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Dichlorvos: A Quantum Mechanical. Study ... parasites in livestock and domestic animals and by controlling ... Calculated ΔH0 (298 K) for Stable Spec...
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Environ. Sci. Technol. 2007, 41, 6109-6116

Mechanism of OH-Initiated Atmospheric Photooxidation of Dichlorvos: A Quantum Mechanical Study

TABLE 1. Calculated ∆H0 (298 K) for Stable Species (Molecules and Radicals) at the TPSSh/6-311+G(3df,2p) Levela

QINGZHU ZHANG, XIAOHUI QU, AND WENXING WANG* Environment Research Institute, Shandong University, Jinan 250100, P. R. China

Dichlorvos (2,2-dichlorovinyl phosphate, DDVP) is a widely used organophosphorus insecticide. DDVP may be released into the atmosphere, where it may be transported for long distances and undergo chemical transformations. The mechanisms of the atmospheric reactions of DDVP have not been fully understood because of the short lifetime of its oxidized radical intermediates and the extreme difficulty in the detection of these species experimentally. In this paper, we carried out molecular orbital theory calculations for the OH-initiated atmospheric photooxidation of DDVP. The profile of the potential energy surface was constructed, and the possible channels involved in the reaction are discussed. Several energetically favorable reaction pathways are revealed for the first time. The calculated results were compared with the available experimental observations. Four product pathways are energetically feasible for DDVP degradation initiated by OH radicals in the atmosphere and are consistent with the experimentally observed products CCl2O and CO, but the additional products CCl2CHO, (CH3O)2P(O)OH, HO2, and a closed-shell organophosphorus compound denoted P10 are also predicted.

1. Introduction Dichlorvos (2,2-dichlorovinyl phosphate, DDVP, (CH3O)2 OOCH ) CCl2) is a synthetic insecticide and belongs to a | P family of chemically related organophosphate pesticides. Since its commercial introduction in 1961, DDVP has been used extensively in many countries and has produced important benefits by controlling internal and external parasites in livestock and domestic animals and by controlling insects in houses and fields (1). But the increasing use of synthetic chemical pesticides is causing worldwide pollution (2-5). The significant problems of human illness and death that follow occupational or accidental exposure to pesticides have been well documented (6-8). The LC50 value shows that DDVP has a high acute toxicity (9). The International Agency for Research on Cancer considers DDVP as a possible carcinogen for human beings. Thus, DDVP has been banned in many countries. However, DDVP is still preferred by some farmers because it is cost-effective, easily available, and displays a wide spectrum of bioactivity. Its annual worldwide sales in 2003 were about 40 million US dollars (10). Dichlorvos can also be released into the environment as a major * Corresponding author e-mail: [email protected]; fax: 86531-8836 4435. 10.1021/es0628001 CCC: $37.00 Published on Web 08/07/2007

 2007 American Chemical Society

a

species

∆ H0

∆H0 (exptl)

CCl2O PCl3 DDVP P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

-51.47 -69.57 -217.40 -13.84 -238.77 -249.34 -78.57 -138.75 -160.30 -176.10 -263.73 -220.52 -244.26

-52.37 -68.59

The experimental values are from refs 28-29.

degradation product of other organophosphate pesticides, such as trichlorfon, naled, and metrifonate (11-13). Pesticides can enter the atmosphere and travel many kilometers (14). This is true not only for the older and more persistent pesticides but also for the newer, currently used ones (15-16). The main input mechanisms of pesticides to the atmosphere are drift during spraying operations, volatilization from ground or leaf surfaces, and wind erosion (17). DDVP is the most frequently detected pesticide in the atmosphere. Its frequency of detection in rainwater is up to 65%, and the highest concentration was found to be 0.33 µg/L in Japan (18). Pesticides exist in air as gases or particles or are distributed between these two phases. The vapor-to-particle ratio (V/P) is controlled by vapor pressure and the total suspended particle concentration. Because of its relatively high vapor pressure (19), DDVP vaporizes quickly. In the atmosphere, the fraction of DDVP in the gas phase can reach 89% (20). This greatly increases the potential for human exposure to this highly toxic material. Particlephase DDVP may be removed from the atmosphere through dry or wet deposition. The tropospheric removal or transformation of gas-phase DDVP involves wet and dry deposition, photolysis, oxidation reactions with OH, NO3, O3, and possibly reaction with gaseous nitric acid (HNO3) in urban areas where the gaseous HNO3 concentration is significant (21-22). Reaction with Cl atoms may also be important in certain locations during certain times of the year (23). The wet and dry deposition of gaseous DDVP is of relatively minor importance as a removal pathway. Among the various oxidants, OH radicals play an essential role in the determination of the oxidizing power of the atmosphere. The reaction of DDVP with OH radicals is considered to be a dominant removal process for gaseous DDVP. To assess the atmospheric behavior of pollutants, it is critical to know their atmospheric reactions. However, current knowledge of the OH-initiated DDVP oxidation mechanism and the major degradation products is very limited. Feigenbrugel et al. (20) conducted an experimental investigation into the degradation products from the reaction of DDVP with OH radicals but only detected phosgene (Cl2CO) and carbon monoxide (CO). Most of the degradation products were not identified and quantified because of the absence of reference spectra. They proposed a possible reaction mechanism (20) to explain the observed products, in which CO formation is initiated by H abstraction from the CH3O group of DDVP, and then the product radical may further VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. TPSSh/6-31G(d,p)-optimized geometries for the reactant, intermediates, transition states. and products involved in the reaction of DDVP with OH radicals. Distances are in angstroms, and angles are in degrees.

react with O2/NO leading to HCO. The observed CO may be formed through the reaction of HCO with O2. They proposed that the formation of Cl2CO is initiated via OH addition to the carbon-carbon double bond. However, there is a shortage of direct experimental data associated with the reaction mechanism, largely, because of the lack of efficient detection schemes for radical intermediate species. Quantum calculation is especially suitable for establishing the feasibility of a reaction pathway. In this paper, we have carried out a theoretical study on the OH-initiated atmospheric photooxidation reaction of DDVP to find favorable reaction pathways and sites. Elucidation of the reaction mechanism is very challenging because of its inherent complexity. The potential energy surface is useful to explain the experimen6110

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tally observed branching ratios, thermochemical properties, and rate coefficients. To our knowledge, this is the first study of the degradation of a pesticide using quantum mechanics.

2. Computational Methods High-level ab initio molecular orbital calculations were carried out for the OH-initiated atmospheric photooxidation of DDVP in the presence of O2 and NO. The Gaussian 03 package (24) was used on an SGI Origin 2000 supercomputer. The choice of computational levels and basis sets requires a compromise between accuracy and computational time. The geometrical parameters of reactants, transition states, intermediates, and products were optimized at the TPSSh level (25-26) with a standard 6-31G(d,p) basis set. The TPSSh/

6-31G(d,p) structures were employed in single-point energy calculations. The vibrational frequencies were also calculated at the TPSSh/6-31G(d,p) level to determine the nature of the stationary points, the zero-point energy (ZPE), and the thermal contributions to the free energy of activation. Each transition state was verified to connect the designated reactants with products by an intrinsic reaction coordinate (IRC) analysis (27). For a more accurate evaluation of the energetic parameters, a more flexible basis set, 6-311+G(3df,2p), was employed to determine the energies of various species. The profile of the potential energy surface was constructed at the TPSSh/6-311+G(3df,2p)//TPSSh/6-31G(d,p) level. Possible secondary reaction pathways were also studied to find the mechanism of formation of secondary pollutants from the OH-initiated atmospheric reaction of DDVP.

3. Results and Discussion The optimized geometries of the reactants, intermediates, transition states, and products are shown in Figure 1 for the reaction of DDVP with OH radicals. Table 1 shows the calculated ∆H0 (298 K) for stable species (molecule and radical) at the TPSSh/6-311+G(3df,2p) level. The profile of the potential energy surface with the zero-point energy (ZPE) correction is presented in Figure 2. The geometrical parameters of the intermediates, transition states, and products are listed in Figure 3 for the atmospheric removal reactions of OH-DDVP adducts, IM1 and IM2, with O2/NO. Figure 4 presents the structures of the intermediates, transition states and products involved in the formation mechanism of secondary pollutant CO. Figure 5 shows the energetically favorable degradation pathways of DDVP in the atmosphere. The first step of this study is to identify the level of theoretical approximation that is not only able to produce accurate results but is also computationally feasible and economical for currently available hardware and software. Because of the absence of experimental information on the thermochemical parameters for the present reaction system, it is difficult to make a comparison of the calculated results with experimental data. Thus, we calculated the standard formation enthalpies, ∆H0 (298 K), of CCl2O and PCl3 at the TPSSh/6-311+G(3df,2p)//TPSSh/6-31G(d,p) level. Table 1 shows that the calculated results are in excellent agreement with the available experimental values (28-29). 3.1. Reaction of DDVP with OH Radicals. The addition of OH to the carbon-carbon double bond is a possible reaction channel for the reaction of DDVP with OH radicals. In addition, OH is a strongly nucleophilic radical, so H abstraction from DDVP by OH radicals should be another possible reaction pathway. This reaction pathway for H abstraction from the CH3O group of DDVP is supported by the work of Tuazon (30), which affirmed that the reaction of a similar compound, (CH3O)3PO, with OH radicals proceeds mainly by H abstraction from the CH3O group. Feigenbrugel (20) suggested that both pathways, that is, OH addition to the >CdC< bond and H abstraction from the CH3O group, contribute to 90% of the overall reaction of DDVP with OH radicals. In this work, we also studied other possible reaction pathways, such as a substitution channel, in which the group attached to CH3 in DDVP is substituted by OH to form CH3OH. Therefore, five possible reaction pathways, R1-R5, were identified for the reaction of DDVP with OH radicals. The reaction scheme can be described as follows:

DDVP + OH f IM1f TS1 f P1 + P2 association-elimination (R1) f IM2 f P3 + C1association-elimination

(R2a)

f IM2 f TS2 f P4 + P5 association-elimination (R2b) f IM3 f TS3 f IM4 + H2O H abstraction

(R3)

f TS4 f P6 + H2O H abstraction

(R4)

f TS5 f P7 + CH3OH substitution channel

(R5)

3.1.1. Association-Elimination Pathways. First, we analyzed the reaction pathway of OH addition to the >C)C< bond. Because the two carbon atoms in carboncarbon double bond are inequivalent, two adduct isomers, IM1 and IM2, were formed. Thus, two reaction pathways, R1 and R2, are found for the addition of OH to the carboncarbon double bond. Calculations show that the addition is a barrierless association. The geometrical parameters of the two OH-DDVP adducts are shown in Figure 1. The evaluation of the vibrational frequencies confirmed that IM1 and IM2 represent two minima on the potential energy surface. Intramolecular hydrogen bonding leads to a six-member cycle in the IM1 isomer. The length of the hydrogen bond is 1.946 Å. No such intramolecular hydrogen bond forms for IM2 because of the molecular configuration. It is interesting to compare the relative stability of the two OH-DDVP adducts. The energy of IM1 is 2.82 kcal/mol lower than that of IM2, suggesting a stabilization effect because of hydrogen bonding in the IM1 isomer. Figure 2 shows that the addition of OH to the carbon-carbon double bond of DDVP is a strongly exothermic process. The energies of IM1and IM2 are 40.30 and 37.48 kcal/mol lower than the total energy of the original reactants (DDVP and OH). The high reaction energies are retained as the internal energy of the adducts. The energy-rich adducts, IM1 and IM2, can react via unimolecular decomposition or with atmospheric O2/NO. Unimolecular decomposition of IM1 results in the products CCl2CHO (denoted as P1) and (CH3O)2P(O)OH (denoted as P2). This process involves H8-migration and cleavage of the C2-O4 bond. The transition state, denoted TS1, corresponding to the decomposition of IM1 is shown in Figure 1. The calculated vibrational frequencies contained only one imaginary component, 381i cm-1, confirming the first-order saddle point configuration. The validity of this transition state was further verified by an IRC calculation showing that the reaction path does connect the designated reactants (DDVP and OH) and products (P1 and P2). This decomposition has a low potential barrier, 2.34 kcal/mol. The process is exothermic by 4.20 kcal/mol, and the overall reaction is strongly exothermic by 44.50 kcal/mol. Thus, the unimolecular decomposition of IM1 would occur readily. CCl2CHO and (CH3O)2P(O)OH are possible products for the reaction of DDVP with OH radicals. Thus, CCl2CHO and (CH3O)2P(O)OH should exist among the reaction products of DDVP with OH radicals, although CCl2CHO is an activated radical and can be further oxidized in the atmosphere. This energetically favorable reaction pathway is revealed for the first time. Further direct experimental observation would be anticipated to identify these products. Two possible unimolecular decomposition channels were found for IM2. One is the loss of a Cl atom bonded to C1 to form a closed-shell molecule, denoted P3. The calculations show the cleavage of the C1-Cl bond is a barrierless process. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Profile of the potential energy surface for the reaction of DDVP with OH radicals at the TPSSh /6-311+G(3df,2p) level.

This channel is strongly endothermic by 21.61 kcal/mol. The other unimolecular decomposition of IM2 occurs by cleavage of the P-O4 bond to form HOCCl2CHO (P4) and (CH3O)2PO (P5) via the transition state TS2. In the TS2 structure, the breaking P-O4 bond is 0.687 Å longer than the equilibrium value of 1.665 Å in IM2. TS2 appears to be a productlike barrier, as expected for a strongly endothermic reaction. This process has a high barrier, 32.68 kcal/mol. The P-O4 bond fission is strongly endothermic by 28.10 kcal/mol. Thus, the unimolecular decomposition of IM2 is less likely to occur under atmospheric conditions. In the troposphere, IM2 will mainly be removed by reaction with O2/NO. 3.1.2. H Abstraction Channels. Two kinds of H atoms exist in DDVP structure: one in the CH3O group and the other attached to the carbon-carbon double bond. Therefore, two primary pathways, R3 and R4, were identified: H abstraction from the CH3O group and abstraction of the H atom attached to the carbon-carbon double bond. For H abstraction from the CH3O group, a hydrogenbonding intermediate, IM3, is formed first. Two intramolecular hydrogen bonds result in a seven-membered ring in IM3. The lengths of the hydrogen bonds are 1.833 and 2.356 Å. The first is a typical hydrogen bond length, and the other is slightly longer than typical hydrogen bonds. The energy of IM3 is 5.01 kcal/mol lower than the total energy of DDVP and OH. After formation of IM3, a hydrogen atom is abstracted from the CH3O group via a low potential barrier, 1.36 kcal/mol. The transition vector clearly shows the motion of H1 between C3 and O5, with an imaginary frequency of 428i cm-1. This process is an exothermic reaction. Thus, H abstraction from the CH3O group of DDVP can occur readily and is expected to play an important role in the degradation of DDVP in the atmosphere. This conclusion is supported by H abstraction from the similar compound (CH3O)3PO by OH (30), which has a large rate constant of (7.7 ( 0.47) × 10-12 cm3 molecule-1 s-1. The product of H abstraction from the CH3O group, denoted IM4, is an open-shell radical and will be further oxidized in the atmosphere. A transition state (TS4) was found for abstraction of the H7 atom attached to the carbon-carbon double bond. In the TS4 structure, the breaking C2-H7 bond is elongated by 25.92%, while the forming H7-O5 bond is longer than the equilibrium value of 0.967 Å in H2O by 27.40%. This process has a high potential barrier of 14.42 kcal/mol, which is 10.6 times higher than that of H abstraction from the CH3O group. 6112

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This indicates that the H atom attached to the carbon-carbon double bond is less activated than the H atoms in the CH3O group. Thus, pathway R4 is not expected to be important. 3.1.3. Substitution Pathway. Reaction pathway R5 can be seen as a substitution process in which the group attached to CH3 in DDVP is substituted by OH to form CH3OH. This process will proceed via the transition state TS5. In the TS5 structure, the breaking C3-O3 bond is longer by 27.90% than the equilibrium value of 1.448 Å in DDVP, while the forming C3-O5 bond is stretched by 30.33%. Figure 2 shows that the substitution pathway is energetically unfavorable for the reaction of DDVP with OH radicals, so this channel may be ignored. 3.2. Secondary Reactions. The above discussion shows that OH addition to the >CdC< bond and H abstraction from the CH3O group are energetically favorable reaction channels for the reaction of DDVP with OH radicals. IM1, IM2, and IM4 are important radical intermediates produced in the degradation process of DDVP initiated by OH radicals. In the atmosphere, they are in an “ocean” of reactive O2 molecules, which represent 21% of the atmosphere. The conventional view is that these radical intermediates, IM1, IM2, and IM4, could react further with O2/NO as their removal from the troposphere. Published work (20) on the products of oxidation of DDVP in smog chambers via hydroxyl chemistry supports this point. These removal reactions are competitive with their unimolecular decomposition reactions. 3.2.1. Atmospheric Reaction Pathway of OH-DDVP Adduct, IM1. The calculated profile of the potential energy surface shows that the reaction of IM1 with O2 is a barrierless association. The structure of OH-O2-DDVP, denoted IM5, is depicted in Figure 3. A comparison of the equilibrium geometry of IM5 with IM1 reveals some intriguing features. The addition of O2 to the OH-DDVP adduct increases the lengths of the C1-Cl1 and C1-Cl2 bonds by 0.051 and 0.054 Å, respectively. The C1-C2 bond adjacent to the site of O2 addition is lengthened by 0.046 Å, as electron density in the π bond is transferred to the newly formed C-O bond. The enthalpy of reaction for the addition of O2 to OH-DDVP is -16.45 kcal/mol. In the troposphere, the OH-O2-DDVP adduct will react immediately with ubiquitous NO. The entrance channel of the reaction is exoergic, leading to a vibrationally excited intermediate (denoted IM6), which promptly reacts via

FIGURE 3. TPSSh/6-31G(d,p)-optimized geometries for the intermediates, transition states, and products involved in the secondary reactions of OH-DDVP adducts with O2/NO. Distances are in angstroms, and angles are in degrees. unimolecular decomposition. The reaction scheme can be described as follows:

IMI + O2 f IM5

∆H ) 16.45 kcal/mol

IM5 + NO f IM6

∆H ) 25.13 kcal/mol

IM6 f TS6 f IM7 + NO2

∆E ) 19.65 kcal/mol, ∆H ) 12.86 kcal/mol

IM7 f IM8 + CCl2O IM8 + O2 f IM9

∆H ) 5.90 kcal/mol

∆H ) 35.81 kcal/mol

IM9 f TS7 f P8 + HO2 ∆E ) 4.42 kcal/mol, ∆H ) 4.26 kcal/mol The optimized geometry of the OH-O2-NO-DDVP adduct, denoted IM6, is presented in Figure 3. When the

equilibrium geometries of IM5 are compared with those of IM6, the torsion angles of C2-C1-O6-O7 are significantly different, implying that addition of the NO group changes the O6-O7 bonding. The corresponding C1-O6 and O6-O7 bond lengths in IM5 and IM6 also change. The addition of NO to IM5 leads to a shortening of the C1-O6 bond from 1.463 to 1.384 Å and an increase of the O6-O7 bond from 1.326 to 1.430 Å. This may be because of redistribution of the electron cloud of the two O atoms in IM6 resulting from formation of the new O7-N bond. Further reaction from IM6 is a direct decomposition via cleavage of the O6-O7 bond to form NO2 and an intermediate, denoted IM7. TS6 represents the transition state for this unimolecular decomposition with a breaking O6-O7 bond length of 1.898 Å. This process has a high potential barrier of 19.65 kcal/mol. IM7 will subsequently decompose to yield VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. TPSSh/6-31G(d,p)-optimized geometries for the intermediates, transition states, and products involved in the secondary reactions of IM4 with O2/NO. Distances are in angstroms, and angles are in degrees.

FIGURE 5. Energetically feasible product channels for the degradation of DDVP initiated by OH radicals in the atmosphere. CCl2O and IM8 via cleavage of the C1-C2 bond. This unimolecular decomposition is a barrierless process. IM8 is an open-shell species and will further react with O2. The potential barrier for IM6 unimolecular decomposition is more than 8 times larger than the barrier to IM1 decomposition. Thus, unimolecular decomposition of IM1 is more favored than its removal reaction with O2/NO and 6114

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CCl2O formed from the reaction of IM1 with O2/NO, which could not account for the large fraction of CCl2O observed in experiments (20). 3.2.2. Atmospheric Reaction Pathway of OH-DDVP Adduct, IM2. The atmospheric reaction pathways of IM2 with O2/NO are similar to the reaction of IM1. The reaction scheme is shown as follows:

IM2 + O2 f IM10 IM10 + NO f IM11

∆H ) 22.16 kcal/mol ∆H ) 21.81 kcal/mol

IM11 f TS8 f IM12 + NO2 ∆E ) 24.90 kcal/mol, ∆H ) -0.57 kcal/mol IM12 f TS9 f P8 + CCl2OH ∆E ) 3.50 kcal/mol, ∆H ) -11.82 kcal/mol CCl2OH + O2 f IM13

∆H ) -24.58 kcal/mol

IM13 f TS10 f CCl2O + HO2 ∆E ) 1.69 kcal/mol, ∆H ) -1.96 kcal/mol The unimolecular decomposition of IM12 produces CCl2OH radicals. The open-shell CCl2OH radical is highly activated, and the removal reaction with an oxygen molecule results in the products CCl2O and HO2. The pathway occurs in two steps. First, CCl2OH is attacked by O2 to form a fivemembered-ring intermediate, denoted IM13, and then HO2 is removed to form CCl2O. The first step is a barrierless association. IM13 has Cs symmetry. The second step has a very low barrier, 1.69 kcal/mol. Comparison shows that the removal reaction of IM2 with O2/NO is more favored than the unimolecular decomposition of IM2. The experimentally observed CCl2O is mainly from the reaction of IM2 with O2/NO. 3.2.3. Atmospheric Reaction Pathways of IM4. The energetic profile of Figure 2 shows that H abstraction from the CH3O group of DDVP is the energetically feasible pathway for photochemical oxidation of DDVP by OH radicals, leading to the products IM4 and H2O. IM4 is an activated radical and can further react with the ubiquitous oxygen molecules in the atmosphere to form the adduct, IM14. In the presence of nitric oxide, IM14 will subsequently react with NO to form an intermediate, IM15. The dominant tropospheric reaction of IM15 is believed to be unimolecular decomposition. The reaction pathway scheme can be described as follows:

IM4 + O2 f IM14 IM14 + NO f IM15

∆H ) -29.17 kcal/mol ∆H ) 20.69 kcal/mol

IM15 f TS11 f IM16 + NO2 ∆E ) 16.38 kcal/mol, ∆H ) -3.65 kcal/mol IM16 f TS12 f P9 + CHO ∆E ) 7.30 kcal/mol ∆H ) -2.60 kcal/mol IM16 + O2 f TS13 f P10 + HO2 ∆E ) 3.17 kcal/mol, ∆H ) 32.03 kcal/mol To evaluate the nature of the entrance channel for the reaction of IM4 with the oxygen molecule, we examined the potential along the reaction coordinate, especially, to determine whether there is a well-defined transition state or if the addition proceeds via a loose transition state without a barrier. The profile of the potential energy surface was scanned by variation of the newly formed C3-O5 bond length. We found no energy exceeding the C3-O5 bond dissociation threshold along the reaction coordinate. This shows that the reaction of IM4 with O2 proceeds via a barrierless association. The process is strongly exothermic by 29.17 kcal/mol. Because of the large number of possible spatial orientations for the O5-O6-N-O7 group in IM15, it is a considerable challenge to locate the global minima of the equilibrium structure. Geometric optimization of IM15 was initially performed with the geometry of IM14, by addition of NO to the terminal O6 atom. The main framework was kept unchanged from IM14. Three important parameters were considered, that is, the torsion angles of O3-C3-O5-O6, C3O5-O6-N, and O5-O6-N-O7. The equilibrium structure of

IM15 is illustrated in Figure 4. The formation of IM15 is exothermic by 20.69 kcal/mol. Unimolecular decomposition of IM15 occurs via cleavage of the O5-O6 bond, forming NO2 and an activated intermediate, denoted IM16. A transition state, TS11, was identified as being associated with the decomposition. The length for the breaking O5-O6 bond is 1.783 Å, which is longer by 24.60% than the equilibrium value of 1.431 Å in IM15. The transition vector clearly shows the motion of O6 between N and O5, with an imaginary frequency of 597i cm-1. Calculations indicate that this unimolecular decomposition has a high potential barrier, 16.38 kcal/mol, and the process is exothermic by 3.65 kcal/mol. IM16 can subsequently decompose to yield a CHO radical and a phosphate compound (P9). This unimolecular decomposition process involves H3 migration and cleavage of the C3-O3 bond. A five-member ring transition state (TS12) was found that has a barrier of 7.30 kcal/mol. The breaking C3-O3 and C3-H3 bonds are longer by 33.91 and 11.74%, respectively, than the equilibrium values in IM16. The exothermicity of the process is predicted to be 2.60 kcal/ mol. The CHO radicals would further react with O2 to form CO and HO2. CO molecules were observed in the chamber in which the OH-initiated oxidation reaction of DDVP was simulated under general atmospheric conditions (20). Another possible tropospheric removal pathway for IM16 was found in this study. IM16 may react easily with ubiquitous oxygen molecules in the atmosphere. This reaction has a very low potential energy barrier, 3.17 kcal/mol. The process is strongly exothermic by 32.03 kcal/mol. The removal pathway can be seen as a simple abstraction reaction in which a hydrogen atom attached to C3 is abstracted by an oxygen molecule to form the products HO2 and a closed-shell organophosphorus compound (P10). The removal reaction of IM16 with O2 is strongly competitive with the unimolecular decomposition of IM16. P10 and HO2 molecules are the possible products for the reaction of DDVP with OH radicals in the presence of O2 and NO. Unfortunately, this removal reaction of IM16 was not considered in the study of Feigenbrugel (20). This energetically favorable removal pathway of IM16 is revealed for the first time. Further direct experimental observation would be anticipated to verify this removal pathway of IM16. Figure 5 shows the energetically feasible product pathways for the degradation of DDVP initiated by OH radicals in the atmosphere. Feigenbrugel et al. (20) conducted an experimental investigation into the degradation products from the reaction of DDVP with OH radicals, but only detected Cl2CO and CO. Most of the degradation products were not identified and quantified because of the absence of reference spectra. Feigenbrugel (20) assumed that CCl2O and CO are the two main carbon-containing products. CO formation is initiated by H abstraction from the CH3O group of DDVP, and the formation of Cl2CO is initiated via OH addition to the carboncarbon double bond. On the basis of the product yields of CCl2O and CO, Feigenbrugel (20) suggested that H abstraction from the CH3O group accounts for 43% of the overall reaction and that OH addition to the >CdC< bond accounts for 47% of the overall reaction. However, our studies show that four product pathways are energetically feasible for the degradation of DDVP initiated by OH radicals in the atmosphere. In addition to CCl2O and CO, HO2 and a closed-shell organophosphorus compound denoted P10 are also easily produced from the pathway initiated by H abstraction from the CH3O group, and CCl2CHO and (CH3O)2P(O)OH are also energetically feasible products from the pathway of OH addition to the >CdC< bond. Thus, it is therefore unreasonable to calculate the branching ratios of H abstraction from the CH3O group and OH addition to the >CdC< bond only from the productivities of CCl2O and CO. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments This work was supported by NSFC (National Natural Science Foundation of China, Project 20507013). The authors thank Dr. Pamela Holt for editing the manuscript.

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Received for review November 25, 2006. Revised manuscript received June 5, 2007. Accepted July 5, 2007. ES0628001