Role of Double Hydrogen Atom Transfer Reactions in Atmospheric

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Role of Double Hydrogen Atom Transfer Reactions in Atmospheric Chemistry Manoj Kumar,† Amitabha Sinha,*,‡ and Joseph S. Francisco*,† †

Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States Department of Chemistry and Biochemistry, University of CaliforniaSan Diego, La Jolla, California 92093-0314, United States

‡

CONSPECTUS: Hydrogen atom transfer (HAT) reactions are ubiquitous and play a crucial role in chemistries occurring in the atmosphere, biology, and industry. In the atmosphere, the most common and traditional HAT reaction is that associated with the OH radical abstracting a hydrogen atom from the plethora of organic molecules in the troposphere via R−H + OH → R + H2O. This reaction motif involves a single hydrogen transfer. More recently, in the literature, there is an emerging framework for a new class of HAT reactions that involves double hydrogen transfers. These reactions are broadly classified into four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv) rearrangement. Hydration and dehydration are classic examples of addition and elimination reactions, respectively whereas tautomerization or isomerization belongs to a class of rearrangement reactions. Atmospheric acids and water typically mediate these reactions. Organic and inorganic acids are present in appreciable levels in the atmosphere and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyltype functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HATbased chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis. Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid. Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key role in the formation of sulfuric acid in the Earth’s stratosphere, Venusian atmosphere, and on heterogeneous surfaces. Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are important in the chemistry of Earth’s atmosphere as well as that of other planets. This Account reviews and puts into perspective some of these atmospheric HAT reactions and their environmental significance.

1. INTRODUCTION Hydrogen bonds are a common feature in many areas of chemistry.1−4 These bonds are much weaker than a typical covalent bond but are of sufficient strength to influence the structural and dynamical properties of many molecular systems. Hydrogen bonds can be of either the intermolecular or intramolecular variety. In this Account, the focus is on exploring the role of intermolecular hydrogen bonds in affecting atmospheric chemistry. An important role played by such hydrogen bonds is in the formation of small atmospheric clusters, which can grow to form a critical nucleus and eventually an aerosol particle, that impacts climate.5 The abundance, volatility, and reactivity of atmospheric species are important factors in determining their potential for contributing to the nucleation process. Because of its low vapor pressure and propensity to form strong hydrogen bonds, a common nucleating species is sulfuric acid. However, depending on the environmental conditions, other nucleating agents are also possible. For example, in the troposphere, volatile organic © XXXX American Chemical Society

compounds emitted from anthropogenic and biogenic sources undergo reactions with OH, NO3 and O3, which result in the formation of oxidized products. The structural features in these oxidized organics impart to the molecule their characteristic chemical and physical properties. In particular, molecules with polar functional groups (e.g., >CO and −OH) possess the ability to participate in hydrogen bonding making these oxidized organics also important contributors to the nucleation process. Apart from affecting molecular aggregation, intermolecular hydrogen bonding can also impact the chemistry of molecules within the clusters by lowering reaction barriers and opening up new reactive pathways. The later topic is the main focus of the current Account. An important mechanism by which hydrogen-bonding lowers the reaction barrier is by forming cyclic molecular complexes that involve lesser ring strain and facilitate intermolecular Received: January 21, 2016

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Accounts of Chemical Research hydrogen atom transfers (HAT). The HAT reactions are quite common and play a prominent role in a variety of environments including biology, industry, combustion and atmospheric chemistry.6 A central feature of atmospheric HAT reactions is the presence of a two-point hydrogen bond, which facilitates the transfer of a hydrogen atom from one location to another within the complex, thus resulting in the formation of new products. Acids are present in appreciable amounts (∼parts per billion) in the atmosphere7 and can readily form hydrogen bonds, which makes them potent HAT catalysts.The main sources of these acids are anthropogenic and biogenic emissions, hydrocarbon gas-phase oxidations, and aqueousphase oxidation of carbonyl compounds. The low molecular weight carboxylic acids are abundantly available in tropospheric aqueous and gaseous phases and in aerosol particles in various environments.7 This Account puts into perspective several examples of atmospherically relevant acid- and water-catalyzed HAT reactions involving neutral and radical molecular complexes, and highlights their impact on a variety of atmospheric processes ranging from the formation of sulfuric acid, organic diols relevant for aerosol growth, and the isomerization and decomposition of free radicals.

Scheme 1. Prereaction Complexes Involved in the Waterand Acid-Assisted Reaction of Acetaldehyde with the Hydroxyl Radical (Upper Panel), and Isomerization of Methoxy Radical (Lower Panel)

2. FEATURES OF THE DOUBLE HYDROGEN BONDING INTERACTION Acids that promote HAT reactions share a common motif; they form two-point hydrogen bonds with substrate in the activated complex. These hydrogen-bonding interactions generally occur between carbonyl (CO) and hydroxyl (OH)-like functionalities and result in pseudo cyclic structures. Note that these cyclic structures contain both covalent and noncovalent bonds and are different from cycloalkanes that are only covalently bonded. The stability of these cyclic structures depends upon the steric flexibility of the noncovalent ring formed by the twopoint hydrogen bonds. For catalysts, in which the hydrogen exchange occurs through a single heteroatom, the smaller sized (up to seven-membered) cyclic structures are formed, which are less stable due to steric congestion (Scheme 1). Water catalysis involves this type of mechanism.8−15On the other hand; there are catalysts, which promote the exchange reaction by employing unique functionalities in both HAT components of the exchange reaction. Under such catalysis, the larger sized (up to 10-membered) and hence, more stabilized cyclic intermediates are formed (Scheme 1), that accounts toward their enormous catalytic efficiency. Monocarboxylic-acid-based catalysis16−28 or inorganic-acid-based catalysis21,26,29−31 belongs to this category. Most catalysts possess the H−O donor motif and the OY (YC, N, S, P) acceptor motif,16−31 except for water, which has a different acceptor motif (O−H).8−15 Depending upon the nature of the reaction and catalyst, the most common donor− acceptor motifs, which are operative in the hydrogen exchange reactions, can be classified into four groups (Scheme 2). For example, C−O(H)···H−O, O−H···OY and O−H···O−H are the donor−acceptor motifs for the decomposition reactions,15,21,24−26 whereas the hydration reactions are only mediated by the O−H···OY and O−H···O−H motifs. The C−H···OY, C−H···O−H, and CO···H−O are the donor− acceptor motifs for the tautomerization of oxygenates.16,24,25 The tautomerizations operative in biological media have the AH···OY, A−H···O−H, and CN···H−O donor−acceptor motifs, respectively.28 The strength of hydrogen bonding in these donor−acceptor motifs depends upon the bond polarity

Scheme 2. General Donor−Acceptor Motifs Involved in Water- and Acid-Assisted Chemical Reactions

and the bond distance. For example, the SO acceptor motif forms better hydrogen bonds than CO or NO because the SO bond is relatively longer than the CO or NO bond.21,26,29−31 On the other hand, the O−H donor motif forms more stable hydrogen bonds than the S−H motif because of its greater polarity.26 To gain molecular level insight into the double hydrogen atom transfer reactions, the nature of interactions in the methanediol···formic acid21 complex within the framework of natural bond orbital (NBO)32 procedure is analyzed. The interactions that most contribute to the stabilization of hydrogen-bonded complex generally involve interactions between the lone pair orbitals of donor oxygen and the σ antibonding orbital of acceptor O−H bond (n → σ*OH). The overlap between the oxygen lone pair and the σ*OH orbital is examined by constructing the pre normalized NBOs. Figure 1a shows the n → σ*OH interaction for the hydrogen bond where a diol acts as the Lewis acid (acceptor) and HCOOH acts as a Lewis base (donor). In the figure, the π-type lone pair of oxygen (nπ) in HCOOH is interacting with the σ*OH orbital of the diol monomer. The nπ → σ*OH interaction contributes 5.8 kcal/mol toward the total second order stabilization energy. There is also a second interaction involving the σ-type lone pair (nσ) of HCOOH and the σ*OH orbital of diol (nσ → σ*OH). B

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Figure 1. Prenormalized NBOs for (a) nπ → σ*OH interaction in the first hydrogen bond, (b) nσ → σ*OH interaction in the first hydrogen bond, and (c) nσ→ σ*OH interaction in the second hydrogen bond of the methanediol···HCOOH complex. The key NBO charges are given in panel (d).

hydration of SO3 and compared it with the uncatalyzed and H2O-catalyzed reactions.17 They found that the unimolecular isomerization of the SO3···H2O···HCOOH complex, whether it is formed from SO3 + H2O···HCOOH or SO3···H2O + HCOOH, is barrierless for producing the H2SO4···HCOOH complex. The calculated barrier for the HCOOH-catalyzed reaction is only 0.1 kcal/mol, which is 6.5 kcal/mol lower than that for the H2O-catalyzed one. This makes acid catalysis an efficient mechanism for the gas-phase hydration of SO3, which may potentially be a major contributor to the atmospheric formation of H2SO4. Long et al.18 also studied this reaction and confirmed that the HCOOH-catalyzed reaction was barrierless. For the HCOOH-catalyzed reaction, they calculated a rate constant of 2.1 × 10−10 cm3 molecule−1 s−1 at a typical atmospheric temperature of 260 K, which is 105 times greater than that for the H2O-catalyzed reaction. Once formed, H2SO4 itself can also catalyze the SO3 hydration. H2SO4 possesses SO and S−O−H functionalities, which allows it to catalytically influence the SO3 hydration. A recent study30 examined the autocatalytic mechanism of H2SO4 formation. The calculated rate constants for the H2SO4catalyzed reaction are 2 orders of magnitude larger than that for the H2O-catalyzed one. The autocatalytic reaction mechanism is expected to be particularly important in the stratosphere, where the concentration of (H2O)2 drops to a level similar to that of the H2SO4···H2O complex. This autocatalytic reaction could also be significant in Venus’ atmosphere, where the estimated concentration of H2SO4··· H2O complexes (8.3 × 1010 molecules·cm−3) is significantly greater than that of (H2O)2 (1.5 × 107 molecules·cm−3).30 ii. Hydration of Aldehydes to Form Diols, Triols, and Tetrols. Organic compounds containing carbonyl moieties can undergo hydration resulting in the formation of more polar OH functionalities that can facilitate aerosol growth. The role of organic acids in catalyzing the hydration of several aldehydes including formaldehyde (HCHO), 19,21,31 acetaldehyde (CH3CHO),20 ketene,22 and glyoxal ((HCO)2)23 to form diols, triols, and tetrols has been analyzed. Here, the results for the hydration of HCHO, CH3CHO, and (HCO)2 are presented. HCHO is the simplest aldehyde and the most abundant carbonyl compound in our atmosphere.36 The reaction between HCHO and H 2 O produces methanediol (CH2(OH)2). This reaction is not only important in the fields of chemistry and biochemistry, but is also atmospherically relevant because CH2(OH)2 is the smallest diol and diols have been implicated in aerosol growth.19,23 Williams et al.37 reported the first theoretical study on the uncatalyzed and H2O-catalyzed hydration reaction in the gas and aqueous phases. The calculated barrier for the uncatalyzed reaction in the gas-phase was 42.2 kcal/mol relative to separated reactants, which was reduced to only 0.8 kcal/mol in the presence of an additional H2O molecule. The inclusion of entropy effects

This interaction, which is shown in the Figure1b, contributes 3.9 kcal/mol toward the second order stabilization energy. Figure 1c shows the n → σ*OH interaction for the second hydrogen bond in the methanediol···formic acid complex, in which the diol acts as a Lewis base and HCOOH acts as a Lewis acid. In this case, the nσ lone pair of oxygen in diol interacts with the σ*OH of the HCOOH monomer. The nσ → σ*OH interaction contributes 23.0 kcal/mol toward the overall second order stabilization energy of the complex. The NBO charge on O6 is 0.13 unit more negative than on O10. Moreover, H12 is bonded to relatively less negative O11. Both these factors account toward the larger second order energy for the interaction shown in Figure 1c.

3. ATMOSPHERIC CHEMICAL PROCESSES UTILIZING DOUBLE HYDROGEN ATOM TRANSFER Many atmospheric reactions involving hydrogen atom transfer can be broadly classified on the basis of structural changes occurring in the reactant molecule. This classification does not require knowledge of reaction path or mechanism. The three main reaction classes discussed below are addition, elimination/ decomposition, and rearrangement/isomerization. a. Addition Reactions

Water vapor is the third most abundant molecule in the Earth’s atmosphere, and hence, many addition reactions involve the incorporation of a water molecule. These hydration reactions can lead to the formation of important atmospheric molecules that can facilitate nucleation and aerosol growth. i. Hydration of SO3 and Sulfuric Acid Formation. Sulfuric acid (H2SO4) is a key contributor to acid rain33 and is known to impact atmospheric nucleation processes.34 In the atmosphere, H2SO4 is mainly formed by the hydration of sulfur trioxide (SO3).35 Both the uncatalyzed and the catalyzed hydrations have been extensively examined computationally.9−12,17,18,30 Morokuma and Muguruma theoretically studied the effect of water catalysis on the reaction and showed that a second water molecule significantly modifies the potential energy surface for the SO3 hydration.9 With two water molecules, the rate-limiting step for H2SO4 formation involves the unimolecular isomerization of the prereactive SO3···H2O··· H2O complex to form H2SO4···H2O. Various calculations have estimated the barrier for this step to be in the range between ∼6.6−13.0 kcal/mol.10−12 The presence of additional water molecules has been shown to further reduce the hydration barrier, with four or more water molecules effectively making the reaction barrierless.12 Since the basic mechanism of water catalysis involves shuttling of a hydrogen atom between H2O and SO3, carboxylic acids, which contain carbonyl and hydroxyl functionalities and are present at the parts per billion level in the troposphere,7 may also catalyze the SO3 hydration. Hazra and Sinha recently examined the formic acid(HCOOH)-catalyzed gas-phase C

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carbonyl compound, HCHO, under H2O catalysis. The barrier for the H2O-catalyzed NH3 reaction is 8.5 kcal/mol, whereas the analogous CH3NH2 addition has only a 0.1 kcal/mol barrier. With (CH3)2NH, the transition state is 5.4 kcal/mol submerged below reactants. Thus, by tuning the R group on the amine nitrogen, the barrier can be varied over a range of ca.14 kcal/mol. The H2O-catalyzed (CH3)2NH addition to HCHO has a rate constant of 1 × 10−6 s−1 under tropospheric temperatures (200−300 K), which is only 1/60th of the bimolecular reaction rate for the OH + (CH3)2NH reaction (6 × 10−5 s−1). This makes the H2O-catalyzed reaction an important loss process for the atmospheric (CH3)2NH at night when OH levels are quite low.

significantly reduced the effect of H2O catalysis; the barrier for the H2O-catalyzed reaction was only reduced by 27 kcal/mol as compared to the uncatalyzed reaction (ΔG⧧ = 53.4 kcal/mol). This led to the conclusion that the catalytic effect of two or more H2O molecules would be nominal. Hazra et al. recently examined the HCHO hydration in the absence and presence of H2O and HCOOH.19 The uncatalyzed and H2O-catalyzed reactions have electronic barriers of 39.2 and 23.1 kcal/mol, respectively at the MP2/6-311++G(3df,3pd) level of theory. The calculated barrier for the HCOOH-catalyzed reaction is dramatically lowered to 9.8 kcal/ mol. Moreover, the transition state is only 1.0−1.5 kcal/mol above the energy of HCHO···H2O + HCOOH reactants, implying that the acid-catalyzed hydration of HCHO could lead to the facile formation of diol under gas-phase conditions. This new mechanism may also be relevant in cold water-rich interfaces such as ice and atmospheric aerosols where the probability of HCHO···H2O complex formation and its subsequent interaction with HCOOH is highly likely. Rypkema et al. also examined the gas-phase hydration of CH3CHO with and without catalysts.20 CH3CHO is an important contributor to the budget of ozone, HOx,38 and peroxyacetyl nitrate,39 and its photochemical tautomerization to vinyl alcohol can serve as a source of organic acids in troposphere.40The barrier for the uncatalyzed CH3CHO hydration is 36.9 kcal/mol. This is substantially reduced in the presence of a catalyst. The H2O-catalyzed reaction has a barrier of 15.5 kcal/mol, whereas the HCOOH-catalyzed one occurs barrierlessly. Additional calculations with other carboxylic acids indicate that the barrier to the catalyzed hydration correlate strongly with the pKa of the acid, providing useful insight into the predictive capacity of the effectiveness of acid catalysts. A qualitative kinetic analysis indicates a lifetime of years for the CH3CHO···H2O complex against the HCOOHcatalyzed reaction, which is suggestive of the fact that these reactions are unlikely to have any atmospheric impact in the free gas-phase. These reactions may, however, be important on surfaces and in the bulk-phase droplets, where the stability of pre- and postreactive complexes would be enhanced by additional stabilization through hydrogen bonding from the solvent cage. Finally, (HCO)2 is the simplest α-dicarbonyl and is produced in the atmosphere through the oxidation of biogenic and anthropogenic oxygen species.41 The two carbonyl groups of (HCO)2 can be hydrated in a stepwise manner leading to the formation of (HCO)2-diol and(HCO)2-tetrol, which may play a role in the formation and growth of secondary organic aerosols. The calculations suggest that the catalytic effect of a HCOOH molecule on the diol formation is enormous: the barriers for the two possible reaction channels, (HCO)2···H2O + HCOOH and (HCO)2 + H2O···HCOOH, are only 0.5 and 1.5 kcal/mol, respectively.23 This implies that HCOOH catalysis could be a viable mechanism for the (HCO)2 hydration in H2O-restricted environments. iii. Addition of Amines to Carbonyls and Carbinolamine Formation. Apart from water, amines can also participate in addition reactions. The water catalyzed reaction between an amine (R1NH2) and a carbonyl compound (R2CHO), leading to carbinolamine R1NHC(R2)(OH) formation, is speculated to be important for certain aerosol growth.42−45A recent theoretical study14 calculated and compared the energetics of the reaction between the simplest amines, NH3, CH3NH2, and (CH3)2NH, and the simplest

b. Radical Isomerizations

Methoxy radical (CH3O) reactions play an important role not only in atmospheric and combustion chemistry, but also in lowtemperature matrix reactions in radiation chemistry.46 In the atmosphere, CH3O is mainly produced by the oxidation of hydrocarbons.47 Because of its gas-phase profile, the isomerization of CH3O into CH2OH has been extensively studied using high-level ab initio theoretical calculations.31,46,48 Buszek et al. examined this isomerization in the catalytic presence of H2O, HCOOH, and H2SO4.29 Though all catalysts lowered the isomerization barrier, HCOOH and H2SO4 produced better catalysis than H2O because of their ability to form sterically more favorable transitions states. The calculated rate constants for HCOOH and H2SO4-catalyzed reactions are 10 and 12 orders of magnitude larger than the H2O-catalyzed one, implying that acid catalysis provides an efficient mechanism for making the radical isomerization energetically accessible under atmospheric conditions. The HOCO radical is another important species in combustion and atmospheric environments. It is formed as an intermediate in the oxidation of CO through its reaction with OH. The existence of HOCO radicals was first experimentally verified through its detection in low-temperature matrices.49 Theoretical calculations and spectroscopic measurements confirmed that the HOCO radical exists in two stable conformers: trans and cis. High-level ab initio calculations showed that the potential well for HOCO is 30 kcal/mol submerged below the separated HO + CO reactants.50,51 The estimated barrier for the trans-HOCO → cis-HOCO interconversion is ∼7−10 kcal/mol depending upon the theoretical method used.51 Among the two conformers, transHOCO is predicted to be more stable. A recent study27 examined the effect of acids on the relative stability of the HOCO conformers as well as on the barrier for their interconversion in the gas-phase. These calculations found that though cis-HOCO forms better complex with acids than trans-HOCO, it would be readily interconverted into the more stable trans-HOCO in acid-rich environments. This may explain why the experimental characterization of cis-HOCO in acidic conditions has been a challenge.52 c. Alkyl Peroxy Radical Decomposition

Alkyl peroxy radicals (RO2) are key intermediates in the hydrocarbon oxidation processes.53A detailed knowledge of peroxy radical chemistry is crucial for understanding the oxidative capacity of atmosphere.54 The decomposition of RO2 radical, which yields the HO2 radical as one of the products, could be an important degradation pathway in troposphere because of its role in O3 cycle and hydrogen peroxide formation. The hydroxymethylperoxy radical, HOCH2O2 is D

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5. CONCLUSION AND OUTLOOK The studies surveyed in this Account show that acid-catalyzed double hydrogen transfer reactions occur across the major classes of atmospheric reactions. There is an underlying theme that runs through these reactions involving the commonality of the potential energy surfaces for the double hydrogen transfer. Traditionally, a chemical reaction is viewed as the passage of reactants from a minimum energy state through a transition state to the minimum energy state of products. The effect of water or acids in the HAT reactions is found to be catalyzing. Changes in the potential energy surface topography under catalysis illustrate how lowering the barrier heights can have a major influence on the reaction rates. There is another unique feature of the acid-catalyzed double hydrogen transfer reactions; they are mediated by deep potential wells. The prereaction and postreaction complexes involved in these systems are significantly more stable than the separated reactants and products due to hydrogen bonding between the oxygenates and acids. Since these entrance/exit complexes are the acid-stabilized forms of the reactants and products, they could be used to trap reactive atmospheric species, which may otherwise be difficult to detect. For example, isolated HO3 is unstable, though when complexed with water it is readily detectable.61,62 In another example, Liu et al.24 have reported the direct detection of stabilized vinyl hydroperoxide formed via the deuterated carboxylic acid-catalyzed tautomerization of alkyl-substituted Criegee intermediates. These acid-stabilized vinyl hydroperoxides may aid in improving our understanding of the mechanism for OH production from olefin ozonolysis in the troposphere. The deep potential energy wells associated with the acidcatalyzed reactions also have potential implications for aerosol formation. Recent reports17,18,30 indicate that the H2SO4 dimers play an important role in atmospheric aerosol formation. Moreover, the nucleation of H2SO4 is enhanced in the presence of aromatic carboxylic acids.63 Recent theoretical calculations17,18,30 show that the postreaction complexes involved in the autocatalytic or acid-catalyzed hydrolysis of SO3 are ∼20−29 kcal/mol more stable than the separated reactants. This makes the subsequent reaction, which may occur between H2SO4···H2SO4 or its hydrates or H2SO4··· HCOOH and SO3, favorable and hence a potential contributor toward aerosol growth in the atmosphere.

an example of the RO2 radicals and its decomposition results in the HO2 radical (eq 1). HOCH 2O2 → HO2 + HCHO

(1) 15

Recent electronic structure calculations investigated this reaction in the absence and presence of water-, water dimer-, the HO2 radical- and the water·HO2 radical complex. The calculations revealed that the bimolecular decompositions involve relatively larger barriers than the uncatalyzed one, and the HO2 radical is generated in the complexed state that must be released into the atmosphere. But, interestingly, the prereaction and postreaction complexes that are more stable than separated reactants and products respectively, mediate these reactions. In these complexes, the peroxy radical and catalyst are stabilized via H-bonds. The key H-bonds are O−H H-bonds, which result in the binding energies of 7−18 kcal/ mol for these complexes. Vibrational overtone excitation of any reagent’s X−H (X = O, N, S, and C) bond can give rise to an increase in the rate coefficient of many orders of magnitude.55 The calculations revealed that the overtone excitation of vOH ≥ 1 would be sufficient to make the HO2 radical- and the H2O· HO2 radical complex-mediated decompositions feasible under atmospheric conditions. Alternatively, the electronic excitation of the organic peroxy species in these complexes occur in the 1044−1182 nm range that might also provide sufficient energy for the decomposition. Results from this study suggested, for the first time, an important chemical role of the H2O·HO2 radical complex that exist in significant abundance in the troposphere.56 These results are expected to help resolve the long-standing discrepancy between measurements and model calculations on the photolytic source of the HOx radical at high solar zenith angles.57

4. WATER-MEDIATED RADICAL-MOLECULE REACTION The reaction of hydrochloric acid (HCl) with the OH radical reactivates chlorine radicals, which destroy stratospheric O3. The experimental rate constant for the reaction fall in the range 6.8−8.5 × 10−13 cm3 molecule−1 s−1 at room temperature. The value recommended by the NASA panel for Data Evaluation58 is 7.8 × 10−13 cm3 molecule−1 s−1 at 298 K with an uncertainty factor of 1.1. Theoretical calculations59,60 indicate that the reaction has a barrier of 2.4−2.6 kcal/mol and the rate constants of 7.9 × 10−13 and 7.8 × 10−13 cm3 molecule−1 s−1, calculated within conventional59 and variational60 transition state theory frameworks, respectively. A recent study examined the effect of H2O catalysis on the chlorine reactivation.13 The calculations suggests that the reaction barrier is reduced from 2.1 to 1.8 kcal/mol in the presence of a H2O molecule, and two new reaction channels open up at even lower energies: 4.2 and 4.4 kcal/mol below the energy of the reactants. These new channels differ in the orientation of the pendant hydrogen of H2O. The possibility of two additional pathways arises from the fact that the three-body reaction, HCl + OH + H2O occurs via three distinct pathways: (i) OH···HCl + H2O → Cl + 2H2O, (ii) H2O···HCl + OH → Cl + 2H2O, and (iii) HCl + H2O···OH → Cl + 2H2O. The pathways (i) and (ii) correspond to the uncatalyzed and the H2O-catalyzed reactions, whereas the pathway (iii) involves a direct HAT between the OH, which is hydrogen-bonded with H2O, and HCl, leading to the two lowest energy reaction channels. Interestingly, H2O does not directly participate in the pathway (iii), but stabilizes the radicals via hydrogen bonding.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Manoj Kumar obtained his Ph.D. with Prof. P. M. Kozlowski at the University of Louisville in 2012. He is currently working as a postdoctoral researcher with Prof. Joseph S. Francisco. Amitabha Sinha obtained his BS degree from Case Western Reserve University and Ph.D. from MIT. After postdoctoral studies at NOAABoulder and the University of WisconsinMadison, he joined U.C.San Diego in 1992. E

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Accounts of Chemical Research

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Joseph S. Francisco is currently Elmer H. and Ruby M. Cordes Chair in Chemistry and Dean of the College of Arts and Sciences at the University of NebraskaLincoln.

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ACKNOWLEDGMENTS We thank Holland computing center of the University of NebraskaLincoln for providing computing resources. A.S. thanks the UCSD Academic Senate for support of his portion of the work.

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DOI: 10.1021/acs.accounts.6b00040 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.accounts.6b00040 Acc. Chem. Res. XXXX, XXX, XXX−XXX