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Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/ F Formation from the Combustion of Cyclodiene Pesticides Nirmala Kumuduni Dharmarathne, John C Mackie, Eric M. Kennedy, and Michael Stockenhuber J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05209 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

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The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides

Nirmala K. Dharmarathne, John C. Mackie, Eric M. Kennedy*, Michael Stockenhuber

Process Safety and Environmental Protection Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

*Corresponding author Phone: (+61 2) 4985 4422 Email: [email protected] Mailing address: Faculty of Engineering and Built Environment, Discipline of Chemical Engineering, School of Engineering, University of Newcastle, University Drive, Callaghan, NSW, 2308, AUSTRALIA.

1 ACS Paragon Plus Environment


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ABSTRACT Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 – 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combine to undergo a series of intramolecular rearrangements and Cl fissions producing principally octachloronaphthalene (8ClNP) and Cl2. This process has been studied by quantum chemical calculation and a reaction potential energy surface developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45×1015 exp(-222±9 kJ mol-1/RT) s-1 between 500 – 2000 K. A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen of O2 content 1, 6, 12 and 20 mol%. Increasing O2 to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O2 content (>~10%) the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB but CO yields increased.


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1. INTRODUCTION Hexachlorocyclopentadiene (HCCP) is the chlorinated reagent which undergoes Diels-Alder addition with a substrate to produce the cyclodiene group of pesticides which include dieldrin, aldrin and endosulfan.1 Because of their long life in soils and plant matter and their action as endocrine disrupters2, most cyclodienes, especially dieldrin and aldrin, were banned many years ago. Endosulfan, however, while also now banned or being phased out by many countries, is still in use as a pesticide in several jurisdictions.3 Large stockpiles, particularly of endosulfan, exist and there are considerable environmental risks associated with accidental fires in storage facilities, and from cyclodiene-contaminated plant matter subjected to wildfire. However, little is known about the toxic products emitted in fires of cyclodienes; one important exception was the 1978 study by Chopra et al4 who identified a large number of chlorinated benzenes and other aromatics in the pyrolysis of endosulfan at 1173 K. It might be expected that the combustion of cyclodienes would produce significant emissions of polychlorinated dibenzodioxins and dibenzofurans (PCDD/F), yet to the best of our knowledge, prior to our recent study5 of PCDD/F from oxidation of endosulfan, there have been no previous reports on dioxins’ emissions from cyclodienes. Researchers have thoroughly investigated the mechanistic pathways of formation of PCDD/F and related molecules via several recently discovered precursors6-8. In order to develop safe strategies for destruction of stockpiled cyclodienes, it is important to obtain a better understanding of the mechanism of formation of toxic products, especially dioxins and their precursors, in the oxidative degradation of cyclodienes. In our previous studies of oxidative5 and non-oxidative9 decomposition of endosulfan, we discovered that reaction was initiated by retro-Diels-Alder decomposition into HCCP and the



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substrate 4,7-dihydro-1,3,2-dioxathiepine-2-oxide (2HDTO). This reaction takes place at about 600 K in a flow reactor of residence time 5 s. The Diels-Alder reaction is reversible, hence decomposition is slow at temperatures below which HCCP does not decompose. Once temperatures are attained at which HCCP can fission a chlorine atom, decomposition of endosulfan is rapidly accelerated as Cl atoms attack unreacted endosulfan abstracting an H atom and leading to rapid bond fissions and to formation of chlorinated aromatics. Under oxidative conditions, these chlorinated aromatics become precursors of PCDD/F. We have observed similar behavior in a study of the decomposition of dieldrin.10 Knowledge of the mechanism and rates of HCCP decomposition under both non-oxidative and oxidative conditions is therefore crucial to understanding the mechanisms of oxidative and nonoxidative degradation of cyclodienes and to the development of detailed kinetic models. The rate constant for Cl atom fission from HCCP has not been previously measured. Furthermore, little is known about the products of decomposition of this molecule. A brief report11 in 1962 stated that gas phase pyrolysis of HCCP at 973 K yielded 90% octachloronaphthalene and 5% bis-(pentachlorocyclopentadienyl). When heated over iron catalyst at lower temperatures, perchlorofulvalene was also detected.11 Herein we report an experimental and theoretical investigation on the kinetics of fission of HCCP into pentachlorocyclopentadienyl radical (PCCP•) and chlorine atom together with a detailed quantum chemical mechanistic study of the reaction potential energy surface leading to major observed products under both pyrolysis and oxidation conditions. 2. EXPERIMENTAL METHODOLOGY Hexachlorocyclopentadiene was purchased from BOC Sciences, USA, of stated purity 97% and used without further purification. Inert gas (He and N2) pyrolysis and oxidative decomposition of HCCP were carried out in a silica flow reactor maintained in a three-zone 4 ACS Paragon Plus Environment

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furnace. The entry zone of the three zone furnace maintained at 533 K to slow evaporation of HCCP, and other zones of the furnace maintained at the desired experimental temperature. The reactor used in this study is a 10 mm inner diameter quartz tube, 50 cm in length (purity 99.5%), and two quartz rods (9.5 mm o.d.) were inserted into the quartz tube to define the reactor volume and to ensure the gas stream reached the reaction zone (12 cm) rapidly. To eliminate the surface effect, we coated the silica reactor with boron oxide (see details in Supplementary Information) and satisfactory boron oxide coating appears transparent. The flow reactor, product collection system and analysis train has been described in detail in our earlier publications5,9 and associated Supplementary Information. A syringe pump was used to introduce reactants into the system and diluted with carrier gas. The experiments proceed for one hour with 5 s residence time and the product species trapped into the condenser submerged in a glycol bath maintained at 263 K. Gaseous species (except for Cl2) were analysed by FTIR spectrometry. Volatile organic compounds have been analysed by GCMS as described previously.5,9 Cl2 was analysed by ion chromatograpy (IC) by first dissolving the gas in dilute aqueous sodium hydroxide in which it forms both Cl-(aq) and ClO-(aq). Both peaks in the IC were quantified to obtain the yield of Cl2. Further details of the analyses are provided in Supplementary Information. Inert gas pyrolysis was carried out in both helium (99.999%) and nitrogen (99.999%). Oxidative studies were carried out in mixtures of nitrogen and oxygen of O2 content 1,6,12 and 20%. Residence times in the reactor were normally 5.0 s although some runs were carried out at 2.0 s and 10.0 s to check for thermal equilibration in the furnace. 3. COMPUTATIONAL METHODOLOGY All quantum chemical calculations were carried out using the Gaussian 09 suite of programs.12 For computation of the rate constant for Cl atom fission from HCCP, relaxed



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potential energy scans were carried out along the dissociating C-Cl bond from the equilibrium separation of 1.781 Å to a distance of 3.30 Å using the density functional M062X and the basis set 6-31+G(d,p). Improved energies were obtained using the GTLarge basis set. As no discrete barrier was located on the potential energy surface (PES), a canonical variational transition state theory (CVTST) computation of the rate constant was employed. Spin-orbit interactions were not taken into account in the evaluation of the PES. Frequency calculations were made on optimized structures along the C-Cl potential surface. Scale factors of 0.979 for frequencies and 0.969 for zero point energies were used.13 As there is no literature value for the enthalpy of formation, ∆fH0298, for PCCP• and the corresponding value for HCCP is very uncertain, the composite method, G4MP2, was employed to obtain more accurate values. Thermochemical and kinetic parameters were computed by the ChemRate program.14 High pressure rate constants were obtained at temperatures from 500 to 2000 K for each optimized structure. Under conditions where the extent of decomposition of HCCP was high, recombinations of pentachloro-cyclopentadienyl (PCCP•) radicals and/or reaction of PCCP• with unreacted HCCP lead to several of the observed products and global PESs have been computed at the M06-2X/GTLarge//M06-2X/6-31+G(d,p) level for both pyrolysis and oxidative conditions. The PCCP• radical is analogous to the cyclopentadienyl (c-C5H5•) radical which has been the subject of many theoretical investigations.15-17 The expected D5h symmetry of c-C5H5• is distorted by Jahn-Teller interaction into two C2v states of nearly equal energies, one of which has an imaginary frequency and can be considered as a transition state (TS) for “pseudorotation”. As c-C5H5• and, by analogy, PCCP•, are best treated by methods such as the Complete Active Space Self-Consistent Field (CASSCF)18, we have carried out comparative computations on PCCP• and c-C5H5• using CASSCF/cc-PVDZ with 5 electrons (the 5 2pπ electrons on C atoms) in 5 orbitals to compare with optimizations using M066 ACS Paragon Plus Environment

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2X/6-31+G(d,p) and with B3LYP/6-31G(2df,p) (the method used for optimization in G4MP2). 4. RESULTS AND DISCUSSION Experimental Study of HCCP Pyrolysis In our flow reactor at a residence time of 5 s, HCCP decomposes between about 690 and 900 K. Runs were carried out in both helium and nitrogen bath gases at initial mole fractions of HCCP of 2.0±0.2 ×10-3. No significant difference in product yields was observed between the two bath gases. Overwhelmingly, the major chlorocarbon product is octachloronaphthalene (8ClNP). It was observed at all temperatures in the studied range and its yield asymptoted to 0.5 (based on initial HCCP). See Figure 1. Molecular chlorine, Cl2, was also a major product which was quantified by ion chromatography. The only other chlorocarbons observed were hexachlorobenzene (maximum yield ≤ 0.04) and 1,3-perchlorobutadiene (yield < 0.001) together with traces of octachlorostyrene and perchlorofulvalene. Figure 1 near here. Electronic Structure Calculations on the PCCP• Radical It is generally accepted that methods such as CASSCF give the best representation of the C2v conformations arising from Jahn-Teller distortions from D5h symmetry in the much studied analogous c-C5H5• radical. We first present results of CASSCF(5,5)/cc-pVDZ computations on PCCP•. These calculations are too computationally intense to be carried out on the many large structures, some containing up to 10 and 11 chlorine atoms, discussed in later sections. Therefore, for consistency of computational methods, we also test the ability of the DFT



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methods, M06-2X and B3LYP, which are less demanding computationally, to calculate the conformations in PCCP•. The CASSCF computations located two optimized C2v structures of very nearly equal energy. One of these, labeled M (following the nomenclature of Bearpark et al16 for c-C5H5•) and exhibiting a dienyl structure, was a true potential energy minimum. The other (labeled TS), possessing an allylic structure, returned a single imaginary frequency. These structures are sketched in Figure 2.

Cl

Cl Cl

.

Cl

Cl

Cl

Cl

.

Cl Cl

Cl

(a)

(b)

Figure 2. Conformers of c-C5Cl5• radical: (a) dienyl structure (M), (b) allylic structure (TS).

Similar CASSCF results have been obtained previously with c-C5H5•. The electronic energy of M is slightly lower than that of TS, however, the situation is reversed when zero point energies are included. Table 1 compares molecular parameters obtained by CASSCF, M062X and B3LYP methods. Spin-orbit interaction has not been taken into account in the CASSCF computations. Table 1 near here. As may be seen from Table 1, all three methods yield very similar molecular parameters and CASSCF and M06-2X are in agreement with the small energy difference between allylic and dienyl conformers. Thus we conclude that the DFT method, M06-2X should be suitable for further analysis of PCCP•. Theoretical Calculation of the Rate Constant for HCCP → PCCP• + Cl


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From the relaxed potential energy scan of the increasing C-Cl bondlength, optimizations at discrete points were obtained. Structures with bondlengths between 2.68 and 3.10 Å exhibited a single imaginary frequency, with the latter distance being the largest returning just a single imaginary frequency. Unrestricted methodology was used in the optimizations in this region of bondlengths to avoid RHF→UHF instabilities to obtain scaled frequencies and rotational constants for subsequent calculation of thermochemical parameters. Energies and enthalpies of reactant, intermediate structures and of radical products were then evaluated at the M062X/GTLarge//M06-2X/6-31+G(d,p) level of theory. Because of the lack of reliable thermochemistry both for HCCP and PCCP• we decided to calculate ∆fH0298 by G4MP2 and the isodesmic method. Although the NIST Webbook19 quotes a single value of ∆fH0298 for HCCP of -11.7±4.4 kJ mol-1 obtained 40 years ago by bomb calorimetry, the following analysis suggests possible revision of this value might be necessary. For HCCP, the following two isodesmic reactions were used to calculate ∆fH0298. c-C5Cl6 + C3H8 +2C2H4 = c-C5H6 + 2cis-CHCl=CHCl + CH3CCl2CH3

(1)

c-C5Cl6 + 2C2H4 + CH4 = c-C5H6 + 2cis-CHCl=CHCl +CH2Cl2

(2)

Details of the experimental values of ∆fH0298 for each of the known components in the two reactions and their sources are provided in Table S1 of Supplementary Information. From eqn (1) we obtain ∆fH0298(HCCP) = 4.4±4.1 kJ mol-1 and from eqn (2), 3.7±3.6 kJ mol-1. Eqns (3) and (4) were used to calculate ∆fH0298 for PCCP•. c-C5Cl5• + CH3• + 2C2H4 = c-C5H5• + 2cis-CHCl=CHCl + CH2Cl• c-C5Cl6 + CH3• = c-C5Cl5• + CH3Cl


(3) (4)


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Eqn (3) yields ∆fH0298(PCCP•) = 129±6 kJ mol-1 and, using the average of the two values calculated above, viz., ∆fH0298(HCCP) = 4.1±4.4 kJ mol-1, we obtain a value of ∆fH0298(PCCP•) = 122±5 kJ mol-1 from eqn (4). The principal source of error arises from the ±4 kJ mol-1 error in the enthalpy of formation of c-C5H5•. Note that the calculation of ∆fH0298(PCCP•) using eqn (3) is independent of the uncertain literature value of ∆fH0298(HCCP) yet there is very good agreement between the values calculated by eqns (3) and (4). If we had used the value of -11.7 kJ mol-1 from Ref. 19, the value from eqn (4) would differ from that obtained by eqn (3) by 23 kJ mol-1. We believe this to be outside the probable error limits of the G4MP2 calculations and have chosen to adopt our averaged G4MP2 values for ∆fH0298 of HCCP and PCCP•. This leads to a value of ∆rH0298 = 243±9 kJ mol-1 for fission of Cl from HCCP as calculated by the G4MP2 method. The comparable value obtained at M06-2X/GTLarge//M06-2X/6-31+G(d,p) was 235 kJ mol-1. For more accurate enthalpies we have chosen to use the G4MP2 values and have consequently scaled the enthalpies calculated by the M06-2X method at discrete points along the reaction path by G4MP2 enthalpies. Scaling factors ranged from 1.12 to 1.09. Rate constants for discrete points along the PES were calculated by ChemRate for temperatures from 500 – 2000 K and listed in Table 2. The minimum rate constant for each Table 2 near here. temperature is shown in bold in the table. This is the position of the transition state at the given temperature. At 500 K, the transition state is found at a C-Cl bondlength of 3.08 Å whereas at 2000 K, the transition state is at the reduced bondlength of 2.78 Å. The variational rate constants can be fitted by the Arrhenius relation k = 1.45×1015 exp(-222 kJ mol-1/RT) s-1.


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Next we consider, the evaluation of the rate constant at the experimental pressure of 1 atm. Computing fall-off curves at reduced pressures for this reaction raises problems. Although critical constants for HCCP are available from literature20 and Lennard-Jones parameters can be calculated from these, no value of the collisional energy transfer probability exists for this molecule. We have therefore chosen a simple collision model – exponential down with a single term α0 = 200 cm-1 together with Lennard-Jones parameters σ = 6.78 Å and ε/kB = 730 K for HCCP in N2 buffer gas. With this choice, calculations found no fall-off effects over the range of experimental temperatures. Reduction of α0 to 100 cm-1 likewise led to no significant fall-off over the experimental temperatures. Before making a comparison with experiment, we should also consider the implications of neglecting spin-orbit interactions in the present calculations. The triple degeneracy of the dissociated Cl atom in its 2P ground state is split by spin-orbit interaction into a 2P3/2 state which is stabilized by ESO/3 and a 2P1/2 state destabilized by 2ESO/3. The presence of a molecular radical, R•, further reduces the degeneracies. Spin-orbit stabilization is very small in the molecule RCl but increases with increase in R-Cl separation. Qualitatively, spin-orbit interaction raises the reaction potential energy surface relative to the reactant, hence reducing the computed reaction rate.21 The magnitude of the spin-orbit interaction is very dependent on the C-Cl bondlength and in our CVTST analysis the position of the transition state is located at differing C-Cl separations at different temperatures. To quantify the varying spin-orbit interactions would require a multireference computational method and this information cannot be obtained by the single reference methods we have employed. At best we can make a crude upper bound estimate of the effect of spin-orbit interaction on our calculated rate constant k using the relation21 kSO = k exp(-ESO/3kBT)


(5)


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where kSO is the computed rate constant adjusted for spin-orbit interaction. For HCCP this would involve a maximum reduction in the high pressure rate constant of a factor of 0.55 at 700 K. Jasper et al21 have made an extensive variable reaction co-ordinate transition state theoretical analysis of the effect of spin-orbit interaction upon the rate constants for R + X → RX where R is methyl or allyl and X = F, Cl and Br. They find that for both radicals and for F and Cl that spin-orbit interaction reduces the computed rate constants by only about 15%. However, it is not clear that their result is applicable to HCCP fission as this takes place over a different range of C-Cl bondlengths. We compare the theoretically derived rate constant with experimental pyrolysis data after a consideration of a theoretical model for products’ formation. Products Formation from HCCP under Pyrolysis and Oxidative Conditions Experimental Results - Pyrolysis In pyrolysis, at temperatures at which decomposition of HCCP is large, significant concentrations of PCCP• are formed and these radicals are able to undergo recombination reactions or, possibly add to HCCP itself. Under these conditions, only one major product, octachloronaphthalene, was detected. In addition, small amounts of hexachlorobenzene, octachlorostyrene and perchlorofulvalene were observed. There were traces of another C10Cl8 isomer which might possibly have been octachloroazulene, but this could not be confirmed. Modeling of Pyrolysis Products’ Formation In developing a model to explain products’ distribution from HCCP we are guided by the considerable volume of theoretical studies on products’ formation from the analogous cC5H6. There are several similarities but also differences. Addition of c-C5H5 to c-C5H6 has been considered to be more important than recombination of two c-C5H5 radicals.22 However,


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in their detailed description of the C10H9 surface, Kislov and Mebel23 considered both radicalmolecule and radical-radical processes might contribute to C10H9 formation – via loss of successive H atoms in the former case, or fission of a single H in the latter case. The situation is less ambiguous in the chlorinated case. Whilst Kislov and Mebel found that both radical-molecule and radical-radical reactions were exoergic, we find from our M062X/GTLarge//M06-2X/6-31+G(d,p) calculations that the addition of c-C5Cl5 to c-C5Cl6 is endoergic by 23 kJ mol-1 with a barrier of 73 kJ mol-1. Hence, we model reactions on a global PES commencing with C10Cl10. In their analysis of the C10H9 surface, Kislov and Mebel23 labeled C10H10 as S0 and we adopt this terminology in our subsequent analysis. Species on our PES which are the direct chlorinated analogues of the more than 40 hydrocarbon species identified by Kislov and Mebel are labeled Sn. Because of the relatively large bond energies, C-H fissions cannot compete with many of the intramolecular rearrangements on the hydrocarbon surface. C-Cl fission energies are considerably smaller so that many fewer intramolecular rearrangements need to be considered on the present PES shown in Figure 3. Figure 3 near here. Fission of a Cl atom from S0 produces S1. S1 primarily rearranges to S2 although a lesser amount can undergo barrierless Cl fission to form the small observed yield of perchlorofulvalene (8ClFL). After S2 rearranges to S3, the latter can either rearrange to S4 or to S11. S4 can either rearrange to S10 or to S6 which can eventually lose a Cl atom to form octachloroazulene. S10 is the source of the major product, octachloronaphthalene (8ClNP) via a facile loss of Cl. Indeed, it is possible that S10 has only a fleeting existence, fission requiring a mere 4.2 kJ mol-1. S11 can rearrange to S10 but can also undergo recombination with a Cl atom leading potentially after further additions of Cl atoms to formation of octachlorostyrene (8Clstyrene), 13 ACS Paragon Plus Environment


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hexachlorobenzene and the chlorocarbons, CCl4 and 1,3-perchlorobutadiene, all of which are observed in trace quantities. (Note that Cl addition to perchlorobuta-1,3-dienylpentachlorobenzene at the ring has a barrier of -1.0 kJ mol-1 at the M06-2X/GTLarge//M062X/6-31+G(d,p) level but exhibited a positive barrier of 1.4 kJ mol-1 at M06-2X/631+G(d,p).) In a direct analogy with the hydrocarbon case, the route of lower barrier to 8ClNP is S3→S4→S10→8ClNP. Comparison of Computed Rate Constant with Experimental Pyrolysis Data As may been seen in Figure 1, at all studied temperatures, decomposition of HCCP is always accompanied by significant yield of 8ClNP. In the 5 s residence time of the flow reactor, appreciable secondary reactions can take place, viz., recombination of Cl atoms with PCCP• (since three-body termination of Cl atoms is relatively slow), and combination between two PCCP• radicals which eventually produce 8ClNP. Abstraction of a Cl atom from HCCP by Cl to form molecular chlorine is also an important reaction and we have optimized a transition state for this reaction. Hence we cannot obtain an experimental first order rate constant for HCCP fission simply by measuring the decrease in HCCP concentration upon passing through the reactor. Instead, we need to model not only the fission reaction but also the important secondary reactions. Since 8ClNP is the only chlorocarbon product of significance, we can restrict our kinetic model to its formation together with Cl2 and to residual HCCP. From the global PES in Figure 3 we can construct the simplified reaction PES shown in Figure 4 beginning with the combination of two Figure 4 near here. PCCP• radicals. We can then model reactant and products by the following mechanism:


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HCCP = PCCP• + Cl

(R1)

PCCP• + PCCP• → S1 + Cl

(R2)

S1 = S2

(R3)

S2 → 8ClNP + Cl

(R4)

Cl + HCCP = PCCP• + Cl2

(R5)

Cl + Cl + M = Cl2 + M

(R6)

Adoption of (R2) needs explanation. From Figure 4 we see that S0 is produced from two PCCP• radicals in a well of depth 121 kJ mol-1. A similar situation takes place with the hydrocarbon analogue. Kislov and Mebel23 argued that because it is a large molecule, the analogous hydrocarbon S0 would have a peak thermal energy of around 200 kJ mol-1 at 1500 K and hence S0 would not be stabilized and would easily surmount the barrier to form S1 + H. The chlorocarbon S0 is an even larger molecule. At 750 K, it has a heat capacity, Cp = 400 J K-1 mol-1 hence would have a peak thermal energy of about 300 kJ mol-1 at this temperature, more than sufficient to overcome the 78 kJ mol-1 barrier to form S1 + Cl. While the barrier for S1 → S2 is only 86 kJ mol-1 the reverse barrier is 23 kJ mol-1 so that equilibration will take place and must be accounted for in the mechanism. The barrier for S2 → S3 is 84 kJ mol-1 but once surmounted, passage to 8ClNP should be facile and take place via chemical activation. Rate constants used in modeling are given in Table 3. Table 3 near here. Kinetic modeling was carried out using CHEMKIN II 24 via the SENKIN module at constant temperature and pressure. Results of modeling with the above model are shown in Figure 1 and can be seen to give good agreement with the experimental profiles of 8ClNP and HCCP



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and Cl2 (for the last two, up to about 850 K whereupon some experimental mass balance problems arise). Sensitivity analysis indicated that both forward and reverse reactions of R1 are sensitive reactions although approaching equilibration in the 5 s residence time. The forward reaction of R5 is also a sensitive reaction. There is also a posteriori justification from the modeling in treating R2 as irreversible – if stabilization of S0 is assumed, reversal takes place and no net decomposition of HCCP is modeled. Modeling of Products’ Formation under Oxidative Conditions Initial studies of oxidative decomposition of HCCP (initial mole fraction of 2.0±0.2 ×10-3) were carried out in an untreated silica reactor of silica content 99.5% to study the effects of various levels of O2 (1, 6, 12 and 20%) in nitrogen on the rate of disappearance of HCCP and yields of major products. Although with 1% O2, the conversion of HCCP was little altered from that observed in pyrolysis, oxygen levels of 6 and 12% considerably increased the decomposition of HCCP and decomposition commenced at temperatures significantly lower than the onset temperature of pyrolysis. Surprisingly, with 20% O2 there was a marked reduction in decomposition at any given temperature between 720 and 920 K. Addition of O2 at all levels decreased the yields of 8ClNP, markedly with addition of 6 and 12%. Again, the effect of 20% O2 was less than that of either 6 or 12% mixtures. Under oxidative conditions hexachlorobenzene (HCB) became a major product while 1,3-perchlorobutadiene, although a minor product, increased significantly compared with pyrolysis. Chlorocarbons CCl4 and C2Cl4 also underwent large increases over their traces in pyrolysis but 8Clstyrene was still only observed in trace quantities. Product profiles obtained from the untreated silica reactor are shown in Supplementary Information (Figures S1 (a)-(g)). Decomposition at temperatures prior to initiation of pyrolysis suggests that oxygen is able to react directly with HCCP. Direct abstraction of a Cl atom by ground state O2 (3Σg) to form


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ClOO• can be discounted because of its high ∆rH0298 =227 kJ mol-1 (∆fH0298 (ClOO•) = 98 kJ mol-1 (Ref. 25)) and low pseudo-first order A-factor relative to the Cl fission of HCCP. We have also made an extensive quantum chemical investigation of adduct formation between ground state O2 and HCCP. Although we have located several weakly bound adducts, none has been found to undergo further reaction of sufficiently low barrier to compete with simple dissociation of the adduct back to HCCP +O2. In earlier studies of oxidation of biphenyl26 and 4-chlorobiphenyl27 in an alumina reactor and an untreated silica reactor similar to the reactor used in the present studies, products which could only be attributed to the presence of singlet delta oxygen (SDO), 1∆g, were obtained. Specifically, benzaldehyde, a low temperature product observed from these reactors, was shown by quantum chemical calculation to arise only from initial addition of SDO to one biphenyl ring with subsequent O-O bridging to the other ring, leading to low barrier rupture of the two-ring structure, rearrangement and benzaldehyde formation.26,27 It is now generally accepted that many surfaces, especially alumina, transition metal oxides and silica can catalyse the formation of SDO at temperatures between about 500 and 700 K.28,29 Thus we attribute the low temperature initiation of oxidation in the present untreated reactor to the presence of SDO. The value of ∆rH0298 for abstraction of Cl from HCCP by SDO is only 128 kJ mol-1 and Cl abstraction by SDO should be able to compete with bond fission of HCCP. When we passified27 the internal surface of the tubular silica reactor with a coating of boron oxide, we no longer observed oxidative decomposition at temperatures lower than the onset temperature of pyrolysis. At the lowest temperatures where product formation was detected, 8ClNP was the principal product but as temperature increased, a maximum in this product was observed, followed by its rapid decline and an increase in HCB was observed. At the



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higher temperatures, HCB and CO were the principal carbon containing products with much smaller amounts of 1,2-perchlorobutadiene, CCl4 and C2Cl2 produced. Major products’ profiles are shown in Figure 5. Qualitatively similar products were observed in both untreated and passified reactors but yields and temperature profiles differed significantly. Figure 5 near here In developing a model for oxidative decomposition of HCCP by ground state oxygen, we start with the experimental observation of initiation by Cl fission from HCCP. Clearly, O2 (3Σg) inhibits the recombination of two PCCP• radicals and/or subsequent reactions which would eventually form 8ClNP. Such inhibition will also diminish the production of hexachlorobenzene and the chlorocarbons via the routes shown in pyrolysis. New, more facile routes to these latter compounds must therefore open up under oxidative conditions. We first investigated the addition of O2 to PCCP• to form a peroxy radical, PCCPOO•. In fact, there are several conformers of PCCPOO• differing in the orientation of the –O-O• moiety with respect to the ring. The most stable of these is the conformer of Cs symmetry, I, which has a barrier of 34 kJ mol-1 for its formation. (See Figure 6 for optimized structures.) Figure 6 near here However they are all formed in very shallow wells, the deepest being only 18 kJ mol-1. The enthalpy change for decomposition to PCCPO• + O is large at 230 kJ mol-1, hence reversal to PCCP• + O2 would be very rapid and we have been unable to find a rearrangement of PCCPOO• of sufficiently low barrier to propagate further reaction. A similar situation has been found with the peroxy adduct of c-C5H5• .31,32 Instead, we have investigated by quantum chemical calculation the possibility of the product of recombination of two PCCP• radicals, S0, possibly in an activated state, reacting with O2 18 ACS Paragon Plus Environment

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(3Σg) to lead to the observed products. Since it would seem that O2 (1∆g) reacts to give similar products, we have also investigated its reaction with S0. Transition states for addition of both O2 and SDO to S0 have been located. Addition takes place at a carbon atom one removed from the bridge atoms of the two C5 rings. As may be noticed in the PES shown in Figure 7, the barrier on the triplet surface is located 103 kJ mol-1 above the isolated reactants and the reaction is endoergic (at 0 K) by 80 kJ mol-1. On the singlet surface, the barrier is 27 kJ mol-1 above S0 + O2 (1∆g) and the reaction is slightly exoergic at -15 kJ mol-1. Structures of the triplet and singlet peroxy adducts, S0-OOtrip and S0-OOsing, (II), have almost identical bondlengths and angles and differ in their energies at 0 K by only -0.8 kJ mol-1, with the singlet adduct lower lying. Figure 7 near here These S0 peroxy biradicals can then form a second bridge between the two rings with the peripheral O atom bonding to the C atom of the opposite ring adjacent to the C-C bridging atom to form a double bridged structure. Barriers for this process are very similar at 69.9 (triplet) and 70.7 (singlet) kJ mol-1. The resulting three-ring endoperoxides, S0-OObridge, (III), again have very similar bondlengths and angles and differ in their energies at 0 K by only 1.1 kJ mol-1 with the singlet once again lower lying. It should be pointed out that in optimizing the singlet species, the keyword guess=(mix,always) was employed and all optimizations have been checked for any wavefunction instabilities. Optimized structures are shown in Figure 6. Both the S0-OO peroxy and the S0-OO bridged species are disjoint biradicals33,34 having a pair of essentially non-interacting nonbonding molecular orbitals occupied by a total of two electrons. Disjoint biradicals have attracted a good deal of interest both from experimentalists and theoreticians in establishing the nature of their ground states which can be confused 19 ACS Paragon Plus Environment


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because they generally exhibit weak to moderate triplet paramagnetic signals. Although accurate computation of the singlet-triplet gap requires a multiconfiguration method with large basis sets, these and DFT methods generally yield very small gaps with singlet ground states and the paramagnetism is now accepted to arise from facile thermally driven intersystem crossing.35,36 With our two disjoint biradicals, not only are the structures of the triplet and singlet conformers nearly identical, only very small torsional movements would be required to shift from triplet to singlet structure. Furthermore, both triplet and singlet forms of the peroxy and bridged species have almost identical harmonic frequencies as computed by M06-2X/631+G(d,p) hence intersystem crossing by enhanced vibration-vibration transfer would be facile. Hence, with triplet O2 as oxidant, reaction would initially take place on the triplet surface but rapid intersystem crossing in S0-OOperoxy and/or S0-OObridge would lead to further reaction on the singlet surface. The only difference, were singlet O2 present, would be the possibility of formation of PCCP• radicals by abstraction at temperatures lower than the onset temperature of pyrolysis. Reaction would take place then entirely on the singlet surface but the same products would result. Further reaction on the PES involves fission of the bridge C-C bond in S0-OObridge with a barrier of 42 kJ mol-1 in a reaction which is -80 kJ mol-1 exoergic. Diperchlorocyclopentadienyl peroxide (IV) so-formed can then undergo O-O fission into two pentachlorocyclopentadieneoxy radicals (V). This fission which does not exhibit a discrete barrier requires 144 kJ mol-1. However, once the barrier to forming S0-OObridge is overcome, the formation of this pair of radicals can take place via chemical activation. It should be noted that the hydrocarbon analogue, cyclopentadieneoxy, has been found to be a key radical intermediate in the pathway to benzene formation in oxidation of


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cyclopentadiene.31,32 We also find that the perchlorinated radical plays an important role in hexachlorobenzene formation from HCCP. There is a facile shift of the chlorine atom ipso to the O atom to the adjacent carbon atom in pentachlorocyclopentadieneoxy (5ClcpdO•). This has a barrier of only 13 kJ mol-1 and forms 2,2,3,4,5-pentachloro- 3-cyclopenten-1-one radical (VI) in a well of depth 149 kJ mol-1 (Figure 7). This radical then undergoes ring fission to open-chain C5Cl5O• (VII) through a TS of barrier 154 kJ mol-1 followed by CO fission through a further barrier of 91 kJ mol-1 yielding n-C4Cl5• (VIII). This radical is analogous to one of several isomeric n-C4H5• radicals which together with the i-C4H5• radicals, have been much studied37-39, because of their importance as intermediates in formation of benzene under combustion conditions. More recently, McIntosh and Russell40 have studied reactions of C4Cl5•, C4Cl3• radicals and their reactions with C2Cl2 with respect to cyclization and other processes. The n-C4Cl5• radical formed in our system has all but one of its chlorine atoms lying on the same side of the carbon backbone. This isomer, following the terminology introduced by Krishtal et al41 is labeled c5-cis. Combination with a Cl atom produces the observed 1,3perchlorobutadiene. We have been unable to locate a TS for fission of c5-cis into C2Cl3• + C2Cl2. Analogous with its C4H5• counterpart, c5-cis must first isomerize through a TS with a barrier of 32 kJ mol-1 to c5-trans, (IX), whereupon fission into C2Cl3• + C2Cl2 takes place with a barrier of 206 kJ mol-1. Addition of C2Cl2 to n-C4Cl5• and cyclization of the resultant open-chain C6Cl7• to form c-C6Cl7• (X) have been studied previously40,41 as have the hydrocarbon analogues.31,32 We find that addition has a barrier of only 4.6 kJ mol-1 and the addition product is formed in a well of depth 203 kJ mol-1. Cyclization to c-C6Cl7• entails a barrier of 31 kJ mol-1 into a well of depth 212 kJ mol-1. Through a TS of barrier of 20 kJ mol-1 a Cl atom is detached to form a 21 ACS Paragon Plus Environment


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post-reaction complex, C6Cl6-Cl, (XI) located 10.5 kJ mol-1 below this TS. Finally, in a barrierless process requiring an additional 20.5 kJ mol-1, equilibrium products hexachlorobenzene and Cl atom are obtained. Kinetic Modeling under Oxidative Conditions Full validation of our oxidation mechanism would require the development of a detailed kinetic reaction mechanism including a multiwell analysis. However, we can use some approximate kinetic analysis to assess its feasibility. Our premise is that with ground state O2, reaction is initiated by reaction R1 – Cl fission as occurs in pyrolysis. Recombination of two PCCP• radicals produces S0 in a highly energized state. In pyrolysis, as discussed above, energized S0 fissioned off a Cl atom to yield S1 as given by the overall reaction R2. However, in the presence of O2 there is competition between Cl fission and reaction, presumably in an energized state, of S0 with O2 to form the peroxy adduct S0-OOtrip. Hence R2 is modified to R2a to explicitly account for the competing reactions of S0. PCCP• + PCCP• → S0

(R2a)

Reaction between S0 and O2 (reaction R7) should produce an energized S0-OOtrip which would be expected to possess on average sufficient energy to overcome the small barrier to forming S0-OObridge whereupon reaction to form the two 5ClcpdO• radicals S0 + O2 → 5ClcpdO• + 5ClcpdO•

(R7)

should take place by chemical activation. Reaction R7 will be in competition with R8 which S0 = S1 + Cl


(R8)

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leads to formation of 8ClNP. From the PES shown in Figure 7, once formed, 5ClcpdO• needs only to surmount relatively small barriers to undergo ring opening and CO fission. In this simplified kinetic modeling we have approximated this process to the one-step equation R9. 5ClcpdO• → trans-C4Cl5• + CO

(R9)

The trans radical can undergo fission into perchlorovinyl radical and perchloroethyne. trans-C4Cl5• = C2Cl3• + C2Cl2

(R10)

Perchlorovinyl may also undergo Cl fission. C2Cl3• = C2Cl2 + Cl

(R11)

Addition of perchloroethyne to trans-C4Cl5• leads to a C6Cl7 radical which can readily cyclize and fission a Cl atom to produce the major product of oxidation, HCB. In our simplified kinetic model, we represent these reactions by the one-step reaction R12. trans-C4Cl5• + C2Cl2 = HCB + Cl

(R12)

Reactions R13 – R17 comprise reactions between chlorine and oxygen and rate constants for these have been taken from the literature Cl + O2 + M = ClOO + M

(R13)

ClO + ClO = Cl + ClOO

(R14)

Cl + ClO + M = ClOCl + M

(R15)

Cl +ClO = Cl2 + O

(R16)

O + O + M = O2 + M

(R17)

Recombination between oxygen atom and PCCP• is given by equation R18.



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O + PCCP• → 5ClcpdO•

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(R18)

Reactions R19 – R22 comprise a set of reactions involving CO and CO2. (Only very small yields of the latter were detected experimentally). CO + ClO = CO2 + Cl

(R19)

CO + O2 = CO2 + O

(R20)

CO + O + M = CO2 + M

(R21)

C2Cl3• + O2 → CO + COCl2 + Cl

(R22)

Reaction R22 needs explanation. This reaction and R11 – Cl fission, compete to set the balance towards either CO or HCB production. Wang et al42 identified several reaction channels in their ab initio study of the C2Cl3• + O2 PES and subsequently Xiang et al43 made a spectroscopic study of this system. Although they did not derive rate constants, three major pathways were identified,43 leading to ClCO• + COCl2, CO + CCl3O• and CO2 + CCl3•. For the first two channels, the C-Cl bonds in both ClCO• and CCl3O• were found to be very weak such that Cl fission can take place rapidly, hence the adoption of the overall reaction R22 in our mechanism. It should be noted, however, that no phosgene was actually detected amongst our experimental products. Since both CO2 and CCl4 were very minor products, we have not incorporated the third channel of Xiang et al43 into our model. Rate constants used in the model are listed in Table 4. Modeling was performed using the SENKIN module and predictions of the model are compared with experiment in Figure 5. Table 4 near here. This simplified model can be seen to give reasonable overall agreement with experiment for the yields of HCCP and the three major products; however, it underpredicts the yield of CO


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at the highest temperatures studied. The results give support to the importance of an energized S0 – the product of recombination of two PCCP• radicals in mediating the flux of reaction to the pyrolysis product, 8ClNP or the oxidation products, HCB and CO. The optimized temperature exponent of the pre-exponential factor of the forward rate constant of reaction R8 was found to be strongly negative, implying a significant falloff in rate with increasing temperature of activated S0 reacting to S1 and ultimately to 8ClNP. This is presumably a result of more rapid de-energizing collisions at higher temperatures. Whilst the simplified model gives a reasonable prediction of extent of decomposition of HCCP and yields of major products at oxygen concentrations up to 6-8%, it fails to predict the decrease in decomposition of HCCP and yield of major products in 20% O2. Experimental temperature profiles of HCCP, 8ClNP, HCB and CO at 6 and 20% are compared in Figures 8(a)-(d). Figures 8(a)-(d) near here. From Figure 8(a) we see that the extent of decomposition of HCCP with temperature is significantly decreased in 20% O2 compared with 6% mixtures and is even lower than in pyrolysis. Our model does not predict a decrease in extent of decomposition in 20% O2 mixtures. In Figure 8(b) the yields of 8ClNP, though lower in 20% mixtures compared with 6%, do not exhibit the maximum yield observed at about 823 K in the 6% mixtures, instead yields continue to rise with temperature. Our model at 20% O2 predicts lower yields than at 6% but still exhibits a maximum yield around 823 K. Yields of HCB decrease while yields of CO increase experimentally at 20% (Figures 8(c) and 8(d)). Our model also predicts this occurrence but this can simply be attributed to the increased flux passing through reaction R22 with higher O2 concentrations compared with flux through the Cl fission reaction R11.



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The reasons for the observed decrease in rate of decomposition of HCCP at high O2 concentrations are uncertain, however, it might be associated with enhanced de-energization. Our model invoked the reactions of an energized S0 – the initial product of recombination of two PCCP• radicals. If O2 were a significantly more efficient de-energizer than N2, the principal bath gas component, then a large proportion of the concentration of S0 might be rapidly de-energized. As such it would be unable to undergo activated fission of a Cl atom on the one hand or reaction with O2 on the other, and significant reversal to PCCP• radicals might result with consequent decrease in rate of decomposition of HCCP. With its large vibrational frequency of 2360 cm-1, several hundred wavenumbers removed from any vibration frequency of S0, N2 collisions are only likely to remove small amounts of energy through translation and rotation. O2 has a vibrational frequency of 1580 cm-1 and, whilst not in close resonance with any frequency of S0, this frequency is nonetheless within 60 cm-1 of two (scaled) frequencies of S0. Thus transfer of relatively large amounts of energy by enhanced vibration-vibration transfer might therefore be possible. Possible Consequences for Dioxins’ Formation from Cyclodienes Although hexachlorocyclopentadiene itself does not produce polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/F) directly, fission of chlorine atoms from HCCP initiates the formation of chlorinated aromatic precursors of PCDD/F from cyclodienes. From our present studies of oxidation of HCCP in both unpassified reactor (where the influences of O2 (1∆g) are implicated) and passified silica reactor, we have found that maximum rates of decomposition of HCCP, formation of Cl and of HCB occur at O2 concentrations in the N2 bath gas of between 6 and 10%. In our earlier study of oxidation of endosulfan, the same range of O2 concentrations was found to produce the highest yields of PCDD/F.5 We now attribute this maximum in PCDD/F yields in endosulfan and other cyclodienes to conditions which maximize the rate of decomposition of hexachlorocyclopentadiene. Hence a 26 ACS Paragon Plus Environment

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knowledge of rates of oxidative decomposition of HCCP is essential to develop safe strategies for incineration of cyclodiene pesticides. Conclusion In an inert bath gas, pyrolysis of hexachlorocyclopentadiene is initiated by chlorine atom fission, forming the pentachlorocyclopentadienyl radical, two of which combine to form an energized bis-(pentachlorocyclopentadienyl), (S0), which undergoes a series of intramolecular rearrangements and Cl atom fissions ultimately to produce principally octachloronaphthalene and Cl2. Under oxidative conditions, Cl fission from HCCP initiates reaction but O2 bimolecular reaction with energized S0 competes with Cl fission such that 8ClNP yields decrease and hexachlorobenzene and CO yields increase with increase in temperature and O2 mole fraction up to about 10 mol%. At higher mole fractions, there is a reduction in rate of decomposition of HCCP. Oxidation conditions which maximize the rate of decomposition of HCCP are also those which maximize the decomposition of cyclodienes and formation of dioxins’ precursors from this group of insecticides. Associated Content Additional information comprises details of quantum chemical isodesmic calculations and experimental enthalpies of formation of species in the isodesmic reactions, HCCP and product yields under oxidative conditions from an unpassified silica flow reactor, analytical methodology, FTIR spectra and Total Ion Chromatogram, boric acid coating of reactor, atomic co-ordinates of species of importance. Table of standard enthalpies of formation and entropies of reactants, products and intermediates.

Acknowledgements



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The authors thank the University of Newcastle, Australia for a postgraduate research scholarship to N.K.D. This study was partly funded by the Australian Research Council. This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. We thank Dr. Glenn Bryant and Mr. Scott Molloy for technical support.


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References 1. Matolcsy, G.; Nadasy, M.; Andriska, V. Pesticide Chemistry, Elsevier: Amsterdam, The Netherlands, 1988. 2. Abreu-Villaca, Y.; Levin, E. D. Developmental Neurotoxicity of Succeeding Generations of Insecticides. Environment International, 2017, 99, 55-77. 3. Stockholm Convention on Persistent Organic Pollutants, Report of the Persistent Organic Pollutants Review Committee, 6th Meeting, Geneva, 11-15 October, 2010. At that meeting endosulfan was placed on the elimination list and usage was expected to halt worldwide by the end of 2017. 4. Chopra, N. M.; Campbell, B. S.; Hurley, J. C. Systematic Studies on Breakdown of Endosulfan in Tobacco Smokes – Isolation of Degradation Products from Pyrolysis of Endosulfan-I in a Nitrogen Atmosphere. J. Agric. Food Chem., 1978, 26, 255-258. 5. Dharmarathne, N. K.; Mackie, J. C.; Kennedy, E. M.; Stockenhuber, M. Gas Phase Thermal Oxidation of Endosulfan and Formation of Dibenzo-p-dioxins and Dibenzofurans. Environ. Sci. Technol. 2016, 50(18), 10106-10113. 6. Altarawneh, M.; Radny, M. W.; Smith, P. V.; Mackie, J. C.; Kennedy, E. M.; Dlugogorski, B. Z.; Soon, A.; Stampfl, C., A first-principles density functional study of chlorophenol adsorption on Cu2O(110):CuO. J. Chem. Phys. 2009, 130, (18), 184505. 7. Altarawneh, M.; Dlugogorski, B. Z., Formation and Chlorination of Carbazole, Phenoxazine, and Phenazine. Environ. Sci. Technol. 2015, 49, (4), 2215-2221. 8. Altarawneh, M.; Dlugogorski, B. Z., Formation of Dibenzofuran, Dibenzo-p-dioxin and their Hydroxylated Derivatives from Catechol. Phys. Chem. Chem. Phys. 2015, 17, (3), 1822-1830.



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9. Dharmarathne, N. K.; Mackie, J. C.; Kennedy, E. M.; Stockenhuber, Gas Phase Pyrolysis of Endosulfan and Formation of Dioxin Precursors of Dibenzo-p-dioxins and Dibenzofurans (PCDD/F). Proc. Combust. Inst. 2017, 36(1), 1119-1127. 10. Dharmarathne, N. K.; Mackie, J. C.; Kennedy, E. M.; Stockenhuber. Pyrolysis of dieldrin and formation of toxic products. To be published. 11. Ginsburg, A. E.; Paatz, R.; Korte, F. Gasphasen-dechlorerung von Hexachlorcyclopentadien: Octachlornaphthalin und Octachlorfulvalen. Tetrahedron Lett. 1962, 17, 779-782. 12. 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, Revision D.01, Gaussian, Inc.: Wallingford CT, 2013 13. Alecu, I. M.; Zheng, J.; Zhao, Y.; Truhlar, D. G. Computational Thermochemistry: Scale Factor Databases and Scale Factors for Vibrational Frequencies Obtained from Electronic Model Chemistries, J. Chem. Theory Comput. 2010, 6, 2872-2887. 14. Mokrushin, V.; Bedanov, V.; Tsang, W.; Zachariah, M.; Knyazev, V.; McGivern, W. S. ChemRate Version 1.5.10, NIST: Gathersburg, MD, 2011. 15. Borden, W. T.; Davidson, E. R. Potential Surfaces for the Planar Cyclopentadiene Radical and Cation, J. Am. Chem. Soc. 1979, 101, 3771-3775. 16. Bearpark, M. J.; Robb, M. A.; Yamamoto, N. A CASSCF Study of the Cyclopentadienyl Radical: Conical Intersections and the Jahn-Teller Effect, Spectrochimica Acta A. 1999, 55(3), 639-646. 17. Kiefer, J. H.; Tranter, R. S.; Wang, H.; Wagner, A. F. Thermodynamic Functions for the Cyclopentadienyl Radical: The Effect of Jahn-Teller Distortion, Int. J. Chem. Kinet. 2002, 33, 834-845, and references therein.


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18. Roos, B.O.; Taylor, P.R.; Siegbahn, P.E.M. A Complete Active Space SCF Method (CASSCF) using a Density-Matrix Formulated Super-CI Approach. Chem. Phys., 1980, 48, 157-173. 19. Linstrom, P. J.; Mallard, W.G. Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology: Gaithersburg MD,

doi:10.18434/T4D303 20. Yaws, C. L., Thermophysical properties of chemicals and hydrocarbons, William Andrew: Norwich, N.Y., 2014. 21. Jasper, A.W.; Klippenstein, S. J.; Harding, L. B. The Effect of Spin-Orbit Splitting on the Association Kinetics of Barrierless Halogen Atom-Hydrocarbon Radical Reactions, J. Phys. Chem. A, 2010, 5759-5768. 22. Wang, D.; Violi, A.; Kim, D. H.; Mullholland, J. A. Formation of Naphthalene, Indene, and Benzene from Cyclopentadiene Pyrolysis: A DFT Study, J. Phys. Chem. A, 2006, 110, 4719-4725. 23. Kislov, V. V.; Mebel, A. M. The Formation of Naphthalene, Azulene, and Fulvalene from Cyclic C5 Species in Combustion: An ab initio/RRKM Study of 9-H-fulvalenyl (C5H5 – C5H4) Radical Rearrangements, J. Phys. Chem. A, 2007, 111, 9532-9543. 24. Kee, R. J.; Millar, J. A.: Jefferson, T. H. CHEMKIN: A general purpose problem independent, transportable FORTRAN chemical kinetics codes package, SANDIA National Laboratories Report no. SAN80-003, 1980. 25. Chase, M.W., Jr., NIST-JANAF Thermochemical Tables, Fourth Edition, J. Phys. Chem. Ref. Data, Monograph, 1998, 9, 1-1951. 26. Summoogum, S.; Dlugogorski, B.Z.; Kennedy, E.M.; Mackie, J.C. Low Temperature Oxidation of Biphenyl in an Alumina Reactor: Possible Initiation by O2 (1∆), Proc. Aust. Combust. Symp., The University of Newcastle, NSW, November, 2011, pp. 183-186.



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27. Altarawneh, M.; Mackie, J. C.; Hou, S.; Kennedy, E. M.; Dlugogorski, B. Z. The Oxidation of a Model PCB (4-chlorobiphenyl) in Catalytic and non-Catalytic Flow Reactors and Formation of PCDF, Proc. Aust. Combust. Symp., The University of Western Australia, Perth, November, 2013, pp. 291-294. 28. Romanov, A.N. Bykhovskii, M. Ya, Rufov, V., Korchak, N., Thermal Generation of Singlet Oxygen on Zeolite ZSM-5, Kinet. Catal. 2000, 41(6), 782-786. 29. Shcherbakov, N. V.; Emel'yanov, A. N.; Khaula, E. V.; Il'ichev, A. N.; Vishnetskaya, M. V.; Rufov, Yu. N. Photo- and Thermogeneration of Singlet Oxygen by the Metal Ions Deposited on Al2O3 and SiO2, Russ. J. Phys. Chem. 2006, 80(5), 799-802. 30. Hou, S.; Mackie, J. C.; Kennedy, E. M.; Dlugogorski, B. Z., Comparative Study on the Formation of Toxic Species from 4-Chlorobiphenyl in Fires: Effect of Catalytic Surfaces, 9th Asia-Oceania Symposium on Fire Science and Technology, Procedia Eng. 2013, 62, 350-358.

31. Zhong, X.; Bozzelli, J. W. Thermochemical and Kinetic Analysis of the H, OH, HO2, O and O2 Association Reactions with Cyclopentadienyl Radical, J. Phys. Chem. A, 1998, 102, 3537-3555. 32. Robinson, R. K.; Lindstedt, R. P. On the Chemical Kinetics of Cyclopentadiene Oxidation, Combust. Flame, 2011, 158, 666-686. 33. Borden, W. T.; Davidson, E.R. Effects of Electron Repulsion in Conjugated Hydrocarbon Diradicals, J. Am.Chem. Soc., 1977, 99, 4587-4594.

34. Borden, W. T.; Iwamura, H.; Berson, J. A. Violation of Hund’s Rule in non-Kekule Hydrocarbons: Theoretical Prediction and Experimental Verification, Acc. Chem. Res., 1994, 27, 109-116.


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35. Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev., 1994, 94, 871-893. 36. Pozun, Z. D.; Su, X.; Jordan, K. D. Establishing the Ground State of the Disjoint Diradical Tetraethyleneethane with Quantum Monte Carlo, J. Am. Chem. Soc., 2013, 135, 13862-13869. 37. Parker, C. L.; Cooksy, A. L. Ab Initio Study of the 1,3-butadienyl Radical Isomers, J. Phys. Chem. A 1998, 102, 6186–6190. 38. Parker, C. L.; Cooksy, A. L. Ab Initio Study of the Most Stable C4H5 Isomers. J. Phys. Chem. A 1999, 103, 2160−2169. 39. Senosiain, J. P.; Miller, J. A. The Reaction of n- and i-C4H5 Radicals with Acetylene. J. Phys. Chem. A 2007, 111, 3740−3747. 40. McIntosh, G.J.; Russell, D. K. Molecular Mechanisms in the Pyrolysis of Unsaturated Chlorinated Hydrocarbons: Formation of Benzene Rings. 1. Quantum Chemical Studies, J. Phys. Chem. A 2013, 117, 4183-4197. 41. Krishtal, S. P.; Mebel, A. M.; Kaiser, R. I. A Theoretical Study of the Reaction Mechanism and Product Branching Ratios of C2H + C2H4 and Related Reactions on the C4H5 Potential Energy Surface, J. Phys. Chem. A 2009, 113, 11112-11128. 42. Wang, H.; Li, J.; Song, X.; Li, Y.; Hou, H.; Wang, B.; Su, H.; Kong, F. Computational Study of the Reaction of Chlorinated Vinyl Radical with Molecular Oxygen (C2Cl3 + O2), J. Phys. Chem. A 2006, 110, 10336-10344. 43. Xiang, T.; Liu, K.; Zhao, S.; Su, H.; Kong, F.; Wang, B. Multichannel Reaction of C2Cl3 + O2 Studied by Time-Resolved Fourier Transform Infrared Emission Spectroscopy, J. Phys. Chem. 2007, 111, 9609-9612.



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44. Baulch, D.L.; Duxbury, J.; Grant, S.J.; Montague, D.C. Evaluated Kinetic Data for High Temperature Reactions. Volume 4 Homogeneous Gas Phase Reactions of Halogen- and Cyanide- containing Species, J. Phys. Chem. Ref. Data 1981, 10, 195-520. 45. Zhu, R.S.; Lin, M.C. Ab Initio Studies of ClOx Reactions. VIII. Isomerization and Decomposition of ClO2 Radicals and Related Bimolecular Processes, J. Chem. Phys. 2003, 119, 2075-2082. 46. Zhu, R.S.; Lin, M.C. Ab Initio Studies of ClOx Reactions. IV. Kinetics and Mechanism for the Self-Reaction of ClO Radicals J. Chem. Phys. 2003, 118, 4094-4106. 47. Xu, Z.F.; Lin, M.C. Ab Initio Chemical Kinetic Study on Cl plus ClO and Related Reverse Processes, J. Phys. Chem. A 2010, 114, 11477-1482. 48. Tsang, W.; Hampson, R.F. Chemical Kinetic Database for Combustion Chemistry. Part I. Methane and Related Compounds, J. Phys. Chem. Ref. Data, 1986, 15, 1087-1279. 49. Louis, F.; Gonzalez, C.A.; Sawerysyn, J.P. Ab Initio Study of the Oxidation Reaction of CO by ClO Radicals, J. Phys. Chem. A 2003, 107, 9931-9936.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


List of Tables

Table 1. Bondlengths (Å) Calculated by Various Methods for the Two C2v Conformers of PCCP• and their Relative Energies (cm-1). Table 2. High Pressure Rate Constants (k/s-1) as a Function of Temperature and Position for the Barrierless c-C5Cl6 → c-C5Cl5• + Cl Reaction. Table 3. Rate constants (k = ATn exp(-Ea/RT) used to model the kinetics of HCCP = PCCP• + Cl and formation of 8ClNP. (Units are cm6 mol-2 s-1, cm3 mol-1 s-1 or s-1 as appropriate, Ea in kJ mol-1.) Table 4. Rate constants (k = ATn exp(-Ea/RT) used to model the oxidation kinetics of HCCP. (Units are cm6 mol-2 s-1, cm3 mol-1 s-1 or s-1 as appropriate, Ea in kJ mol-1.)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

List of Figures Figure 1. Yield profiles in pyrolysis. Symbols are experimental data, lines are modeled. HCCP: squares and unbroken line, 8ClNP: triangles and dashed line, Cl2: circles and dot-dash line. Figure 2. Conformers of c-C5Cl5• radical: (a) dienyl structure (M), (b) allylic structure (TS). Figure 3: Rearrangements arising from recombination of two PCCP• radicals. First number: Reaction energy. Second number: Barrier energy. Both in kJ mol-1 at 0 K. Figure 4. Reaction PES at 0 K for recombination of two PCCP• radicals leading to 8ClNP product. (Refer to Figure 3 for species identification.) Figure 5. Yield profiles in oxidation (6% O2 in N2). Symbols are experimental data, lines are modeled. HCCP: circles and unbroken line. 8ClNP: triangles and small dashed line. HCB: diamonds and dot-dash line. CO: squares and large dashed line. Figure 6. Optimized structures of important intermediates in the oxidation of HCCP. Figure 7. Top: PES for addition of O2 (3Σ) and O2 (1∆) to S0. Middle: PES for decomposition of 5ClcpdO•. Bottom: PES for addition of C2Cl2 to C4Cl5 and subsequent reaction to hexachlorobenzene. Figure 8(a). Variation in yields of HCCP with O2 concentration. Figure 8(b). Variation in yields of 8ClNP with O2 concentration. Figure 8(c). Variation in yields of HCB with O2 concentration. Figure 8(d). Variation in yields of CO with O2 concentration.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Bondlengths (Å) Calculated by Various Methods for the Two C2v Conformers of PCCP• and their Relative Energies (cm-1)

CASSCF/

M06-2X/

B3LYP/

Bond

cc-pVDZ

6-31+G(d,p)

6-31G(2df,p)

C1-C2

1.440

1.433

1.434

C2-C3

1.365

1.368

1.372

C3-C4

1.484

1.473

1.476

C1-Cl

1.699

1.686

1.692

C2-Cl

1.714

1.708

1.714

C3-Cl

1.707

1.696

1.700

C1-C2

1.397

1.395

C2-C3

1.473

1.462

C3-C4

1.354

1.358

C1-Cl

1.717

1.711

C2-Cl

1.701

1.689

C3-Cl

1.711

1.703

93

115

a

Dienyl (M )

Allyl (TSa)

∆E(dienyl-allyl) a

Nomenclature employed by Bearpark et al16 for analogous c-C5H5• radical.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

Table 2. High Pressure Rate Constants (k/s-1) as a Function of Temperature and Position for the Barrierless c-C5Cl6 → c-C5Cl5• + Cl Reactiona

c-C5Cl5-Cl distance (Å) T/K 2.68 2.78 2.83 500 7.20E-07 9.04E-08 4.86E-08 600 1.94E-03 3.73E-04 2.10E-04 700 5.53E-01 1.44E-01 9.17E-02 800 3.86E+01 1.26E+01 8.82E+00 900 1.05E+03 4.11E+02 3.09E+02 1000 1.48E+04 6.69E+03 5.32E+03 1100 1.29E+05 6.57E+04 5.48E+04 1200 7.86E+05 4.41E+05 3.83E+05 1300 3.63E+06 2.21E+06 1.99E+06 1400 1.35E+07 8.81E+06 8.15E+06 1500 4.19E+07 2.92E+07 2.77E+07 1600 1.13E+08 8.35E+07 8.09E+07 1700 2.73E+08 2.11E+08 2.08E+08 1800 5.96E+08 4.80E+08 4.83E+08 1900 1.20E+09 1.00E+09 1.03E+09 2000 2.25E+09 1.95E+09 2.02E+09 a Variational rate constant shown in bold.

2.88 2.11E-08 1.23E-04 6.07E-02 6.41E+00 2.41E+02 4.41E+03 4.76E+04 3.47E+05 1.86E+06 7.86E+06 2.74E+07 8.18E+07 2.15E+08 5.07E+08 1.09E+09 2.18E+09






Page 39 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Rate constants (k = ATn exp(-Ea/RT) used to model the kinetics of HCCP = PCCP• + Cl and formation of 8ClNP. (Units are cm6 mol-2 s-1, cm3 mol-1 s-1 or s-1 as appropriate, Ea in kJ mol1 .)

Reaction No. R1 R2 R3 R4 R5 R6

A 1.45×1015 5.00×1013 2.90×1011 3.10×1013 2.66×1018 2.23×1014

kf n 0. 0. 0. 0. -1.85 0.

Ea 222.0 12.6 86.2 96.2 65.9 -7.5

Ref.

A 3.68×1012

kr n 0.

Ea -16.4

1.80×1013

0.

26.0

3.60×1010 1.47×1015

0. 0.

57.5 232.9

PWa,b PWc PWa,b PWa PWb,d 44b

a

Present work – rate constant(s) calculated from quantum chemical structure determination.

b

Reverse rate constant calculated from equilibrium constant.

c

Rate constant optimized in modeling.

d

Ea calculated from quantum chemical structure determination, A optimized in modeling.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

Table 4. Rate constants (k = ATn exp(-Ea/RT) used to model the oxidation kinetics of HCCP. (Units are cm6 mol-2 s-1, cm3 mol-1 s-1 or s-1 as appropriate, Ea in kJ mol-1.)

Reaction No. R1 R2a R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22

A 1.45×1015 5.00×1013 2.90×1011 3.10×1013 2.66×1018 2.23×1014 1.00×10-2 2.00×1055 5.00×1013 5.00×1015 1.00×1014 5.00×1014 6.02×1028 8.19×1010 3.83×1023 1.34×1010 1.89×1013 6.00×1013 2.41×105 2.53×1012 6.17×1014 7.00×1013

kf n 0. 0. 0. 0. -1.85 0. 6.0 -15.0 0. 0. 0. 0. -5.34 0.77 -2.87 1.00 0. 0. 2.02 0. 0. 0.

Ea 222.0 5.9 86.2 96.2 65.9 -7.5 108.8 102.5 125.5 209.5 134.0 4.6 5.6 18.0 4.2 39.6 -7.5 0. 43.8 199.5 12.6 105.0

Ref.

A 3.68×1012

kr n 0.

Ea -16.4

1.80×1013

0.

26.0

3.60×1010 1.47×1015

0. 0.

57.5 232.9

7.00×1013

0.

0.

1.02×1013 1.63×1013

0. 0.

19.3 -27.5

1.92×1011 3.90×1013 1.32×1015 3.58×1013 5.22×1014

0. 0. 0. 0. 0.

-5.4 176.6 149.5 39.5 489.5

1.12×1014 4.64×1013 3.13×1017

0. 0. 0.

338.0 231.7 541.6

a

Present work – rate constant(s) calculated from quantum chemical structure determination.

b

Reverse rate constant calculated from equilibrium constant.

c

Rate constant optimized in modeling.

d

Ea calculated from quantum chemical structure determination, A optimized in modeling.


PWa,b PWc PWa,b PWa PWb,d 44b PWc PWb,c PWc PWb,c PWb,c PWc 45b 46b 47b 47b 48b PWc 49b 48b 48b PWc

Page 41 of 50

1.2

1

Yield (mol/mol of HCCP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4

0.2

0

650

700

750

800 Temperature/K

850

900

Figure 1. Yield profiles in pyrolysis. Symbols are experimental data, lines are modeled. HCCP: squares and unbroken line, 8ClNP: triangles and dashed line, Cl2: circles and dot-dash line.


950


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cl

Cl Cl

.

Cl Cl

Cl

(a)

Cl

.

Cl Cl

Cl

(b)

Figure 2. Conformers of c-C5Cl5• radical: (a) dienyl structure (M), (b) allylic structure (TS).


Page 42 of 50

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cl

Cl

.

Cl

.

Cl

Cl

Cl

+ Cl

.

Cl

Cl

Cl

PCCP

-121/0

Cl

Cl

Cl

Cl

Cl Cl

Cl

-Cl

Cl

Cl

Cl

Cl S0

Cl

Cl

Cl Cl

Cl Cl

Cl

Cl

Cl

. Cl

-360/0

8ClFL

Cl

Cl

S2

Cl Cl

Cl

.

Cl

Cl

Cl

Cl Cl

Cl

Cl

S11

Cl

Cl

Cl

Cl

S3 49/89

Cl

Cl

Cl

Cl

Cl

Cl

Cl

-25/6.3 +Cl

Cl

Cl

Cl

Cl

Cl

Cl Cl

.

-35/46

.

Cl

Cl

Cl

Cl

Cl Cl

Cl

83/103

Cl

+Cl

Cl

-69/42

Cl

Cl

-93/84

Cl

Cl

Cl

Cl

Cl

Cl S1

-Cl

.

Cl 63/86

Cl

Cl

167/0

Cl

Cl

.

199/0

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

S4

S10 Cl

Cl

.

Cl

Cl Cl

Cl

-Cl Cl

Cl

Cl Cl

Cl

+Cl

Cl

Cl

Cl

Cl

-22/-1.0*

103/107

Cl

-30.5/23.8

4.2/0

Cl

Cl Cl

Cl

+ CCl

Cl

Cl

Cl 8Clstyrene

Cl

CCl4

Cl

Cl

Cl

+Cl

Cl

Cl

Cl

S6

Cl Cl

.

Cl

Cl

Cl

Cl

+ Cl Cl

Cl Cl

98/114

Cl

Cl

Cl

Cl

Cl

Cl

151/0

Cl

Cl Cl

Cl

Cl

-Cl

3

Cl

.

Cl

8ClNP

.

Cl

Cl

Cl

Cl

Cl

Cl

.

Cl

Cl

8ClAZ

C 4Cl 5 +Cl

C4Cl 6

HCB

Figure 3: Rearrangements arising from recombination of two PCCP• radicals. First number: Reaction energy. Second number: Barrier energy. Both in kJ mol-1 at 0 K.



300 TS S2→ S3

200

TS S1→S2

TS S4→S10

TS S3→S4

Potential Energy kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 50

S2+Cl

100

S4+Cl S1+Cl S3+Cl

S10+Cl

8ClNP+2Cl

0 2PCCP

-100 S0

-200 Reaction Coordinate

Figure 4. Reaction PES at 0 K for recombination of two PCCP• radicals leading to 8ClNP product. (Refer to Figure 3 for species identification.)


Page 45 of 50

1.5 1.25

Yield (mol/mol HCCP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.75 0.5

0.25 0 700

750

800 850 Temperature / K

900

950

Figure 5. Yield profiles in oxidation (6% O2 in N2). Symbols are experimental data, lines are modeled. HCCP: circles and unbroken line. 8ClNP: triangles and small dashed line. HCB: diamonds and dot-dash line. CO: squares and large dashed line.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

I

III

II

V

IX

Page 46 of 50

IV

VIII

VII

VI

X

XI

Figure 6. Optimized structures of important intermediates in the oxidation of HCCP.


Page 47 of 50

80

TS peroxybridge

40

TS for S0-OO

0 O2 (1∆) + S0

-40

II

TS bridge C-C fission V+V

-80 -120

O2 (3Σ ) + S0 III

-160 -200

IV TS C4Cl5 fission

300 Relative Potential Energy at 0 K (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

C2Cl2 + C2Cl3+CO TS cis-trans

TS CO loss

100

TS ring opening

TS Cl shift

VIII + CO

0

IX + CO

V VII

-100 VI

-200

100 TS for add.

0 IX + C2Cl2

-100

TS for cycliz.

-200 C6Cl7

-300 TS for Cl fission

-400 X

-500

XI (post reaction complex)

C6Cl6 + Cl

Figure 7. Top: PES for addition of O2 (3Σ) and O2 (1∆) to S0. Middle: PES for decomposition of 5ClcpdO•. Bottom: PES for addition of C2Cl2 to C4Cl5 and subsequent reaction to hexachlorobenzene.



1 ♦ 0% O2 • 6% O2 ∆ 20% O2

0.8 0.6 0.4 0.2 0 700

750

800

850

900

950

Temperature/K

Figure 8(a). Variation in yields of HCCP with O2 concentration.

8ClNP Yield (mol/mol of HCCP)

HCCP Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 50

0.6 0.5

♦ 0% O2 • 6% O2 ∆ 20% O2

0.4 0.3 0.2 0.1 0 700

750

800

850

900

950

Temperature/K

Figure 8(b). Variation in yields of 8ClNP with O2 concentration.


0.6 • 6% O2 ∆ 20% O2

0.5 0.4 0.3 0.2 0.1 0 700

750

800

850

900

950

Temperature/K

Figure 8(c). Variation in yields of HCB with O2 concentration.

1.6

CO Yield (mol/mol of HCCP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HCB Yield (mol/mol of HCCP)

Page 49 of 50

• 6% O2 ∆ 20% O2

1.4 1.2 1 0.8 0.6 0.4 0.2 0 700

750

800

850

900

950

Temperature/K

Figure 8(d). Variation in yields of CO with O2 concentration.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic

Cl Cl Cl

Cl Cl O

Cl

Cl

Cl

Cl

Cl

+ Cl2

Cl

S

Cl

O

Cl

O

Cl Cl

Endosulfan

Cl

Cl

Cl Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

+ CO

O Cl Cl

Cl

Cl Cl

Dieldrin


Page 50 of 50

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