Theoretical study on mechanisms and pathways of the atmospheric CF3O2 + BrO reaction

Journal of Fluorine Chemistry 180 (2015) 110–116

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Theoretical study on mechanisms and pathways of the atmospheric CF3O2 + BrO reaction Yizhen Tang a,*, Wei Zhang a, Zhiwen Song a, Xiangyu Wang a, Jingyu Sun b, Rongshun Wang c a b c

School of Environmental and Municipal Engineering, Qingdao Technological University, Fushun Road 11, Qingdao, Shandong 266033, PR China College of Chemistry and Chemical Engineering, Hubei Normal University, Cihu Road 11, Huangshi, Hubei 435002, PR China Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Renmin Road 5268, Changchun, Jilin 130024, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 26 April 2015 Received in revised form 31 August 2015 Accepted 7 September 2015 Available online 9 September 2015

Using quantum chemistry methods, mechanisms and reaction pathways of the atmospheric CF3O2 + BrO reaction have been investigated. The result indicates that the most important products include CF3OOOBr, CF3OOBrO and CF3O + BrOO in the atmospheric conditions below 300 K. While other products including CF3OBrO2, CF2O + FBrO2, CF2O + FOBrO, CF2O2 + FOBr, CF2O + FOOBr, CF3OOBr + O(3P), CF3OBr + O2 (1D) and CF3O + OBrO are negligible due to high barriers and/or unstable formations. Moreover, some roles relative to hydrogen are found in the CX3O2 + BrO (X = H and F) reactions; while halogen substitution makes a certain contribution to the CF3O2 + YO (Y = Cl, Br and I) reactions. ß 2015 Elsevier B.V. All rights reserved.

Keywords: CF3O2 Atmospheric reaction Bromine oxide Halogen substitution

1. Introduction CF3O2 radical was formed in the atmosphere from the reaction of O2 with CF3 radical, which is generated in stratosphere via the photolysis of CF3Cl and CF3Br, and in troposphere by the degradation of CF3 containing hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Although its concentration is much lower than that of HO2 and CH3O2 radicals, and might be only a few pptv in the atmosphere, researches indicate that CF3O2 radicals cause the formation and breakage of ozone, and have an important effect on ozonosphere. The reactions of CF3O2 with many radicals and molecules have been investigated experimentally [1–13]. And more attentions have been paid to mechanisms or kinetics theoretically recent years [14–20]. It is known that BrO, several pptv in the atmosphere, plays an important role in O3 destruction as well, and its concentration increases rapidly in recent years. Moreover, the known catalytic reactions between bromine monoxide and peroxy radicals (HO2 and CH3O2) leading to destruction of ozone layer in the upper atmosphere have been investigated extensively by experimental and theoretical methods [21–26]. However, no literature was found involving CF3O2 with BrO reaction, no matter the possible products or reaction channels.

* Corresponding author. Tel.: +86 532 85071262. E-mail address: [email protected] (Y. Tang). http://dx.doi.org/10.1016/j.jfluchem.2015.09.005 0022-1139/ß 2015 Elsevier B.V. All rights reserved.

Considering the significant roles of CF3O2 and BrO in the atmosphere, the possible reaction pathways and the products along with some sequence reactions are necessary to be probed for the CF3O2 + BrO reaction to reveal their atmospheric chemistry. Thus, the CF3O2 + BrO reaction is investigated theoretically using quantum chemistry methods for the first time. We expect to give a detailed description of the title reaction, provide useful information for further experimental researches, and understand the reactivity of these species in the atmospheric conditions.

2. Computational methods The geometries of reactant (R), products (P), intermediates (IM), and transition states (TS) were optimized at the B3LYP/6311++G(d,p) levels of theory. Harmonic vibrational frequencies calculations were performed at the same level of theory in order to determine the nature of the various stationary points, as well as zero-point-energy (ZPE) corrections. All stationary points have been identified for minimum with number of imaginary frequencies (NIMAG = 0) or transition states (NIMAG = 1). The transition states were verified by intrinsic reaction coordinate (IRC) calculations to connect the designated reactants and products. In order to obtain more reliable relative energy of each stationary point on the potential energy surface (PES), single-point energy calculations have been refined by CCSD(T)/6-311++G(2d,2p). All

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the calculations were carried out using the GAUSSIAN 09 program package [27]. 3. Results and discussions Geometries of all reactants, products and possible intermediates (IM) involved in the title reaction are shown in Fig. 1, and all possible transition states (TS) are exhibited in Fig. 2. The energy profile of the singlet and triplet PESs for the CF3O2 + BrO reaction at the CCSD(T)//B3LYP level is depicted in Fig. 3. The possible reaction routine is drawn in Fig. 4 to clarify the reaction process. Table 1 exhibits the relative energies and ZPE corrections at the B3LYP and CCSD(T) levels for all species involved in the title reaction. For the current radical-radical reaction, it can take place on both the

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singlet and triplet PES, in order to differentiate the singlet and triplet species, the triplet species are signed as 3 as the superscript. The relative energy obtained at the CCSD(T) level is used in the following discussion unless otherwise stated. 3.1. Adducts formed in the CF3O2 + BrO reaction on the singlet PES Calculations show that two adducts, CF3OOOBr (IM1) and CF3OOBrO (IM2), are formed directly on the singlet PES by addition of BrO and CF3O2 radicals. As shown in Fig. 1, the O–Br bond is 1.944 A˚ in IM1, and the BrOO angle is about 112.78. In IM2 the newly formed Br–O bond is 1.972 A˚, while the ‘‘original’’ Br–O bond is shorten by 0.076 A˚ from that in reactant. Calculation shows that IM1 and IM2 can interconvert to each other via a triangular

Fig. 1. The optimized geometries for all reactants, products and intermediates (IM) in the CF3O2 + BrO reaction at the B3LYP/6-311++G(d,p) level. Bond distances are in angstrom and bond angles are in degrees.

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Y. Tang et al. / Journal of Fluorine Chemistry 180 (2015) 110–116

Fig. 2. The optimized geometries for all transition states (TS) in the CF3O2 + BrO reaction at the B3LYP/6-311++G(d,p) level. Bond distances are in angstrom and bond angles are in degrees.

transition state TS1. The broken O–O bond is 2.328 A˚ in TS1, about 73.6% longer than that in IM1; while the formed Br–O bond is stretched by 0.225 A˚ from the equilibrium distance in IM2. Simultaneously, the terminal Br–O bond is reduced to be the equilibrium length of that in isolated BrO radical. The barrier height for IM1 to IM2 is about 113 kJ/mol; however, TS1 is 33.6 kJ/ mol higher than the initial reactants, which is moderate to overcome in the atmospheric conditions. Energetically, IM1 and IM2 are about 79.4 and 15.2 kJ/mol lower than the initial reactants, respectively. Due to different dihedral angles of BrOOO (OBrOO) and OOOC (BrOOC) with internal rotation around the Br–O and O–O bonds in IM1 and IM2, it is mentioned that several conformers of CF3OOOBr and CF3OOBrO exist. According to our calculations, the conformers of IM2, namely, IM2a and IM2b, were involved in title reaction. In IM2a and IM2b, the terminal Br–O bond is about 1.68 A˚, which is close to that in IM2. The O–O bond and the middle Br–O bonds

change from IM2, IM2a and IM2b. The stabilization energy of IM2a and IM2b is around 15 kJ/mol. With around 79.4 and 15 kJ/mol available as internal energy CF3OOOBr and CF3OOBrO might take place further reactions before being quenched by collisions. Several conceivable isomerization channels are located, and will be discussed in the following parts. Due to structure limitation CF3OOOBr cannot occur further isomerization expect TS1 stated above, and therefore no more consideration was made from CF3OOOBr (IM1). With bromine atom migrating from the central oxygen atom to the oxygen atom connected with CF3 group, CF3OOBrO (IM2) can isomerize to CF3OBrO2 (IM3) via TS2, in which the formed Br–O bond is 2.547 A˚, and the broken O–O bond is stretched to be 2.157 A˚. Simultaneously, the central Br–O bond is shorten by 0.3 A˚ from its equilibrium distances in IM2. On the other hand, the two non-reactive Br–O bonds are close to that in IM3, taking values around 1.63 A˚; and the newly formed Br–O bond is 2.020 A˚. IM3 is

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product formed, even in high temperature and pressure conditions. Therefore, further isomerization and dissociation reaction starting from IM4 will be not considered here. Thus, with abundant internal energy available from intermediates except for IM4, many possible dissociation reaction channels are determined, and we will give detailed description in the following part. 3.2. Reaction channels of the CF3O2 + BrO reaction on the singlet PES

Fig. 3. The energetic reaction routes of the CF3O2 + BrO reaction on singlet and triplet PES at the CCSD(T)//B3LYP level.

rather stable, with its relative energy of 47.3 kJ/mol lower than the initial reactants, and about 32.1 kJ/mol lower than IM2. The activation energy of TS2 is 70.5 kJ/mol; the barrier height of this isomerization step reaches to be 85.7 kJ/mol. After IM3 formed, it is possible to undergo further isomerization giving out CF3BrO3 (IM4), with Br atom shifting from O atom to C atom via a three-membered-ring transition state TS3. The broken C–O bond is elongated to be 2.374 A˚; while the formed C–Br bond is 0.366 A˚ longer than its equilibrium distance in IM4. With stable C–O bond destroyed in IM3 and weak C–Br bond formed in IM4, it is reasonable that the barrier height of IM3 ! IM4 reaches to be 412.1 kJ/mol, and TS3 is even 364.8 kJ/mol higher than the initial reactants. IM4 is hereby also unstable with 247.0 kJ/mol on the PES, about 294.3 kJ/mol higher than IM3. Obviously, the above process is different to take place with so high barrier and unstable

After IM1, IM2 and IM3 formed, it is possible to undergo further dissociation reactions to form different products, besides IM1 and IM2 go back to reactants directly via barrierless processes. The result shows that four scenarios from IM1, four from IM2 and two from IM3 involved in the title reaction. Firstly, the direct cleavage of O–O bond in IM1 leading to CF3O + BrOO without any barrier, confirmed by scanning calculations of energy variations along with O–O bond. The formation of CF3O + BrOO is exothermic by 43.1 kJ/mol at the B3LYP level, and its relative energy is 29.1 kJ/mol lower than initial reactants at the CCSD(T) level. It should be noted that BrOO is rather unstable and readily decomposes to releasing radical bromine, which plays a significant role in the atmosphere to deplete ozone as well. Starting from IM1 two similar four-centered-ring transition states (TS4 and TS5) are located with cleavage of O–O bond and one F atom migration from CF3 group to O atom producing CF2O + FOOBr. The ruptured O–O bonds are stretched to be 2.584 and 2.278 A˚ in TS4 and TS5, respectively. The formed F–O bonds are as long as 2.049 and 2.029 A˚, respectively, while the broken C–F bonds are elongated by 0.3 A˚ from that in IM1. It is noticed that the terminal Br–O bond in TS4 and TS5 is 2.041 and 2.084 A˚, close to the equilibrium length of 2.011 A˚ in final product FOOBr. Energetically, TS4 and TS5 are 139.7 and 150.5 kJ/mol, and the barrier heights are 219.1 and 229.9 kJ/mol, respectively. The relative energy of CF2O + FOOBr is 3.5 kJ/mol, and the overall channel is exothermic by 13.1 kJ/mol at the B3LYP level. With so high barriers surmounted it is apparently of no importance under

Fig. 4. Reaction routes of the CF3O2 + BrO reaction on the singlet and triplet PES.

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Table 1 The zero-point energy correction (ZPE), relative energies (DE) and reaction enthalpies (DH) (in kJ/mol) of various species at the different methods and basis sets in the CF3O2 + BrO reaction. Species

CF3O2 + BrO CF3BrO + O2(3S) CF3BrO + O2(1D) CF3OBr + O2(1D) CF3OOBr + O(3P) CF3O + BrOO CF3O + OBrO CF2O + FOOBr CF2O + FOBrO CF2O + FBrO2 CF2O2 + FOBr IM1 IM2 IM2a IM2b IM3 IM4 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10 TS11 TS12 3 TS1 3 TS2 3 TS3

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

B3LYP/6-311++G(d,p)

DE

DE

DH

ZPE

0.0 53.7 182.1 90.9 86.5 29.1 58.3 3.5 88.2 41.7 304.6 79.4 15.2 15.3 15.4 47.3 247.0 33.6 70.5 364.8 139.7 150.5 251.0 102.6 214.6 130.3 329.7 381.9 302.4 267.4 124.9 191.7

0.0 56.7 218.0 47.8 126.1 45.0 73.9 13.6 90.7 16.8 295.9 66.2 12.0 12.3 12.3 3.0 301.7 80.2 108.3 407.4 102.0 111. 3 251.2 120.6 213.9 152.2 271.8 357.7 292.4 209.8 111.5 195.8

0.0 59.6 220.9 47.0 127.5 43.1 74 13.1 92.2 16.6 296.8 67.8 11.3 11.6 11.6 3.7 301.1 77.8 107.0 406.6 101.8 111.3 250.6 119.9 212.5 152.5 272.6 360.7 293.7 215.2 111.7 196.2

56.2 51.6 51.6 57.3 55.7 52.3 52.1 55.4 52.6 55.7 52.8 63.0 61. 4 61. 4 61.4 61.3 60.2 60.8 57.6 56.2 55.9 56.3 56.3 55.1 57.8 54.9 55.3 52.0 53.6 48.4 53.3 52.3

the atmospheric conditions below 300 K, and two channels via TS4 and TS5 could be neglected to the overall reaction. With a concerted step involving O atom in BrO group shifting to the C center and rupture of the C–O and O–O bonds, CF3OBr + O2(1D) is generated via TS6, which is about 251 kJ/mol higher than the initial reactants. In TS6 the broken O–O and C–O bond is stretched by 0.553 and 0.816 A˚ from that in IM1, respectively. The newly formed C–O bond is elongated by 0.791 A˚ from that in isolated CF3OBr molecule. With stable C–O and O–O bonds destroyed it is easily to be understood that TS6 is rather unstable. However, the relative energy of CF3OBr + O2(1D) is 90.9 kJ/mol, about 11.5 kJ/mol lower than IM1. However, this channel is difficult to occur kinetically in the atmospheric conditions due to high barrier. CF3OBr + O2(1D) also could be formed via other channels discussed later. Here it is mention that singlet O2(1D) is difficult to be described with single determinant wave function method in calculations. Considering the methods employed in this work, the uncertainty may cause overestimation its energy. Fortunately, all singlet O2 channels are out of significance due to high barriers involved. From IM2 the first possible dissociation with barrierless O–O bond cleavage will give out CF3O + OBrO, which is 58.3 kJ/mol on the PES energetically. With unstable products formed, CF3O2 + BrO ! IM2 ! CF3O + OBrO channel is unfavorable thermodynamically. Similar to IM1, a concerted step could also occur from IM2 via TS7 leading to CF2O + FBrO2. In TS7, with F atom migrating from C atom to Br atom to produce FBrO2, the broken C–F bond is stretched to be 1.628 A˚, and the formed F–Br bond reaches to

2.393 A˚. The O–O bond becomes longer and longer generally and then dissociates with its bond length around 2.274 A˚. The IRC calculations indicate that TS7 is connected with IM2a and CF2O + FBrO2. The barrier height of this step is 117.9 kJ/mol, and TS7 is 102.6 kJ/mol. The product CF2O + FBrO2 is about 41.7 kJ/mol lower than the initial reactants. From IM2 another transition state TS8 was located to produce CF3OBr + O2(1D), involving cleavage of the Br–O and C–O bonds with terminal O atom approaching to C center. In TS8, a fivemember-ring structure, the broken C–O and Br–O bond is elongated to be 2.027 and 2.794 A˚, respectively; and the formed C–O bond is 2.046 A˚. IRC calculation suggests that the right reactant connected by TS8 is IM2b, one of conformers from CF3OOBrO. The barrier of TS8 (230 kJ/mol) is too high to be overcome, and thus this channel is negligible to the overall reaction. The last product from IM2 is CF2O2 + FOBr generated via TS12, which barrier is about 317.6 kJ/mol. In TS12 the C–F (2.224 A˚) and middle Br–O bond (2.951 A˚) is broken and F–O bond (1.632 A˚) is formed. Although Criegee intermediate is focused by many researchers recently, no attention was paid to their halogen substitutions. The relative energy of CF2O2 + FOBr is high to be 304.6 kJ/mol, and could be negligible in the atmosphere. Certainly, with high barriers and/or unstable production all channels from IM2 are really hard to take place under the ‘‘normal’’ atmospheric condition. Once IM3 (CF3OBrO2) formed, it is also feasible to give out CF3O + OBrO with rupture of the Br–O bond. No transition state was obtained in this process although many attempts made. From IM3 only TS9 was located to form CF2O + FOBrO. In TS9 the broken O–Br and C–F bond is elongated to be 2.583 and 1.923 A˚, respectively; and C–F bond is extended to 1.998 A˚. The barrier is 177.6 kJ/mol, and the relative energy of TS9 reaches to 130.3 kJ/ mol. As stated above it is concluded due to high barriers and/or unstable products, all pathways from IM3 are of no significance in the atmosphere, and even in high temperature conditions like combustion. On the singlet PES, substitution mechanism was found to produce CF3OBr + O2(1D) and CF3BrO + O2(1D). The corresponding transition states are TS10 and TS11, in which the newly formed C–O/C–Br bond takes values of 2.132 and 2.679 A˚; and the broken C–O bond is stretched by 0.477 and 0.701 A˚ from its equilibrium distance in reactant CF3O2, respectively. The barriers of TS10 and TS11 are high to be 329.7 and 381.9 kJ/mol, respectively. At the B3LYP//6-311++G(d,p) level, production of CF3OBr + O2(1D) is exothermic by 47 kJ/mol; while CF3BrO + O2(1D) is endothermic by 220.9 kJ/mol. With so high barriers it is apparently that the two substitution channels play no importance in the atmosphere. From the reaction channels discussed above, it is presumed that the predominant products should be CF3OOOBr (IM1), CF3OOBrO (IM2) and CF3O + BrOO in the atmosphere below 300 K. With high barriers or/and unstable products, other dissociation channels from these intermediates and substitution pathways are difficult to undergo in the atmospheric conditions kinetically and/or thermodynamically. 3.3. Reaction channels of on the CF3O2 + BrO triplet PES On triplet PES, both substitution and abstraction mechanisms were located to form several products. As shown in Fig. 2, with transition states 3TS1, 3TS2 and 3TS3 surmounted CF3BrO + O2(3S), CF3O + OBrO and CF3OOBr + O(3P) were produced. The broken C–O or O–O bonds are stretched to be 2.350, 1.735 and 1.842 A˚; and the formed C–Br, O–O or O–Br bonds are elongated by 0.908, 0.202 and 0.358 A˚, in 3TS1, 3TS2 and 3TS3, compared with the corresponding bond length in products CF3BrO, OBrO and CF3OOBr, respectively.

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The energy of corresponding products is 53.7, 58.3 and 86.5 kJ/mol, and the barrier heights for the three transition states are 267.4, 124.9 and 191.7 kJ/mol, respectively. Although CF3O + OBrO formed via 3TS2 is possible to take place kinetically with lowest barrier on the triplet PES, it is difficult to occur actually in the atmosphere below 300 K. In a word, due to high barriers and unstable formation of products, all channels on the triplet PES are negligible under the normal atmospheric condition with low temperature (e.g., T < 300 K). Due to high barriers on the singlet and triplet PES, all reaction channels are out of significance except for CF3O2 + BrO ! CF3OOOBr ! CF3O + BrO2 and CF3O2 + BrO ! CF3OOBrO. It is noted that conical points may exist in the entrance and exit of intermediate IM1 to form biradicals, i.e., CF3O2 + BrO and CF3O + BrOO. In order to clarify this issue, we scanned the singlet PES along the O–O bond from its equilibrium bond length 1.341 (1.489) to 3.041(3.089) A˚ in IM1 for entrance and exit reactions, respectively. At the same time, the triplet PES along the identical distance between BrO (BrOO) and CF3OO (CF3O) was scanned with the same methods. The results indicate that a conical point is located in CF3OO. . .OBr being 2.016 A˚ for entrance, and CF3O. . .OOBr being 2.144 A˚ for exit, and the relative energy is about 81.6 and 30.0 kJ/mol above the initial reactants of CF3OO + BrO. Unfortunately, both conical points were not obtain with CASSCF method. However, it is true that singlet-triplet intersection exists for the entrance and exit reactions. Thus, on the analogy of the similar peroxyl radicals (HO2 and CH3O2) with BrO reactions, it is assumed that the overall rate constant should be around (1–10)  1012 cm3 molecule1 s1 under the normal atmospheric conditions. Moreover it is expected that the yield of CF3OOOBr and CF3OOBrO will be dominant at high pressure, and CF3O + BrOO is predominant at low pressure. Further experimental investigations on kinetics are expected. 3.4. Comparisons with the CH3O2 + BrO reaction Comparisons between PES features of the CF3O2 + BrO and its analogous reactions CX3O2 + YO (X = H, F; Y = Cl, Br, I) are important and useful to understand the halogen-substitution effect on these reactions. For the CH3O2 + BrO reaction, CCSD(T)//MP2 methods were used by Francisco [26]. They concluded that CH3OBrO2 is the most stable species, and the dominant products include CH3OOOBr and CH3OOBrO, which is in accordance with our result in this work. It is noted that the newest experiment by Shallcross et al. indicates that CH2OO + HOBr is the major product in the CH3O2 + BrO reaction [28], unfortunately, this reaction channel was not investigated by Francisco [26]. However, in our work CF2OO + FOBr plays of no importance in the title reaction with high barrier and unstable product. In order to check the dramatic difference in

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Fig. 5. The Mulliken charge on each atom of transition states in the CF3O2BrO ! CF2O2 + FOBr and CH3O3Br ! CH2O2 + HOBr reactions.

barriers between the two transition states, we have optimized transition state involving in the CH3O3Br ! CH2OO + HOBr reaction at the B3LYP/6-311++G(d,p) level. The Mulliken charge of all atoms in both transition states is listed in Fig. 5, in which it could be seen that the charge on F and H atom is 0.170 and 0.303e, respectively. While the reacting O atom is 0.167 and 0.369e in transition state of CF3O2BrO ! CF2OO + FOBr and CH3O3Br ! CH2OO + HOBr reactions, respectively. Thus it is reasonable to conclude that negative charged O atom prefers to react with positive charged H atom in the CH3O2 + BrO reaction rather than react with negative charged F atom in the CF3O2 + BrO reaction, and leading to different barriers involved in the two reactions. It indicates that F-substitution effect makes a certain contribution to the CH3O2 + BrO reaction. In our previous works, the reactions of CF3O2 + ClO [19] and CF3O2 + IO [20] were also studied theoretically with the same levels of theory. Comparisons indicate that similar PESs exist in the three reactions. The order of relative stability among the CF3OOOY isomers from the most stable to the least stable structure is CF3OYO2 > CF3OOOY > CF3OOYO (Y = Cl, Br, I). Comparing the relative energy of these relevant intermediates, it could be found that stability is in order of I > Br > Cl, indicating that the reactivity increases from Cl to I toward CF3O2 radicals. However, besides CF3O + YOO and CF3OOOY are predominant in these reactions, other significant products change obviously according to different halogen involved in YO. As for the CF3O2 + ClO reaction, other products are negligible to the overall reaction; and CF3OOBrO is dominant as well in the title reaction; while CF3OOIO, CF3OIO2 and CF2O + FIO2 in the CF3O2 + IO reaction are of significance in the atmospheric conditions. It is suggested that halogen substitution makes a certain contribution to CF3O2 + YO reactions.

Table 2 The excitation energy TV (in eV), oscillator strength f (in atomic units) and wavelength l (in nm) of the first ten excited states of CF3OOOBr, CF3OOBrO and CF3OBrO2. Excited states

1 2 3 4 5 6 7 8 9 10

CF3OOOBr

CF3OOBrO

CF3OBrO2

TV

f

l

TV

f

l

TV

f

l

2.78 3.40 4.70 5.01 5.12 5.33 5.49 5.72 6.29 7.11

0.0000 0.0000 0.0883 0.0039 0.0175 0.0009 0.0226 0.1640 0.0023 0.0009

445.67 365.15 264.03 247.33 242.05 232.52 225.66 216.62 197.12 174.26

1.79 3.87 4.12 4.42 5.41 5.72 5.82 5.94 6.50 6.86

0.0001 0.0100 0.0007 0.1289 0.0016 0.0155 0.0035 0.0347 0.0034 0.0087

690.62 320.70 301.18 280.38 229.06 216.85 213.16 208.70 190.82 180.71

3.58 3.77 4.02 4.90 5.20 5.82 6.01 6.09 6.35 6.49

0.0028 0.0113 0.0021 0.0159 0.1300 0.0007 0.0357 0.0147 0.0028 0.0011

345.99 329.13 308.56 252.89 238.30 212.98 206.20 203.64 195.28 192.26

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3.5. Vertical excitation energy TV of CF3OOOBr, CF3OOBrO and CF3OBrO2 As studied above intermediates IM1, IM2 and IM3 are stable, and they may take place photo-oxidation in the sunlight. In order to reveal their photolysis in the atmospheric conditions, we calculated the vertical excitation energy (TV) of the first ten excited states for CF3OOOBr, CF3OOBrO and CF3OBrO2 using TD-DFT method. i.e., B3LYP/6-311++G(d,p), and the result is listed in Table 2. It is known that the TV value is smaller than 4.13 eV (about 300 nm of threshold in the sunlight), the compound is considered to take photolysis in the sunlight. From Table 2 it could be seen that the TV value for the first two excited states of CF3OOOBr is 2.78, 3.40 eV, respectively, indicating the photolysis in the sunlight is feasible to undergo. However, the oscillator strength is 0.0, thus the photolysis does not happen actually. As for CF3OOBrO (CF3OBrO2), TV values of the first three excited states are smaller than 4.13 eV, i.e., 1.79 (3.58), 3.87 (3.77) and 4.12 (4.02) eV, certainly, both of them could photolyze in the sunlight. The oscillator strength of the second excited state is 0.01 for both species, implying strong photolysis happened at about 320 nm. It is speculated that CF3OOBrO and CF3OBrO2 might be one of the sources of bromine-containing compounds by their photolysis in the sunlight, and then take further reaction in bromine recycle in atmosphere. 4. Conclusions Mechanisms and channels for the atmospheric CF3O2 + BrO reaction on the singlet and triplet PESs were studied in details by CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) levels of theory. The result reveals that the addition and substitution mechanism was located on the singlet PES. The dominant products are CF3OOOBr, CF3OOBrO, and CF3O + BrOO. All channels on the triplet PES are out of significance with high barrier and unstable products. Moreover, TDDFT calculation indicates that CF3OOBrO and CF3OBrO2 take photolysis easily in the sunlight. The comparison implies that F-substitution effect plays a significant role on the CX3O2 + BrO (X = H and F) reactions. While halogen-substitution make a certain contributions to the CF3O2 + YO (Y = Cl, Br and I) reactions. Acknowledgements This work has been supported by the http://dx.doi.org/ 10.13039/501100001809National Natural Science Foundation of China (Nos. 21207075, 31170509 and 21507027) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM), the Shandong Youth Scientist Award Fund (BS2013HZ022), Shandong Province

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