Computational Study on the Mechanisms and Rate Constants of the Cl

Article pubs.acs.org/JPCA

Computational Study on the Mechanisms and Rate Constants of the Cl-Initiated Oxidation of Methyl Vinyl Ether in the Atmosphere Dandan Han, Haijie Cao, Mingyue Li, Xin Li, Shiqing Zhang, Maoxia He,* and Jingtian Hu Environment Research Institute, Shandong University, Jinan 250100, People’s Republic of China S Supporting Information *

ABSTRACT: The Cl-initiated oxidation reactions of methyl vinyl ether (MVE) are analyzed by using the high-level composite method CBS-QB3. Detailed chemistry for the reactions of MVE with chlorine atoms is proposed according to the calculated thermodynamic data. The primary eight channels, including two Cl-addition reactions and six H-abstraction reactions, are discussed. In accordance with the further investigation of the two dominant additional routes, formyl chloride and formaldehyde are the major products. Over the temperature range of 200−400 K and the pressure range of 100−2000 Torr, the rate constants of primary reactions are calculated by employing the MESMER program. H-abstraction channels are negligible according to the value of rate constants. During the studied temperature range, the Arrhenius equation is obtained as ktot = 5.64 × 10−11 exp(215.1/T). The total rate coefficient is ktot = 1.25 × 10−10 cm3 molecule−1 s−1 at 298 K and 760 Torr. Finally, the atmospheric lifetime of MVE with respect to Cl is estimated to be 2.23 h. pollutants, Cl atoms are also detected at a similar level.22 This high concentration of Cl atoms originates from the heterogeneous reaction of chloride in the coastal and highly polluted area. What is more, most organic compounds react with Cl atoms with rate constants higher than that with OH radical. Therefore, despite the lower concentration, chlorineinitiated reactions dominant the tropospheric degradation of VEs in some special area. Although numbers of investigations have been done on the reactions of chlorine atoms with organic compounds,12,23−26 only one study is about the reactions of VEs and atomic chlorine. Wang et al.12 have carried out the kinetic study for the reactions of chlorine atoms with ethyl vinyl ether (EVE) and propyl vinyl ether (PVE). Although rate constants of EVE + Cl and PVE + Cl were obtained at different temperatures and the atmospheric lifetimes of EVE and PVE are calculated in this literature, there still exist several insufficiencies. First, the reaction mechanisms and products, which are necessary for deeply understanding the reaction processes, are not provided. Second, the rate constants just were computed at a low pressure (1 Torr), which is unreasonable in the real atmospheric surrounding. The theoretical method, which has been employed in environmental chemistry successfully,27−34 is capable of solving the deficiencies mentioned above. Thus, we intended to adopt a theoretical approach for the thermodynamic and kinetic studies on the reactions of VEs with Cl atoms.

1. INTRODUCTION Oxygenated volatile organic compounds (OVOCs) are emitted into the troposphere mainly through manufacturing and biogenic processes.1−3 In addition, the oxidation of the hydrocarbons existing in the troposphere could generate OVOCs directly.4 The high vapor pressures and low boiling points5,6 make these compounds gas state in the atmosphere, which readily play a key role in the atmospheric chemical processes.7 Through the reactions with oxidants (OH, NOx, O3, Cl, etc.), several secondary pollutants that have crucial contribution to the generation of secondary organic aerosols (SOAs)8,9 are formed. As a main ingredient of PM 2.5 and PM 10, the formation processes of SOA should draw our extraordinary attention. Vinyl ethers (CHCHOR, VEs) are an essential type of VOC. Along with the general application of VEs in different industries,10 particularly as friendly solvents, these compounds are released into the troposphere increasingly. The existence of the active carbon−carbon double bond makes VEs rapidly oxidized by tropospheric oxidations once in the atmosphere. In accordance with the investigations conducted on the oxidation reactions of VEs with OH, NO3, O3, and Cl,4,11−14 the hydroxylation reactions are considered to be the main removal pathway for the atmospheric VEs ordinarily. However, in the marine boundary layer and urban regions, Cl-initiated oxidation reactions of these compounds are competitive with those initiated by OH radical.15−17 Although the global average concentration of chlorine atoms is estimated to be smaller than 103 atoms cm−3, 3 orders of magnitude lower than that of OH (12 h daytime concentration of 2.0 × 106),18 it increases to be the peak of 105 atoms cm−3 in the marine boundary layer at sunrise.19−21 In the district with heavy anthropogenic © 2015 American Chemical Society

Received: November 11, 2014 Revised: January 4, 2015 Published: January 7, 2015 719

DOI: 10.1021/jp5112839 J. Phys. Chem. A 2015, 119, 719−727

Article

The Journal of Physical Chemistry A

Figure 1. Primary reaction paths of MVE with Cl at the CBS-QB3 level of theory; the energy barriers ΔE (kcal/mol) and reaction enthalpy ΔH (kcal/mol) are calculated at 298 K. Values recorded in parentheses are ΔE and ΔH calculated using the B3LYP method.

to investigate the statistical mechanics of the title reaction. The form of the ME used in this work is

Methyl vinyl ether (MVE), the simplest VE, has been studied on the reactions with OH, NO3, and O3 in both the experimental and theoretical fields,35−38 while no data exist on the Cl-initiated reaction of MVE according to our knowledge. In order to achieve a better understanding of the reaction mechanism as well as the reactivity of MVE toward Cl atoms, we conducted further theoretical investigation on the Cl-initiated oxidation reaction of MVE. This study is also to be a complement of our previous work on the degradation reactions of VEs.39,40

dp = Mp dt

(1)

Here, M is a collision/reaction matrix that represents collisional energy transfer between the states and reaction from the energized states, and p is the population distribution vector. The microcanonical rate coefficient k(E) for each elementary reaction in this work is depicted as

2. COMPUTATIONAL METHODS 2.1. Electronic Structures. The Gaussian 09 package of programs41 was employed for all calculations reported here. The reaction mechanisms of the title reaction were explored by using the high-level composite CBS-QB3 computing method. CBS-QB3 involves five steps; the first one is a geometrical optimization at the B3LYP level with the basis of 6311G(2d,d,p), the second step is thermal corrections, the third one is a frequency calculation, the fourth step is singlepoint calculations employing CCSD(T), MP4SDQ, and MP2 methods, and the last step is a CBS extrapolation.42 Whether the stationary points are minima or transition states was judged by the number of imaginary frequencies (0 or 1, respectively). Intrinsic reaction coordinate (IRC) calculations43 were performed by UB3LYP\6-311G(2d,d,p) to identify the rationality of the transition states for each elementary reaction. 2.2. Rate Constants. The MESMER program44 is employed in this work for the calculation of rate constants. This program has been successfully applied in several investigations.45,46 The master equation (ME) was executed

k(E) =

W (E ) hρ(E)

(2)

where ρ(E) is the density of rovibrational states of the reactant and W(E) is the sum of rovibrational states at the optimized transition state geometry. Then, the canonical rate coefficient k(T) follows the equation k(T ) =

1 Q (T )

∫ k(E)ρ(E) exp(−βE) dE

where Q(T) is the reactant partition function. Helium (the molecular weight is 4.0; Lennard-Jones parameters are σ = 2.55 and ε = 10.2) is used as the bath gas during the use of the MESMER program for the rate constant calculations. The grain size used is 100 cm−1, and the maximum grain energy is 25kT in the calculation process. Classical method is used to calculate the density of states. Simple RRKM or the MesmerILT method was provided for the investigation of rate constants. The exponential down model was implemented for describing collisional transfer probabilities 720


Article


Figure 2. Further reaction paths of IM1 at the CBS-QB3 level of theory; the energy barrier ΔE (kcal/mol) and reaction enthalpy ΔH (kcal/mol) are calculated at 298 K.

lengths and labeled atoms, are depicted in Figures SM1−SM3 in the Supporting Information. 3.1.1. Initial Reaction of MVE with Cl Atoms. Similar to the hydroxylation decomposition of VEs, the Cl-initiated reactions with VEs involve addition and abstraction pathways. Therefore, for the reaction of MVE, two Cl-addition channels and six Habstraction channels are taken into account in this study. Activation energies (ΔE) and reaction enthalpies (ΔH) recorded for each elementary reaction in Figure 1 are calculated by employing the CBS-QB3 method. Here, we list the initial reaction pathways and name them R1−R8.

in this calculation. The calculation of the quantum mechanical energy levels of a hindered rotation is based on a onedimensional rotational eigenfunction. The Eckart coefficient approximates tunneling using a one-dimensional asymmetric Eckart potential. The precision used for computation is double.

3. RESULTS AND DISCUSSION 3.1. Mechanism of Reactions. All of the possible reaction pathways mentioned in this investigation are revealed in Figures 1−3. Figure 4 plots the reaction path profiles of the premier title reaction. Also, the optimized geometric structures of all compounds taking part in the reactions, with main bond MVE + Cl → IM1

OH‐addition to C1 atom

(R1)

→ IM2

OH‐addition to C2 atom

(R2)

→ IM03 → TS3 → IM3 + HCl the abstraction of H1 atom

(R3)


(R4)


(R5)


(R6)


(R7)


(R8)

According to the value calculated by CBS-QB3, both addition reactions R1 and R2 are barrier-free pathways, leading to IM1 and IM2, with 22.77 and 17.78 kcal mol−1 heat released, respectively. All of the H-abstraction reactions R3−R8 could occur via the prereactive complexes, of which IM04 and IM05

share the same geomertrical structure (Figure SM1, Supporting Information). Also, an inexplainable phenomenon that the energy of the intermidiate is higher than the transition state appears in reactions R3, R4, R6, and R7. Finally, we failed to find the correct geomertric structures for the postreactive 721


Article


With respect to IM11, the simplest reaction pathway is Helimination, which contains direct and indirect processes. The direct H-elimination reaction could proceed by overcoming a 10.14 kcal mol−1 energy and absorbing a 0.36 kcal mol−1 heat quantity, resulting in P1. With the help of the O2 molecule, the indirect process could occur. However, the transition state is located 27.29 kcal mol−1 lower than IM11, and no reasonable structure of the prereactive complex has been identified; this course is unreasonable. In addition, three dissociation patterns of IM11 are investigated in this work. The first is the rupture of the C1− C2 single bond and the synchronous formation of the C2O2 double bond. This course could take place by overcoming a low barrier of 3.60 and releasing 10.13 kcal mol−1 of energy, with the formation of methyl formate (P2) and ClCH2 radical (IM12). The subsequent step is the barrier-free association of IM12 and O2, with a high exothermic heat of 57.68 kcal mol−1. Similar to IM9, IM13 owns a peroxyl radical and could react with both NO and itself. The premier processes proceed by releasing 25.97 and 16.40 kcal mol−1 of energy, resulting in IM14 and IMb2, respectively. The possible further reaction of IMb2 is the crack of O7−O8, O7′−O8′, and C1−H1 bonds simultaneously with the generation of an O8′−H1 bond (Figure SM2, Supporting Information). Nevertheless, the energy barrier of this process is a negative value of −1.76 kcal mol−1, and no correct optimized geometrical structure of the prereactive compound has been found. Thus, this reaction course is preposterous. The following reaction step of IM14 could generate formyl chloride (P3) and IM15 via TS12-HNO2 and TS12, respectively. Although it overcomes a similar energy barrier at around 20 kcal mol−1, HNO2-elimination could release 49.47 kcal mol−1 compared with the absorption of 8.55 kcal mol−1 for NO2-eliminaion. Formyl chloride is the major product of the further reaction of IM14. Because of the existence of a single electron in IM15, the formation of a CO double bond along with the elimination of a H or Cl atom is ready to occur. Despite the unreasonable channel with the participation of O2, the direct H-elimination is the only process to produce formyl chloride, with the barrier energy and reaction heat of 18.43 and 11.81 kcal mol−1. The Cl-elimination of IM15 could proceed via the transition state TS14 by overcoming a 9.26 kcal mol−1 energy barrier and releasing 7.32 kcal mol−1 of heat, leading to formaldehyde (P4). By comparing the above elimination pathways, the Cl atom is easier to remove, and formaldehyde is a main product. The second decomposition channel of IM11 is the rupture of a C2−O1 bond and the formation of a C2O2 double bond, resulting in 2-chloroacetaldehyde (P5) and OCH3 radical (IM16). This process could occur by overcoming 17.02 kcal mol−1 of energy and releasing 11.09 kcal mol−1 of heat. The only reasonable H-elimination reaction of IM16 is the direct elimination, with the energy barrier of 23.15 kcal mol−1 and endothermic energy of 19.92 kcal mol−1. The product of this reaction process is formaldehyde. The last dissociation reaction of IM11 is the H-shift process simultaneously with the rupture of a C2−O1 bond. During this process, the H3 atom transfers from C2 to O1 to form methanol (P6) and IM17, with a barrier of 13.69 kcal mol−1 and exothermic by 2.16 kcal mol−1. As depicted in Figure 2, IM17 first combines O2 and then NO, with the successive released heat of 61.88 and 31.04 kcal mol−1. Such a high exothermicity makes the following NO2-elimination reaction easy to occur. Through the disruption of the C1−C2 bond,

complexes. This offers strong evidence for the low reactivities of these four reaction channels mentioned above. Although the energies of the reaction channels R5 and R8 are resonable, the following calculation of rate constants validates the remote possibility for the occurrence of the two pathways. As a supplement, we optimize all of the structures of complexes involved in the title reactions by employing the B3LYP/631+g(d,p) method and acquire the other set of energy datum written in parentheses in Figure 1. Similarly, reactions R1 and R2 are barrier-free, while all H-abstraction channels could occur via the prereactive compounds. Although the energies of all Habstraction pathways are normal, the rate coefficients obtained based on the B3LYP method also attest to the difficulty of the abstraction reactions. In summary, the Cl atom attacking the carbon−carbon double bond is a favorable initial channel, and the further reaction processes of IM1 and IM2 are selected to discuss. 3.1.2. Secondary Reactions. As seen in Figure 2, there exists a single electron in the C2 atom; thus, the O2 molecule prevalent in the surrounding attacks this atom immediately, with a high exothermic heat of 61.86 kcal mol−1. This process is a barrier-free pathway, with the generation of IM9. The subsequent reaction of IM9 associated with NO is also barrierfree, with 24.31 kcal mol−1 of heat released. Once IM10 is formed, HNO2-elimination or NO2-elimination processes are unavoidable, with the transition states TS9-HNO2 and TS9, respectively. The energy barriers and reaction heats of the two pathways are 24.75, 22.85 kcal mol−1 and −58.40, 11.07 kcal mol−1. Methyl 2-chloroacetate (P1) and IM11 are the corresponding products of the two elimination elementary reactions. It is clear to reveal that P1 is more favorably resultant during the further reaction process of IM9 in the presence of NO. In the absence of NO, as an intermediate with a peroxy radical, IM9 could associate with itself via a barrier-free process, resulting in IMb1. Compared with the combination of IM9 and NO, the self-associated reaction of IM9 owns the lower exothermic heat of 15.82 kcal mol−1. However, along with the low energy barrier of 8.55 kcal mol−1 and the high heat release of 30.91 kcal mol−1, P1 and IM11 could also be generated. Consequently, the self-associated reaction is another feasible process for the decomposition of IM9. Another reaction method of IM9 without NO is the H-shift process, depicted in Figure 2. Via a transition state TS9Hs with a six-membered ring, the H5 atom linked to C2 transfers to O3 and results in IM9Hs. The energy barrier and reaction heat are 19.18 and 9.92 kcal mol−1, which makes this reaction difficult to carry out. However, the following barrier-free association with O2 could active the whole reaction because of such a high exothermic heat of 63.20 kcal mol−1. Following is another H-shift process via the transfer of the H3 atom from C2 to O5, with the energy barrier and reaction heat of 18.51 and 8.50 kcal mol−1. The intermediate IM9Hs2 could eliminate the OH radical linked to O4 and lead to IM9Hs3. This is a barrier-free process, with the exothermic heat of 47.57 kcal mol−1. As a peroxy compound, IM9Hs3 could self-dissociate into IM9Hs4 and a hydroxy. This process is an endothermic barrier-free reaction, and the high absorption energy of 43.74 certifies the tiny possibility of this pathway. Once the first step occurs, H-elimination of IM9Hs4 is unavoidable, accompanied by the formation of 2-chloroacetic formic anhydride (Padd1). According to the above anaylsis, the main products of the decomposition of IM9 are P1 and IM11 in any case. 722


Article


Figure 3. Further reaction paths of IM2 at the CBS-QB3 level of theory; the energy barrier ΔE (kcal/mol) and reaction enthalpy ΔH (kcal/mol) are calculated at 298 K.

Figure 4. Reaction path profiles of the primary reactions of MVE + Cl.

NO, IM22 could easily be formed by releasing 23.57 kcal mol−1 of heat. By overcoming 20.04 and 22.67 kcal mol−1 barriers, IM23 and 2-chloro-2-methoxyacetaldehyde (P7) are formed, respectively. Although overcoming similar energy barriers, the generation of P7 releases such a high heat of 41.07 kcal mol−1 that HNO2-elimination is the favorable channel. Through the combination of two IM21 molecules, IMb3 is formed without an energy barrier. According to Figure SM3 (Supporting Information), the rupture of O2−O3 and O2′−O3′ bonds along with the H1′ transfers from C1′ to O3 via TSb3 lead to the generation of P7 and IM23, with the energy barrier and reaction heat of 6.33 and −16.22 kcal mol−1. In terms of the last reaction course of IM21, H-shift occurs via TS21Hs with a seven-numbered ring in it. Although the energy barrier is 22.58

CO2 and IM12 are generated. The low energy barrier (4.66 kcal mol−1) and high exothermicity (21.57 kcal mol−1) make it ready to carry out. Comparing the energy barrier and heat release of the further reactions of IM11, the rupture of C1−C2 is the most favorable process, and the major products are methyl formate (P2) and formaldehyde (P4). In summary, 2-chloroacetate (P1), methyl formate (P2), and formaldehyde (P4) are the major products for the whole reaction process of IM1, while the minor resultants are formyl chloride (P3), 2-chloroacetaldehyde (P5), and 2-chloroacetic formic anhydride (Padd1). As shown in Figure 3, the further reaction processes of IM2 are similar to those of IM1. First, the barrier-free association with O2 results in a peroxyl radical, IM21. In the presence of 723


Article


Table 1. Individual Rate Constants (cm−3 molecule−1 s−1) of the Primary Reaction of MVE + Cl at Different Temperatures (K) Based on the CBS-QB3 Method T (K) 200 225 250 275 298 300 325 350 375 400

kR1 7.28 7.18 7.36 7.02 6.71 6.69 6.41 6.04 5.60 5.09

× 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11

kR2 7.89 6.86 6.59 6.19 5.75 5.71 5.14 4.52 3.88 3.25

× 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11

kR5 1.49 3.40 6.41 1.07 1.55 1.59 2.18 2.87 3.56 4.23

× 10−20 × 10−20 × 10−20 × 10−19 × 10−19 × 10−19 × 10−19 × 10−19 × 10−19 × 10−19

kR8 1.09 1.08 1.07 1.07 1.07 1.07 1.06 1.06 1.05 1.04

× 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18

kadd 1.52 1.40 1.39 1.32 1.25 1.24 1.15 1.06 9.48 8.34

× 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−11 × 10−11

kabs 1.11 1.12 1.14 1.18 1.22 1.22 1.28 1.34 1.41 1.47

× 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18

ktotal 1.52 1.40 1.39 1.32 1.25 1.24 1.15 1.06 9.48 8.34

× 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−11 × 10−11

Table 2. Individual Rate Constants (cm−3 molecule−1 s−1) of the Primary Reaction of MVE + OH at Different Pressures (Torr) Based on the CBS-QB3 Method P (Torr) 100 300 500 600 700 740 760 800 900 1000 1200 1500 2000

kR1 5.46 6.27 6.55 6.63 6.68 6.70 6.71 6.73 6.76 6.80 6.85 6.90 6.97

× 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11

kR2 3.58 4.85 5.37 5.54 5.68 5.73 5.75 5.79 5.89 5.97 6.10 6.24 6.41

× 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11 × 10−11

kR5 5.46 6.27 6.55 6.63 6.68 6.70 6.71 6.73 6.76 6.80 6.85 6.90 6.97

× 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000

kR8 3.58 4.85 5.37 5.54 5.68 5.73 5.75 5.79 5.89 5.97 6.10 6.24 6.41

× 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 × 1000

kadd 9.04 1.11 1.19 1.22 1.24 1.24 1.25 1.25 1.27 1.28 1.29 1.31 1.34

× 10−11 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10

kabs 1.07 1.12 1.17 1.19 1.21 1.22 1.22 1.23 1.25 1.25 1.31 1.37 1.47

× 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18

ktotal 9.04 1.11 1.19 1.22 1.24 1.24 1.25 1.25 1.27 1.28 1.29 1.31 1.34

× 10−11 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10 × 10−10

of IM26 include two channels, and the major product is 2hydroxyacetyl chloride (P8). The other dissociation pathway of IM23 is the fracture of the C1−C2 bond with the formation of the C1O2 double bond, synchronously. The generation of formaldehyde and IM29 could overcome the 10.16 kcal mol−1 barrier and absorb 10.74 kcal mol−1 of heat. The following reactions of IM29 contain two channels, the barrier-free combination with O2 and the dissociation process. The former pattern is the formation of IM33, with the released heat of 60.75 kcal mol−1. Such a great exothermicity makes it ready to undergo. As depicted in Figure 3, the subsequent step of IM33 is divided into two channels, and the possible products are (methyl carbonochloridate) P9 and IM35. However, the structure of IM35 could not be identified. The only reaction pathway is IM33 → IM34 → TS29-HNO2 → P9. For the dissociation of IM29, the break of the C3−O1 bond and the generation of the C2O1 double bond occur simultaneously. The products of this elementary reaction are formyl chloride (P3) and methyl radical (IM30), with an energy barrier and reaction heat of 18.35 and −0.53 kcal mol−1. The following two barrier-free association reactions make the decomposition reaction of IM29 proceed absolutely. According to the above investigation of IM2, the major products are formyl chloride (P3), formaldehyde (P4), and 2chloro-2-methoxyacetaldehyde (P7), while the minor products are (methyl carbonochloridate) P9 and 2-chloroacetic formic anhydride (Padd2). 3.2. Kinetic Properties. By employing the MESMER program, rate constants of the primary reaction pathways based on the calculations of CBS are calculated over the temperature

and this process is endothermic, the following reaction with such high exothermicity (63.51 kcal mol−1) could promote the former reaction in turn. The second H-shift reaction could proceed once IM21Hs-O2 is formed, with the energy barrier and endothermic heat of 19.30 and 7.36 kcal mol−1, resulting in IM21Hs2. The subsequent step is the remove of OH radical with no transition state determined. This step could produce the intermediate IM21Hs3 and release 28.06 kcal mol−1 of heat. As a endothermic barrier-free process, the following OHelimination reaction hardly occurs because of the high absorption heat of 39.64 kcal mol−1. Thus, 2-chloroacetic formic anhydride (Padd2) produced via the H-elimination of IM21Hs4 is the minor product of the further reactions of IM21. In terms of IM23, two H-elimination reactions and two decomposition channels are taken into account in this work. Similarly, H-abstraction with the help of the O2 molecule is nonexistent because of the unreasonable energy value. The H atom could be removed directly by overcoming a 24.25 kcal mol−1 barrier and absorbing 19.04 kcal mol−1 of heat, leading to P7. According to Figure 3, the first decomposition pathway of IM23 is the H4 atom linked to C3 migrating to the O2 atom via a six-membered ring in TS22. The energy barrier and exothermic energy of this process are 11.00 and 7.22 kcal mol−1, with the generation of intermediate IM24. Subsequently, 21.08 kcal mol−1 should be overcome, and 11.35 kcal mol−1 of heat should be absorbed in order to produce formaldehyde and IM25. Once the first step occurs, the following O2-association step with 61.34 kcal mol−1 of heat released could promote the whole reaction processes. The same as the analysis of other peroxyl radicals (IM9, IM13, and IM21), the further reactions 724


Article


760 Torr, the total rate coefficient is ktot = 1.25 × 10−10 cm3 molecule−1 s−1. Although there are no experimental data for the reaction of MVE with Cl, Cl-initiated oxidations of EVE and PVE have been investigated by Wang et al.12 Also, the rate constants of these processes at 298 K were presented in that study: kEVE+Cl = (2.43 ± 0.62) × 10−10 cm3 molecule−1 s−1 and kPVE+Cl = (4.23 ± 0.48) × 10−10 cm3 molecule−1 s−1. Therefore, the rate constant obtained in this work is credible via reasonable predication. For the atmospheric reactions of MVE + Cl, the lifetime of the reactant τ is the reciprocal of k[Cl], that is, τ = (1/k[Cl]), where k is the total rate coefficient for the primary reactions of MVE + Cl and [Cl] is given the value of 1.0 × 105 molecules cm−3. Thus, the lifetime of MVE with respect to Cl is calculated to be 2.23 h. Compared with the lifetimes or the half-lives of VEs with respect to OH and O3,13 a conclusion deduced is that the reactions of MVE with Cl atoms are competitive among the atmospheric reactions of VEs.

range of 200−400 K and pressure range of 100−2000 Torr. The rate coefficients under different temperatures and pressures are listed in Tables 1 and 2, respectively. According to Table 1, there is no value for the pathways R3, R4, R6, and R7 because of the unreasonable energy calculated by CBS, which was mentioned in section 3.1.1. Compared with the rate constants of reactions R1, R2, R5, and R8, we can see that the two abstraction pathways should be ignored because the 7−9 orders of magnitude are smaller than that of addition channels. In order to acquire a reasonable value of reactions R3, R4, R6, and R7, the computation of rate constants is repeated based on the B3LYP calculation. According to Tables SM1 and SM2 (Supporting Information), there is the same condition that addition pathways dominate the whole reaction, even if the rate constants of H-abstraction channels are comprehensive. Without consideration of H-abstraction channels, the whole and individual rate constants of Cl-addition pathways versus 1000/T are depicted in Figure 5a. All three curves decrease

4. CONCLUSIONS The detailed reaction mechanisms and rate constants of reactions are presented in this paper. According to the discussion of all of the elementary reactions, the major products of the reaction of MVE and Cl atoms are 2chloroacetate (P1), methyl formate (P2), formyl chloride (P3), formaldehyde (P4), and 2-chloro-2-methoxyacetaldehyde (P7). Additionally, deduced from the value of rate constants, the atmospheric lifetime of MVE with respect to Cl atoms is 2.23 h. Such a short lifetime makes the degradation of MVE initiated by chlorine atoms essential during the atmospheric processes. The comprehensive mechanisms of these reactions provide a direction for experimental investigations. Also, this study is a supplement of our previous work.

■

ASSOCIATED CONTENT

* Supporting Information S

Optimized geometries and rate constants for the reaction of EVE + Cl. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-531-8836 1990. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (21337001, 21477065, and 20877049) and the Fundamental Research Funds of Shandong University (2014JC014). We thank Dr. Struan H. Robertson for providing the MESMER program.

Figure 5. Whole rate constants (ktot) for the Cl-initiated reactions of MVE (a) at different temperatures and (b) under different pressures.

with the increase of temperature in the study range (200−400 K). In detail, the rate constant of reaction R2 decreases more quickly than that of reaction R1. Thus, reaction R1 is the favorable pathway in the temperature range of 216−400 K. In summary, the primary reaction of MVE + Cl displays an obviously negative temperature dependence. Figure 5b shows us the rate coefficients of reactions R1, R2, and the total reaction over the pressure range of 100−2000 Torr and 298 K. Obviously, the rate coefficient displays positive pressure dependence, and reaction R1 governs the whole reaction process over the studied pressure. According to the value of rate constants in Table 1, the Arrhenius equation is obtained as ktot = 5.64 × 10−11 exp (215.1/T). At room temperature and

■

REFERENCES

(1) Fehsenfeld, F.; Calvert, J.; Fall, R.; Goldan, P.; Guenther, A. B.; Hewitt, C. N.; Lamb, B.; Liu, S.; Trainer, M.; Westberg, H.; Zimmerman, P. Emission of volatile organic compounds from vegetation and the implications for atmospheric chemistry. Global Biogeochem. Cycles 1992, 6, 389−430. (2) Williams, D. C.; O’Rji, L. N.; Stone, D. A. Kinetics of the reactions of OH radicals with selected acetates and other esters under simulated atmospheric conditions. Int. J. Chem. Kinet. 1993, 25, 539− 548.

725


Article


(24) Ragains, M. L.; Finlayson-Pitts, B. J. Kinetics and mechanism of the reaction of Cl atoms with 2-methyl-1,3-butadiene (isoprene) at 298 K. J. Phys. Chem. A 1997, 101, 1509−1517. (25) Blanco, M. B.; Bejan, I.; Barnes, I.; Wiesen, P.; Teruel, M. A. The Cl-initiated oxidation of CH3C(O)OCHCH2, CH3C(O)OCH2CHCH2, and CH2CHC(O)O (CH2)3CH3 in the troposphere. Environ. Sci. Pollut. Res. 2009, 16, 641−648. (26) Dalmasso, P. R.; Taccone, R. A.; Nieto, J. D.; Cometto, P. M.; Cobos, C. J.; Lane, S. I. Reactivity of hydrohaloethers with OH radicals and chlorine atoms: Correlation with molecular properties. Atmos. Environ. 2014, 91, 104−109. (27) Martell, J. M.; Boyd, R. J. Ab initio studies of reactions of hydroxyl radicals with fluorinated ethanes. J. Phys. Chem. 1995, 99, 13402−13411. (28) Sekušak, S.; Güsten, H.; Sabljić, A. An ab initio investigation on transition states and reactivity of chloroethane with OH radical. J. Chem. Phys. 1995, 102, 7504−7518. (29) Sekušak, S.; Güsten, H.; Sabljić, A. An ab initio study on reactivity of fluoroethane with hydroxyl radical: Application of G2 theory. J. Phys. Chem. 1996, 100, 6212−6224. (30) Suh, I.; Zhang, R.; Molina, L. T.; Molina, M. J. Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH−toluene reactions. J. Am. Chem. Soc. 2003, 125, 12655−12665. (31) Zhang, Q. Z.; Qu, X. H.; Wang, H.; Xu, F.; Shi, X. Y.; Wang, W. X. Mechanism and thermal rate constants for the complete series reactions of chlorophenols with H. Environ. Sci. Technol. 2009, 43, 4105−4112. (32) Zhang, Q. Z.; Yu, W. N.; Zhang, R. X.; Zhou, Q.; Gao, R.; Wang, W. X. Quantum chemical and kinetic study on dioxin formation from the 2,4-TCP and 2,4-DCP precursors. Environ. Sci. Technol. 2010, 44, 3395−3403. (33) Zhou, Q.; Shi, X.; Xu, F.; Zhang, Q.; He, M.; Wang, W. Mechanism of OH-initiated atmospheric photooxidation of the organophosphorus insecticide (C2H5O)3PS. Atmos. Environ. 2009, 43, 4163−4170. (34) Gao, R.; Xu, F.; Li, S. Q.; Hu, J. T.; Zhang, Q. Z.; Wang, W. X. Formation of bromophenoxy radicals from complete series reactions of bromophenols with H and OH radicals. Chemosphere 2013, 92, 382−390. (35) Perry, R. A.; Atkinson, R.; Pitts, J. N., Jr. Rate constants for the reaction of OH radicals with dimethyl ether and vinyl methyl ether over the temperature range 299−427 K. J. Chem. Phys. 1977, 67, 611− 614. (36) Grosjean, D.; Williams, E. L., II. Environmental persistence of organic compounds estimated from structure−reactivity and linear free-energy relationships. Unsaturated aliphatics. Atmos. Environ., Part A 1992, 26, 1395−1405. (37) Mellouki, A. 2006. Proceedings of the NATO ARW “Environmental Simulation Chambers − Application to Atmospheric Chemical Processes”, Zakopane, Poland, 1−4 October 2004; NATO Science Series, IV. Earth and Environmental Sciences; Springer: Dordrecht, The Netherlands; pp 163−170. (38) Sun, X. Y.; He, M. X.; Zhang, Q. Z.; Wang, W.; Jalbout, A. F. Quantum chemical study on the atmospheric photooxidation of methyl vinyl ether (MVE). J. Mol. Struct: THEOCHEM. 2008, 868, 87−93. (39) Han, D. D.; Cao, H. J.; Li, J.; Li, M. Y.; He, M. X.; Hu, J. T. Computational study on the mechanisms and rate constants of the OH-initiated oxidation of ethyl vinyl ether in atmosphere. Chemosphere 2014, 111, 61−69. (40) Han, D. D.; Cao, H. J.; Sun, Y. H.; Sun, R. L.; He, M. X. Mechanistic and kinetic study on the ozonolysis of n-butyl vinyl ether, i-butyl vinyl ether and t-butyl vinyl ether. Chemosphere 2012, 88, 1235−1240. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09; Gaussian Inc: Wallingford, CT, 2009.

(3) Van Ornum, S. G.; Champeau, R. M.; Pariza, R. Ozonolysis applications in drug synthesis. Chem. Rev. 2006, 106, 2990−3001. (4) Mellouki, A.; Le Bras, G.; Sidebottom, H. Kinetics and mechanisms of the oxidation of oxygenated organic compounds in the gas phase. Chem. Rev. 2003, 103, 5077−5096. (5) Lide, D. R., Kehiaian, H. V. CRC handbook of thermophysical and thermochemical data; CRC Press: Boca Raton, FL, 1994. (6) TRCVP, Vapor Pressure Database, version 2.2P, Thermodynamic Research Center, Texas A&M University: College Station, TX, 1989. (7) Singh, H.; Chen, Y.; Staudt, A.; Jacob, D.; Blake, D.; Heikes, B.; Snow, J. Evidence from the Pacific troposphere for large global sources of oxygenated organic compounds. Nature 2001, 410, 1078−1081. (8) Jang, M.; Kamens, R. M. Newly characterized products and composition of secondary aerosols from the reaction of α-pinene with ozone. Atmos. Environ. 1999, 33, 459−474. (9) Kalberer, M.; Yu, J.; Cocker, D. R.; Flagan, R. C.; Seinfeld, J. H. Aerosol formation in the cyclohexene−ozone system. Environ. Sci. Technol. 2000, 34, 4894−4901. (10) George, C.; Sidebottom, H.; Mellouki, A.; Herrmann, H.; Wortham, H.; Kirchner, F.; Carlier, P. Mutiphase chemistry of oxygenated species in the troposphere. MOST, Final Report EU contract EVK2-CT-2001-00114; 2005. (11) Thiault, G.; Thévenet, R.; Mellouki, A.; Le Bras, G. OH and O3initiated oxidation of ethyl vinyl ether. Phys. Chem. Chem. Phys. 2002, 4, 613−619. (12) Wang, L.; Ge, M.; Wang, W. Kinetic study of the reactions of chlorine atoms with ethyl vinyl ether and propyl vinyl ether. Chem. Phys. Lett. 2009, 473, 30−33. (13) Zhou, S.; Barnes, I.; Zhu, T.; Bejan, I.; Benter, T. Kinetic study of the gas-phase reactions of OH and NO3 radicals and O3 with selected vinyl ethers. J. Phys. Chem. A 2006, 110, 7386−7392. (14) Zhou, S.; Barnes, I.; Zhu, T.; Klotz, B.; Albu, M.; Bejan, I.; Benter, T. Product study of the OH, NO3 and O3 initiated atmospheric photooxidation of propyl vinyl ether. Environ. Sci. Technol. 2006, 40, 5415−5421. (15) Singh, H. B.; Kasting, J. F. Chlorine−hydrocarbon photochemistry in the marine troposphere and lower stratosphere. J. Atmos. Chem. 1988, 7, 261−285. (16) Oum, K. W.; Lakin, M. J.; DeHaan, D. O.; Brauers, T.; Finlayson-Pitts, B. J. Formation of molecular chlorine from the photolysis of ozone and aqueous sea-salt particles. Science 1998, 279, 74−76. (17) Mishra, B. K.; Chakrabartty, A. K.; Deka, R. C. Theoretical investigation of the gas-phase reactions of CF2ClC(O)OCH3 with the hydroxyl radical and the chlorine atom at 298 K. J. Mol. Model. 2013, 19, 3263−3270. (18) Atkinson, R.; Arey, J. Atmospheric degradation of volatile organic compounds. Chem. Rev. 2003, 103, 4605−4638. (19) Keene, W. C.; Pszenny, A. A. P.; Jacob, D. J.; Duce, R. A.; Galloway, J. N.; Schultz-Tokos, J. J.; Boatman, J. F. The geochemical cycling of reactive chlorine through the marine troposphere. Global Biogeochem. Cycles 1990, 4, 407−430. (20) Rudolph, J.; Koppmann, R.; Plass-Dülmer, C. The budgets of ethane and tetrachloroethene: Is there evidence for an impact of reactions with chlorine atoms in the troposphere? Atmos. Environ. 1996, 30, 1887−1894. (21) Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.; Plastridge, R. A.; Hubbe, J. M.; Fast, J. D.; Berkowitz, C. M. Unexpectedly high concentrations of molecular chlorine in coastal air. Nature 1998, 394, 353−356. (22) Thornton, J. A.; Kercher, J. P.; Riedel, T. P.; Wagner, N. L.; Cozic, J.; Holloway, J. S.; Brown, S. S. A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry. Nature 2010, 464, 271−274. (23) Bilde, M.; Sehested, J.; Nielsen, O. J.; Wallington, T. J.; Meagher, R. J.; McIntosh, M. E.; Wine, P. H. Kinetics and mechanism of the gas phase reaction of atomic chlorine with CH2ICl at 206−432 K. J. Phys. Chem. A 1997, 101, 8035−8041. 726


Article

The Journal of Physical Chemistry A (42) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. Accuracy of computational solvation free energies for neutral and ionic compounds: Dependence on level of theory and solvent model. J. Chem. Phys. 1999, 110, 2822. (43) Gonzalez, C.; Schlegel, H. B. An improved algorithm for reaction path following. J. Chem. Phys. 1989, 90, 2154−2161. (44) Glowacki, D. R.; Liang, C. H.; Morley, C.; et al. MESMER: an open-source master equation solver for multi-energy well reactions. J. Phys. Chem. A 2012, 116, 9545−9560. (45) Gannon, K. L.; Blitz, M. A.; Liang, C. H.; Pilling, M. J.; Seakins, P. W.; Glowacki, D. R. Temperature dependent kinetics (195−798 K) and H atom yields (298−498 K) from reactions of 1CH2 with acetylene, ethene, and propene. J. Phys. Chem. A 2010, 114, 9413− 9424. (46) Gannon, K. L.; Blitz, M. A.; Liang, C. H.; Pilling, M. J.; Seakins, P. W.; Glowacki, D. R.; Harvey, J. N. An experimental and theoretical investigation of the competition between chemical reaction and relaxation for the reactions of 1CH2 with acetylene and ethene: Implications for the chemistry of the giant planets. Faraday Discuss. 2010, 147, 173−188.

727


Computational Study on the Mechanisms and Rate Constants of the Cl

Recommend Documents