Article pubs.acs.org/JPCA
Ozone Dissociation to Oxygen Affected by Criegee Intermediate Wen-mei Wei,† Ren-hui Zheng,*,‡ Yue-li Pan,† Yun-kai Wu,† Fan Yang,† and Shi Hong† †
Department of Chemistry, College of Basic Medicine, Anhui Medical University, Hefei, Anhui 230032, P. R. China Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, P. R. China
‡
ABSTRACT: The detailed potential energy surfaces for the reactions of Criegee intermediate (CI, H2COO) and formaldehyde (H2CO) with ozone (O3) have been investigated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(2d,2p) level of theory, respectively. New alternative reaction mechanisms, to the one previously proposed (J. Phys. Chem. Lett. 2013, 4, 2525) have been found. The lower barrier of the new mechanism shows that it is easy for H2COO + O3 to dissociate to formaldehyde and oxygen. For the reactions of H2CO with O3 to produce H2COO and O2, we find relatively high energy barriers, which makes the ozone dissociation to oxygen unlikely to be catalyzed by CI.
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INTRODUCTION Ozone is very important in the atmosphere to absorb UV light that may be harmful to humans and living creatures on the earth. Ozone in the stratosphere can be destroyed by chlorine (Cl) and bromine (Br) free radicals from the chemical material freon. Most recently, Kjaergaard et al.1 theoretically found that H2COO (Criegee intermediate, formaldehyde oxide) can react with ozone to produce H2CO (formaldehyde) and molecule O2 (oxygen), which may cause the loss of ozone and Criegee intermediate (CI). In their studies, the key intermediate (H2CO5) is a six-membered ring structure with one carbon atom and five oxygen atoms. They tried many methods to find the transition state connecting the reactant (CI and ozone) and the key intermediate. They also successfully obtained the corresponding saddle point with the RB3LYP/aug-cc-pVTZ method, which is not a stationary point by the UB3LYP method. The other high level methods can not solve the problem.1 Here, we try to get the corresponding transition state using density functional theory (DFT). CIs are produced through the reaction of ozone with alkenes.2 H2COO is the simplest CI, and it can produce the stable molecules include HC(O)H, CO, CO2, H2O, and HC(O)OH from unimolecular reaction.3 Recently, experimental and theoretical studies indicated that the CI could initiate oxidation reactions just as the small chemical molecules such as Cl, Br, O3, OH, and NO3 do.4−9 CIs can also react with NO2,3 SO2,4,5,10 H2O,5 or carbonyl compounds through bimolecular reactions.11 Besides the above studies, there are some interesting investigations on CIs recently.12−18 In ref 1, H2CO is formed after the reaction of H2COO with O3. The further reactions of the products H2CO with O3 are investigated in this article. In ref 19, Wang et al. theoretically studied the reaction pathways of H2CO and O3. They found that the products H2COO and O2 from the reactants are through a © 2014 American Chemical Society
chain-typed transition state with a barrier of 42.6 kcal/mol by BHandHLYP computation and 63.5 kcal/mol by BMC−CCSD method. We want to investigate whether there is a transition state with lower barrier for the products O2 and H2COO, where H2COO can be a catalyst to make ozone dissociation to oxygen just as Cl and Br free radicals do. The total reaction can be given as follows:
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H 2COO + O3 → CH 2O + 2O2
(R1)
H 2CO + O3 → H 2COO + O2
(R2)
COMPUTATIONAL DETAILS Using density functional theory (B3LYP/6-311++G(2d,2p),20,21 we optimize the structures for the reactants, products, transition states (TS), and intermediates (IM). Then we calculate their vibrational frequencies and the corresponding zero-point energies (ZPE), which are scaled by a factor of 0.96.22 When all the frequencies are real, the molecule is taken to be a reactant, intermediate, or product; when there is only one imaginary frequency, the molecule is taken as a transition state. Intrinsic reaction coordinate (IRC)23 computations are applied to confirm the transition states connecting proper reactants and products. With coupled-cluster theory (CCSD(T)/aug-ccpVDZ),24,25 we calculate the electronic energies. The corrected relative Gibbs free energies, GCCSD(T)/aug‑cc‑pVDZ, are applied in the following energy discussion (in the unit of kcal/mol) if there is no special explanation. The correction is GCCSD(T)/aug‑cc‑pVDZ = ECCSD(T)/aug‑cc‑pVDZ + [GB3LYP/6‑311++G(2d,2p) − EB3LYP/6‑311++G(2d,2p)] + ZPE. Received: October 13, 2013 Revised: February 13, 2014 Published: February 14, 2014 1644
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Figure 1. Geometric parameters of related molecules on the H2COO + O3 energy surfaces at the B3LYP/6-311++G(2d,2p) level of theory. Bond lengths are in angstroms; bond angles are in degrees.
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RESULTS AND DISCUSSION Reactions of H2COO with O3. Three pathways (including seven channels) to produce H2CO and two 3O2 have been
When we calculate the Gibbs free energy, the pressure is taken as 1 atm and the temperature is 298.15 K. All the computations are done with the software Gaussian09.26 1645
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studied for the reactions of H2COO with O3. The optimized geometries of stationary points are depicted in Figure 1. The relative electronic energies and Gibbs free energies at CCSD(T)/ aug-cc-pVDZ level, zero-point vibrational energies, entropies, and Gibbs free energies at the B3LYP/6-311++G(2d,2p) level are listed in Table 1, and the energies of the reactions of H2CO with O3 are listed in Table 2.
Table 2. Zero-Point Vibrational Energies (ZPE, in kcal/mol), Entropy S (in cal mol−1 K−1), Gibbs Free Energies (ΔG1, in kcal/mol) at 298.15 K, and Relative Energies (RE1, in kcal/mol) of the Reactants, Intermediates, Transition States, and Products Calculated at the B3LYP/6-311++G(2d,2p) Level
Table 1. Zero-Point Vibrational Energies (ZPE, in kcal/mol), Entropy S (in cal mol−1 K−1), Gibbs Free Energies (ΔG1, in kcal/mol) at 298.15 K, and Relative Energies (RE1, in kcal/mol) of the Reactants, Intermediates, Transition States, and Products Calculated at the B3LYP/6-311++G(2d,2p) Level B3LYP/6-311++G(2d,2p) species
ZPE
a
S
ΔG1
RE1
H2COO + O3 IM1 IM2 IM3
23.2 26.4 26.4 27.0
117.8 73.5 73.5 70.8
0.0 −17.3 −17.3 −26.8
0.0 −32.7 −32.7 −43.3
TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10 H2CO + 23O2
24.5 24.5 24.5 24.7 24.5 24.3 26.1 24.8 24.0 24.0 20.5
80.3 75.7 76.3 76.1 76.3 76.7 72.3 75.7 78.1 78.1 151.6
12.0 −12.4 −13.1 17.2 −13.1 −6.8 0.3 1.1 −9.4 −9.4 −108.1
0.1 −25.6 −26.1 4.0 −26.1 −19.5 −14.9 −12.2 −21.6 −21.6 −96.6
CCSD(T)/aug-cc-pVDZ RE2b 0.0 −36.9 −36.9 −46.5 [−47.3]e 0.8 −21.1 −20.4 5.1 −20.4 −16.3 −22.8 −452.0 −14.9 −14.9 −92.8 [-87.7]
CCSD(T)/augcc-pVDZ
B3LYP/6-311++G(2d,2p)
ΔG2c d
0.0 −18.2 −18.2 −26.2[−32] 14.1 −6.6 −6.0 19.8 −6.1 −2.6 −4.7 −437.1 −2.0 −2.0 −106.9
species
ZPEa
S
ΔG1
RE1
RE2b
ΔG2c
H2CO + O3 p-IM1 p-IM2 p-IM3 p-IM4 p-TS1 p-TS2 p-TS3 p-TS4 p-TS5 p-TS6 p-TS7 p-TS8 H2COO + 3O2
20.4 24.4 23.7 24.4 23.7 22.9 21.7 23.2 22.5 22.9 21.7 23.2 22.5 21.0
111.8 69.9 72.5 69.9 72.5 68.7 76.9 69.4 72.1 68.7 76.9 69.4 72.1 108.6
0.0 16.7 46.5 16.7 46.5 26.1 53.0 57.4 55.5 26.1 53.0 57.4 55.4 8.0
0.0 1.4 32.3 1.4 32.3 12.1 41.8 43.3 42.5 12.1 41.8 43.3 42.5 6.5
0.0 1.9 12.0 1.9 12.0 11.9 41.7 9.0 47.4 11.9 41.7 9.0 47.4 13.2
0.0d 21.2 29.5 21.2 29.5 28.4 54.3 25.8 62.3 28.4 54.3 25.8 62.3 15.3
a
Scaled by a factor of 0.96. bThe relative energies obtained at the CCSD(T)/aug-cc-pVDZ level. cThe corrected relative Gibbs free energy obtained at the CCSD(T)/aug-cc-pVDZ level. G2 = ECCSD(T)/aug‑cc‑pVDZ + [GB3LYP/6‑311++G(2d,2p) − EB3LYP/6‑311++G(2d,2p)] + ZPE. dThe total energy is −339.191329 hartree.
a
Scaled by a factor of 0.96. bThe relative energies obtained at the CCSD(T)/aug-cc-pVDZ level. cThe corrected relative Gibbs free energy. G2 = E CCSD(T)/aug‑cc‑pVDZ + [G B3LYP/6‑311++G(2d,2p) − EB3LYP/6‑311++G(2d,2p)] + ZPE. dThe total energy is −414.110709 hartree. eThe values reported by Kjaergaard et al. are listed in square brackets.1
Pathway A. In this pathway, the reactants H2COO and O3 first combine to a six-membered ring intermediate IM1 via a transition state TS1 with a small imaginary frequency of 196.8 cm−1. In the structure of TS1, the forming C1···O8 and O5···O6 distances are shortened to 2.039 and 3.423 Å, respectively. The schematic profile of the potential energy surface for pathway A is plotted in Figure 2, which shows that this process has a low energy barrier of 14.1 kcal/mol, indicating that it is easy to form IM1. Then IM1 can dissociate to the final products H2CO and 3O2 via two paths. The first one is via a six-membered ring transition state TS2 with an imaginary frequency of 243.8 cm−1, and the second one is through the other six-membered ring transition state TS3 with an imaginary frequency of 355.3 cm−1. From Figure 1 we can see that the structures of TS2 and TS3 are different. In TS2, the O4O5, O6O7, and C1O8 bonds are elongated to 1.982, 2.124, and 1.823 Å, respectively, while the corresponding bonds in TS3 are stretched to 2.011, 2.017, and 1.872 Å, respectively. The energy barriers of these two paths are only 11.6 and 12.2 kcal/mol, respectively. Among all the paths investigated, the first path has the lowest barriers. Thus, it is the most feasible channel for IM1 decomposition. Furthermore, because the barrier of the second path is only 0.6 kcal/mol higher
Figure 2. Potential energy diagram for pathway A. The most feasible recombination routes are delineated in bold lines. The energies are in kcal/mol.
than that of the first one, it is also a feasible channel for IM1 dissociation. We also calculate the spin contamination indicated by the ⟨S2⟩ value for the product 3O2, which is 2.01 before annihilation and 2.00 after annihilation based on UB3lYP/ 6-311++G(2d,2p) optimized geometry, and the corresponding multireference diagnostic value T1 is 0.0177 based on UCCSD(T)/ aug-cc-pVDZ//UB3lYP/6-311++G(2d,2p) calculation. The T1 diagnostic values of all the molecules are listed in Table 3. From Table 3, we find that the T1 values for many of the TS structures are large. Thus, the multireference character of these reactions should be included. The calculations also show that the T1 values of O3, IM1, and IM2 are 0.02760768, 1646
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Table 3. T1 Diagnostic Values for the Reaction between H2COO and O3 and the Reaction between H2CO and O3 Using the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(2d,2p) Method H2COO
O3
IM1
IM2
IM3
TS1
TS2
TS3
0.0456 TS4
0.0276 TS5
0.0276 TS6
0.0276 TS7
0.0211 TS8
0.0502 TS9
0.0488 TS10
0.0436 H2CO
0.0469 3 O2
0.0437 p-IM1
0.0649 p-IM2
0.0213 p-IM3
0.2275 p-IM4
0.0313 p-TS1
0.0313 p-TS2
0.0164 p-TS3
0.0177 p-TS4
0.0210 p-TS5
0.0319 p-TS6
0.0210
0.0407
0.0364
0.0400
0.0472
0.0407
0.0364
0.0319 p-TS7
p-TS8
0.0401
0.0472
On the basis of the discussions above, we can draw the conclusion that pathway A is the most feasible channel for the reactions of H2COO and O3 to produce H2CO and two 3 O2 and that the Gibbs free energy of the overall process is 106.9 kcal/mol. Pathway B. Similar to pathway A, the two reactants first form a six-membered ring intermediate IM2 via TS4, then IM2 decomposes to the final products via two different transition states, TS5 and TS6. The schematic profile of the potential energy surfaces for this pathway is shown in Figure 3. Notice that IM2 and IM1 are a pair of mirror image isomers. They almost have the same bond lengths and bond angles, opposite values of dihedral angles. For example, the dihedral angle among O5, O4, C1, and O8 atoms is 53.5° in IM1, while it is −53.5° in IM2. The dihedral angle among O6, O5, O4, and C1 atoms is −68.3° in IM1, while it is 68.3° in IM2. Thus, this pair of mirror image isomers has the same energies from the calculated results. The transition state TS4 has an imaginary frequency of 335.8 cm−1, and in its structure, the forming C1···O8 and O5··· O6 distances are reduced to 2.114 and 2.104 Å, respectively. The energy barrier for the formation of IM2 is 19.8 kcal/mol, which is 5.7 kcal/mol higher than that of IM1. There are two paths for IM2 dissociation, the first one is via a six-membered-ring transition state TS5, and the second one is over the other six-membered ring transition state TS6. The imaginary frequencies of TS5 and TS6 are 355.5 and 265.5 cm−1, respectively. In the structure of TS5, the O4O5, O6O7, and C1O8 bonds are stretched to 2.011, 2.016, and 1.872 Å,
Figure 3. Potential energy diagram for pathway B. The energies are in kcal/mol.
0.02762406, and 0.02761778, resectively. They are a little different, and the T1 value of TS8 is 0.2275, which is abnormally large. The corresponding energy of TS8 by CCSD(T) is also abnormal. Thus, the CCSD(T) energy by Gaussian for TS8 is incorrect.
Figure 4. Potential energy diagram for pathway C. The energies are in kcal/mol. aThe Gibbs free energies were calculated at the B3LYP/6-311++ G(2d,2p) level. 1647
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Figure 5. Geometric parameters of related molecules on the H2CO + O3 energy surfaces at the B3LYP/6-311++G(2d,2p) level of theory. Bond lengths are in angstroms; bond angles are in degrees.
The energy barrier of the first path is 12.1 kcal/mol, which is 3.5 kcal/mol lower than that of the second path.
respectively. In TS6, the C1O4, O5O6, and O7O8 bonds are lengthened to 1.773, 2.073, and 1.913 Å, respectively. 1648
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Pathway C. In this pathway, a new six-membered ring intermediate IM3 appears (see Figure 4), which has been reported by Kjaergaard et al.1 They reported that the formation of the interesting singlet cyclic adduct was found to be exothermic by 47.3 kcal/mol, and the values are calculated by UCCSD(T)/aug-cc-pVTZ//UCCSD(T)/cc-pVDZ.1 From Table 1 we can see that the formation of IM3 is exothermic by 46.5 kcal/mol at CCSD(T)/aug-cc-pVDZ level, in good agreement with Kjaergaard’s result. The relative Gibbs free energy (see Figure 4) we obtained is −26.2 kcal/mol, about 5.8 kcal/mol smaller than the value given by Kjaergaard et al. (ΔG = −32 kcal/mol). Kjaergaard et al. 1 found the corresponding transition with RB3LYP/aug-cc-pVTZ computation, which is not confirmed by the UB3LYP method. Thus, they could not determine whether there is a saddle point connecting this intermediate and reactants. In this study, we have also tried our best to find the transition state for the reactants to form IM3 directly, but we fail. However, we have found the saddle point, TS7, from the intermediate IM2 to form IM3. This process has an energy barrier of 13.5 kcal/mol. In a word, the mechanism to produce IM3 is divided into two steps: the reactants first combine to form IM2 via TS4, then IM2 rearranges to produce IM3 via TS7. Three paths have been investigated to obtain the final products from IM3. The saddle points are named TS8, TS9, and TS10, respectively. In TS8 and TS10, the breaking bonds are O4O5, O6O7, and C1O8, yet in TS9 the rupture bonds are C1 O4, O5O6, and O7O8. We have to say there is something strange about the energy of TS8. The calculated absolute electronic energy of TS8 is −414.8585352 hartree at the CCSD(T)/aug-cc-pVDZ level; thus, the relative energy is abnormally −452.0 kcal/mol and the relative Gibbs free energy is −437.1 kcal/mol. After checking the output file carefully, we find that the CCSD energy is −414.0781485 hartree, and it is impossible for these two energies to have such a large difference. Thus, we infer that maybe there is something wrong about the CCSD(T) energy. Then, in Figure 4, we plot the Gibbs free energy calculated at the B3LYP/6-311++G(2d,2p) level. The barriers for IM3 dissociation to form the final products via TS9 or TS10 are both 24.2 kcal/mol. Reactions of H2CO with O3. In order to study whether ozone dissociation to oxygen can be catalyzed by Criegee intermediates, we carry out further studies on the reactions of the product H2CO with O3. Four intermediates and two pathways to obtain H2COO and 3O2 have been investigated. Figure 5 depicts the geometries of stationary points. The corresponding data are listed in Table 2. Pathway D. In this pathway, the reactants first connect to form an intermediate p-IM1 with five oxygen atoms in the same ring via a five-membered ring transition state p-TS1 with an imaginary frequency of 382.3 cm−1. In the structure of p-TS1, the forming C1···O7 and O4···O5 distances are shortened to 1.727 and 2.002 Å, respectively. The energy barrier of this process is 28.4 kcal/mol (see Figure 6). Then p-IM1 dissociates to the products H2COO and 3O2 via p-TS2 with a relative lower barrier of 33.1 kcal/mol, which indicates that this is a competing channel for p-IM1 decomposition. In p-TS2, the opening C1O7 and O5O6 bonds are lengthened to 3.234 and 2.314 Å, respectively. This loss of 3O2 reaction from p-IM1 is endothermic by 5.9 kcal/mol, and the whole process is endothermic by 15.3 kcal/mol. p-IM1 can isomerize to another intermediate, p-IM2, via pTS3 with a low barrier of 4.6 kcal/mol. The breaking O5O6
Figure 6. Potential energy diagram for pathway D. The most feasible recombination routes are delineated in bold lines. The energies are in kcal/mol.
bond in p-TS3 is elongated to 3.379 Å. Note that the energy of pTS3 is abnormally 3.7 lower than p-IM2, which may be caused by the calculation inaccuracy. Finally, p-IM2 decomposes to the products via p-TS4 with a small imaginary frequency of 205.9 cm−1. The rupture C1O4 bond in p-TS4 is stretched to 2.074 Å. Because this channel has the lowest energy barrier of 32.8 kcal/mol, it is the most feasible path for the title reactions. Wang et al. have investigated the potential energy surfaces for the reaction of H2 CO and O3 at the BMC−CCSD// BHandHLYP/6-311+G(d,p) level.19 The mechanism that they obtained to produce H2COO and 3O2 is via a chain-typed transition state (H2C−O···O···O−O) with a high barrier of 63.5 kcal/mol. However, the lowest energy path through the five-membered ring intermediate we obtained has a barrier of 32.8 kcal/mol, about 30.7 kcal/mol lower than the Wang’s values.19 Thus, we think that our obtained path is more feasible than that by Wang et al. to produce H2COO and 3O2. The relative energy of the products we calculated is 15.3 kcal/mol, which agrees very well with the value of 15.11 kcal/mol given by Wang et al.19 However, according to Wang’s studies, the most feasible path for the reaction of H2CO with O3 is to produce HCO and HOOO via a transition state with a barrier of 18.34 kcal/mol,19 which is much lower than our result (32.8 kcal/mol). Then the major products are HCO and HOOO, not H2COO and 3O2. Thus, ozone dissociation to oxygen cannot be catalyzed by CI. Pathway E. This pathway has the similar process to pathway D. The intermediates and transition states that appear in this pathway are the corresponding mirror images in pathway D. Thus, we shall not discuss and present them further.
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CONCLUSIONS The results discussed above show that the lowest energy path for the reactions of H2COO with O3 is as follows (the energy barriers are listed in parentheses): TS1
TS2
H 2COO + O3 ⎯⎯⎯→ IM1 ⎯⎯⎯→ H 2CO + 23O2 (14.1 kcal/mol, imposed by TS1)
The following channel is the competitive one: TS1
TS3
H 2COO + O3 ⎯⎯⎯→ IM1 ⎯⎯⎯→ H 2CO + 23O2 (14.1 kcal/mol, imposed by TS1)
Because of the small energy barriers, these reactions easily occur in the atmosphere. 1649
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For the reactions of the product H2CO with O3 to form H2COO again, the lowest energy paths we studied are p‐TS1
p‐TS3
p‐TS4
H 2CO + O3 ⎯⎯⎯⎯⎯→ p‐IM1 ⎯⎯⎯⎯⎯→ p‐IM2 ⎯⎯⎯⎯⎯→ H 2 COO + 3O2
(32.8 kcal/mol, imposed by p‐TS4)
p‐TS5
p‐TS7
p‐TS8
H 2CO + O3 ⎯⎯⎯⎯⎯→ p‐IM3 ⎯⎯⎯⎯⎯→ p‐IM4 ⎯⎯⎯⎯⎯→ H 2 COO + 3O2
(32.8 kcal/mol, imposed by p‐TS8)
The competing channels are p‐TS1
p‐TS2
H 2CO + O3 ⎯⎯⎯⎯⎯→ p‐IM1 ⎯⎯⎯⎯⎯→ H 2COO + 3O2 (33.1 kcal/mol, imposed by p‐TS2) p‐TS5
p‐TS6
H 2CO + O3 ⎯⎯⎯⎯⎯→ p‐IM3 ⎯⎯⎯⎯⎯→ H 2COO + 3O2 (33.1 kcal/mol, imposed by p‐TS6)
These reactions have relatively higher energy barriers; thus, they are more difficult to have occur in atmosphere. Therefore, ozone can react with CI to produce formaldehyde. However, their decomposition cannot be catalyzed by CI. The reaction of CI with ozone to produce formaldehyde can cause the loss of CI and ozone. However, further reaction of formaldehyde with ozone is difficult to make happen, which indicates that CI can not severely destroy ozone just as Cl and Br free radicals do. CI is produced from the reaction of ozone with alkenes. Hence, the alkenes are not as dangerous as freon gas.
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AUTHOR INFORMATION
Corresponding Author
*(R.Z.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (NNSF) (No. 21103003 and 20903101). REFERENCES
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