60-65

4. Results and Discussion: The reagent gem diols considered in this study can exist in several stable conformers that differ slightly in energy and the relative orientation of the two hydroxyl groups.66-69 Figure 1 shows the structures of the most stable conformer for each of the gem diols while the other higher energy conformers are given in Fig.S1 of the Supporting Information. The conformers are labeled as cis, trans, and trans’, as shown in Fig. 1 and Fig.S1, and only stable conformers found at the MP2/6-311++G(3df,3pd) level are listed. In investigating the gem diol oxidation reactions, we have only considered reactions occurring via its lowest energy conformer. In the atmosphere, the R-CH(OH)2 gem diols can react with OH radicals resulting in the abstraction of a hydrogen atom from the diol. In principle, the OH radicals can abstract either a hydrogen atom from the C-H moiety associated with the α position of the diol as shown in Eq. 1 or a hydrogen atom from the attached R group; it is also possible for the OH radical to abstract the hydrogen atom from the OH moiety of the diol. Using ab initio calculation, we have verified that for most of the diols considered here, the removal of the α hydrogen of the diol CH group, as shown in Eq.1, is typically the lowest energy pathway. In order to discuss these results, we note that the possible hydrogen abstraction pathways associated with the OH radical are: R-CH(OH)2 + OH → R-C•(OH)2 + H2O

(5a)

R-CH(OH)2 + OH → R-CH(OH)O• + H2O

(5b)

R-CH(OH)2 + OH → R•-CH(OH)2 + H2O

(5c)



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Further, as there are multiple sites of the gem diol from which a C-H hydrogen atom can be removed, we label these sites with the Greek letters α, β, or γ as shown below in Scheme I:

Scheme I. Labeling of carbon sites associated with C-H hydrogen abstraction by OH radicals.

The potential energy surface associated with the various OH + gem diol reactions are shown in Figure S2 of the Supporting Information. As an example, Fig. 2 shows the potential associated with

the

OH

+

methylglyoxal

diol

reaction,

computed

at

the

CCSD(T)/6-

311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory. From the figure we see that the barrier for abstracting the α hydrogen atom from the C-H moiety of the diol is lower by ~2.8 kcal/mol relative to abstraction of the hydrogen atom from its OH moiety and ~3.2 kcal/mol lower compared to that for removing the γ C-H hydrogen from the attached methyl group. We see a similar trend for the other gem diols as well and in most cases the abstraction of the α hydrogen by the OH radical has the lowest barrier. Only in the case of glyoxal diol is the barrier for α hydrogen abstraction, though negative, slightly higher than that for abstracting the C-H hydrogen from the side group. Table 1 summarizes the computed rate constants for the various OH + gem diol abstraction reactions at 300 K. The rates at other temperatures covering between 200 and 300 K are given in Table S10 and S11 of the supporting information. For these Mesmer rate calculations, the required ILT pre-exponential factors were taken from references: 10 ACS Paragon Plus Environment

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58,70, and 71. These results confirm that α-hydrogen abstraction from these geminal diols by OH radicals will be fast and typically the dominant abstraction pathway. Thus, based on the above analysis we expect the initial reaction between atmospheric OH radicals and the RCH(OH)2 gem diols to produce the RC(OH)2 radicals. Once formed, the RC(OH)2 radicals are expected to rapidly react with the large concentration of 3O2 molecules present in the atmosphere. As shown in Eqs. 6a and 6b, there are potentially two possible pathways for the reaction between O2 and the RC(OH)2 gem diol radical. RC(OH)2 + 3O2  RC(O)OH + HO2

(6a)

RC(OH)2 + 3O2 + M  (R-C(OO)(OH)2) + M

(6b)

In the first, Eq. 6a, the 3O2 molecule abstracts a hydrogen atom from one of the -OH groups of the RC(OH)2 radical to directly form HO2 and an organic acid. In the second pathway, Eq. 6b, the 3O2 molecule adds onto the RC(OH)2 radical in the presence of a third body M (e.g. N2), which takes away the excess energy, to form the R-C(OO)(OH)2 radical adducts. Although we searched extensively, we did not find a transition state associated with the direct hydrogen abstraction pathway (Eq. 6a). We note that earlier studies for the analogous CH2OH + 3O2 reaction also did not find a transition state associated with the direct hydrogen abstraction channel.72 Thus, based on our present search and earlier studies on the CH2OH radical, we conclude that direct hydrogen atom abstraction by the 3O2 molecule from the -OH moiety of RC(OH)2 does not occur and that the addition of 3O2 on to the radical to form the R-C(OO)(OH)2 adduct, Eq. 6b, is the dominant pathway. Further, we have explored the potential energy surface associated with the 3O2 addition pathway at the M06-2X, MP2, and QCISD levels and did not find a TS associated with them, consistent with these reactions being barrierless. Our findings are



in line with results of both theoretical and experimental studies on analogous reactions involving the addition of 3O2 onto other organic radicals, which suggest that these reactions are generally barrierless.72-80 Figures 3-6 show the potential energy curves associated with these 3O2 addition reactions for the four different R-C(OH)2 diol radicals (where R is either -H, -CH3, -HC(O), CH3C(O)). As the figures show, these addition reactions generate the corresponding peroxy radical adduct (R-C(OO)(OH)2) which are typically lower in energy relative to the separated starting R-C(OH)2 + 3O2 reagents by ~18-40 kcal/mol, depending on the R group (see Fig. 3-6). It is interesting to compare the stability of the peroxy radical adduct from the gem diol reactions with those of other peroxy radical adducts available in the literature.72-75,79,80 For example our findings that the R-C(OO)(OH)2 diol radical adduct are typically 18-40 kcal/mol lower in energy compared to the separated R-C(OH)2 + 3O2 reagents is comparable to that found for adducts generated in the CH2OH + 3O2 and CH3CHOH + 3O2 reaction system where the adduct is reported to be of lower energy by respectively ~35 and ~37 kcal/mol.72-75 Our results are also similar to those found for the CH2C(O)H + 3O2 and CH3C(O) + 3O2 reaction where the oxygenated adducts are found to be of lower in energy relative to the separated starting reagents by ~28 and ~34 kcal/mol.79,80 After the R-C(OO)(OH)2 diol radical adducts are formed, they contain sufficient energy to undergo unimolecular reaction, corresponding to a 1,4-H-shift, that results in the formation of a double bond between carbon and oxygen atoms in the adduct and the elimination of the organic acid and HO2 radical as shown in the exit channel portion of Figs. 3-6. We find that the intramolecular 1,4-hydrogen shift in the R-C(OO)(OH)2 radical adduct occurs through a proton coupled electron transfer (PCET) mechanism; this is illustrated in Scheme-II using the methane diol peroxy adduct as an example. Detail discussion of PCET mechanisms associated with both


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intermolecular and intramolecular hydrogen transfer reactions have been reported in the literature.36-43 As illustrated in Scheme-II, in the present PCET process the hydrogen atom on the terminal OH group of the RC(OO)(OH)2 adduct departs towards the radical center, located on the terminal peroxy oxygen atom of the adduct, as a proton (H+). This, then induces a simultaneous shift in the electron distribution within the adduct, as shown by the arrows, and leads to C-O bond cleavage and the formation of the R-C(O)OH and HO2 products.

Scheme II. Illustration of the PCET mechanism for the HC(OH)2 + 3O2 → HCOOH + HO2 reaction. The natural orbitals shown have electron occupancies of 1.91 (left) and 1.04 (right) in the transition structure TS1 for the acidic hydrogen atom abstraction from HC(OH)2 by 3O2. Here n stands for the electron occupation of the natural orbitals of the QCISD wave function. The natural orbital analysis was performed at the QCISD/aug-cc-pVTZ level of theory.



To gain deeper insights into the PCET mechanism for the HC(OH)2-3O2 interaction, we analyzed the QCISD wave function of the transition state (TS1) in terms of the natural orbitals obtained from the first-order density matrix. The natural orbitals for TS1 were calculated at the QCISD/aug-cc-pVTZ level of theory. The key natural orbitals arising from the bonding and antibonding interactions involving the HC(OH)2 and 3O2 species are shown in the lower portion of Scheme II. The HOMO clearly represents the bonding interaction between the C1=O5-based π orbital of the HC(OH)2 moiety and 2p orbital of the O7 atom of 3O2, which are suggestive of an electron transfer event between HC(OH)2 and 3O2. The SUMO orbital shows the presence of significant spin population on the O7 and O8 atoms of the 3O2, and absence of any spin population on the O5 and H6 atoms of the HC(OH)2 moiety. This nullifies the triplet repulsion between the two oxygens atoms (O7 and O8) involved in the hydrogen atom transfer in TS1. This natural orbital analysis further supports our viewpoint that during the course of the reaction, a proton and a single electron are transferred simultaneously from the HC(OH)2 moiety to the terminal oxygen of the 3O2, O8. These electronic features indicate that the mechanism of the HC(OH)2 + 3O2 → HCOOH + HO2 reaction is of a proton-coupled electron-transfer (PCET) type. It is worth mentioning here that the PCET mechanism of the HC(OH)2 + 3O2 → HCOOH + HO2 reaction shows the same electronic features described for the gas-phase oxidation of atmospheric acids by hydroxyl radicals.36,39-43 That the peroxy radical adduct unimolecular reaction involves a PCET mechanism and not just an ordinary hydrogen atom transfer (HAT), is also verified by examining the atomic spin densities on the RC(OO)(OH)2 adduct and the corresponding transition state. As an illustration of this, Fig. 7 shows the computed spin densities associated with the methane diol peroxy adduct (HC(OO)(OH)2) and its corresponding transition state (TS1) for the intramolecular hydrogen 14 ACS Paragon Plus Environment

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transfer. In the figure we see that the spin densities on oxygen atoms O7 and O8 of HC(OO)(OH)2 are respectively 0.32 and 0.68. While in the transition state, the spin densities on atoms O7 and O8 are respectively 0.54 and 0.49. Thus, in going from the reactant to the TS the spin density changes from being predominantly localized on a single oxygen atom of the adduct to being fairly evenly distributed between the two-oxygen atoms in the TS. This change in spin density is consistent with the electron movement described in Scheme-II. The atomic spin densities associated with the other gem diol oxidation reactions behave similarly and are given in Table 2. Examining the barriers associated with the unimolecular reactions of the R-C(OO)(OH)2 radical adducts, shown in Fig. 3-6, also reveals an interesting trend. We find that the barrier heights for these reactions decrease along the R substitution series: -H, -CH3, -HC(O), CH3C(O). Figure 8 summarizes these findings at both the CCSD(T)//M06-2X and CCSD(T)//MP2 levels. The reaction associated with -CH3CO substitution has the lowest unimolecular reaction barrier with the transition state being respectively 4.3 and 3.2 kcal/mol above the potential well of the CH3C(O)-C(OO)(OH)2 peroxy radical adduct at both levels of theory. The decrease in barrier height along the R substitution series is consistent with an increase in internal hydrogen bonding along the series, which stabilizes the TS relative to the reactant adduct complex. The presence of hydrogen bonding in the various peroxy radical adducts is indicated by the dashed lines appearing in the structures shown in the bottom of Fig. 8. The fact that increased internal hydrogen bonding can lead to a lower reaction barrier through enhanced stabilization of the TS, was confirmed by considering a higher energy conformer of the glyoxal diol and methylglyoxal diol radical adducts where internal hydrogen bonding between the OH moiety and the carbonyl group was absent. These results are shown in Fig. S3 of the 15 ACS Paragon Plus Environment


Supporting Information. A similar trend arising from internal hydrogen bonding has recently been reported in studies examining the reactions of amines with oxygenated atmospheric molecules.81 To better quantify the atmospheric impact of the gem diol oxidation reactions, we have carried out rate calculations using the MESMER kinetic code. The required input parameters for the rate

calculations

were taken

from

the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-

311++G(3df,3pd) results and the rates calculated over the temperatures between 200 and 300 K and bath gas pressures ranging from 100 to 7600 Torr. As discussed in the theoretical kinetic analysis section, we used the MESMER ILT method for handling the barrierless association and reverse dissociation steps and the RRKM method for handling the unimolecular reaction step with well-defined barrier, associated with these multi-well reactions. In addition, the MESMER calculations applied Eckart tunneling correction to the reaction rates, although ultimately the impact of tunneling was found to be negligible. The entire reaction was treated as a pseudo first order reaction, as the glyoxal diol radical concentration is several orders of magnitude lower than the O2 concentration under atmospheric conditions. The calculated effective bimolecular rate constants, in units of cm3 molecule-1 s-1, over the temperatures from 200-300 K and bath gas pressures at 100 Torr and 760 Torr for glyoxal diol and methyl glyoxal diol given in Table 3. The rates for methane and ethane diols are similar. For these reactions the rate constant at 250 K is found to be around 1.0 х 10-11 cm3 molecule-1 s-1 with no observed pressure dependence over the 100-7600 Torr range. The fact that all the rate constants are roughly similar, even though the unimolecular reaction steps for the various diol radical adduct have different barrier heights, basically indicates that the large excess energies associated with these oxidation reactions control their kinetics. Further, since collisional relaxation is included in the master equation treatment,


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the large rate constants and their lack of pressure dependence suggests that the peroxy radical adducts are formed with considerable excess energies, so much so that even after undergoing collisions with N2 buffer gas, they still retain sufficient energy to readily undergo unimolecular reaction over their respective barriers. The present rate results suggest that the oxidation reaction pathway will be an important contributor to the gas phase degradation of gem diols in the atmosphere. For example, the present reaction mechanism is significantly more effective than that associated with the dehydration and dehydrogenation pathways for diol removal, which have significantly higher barriers.7,69 Photodecomposition is another possible degradation mechanism for atmospheric gem diols. The recent study of the ultraviolet spectrum of methylglyoxal diol suggests that its absorption spectrum is centered at ~240 nm, and thus falls outside the tropospheric actinic window.35 Hence for methylglyoxal diol, tropospheric photodissociation is not expected to be efficient, leaving the present oxidation reaction pathway as the most likely dominant gas phase loss mechanism. The UV absorption spectrum of the other gem diols considered in this work are not known; however, the lack of any significant chromophores on these diols suggests that their electronic absorption bands will also fall outside the tropospheric actinic window, and hence the present oxidation mechanism is also expected to be a very important degradation pathway for these other diols as well. Since these oxidation reactions are very fast and produce organic acids as a byproduct, the present results suggest that the gas phase oxidation of gem diols will be an important source of atmospheric organic acids and HO2 radicals.

5. Conclusions: In the present work, the gas phase oxidation of four gem diols in the presence of OH radicals and molecular oxygen (3O2) is investigated using high level computational methods. The 17 ACS Paragon Plus Environment


four gem diols are represented generically as R-HC(OH)2, with R being either -H, -CH3, -HC(O), and -CH3C(O). Hydrogen abstraction from the gem diol by the OH radicals followed by addition of 3O2 results in the production of the R-C(OO)(OH)2 peroxy radical adduct. A key step in the oxidation process is the unimolecular reaction of this R-C(OO)(OH)2 radical adduct. This unimolecular reaction occurs through a proton coupled electron transfer mechanism. We find that the unimolecular reaction barrier height, decreases along the substation series: -H, -CH3, HC(O), -CH3C(O). The rate constants for the R-C(OO)(OH)2 + O2 reactions over the temperatures between 200 and 300 K and 1 atm pressure is found to be ~1.0x10-11 cm3 molecule1

s-1. Our results suggest that these oxidation reactions are an important mechanism for the

degradation of gas phase gem diols in the atmosphere and will also be an important source of organic acids.

Acknowledgement: A.S. thanks the National Science Foundation for support of this work under the Grant CHE-1566272. AS also thanks the W. M. Keck Foundation, through computing resources at the W. M. Keck Laboratory for Integrated Biology, for allowing use of their computers. We also thank Chanin B. Tangtartharakul for his help with the supporting material preparation.

Supporting Information: Table S1-S2 contains total electronic energies including zero-point energy correction at different levels of theory. Table S3-S8 contains barrier heights, optimized geometries of reactants, products, and transition states with their z-matrices, vibrational frequencies, rotational constants, imaginary frequencies of all the transition states. Table S9-S10 contains the site-specific rate constants for the reaction of OH radicals with various gem diols.


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Figure Captions Figure 1. The optimized lowest energy stable conformer respectively for methane diol, ethane diol, glyoxal diol, and methylglyoxal diol, calculated at the MP2/6-311++G(3df,3pd) level. Figure 2. Potential energy diagram for the OH + methylglyoxal diol reaction calculated at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The energies (kcal/mol) correspond to the abstraction of a hydrogen atom by the OH radical from the α C-H, O-H, and γ C-H groups. The symbols RCα, RCo, RCγ correspond to reactant complexes, TSα, TSo, TSγ to the transition states, and PCα, PCo, PCγ to the product complexes. Figure 3. Potential energy diagram for the methane diol radical + 3O2 reaction to form formic acid and HO2 at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The energies (kcal/mol) given in parenthesis correspond to values computed at the CCSD(T)/6311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level. The symbols correspond to: RC1 (methane diol peroxy adduct), TS1 (transition state), and PC1 (product complex). Figure 4. Potential energy diagram for the ethane diol radical + 3O2 reaction to form acetic acid and HO2 at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The energies (kcal/mol) given in parenthesis correspond to values computed at the CCSD(T)/6311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level. The symbols correspond to: RC2 (ethane diol peroxy adduct), TS2 (transition state), and PC2 (product complex). Figure 5. Potential energy diagram for the glyoxal diol radical + 3O2 reaction to form glyoxalic acid and HO2 at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The energies (kcal/mol) given in parenthesis correspond to values computed at the CCSD(T)/6-


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311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level. The symbols correspond to: RC3 (glyoxal diol peroxy adduct), TS3 (transition state), and PC3 (product complex). Figure 6. Potential energy diagram for the methylglyoxal diol radical + 3O2 reaction to form pyruvic acid and HO2 at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. The energies (kcal/mol) given in parenthesis correspond to values computed at the CCSD(T)/6311++G(3df,3pd)//MP2/6-311++G(3df,3pd)

level.

The

symbols

correspond

to:

RC4

(methylglyoxal diol peroxy adduct), TS4 (transition state), and PC4 (product complex). Figure 7. The atomic spin densities on carbon (C1) and both oxygen atoms (O7 and O8) in the methane diol peroxy adduct, transition state, and product complex involved in the unimolecular reaction of HC(OO)(OH)2 calculated at M06-2X/6-311++G(3df,3pd) level. Figure 8. Changes in unimolecular barrier height for the formation HO2 and the corresponding acid from the R-C(OO)(OH)2 adduct as a function of -H, -CH3, -HC(O), -CH3C(O)) substitutions. The red and blue color dotted lines in the graph correspond to results obtained respectively at the CCSD(T)//M06-2X and CCSD(T)//MP2 levels of theory. Internal hydrogen bonding occurring in the radical adduct, shown at the bottom of the graph, is indicated for each structure by their respective dashed lines.



Fig. 1


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Fig. 2



Fig. 3


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Fig. 4



Fig. 5


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Fig. 6



Fig. 7


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Fig. 8



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Table 1. Mesmer calculated site-specific rate constants (in cm3 molecule-1 s-1) for hydrogen atom abstraction from the four geminal diol by OH radicals at 300 K.a

reaction site

kα kO kβ kγ

2.20 х 10-13 1.89 х 10-15

1.48 х 10-12 2.98 х 10-15 1.67 х 10-13

1.24 х 10-12 1.94 х 10-14 2.81 х 10-12

1.02 х 10-12 2.68 х 10-14 2.68 х 10-14

The Arrhenius pre-exponential factor (A) used in the Mesmer ILT method was set to 9.0 × 10−13 (for -C-H) , 5.0 × 10−14 cm3 molecule-1 s-1 (for O-H) hydrogen abstraction from methane diol and they were taken from Ref.58,70. The A values 4.5 × 10−12, 1.6 × 10−13, 1.0 × 10−13, and 1.5 × 10−11 cm3 molecule-1 s-1 are used for α C-H, -O-H, -CH3, and -C(O)H hydrogen abstractions of ethane, glyoxal and methylglyoxal diols respectively, and are are taken from Ref.58,71. a


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Table 2. The atomic spin densities on carbon (C1) and the two oxygen atoms (O7 and O8) of the leaving HO2 group (see Fig. 7) for the four diol peroxy radical adducts and their corresponding transition states calculated at M06-2X/6-311++G(3df,3pd) level of theory.

Radical methane diol

ethane diol

glyoxal diol

methylglyoxal diol

Selected atom C1 O7 O8 C1 O7 O8 C1 O7 O8 C1 O7 O8

Peroxy radical 0.01 0.32 0.68 0.003 0.33 0.66 -0.02 0.34 0.67 -0.01 0.34 0.65


TS -0.03 0.54 0.49 -0.03 0.54 0.49 -0.04 0.53 0.52 -0.04 0.54 0.51


Table 3. Bimolecular rate constant (cm3 molecule-1 s-1) for the HCO-C(OH)2 + O2 → HC(O)C(O)OH + HO2 and CH3CO-C(OH)2 + O2 → CH3C(O)C(O)OH + HO2 reactions calculated using MESMER at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory.a

T (K) 200 220 240 250 270 300

HCO-C(OH)2 + O2 k (cm3 molecule-1 s-1) 100 Torr 760 Torr -11 1.03 х 10 1.03 х 10-11 -11 1.02 х 10 1.02 х 10-11 1.01 х 10-11 1.01 х 10-11 1.00 х 10-11 1.00 х 10-11 9.94 х 10-12 9.94 х 10-12 9.79 х 10-12 9.79 х 10-12

CH3CO-C(OH)2 + O2 k (cm3 molecule-1 s-1) 100 Torr 760 Torr -11 1.03 х 10 1.03 х 10-11 -11 1.02 х 10 1.02 х 10-11 1.02 х 10-11 1.02 х 10-11 1.01 х 10-11 1.01 х 10-11 1.01 х 10-11 1.01 х 10-11 1.00 х 10-11 1.00 х 10-11

The Arrhenius pre-exponential factor used in the Mesmer ILT method was set to 1.0 × 10−11 cm3molecule-1 s-1 and it was taken from Ref.58. a


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TOC Graphic


Organic Acid Formation from the Atmospheric Oxidation of Gem Diols

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