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Environmental Processes

Mechanism of the Thermal Decomposition of Chlorpyrifos and Formation of the Dioxin Analog, 2,3,7,8-Tetrachloro-1,4-dioxino-dipyridine (TCDDpy) Eric M. Kennedy, and John C Mackie Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01626 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Environmental Science & Technology

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Mechanism of the Thermal Decomposition of

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Chlorpyrifos and Formation of the Dioxin Analog,

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2,3,7,8-Tetrachloro-1,4-dioxino-dipyridine

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(TCDDpy)

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Eric M. Kennedy* and John C. Mackie

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Process Safety and Environmental Protection Group

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Discipline of Chemical Engineering, School of Engineering,

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University of Newcastle, Callaghan, NSW 2308, AUSTRALIA.

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ABSTRACT:

Thermal decomposition of the pesticide, chlorpyrifos (CPf) and its major

12

degradation product, 3,5,6-trichloro-2-pyridinol (TCpyol), has been studied by quantum

13

chemical calculation using density functional methods at the M06-2X/GTLarge//M06-2X/6-

14

31+G(d,p) level of theory. Chlopyrifos was found to undergo a series of unimolecular stepwise

15

elimination reactions releasing two molecules of ethylene and finally HOPOS to form TCpyol.

16

TCpyol underwent oxidative decomposition initiated by abstraction of its phenolic H atom by

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O2. Two phenoxy radicals so produced underwent combination leading to the formation of

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2,3,7,8-tetrachloro-[1,4]dioxinodipyridine (TCDDpy). Via Smiles rearrangement both cis and

19

trans TCDDpy are formed. Kinetic models have been constructed to model the decomposition of

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CPf into TCpyol and of the latter into cis and trans TCDDpy. Modeled results are compared with

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the experiments of Sakiyama et al (Organohalogen Compounds, 2012, 74, 1441-1444).

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INTRODUCTION

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Chlorpyrifos (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate, CAS: 2921-88-2)

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(CPf) is one of the most commonly used pesticides in the world. Organophosphate pesticides

26

were introduced to replace the long persisting, soil contaminating chlorinated cyclodienes (e.g.,

27

aldrin, dieldrin, endosulfan). CPf has a half-life in soils generally ranging from 60 to 120 days,

28

but variations, depending on soil type, can occur from 14 days to 1 year (1). Despite its lower

29

persistence, however, CPf exhibits acute neurotoxicity because of its ability to suppress

30

acetylcholine esterase (2). This toxicity is due, in part, to its product of soil hydrolysis of 3,5,6-

31

trichloro-2-pyridinol (TCpyol). Recently a US EPA study (3) has shown that both CPf and

32

TCpyol are toxic to human kidney cells, with the former being the more toxic. A planned EPA

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removal of CPf from the list of permitted pesticides for food crops has now been postponed for

34

further consideration in 2022 (4). Molecular structures of CPf and TCpyol are shown below.

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Little is known about the effects of heat or fire upon CPf. Vegetation recently treated with CPf

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may be subjected to wildfire and storage facilities can undergo accidental fires. Wang et al (5,6)

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estimated that in 2017 the annual emission of chlorpyrifos into the Australian atmosphere as a

40

result of wildfire was 1400 kg. They also emphasized that because of its thermal instability, the

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initial emission of CPf prior to decomposition would be much higher. We have little knowledge

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of possible toxic products emitted from the thermal decomposition of CPf. A major exception to

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this lack of knowledge may be found in the work of Sakiyama et al (7,8). This group studied the

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thermal decomposition (individually) of CPf or TCpyol with air in sealed glass capsules at

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temperatures between 300 and 380°C and a residence time of 15 min. They discovered that

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TCpyol was the major product of decomposition of CPf but they also detected (by GC-MS) very

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small amounts of both cis and trans 2,3,7,8-tetrachloro-[1,4]dioxinodipyridine (TCDDpy) – the

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nitrogen containing analogs of the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin.

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Significantly higher yields of TCDDpy were obtained commencing with TCpyol as initial

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reactant. From DR-CALUXTM studies (9) of the product mixtures obtained at 380°C, Sakiyama

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et al (8) concluded that 2,3,7,8-TCDDpy exhibited extremely high dioxin-like toxicity.

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More recently, Moriwaki et al (10) thermally decomposed CPf and studied products by LCMS

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and GCMS. They established that CPf undergoes a thermal elimination of phosphate and showed

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that 2,3,7,8-TCDDpy arises from thermal decomposition of CPf.

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Apart from these studies, to date we have little or no understanding of the mechanism of thermal

56

decomposition of chlorpyrifos or of its initial product of decomposition, 3,5,6-trichloro-2-

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pyridinol. The aims of this study are to investigate the reaction potential energy surface (PES) of

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CPf, TCpyol and TCDDpy using quantum chemical calculation. From this PES we derive a


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chemical kinetic reaction mechanism to model the observations of Sakiyama et al (8). In

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understanding the evolution of TCDDpy from TCpyol we are guided by the large body of works

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modeling the formation of polychlorinated dibenzo-p- dioxins (PCDD) (11,12,13 and references

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therein). As Sakiyama et al (8) employed sealed glass capsules in their study, we also need to

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consider the possible influences of wall-assisted reactions. These aspects have been studied for

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the analogous PCDD reactions (14,15). The development of a detailed mechanism will enable a

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better understanding to be made of the nature and extent of toxicants resulting from major events

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such as the burning of vegetation in crops and post-harvest residues treated with chlorpyrifos and

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in accidental fires in stockpiles of this pesticide.

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COMPUTATIONAL METHODOLOGY

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All quantum chemical calculations were carried out using the Gaussian 09 suite of programs

70

(16). For development of the reaction PES, all reactant, products and transition state (TS)

71

structures were optimized using the M06-2X density functional (17) together with the basis set 6-

72

31+G(d,p). Intrinsic reaction co-ordinate (IRC) calculations (16) have been employed to ensure

73

TSs are properly linked to reactant(s) and product(s). Improved energy calculations have been

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carried out at the M06-2X/GTLarge//M06-2X/6-31+G(d,p) level of theory. We have chosen to

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use this method because of its previous success in the computation of large polychlorinated

76

molecules such as endosulfan (18), dieldrin (19), hexachlorocyclopentadiene (20) and their

77

products. Rate constants at the high pressure limit for unimolecular reactions have been

78

calculated by the ChemRate program (21) using thermochemical parameters obtained from the

79

quantum chemical computations. Where internal rotations have been identified, using the

80

freq=hinderedrotor keyword in Gaussian 09 (16), their harmonic frequency partition functions

81

have been replaced with hindered rotor partition functions calculated by ChemRate (21) using


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reduced moments of inertia and internal rotation frequencies evaluated by Gaussian 09. Certain

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transition states involve H atom transfer and we have computed tunneling transmission

84

coefficients for these reactions using the unsymmetric Eckart method (22,23). For certain key

85

reactions, reaction enthalpies have also been computed using the composite G4MP2 method (24)

86

which has been found, for a large training set, to be accurate to ±~ 4 kJ mol-1. Application of the

87

G4MP2 method to chlorpyrifos itself, however, was found to be too computationally demanding.

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For the key species, TCpyol, its enthalpy of formation has been evaluated by the isodesmic

89

method (25). Details are provided in Supporting Information as are optimized structures for all

90

species and computed enthalpies of formation at 298 K for all molecules.

91

Because of the large number of differing chemical elements {7} comprising CPf, the public

92

domain Chemkin 2 kinetic modeling program (26), limited to 4 different elements, could not be

93

used. Instead, the public domain software R (27) was programmed with the specific differential

94

rate equations and solved with deSolve which uses the stiff lsodes integrator (28). Modeling was

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carried out under constant temperature and pressure conditions.

96

RESULTS AND DISCUSSION

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Reaction Potential Energy Surface. A preliminary calculation of bond enthalpies indicated

98

that bond fission was highly unlikely at the relatively low temperatures to be modeled in this

99

study. Specifically, rupture of the O-P bond which would lead to the production of a

100

heteroaromatic chlorinated phenoxy radical had a reaction enthalpy at 298 K of 370 kJ mol-1.

101

Likewise, fission of a Cl atom from the aromatic ring required around 380 kJ mol-1. Because of

102

the low concentrations of CPf, initiation via bimolecular abstraction reaction involving O2 (3Σg)

103

is uncompetitive with a unimolecular initiation.


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104

Instead, we find that initiation takes place by a hydrogen atom transfer from the peripheral

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methyl group of one of the ethoxy (-OCH2CH3) groups. There are several possible transfers. In

106

the equilibrium structure of CPf, a H atom on the adjacent ethoxy group is only 2.83 Å distant

107

from the ring N atom. Via an eight-centered transition state, transfer to the N atom can take place

108

as shown in Figure 1 (upper left). The barrier for the initiation is 225 kJ mol-1, significantly

109

lower than that for bond fission. IRC analysis of the transition state (shown in Supplementary

110

Information) indicates that as a consequence of the H-transfer, an ethylene molecule is expelled

111

and the resultant structure of the intermediate IM1a exhibits a long O-P bond of bondlength

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1.815 Å, shown as the dotted bond in Figure 1 and best described as a dative bond between O and

113

P. The N-H bond is also elongated to 1.072 Å. In the TS for IM1a → IM2a, H transfer from the

114

remaining ethyl moiety leads to the expulsion of a second ethylene molecule, again leading to a

115

structure (IM2) with elongated O-P and N-H bonds. Finally, H transfer from the N atom to the

116

ring-bonded O atom leads to the expulsion of HOPOSrot species and formation of TCpyol.

117

A second possible H transfer via a four-centered transition state involves a methyl H transferring

118

to the O atom of the same ethoxy group. This route is shown in Figure 1 (upper right). The

119

barrier for initiation is 228 kJ mol-1, very similar to that of the first route. A subsequent step

120

eliminates a second C2H4 and finally TCpyol is formed together with HOPOS. Barriers in this

121

second route (b) are comparable with those of route (a). The HOPOS molecule differs from its

122

isomer, HOPOSrot only in the orientation of the –OH group. The former is more stable by just

123

1.2 kJ mol-1 at 298 K and the barrier for interconversion is only 27.5 kJ mol-1.

124

We have discovered a third route (c) shown in Figure 1 (lower).

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Figure 1. Three possible routes for the thermal decomposition of CPf into TCpyol. Numbers in italics are reaction enthalpies, numbers in bold are barrier heights, both in kJ mol-1 at 298 K. ACS Paragon Plus Environment

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This route (c) involves transfer of a methyl H via a six-centered transition state to the sulfur

128

atom. The barrier for initiation was found to be 192 kJ mol-1 at 298 K, significantly lower than

129

either initiation barrier for routes (a) or (b). The following two steps in route (c) also have

130

considerably smaller barriers than their counterparts in (a) or (b). They comprise a methyl H

131

transfer to O= via a six-centered transition state and H transfer from sulfur to the ring-bonded O

132

through a four-centered transition state, respectively.

133

Two further routes might be postulated. Via a six-centered transition state, a methyl hydrogen

134

might transfer from one ethoxy group to the oxygen atom of the other ethoxy group. This

135

transfer has been observed in pyrolysis of diethyl phosphate (29). In our case, the products of this

136

transfer would be C2H5OH, C2H4 and trichloropyridinyl-OPOS residue. However, we can

137

discount this route on energetic grounds as the reaction enthalpy computed at 298 K is 280 kJ

138

mol-1 at the M06-2X/GTLarge//M06-2X/6-31+G(d,p) level of theory – significantly greater than

139

the barriers for routes (a)-(c).

140

The final route would involve H-transfer, again by a six-centered transition state, to the O atom

141

bonded to the ring. We have been unable to locate a transition state for this transfer. However,

142

the products would be TCpyol, C2H4 and C2H5OPOS. The computed reaction enthalpy at 298 K

143

is 227 kJ mol-1 and is considerably larger than initiation via route (c). All optimized transition

144

state structures are shown in Supplementary Information.

145

As found experimentally by Sakiyama et al (8), TCpyol is the precursor of the TCDDpy species.

146

Initially, in an investigation of the PES from TCpyol to TCDDpy, we discovered a route

147

involving stable closed shell molecules. This involved first forming an ether linkage between two

148

TCpyol molecules with expulsion of an HCl molecule, followed by a second elimination of HCl


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149

with further cyclization to TCDDpy trans. However, this route was discarded since the activation

150

energy of the bimolecular step was found to be in excess of 250 kJ mol-1, and as the

151

concentration of TCpyol would be no greater than 1×10-6 mol cm-3, the rate of this bimolecular

152

reaction would be negligible even at temperatures of 1500 K.

153

A similar situation has previously been identified in the formation of polychlorinated dibenzo-p-

154

dioxins (PCDD). There the precursor route involving chlorinated phenols is only operative with

155

chlorinated phenoxy radicals (and not phenol molecules) since two such radicals can initially

156

undergo a barrierless recombination reaction sufficiently rapidly to lead to PCDD (14,15). Hence

157

in the current study, we investigate the formation of the phenoxy radical of TCpyol.

158

We have studied the unimolecular fission of the hydroxyl H atom in TCpyol as well as the

159

bimolecular abstraction of this H atom by triplet O2. The reaction enthalpy at 298 K, ∆rH0298, was

160

found to be 397 kJ mol-1 at the M06-2X/GTLarge//M06-2X/6-31+G(d,p) level of theory. A

161

G4MP2 calculation yielded a value for ∆rH0298 = 386 kJ mol-1. Clearly, with such a high reaction

162

enthalpy, unimolecular fission is very unlikely to contribute significantly to initiation of reaction

163

at temperatures as low as 380°C.

164

A transition state structure for H abstraction by O2 was located at 151 kJ mol-1 above the

165

reactants TCpyol + O2 (3Σg). However, this transition structure lies 48 kJ mol-1 below the

166

separated products 3,5,6-trichloro-2-pyridinyloxy (TCpyoxy) + HO2, both calculated at the

167

M06-2X/GTLarge//M06-2X/6-31+G(d,p) level of theory and at 298 K. We have previously

168

encountered a similar phenomenon in an earlier study of H abstraction by O2 from 2-

169

chlorophenol (30). This behavior is indicative of a barrierless reaction in which reactants pass to

170

products without traversing an intrinsic barrier. As this reaction is the key initiation reaction of


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TCpyol, we need to evaluate its rate constant by canonical variational transition state (CVT)

172

methods (31). To this end, from an IRC calculation, we have generated the minimum energy

173

pathway (MEP) centered at the transition structure to find that only those structures lying

174

between -0.28 and +0.28 amu½ bohr exhibited a single imaginary frequency. Only these

175

structures were used in the CVT calculation of the rate constant. We have recalculated the

176

reaction enthalpy by the G4MP2 method which yields a value of ∆rH0298 =176 kJ mol-1 and

177

rescaled the enthalpies along the MEP with G4MP2 values.

178

A global PES for conversion of TCpyol into TCDDpy is shown in Figure 2. TCpyol is first

179

converted into the phenoxy radical, TCpyoxy, by abstraction reactions, initially by triplet O2.

180

Two TCpyoxy radicals can then react in a barrierless reaction of exothermicity 150 kJ mol-1 to

181

form the closed-shell intermediate IM3. Only an additional 34 kJ mol-1 is required to liberate a

182

Cl atom and form IM4. In a reaction of barrier 121 kJ mol-1, closure to the tri-ring TCDDpy trans

183

can take place with elimination of a second Cl.

184 185 186 187 188 189 190

Figure 2. Global PES for conversion of TCpyol into TCDDpy. Numbers in italics are reaction enthalpies, numbers in bold are barrier heights, both in kJ mol-1 at 298 K. ACS Paragon Plus Environment

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191 192

Sakiyama et al (8) also observed TCDDpy cis in their products. This can arise via a Smiles

193

rearrangement (32). We find the barrier for formation of the Smiles intermediate to be only 110

194

kJ mol-1 and this reaction is competitive with closure of IM4 to TCDDpy trans. With a barrier of

195

only 56.5 kJ mol-1, the Smiles intermediate can open the center ring to form IM5. The latter, with

196

a barrier of 120 kJ mol-1 (almost the same barrier as for IM4) can close to TCDDpy cis, also with

197

loss of a Cl atom. The reactions between IM4, TCDDpy trans, Smiles intermediate, IM5 and

198

TCDDpy cis are all reversible reactions. TCDDpy cis has a slightly lower Gibbs free energy than

199

the trans isomer at 298 K, but because the trans isomer has a larger entropy, its free energy drops

200

below that of the cis above about 800 K. Hence the cis should predominate at lower temperatures

201

and the cis:trans ratio should decrease with increase in temperature. This agrees with the results

202

of Sakiyama et al who found higher relative levels of the cis isomer in their products at lower

203

temperatures. Incidentally, Moriwaki et al (10) also indentified structures in their GCMS studies

204

which appear to be the protonated versions of IM4 and IM5 radicals.

205

Chemical Kinetic Modeling. We first attempt to model the thermal decomposition of CPf into

206

TCpyol, whose PES is shown schematically in Figure 1. In their experiments, Sakiyama et al (8)

207

decomposed about 2 mg of reactant in a 10 mL glass ampoule containing air. This corresponds to

208

an initial concentration of gaseous CPf of approximately 5.7×10-7 mol cm-3. Unfortunately, they

209

do not provide yield data with which we might make direct comparison with values predicted by

210

our kinetic model.

211

Initially, we derived rate constants from the quantum chemical data for the nine reactions shown

212

in Table 1. At the highest temperature (380°C, 653 K) studied by Sakiyama et al and at their


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213

residence time of 900 s, we calculated an essentially complete decomposition of CPf and an 18%

214

decomposition at 300°C. However, essentially the same modeled results were obtained by

215

considering just the last three reactions of Table 1 – i.e., the pyrolysis is driven by route (c) of

216

Figure 1.

217

Table 1. Rate Constants for Modeling the Thermal Decomposition of Chlorpyrifos.

218 Forward Rate Constanta A/s-1

n

Ea/kJ mol-1

A/s-1

n

Ea/kJ mol-1

CPf → IM1a

3.9×1012

0.42

237

1.8×109

0.42

89.5

IM1a → IM2a

1.1×1015

-0.006

255

2.5×1011

-0.006

193

IM2a → TCpyol

3.5×108

1.45

174

2.3×105

1.45

107

CPf → IM1b

3.1×1015

-0.119

233

1.3×1011

-0.119

173

IM1b → IM2b

5.8×1012

0.31

242

1.1×1010

0.31

171

IM2b → TCpyol

9.6×1012

0.009

172

7.2×109

0.009

34.3

CPf → IM1c

1.1×1014

0.134

198

1.2×1010

0.134

102

IM1c → IM2c

1.8×1012

0.624

179

1.6×108

0.624

114

IM2c → TCpyol

9.5×1010

0.351

145

5.9×108

0.351

52.7

Reaction

219

Reverse Rate Constanta

a

Rate constant k = ATn exp(-Ea/RT). Reverse rate constant calculated from equilibrium constant.

220 221

Figure 3 shows typical predicted reactant and product profiles at an intermediate temperature of

222

600 K. The yield of HOPOS (not shown) equals that of TCpyol. At higher temperatures (≥ 650

223

K), both CPf and IM1c are completely decomposed into TCpyol and HOPOS. The yield of

224

ethylene is twice the initial CPf concentration at these temperatures.

225


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226 227 228 229 230 231 232 233 234 235 236 237

Figure Figure3.3.Product Productprofiles profilesininthe thethermal thermaldecomposition decompositionofofchloropyrifos chloropyrifosatat673 600K.K. Unbroken Unbrokenblack blackline line– –CPf, CPf,dotted dottedblue blueline line– –CC dashedred redline line– –IM1c, IM2c,dot-dash dot-dash 2H 2H 4, 4,dashed purple purpleline line– –TCpyol. TCpyol.

238

Turning now to a consideration of the kinetics of decomposition of TCpyol into the two isomers

239

of TCDDpy, our premise is that two TCpyoxy radicals, initially formed through O2 abstraction of

240

the phenolic H atom of TCpyol, combine, leading eventually to TCDDpy formation as shown

241

schematically in Figure 2. Rate constants for this mechanism are given in Table 2.

242 243 244


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Table 2. Modified Arrhenius parameters for oxidative decomposition of TCpyol to TCDDpy. Aa

Reaction

n

Ea/kJ mol-1

Ref.

TCpyol + O2 → TCpyoxy + HO2

2.8×1012

0.

141

PWb

TCpyoxy + HO2 → TCpyol + O2

1.6×1011

0.

-11.3

PWc

TCpyol + Cl → TCpyoxy + HCl

1.1×109

1.645

-4.85

PWd

TCpyoxy + HCl → TCpyol + Cl

1.5×107

1.645

12.3

PWc

TCpyoxy + TCpyoxy → IM4 + Cl

2.3×1013

0.

21

PWe

IM4 → TCDDpy-trans + Cl

4.1×1011

0.

125

PW

TCDDpy-trans + Cl → IM4

9.4×1011

0.

56.1

PWc

IM4 → Smiles intermediate

3.6×1010

0.562

105

PW

Smiles intermediate → IM4

4.8×1011

0.562

61.9

PWc

Smiles intermediate → IM5

8.0×1011

0.456

55.2

PW

IM5 → Smiles intermediate

6.5×1010

0.456

106

PWc

IM5 → TCDDpy-cis + Cl

3.7×1011

0.

123

PW

TCDDpy-cis + Cl → IM5

2.1×1012

0.

53.7

PWc

Cl + Cl + M → Cl2 + M

2.2×1014

0.

-7.53

Cl2 + M → Cl + Cl + M

1.5×1015

0.

233

HO2 + HO2 → H2O2 + O2

1.9×1012

0.

6.44

246

PW: Present work – rate parameters obtained from ab initio calculation.

247

a

248

b

249

c

Reverse rate parameters calculated from equilibrium constant.

250

d

Estimated by analogy from Cl + 2-chlorophenol (Ref. 26)

251

e

Estimated in present work.

252

f

Ref. 33

253

g

Ref. 34

f

PWc g

Units are s-1, cm3 mol-1 s-1, cm6 mol-2 s-1 as appropriate. Ea reduced by20 kJ mol-1 from ab initio calculation.


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254

In the global PES shown in Figure 2, two TCpyoxy radicals are shown combining to produce

255

IM3 in a reaction of exothermicity 150 kJ mol-1. IM3 is a large molecule whose heat capacity,

256

CP, is approximately 400 J K-1 mol-1 at 600 K. To fission a Cl atom and form IM4 requires only

257

another 34 kJ. At 600 K, IM3 would possess a peak thermal energy of 240 kJ mol-1 such that

258

IM3 would not be stabilized and would possess more than sufficient energy to surmount the

259

small barrier to form IM4. Kislov and Mabel (35) used similar reasoning in developing a kinetic

260

model for combination of two cyclopentadienyl radicals and we have been able to develop an

261

analogous kinetic model for combination of two pentachlorocyclopentadienyl radicals (20) with

262

a similar assumption. We have allowed a small barrier of 21 kJ mol-1 for the chemically activated

263

combination of two TCpyoxy radicals to form IM4 + Cl. As reaction proceeds, Cl atom

264

concentrations build up so that the possibility of Cl abstraction reaction to form TCpyoxy is

265

taken into account. Termination reactions for Cl atoms and HO2 radicals are included in the

266

model with rate constants taken from the literature.

267

At 380°C (653 K) we model an extent of decomposition of 5.5% for TCpyol with TCDDpy cis

268

and TCDDpy trans in approximate ratio of 4:1 being the only carbon-containing products of

269

significance. H2O2 was the only other significant product, produced at a comparable level with

270

TCDDpy cis. Sakiyama et al (8) did not provide any yield data with which we might compare,

271

only stating that, after their experiments, TCpyol “remained partly unreacted in the ampoules”.

272

They also found that at 340°C, the level of TCDDpy was one fifth of that at 380°C. In

273

comparison, we find an extent of decomposition at 340°C to be 0.9%. Our results for these two

274

temperatures are in nearly the same ratio as those of Sakiyama et al.


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275

Initial reaction between O2 and TCpyol is by far the most sensitive reaction in the model and is

276

rate determining.

277

temperature of 720 K at which decomposition is greater and show profiles in Figure 4.

To present a clearer picture of reactant and products, we consider the

278 279 280 281 282 283 284 285

Figure 4. Product profiles the thermal decomposition of TCpyol at 720 black At this temperature, TCpyolin decomposition reaches 53% with a cis:trans ratioK. of Unbroken 3.6:1. line – TCpyol, dot-dash blue line – TCDDpy cis, dashed red line – TCDDpy trans, dotted purple line – H2O2.

286 287

Sakiyama et al considered that the low yield of TCDDpy from chlorpyrifos might be a

288

consequence of inhibition by HOPOS present in the products. There is, however, an alternative

289

explanation. From our modeling, at 380°C, at their maximum temperature studied,

290

decomposition of TCpyol is slow, so a residence time of 900 s is probably too short to produce

291

significant concentrations of TCDDpy commencing with CPf.

292

As we do not have experimental yield data with which to compare our modeled yields of

293

TCDDpy we should consider the possibility that the experimental decomposition studies (8)


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Page 17 of 25


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conducted in sealed glass ampoules, might have experienced some catalytic surface assistance.

295

It is well-known that promotion of PCDD formation can result from the facile formation and

296

desorption of chlorophenoxy radicals from chlorophenolic precursors on surfaces of copper and

297

its oxides (11,12,14). Another possible source of enhanced reactivity is that impurities of

298

transition metal oxides in silica at around 300°C have been found (36) to promote singlet delta

299

oxygen (O2 1∆g) formation which can significantly lower the barrier to gas phase formation of

300

chlorophenoxy radicals from chlorophenol.

301

Apart from the work of Sakiyama et al (8) we cannot find other references to the toxicity of

302

TCDDpy. However, because of the marked similarity in structures of both cis and trans isomers

303

with the highly toxic 2,3,7,8-TCDD, the two TCDDpy isomers would be expected to be of

304

comparable toxicity. A temperature of 653 K is well below the temperatures that might be

305

obtained in fires of chlorpyrifos contaminated vegetation or in stockpiles of this pesticide.

306

Another high temperature situation would be expected in the smoking of tobacco treated with

307

chlorpyrifos (37). We would expect these elevated temperatures to produce large yields of

308

TCDDpy at short residence times. Our present model predicts that at 900 K and a residence time

309

of 10 s, 71% TCpyol would be converted into the cis and trans TCDDpy isomers in a ratio of

310

2.6:1. However, the actual conversion at this temperature could be higher, since at 900 K and

311

above, H2O2, produced along with TCDDpy, would be significantly decomposed into 2OH

312

radicals. These radicals would be expected to enhance TCpyol decomposition through

313

abstraction but might also produce additional products through their propensity to add to the

314

heteroaromatic rings and undergo further ring decomposition reactions. To clarify the thermal

315

decomposition mechanisms of both CPf and TCpyol at temperatures attained in combustion and


17


316

to eliminate possible effects of surface catalysis, we plan a series of experiments at 900 K and

317

above in a passified flow reactor (18).

Page 18 of 25

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ASSOCIATED CONTENT

320

Supporting Information. Atomic co-ordinates and structures of all optimized molecules.

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Computed enthalpies of formation at 298 K.

322

Programs written in R for integrating rate equations for decomposition of CPf and TCpyol.

323

AUTHOR INFORMATION

324

CORRESPONDING AUTHOR

325

*Corresponding author Eric M. Kennedy

326

Phone: (+61 2) 4985 4422

327

Email: [email protected]

328

Mailing address: Faculty of Engineering and Built Environment, Discipline of Chemical

329

Engineering, School of Engineering

330

AUTHOR CONTRIBUTIONS

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The manuscript was written through contributions of all authors. All authors have given approval

332

to the final version of the manuscript.

333

ACKNOWLEDGMENT


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This research was undertaken with the assistance of resources from the National Computational

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Infrastructure (NCI), which is supported by the Australian Government.

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