Hydroxyl Radical Substitution in Halogenated ... - ACS Publications

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J. Phys. Chem. A 2010, 114, 2806–2820

Hydroxyl Radical Substitution in Halogenated Carbonyls: Oxalic Acid Formation Carrie J. Christiansen, Shakeel S. Dalal, and Joseph S. Francisco* Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue UniVersity, West Lafayette, Indiana 47909

Alexander M. Mebel Department of Chemistry and Biochemistry, Florida International UniVersity, Miami, Florida 33199

Jeffrey S. Gaffney Department of Chemistry, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204 ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: January 11, 2010

An ab initio study of OH radical substitution reactions in halogenated carbonyls is conducted. Hydroxyl radical substitution into oxalyl dichloride [ClC(O)C(O)Cl] and oxalyl dibromide [BrC(O)C(O)Br], resulting in the formation of oxalic acid, is presented. Analogous substitution reactions in formyl chloride [ClCH(O)], acetyl chloride [ClC(O)CH3], formyl bromide [BrCH(O)], and acetyl bromide [BrC(O)CH3] are considered. Energetics of competing hydrogen abstraction reactions for all applicable species are computed for comparison. Geometry optimizations and frequency computations are performed using the second-order Møller-Plesset perturbation theory (MP2) and the 6-31G(d) basis set for all minimum species and transition states. Single point energy computations are performed using fourth-order Møller-Plesset perturbation theory (MP4) and coupled cluster theory [CCSD(T)]. Potential energy surfaces, including activation energies and enthalpies, are determined from the computations. These potential energy surfaces show that OH substitution into ClC(O)C(O)Cl and BrC(O)C(O)Br, resulting in the formation of oxalic acid and other minor products, is energetically favorable. Energetics of analogous reactions with ClCH(O), BrCH(O), ClC(O)CH3, and BrC(O)CH3 are also computed. 1. Introduction Atmospheric acids, including organic acids, have long been considered an environmental threat. Acid precipitation, resulting from atmospheric acids, has received much attention in the past and is still a significant concern, prompting environmental policy to curb the problem.1-4 Organic acids, such as formic acid, acetic acid, and oxalic acid, have been measured in precipitation worldwide.5-8 Organic acids can also cause other environmental problems. For example, oxalic acid, the smallest and most abundant dicarboxylic acid,9,10 has been identified as a major component of water-soluble organic mass in aerosols.10-12 The detrimental environmental impacts of organic aerosols, including their effects on climate, is well documented.13 As such, the formation reactions of aerosol precursors, like oxalic acid, is of particular concern. The concentration of oxalic acid in aerosols in urban environments has been observed as high as 900 ng · m-3, with levels in remote locations much lower, 10-50 ng · m-3.10,14-22 As with all diacids, the volatility of oxalic acid is quite low; therefore, most oxalic acid ultimately associates with aqueous phase droplets or particulate solids, typically leaving gas phase levels near or below most detection limits.23-26 The sources of this highly abundant dicarboxylic acid are not completely understood. Primary sources include vehicle exhaust16 and biomass burning.27,28 Secondarily, oxalic acid is thought to be generated through aqueous phase photooxidation reactions of anthropogenic pollutants or pollutant oxidation products,9,29,30 particle surface reactions,31,32 and in cloud/fog

reactions.14,22,33 In addition to aqueous and particle reactions, the gas phase formation mechanisms of organic acids, including oxalic acid, has been of ongoing interest.9,34-36 Previous work by Christiansen and Francisco investigated the atmospheric breakdown of 1,2-dibromoethane (EDB), a CFC replacement compound used as a fumigant and pesticide, as well as an oil and gasoline additive.37 One of the products of the EDB oxidation is oxalyl dibromide [BrC(O)C(O)Br]. A literature search shows that no studies, other than the work by Christiansen and Francisco, indicate any atmospheric source of oxalyl dibromide. Other sources may exist but have not yet been determined, as the study of the degradation of brominated CFC replacements is limited. Only a small number of publications indicate atmospheric sources of a sister compound, oxalyl dichloride [ClC(O)C(O)Cl]. One study hypothesizes the mechanism of the OH initiated oxidation of C2Cl4 and suggests oxalyl dichloride as a product of this reaction.38 It is likely that oxalyl dichloride, by mechanisms similar to oxalyl dibromide formation from 1,2-dibromoethane, is produced in the atmosphere through degradations of other chlorinated hydrocarbons as well. Oxalyl dibromide and oxalyl dichloride are both used extensively as reagents in organic reactions. As such, both compounds have been characterized.39-42 In aqueous solutions, these oxalyl dihalides will hydrolyze to oxalic acid; however, this conversion has not been considered in the gas phase.15 This work uses oxalic acid as a case study to illustrate a novel mechanism for the atmospheric production of carboxylic acids, particularly diacids. We propose the gas

10.1021/jp9045116  2010 American Chemical Society Published on Web 02/04/2010

OH Radical Substitution in Halogenated Carbonyls

J. Phys. Chem. A, Vol. 114, No. 8, 2010 2807 phase OH substitution reactions of oxalyl dihaldes as a possible source of oxalic acid. Additionally, according to the same mechanism, other halogenated carbonyls may result in the formation of various carboxylic acids. 2. Computational Methods

Figure 1. Reactions pathways for ClC(O)C(O)Cl + OH.

All optimization and frequency computations are performed using the Gaussian 03 suite of programs.43 Optimized geometries and corresponding energies for reactants, reactive intermediates, products, and transition states are determined using the secondorder Møller-Plesset perturbation theory (MP2) with the 6-31G(d) basis set. All electrons are correlated in the MP2 optimization. Frequency computations are also performed with this theory and basis set, giving vibrational mode frequencies, as well as thermochemistry, including the zero-point energy (ZPE) correction. The optimized geometries are used to compute single point energies with the following theory and basis set combinations: MP4/6-311++G(2d,2p), MP4/6-311++G(2df,2p),

Figure 2. Structures of reactants, reactive intermediates, products, and transition states in ClC(O)C(O)Cl + OH and resulting reactions. Bond distances given in Å and angles in degrees.

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Figure 3. Energy diagram for ClC(O)C(O)Cl + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6311++G(2df,2p) level.

TABLE 1: Energetics for OH + ClC(O)C(O)Cl and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G (d)

ClC(O)C(O)Cl + OH f ClC(O)OC(O)Cl ClC(OH)OC(O)Cl f C(O)(OH)C(O)Cl + Cl

MP4/6-311++G (2d,2p)

MP4/6-311++G (2df,2p)

CCSD(T)/6-311++G (2d,2p)

CCSD(T)/6-311++G (2df,2p)

-7.4

-8.5

-10.6

Enthalpy of Reactions -7.2 -5.1 -24.3

-20.2

-18.3

-16.6

-15.2

-7.3

-5.0

-7.3

-8.5

-10.6

C(O)(OH)C(OH)ClO f C(O)(OH)C(O)(OH) + Cl

-25.8

-23.9

-22.5

-20.2

-19.2

ClC(OH)OC(O)Cl f ClC(O)(OH) + CO + Cl

-25.3

-25.5

-20.8

-22.0

-17.8

C(O)(OH)C(OH)ClO f C(O)(OH) + ClC(O)(OH)

-18.9

-19.1

-18.4

-15.3

-14.8

C(O)(OH)C(O)Cl + OH f C(O)OC(O)Cl + H2O

2.6

-1.6

-1.5

-3.2

-3.1

15.8

11.5

10.0

C(O)(OH)C(O)Cl + OH f C(O)(OH)C(OH)ClO

ClC(O)C(O)Cl + OH f ClC(OH)OC(O)Cl ClC(OH)OC(O)Cl f C(O)(OH)C(O)Cl + Cl

Activation Energy of Reactions 19.9 17.4 3.4

1.1

1.1

-0.8

-0.7

18.8

16.9

15.5

11.0

9.6


1.2

-1.6

-1.7

-3.7

-3.8


6.3

3.5

2.9

1.5

0.8


6.2

3.8

3.2

2.5

1.9


14.5

12.0

11.5

10.3

9.8


CCSD(T)/6-311++G(2d,2p), and CCSD(T)/6-311++G(2df,2p). These energies are corrected with the MP2/6-31G(d) level ZPE correction to obtain total energies. Finally, using corrected energies, the enthalpy and activation energy barriers are calculated for individual reactions. 3. Results and Discussion 3.1. Chlorinated Carbonyls. 3.1.1. ClC(O)C(O)Cl + OH Reactions. The oxalyl dichloride with hydroxyl radical reactions are shown in Figure 1. Optimized geometries of all species involved in these pathways are given (Figure 2). An energy diagram for this set of reactions is also presented (Figure 3).

Enthalpies and activations energies (Table 1) for these reactions are calculated using the calculated zero-point corrected energies. In the text, the values discussed for this set of reactions, and all others, are at the CCSD(T)/6-311++G(2df,2p) level. Zero-point corrected energies of all species of this set of reactions, as well as vibrational frequencies, are available in Supplementary Table 1 and Supplementary Table 2 in the Supporting Information, respectively. Oxalyl dichloride is a planar, symmetric molecule (Figure 2a). The C-Cl bond is 1.744 Å, the C-O bond length is 1.202 Å, and the C-C bond length is 1.547 Å. The OCCl angle and the OCC angle are both 123.9°, while the ClCC angle is 112.1°.


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ClC(O)C(O)Cl + OH f ClC(OH)OC(O)Cl

Figure 4. Reaction pathways for ClCH(O) + OH.

The first reaction in the pathway is the addition of the OH radical into the carbonyl of ClC(O)C(O)Cl, creating a tetrahedral intermediate (reaction 1).

(1)

The activation energy of this reaction is 10.0 kcal · mol-1 with an enthalpy of -10.6 kcal · mol-1. The associated transition state (Figure 2i) has a vibrational frequency of 991i cm-1, and a C-O bond length of 1.763 Å. In the intermediate, ClC(OH)OC(O)Cl (Figure 2b), the carbon where the addition occurred now has a near tetrahedral geometry. The C-O bond length has increased to 1.361 Å and the C-Cl bond length has increased to 1.824 Å. The existence and stability of this intermediate shows that the OH substitution reaction of this compound is a two step process, first addition of the OH (reaction 1), and second, the elimination of the chlorine (reaction 2).

ClC(OH)OC(O)Cl f C(O)(OH)C(O)Cl + Cl

(2)

Figure 5. Structures of reactants, reactive intermediates, products, and transition states in ClCH(O) + OH and resulting reactions. Bond distances given in Å and angles in degrees.

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TABLE 2: Energetics for OH + ClCH(O) and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G (d) ClCH(O) + OH f ClCH(OH)O

-7.6

MP4/6-311++G (2d,2p)

MP4/6-311++G (2df,2p)

Enthalpy of Reactions -4.8 -7.1

CCSD(T)/6-311++G (2d,2p)

CCSD(T)/6-311++G (2df,2p)

-7.9

-10.1

ClCH(OH)O f CH(O)(OH) + Cl

-24.4

-20.6

-18.8

-17.1

-15.7

ClCH(OH)O f Cl(O)(OH) + H

-14.8

-4.9

-5.8

0.3

-0.5

ClCH(O) + OH f Cl + CO + H2O

-21.7

-27.0

-23.1

-25.8

-22.2

Activation Energy of Reactions 14.2 13.9

11.7

11.6

ClCH(O) + OH f ClCH(OH)O ClCH(OH)O f CH(O)(OH) + Cl ClCH(OH)O f Cl(O)(OH) + H ClCH(O) + OH f Cl + CO + H2O

16.5 2.2

0.2

0.3

-1.5

-1.4

14.8

14.4

13.4

13.8

12.7

8.2

3.6

3.1

0.2

-0.2

This elimination reaction has a calculated activation energy of -0.7 kcal · mol-1. The negative activation energy indicates that the transition state of this reaction is separated from the products by little or no barrier. The enthalpy of the reaction is -15.2 kcal · mol-1. The transition state corresponding to this reaction has a C-Cl bond length of 2.027 Å (Figure 2j) and a vibrational frequency of 732i cm-1. The product, C(O)(OH)C(O)Cl (Figure 2c), has a geometry quite similar to the parent molecule; a planar molecule with trigonal planar geometry around each carbon. This species now undergoes another two reactions to displace the remaining chlorine with the hydroxyl group (reactions 3 and 4).



(3)

(4)

Reaction 3, addition of the OH, has activation energy similar to the previous OH addition, 9.6 kcal · mol-1, with the same enthalpy, -10.6 kcal · mol-1. The transition state (Figure 2k) is similar as well, with a C-O bond length of 1.769 Å and vibrational frequency associated with the C-O stretch of 988i cm-1. The produced intermediate (Figure 2d) resembles the previous tetrahedral intermediate with slightly extended C-O and C-Cl bond lengths. Finally, in reaction 4, with an activation energy of -3.8 kcal · mol-1, the chlorine is extruded to produce oxalic acid. The enthalpy of this reaction is -19.2 kcal · mol-1. The transition state (Figure 2l) mimics the previous chlorine elimination transition state with a C-Cl bond length of 2.052 Å and associated vibrational frequency of 719i cm-1. The produced oxalic acid (Figure 2e) is also planar and symmetric. It has a C-C bond length of 1.534 Å, a C-O bond length of 1.219 Å, and a C-(OH) length of 1.330 Å. The OCO angles are 125.4° with (OH)CC angles of 113.3° and the OCC angles at 121.3°. Other possible reactions, leading to products other than oxalic acid, exist. Both tetrahedral intermediates can undergo C-C bond breakage (reactions 5 and 6).


(5)


(6)

Reaction 5 has a small activation energy, 0.8 kcal · mol-1 and an exothermic enthalpy, -17.8 kcal · mol-1. The transition state is available (Figure 2m). The carbon monoxide and chlorine radical readily split, resulting in products of ClC(O)(OH) (Figure 2f), CO, and Cl. Reaction 6 is similar with an activation energy of 1.9 kcal · mol-1 and an enthalpy of -14.8 kcal · mol-1. The transition state is given in Figure 2n. This reaction results in the production of C(O)(OH) (Figure 2g) and ClC(O)(OH) (Figure 2f). As both reactions have small activation energies and the enthalpy change is favorable, it is likely that these reactions will compete with the chlorine elimination reactions and may result in all products, rather than the sole production of oxalic acid. One other reaction that must be considered in competition with the OH addition is the abstraction of hydrogen by OH (reaction 7).


(7) The activation energy of this reaction is very close to that of the OH addition, 9.8 kcal · mol-1 but with a less exothermic enthalpy -3.1 kcal · mol-1. For the optimized geometry of the transition state, see Figure 2o. The product, C(O)OC(O)Cl, is shown in Figure 2h. This reaction may compete with the OH addition reaction. 3.1.2. ClCH(O) + OH Reactions. Analogous systems, and their reactions with OH are investigated, allowing for tests of the validity of extending the OH substitution mechanism to other halogenated carbonyls. Formyl chloride, ClCH(O), is the simplest chlorinated carbonyl. This system allows for the investigation of the influence of a hydrogen, alpha to the carbonyl, on the OH substitution mechanism. Optimized geometries of all species involved in the hydroxyl radical with formyl chloride reactions (Figure 4) are given in Figure 5. Zero-point corrected energies of all species as well as vibrational frequencies are available (Supplementary Table 3 and Supplementary Table 4, Supporting Information). Using these energies, enthalpies and activations energies are calculated and presented in Table 2. An energy diagram, visually summarizing these energetics, is presented (Figure 6).



Figure 7. Reactions pathways for ClC(O)CH3 + OH. Figure 6. Energy diagram for ClCH(O) + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6311++G(2df,2p) level.

Formyl chloride (Figure 5a) is a trigonal planar molecule. It has a C-O bond length of 1.200 Å, a C-Cl bond length of 1.767 Å, and a C-H bond length of 1.096 Å. The optimized angles are OCCl ) 123.7°, OCH ) 126.3°, and HCCl ) 110.0°. The OH substitution reaction in formyl chloride follows the same mechanism as in oxalyl dichloride. It begins with the addition of OH (reaction 8) to produce the tetrahedral intermediate, which then undergoes a chlorine elimination (reaction 9).

ClCH(O) + OH f ClCH(OH)O

(8)

ClCH(OH)O f CH(OH)O + Cl

(9)

The activation energy of reaction 8 is 11.6 kcal · mol-1 and the enthalpy is -10.1 kcal · mol-1. The transition state (Figure 5e) has a C-(OH) bond of 1.856 Å and an imaginary vibrational frequency, associated with that stretch, of 641i cm-1. The producedtetrahedralintermediate(Figure5b)hascarbon-substituent angles ranging from 99.5° to 114.9°. As before, the C-O bond length increases slightly to 1.366 Å and the C-Cl bond increases to 1.821 Å. This intermediate next undergoes the chlorine elimination (reaction 9) with the transition state shown in Figure 5f. The barrier of this reaction, as expected, is small, at -1.4 kcal · mol-1 with an enthalpy of -15.7 kcal · mol-1. The product of this reaction is formic acid (Figure 5c). It is also trigonal planar, as the starting molecule, formyl chloride. As in the previous system, the tetrahedral intermediate created from formyl chloride can also undergo a competing reaction (reaction 10).

ClCH(OH)O f ClC(O)(OH) + H

(10)

The transition state, with a vibrational frequency of 1853i cm-1, is shown in Figure 5g. This reaction has significant activation energy, 12.7 kcal · mol-1, and an enthalpy of -0.5 kcal · mol-1. The product of this reaction, ClC(O)(OH), is a planar molecule and is shown in Figure 5d. Comparison of the energetics indicates that the chlorine elimination will dominate. The other competing reaction of this system is of great importance (reaction 11).

ClCH(O) + OH f Cl + CO + H2O

(11)

This hydrogen abstraction of the forrmyl chloride has a small activation energy, -0.2 kcal · mol-1 and a large exothermic enthalpy, -22.2 kcal · mol-1. Figure 5h shows the transition state

for the reaction. Comparison of these energetics with the OH addition reaction (reaction 8) indicates that in this system, and likely in other systems containing a hydrogen R to the carbonyl, the hydrogen abstraction will dominate and the OH addition will not occur. Therefore, the formation of formic acid is unlikely and the production of chlorine radical, carbon monoxide, and water will result. 3.1.3. ClC(O)CH3 + OH Reactions. The final chlorinated system studied is ClC(O)CH3. As with the previous compound, ClCH(O), this system contains a nearby hydrogen. However, this hydrogen is now β to the carbonyl. The position of the hydrogen, relative to the carbonyl, may affect the energetics of the reactions and subsequently the OH substitution reaction’s likelihood compared to the hydrogen abstraction. Figure 7 shows the reactions considered for this system. Optimized geometries of all species involved are given (Figure 8). Using the computed zero-point corrected energies, enthalpies and activations energies are calculated (Table 3). Figure 9 shows the energy diagram for these reaction pathways. Supplementary Tables 5 and 6 (Supporting Information) contain the zero-point corrected energies as well as the vibrational frequencies for all computed species. Acetyl chloride (Figure 8a) is trigonal planar around the chlorinated carbon, with a methyl substituent. The bond lengths are as follows: C-Cl ) 1.798 Å, C-O ) 1.200 Å, and C-C ) 1.498 Å. The bond angles surrounding the chlorinate carbon range from 112.0° to 127.4°. The reactions of ClC(O)CH3 and OH will proceed with the same mechanism as in other systems (reaction 12 and 13).

ClC(O)CH3 + OH f ClC(OH)OCH3

(12)

ClC(OH)OCH3 f C(O)(OH)CH3 + Cl

(13)

The barrier of reaction 12 is 10.7 kcal · mol-1 and has an enthalpy of -8.9 kcal · mol-1. The transition state (Figure 8g) leads to the tetrahedral intermediate (Figure 8b). This intermediate features geometry changes as seen in the other intermediates. It then undergoes the chlorine elimination shown in reaction 13. This reaction, as expected, has a slightly negative activation energy of -1.7 kcal · mol-1 and enthalpy of -15.8 kcal · mol-1. The transition structure for this reaction (Figure 8h) has a C-Cl bond length of 2.026 Å. The associated vibrational frequency is 625i cm-1. The resulting product, acetic acid, C(O)(OH)CH3, is seen in Figure 8c. A competing reaction stemming from the tetrahedral intermediate occurs in this system as well (reaction 14).

ClC(OH)OCH3 f ClC(O)(OH) + CH3

(14)

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Figure 8. Structures of reactants, reactive intermediates, products, and transition states in ClC(O)CH3 + OH and resulting reactions. Bond distances given in Å and angles in degrees.

This reaction, through the transition state shown in Figure 8i, produces ClC(O)(OH) (Figure 8d) and a methyl radical (Figure

8e). The activation energy of this reaction though is quite high when compared with that of the chlorine elimination, 11.2



TABLE 3: Energetics for OH + ClC(O)CH3 and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G (d) ClC(O)CH3 + OH f ClC(OH)OCH3

MP4/6-311++G (2d,2p)

MP4/6-311++G (2df,2p)


-6.9

CCSD(T)/6-311++G (2d,2p)

CCSD(T)/6-311++G (2df,2p)

-6.8

-8.9


-23.3

-20.4

-18.8

-17.0

-15.8


-10.6

-10.5

-10.2

-6.5

-6.3

ClC(O)CH3 + OH f ClC(O)CH2+H2O

-9.2

-15.4

-16.0

-17.5

-18.3

ClC(O)CH3 + OH f ClC(OH)OCH3

15.5


10.8

10.7


1.6

-0.3

-0.2

-1.8

-1.7


14.6

11.6

11.2

11.6

11.2

ClC(O)CH3 + OH f ClC(O)CH2 + H2O

12.0

6.6

5.9

5.2

4.4

Figure 9. Energy diagram for ClC(O)CH3 + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6-311++G(2df,2p) level.

kcal · mol-1. The enthalpy is exothermic, at -6.3 kcal · mol-1. The comparatively large activation energy is likely to eliminate the occurrence this reaction. The final competing reaction to consider for this system is the hydrogen abstraction of the hydrogen beta to the carbonyl (reaction 15).

ClC(O)CH3 + OH f ClC(O)CH2 + H2O

(15)

This reaction is important as it, combined with the hydrogen abstraction reaction that occurred in the formyl chloride system, allows for predictions of the influence of neighboring hydrogens on the addition of the hydroxyl radical. The transition state for this reaction is given in Figure 8j and the resulting product, ClC(O)CH2, is given in Figure 8f. The energy barrier of reaction 15 is 4.4 kcal · mol-1, and the enthalpy is -18.3 kcal · mol-1. Comparison with the energetics for the OH addition indicates this reaction will be competitive with the OH addition reaction, and may dominate. 3.2. Brominated Carbonyls. 3.2.1. BrCH(O) + OH Reactions. Formyl bromide, BrCH(O), is the simplest brominated carbonyl. The reaction of acetyl bromide + OH produces acetic acid, the same product as in the acetyl chloride + OH reaction. However, the mechanism of substitution of the OH proceeds in an unusual way. To understand the nature of the transition state, TS1 (Figure 12d), for the OH addition to BrCH(O), intrinsic reaction

coordinate calculations in the forward (product) direction were run. To not miss any local minimum on the potential surface, the step size in IRC calculations was chosen as 0.02 bohr amu1/2 with the “verytight” option, which corresponds to the most stringent convergence criteria for geometry optimization. Results of these IRC calculations are illustrated in Figure 10. The energy monotonically decreases, although it does exhibit a plateau at the reaction coordinate values around 3.9-4.0 bohr amu1/2. However, no local minimum could be found in this area. The C-(OH) bond distance corresponding to forming the C-O bond decreases from approximately 1.75 Å in TS1, initially at a fast rate and then slowly, eventually attaining a value of approximately 1.35 Å, indicating that the single C-(OH) bond is indeed formed in the reaction. However, the behavior of the C-Br bond length is not monotonic. It increases first, then goes via a maximum and a shallow minimum at the reaction coordinate values where the energy plateau is observed but, after that, again starts to increase at a faster rate. This indicates that the C-Br bond actually breaks during the reaction. Finally, the C-O distance first increases from a typical double-bond length to a single-bond length but then starts to rapidly decrease back in the direction of a double bond. This means that the double bond character is first broken but then restored during the reaction. All these results point to the fact that the reaction products are CH(O)(OH) + Br rather than BrCH(OH)O. Geometry optimization carried out after 365 steps of IRC calculation converged to a CH(O)(OH) · · · Br (to be discussed below) and also showed that a covalently bound BrCH(OH)O species does not exist as a local minimum on the potential energy surface. The question then becomes the nature of TS2, which seemingly connects the CH(O)(OH) + Br products with a BrCH(OH)O local minimum. IRC calculations for TS2 allowed us to address this question. IRC calculations were run in both the forward and reverse directions, and results are also illustrated in Figure 10. The options applied were similar to those used in IRC calculations for TS1. First, the energy behavior in the forward and reverse directions is different, so that the potential is not symmetric. The C-Br distance steadily increases in the forward direction, indicating that this bond breaks on the reaction course. The C-Br behavior in the reverse direction is more peculiar; the distance first slightly decreases, goes through a shallow minimum, but then starts to increase again. It is clearly seen

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Figure 10. Results of IRC calculations for transition states in the BrCH(O) + OH f Br + CH(O)(OH) reaction: left column, TS1 connecting BrCH(O) + OH and CH(O)(OH) · · · Br; right column, TS2, a “bounce” transition state connecting the CH(O)(OH) · · · Br complex with itself. The plots show the behavior of the total energy, R(C-Br), R(C-O(H)), and R(CdO) distances, and the Br-C-O angle as a function of the reaction coordinate.

that no local minimum exists on the potential energy surface in the region where the C-Br distance reaches its minimums another indication that there is no BrCH(OH)O local minimum. As the C-Br distance also increases at larger reaction coordinate values in the reverse direction, the C-Br bond also breaks in the reverse direction. So, apparently TS2 is a “rebound” or “bounce” transition state; the Br atom attacks the CH(O)(OH) molecule but cannot form a bond,

bounces off, and leaves by another trajectory. This hypothesis is supported also by the behavior of the C-O distance and Br-C-O angle. C-O monotonically decreases from a single to a double bond value in the forward direction, and in the reverse direction it goes via a shallow maximum and then decreases. The Br-C-O angle decreases in the forward direction, increases in the reverse direction, but then also goes through a maximum and starts to decrease. So, the two


J. Phys. Chem. A, Vol. 114, No. 8, 2010 2815 CH(O)(OH) · · · Br complex (Figure 12c)sthe same complex that was found by after-IRC optimization for TS1.

Figure 11. Reaction pathways for BrCH(O) + OH.

trajectories differ by the angle of Br’s attack toward CH(O)(OH). The fact that TS2 is a bounce transition state is confirmed by after-IRC geometry optimizations in the forward and reverse directions, which both converge to the same

We conclude that only TS1 is relevant to the reaction considered here and the reaction mechanism can be described as OH + BrCH(O) f TS1 f CH(O)(OH) · · · Br complex f CH(O)(OH) + Br. The reaction pathways of BrCH(O) + OH are given in Figure 11. Figure 12 shows all optimized geometries of the species involved in the BrCH(O) + OH reactions. Enthalpies and activation energies for the reactions are calculated using zeropoint corrected energies and are given in Table 4. The zeropoint corrected energies and vibrational frequencies are available in Supplementary Table 7 and Supplementary Table 8 (Supporting Information), respectively. The energy diagram for this set of reactions is given in Figure 13. Formyl bromide (Figure 12a) is transformed to formic acid [CH(O)(OH)] (Figure 12b) through the following reaction (reaction 16).

Figure 12. Structures of reactants, reactive intermediates, products, and transition states in BrCH(O) + OH and resulting reactions. Bond distances given in Å and angles in degrees.

TABLE 4: Energetics for OH + BrCH(O) and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G (d) BrCH(O) + OH f CH(O)(OH) + Br

-40.7

BrCH(O) + OH f Br + CO + H2O

-30.4

BrCH(O) + OH f complex BrCH(O) + OH f Br + CO + H2O

19.9 7.1

MP4/6-311++G (2d,2p)

MP4/6-311++G (2df,2p)

CCSD(T)/6-311++G (2d,2p)

CCSD(T)/6-311++G (2df,2p)

-36.9

-38.7

-35.9

-37.7

-35.2


13.2

11.6

-0.6

-0.9


3.0

2.6

2816


Christiansen et al.

Figure 14. Reactions pathways for BrC(O)CH3 + OH.

All optimized geometries of species involved in the BrC(O)CH3 + OH reaction are given (Figure 15). Table 5 gives enthalpies and activation energies. The zero-point corrected energies and vibrational frequencies are available (Supplementary Tables 9 and 10, Supporting Information). The energy diagram for acetyl bromide + OH is presented in Figure 16. Acetic acid can be produced through the OH substitution reaction of acetyl bromide (reaction 18).

BrC(O)CH3 + OH f C(O)(OH)CH3 + Br

Figure 13. Energy diagram for BrCH(O) + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6311++G(2df,2p) level.

BrCH(O) + OH f CH(O)(OH) + Br

(16)

This reaction, as discussed above, does proceed through TS1 (Figure 12d), then onto a complex of CH(O)(OH) with Br (Figure 12c), and finally producing CH(O)(OH) and bromine. The activation energy from BrCH(O) + OH to TS1 is 11.6 kcal · mol-1. Reaction 16 has an enthalpy of -38.7 kcal · mol-1, with the complex lying 2.9 kcal · mol-1 below the product. The geometry change from formyl bromide to CH(O)(OH) is slight. The HC(O) angle decreases from 127.2° to 125.5°. The closer proximity of the OH substituent, versus the bromine, necessitates the decrease of this angle to accommodate the hydroxyl group. Formyl bromide, as with formyl chloride, has a hydrogen R to the carbonyl, allowing for the investigation of the competition between the OH substitution into the carbonyl and the OH abstraction of the alpha hydrogen. The hydrogen abstraction proceeds as shown below (reaction 17).

BrCH(O) + OH f Br + CO + H2O

(17)

The enthalpy of this reaction is slightly less exothermic than the OH substitution, -35.2 kcal · mol-1. However, it has an activation energy of -0.9 kcal · mol-1, indicating that this reaction has little or no barrier. The transition state of this reaction (Figure 12e) has a vibrational frequency of 2563i cm-1. The C-H bond length is 1.208 Å, and the H-O bond length is 1.315 Å. Due to the minimal barrier for the hydrogen abstraction reaction and its large negative enthalpy, this reaction will dominate over the hydroxyl radical substitution. Final products will therefore be bromine radicals and carbon monoxide, rather than formic acid. 3.2.2. BrC(O)CH3 + OH Reactions. A slightly more complex brominated carbonyl system is also computed. The reactions of acetyl bromide, BrC(O)CH3, and OH are given in Figure 14. We have determined that this substitution reaction will proceed in the same manner as the BrCH(O) + OH reaction.

(18)

The activation energy for the addition of OH is 12.2 kcal · mol-1. Figure 15d depicts this transition state. Reaction 18 has an enthalpy of -37.3 kcal · mol-1. Again, as in the other brominated OH substitution reactions, the geometry changes from reactant to product is slight (Figure 15a and 15b). Having a reactant with no R hydrogen, but one that does contain a hydrogen β to the carbonyl, allows for an investigation of the competition between an OH substitution and a hydrogen abstraction, now with respect to a β hydrogen.

BrC(O)CH3 + OH f BrC(O)CH2 + H2O

(19)

This hydrogen abstraction (reaction 19) has an enthalpy of -18.3 kcal · mol-1 and an activation energy of 4.5 kcal · mol-1. The transition state has a vibrational frequency of 2376i cm-1. The optimized geometry of BrC(O)CH2 and of the associated transition state are given in Figure 15c,e, respectively. The energetics show that the hydrogen abstraction reaction of a β hydrogen is favorable and will likely dominate over the OH substitution. However, it is important to note that the difference in barrier heights of the hydrogen abstraction and the OH substitution are closer than in the BrCH(O) system. 3.2.3. BrC(O)C(O)Br + OH Reactions. The substitution reaction of BrC(O)C(O)Br + OH follows the same reaction mechanism as the previously discussed brominated carbonyls, ultimately resulting in the formation of oxalic acid. Figure 17 shows the pathways for the reaction of oxalyl dibromide with hydroxyl radical. Optimized geometries of all species involved in the BrC(O)C(O)Br + OH reactions are given in Figure 18. Using the zero-point corrected energies for all species, enthalpies and activation energies for the reactions involved are calculated and given in Table 6. The zero-point corrected energies of all species are available in Supplementary Table 11 (Supporting Information). Supplementary Table 12 (Supporting Information) contains the vibrational frequencies for all computed species. A visual depiction of the energetics for this set of reactions is given (Figure 19). The optimized geometry of oxalyl dibromide (Figure 18a) is almost identical to that of oxalyl dichloride. As expected, the C-Br bond, at 1.930 Å, is slightly longer than the C-Cl bond. Additionally, the angle BrCC is slightly smaller, 111.4°, than the ClCC angle in oxalyl dichloride. The first substitution of OH into the carbonyl, and simultaneous



Figure 15. Structures of reactants, reactive intermediates, products and transition states in BrC(O)CH3 + OH and resulting reactions. Bond distances given in Å and angles in degrees.

elimination of the corresponding bromine, results in the formation of C(O)(OH)C(O)Br (Figure 18b), as seen in reaction 20.

BrC(O)C(O)Br + OH f C(O)(OH)C(O)Br + Br

(20)

The geometry of the transition state for the addition of OH to BrC(O)C(O)Br is given (Figure 18e). The activation energy to this transition state is 9.9 kcal · mol-1. Reaction 20 has an enthalpy of -37.8 kcal · mol-1. C(O)(OH)C(O)Br maintains much of the geometry of the parent molecule, remaining planar with only minor changes in angles and bond lengths. Most notably, the OH group is more tightly bound to the carbon than was the bromine, with a C-O bond length of 1.343 Å. Additionally, the OCO angle is 126.4°, 2.4° larger than was the OCBr angle. This species now undergoes another OH substitution and bromine elimination to form the final product, oxalic acid, C(O)(OH)C(O)(OH) (Figure 18c) (reaction 21).

C(O)(OH)C(O)Br + OH f C(O)(OH)C(O)(OH) + Br (21) The activation energy for the addition of OH is 9.8 kcal · mol-1. The geometry for the corresponding transition state is given in

Figure 16. Energy diagram for BrC(O)CH3 + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6-311++G(2df,2p) level.

Figure 17. Reactions pathways for BrC(O)C(O)Br + OH.

2818


Christiansen et al.

Figure 18. Structures of reactants, reactive intermediates, products and transition states in BrC(O)C(O)Br + OH and resulting reactions. Bond distances given in Å and angles in degrees.

Figure 18f. The second substitution reaction has a slightly larger enthalpy than the first, -42.4 kcal · mol-1. Again, the resulting geometry of oxalic acid does not change significantly from both the previous intermediate and the parent molecule. Another possible reaction can proceed from the species produced after the first substitution reaction has occurred (reaction 22).

C(O)(OH)C(O)Br + OH f C(O)OC(O)Br + H2O (22)

Rather than the addition of OH into the carbonyl, the hydroxyl radical may abstract a hydrogen from the recently added OH group. The optimized geometry of the resulting product, C(O)OC(O)Br, is seen in Figure 18d, while Figure 18g shows the geometry of the transition state. The activation energy for this process is 9.6 kcal · mol-1 with an associated vibrational frequency of 2951i cm-1. The enthalpy is small but still exothermic at -3.4 kcal · mol-1. With a propagatable barrier, this reaction may compete with the second substitution, and therefore the formation of both products, oxalic acid and C(O)OC(O)Br, is likely to occur.



TABLE 5: Energetics for OH + BrC(O)CH3 and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G (d) BrC(O)CH3 + OH f C(O)(OH)CH3 + Br

-37.5


-9.2

BrC(O)CH3 + OH f complex

19.5


10.4

MP4/6-311++G (2d,2p)

MP4/6-311++G (2df,2p)

CCSD(T)/6-311++G (2d,2p)

CCSD(T)/6-311++G (2df,2p)

-35.4

-37.3

-15.9

-17.6

-18.3


13.7

12.2

5.3

4.5


6.6

5.8

TABLE 6: Energetics for OH + BrC(O)C(O)Br and Resulting Reactions (Energies in kcal · mol-1) MP2/6-31G MP4/6-311++G MP4/6-311++G CCSD(T)/6-311++G CCSD(T)/6-311++G (d) (2d,2p) (2df,2p) (2d,2p) (2df,2p)

BrC(O)C(O)Br + OH f C(O)(OH)C(O)Br + Br


-37.1

-36.2

-37.8

C(O)(OH)C(O)Br + OH f C(O)(OH)C(O)(OH) + Br

-40.9

-40.1

-42.1

-40.2

-42.4

2.4

-1.9

-1.8

-3.6

-3.4

15.7

11.5

9.9

C(O)(OH)C(O)Br + OH f C(O)OC(O)Br + H2O BrC(O)C(O)Br + OH f complex


C(O)(OH)C(O)Br + OH f complex

18.7

17.2

15.7

11.2

9.8

C(O)(OH)C(O)Br + OH f C(O)OC(O)Br + H2O

14.4

11.8

11.3

10.1

9.6

4. Conclusions The energetics of the studied systems, including possible competing reactions, shows that the formation of oxalic acid from the substitution of OH into oxalyl dihalides is energetically favorable and may occur in the atmosphere. However, our results show that the type of halide will determine the specifics of the mechanism. As described above, oxalyl dichloride reacts in a two-step process, with a tetrahedral intermediate. The OH adds into the carbonyl, forming a tetrahedral geometry around the carbon. Then, in a separate step, the chlorine is extruded, reforming the carbonyl and leaving a geometry similar to that of the parent molecule. In the brominated species, the addition of the OH and the elimination of bromine occur through a single transition state (TS), which appears like a TS corresponding to the OH addition. A second transition state, which looks like a TS for Br elimination, is actually proven to be a “bounce” or “rebound” transition state for Br atom attacking CH(O)(OH), as no stable BrCH(O)(OH) local minimum (or its substituted analogs) exist. The reaction mechanism can be described as OH + BrCH(O) f TS1 f CH(O)(OH) · · · Br complex f CH(O)(OH) + Br. Because of bromine’s large size and subsequent effectiveness as a leaving group, there is no energetic barrier for elimination of the bromine. Therefore, no stable intermediate exists, as well as the absence of a true transition state for the elimination. As chlorine is smaller and less effective as a leaving group, there is an energetic barrier for the chlorine elimination during the addition of the OH and therefore the chlorinated species reacts in a two-step process. We propose that an iodated species would undergo the same reactions, with a mechanism similar to that of the brominated species. Computations of the reaction energetics of analogous systems allows for the investigation of the effects of a neighboring hydrogen on the production of a carboxylic acid, through the substitution of a hydroxyl radical for a halogen. The results show

that with a hydrogen R to the carbonyl, the hydrogen abstraction with OH is more favorable than the OH substitution. However, with a hydrogen β to the carbonyl, the hydrogen abstraction and OH substitution reactions have more similar barrier heights.

Figure 19. Energy diagram for BrC(O)C(O)Br + OH and resulting reactions. Energies are calculated using values computed at the CCSD(T)/6-311++G(2df,2p) level.

2820


We propose that the further a hydrogen is from the carbonyl, the more likely the OH substitution reaction will occur. This study proposes that halogenated compounds, including CFC replacements, which may oxidize into halogenated carbonyls, could have atmospheric implications beyond ozone depletion. Halogenated carbonyls, have been shown to have the potential to be converted into atmospheric organic acids, including aerosol precursors, like oxalic acid. Further studies of similar systems are necessary to fully determine the contribution that halogenated carbonyls may have to the production of aerosol precursors and organic acids in general. Acknowledgment. We express thanks to the U.S. Department of Energy, Global Change Education Program for financial support through the Graduate Research Environmental Fellowship awarded to Carrie J. Christiansen. Supporting Information Available: Zero-point corrected energies and vibrational frequencies of all species are available as supplementary tables. This material is available free of charge via the Internet at http://pubs.acs.org. Note Added after ASAP Publication. This article posted ASAP on February 4, 2010. Errors in the reaction numbering have been corrected. The correct version posted on February 10, 2010. References and Notes (1) Likens, G. E.; Bormann, F. H. Science 1974, 184, 1176. (2) Grant, W. B. Acid rain and deposition. In Handbook of Weather, Climate and Water, Potter, T. D., Colman, B. R., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2003; Vol. 1, pp 269-284. (3) Menz, F. C.; Seip, H. M. EnViron. Sci. Policy 2004, 7, 253. (4) Norton, S. A.; Vesely, J. Acidification and acid rain. In Treatise on Geochemistry; Holland, H. D., Turekian, K. K., Eds.; Elsevier Ltd.: Oxford, U.K., 2004; 9, 367-406. (5) Kawamura, K.; Steinberg, S.; Kaplan, I. R. Atmos. EnViron. 1996, 30, 1035. (6) Pen˜a, R. M.; Garcı´a, S.; Herrero, C.; Losada, M.; Va´zquez, A.; Lucas, T. Atmos. EnViron. 2002, 36, 5277. (7) Avery, G. B., Jr.; Kieber, R. J.; Witt, M.; Willey, J. D. Atmos. EnViron. 2006, 40, 1683. (8) dos Santos, M. A.; Illanes, C. F.; Fornaro, A.; Pedrotti, J. J. Water, Air, Soil Pollut. 2007, 7, 85. (9) Kawamura, K.; Ikushima, K. EnViron. Sci. Technol. 1993, 27, 2227. (10) Sempere, R.; Kawamura, K. Atmos. EnViron. 1994, 28, 449. (11) Mochida, M.; Kawabata, A.; Kawamura, K.; Hatsushika, H.; Yamazaki, K. J. Geophys. Res. 2003, 108, 4193. (12) Mader, B. T.; Yu, J. Z.; Xu, J. H.; Li, Q. F.; Wu, W. S.; Flagan, R. C.; Seinfeld, J. H. J. Geophys. Res. 2004, 109, D06206. (13) Hoffmann, T.; Warnke, J. Organic aerosols. In Volatile Organic Compounds in the Atmosphere; Koppmann, R., Ed.; Blackwell Publishing Ltd.: Oxford, U.K., 2007; pp 342-387. (14) Warneck, P. Atmos. EnViron. 2003, 37, 2423. (15) Norton, R. B.; Roberts, J. M.; Huebert, B. J. Geophys. Res. Lett. 1983, 10, 517.

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