Formation of Benzene Rings. 2. Experimental and K - ACS Publications

Article pubs.acs.org/JPCA

Molecular Mechanisms in the Pyrolysis of Unsaturated Chlorinated Hydrocarbons: Formation of Benzene Rings. 2. Experimental and Kinetic Modeling Studies Grant J. McIntosh* and Douglas K. Russell Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand S Supporting Information *

ABSTRACT: The mechanism of formation of benzene rings during the pyrolysis of dichloro- and trichloroethylenes has been investigated by the method of laser powered homogeneous pyrolysis coupled with product analysis by gas chromatography. Additionally, selected (co)pyrolyses between the chlorinated ethylenes, CH2Cl2, C4Cl4, C4Cl6, and C2H2 have been performed to explicitly probe the roles of 2C3 and C4/C2 reaction pairs in aromatic growth. The presence of odd-carbon products in neat C4Cl6 pyrolyses indicates that 2C3 processes are operative in these systems; however, comparison with product yields from C2HCl3 suggests that C4/C2 processes dominate most other systems. This is further evidenced by an absence of C3 and other odd-carbon species in (co)pyrolyses with dichloromethane which should seed C3-based growth. The reactions of perchlorinated C4 species C4Cl5, C4Cl3, and C4Cl4 with C2Cl2 were subsequently explored through extensive kinetic simulations of the possible reaction pathways based on previous kinetic models and the exhaustive quantum chemical investigations of our preceding work. The experimental and theoretical results strongly suggest that, at moderate temperatures, aromatic ring formation from chlorinated ethylenes normally follows a Diels−Alder coupling of C4 and C2 molecular units followed by internal shifts; the one exception is the C4Cl4 + C2Cl2 system, where steric factors lead to the formation of nonaromatic products. There is little evidence for radical-based routes in these systems.

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INTRODUCTION The gas-phase formation of molecules containing several carbon atoms from smaller precursors has long been recognized as of both practical and fundamental significance. Many simple hydrocarbons lead to complex systems such as aromatic and polyaromatic hydrocarbons (PAHs) and even fullerenes on photolysis, combustion, or pyrolysis.1−16 Despite extensive experimental and theoretical studies, the detailed mode of formation of these compounds remains controversial. Benzene itself is normally regarded as the key precursor in the formation of soot and PAHs, and its formation is usually seen as the ratelimiting step in the growth of higher molecular weight products. High yields of benzene are produced under high temperature conditions from a wide range of starting materials, and there is an extensive body of work relating to its formation.17−36 Less widely studied are the chlorinated benzenes formed in the decomposition of small chlorinated precursors, but their formation is typically assumed to follow extensions of their nonchlorinated counterparts. On the mechanism of aromatic formation, a number of early studies advocated molecular processes, alkene trimerization,37 or Diels−Alder cyclizations between 1,3-butadiene and ethylene or acetylene in particular.37−39 While these routes were appealing on the grounds of simplicity and the clear relationship between reactants and products, radical-based routes have acquired more prominence.40,41 Early advocates of © 2013 American Chemical Society

radical channels argued that reaction proceeds via vinyl addition to 1,3-butadiene;24,31,42,43 C4H4 as a base was neglected on the grounds that all such channels required an unfavorable 1,3-H shift. However, such channels lead to overprediction of C6H8 while underpredicting benzene yields, and the addition of 1,3butadienyl radicals to acetylene (reactions a−c)44 is now viewed as the major C4/C2 radical addition channel. 1,3‐C4 H5 + C2H 2 → C6H6

(a)

C6H 7 → c‐C6H 7

(b)

c‐C6H 7 → C6H6 + H

(c)

Formation of 1,3-C4H5 is feasible via H-initiated Habstraction from C4H6 with a barrier of 25 kJ mol−1.45 However, bimolecular reaction between acetylene and vinyl radicals may also play a major role in other systems.45−47 At higher temperatures, analogous pathways via C4H3 radicals leading to phenyl radicals have often been invoked.14,22,42,47 C4H3 may be formed through H-loss from vinylacetylene,47 ethynyl addition to acetylene,47 H-initiated Habstraction from vinylacetylene,47,48 or the dimerization of Received: December 6, 2012 Revised: April 11, 2013 Published: April 18, 2013 4198

dx.doi.org/10.1021/jp3120385 | J. Phys. Chem. A 2013, 117, 4198−4213

The Journal of Physical Chemistry A

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acetylene, accompanied by H-loss.15 However, doubts over these mechanisms were voiced by Miller and Melius49 in their 1992 reexamination of the modeling of acetylene flames by Bastin et al.50 The authors argued that the thermodynamics of the isomers of C4H5 and C4H3, a point that had largely been ignored hitherto, could not be neglected. Shown in Figure 1,

environments at high temperatures. The authors observed small amounts of pentachlorobenzene (pCB), while hexachlorobenzene (hCB) dominates products; this is consistent with a number of other works.61−64 Tetrachloroethylene also readily yields hCB,65−67 while dichloroethylene (DCE) produces trichlorobenzene (triCB), 1,2,4-triCB in particular.68 In their analysis of TCE, Taylor et al.60 modified reaction a, and its analogue involving C4Cl3 in the formation of phenyl radicals, to fully chlorinated systems. Sensitivity coefficients showed that C4Cl5 plays a major role at lower, and C4Cl3 at higher, temperatures. Further studies by this group into tetrachloroethylene66 and hexachlorobutadiene69 pyrolysis also advocated C4Cl5 and C4Cl3 radical routes. While no other reactions forming phenyl or benzene were included in the analysis of products from C2 precursors, C3Cl3 dimerization was included and was found to be an important process during the pyrolysis of hexachloropropene.70 The role of i- and n-C4Cl5/C4Cl3 isomers was partially addressed in these works; in line with nonchlorinated systems, n-isomers were assumed to be predominantly involved in aromatic formation. The more stable i-isomers, less reactive in addition reactions, were claimed to represent a continuous source of the n-isomer, and growth reactions explicitly involving i-radicals were omitted entirely. Consequently, it is apparent that the current models of chloroaromatic growth from C2 precursors, or species that crack to give C2 species, are direct analogues of the C4/C2 radical growth pathways suggested for hydrocarbons, but have by no means been as extensively explored. However, the above works do not address several nonaromatic C6Cl6 isomers at all. Although unacknowledged, small quantities of a second isomer eluted, with a retention time similar to that of pCB, in the GC−MS separation of TCE soots formed at 1110 K by Mulholland et al.63 On the basis of IR and mass-spectroscopic standards, Earl and Titus unambiguously identified hexachloro-1,5-hexadien-3-yne (which has a similar retention time to pCB) in TCE soots, finding that it formed in yields comparable to hCB; perchlorofulvene was also detected in lower quantities.64 We also note that the energy barriers of 143 and 204 kJ mol−1 calculated for the decomposition of C4Cl5 and C4Cl3 to C4Cl4 and C4Cl2 respectively, used by Taylor et al.60,66 in their kinetic models, are based on estimation methods similar to those they employed to estimate the barriers to decomposition of C2Cl3 to C2Cl2;66 we have previously shown for C2Cl3 that this overpredicts their stability and profoundly influences chlorinated hydrocarbon growth.71 The possibility of erroneous kinetic data in the current models, and their inability to describe nonaromatic products, suggests that closer scrutiny is required. In the present work (hereafter referred to as part 2), we explore a variety of judiciously chosen mixed chloroethylene/C4 systems through experimental and kinetic modeling approaches. Our kinetic models are supplemented with data from exhaustive ab initio studies of potential energy surfaces (PESs) of representative C4/C2 radical and nonradical systems, covering partially to fully chlorinated precursors. This is described in a preceding accompanying work, hereafter referred to as part 1.72 We reexplore C3 and C4/C2 radical processes, but ultimately argue that nonradical C4/C2 processes (both Diels−Alder mechanisms and novel molecular steps, considered for the first time in part 172) are the dominant channels to chlorobenzene production.

Figure 1. (left) n- and (right) i-isomers of C4H3 and C4H5 showing possible resonance structures.

both C4H5 and C4H3 possess a resonantly stabilized i-form which is both more stable and less reactive than the n-isomer involved in C6 formation chemistry. Further, i-isomers lend themselves much more readily to the C6H6 isomer fulvene, rather than benzene, as their resonance forms with the radical site on the interior carbon are the more reactive of the two structures.51,52 However, due to their stability, i-isomers should also dominate C4 yields. Recombination of C3H3 (propargyl) radicals was suggested as an alternative.49 Dominant in aromatic ring formation during the pyrolysis of C3 reagents,26,44,53,54 it was suggested that this mechanism may operate in C2 and C4 based systems as C3H3 is also resonance stabilized and, therefore, relatively abundant in flames. However, as reaction now proceeds via radical−radical recombination, dimerization is both kinetically and thermodynamically far more favorable than the analogous C4 + C2 steps. Furthermore, C3H3 efficiently leads to benzene25,55−58 without invoking isomerization of fulvene. Diels−Alder cyclizations have of late, however, been reintroduced into kinetic models and found to be quite successful. Originally neglected primarily due to energy considerations, it appears that C4/C2-based molecular routes may have non-negligible rates as reactant concentrations are higher than in radical reactions. In fact, recent kinetic models of ethylene, acetylene, and propylene pyrolyses indicate that 95% of benzene production in acetylene systems proceeds via nonradical cycloaddition of acetylene to vinylacetylene.31−33 Interestingly, these works also disregard the role of C4H5 and C4H3 radicals from all three precursors, and they suggest that ethylene and propylene yield aromatic products by way of C3H3 dimerization. This strongly suggests that explicit and complete kinetic models, rather than solely relying on energetics from ab initio studies, are needed to understand these complex high temperature systems. Some works have extended these reactions to chlorinated systems, which are typically seen as following C4/C2 radical addition pathways analogous to those proposed for nonchlorinated systems above. Some of the earliest work explored trichloroethylene (TCE). Chang and Senkan59 modeled fuelrich C2HCl3/O2/Ar flames. However, the products were predominantly CO and CO2;59 thus the assessment of aromatic formation reactions was somewhat difficult. The seminal work in chloroaromatic formation from TCE came from Taylor et al.60 in comprehensive kinetic modeling studies of oxygen-free 4199



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C. Analytical Methods. The FTIR spectra presented in this work were recorded using a Digilab FTS-60 Fourier transform spectrometer at a resolution of 1 cm−1 and averaged over 10 scans per spectrum. Win-IR software was used to acquire and process spectral data. The sample compartment of the spectrometer, which contained the pyrolysis cell, was purged with N2 to reduce/eliminate contributions of CO2 and water from the ambient atmosphere. All spectra were collected against a background spectrum of an evacuated pyrolysis cell. FTIR spectra were used primarily to characterize small gaseous products such as acetylenes or HCl, where resolved vibration− rotation structure permits unambiguous identification even from a single band; in such cases, comparison with published reference data was used for verification. The technique is less useful for larger molecules, in which similar vibrations often have almost identical bands, and it also lacks the sensitivity necessary for quantitative analysis. GC−MS analyses were conducted using a Hewlett-Packard 6890 series gas chromatograph interfaced with a HewlettPackard 5793 mass selective detector (MSD). The base pressure of the MSD was maintained at 1 × 10−5 Torr by a turbomolecular pump backed by a rotary pump. An “HP5-MS” cross-linked phenyl methyl siloxane gum capillary column (i.d. 0.25 mm; length 29.2 m; film thickness 0.25 μm) was employed. Helium was used as the carrier gas in this GC− MS system. Gas samples were extracted from the pyrolysis cell, via a septum port, with a Hamilton 2.5 mL gastight syringe fitted with a lockable valve. The contents of the cell were at relatively low pressure (20−30 Torr); typically, therefore, 2.5 mL of the gas sample was extracted, and after the syringe valve was engaged, the sample was compressed to 0.25 mL to achieve a pressure closer to atmospheric. One drawback of the sample extraction method described above is that products of low volatility are not easily recovered from the pyrolysis cell, and therefore a different method was developed for the analysis of solid deposits. Pyrolysis mixtures in these experiments led to sooty or tarlike black solids. The cell was refilled and more deposits were generated prior to sample extraction; typically, three successive fills and pyrolyses were performed to accumulate enough solid for analysis. A 10− 20 mL volume of toluene was then added to an evacuated cell after pyrolysis, and analyte dissolution was aided by around 5 min of vigorous shaking. Finally, to ensure all extractable materials were removed, the cell walls were scraped down with a metal spatula, with all removed solid dropping into the toluene extract. The liquid sample and suspended solid was removed with a Pasteur pipet and placed into labeled sample vials. The vials were allowed to sit for at least a week to ensure that any extractable material still remaining in the suspended solid had dissolved. The extract was filtered into clean vials prior to GC−MS analysis. Unless explicitly stated otherwise, all samples were obtained by the toluene wash, rather than gas extraction, method. D. Computational Methodology. Rate constants have been estimated within the framework of transition state theory. All energetic and structural data were obtained from M06-2X/ 6-311+G(3df,3p)//B3LYP/6-31G(d) level calculations (see part 172). The generalized transition state theory rate constant is given by eq 1:79

EXPERIMENTAL AND THEORETICAL METHODOLOGY A. Chemicals. All chlorocarbons and other compounds used were of analytical grade quality and obtained from Sigma Aldrich. These and SF6 (British Oxygen Co.) were purified before use by repeated freeze−pump−thaw cycles. Materials were handled on Pyrex vacuum lines fitted with greaseless taps; before use, the line was preconditioned by exposure to the vapor under study and reevacuated. B. Infrared Laser Powered Homogeneous Pyrolysis. All static cell pyrolyses utilized the IR LPHP technique. Since this method has been described in detail elsewhere, only a brief description is given here.73−76 Pyrolysis is performed in a twopiece cylindrical Pyrex cell (total length 120 mm; diameter 38 mm). The cell allows easy disassembly for sample extraction, with the join biased more toward one end to minimize disruption to the gas flow around the hot zone.77 The two pieces are flanged to allow them to fit against one another, forming an airtight seal with the aid of a rubber O-ring and vacuum grease. The cell is held together with a metal clamp attached across the join, and is enclosed by ZnSe windows. Although ZnSe is opaque to infrared radiation below 500 cm−1, it has several distinct advantages over cheaper materials, such as NaCl. ZnSe is strong and thermally stable, and nonhygroscopic. Most significantly, ZnSe is highly transparent to the CO2 laser radiation. The pyrolysis cell is filled with between 10 and 20 Torr (1 Torr = 133.3 Pa) of the vapor under study and approximately 8 Torr of SF6. The contents of the cell are then exposed to the output of a 70 W free-running CW CO2 laser operating at 10.6 μm. The laser power level (which determines the temperature) is generally set close to the threshold required for measurable decomposition of the target compound, with an exposure time sufficient to provide an analyzable yield of products, typically a few percent decomposition. Incident power is controlled by attenuation of the beam exiting the cavity by a variety of different sized apertures. As shown elsewhere,73,76 SF6 strongly absorbs the laser radiation, which is then rapidly converted to heat via efficient intermolecular and intramolecular relaxation. The low thermal conductivity of SF6 ensures that a strongly nonuniform temperature profile is produced in which the center of the cell may reach temperatures of the order of 1500 K while the cell wall remains at room temperature.78 IR LPHP has a number of well-documented advantages. The first of these is that pyrolysis is initiated directly in the gas phase, thereby eliminating the complications frequently introduced by competing surface reactions. Since surfaceinitiated reactions frequently involve free radicals, this factor enhances the role of molecular mechanisms. The second is that the primary products of pyrolysis are rapidly ejected into the cooler regions of the cell, inhibiting their further reaction. In favorable cases, these products may be accumulated for further investigation. One substantial disadvantage of IR LPHP is that the temperature of the pyrolysis is neither well-defined nor easily determined, despite many experimental and theoretical approaches;73,76 kinetic analysis and comparison with more conventional methods of pyrolysis are thus difficult. However, an approximate effective temperature may often be estimated by the use of a “chemical thermometer”, i.e., a noninteracting reaction of known kinetic parameters; we apply the concept of an effective temperature in our kinetic models.

k(T , s) = κ(T ) 4200

kBT Q (T , s) −VMEP(s)/ kBT e h Φ(T )

(1)



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where a generalized transition state is defined at each point s along the minimum energy path (MEP) and is perpendicular to the MEP while intersecting it at s; kB and h are the Boltzmann and Planck constants, respectively; Φ(T) is the partition function of the reactants; and Q(T,s) is the transition state quasi-partition function (the imaginary frequency has been projected out). Partition functions are evaluated under rigid rotor and harmonic oscillator approximations. The transmission coefficient is denoted κ(T) and accounts for tunneling effects. We have estimated the transmission factor with the Wigner correction80 for tunneling through the barrier, where ω‡ represents the (imaginary) vibrational frequency corresponding to the reaction coordinate, deemed appropriate as this term deviated negligibly from unity.

Figure 2. Experimental yields of C6H6−xClx, 3 ≤ x ≤ 6, in mixed DCE/TCE pyrolyses. Also shown are normalized statistical yields based on C2HCl/C2Cl2 trimerization reactions.

‡ 2

κ (T ) = 1 +

1 ℏϖ 24 kBT

(2)

eluting scaled by a factor (equal to the molar mass divided by the molar mass of C6Cl6) such that the areas are more representative of molar yields. Reasonably close agreement is apparent regarding positions of concentration maxima with respect to the Cl content of the reaction. Time-dependent yield changes determined by IR spectroscopy provide an estimate of the approximate times for chlorobenzenes to attain equilibrium and their equilibrium concentrations. Peak areas of pertinent spectral features were converted to concentrations using molar absorptivities computed from DFT/B3LYP/6-31G(d) calculations. When benchmarked against measured absorptivities for the cis−trans isomerization of C2H2Cl2, we found a deviation of no more than 40%;83 ref 83 also provides details for the conversion of concentration in the TCE system depicted in Figure 3. These

We assume that variational minimization of the rate constants does not lead to a significant improvement, and the rate constants are evaluated only at s = 0 in the current work. Once rate constants were compiled, simulation of the various pyrolysis systems was carried out with the Kintecus package.81,82 This constructs, then solves, the differential equations required to determine the concentrations of pertinent species as a function of time, subject to given initial conditions. Kintecus also allows for the application of sensitivity analysis; normalized sensitivity coefficients (NSCs) may be generated for species x in a given simulation, at a given time: ⎛ ∂[x] ⎞ ⎛ ∂ ln[x] ⎞ ⎜ [x] ⎟ ⎟ NSC = ⎜ ∂k ⎟ = ⎜ ⎝ ∂ ln k ⎠k ⎝ k ⎠k

(3)

The sign denotes whether the reaction with rate constant k is a source (positive) or sink (negative) of x; the magnitude ranks the influence of reaction with rate constant k on x. The use of NSCs greatly simplifies the identification of the key reactions in complex, multistep, and multispecies systems such as (chloro)hydrocarbon pyrolysis.

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EXPERIMENTAL RESULTS A. Chromatographic and IR Spectroscopic Evidence. Forty Torr of mixed DCE and TCE pyrolyses (aperture diameter 6.0 mm; duration 6 × 30 s exposures, with 30 s cooling in between, per fill of reagent) yielded a number of solid products. DCE and TCE are formed abundantly in the initial stages of dichloromethane (DCM) pyrolysis, and these systems,77 as we will justify soon, should be good analogues for growth in single carbon systems. It should be noted that transDCE is the reagent used in all experiments; however, rapid equilibration between this and the cis isomer prior to much appreciable growth-related chemistry makes distinguishing between isomers unnecessary, in general. The degree of chlorination of C6 products as the reaction mixture changed from pure TCE to DCE is shown in Figure 2; only the yields of aromatic species have been included here. Also included are the yields expected from acetylene trimerization (assuming that acetylene is formed via efficient HCl elimination from the parent ethylene onlythis is justified on the basis of our previous studies).71,83 Theoretical yields have been scaled to attain, at their maxima, the same values as the experimental maxima, which are the percentage peak areas of products

Figure 3. Yields, determined by IR spectroscopy,83 of C2 to C6 products from a typical TCE pyrolyses (8 s exposures, 30 s cooling in between shots, with a 7.6 mm aperture diameter).

studies will allow for a comparison of model systems with experiments regarding yields and overall reaction rates with the elucidation of plausible temperatures. This is considered in detail under Kinetic Modeling Results; however, experiments indicate that equilibration can be achieved in a few tens of seconds with maximum hCB yields of a few tenths of a percent of the initial concentration of TCE (see Figure 3). Note that hCB yields are much lower in Figure 3 than in Figure 2; while 4201



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Figure 4. C6H6−xClx, 3 ≤ x ≤ 6, isomer yields from copyrolysis of equimolar mixtures of DCE and TCE pyrolyses (aperture diameter 6.0 mm; duration 6 × 30 s exposures, with 30 s cooling in between, per fill of reagent).

IR spectroscopic studies examine all products, GC−MS results were restricted to just the involatile and toluene-soluble fraction of products, and hence the “total” to which product yields are compared in Figure 2 is considerably lower than the true total product yield. Specific isomer analyses of C6 species are shown in Figure 4. Products have been identified by comparison with reference standards, where available, and to relative retention times otherwise; see McIntosh83 for further details. In most cases, the products are isomeric chlorinated benzenes. Smaller peaks are unidentified isomers, but are all negligible. The one exception is C6Cl6; although hCB dominates in the chromatogram provided, two less abundant isomers are also produced in significant quantities at lower laser powers (i.e., temperatures). These additional isomers, in comparable relative abundances during pyrolyses with similar aperture sizes (temperatures), have been found in the pyrolysis of dichloromethane.77 The larger of these two is identified as 1,1,2,5,6,6-hexachloro-1,5hexadien-3-yne, and the other, eluting just before hCB, is perchlorofulvene. These assignments are based on the retention times of products during trichloroethylene decomposition studies of Earl and Titus,64 where unambiguous structural assignments were made from IR spectroscopic evidence. Further, peaks identified with those of 1,1,2,5,6,6-hexachloro1,5-hexadien-3-yne are also observed in the IR spectrum of our deposits (see Figure 5), 84 but none associated with perchlorofulvene85 were detectable. At higher temperatures, the yield of the nonaromatic isomers is reduced; in fact, during pyrolyses with an 8.4 mm diameter aperture (therefore, relatively high temperatures), we find ratios of chromatographic peaks (which we equate to product ratios) of 13:3.5:83 for 1,1,2,5,6,6-hexachloro-1,5-hexadien-3- yne:perchlorofulvene:hexachlorobenzene (compare with Figure 4, where aromatic formation is only 4 times that of the linear species, and, indeed,

Figure 5. IR spectrum of TCE soots. hexadien-3-yne.

◊,

hCB; ●, perchloro-1,5-

the near-equimolar yields of linear to aromatic species in the TCE/C4Cl6 copyrolyses depicted in Figure 9 performed with 6 and 5.3 mm aperture diameters, i.e. decreasing temperatures, respectively). These results are entirely consistent with the work of Mulholland et al.,63 who, in the GC−MS separation of TCE tars, found low concentrations of nonaromatic C6Cl6 isomer (in yields of approximately 1/10 that of hCB) at 1110 K. However, when tars are produced at 1225 K (or hotter), this isomer was absent. While this isomer was unidentified in the 4202



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study of Mulholland et al.,63 its relative retention time is consistent with 1,1,2,5,6,6-hexachloro-1,5-hexadien-3-yne. In order to further elucidate the chemistry of chloromethane and chloroethylene pyrolysis, we have conducted a series of copyrolyses of pairs of relevant C1−C4 species. As noted with comparison to hydrocarbon systems, particular emphasis should be placed on first distinguishing between the influence of C3 dimerization and C4/C2 channels as the dominant routes to growth. Consequently, we first explored the role of DCM with ethylenes to investigate the relative concentrations of C3 and C4 precursors. The products formed by the copyrolyses of DCM/DCE and DCM/TCE are shown in Figures 6 and 7, respectively.

Figure 7. Gas-sample chromatograms of equimolar DCM/TCE copyrolysis products (20 Torr each of DCM and TCE); products generated using a laser aperture diameter of 7.6 mm after 2 × 8 s exposures, with 30 s cooling in between. Figure 6. Gas-sample chromatogram of equimolar DCM/trans-DCE copyrolysis products (20 Torr each of DCM and DCE); products generated using a laser aperture diameter of 7.6 mm after 2 × 8 s exposures, with 30 s cooling in between.

1,3-butadiene, both alone and with additional C2 reactants. The chromatogram provided in Figure 8 depicts the major products formed during the pyrolysis of C4Cl6 (aperture diameter 8.4

Each experiment was approximately equimolar in DCM and the ethylene (20 Torr each), and pyrolyses were performed in a series of 8 s exposures, with 30 s cooling between exposures, and using a 7.6 mm diameter aperture. The chromatograms show the gaseous products prior to pyrolysis and after two exposures, and were sampled with a gastight syringe. The formation of DCE isomers, C2HCl, and C4H2Cl2 in Figure 6 are typical of both neat DCM and neat DCE systems; the cis− trans isomerization of the initially trans-DCE reagent is evident here. Additionally, we note that C2HCl yields are reasonably low (IR spectroscopic investigations find that C2HCl is a major product), which is a consequence of its rapid consumption to form dimers.71,83 Similarly, the products of Figure 7 are largely those expected during individual pyrolyses of DCM and TCE.71,83 The most significant perturbation appears in the DCM/TCE system, where the higher concentration of chlorine has led to the trapping of methyl radicals, resulting in small quantities of highly chlorinated methanes CHCl3 and CCl4. The C3 products expected in a forced C1/C2 reaction system are conspicuously absent; note, however, at the bottom of Figure 7 in particular where C4Cl4 is instead seen to dominate heavier (less volatile) products. We also explored hexachloro-

Figure 8. Major products formed during the pyrolysis of 1,3-C4Cl6. 4203



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mm; duration 6 × 10 s exposures, with 20 s cooling in between, per fill of reagent). These experiments required much higher laser powers (temperatures) than the ethylene experiments. The postpyrolysis chromatograms are dominated by unreacted hexachloro-1,3-butadiene; there is also a large isomer peak at 26.6 min assigned to perchlorocyclobutene.86 The sole C6Cl6 isomer observed is hCB. Somewhat unexpectedly, we detect substantial quantities of hexachlorocyclopentadiene, also observed (but not modeled) in the C4Cl6 studies of Taylor et al.69 A number of heavier odd-carbon species, C9Cl8 in particular, were also observed; such species are negligible in DCM or chloroethylene pyrolysis systems. Copyrolysis of C4Cl6 with TCE (at considerably lower laser powers; aperture diameter 5.3 mm; duration 6 × 10 s exposures, with 20 s cooling in between, per fill of reagent), which should ensure C4/C2 pathways, restored the nonaromatic C6Cl6 isomer yields (Figure 9) and suppressed odd-carbon products from forming.

Figure 10. (top to bottom) C6H2Cl4 products of C4Cl6/C2H2 and C4Cl4/C2H2 soot.

materials yield different isomeric distributions.87 Furthermore, the observed yields of triCB isomers are not in keeping with thermodynamic expectations88−91 (the relative energies of the 1,3,5, 1,2,4 and 1,2,3 isomers are 0, 9, and 20 kJ mol−1 respectively at the DFT/B3LYP/6-31G(d) level), and consequently analysis of these yields provides insight into their mechanism of formation. The results of Figure 2 clearly demonstrate that the C6 product stoichiometry of DCE + TCE copyrolyses is well described by HCl loss followed by a formal acetylene trimerization. However, direct trimerization may be dismissed, since termolecular reactions have low probability, and such routes cannot realistically account for the formation of 1,2,3-triCB from DCE alone. One is therefore directed toward a C4 + C2 route, since our earlier work revealed substantial yields of C4 products;71,83 these themselves could be viewed formally as acetylene dimers, therefore explaining the acetylene trimerization profile. It is difficult to conceive of a C3 dimerization model by which these results may be described as convincingly; the formation of tetrachlorobenzenes during C4Cl4/C2H2 and C4Cl6/C2H2 copyrolyses further supports this hypothesis, as these pairs explicitly imply the reaction of C2H2 with a perchlorinated C4 backbone, and again appears inconsistent with C3 dimers. There is more direct evidence for the neglect of C3 channels in chlorinated systems. The copyrolysis systems that we have studied are very effective at producing specific products. For

Figure 9. Major products formed during the copyrolysis of TCE/1,3C4Cl6 with a 5.3 mm diameter aperture.

Experiments involving copyrolysis with C2H2 (20 Torr) were also conducted to further examine the interaction of acetylenes, formed abundantly in chloroethylene pyrolysis. A sample of C4Cl4 (prepared by several distillation steps following pyrolysis of TCE) was copyrolyzed with C2H2, as was C4Cl6 (aperture diameter 8.4 mm; duration 6 × 10 s exposures, with 20 s cooling in between, per fill of reagent)see Figure 10. C6Cl4H2 isomers were produced in both experiments as some of the most important partially chlorinated products. As shown in Figure 10, we find that 1,2,3,4-tetCB (rather than 1,2,3,5tetCB, as found in neat TCE pyrolyses) is now the dominant isomer. B. Preliminary Discussion of Experimental Results. The experimental systems studied above provide vital clues to the relative roles of C4/C2 and 2C3 channels in the growth of chlorinated aromatic hydrocarbons. DCE pyrolysis results in substantial quantities of triCB, giving way to more highly chlorinated benzenes as the fraction of TCE in the precursor mix is increased. Our observation that 1,2,4-triCB dominates is consistent with that of other workers;68 however, other starting 4204



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Figure 11. Nonradical growth pathways to C6Cl6 isomers.

where the distinctive C6Cl6 distributions demonstrated in TCE pyrolyses are found in DCM pyrolyses, too.77 The pyrolysis of C4Cl6 also appears to rule out C3 routes in chloroethylene pyrolysis. By analogy with nonchlorinated congeners, 1,3-C4Cl6 should isomerize to 1,2-C4Cl6 and then decompose to CCl 3 and perchloropropargyl radicals 23,24,26,93−98 (as opposed to decomposition to 2C 2 moieties).99,100 Unlike the C1/C2 system, where rapid C1 dimerization precludes C3 precursors from forming in the radical pool, these C4Cl6 decomposition processes would lead directly to C3 species. The expected dominance of C3 radical routes in 1,3-C4Cl6 pyrolysis appears justified by the appearance of odd-carbon species such as C5Cl6 and C9Cl8, and based on analogy with chemistry of nonchlorinated systems. Consequently, the production of hCB as the sole C6Cl6 product during C4Cl6 pyrolysis, when contrasted with the appearance of perchlorofulvene and perchlorohexa-1,5-dien-3-yne additionally produced during TCE, DCM, and TCE/DCM pyrolyses, is indicative of fundamentally different chemistry during neat C4Cl6 pyrolyses. The reappearance of all C6Cl6 isomers in the copyrolysis of C4Cl6 with TCE also suggests that the C3 routes are slower than the presumed C4 + C2 channel (either radical or nonradical) believed operative in TCE pyrolysis. Consequently, (at least chlorinated) systems dominated by C2 species (either as explicitly added reagents or from precursors which lead to C2 species) proceed to aromatic hydrocarbons by a C4/C2 channel rather than 2C3 processes. However, the relative roles of molecular vs radical processes still need to be clarified. The unique products formed during neat TCE pyrolysis, while complicated, can be used to provide a means of comparing potential C6 formation mechanisms. In agreement with Earl and Titus,64 substantial amounts of nonaromatic

example, the mixed C2/C2 systems which we have explored in both this work and our previous studies show that very good control of chlorovinylacetylenes and chlorodiacetylenes (C4 species) products is achievable.71,83 Similarly, as discussed below, C2/C4 pyrolyses lead to controlled formation of C6 products. These observations strongly support the hypothesis that Cx/Cy copyrolyses drive Cx+y species formation, as one may reasonably expect. However, both Figures 6 and 7 show that controlled C1/C2 mixtures do not demonstrate the expected enhancement of C3 species, which are in fact scarcely formed at all. Thus, chloromethane, chloroethylene, and chloroacetylene systems, or their mixtures (which are, incidentally, the typical cracking products of a number of chlorinated hydrocarbons and polymers and are therefore expected to dominate a wide variety of pyrolytic and combustion systems), yield very little C3 product. This is true even in instances in which the system has been specifically designed to favor such species. The absence of C3 species is most likely a consequence of the rapid radical−radical recombination of methyl radicals rather than the slower radical−molecule steps needed to form C3 products. Radical−radical recombination is a possibility for methyl radicals, while we discount this pathway for C2/C2 systems, since methyl radicals cannot decompose unimolecularly; once formed, they must undergo bimolecular reaction. C2 radicals are undoubtedly dominated by vinyl radicals (C2X, X = H, Cl, radicals will be prohibitively high in energy)83,92 which rapidly decompose via Cl-loss to chloroacetylene congeners. Thus, C1-doped systems very quickly react to provide a C2-rich system, and therefore neat DCM, DCM/DCE, and DCM/TCE should provide aromatic product distributions comparable to the ethylene systems. This is consistent with observations 4205



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therefore the 900−1250 K range appears reasonable. Residence times of 60 s are utilized, similar to the duration of total exposure in the production of each set of samples. Strict time dependence of product yields was therefore not sought; however, good agreement of product yields and reaction times with those observed (discussed shortly) was still found. The initial concentration of TCE, 3.5 × 10−4 mol L−1, was calculated from its pressure. The kinetic model data are given in the Supporting Information (Table S1). To provide accurate instantaneous yields of important precursors in the formation of C6 products during C2HCl3 pyrolysis, we have included a number of C2 degradation and additional C4 formation/consumption/isomerization reactions compiled in previously published kinetic models.60,66,102,103 These reactions are almost invariably radical reactions. Arguments for the accuracy of these models have been quite persuasive; however, the Arrhenius parameters were in a number of instances necessarily estimates. In particular, we have previously pointed out71 that more recent combined highlevel theoretical and experimental data imply that the lifetime of C2Cl3, a key radical invoked in the growth of C4 (and larger species), has been severely underestimated. As a key precursor in previous kinetic models, this will have a significant impact on the predicted dominant mechanisms. Therefore, the previous estimates used by Taylor et al.60,66 for C2Cl3 decomposition to C2Cl2 + Cl have been replaced with recent direct data of Bryukov et al.103 As a consequence, we have further supplemented the model with nonradical C2Cl2 dimerization routes71,83 utilizing B3LYP/6-31G(d) geometries and CCSD(T)/6-31G(d)//B3LYP/6-31G(d) energies in conjunction with CTST to supplement C4 growth routes. As shown in Figure 12, these novel molecular routes are essential in forming sufficient quantities of C4 intermediates required for the growth of C6, and higher, products as the inclusion of measured C2Cl3 decomposition rates effectively removes this radical from the reactant pool. Further, the inclusion of nonradical C 4 dimerization steps leads to product yields which are much more in line with our experimental results in which we detect C2Cl2 in only very low quantities (only observed via GC−MS), with C4Cl4 being the dominant product83 (see Figure 3 for example). The relative yields of the major predicted C6Cl6 isomers as a function of temperature are given in Figure 13. The simulated yields match experiment very well with perchloro-1,5-hexadien3-yne and hexachlorobenzene predicted in comparable amounts (with small quantities of perchlorofulvene) in lower temperature regimes (900−1150 K), giving way to aromaticdominated yields at higher temperatures. Approximate isothermal temperatures agree well here, too; the aperture diameter used to form the products depicted in Figure 4 (6 mm) represents some of the cooler pyrolysis runs we have undertaken, but are still much hotter than the slow benchmarking DCE isomerization reactions (using a 3.4 mm diameter aperture) which yield a temperature of ∼800 K; therefore, temperatures of ∼950−1050 K are probably quite reasonable estimates. Further, these channels appear kinetically reasonable; simulations at 1200 K reveal that hCB reaches equilibrium after ∼30 s with a final yield of 3.02% of initial TCE concentration; this agrees well with the data in Figure 3, where equilibrium yields of ∼0.3% of the initial TCE yields are attained at approximately 30 s. (Note, higher equilibrium yields are found in the model due to an absence of C 6 Cl 6 consumption channels.) Model C6Cl6 yields at 1200 K, as a

C6Cl6 isomers are produced. It is unlikely that this is indicative of a different fundamental chemistry involved in the formation of pCB and other less chlorinated congeners, when formed from similar precursors, and we therefore require the production of nonaromatic isomers to arise naturally from a general model of C6 formation. This should also describe aromatic congeners formed in all chloroethylene systems, and in forced C4/C2 systems such as C2H2 with C4Cl6 or C4Cl4. In order to assess the relative merits of a molecular C4 + C2 mechanism and radical-based processes, we have constructed a detailed kinetic model of C6Cl6 formation based on our previous extensive theoretical study. We will use these results, as well as our ab initio work, to rationalize all experimental observations regarding the formation of C6H6−xCl6, 3 ≤ x ≤ 6, species.

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KINETIC MODELING RESULTS Classical transition state theory (CTST) has been employed to develop a full kinetic model of the reactions in the TCE pyrolysis system. As this is shown to lead to largely perchlorinated systems, this should be representative of a number of heavily chlorinated precursors, or systems where cracking reactions lead to high concentrations of heavily chlorinated C2 species. Further, it will allow for the direct comparison of radical and nonradical growth mechanisms involving C2Cl2, C4Cl3, C4Cl4, and C4Cl5. All reaction pathways considered in this work for the C4Cl3 and C4Cl5 radicals were initially considered in part 172 and have been compiled in the final model. In the case of the ambiguous step describing the formation of the initial molecular C4Cl4−C2Cl2 dimer, the reaction leading to 3C (see Figure 11 for reference) is now considered as a single concerted dimerization step, as suggested by our M06-2X calculations. As a consequence, the linear intermediate 1C is no longer included, and therefore neither is the Cl migration TS2C that leads to perchloro-1,5-hexadien-3yne. Similarly, only the lower energy addition pathway is considered (see part 172 for details). For the reaction pathways explicitly considered in our previous work, M06-2X/6-311+G(3df,3p)//DFT/B3LYP/6-31G(d) energies have been approximated to the activation energies, where available; in rare cases concerning “barrierless” processes, activation energies have been estimated. Similarly, only the dominant reaction pathways to nonradical growth located in part 172 have been included; these reactions are depicted in Figure 11. Several assumptions and approximations have been made in the modeling of these reactions. No attempt to produce an accurate temperature profile has made, given the relative simplicity of the current model and the well-known difficulties associated with defining temperatures in IR LPHP.76 Instead, the reaction mixture was assumed to be isothermal, and the temperature was varied in 50 K increments from 900 to 1250 K. As noted under Experimental and Theoretical Methodology, the isothermal approximation has been shown to be a good approximation in chemical thermometer experiments. Approximate isothermal temperatures for pyrolyses with a given aperture size may be determined by pyrolysis of trans-DCE71,83 by comparing the time required for cis−trans isomerization to occur with the experimental kinetic data provided by Jeffers.101 For example, relatively low temperature pyrolyses (with an aperture diameter of 3.4 mm) of DCE indicate temperatures of ∼800 K; similar pyrolyses with apertures of 7.6 mm diameter yield temperatures in excess of 1000 K.83 The latter represents some of the higher temperatures explored in this work, and 4206



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depicted in Figure 3 was considerably larger (7.6 mm) and is one of the highest temperatures utilized in this work, clearly in excess of the 950−1050 K range used to generate the solid deposits examined in Figure 4. To explore the reaction mechanism in detail, we have determined the NSCs of all species at 1, 30, and 60 s into the reaction at representative temperatures of 900, 1150, and/or 1250 K. NSCs for C2Cl2 and C4Cl4 are very similar (see the Supporting Information, Table S2). Both show that the molecular dimerization reactions of C2Cl2 are the dominant elementary reaction with respect to C2Cl2 and C4Cl4 yields. In fact, the only radical routes of importance are the unimolecular decomposition of TCE, Cl-trapping of C2Cl3 radicals, and, at high temperatures and longer reaction times, Cl-loss from C4Cl4 to give i-C4Cl3. The latter reaction may indicate that radical processes could dominate at the highest temperatures. Table 1 shows the NSCs for the three stable C6Cl6 isomers modeled in this study at 1, 30, and 60 s and at the temperatures where each isomer is the most importantsee Figure 13. The NSCs for hexachlorobenzene at all temperatures are included as this species makes up ∼50% or more of the C6Cl6 yield at all temperatures studied. Perchloro-1,5-hexadien-3-yne is studied at 900 and 1150 K as it is negligible at 1250 K, and its formation is found to be primarily influenced by C2Cl2 + C4Cl4. That is, it is formed solely through nonradical routes. In the earliest stages of the reaction at lower temperatures, steps involving 13C are particularly influential, suggesting that this product is strongly influenced by the addition of Cl atoms to, and subsequent rearrangement of, important precursors to this linear adduct. However, at later times and higher temperatures, the conversion of 5C, perchlorovinylcyclobutadiene, to 20C is extremely influential; with reference to Figure 11, we see that this is because this represents the step at which the molecular routes diverge into channels leading to the linear species and hexachlorobenzene. This implies that it is competition with aromatic formation routes that dictates the success of perchloro-1,5-hexadien-3-yne forming channels at longer times and higher temperatures. Hexachlorofulvene, on the other hand, is entirely radically driven. The reactions i-C4Cl5 + C2Cl2 → 2B (dichloroacetylene addition to an interior carbon of the linear C4 radical, a route leading exclusively to perchlorofulvene) and C4Cl4 + Cl → iC4Cl5 have the greatest influence on product formation. Meanwhile i-C4Cl5 → C4Cl4 + Cl, the molecular addition of C2Cl2 to C4Cl4, and nonradical perchloro-1,5-hexadien-3-yne formation channels (13C + Cl→ 20C + Cl2) are the major steps that compete with hexachlorofulvene production. This confirms early indications that the i-radicals are responsible for any radical-carried growth, in analogy with findings for nonchlorinated systems. Further, it suggests that, at least for TCE and at the temperatures explored, only C4Cl5 and not C4Cl3 is responsible for growth. This is also hypothesized for nonchlorinated systems. Finally, NSCs suggest that hexachlorobenzene production is almost entirely driven by nonradical processes at all temperatures studied, with C2Cl2 + C4Cl4 → 3C being the most influential source elementary step at every point considered. Further, it should be noticed that only at the highest temperatures do radical C4 routes become influential, and only by removing perchlorovinylacetylene from the reactant pool; that is, despite the potential for C6Cl6 formation through i-C4Cl3, these species are found to represent a sink and not a source.

Figure 12. (top) Modeled yields of C2HCl3, C2Cl2, and C4Cl4 as a function of time at T = 850 K, based solely on kinetic parameters employed in previous modeling attempts,60,66,102,103 but including recently revised rates of trichlorovinyl radical decomposition103 (C6Cl6 formation routes are not yet included). (bottom) molecular C2Cl2 routes from McIntosh,83 and McIntosh and Russell,71 are included, yielding a description of C2Cl2 and C4Cl4 that is in much closer agreement with experiment.

Figure 13. Relative yields of major C6Cl6 isomers as a function of temperature, as determined by kinetic modeling. A 60 s residence time is employed.

function of time, are given in the Supporting Information (Figure S1). The higher temperature for these simulations seems justified as the aperture diameter used in the experiments 4207



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Table 1. Four Highest Magnitude NSCs of C6Cl6 Isomers at 1, 30, and 60 s into the Reaction at 900, 1150, and/or 1250 Ka

a

See Figure 11 for the structures of numbered species. 4208



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DISCUSSION AND CONCLUSIONS Analogy with nonchlorinated systems suggests any of three classes of reaction may be operative in the growth of chlorinated C6 species from smaller precursors: the dimerization of C3 radicals, addition of acetylenes to radical C4 moieties, and (as indicated by more recent studies) nonradical C4/C2 reactions, presumably in the form of Diels−Alder reactions. The few studies performed on chlorinated systems only focus on hexachlorobenzene, and favor reactions with C4 radicals.60,66,69 In this study and our preceding work (part 172), we have considered a number of reaction systems both experimentally and computationally. In partially chlorinated systems, we may use the chlorine and hydrogen atoms as an approximate means of “labeling” carbons; this, and the unique product distributions obtained in fully chlorinated species, provides us with a rich source of data to identify the dominant mechanism of growth. Several experimental observations preclude C3 dimerization in systems where C2 species are favored. The C6Hx−6Clx (3 ≤ x ≤ 6) homologue yields as a function of chloroethylene precursor ratio (Figure 2) are most readily described within an acetylene trimerization scheme. While this observation does not in itself preclude C3 routes, it is less clear how one may describe these observations within the C3 dimerization framework. Further, it is not immediately obvious how C3 moieties may form during ethylene decomposition. More convincing arguments against 2C3 channels are made when examining fully chlorinated systems; the appearance of oddcarbon PAH species during perchloro-1,3-butadiene pyrolysis is far more readily described with C3 radicals than by the reaction of even-carbon species, but a unique C6Cl6 isomer distribution is found in these systems. This suggests that a fundamentally different chemistry underlies the C4Cl6 system, the only case that we have considered that exhibits evidence for C3 dimerization. Several experimental observations from copyrolysis also support C4/C2 (rather than 2C3) channels. C1/C2 systems designed to explicitly drive C3 formation (see Figures 6 and 7) show very little evidence of such species. Restoration of nonaromatic isomers in TCE/C4Cl6 pyrolyses, relative to neat C4Cl6, indicates that addition of C2 to C4 systems forces C4/C2 channels. Similar arguments can be made of the C4Cl6/C2H2 and C4Cl4/C2H2 systems. Both reaction pairs “force” C4/C2 reactions, if they are operable; in both cases, 1,2,3,4-tetCB is formed, in agreement with a C4/C2 mechanism where the C2H2 fragment would slip into an otherwise perchlorinated carbon backbone. Finally, kinetics in chlorinated systems do not seem to favor C3 species, given the high propensity of chlorinated systems to form acetylenes.71,83 Abundant generation of acetylenes, at far lower temperatures than for nonchlorinated systems, is due to lower energy of Cl-loss than H-loss, which through Cl-initiated H-abstraction leads to greater yields of vinyl radicals which are, in turn, more susceptible to decomposition to acetylenes in chlorinated systems (see part 172 for examples). Kinetic models of hydrocarbon pyrolysis systems predict that benzene forms predominantly through nonradical (Diels−Alder) processes during acetylene pyrolysis, and C3 dimerization during ethylene pyrolysis. If one were to apply a mechanism of C6 formation from a nonchlorinated analogue, it would appear that the acetylene system (proceeding through nonradical pathways)

would be the closest match to a majority of typical chlorinated systems. With C3 dimerization ruled out, growth must proceed via either radical or nonradical C4/C2-based channels. As discussed in the text, a number of both experimental and theoretical observations rule out radical channels. First, kinetic models indicate that radical processes are too slow, and too low in yield, to describe C6 formation. While the elementary steps in a radical process are far faster than typical molecular channels, the effect here can be traced to the relative rates of growth by, or decomposition of, the key C4 radicals C4Cl5 and C4Cl3. Clloss from these intermediates is shown here to be lower in energy than previous estimates suggested;60,66 as a consequence, the radical pool is considerably smaller, and growth reactions much slower, than previously thought. This effect is large enough to render radical channels minor at best. C6 isomer yields also rule out radical channels. In both perchlorinated (see part 172) and nonchlorinated systems, iisomers are expected to dominate C4 radical profiles, contrary to the findings of Taylor et al.;60,66 and both i-C4Cl3 and iC4Cl5 are expected to lead to fulvenes. While isomerization routes to the thermodynamically most stable isomer, hCB, are available, part 172 argues that they are far higher in energy for fully chlorinated species and will therefore probably be inactive at the temperatures that we have explored. Further, no viable channels to perchlorohexa-1,5-dien-3-yne, a major C6Cl6 isomer formed in our lower temperature experiments, are present in the radical routes. The rates of product formation and isomer yields are, on the other hand, consistent with nonradical pathways. Simulations of C 6 yields (for C6Cl6 isomers, at least) at isothermal temperatures expected to match the experimental conditions of Figure 3 closely suggest that the reaction system is equilibrated within tens of seconds and reaches concentrations around 100 times less than initial reagent concentrations. This is in good agreement with the equilibration time and final hCB yields noted in Figure 3. This (at least order-of-magnitude) agreement strongly suggests that molecular routes are very feasible as the carriers of growth reactions, refuting earlier suggestions that nonradical routes should be many orders of magnitude too slow to describe aromatic formation. These nonradical pathways have been modeled as Diels−Alder cyclizations and provide excellent agreement with observed major isomer trends. The observations that give the most immediate confirmation of these processes are produced during experiments where C4Cl6 or C4Cl4 is pyrolyzed with C2H2. The congener produced is indicative of a C4/C2 addition, as hypothesized. The model calls for acetylene addition to an exclusively chlorinated C4 unit, which leads to tetCB as required. Further, 1,2,3,4-tetCB formation is expected to dominate the largely aromatic products; addition of C2H2 does not perturb the Diels−Alder transition state dramatically, and rearrangements should not scramble any of the substituents in the original acetylene. This is in very good agreement with experiment, particularly when contrasted with DCE/TCE mixed pyrolyses in which 1,2,3,5-C6H2Cl4 now dominates (see Figure 4), where C4H2Cl2/C2Cl2 and C4HCl3/C2HCl are the operative Diels−Alder pairs. TriCB isomer yields from DCE pyrolyses are also in very good agreement with a Diels−Alder model. The 1,2dichlorovinylacetylenes explored in part 172 are the expected dominant C4 isomers in DCE systems regardless of whether formed by molecular routes83 or via radical channels (addition 4209



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of C2HCl to CHClCCl• radicals, with Cl-loss from the acetylene, would be the anticipated major channel here). Consequently, the reactions depicted in part 172 should describe the vast majority of the C6H3Cl3 formation routes. The reaction pathways are almost isoenergetic, and little differences in pre-exponential factors are expected, and therefore all pathways should proceed with very similar rates. The yields of isomers produced via Diels−Alder cycloaddition are therefore also likely to be reflected by statistical factors in addition to thermodynamic considerations. The relative numbers of shift routes leading to 1,3,5, 1,2,4, and 1,2,3 isomers greatly favor 1,2,4-triCB as the dominant isomer, in good agreement with experimental observation. However, when accounting for energies we find (see part 172) that there are two nearly isoenergetic channels that represent the lowest energy routes to C6H3Cl3, with one leading to 1,2,4triCB and the other to 1,2,3-triCB. Consequently, in considering both statistical and thermodynamic factors together, we expect 1,2,4-triCB to dominate with lesser quantities of 1,2,3-triCB and negligible quantities of 1,3,5triCB, which is neither statistically or energetically favorable. This is in excellent agreement with experimental observation. Note, too, that only aromatic species are predicted, as Diels− Alder cyclization, not the “failed” Diels−Alder process to which C6Cl6 isomers are limited, is by far the easiest pathway for reaction. Pentachlorobenzene is also described well by Diels−Alder reactions; however, due to the lack of isomers, no additional mechanistic information can be obtained for these species. Therefore, while we have neglected explicit study of these products, they are expected to be readily described within a molecular addition framework. Finally, the unique but previously unaddressed C6Cl6 isomer distribution is also very well described by a somewhat different nonradical mechanism from other chlorobenzene congeners, but a mechanism that nonetheless arises naturally from Diels− Alder process. Part 172 demonstrates that steric hindrance prevents the Diels−Alder transition state closing in the usual fashion, leading to the rich and novel chemistry which our kinetic simulations show describe temperature dependence of the product isomers extremely well. Our model predicts that linear isomers should be important up to ∼1050 K (we have not considered temperatures lower than 900 K), forming in increasingly low yields thereafter as the temperature increases until ∼1175 K, at which point nonaromatic products are essentially absent. Earlier, we argued that, based on DCE cis− trans equilibration times, the temperature regime used to generate the products shown in Figure 4 is consistent with temperatures of ∼950−1050 K; these C6Cl6 isomer yields are reasonably consistent with our predictions (Figure 13) in the upper region of this estimated range. Also in line with these results are the C4Cl6/TCE copyrolyses (depicted in Figure 9); these were performed with a smaller diameter aperture (lower temperature) of 5.3 mm with temperatures likely to correspond to the lower region of our estimated range, and they show higher yields of linear isomers relative to aromatic products again, also in good (qualitative) agreement with theory. Similarly, we noted that increasing aperture size (therefore, increasing temperature) results in higher hCB content at the expense of linear isomers, with linear C6Cl6:hCB forming in ratios of approximately 1:6, in the highest temperature pyrolyses we have performed. This, too, is consistent with our model predictions (strict quantitation is impossible due to

the difficulties in determining temperature in IR LPHP). Therefore, experiment shows decreasing linear content as pyrolysis temperature is increased, as predicted. More rigorous quantitative agreement can be sought, however, upon close examination of the additional C6Cl6 isomer observed by Mulholland et al.,63 the relative retention times of which allow us to confidently identify it as 1,1,2,5,6,6-hexachloro-1,5hexadien-3-yne. This species forms in yields of approximately 5−10% that of hCB at 1110 K, in reasonable agreement with our model (we predict a product ratio of 1:10 in favor of hCB); similarly, at 1225 K (and higher) this product can no longer be identified in the chromatograms, which is entirely consistent with our model (which predicts hCB forms in yields of approximately 4 × 108 times greater than 1,1,2,5,6,6hexachloro-1,5-hexadien-3- yne at 1225 K). While far from a quantitative study of the yield dependence on temperature, the qualitative and semiquantitative successes of our model provide very strong additional evidence toward the nonradical channels developed in part 172 being the dominant growth channels in the temperature regimes we have explored here. In conclusion, DCE and TCE, and therefore DCM,83 pyrolysis systems do not appear to form C6 products through C3 or C4 radical routes, instead proceeding via Diels−Alder cyclization between vinylacetylenes and acetylenes, as observed for C2H2 pyrolysis.31−33 This occurs regardless of the presence of radicals as we have found through explicit kinetic modeling and experimental observation that key radicals either decompose too rapidly (C4Cl3 and C4Cl5) or are not even formed (C3-based species), and therefore nonradical C6 formation is found to be the fastest and highest yield channel to C6 products. Similarities with acetylene are entirely consistent with the conclusions above, where we argue that chlorinated vinyl radicals decompose to acetylenes far too rapidly to facilitate growth reactions.71,83 Such cyclizations are both consistent with the observed similarities of stoichiometry with acetylene trimerization and the formation of strictly aromatic products. In the highest degrees of chlorination, however, the steric properties of Cl atoms lead to inhibited ring closure, consistent with the recent claims that the chemistry of highly chlorinated species may not necessarily proceed through the same channels as their generally well-studied nonchlorinated analogues.104 Failed cyclization leads to cyclobutadiene-based formation channels which either ring-open and (aided by Cl-radicals) isomerize to linear products at low temperature, or cyclize to benzene at higher temperatures, leading for the first time to a time- and temperature-consistent description (so far, at least qualitatively or semiquantitatively) of the formation of important aromatic and nonaromatic isomers in our, and other published, work.63,64 We conclude that the yields of all C6H3−xCl6−x species, both aromatic and nonaromatic, formed during DCM, DCE, and TCE (co)pyrolyses (as well as a number of other chlorinated hydrocarbon systems) can be describable solely within the molecular addition framework developed both here and in our preceding publication (part 172).

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ASSOCIATED CONTENT

S Supporting Information *

All kinetic parameters employed in this study are tabulated, and normalized sensitivity coefficients of dichloroacetylene and tetrachlorovinylacetylene at 1, 30, and 60 s for simulations run at 900, 1150, and 1250 K are given. This material is available free of charge via the Internet at http://pubs.acs.org. 4210



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +64 9 928 8302. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Auckland, the Marsden Fund, and Lottery Science for grants toward equipment. We also gratefully acknowledge the University of Auckland for financial support of G.M. through a Guaranteed Doctoral Scholarship. Finally, we are also very grateful for the support provided by the Computational Chemistry Group at the School of Chemical Sciences, the University of Auckland.

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ABBREVIATIONS CTST, classical transition state theory; MEP, minimum energy path; NSC, normalized sensitivity coefficient; PES, potential energy surface; PAH, polycyclic aromatic hydrocarbon; DCM, dichloromethane; DCE, dichloroethylene; TCE, trichloroethylene; IR LPHP, infrared laser powered homogeneous pyrolysis; ZPE, zero point energy; IRC, intrinsic reaction coordinate; tri(/tet/p/h)CB, tri(/tetra/penta/hexa)chlorobenzene

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