Role of Hydrogen Abstraction Acetylene Addition Mechanisms in the

Article pubs.acs.org/JPCA

Role of Hydrogen Abstraction Acetylene Addition Mechanisms in the Formation of Chlorinated Naphthalenes. 2. Kinetic Modeling and the Detailed Mechanism of Ring Closure Grant J. McIntosh* and Douglas K. Russell School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand S Supporting Information *

ABSTRACT: The dominant formation mechanisms of chlorinated phenylacetylenes, naphthalenes, and phenylvinylacetylenes in relatively low pressure and temperature (∼40 Torr and 1000 K) pyrolysis systems are explored. Mechanism elucidation is achieved through a combination of theoretical and experimental techniques, the former employing a novel simplification of kinetic modeling which utilizes rate constants in a probabilistic framework. Contemporary formation schemes of the compounds of interest generally require successive additions of acetylene to phenyl radicals. As such, infrared laser powered homogeneous pyrolyses of dichloro- or trichloroethylene were perturbed with 1,2,4- or 1,2,3-trichlorobenzene. The resulting changes in product identities were compared with the major products expected from conventional pathways, aided by the results of our previous computational work. This analysis suggests that a Bittner−Howard growth mechanism, with a novel amendment to the conventional scheme made just prior to ring closure, describes the major products well. Expected products from a number of other potentially operative channels are shown to be incongruent with experiment, further supporting the role of Bittner−Howard channels as the unique pathway to naphthalene growth. A simple quantitative analysis which performs very well is achieved by considering the reaction scheme as a probability tree, with relative rate constants being cast as branching probabilities. This analysis describes all chlorinated phenylacetylene, naphthalene, and phenylvinylacetylene congeners. The scheme is then tested in a more general system, i.e., not enforcing a hydrogen abstraction/acetylene addition mechanism, by pyrolyzing mixtures of di- and trichloroethylene without the addition of an aromatic precursor. The model indicates that these mechanisms are still likely to be operative.

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INTRODUCTION Chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) are a class of environmental toxins that have been identified in a number of environmental and biological samples;1−11 of these, polychlorinated naphthalenes (PCNs) and styrenes have also been of particular interest given their toxicity and persistence.2−5,12−14 Studies into the growth of these compounds at high temperatures are necessary for the development of strategies to mitigate their formation. Additionally, PCNs represent the smallest Cl-PAHs, and as such the mechanism of their formation will likely represent prototype reactions for larger PAHs. These reaction classes may also be operative during the formation of fullerenes and nanotubes. However, despite their importance, little is known regarding the mechanism of PCN formation from chlorinated precursors. This is likely due to the complexity of this system; there are 76 distinct PCN congeners that a growth model must account for, with a large number of possible candidate reaction classes that could be implicated in their formation. Complicating matters, these systems involve species with many heavy atoms, and as such complete first-principles explorations are precluded; further experimental kinetic data are extremely scarce. This has been a major inhibitor in the development of kinetic models. © 2014 American Chemical Society

Some mechanistic insight has, however, been gained on recognizing that the distinctive PCN isomer profiles can be used advantageously by comparing the major predicted and observed PCN isomers, often allowing for the construction of counterarguments to rule out various alternative pathways.15−19 This approach has been used to argue for a wide range of routes, such as successive chlorination/dechlorination of PAHs16,18 or condensation reactions of chlorophenols.17 These channels may be active for PCN production in municipal waste incinerators (MWIs),19 shown to be important sources of PCNs in the environment.12,20,21 Technical mixtures of PCNs such as halowax, formed in the chlorination of C10H8 with Cl2 over FeCl3 or SbCl5 catalysts,22 show a distinct isomer pattern consistent with electrophilic and nucleophilic substitutions; because PCN distributions from MWIs and other production methods differ from halowax,5,13,15,21,23,24 these substitution channels must be inactive here. Computed thermodynamics demonstrate that MWI profiles are not generally thermodynamically distributed,16,25 and therefore kinetics must play the decisive role. That said, there is evidence that both electrophilic Received: September 4, 2014 Revised: November 13, 2014 Published: November 24, 2014 12205

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strated a correlation between phenylacetylene and naphthalene, confirmed by subsequent studies employing isotopic labeling.39 Branching between H-loss and C10-growth from the C8H7 intermediates formed by C2H2 + C6H5 has been studied extensively,40−43 revealing important impacts of temperature and pressure. Several studies also advocate C4 + C6H5 reactions by analogous processes. C4H4 is generally implicated in naphthalene formation,44−46 (shown feasible by naphthalene formation from the decomposition product of one of the initial adducts, the phenylvinylacetylenes).47 The mechanisms are addressed more fully in part 1.37 There are two possible addition modes with vinylacetylene, a vinylic and acetylenic mode, with recent studies indicating the former is more energetically feasible on the non-chlorinated PES.46 Other C4 addition channels, such as 1,3-butadiene addition to phenyl radicals. have also been subject to study and also appear feasible with non-chlorinated precursors;48−50 the general conclusion from these studies indicates that C4 addition channels only dominate when [C4] > [C2]. Chlorinated systems, the focus of the current work, have been subject to far less study with regard to C2 or C4 addition pathways; however, HACA channels should be extremely important when PAH clusters are not already abundant and/or catalysts are absent (thereby ruling out mechanisms mentioned at the open of this work). Analogues of HACA channels appear the most reasonable starting point. This is indeed the assumption behind the few studies into chlorinated systems that have been undertaken, which have invariably focused exclusively only on fully chlorinated systems. Taylor et al. have studied trichloroethylene (TCE),51 tetrachloroethylene,52 hexachlorobutadiene,53 and hexachloropropene54 systems in combined experimental/kinetic model works (summarized in the work of Taylor and Lenoir).55 These models assumed a Bittner−Howard mechanism is operative and is lifted almost directly from hydrocarbon analogues. The model results for C8 and C10 species are seldom discussed, if at all. C6Cl6 pyrolysis systems56 modeled with the same core model were also shown to perform well overall but, again, fidelity of the mechanism to describe C8 and C10 products is not discussed. In this work, we explore the mechanism of C8 and C10 formation in partially to fully chlorinated systems in explicit detail for the first time. We focus on dichloro- and trichloroethylene (DCE and TCE, respectively) pyrolysis systems, perturbed in some instances by the addition of trichlorobenzene (triCB) isomerers. We have previously shown57−60 that C2 oligomerization reactions dominate chemistry in analogous reactions up to C6 formation, indicating continued C2 additions (analogous with HACA processes) are the most likely. This is supported by the apparent success of the models of Taylor et al.51−55 which use HACA analogues. We have shown in a companion paper (part 137), for what we believe is the first time, that these analogues are highly energetically feasible. This study is presented in several parts. First, we explore perturbations in congener yields experimentally (Results and Discussion, section A). Then to reveal the likely underlying mechanism, we first eliminate many classes of reaction that are unlikely to be operative by way of counterexample (Results and Discussion, section B); section C outlines a simplified modeling approach to yield prediction by a probabilistic analogue of kinetic modeling that is applied in sections D and E to test the yield predictions of short-listed reaction channels, based on HACA and C4 addition processes.

substitution and thermodynamic control may be active in polychlorinated dibenzodioxins and -furans (PCDD and PCDF, respectively) formation in MWIs;26 and as such these channels should be ruled out entirely for PCN formation under such conditions. Further channels still, such as successive H/Cl disproportionation reactions with naphthalene27 or C2H2 loss from C12 intermediates during chlorobenzene pyrolysis,28−30 have also been considered. The key commonalities of all of these reactions are the required presence of a C10 (or larger) skeleton and potentially a catalytic agent for H/Cl scrambling. In more general systems, however, a rich supply of naphthalenes and/or chlorinating catalysts are likely to be absent. In such cases, mechanisms that involve the growth of C8 and C10 backbones are required. While molecular processes such as Diels−Alder addition of chlorinated 1,3-butadienes to chlorobenzynes have received a little attention,31 the most likely candidates, particularly in our studies where C2 precursors are abundant, will be extensions of the radical driven hydrogen abstraction/acetylene addition (HACA) reactions which have been developed for, and extensively studied in, non-chlorinated systems; these are summarized in Scheme 1, with extensions to chlorinated systems discussed Scheme 1. Frenklach and Bittner−Howard Mechanisms of Naphthalene Growtha

a

Competitive H-loss leading to phenylacetylene is also shown.

shortly. Considering C10 formation, C2H2 addition to C6H5 initiates growth. This may decompose to phenylacetylene, or undergo a ring → acetylene H-shift stabilizing against further decomposition;32,33 further C2H2 addition to this stabilized intermediate initiates the Frenklach channel, while addition to the unstabilized C8H7 radical is known as the Bittner−Howard channel34 (see Scheme 1). Additional mechanisms of stabilizing 2-phenylvinyl radicals against H-loss, such as their rearrangement to various [4.2.0]-bicyclic35 or [3.3.0]-bicyclic36 structures, have been explored but to our knowledge, only our preceding companion paper (referred to henceforth as part 137) has considered their continued reaction with acetylene. Several studies have indicated such channels are extremely important in many (non-chlorinated) combustion systems, and as such, these routes have been extensively examined. CH4/O2 flames doped with benzene or alkylated benzenes38 demon12206

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EXPERIMENTAL AND COMPUTATIONAL METHODOLOGY A. Experimental Methodology. 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. All static cell pyrolyses utilized the infrared laser powered homogeneous pyrolysis (IR LPHP) technique. Since this method has been described in detail elsewhere, only a brief description is given here.61−64 Pyrolysis is performed in a two piece 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.65 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, 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 continuous wave (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,61,64 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.66 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 reaction. 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;61,64 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.

Following pyrolysis, the cell is evacuated to purge volatile products. Solids were removed from the cell walls with a metal spatula and sat in toluene for several days to extract Cl-PAHs. Samples were filtered prior to analysis via gas chromatography− mass spectrometry (GC-MS). These analyses have been conducted using a Hewlett-Packard 6890 series gas chromatograph interfaced with a Hewlett-Packard 5793 mass selective detector (MSD). Base pressure of the MSD was maintained at 1 × 10−5 Torr by a turbomolecular pump backed by a rotary pump. A 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 for the analysis of neat-chloroethylene products. Products formed upon co-pyrolysis with trichlorobenzene were analyzed on a Restek Rtx-5MS capillary column, with 0.25 mm i.d., 29.9 m length, and a 0.25 μm film thickness. In all experiments, helium was used as the carrier gas. B. Computational and Isomer Analysis Techniques. The geometries of all stationary points were located with the Gaussian 09 suite.67 The B3LYP hybrid density functional68,69 was utilized with a 6-31G(d) basis set. Zero point energy (ZPE) corrections were computed from harmonic frequencies obtained at the same level of theory as that employed for optimization and scaled by 0.9806.70 Rate constants have been estimated within the framework of transition-state theory. All energetic and structural data were obtained from M06-2X/6311+G(3df,3p)//B3LYP/6-31G(d) level calculations (see part 137). The generalized transition-state theory rate constant is given by eq 1:71 k(T , s) = κ(T )

kT Q (T , s) −VMEP(S)/ kT e h Φ(T )

(1)

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; k 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 correction72 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. 1 ℏϖ⧧ κ (T ) = 1 + 24 k bT

2

(2)

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. Identification of eluting compounds is necessary for the development of a kinetic model. PCN products were identified by comparison of retention times we have observed with published retention times21 and relative retention indices.73 In temperature-programmed GC investigations, the Kovats retention index is denoted I: ⎛ t − tz ⎞ + z⎟ I = 100⎜ x ⎝ tz − 1 − tz ⎠ 12207

(3)

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where tx is the adjusted retention time of the unknown and tz and tz−1 are the adjusted retention times of two n-alkanes with z and z − 1 carbon atoms, respectively. The quantity I is assumed invariant with respect to all parameters such as the use of differing columns or temperature programs. The retention time of the unknown is clearly linearly related to I. Similarly, two sets of retention times can also be shown to be linearly related from eq 3 by eliminating I for the two sets. tx′ =

⎛ [tz′− 1 − tz′] [t ′ − tz′] ⎞ tx + ⎜tz′ + z − 1 ⎟ [tz − 1 − tz] [tz − 1 − tz] ⎠ ⎝

Visual comparison is best supported with a solid quantitative basis; for this we will employ the similarity index, SI. This compares two profiles of isomers and is calculated with eq 7.76 S(A , B ) =

area ≈ hmax

(7)

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(4)

RESULTS AND DISCUSSION A. Experimental Results of Chloroethylene/Trichlorobenzene (Co-)Pyrolyses. During the neat-chloroethylene pyrolyses (laser aperture diameter of 6.0 mm and pyrolysis duration of 6 × 30 s exposures, with 30 s cooling in between), chlorinated benzene, phenylacetylene, and naphthalene congeners are produced abundantly. With the exception of C8Cl8, chlorinated styrenes are produced in negligible quantities. Total homologue yields of both phenylacetylene and naphthalene congeners are reproduced well by a binominal distribution model based on the respective tetramerization and pentamerization (assuming facile HCl elimination from the C8H8−xClx and C10H10−yCly products) of C2HCl and C2Cl2 (see eq 8). The acetylenes are produced abundantly through HCl elimination channels in DCE and TCE, respectively.57 Calculated and experimental naphthalene homologue yields are depicted in Figure 1 assuming approximately half of the C10Cl10 modeled undergoes two Cl-losses to give octachloronaphthalene. [C2nHn − xCl n + x] =

n! [C2HCl]n − x [C2Cl 2]x x ! (n − x )!

(8)

(5)

The retention time of the fitted peak is given by tr, hmax is the peak height, and k and n are Poisson model parameters. The parameters tr, k, and n are derived exclusively from the shapes and positions of the experimental peaks.75 Thereafter, the peak height parameter hmax is dictated solely by the predicted relative product yields from our model. Integration of eq 5 yields eq 6. 2πn k

∑ (Ai )2 ∑ (Bi )2

A value of 1 would indicate that the two data sets are identical. Typically, values greater than 0.7 have been taken to indicate an acceptable level of similarity between two distributions.16,76

We can now describe the process used to identify the PCN congeners. A set of probable naphthalene congeners from each experiment was compiled by examining the mass spectrum of eluting compounds. The chlorine isotope profiles, which depend on the number of chlorine atoms, are distinctive and allow the chlorine content of the structure to be readily determined. This, coupled with the mass of the parent ion is sufficient information to derive a molecular formula.74 For unambiguous isomer identification, we first match experimental elution times to a reference set; for this, it is sufficient for a set of experimental retention times and reference retention times or indices to be shown to be linearly related (see eq 4), assuming molecular formulas for each data point from the two data sets agree and that at least one experimental data point can be unambiguously matched to a corresponding reference time. This is generally readily achieved as perchlorinated compounds generally have only one (at least probable) isomer. Once experimental and reference times are matched, any unambiguous structural identification made for eluents in the reference set is then inherited by our experimental data set. The linearity of two sets of retention times was also particularly useful for correlating our experimental data from the two different columns. The retention indices and times utilized in this work are provided in the Supporting Information. For easy visual comparison between theory and experiment, we adopt the device of plotting experimental chromatographic data alongside fitted theoretical curves of major products. The chromatographic peaks modeled tend to be rather asymmetric; chromatograms have therefore been simulated with modified Poisson functions of the form given in eq 5:75 ⎤n ⎡ k h(t ) = hmax e−k(t − tr)⎢1 + (t − tr)⎥ ⎦ ⎣ n

∑ Ai Bi

Figure 1. Experimental C10H8−xClx yields as a function of the fraction of DCE in the starting material. Dashed lines are theoretical yields; experimental data are labeled as follows: ◆, C10Cl8; ▲, C10HCl7; •, C10H2Cl6; *, C10H3Cl5; , C10H3Cl4.

Within each homologue family several phenylacetylene and naphthalene isomers are formed. Additional, generally highly chlorinated, C10H8−xClx isomers which do not possess a naphthalenic carbon skeleton are also observed. These species only tend to form in 75−100% TCE systems. At a DCE:TCE ratio of 25:75, three small C10HCl7 isomers are formed in trace amounts; an additional C 10 Cl 8 isomer, with an area approximately one-third of octachloronaphthalene peaks, is formed in neat-TCE pyrolyses. This second C10Cl8 isomer has been observed in other works and is either assumed to be perchlorofulvalene77 or is left undiscussed.78

(6)

Thus, once n and k have been determined from the peak shapes, the relative height of the fitted curve is determined with eq 6 by equating relative areas to the fractional yields predicted by our probabilistic reaction tree approach. To aid comparisons, we have subsequently scaled the modeled profile such that the experimental and modeled peaks of the dominant congener are the same height. 12208

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The integral role of the phenyl radical as a substrate for growth, as insisted by HACA schemes, was tested by perturbing the DCE and TCE pyrolysis systems with either 1,2,4- or 1,2,3trichlorobenzene (laser aperture diameter of 6.0 mm and pyrolysis duration of 6 × 30 s exposures, with 30 s cooling in between). Perturbation of the DCE system did not change C8H3Cl3 and C10H4Cl4 as the major phenylacetylene and naphthalene congeners produced; on the other hand, TCE systems produced less C8Cl6 and C10Cl8 in favor of C8H2Cl4 and C10H2Cl8. This is strong evidence toward HACA mechanisms: DCE naturally produces triCB, and therefore changes are not expected, while in TCE-doped systems, successive addition of C2Cl2, with Cl-loss, to C6H2Cl3 radicals (formed as the H-abstraction products of the chlorobenzenes) should yield the observed homologues. Co-pyrolysis of trichlorobenzene with TCE also generates additional nonnaphthalenic C10H2Cl6 not observed during DCE/TCE copyrolyses. Individual congener yields within a given homologue family are again highly dependent on the trichlorobenzene additive used. These changes in the congener profiles upon addition of C6H3Cl3 provide compelling evidence toward chlorobenzenes seeding PAH growth. Modeling the changes in individual congener yields will now be examined. B. Growth Based on Frenklach HACA Mechanisms and Acetylene Addition to [4.2.0]-Bicyclic Intermediates: Counterexamples from Experimental C10HxCl8−x Yields. The introduction presents a large number of previously proposed mechanisms of PCN or Cl-PAH growth, as well as a number of pathways believed active in non-chlorinated systems but largely untested in partially chlorinated systems. The number of possible reactions will make model construction extremely challenging; this section addresses the exclusion of most of these channels by presenting counterexamples. In this way, later (more explicit) study can proceed with a much tighter focus. In part 137 we demonstrated that the various cyclization pathways produce distinctive sets of products in partially chlorinated systems. Arguments against various mechanisms of ring closure can therefore be developed by using each reaction mechanism to predict the dominant congener from a simple reactant pair and then demonstrating that certain models make incorrect predictions; this simplifying screening approach was previously shown to be effective in reducing the size of the final probability tree model.60 Given the high symmetry of the starting materials, the addition of C2HCl and C2Cl2 to 1,2,3-trichlorobenzene-based radicals provides a useful system around which to structure counterarguments. The C10H2Cl6 and C10H4Cl4 congeners formed in C2HCl3- and trans-1,2-C2H2Cl2-doped systems, respectively, are given in Figure 2, with the dominant congeners identified. In our previous studies we have observed that each of these ethylenes readily undergo HCl elimination and therefore represent convenient in situ sources of C2Cl2 and C2HCl, respectively.57,58,74 The first, and simplest, potential explanation of congener yields is that they are thermodynamically controlled. This can be disregarded immediately since product yields are found to be extremely sensitive to the reagent composition; we will see explicit examples of this in following sections. We can also screen out the entire class of non-HACA mechanisms, which represent a major subgroup of possible reactions. Many proposed mechanisms involve hydrogen/chlorine scrambling processes in, for analogues in our study, naphthalene structures. These mechanisms still require a source of C10 (or higher)

Figure 2. Hexachloronaphthalene (top) and tetrachloronaphthalene (bottom) products formed during co-pyrolyses of 1,2,3-trichlorobenzene with C2HCl3 and trans-1,2-C2H2Cl2, respectively.

formations to work from in the absence of these structures in the reactants; these mechanisms therefore have limited explanatory scope. At any rate, the close correspondence of homologue yields to acetylene oligomerization processes (Figure 1) is difficult to reconcile with another mechanism; for example, consider metal chloride catalyzed substitution reactions advocated by Wehrmeier et al.79 where the authors demonstrate that the presence of HCl leads to an increased degree of chlorination. DCE possesses a greater quantity of HCl than TCE, so should lead to more highly chlorinated products, contrary to experiment. The role of Cl2 liberated is unclear, but with HCl an effective chlorinating agent, the strict adherence to the chlorination pattern that acetylene oligomerization explains so well is difficult to conceive of. However, more convincing counterarguments again can, on the other hand, be constructed that rely on congener patterns. The electrophilic substitution reactions are dictated largely by the nature of the products (particularly the HOMO and LUMO energies of the products);26 as such, congener yields should only have a minor relationship with the reactants. We instead have found that product identification depends very sensitively on the nature of the reactants. Having restricted our focus primarily to the HACA pathways, or analogous processes that do not rely on the presence of an already formed C8 or C10 backbone, we consider the Frenklach mechanism of ring closure first. Reaction a demonstrates the expected mechanism of C10H2Cl6 formation in the successive addition of two C2Cl2 units to 3,4,5-trichlorophenyl radicals. Part 137 suggests that few significant barriers are present, and thus this should be a relatively facile route. The chlorinated 2vinylphenyl radicals to which the second acetylene adds are typically found to be higher in energy than the 2-phenylvinyl radicals which serve as the base in Bittner−Howard routes, but far less susceptible to the stabilizing H- or Cl-loss reactions that shut down naphthalene formation. Similarly, our previous results also indicate that Cl-loss at the ring closure site should exhibit appreciably lower dissociation energies than any 12209

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competing H-loss steps and should consequently dominate (see part 137). The prediction of 1,2,3,5,7,8-hexachloronaphthalene as the major product is, however, contrary to observation, which suggests that Frenklach-based cyclization channels are only minor contributors at best. We note from part 137 that the necessary 2-vinylphenyl radicals are appreciably higher energy than 2-phenylvinyl with chlorinated analogues (by 20−30 kJ mol−1; compare with ∼5 kJ mol−1 with non-chlorinated analogues) which may limit the contribution of the Frenklach mechanism. Further, the C6H2Cl3CClCCl radical in reaction a should also be far too short-lived to isomerize and subsequently initiate growth reactions, given the far lower barriers associated with Cl-loss relative to H-loss. Thus, that the Frenklach mechanism does not appear active with chlorinated analogues is entirely consistent with our previous computational work (part 137). C4 addition mechanisms have also been explored in part 1,37 and particularly C4Cl4 addition given the tendency of C2Cl2 to dimerize. Explicit computational study indicates that the perchlorovinylacetylene adds to phenyl radicals preferentially through the acetylene moiety rather that the vinylic group, in contrast with non-chlorinated systems in which the opposite is observed.46 This appears to be a consequence of the steric perturbations introduced by chlorine. This would be an important point to test experimentally given the clear differences between chlorinated and non-chlorinated systems suggested by computational studies. Direct extension of the vinylic addition pathways advocated for non-chlorinated systems by Parker et al.46 to the dominant channel in the 1,2,3-triCB + TCE reaction system is given in reaction e; contrasting this with Figure 2 (top), we see that the predicted products are not consistent with observation. Analogous mechanisms initiated through the 2,3,4-trichlorophenyl radicals lead to 1,2,3,5,7,8-hexachloro- and 1,2,3,5,6,8-hexachloronaphthalene via the H- or Cl-migration channels, respectively; neither is in line with experiment. This is strong support for our theoretical studies favoring acetylenic, over vinylic, addition; the former will, of course, be considered explicitly in the more rigorous tests presented in the following sections.

The reactions of bicyclo[4.2.0]octa-1,3,5-trien-7-yl-based C8H7 intermediates, and their chlorinated congeners, are considered next. Part 137 demonstrated that acetylene addition to these species leads to two distinct routes of naphthalene formation. Both proceed through Dewar-benzene-based intermediates. Importantly, these C8H7 intermediates are found to be the lowest energy isomers and consequently may represent the largest fraction of C8H7 radicals. The first cyclization mechanism proceeds via an internal Frenklach-like H-abstraction immediately prior to ring closure. As demonstrated in reaction b, the extension to the 3,4,5-trichlorophenyl/ C2Cl2 system, this channel also fails to describe the major products.

Analogous arguments for the same reaction system, but proceeding through a second more direct ring closure mechanism, predicts 1,2,3,5,6,7-hexachloronaphthalene is the dominant congener, as observed. However, as shown in reactions c and d, this successful channel, when applied to C2HCl addition to trichlorophenyl radicals, fails to describe experimental observation. These reactions are developed on the results of both part 137 and the computational work in the following section which indicate that C2HCl addition leads predominantly to the β-chlorinated product and that Frenklachlike H-abstraction is faster than analogous Cl-abstraction. This example in particular demonstrates the power of studying partially chlorinated systems; simple comparison between expected and observed product distributions provides a straightforward assessment of the importance of a given pathway as chlorine effectively labels the various carbon atoms. As such, we can conclude that pathways from bicyclo[4.2.0]octa-1,3,5-trien-7-yl-based intermediates do not appear active.

The Bittner−Howard mechanism is the final process we have tested. Its original form,34 when applied to chlorinated systems, predicts the formation of 1,2,3,4,5,6,7-heptachloronaphthalene in the 3,4,5- trichlorophenyl radical/C2Cl2 system (reaction f). Similarly, application of the Bittner−Howard route to the trichlorobenzene + 2C2HCl reaction predicts that pentachloronaphthalenes should dominate. This scheme is unable to describe the dominant homologue let alone deliver an accurate isomer prediction, and this also still ignores the short lifetime of α-chlorinated 2-phenylvinyl radicals. As such, this pathway must also be negligibly slow and is discounted. 12210

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Scheme 2. Probability Tree Fragment Depicting the Likelihood of Encountering a Particular Chlorophenyl Radical during DCE/TCE (Co-)Pyrolysis Our computational study in part 1,37 however, developed a minor amendment to the conventional Bittner−Howard route where the δ-carbon abstracts the substituent from the orthoring position immediately prior to ring closure (see reaction g as an example of this process).

Energies derived indicated that these channels may be feasible. Initial examples, analogous to those derived previously, further suggested that these pathways may describe our observed product yields. The following section outlines the construction of the probabilistic reaction tree for chloronaphthalene growth within the modified Bittner−Howard ring closure frameworksee McIntosh and Russell60 for a detailed overview and benchmarking of the probability tree approach. The predictions of this model will provide quantitative support of the modified Bittner−Howard schemes in ring closure and should provide stronger evidence for their activity than the lack of counterexample alone. C. Construction of a Probabilistic Reaction Tree for the Modeling of the Modified Bittner−Howard/C4Cl4 Addition Ring Closure Mechanism. While the construction of a full kinetic model is the ultimate goal, this is a very timeand labor-intensive process, particularly for partially chlorinated systems. We face additional difficulties in applying rigorous kinetic models to IR LPHP systems given the problems in assessing temperature,64 and the obvious expense associated with explicitly deriving rate parameters, from first-principles, for all of the elementary reaction steps in this system, particularly given the number of heavy-atom-rich species. Therefore, conventional kinetic models are simply not feasible in the current work, particularly as the goal so far is only to perform preliminary studies in which we conclude which cyclization schemes are the most likely. The probability tree approach is therefore ideally suited to quantitatively supporting the feasibility of our short-listed (Bittner−Howard) channels. Again, we stress this is primarily a screening tool; given the huge variety of possible growth reactions, however, such an approach is vital for practical future work. The probability tree approach views reaction branching as branching on a probability tree, with the weightings computed from the relative rates along each reaction pathway. The first of the branching parameters required for a description of the modified Bittner−Howard mechanism, beginning with the first acetylene addition to phenyl radicals, quantifies the probabilities of encountering various chlorophenyl radical congeners which subsequently act as a Cl-PAH base. Scheme 2 depicts the possible reaction branches. The reaction system is weighted by 5Cl, 4Cl, or 3Cl, the relative likelihood that growth is seeded by either a penta-, tetra-, or trichlorobenzene, respectively. C6Cl6 has not been considered as it is appreciably more stable against Cl-abstraction.80,81 These parameters are chosen as the final relative yields of each benzene homologue in a given experiment, which are taken from our chromatograms; equally, they might have been obtained from the binomial distribution-

based model (similar to that provided in Figure 1) shown to be valid in our previous work.60 The experimental values have been weighted by a molar mass factor and consequently should be more representative of molarity. We have also corrected for the relative reactivities; for example, C6H3Cl3 is around 3 times more reactive than C6HCl5 as it has three times the number of hydrogen atoms available for abstraction. (This accounts for the neglect of C6Cl6, with no abstractable hydrogens). This method assumes that the final chlorobenzene yields reflect the instantaneous proportions present during reaction. The final parameters are given in the Supporting Information. Once a chlorobenzene has been encountered in the reaction atmosphere, hydrogen abstraction yielding the phenyl radical base can proceed, and more probability tree branching may occur. While the C6Cl5 reaction path has only one isomer, C6HCl4 possesses three. The probability of encountering each is labeled 1234tet, 1235tet, and 1245tet in Scheme 2. These parameters are also assigned numerical values based on relative measured abundances from DCE/TCE pyrolyses;74 equally, the predicted values from our previously published probability tree model of benzene formation could also have been used.60 Three trichlorobenzene isomers also exist, and the values of 124tri, 123tri, and 135tri are defined and assigned similarly for mixed DCE/TCE systems. Values of 0.9, 0.1, and 0 are utilized during 1,2,4-trichlorobenzene copyrolyses and 0.1, 0.9, and 0 in 1,2,3-trichlorobenzene copyrolyses. The observed formation of a small quantity of additional isomers is due to H/Cl scrambling. Finally, C6H3Cl3 isomers must be weighted by a further parameter to uniquely describe a phenyl radical base. Unlike C6HCl5 and C6H2Cl4, there may be several symmetrically unique H-atoms in each trichlorobenzene isomer. Probabilities based on classical transition-state theory (CTST) rate constants for the Cl-initiated H-abstraction, using DFT/B3LYP/631G(d) geometries and energies (see Supporting Information), have been derived. We feel this level of theory is sufficiently accurate for the order of magnitude predictions that are the aim of the current study; thermochemistry is treated in greater detail in part 137 (although this work suggests the B3LYP/612211

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valid in the analogous description of benzene congener formation.60 While C2Cl2 addition is relatively straightforward, C2HCl exhibits additional branching as the acetylene group of the product may be either α- or β-chlorinated. Potential energy surfaces explored in part 137 suggest that this first scenario is typically ∼15 kJ mol−1 lower in energy than the second. This appears to be true regardless of whether the acetylene adds to a ring or a C2 fragment, again consistent with the findings in the benzene congener probability tree model.60 DFT/B3LYP/631G(d) energy barriers for a number of systems, shown in Table 1, provide further evidence that this energy disparity is

31G(d) approach should provide reasonable accuracy), and careful consideration will be required in later extension of this work focusing on the most important pathways advocated here, but beyond the scope of the current work which is to access reaction feasibility. In particular, as broad deductions are made from the results of an exemplar study for a single (relatively lightly) chlorinated congener; errors with this generalization will likely be more important than the specific level of theory employed. 1,2,4-Trichlorobenzene is consequently weighted by 0.16, 0.42, or 0.42 in the formation of 2,3,6-, 2,3,5-, or 2,4,5trichlorophenyl radicals. Similarly, 1,2,3-trichlorobenzene has two possible radical isomers, with CTST estimates giving ∼0.23 and 0.77 as the representative relative proportions of 2,3,4- and 3,4,5-trichlorophenyl radicals. However, these estimates are based on B3LYP/6-31G(d)//AM1 level of theory calculations as the transition state could be located at the desired level of theory. Values have therefore been empirically revised slightly, with respect to chloroethynyltrichlorobenzene yields during 1,2,3-trichlorobenzene/TCE pyrolyses (discussed shortly), yielding 0.4 and 0.6, respectively. The next branches of the probabilistic reaction tree involve the acetylene addition stepssee Scheme 3. We assume that

Table 1. Energy Barriers of C2HCl Addition, Comparing αand β-Chlorinated Attack Modes, to Various Phenyl Radicalsa E/(kJ mol−1)

Scheme 3. Probability Tree Fragment Depicting the Chances of Adding Certain Acetylene Fragments to Phenyl Radicals, and Their Probabilities of Survival To Form Naphthalene

phenyl radical base

β-addition

α-addition

phenyl o-chlorophenyl m-chlorophenyl p-chlorophenyl o,o′-dichlorophenyl pentachlorophenyl

7.6 1.8 6.0 6.0 1.8 −1.0

20.0 18.5 18.7 18.6 16.1 13.2

a

All barriers are in kilojoules per mole and calculated at the DFT/ B3LYP/6-31G(d) level.

largely independent of the benzene nucleus. Denoting the probability of α-hydrogenated acetylene addition as q, and assuming both addition modes have approximately equivalent preexponential factors, a value of q = 0.85 at T = 1000 K may be obtained (ϕ denotes a phenyl radical). q= =

rate(ϕ‐CHCCl) rate(ϕ‐CHCCl) + rate(ϕ‐CClCH) [ϕ][C2HCl]A e−E / RT [ϕ][C2HCl]A e−E / RT + [ϕ][C2HCl]A e−(E + 15)/ RT 1 = (1 + e−15/ RT )

(9)

Under the assumption that phenylacetylenes form as the result of “failed” naphthalene formation routes, the next parameter, Ph, may be introduced. This represents the relative proportion of ϕ-CClCX (α-chlorinated) species that decay to phenylacetylenes, ϕ-CCX. We assume that the addition of the second acetylene, to give ϕ-CClCXCYCZ radicals (X, Y, and Z are H or Cl), is the only competitive reaction and that ϕ-CHCY does not decompose appreciably relative to the α-chlorinated analogue. This leads to eq 10: Ph =

rate(ϕ‐CClCX Cl‐loss) rate(ϕ‐CClCX Cl‐loss) + rate(ϕ‐CClCX C2 YZ‐addn) A Cl e−E(Cl)/ RT

= A Cl e

only C2HCl and C2Cl2 form from DCE and TCE, respectively, in good agreement with our previous findings.57,74 We have weighted reaction branches with a probability p of reacting with C2Cl2 and 1 − p of undergoing C2HCl addition. The value of p is simply assigned by assuming the initial proportion of the parent ethylene reflects the instantaneous proportions of each acetylene; for example, the 75:25 C2HCl3:C2H2Cl2 system implies p = 0.75. This approximation has been shown to be

−E(Cl)/ RT

+ A acee−E(ace)/ RT [ace]

(10)

Unimolecular Cl-losses typically have a preexponential factor of around ACl ∼ 5 × 1013 s−1, and the energies derived at the DFT/B3LYP/6-31G(d) level are typically around 60 kJ mol−1; see Table 2. It has been noted that, in chlorinated ethanes, similar levels of theory underestimate the Cl-loss barrier by successively larger amounts as the Cl content is increased,82 12212

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Table 2. Chlorine-Dissociation Energies from Several Representative Chlorinated 2-Phenylvinyl Radicalsa reaction

barrier

reaction

barrier

phenyl o-chlorophenyl m-chlorophenyl

58.2 51.6 57.6

p-chlorophenyl o,o′-dichlorophenyl pentachlorophenyl

57.7 53.9 53.3

Scheme 4. Probability Tree Fragment Depicting the Chances of Certain Cyclization Routes When H and Cl Are Present on the o-Carbon

a

All energies, in kilojoules per mole, are derived from ZPE-corrected DFT/B3LYP/6-31G(d) level calculations.

traced to an incomplete description of dispersion forces in density functional theory. Underestimates up to 60 kJ mol−1 in C2Cl6 were noted, and therefore we employ a revised value of E(Cl) = 120 kJ mol−1. This, however, probably significantly overestimates the stability of these compounds as the reaction site is not particularly highly chlorinated, and therefore our values will probably underestimate the degree of Cl-loss. Bimolecular acetylene addition, Aace, has an assumed approximate preexponential factor of 1 × 109 L mol−1 s−1, and the barrier to addition, E(ace), is around 10 kJ mol−1 (see part 137). Finally, with typical chloroacetylene concentrations57 of around 1 × 10−4 mol L−1 in our pyrolysis systems, we find a value of (at least) 0.99 for Ph. This implies that essentially all ϕ-CClCX intermediates decompose to phenylacetylene even with values for Cl-loss that probably significantly overestimate the stability of these radicals. This is in good agreement with our findings concerning C4 formation57 and production of benzenes58,59 where we note that high rates of C−Cl bond cleavage can render certain radicals, otherwise assumed central to growth, unreactive. Addition of the second acetylene fragment is assumed to proceed in an analogous fashion to the first. There is a probability p that C2Cl2 adds, and q again dictates the probability that addition of C2HCl proceeds with H on the α-carbon. Table 3, based on the energy surfaces derived in part

or Cl-site. Loss of the ortho-substituent completes PCN formation. We find these processes have typical barriers of 15 and 40 kJ mol−1, respectively (see part 137), and our CTST estimates put the preexponential factor of cyclization at ∼1 × 1012 s−1; these probabilities are denoted Hl and Cl, respectively. Competing with these steps is a novel channel developed in part 137 involving H-abstraction by the δ-carbon of the C4chain. Ring closure follows, and Cl-loss from the doubly occupied carbon yields the PCN product. This probability is denoted Ha, and A values of 1 × 1013 s−1 and energies of around 30 kJ mol−1 are typical (see part 137). In analogous calculations to those shown in eqs 9 and 10, we find values of 0.02, 0.37, and 0.61 for Cl, Hl, and Ha, respectively. For scenarios in which both ortho-positions are occupied by hydrogen, Ha and Hl are renormalized to 0.62 and 0.38. It is worthwhile noting that the branching probabilities derived indicate that the modified Bittner−Howard route (with probability Ha) naturally arises as the dominant PCN formation mechanism. This is despite the higher energy barriers and is therefore a consequence of more favorable entropic barriers. This agrees well with the counter-examples against the Bittner−Howard routes developed in the preceding section and explicit demonstration in the following sections that the modified route is consistent with the observed products. One final scenario that may arise involves the occupation of both ortho-positions exclusively by chlorine. Note that the modified Bittner−Howard route is not operative here as the requisite Cl-migrations cannot be located. Branching, denoted Cl1, was initially expected to be 0.5; i.e., both reactions occur with similar ease. However, in the majority of cases the δcarbon substituent (Y in Scheme 4) will be Cl, and steric factors may be important. The meta-substituents were therefore considered as well. If both are identical, Cl1 is set to 0.5; however, if one is H and the other Cl, the potential added steric hindrance of ring closure along this Cl-rich side of the phenyl ring will make this process, 1 − Cl1, slightly less favored. In these cases, Cl1 is set at 0.7; that is, ring closure is slightly more preferred at the o-Cl, m-H side of the ring. In the following section we will use the relatively simple triCB-doped systems to benchmark our model and justify that the probabilistic reaction tree requires supplementary reactions between chlorinated phenyl radicals and perchlorovinylacetylene (C4Cl4). This is entirely reasonable on physical grounds. These are found to be facile steps (see part 137) and are arguably competitive provided concentrations of C4Cl4 and its monomer, C2Cl2, are comparable. C2Cl2 has been observed to

Table 3. Energy Barriers for the Second C2HCl Addition for Representative Reactions (See Part 137), Comparing α- and β-Chlorinated Attack Modesa E/(kJ mol−1) reaction

β-addition

α-addition

phenyl-CHCCl···CHCCl phenyl-CClCH···CHCCl

14.0 5.6

32.0 21.2

a

All barriers are in kilojoules per mole and calculated at the DFT/ B3LYP/6-31G(d) level.

1,37 justifies that α-chlorinated addition of the second C2HCl unit again has a 15 kJ mol−1 higher barrier than the βchlorinated analoguecompare with Table 1 for the first C2HCl addition. As the two routes have the same energy difference as in the first addition step, and assuming again that both α- and β-chlorinated addition modes have approximately the same preexponential factor as one another (although they will likely differ from addition to the phenyl radical), the same parameter used to describe the first acetylene addition, q, will be equally valid for the second. With the second acetylene bound, ring closure may proceed and, depending on the ortho-substituents of the ring, several processes are possible. The first situation involves H- and Clatoms on the two ortho-sites. Depicted in Scheme 4 are three energetically feasible options, all explored in part 1.37 The first two are Bittner−Howard ring closure channels at either the H12213

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dimerize rapidly,57,83−85 possibly a consequence of the lowering of the singlet−triplet splitting gap upon chlorination,74 and is indeed found in similar yields to C4Cl4 in TCE pyrolysis systems.57,74 Inclusion of these reactions requires the addition of three more parameters, B, V, and vinH (see Scheme 5). The

and the elucidation of detailed mechanistic features. The model we have developed assumes an integral role is played by the phenyl radical. While very probable, and we have argued strongly against competing processes (section C), there is still little direct evidence to confirm this claim in general for partially chlorinated systems. As such, and with explicit kinetic models prohibitively expensive at this early stage where even the fundamentals of the reaction mechanism are still unclear, the probabilistic reaction tree approach60 was benchmarked against the relatively simple trichlorobenzene-doped chlorinated ethylene and neat-TCE systems. Increased concentration of aromatic precursors should increase the likelihood of HACA processes in the chlorobenzene systems and therefore indicate any serious deficiencies of the model. Similarly, the neat-TCE system is also particularly simple as only perchlorinated products form appreciably. An initial model originally compiled neglected C4Cl4 additions. This was found to be inadequate, predicting negligible quantities of perchloronaphthalene (C10Cl8) relative to perchlorophenylacetylene (C8Cl6). This was traced to the high value associated with the parameter Ph (eq 10 and Scheme 3). That is, C6Cl5CClCCl decomposes far too rapidly, yielding C8Cl6, to undergo a second C2Cl2 addition required to form C10Cl8. Inclusion of C4Cl4 addition rectified these issues as the analogous Cl-dissociation parameter, V, indicates a far greater likelihood of cyclization (40% of C4Cl4 addition products are expected to proceed to naphthalenes, contrasted with a mere 1% of all α-chlorinated acetylene additions). Additionally, Cl-dissociation from C4Cl4 addition products, yielding chlorinated phenylvinylacetylene congeners, provides a candidate structure for the unidentified non-naphthalenic C10H8−yCly isomers that are also formed during several of our experiments. This hypothesis will be discussed shortly. The key result is that C4Cl4 addition reactions are essential. Returning to C6H3Cl3-doping experiments with the fuller (C4Cl4 addition inclusive) model, DCE/trichlorobenzene systems are predicted to yield C8H3Cl3 and C10H4Cl4 as major products, which matches observation well. This observation demonstrates the necessity of the amendment to the Bittner−Howard channel which would otherwise predict C10H3Cl5 congeners dominate. Given the possibility of coelution, and absence of calibration standards, the yields of C8H3Cl3 isomers were difficult to analyze further. On the other hand, isomeric identification of tetrachloronaphthalene isomers (for which retention indices are available) should be relatively sound. Despite significant profile shifts with the addition of 1,2,4- and 1,2,3-trichlorobenzene, the modeled profiles match experiment closely (SI values are 0.9632 and 0.9103, respectively). These are depicted in Figure 3. The predicted major products in TCE/trichlorobenzene pyrolysis systems are C8H2Cl4 and C10H2Cl6, again in good agreement with experiment. Tetrachlorophenylacetylenes, although lacking reference retention data, appear to suffer less from coelution than C8H3Cl3 and possess far simpler profiles. We find that our model predicts the correct number of isomers, with very distinctive yields that coincide closely with those observed, suggesting our model is likely correct; this in turn may allow for one to deduce the identity of the observed C8H2Cl4 species. The results are depicted in Figure 4, where we have assumed the identity of the isomers produced, but find an extremely good fit. Very similar profiles are observed experimentally for several non-naphthalene C10H2Cl6 isomers to those of the C8H2Cl4

Scheme 5. Probability Tree Fragment Depicting the Branching Probabilities Associated with C4Cl4 Addition to Chlorinated Phenyl Radicals and Subsequent Cyclization

first represents the probability that reaction proceeds via the Bittner−Howard route, B, or by C4Cl4 addition, 1 − B. B is set at 0.4 and represents an estimate of relative concentrations of C2Cl2 and C4Cl4 in the reaction atmosphere.57,74 Once formed, we find the ϕ-C4Cl4 adduct either loses the α-Cl atom to form vinylphenylacetylene congeners or undergoes an internal Hshift which initiates cyclization. V represents the vinylacetylene equivalent of the parameter Ph (see Scheme 3), loss of the Clatom from the α-carbon of the newly bound C4Cl4 moiety leading to a ϕ-CCCClCCl2 species isomeric with the PCNs. DFT energies of this Cl-loss are higher than those from phenylvinyl radicals at around 100 kJ mol−1, and may be empirically corrected (as with Ph) to ∼160 kJ mol−1. The stabilizing internal Frenklach-like H-abstraction step (see Scheme 5), with which Cl-loss is competitive, has an energy of about 150 kJ mol−1. Analogous calculations to those performed with eq 10 (with the preexponential factor of the abstraction process set at ∼1 × 1013 s−1) leads to 60% Cl-loss. This substantially lowered fraction of Cl-loss, when contrasted with Ph, is a consequence of both a higher C−Cl bond dissociation energy and the fact that the competing growth process in this case is unimolecular (not bimolecular), and thus has a much higher preexponential factor. Finally vinH is the relative probability of undergoing an internal hydrogen abstraction if the option of abstracting Cl is also present. The energy of H abstraction is lower, typically by around 20 kJ mol−1, and this leads to about a 90% chance that H-abstraction occurs if there is also a Cl-atom adjacent. Note, all probabilistic reaction schemes are explicitly given in the Supporting Information. D. Application of a Probabilistic Reaction Tree for the Modeling of the Trichlorobenzene-Doped DCE and TCE Pyrolysis Systems. With the compilation of all relevant branching parameters, the model is ready for rigorous testing 12214

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Figure 5. Non-naphthalenic C10H2Cl6 isomers observed (black line) and predicted (red boxes) during (top) 1,2,4- and (bottom) 1,2,3trichlorobenzene-doped TCE pyrolyses. Figure 3. C10H4Cl4 isomers observed (black line) and predicted (red boxes) during (top) 1,2,4- and (bottom) 1,2,3-trichlorobenzene-doped DCE pyrolyses.

Hexachloronaphthalene yields, shown in Figure 6, are similarly reproduced very well by this model (SI = 0.9668

Figure 4. C8H2Cl4 isomers observed (black line) and predicted (red boxes) during (top) 1,2,4- and (bottom) 1,2,3-trichlorobenzene-doped TCE pyrolyses.

Figure 6. C10H2Cl6 isomers observed (black line) and predicted (red boxes) during (top) 1,2,4- and (bottom) 1,2,3-trichlorobenzene-doped TCE pyrolyses.

isomers in Figure 4. Given similarities between the formation mechanisms of phenylacetylenes and phenylvinylacetylenes, it was presumed that the latter would probably explain the yields of these additional C10H2Cl6 products. Presented in Figure 5, again making assumptions regarding retention times, the comparison of modeled trichlorophenyltrichlorovinylacetylene and unidentified (non-naphthalene isomer) yields is very good. Further, analogous structures were not observed (or predicted) in DCE-based systems as it is the particularly rapid dimerization57,59,74 of C2Cl2 that ensures sufficient concentrations of the dimer and driving this class of reaction. These observations are all consistent with non-naphthalene isomers accounted for as phenylvinylacetylenes, and not fulvalenes, as previously hypothesized.77

and 0.9543 for 1,2,4- and 1,2,3-trichlorobenzene-doped systems, respectively), in spite of the vastly different profiles introduced by providing a new aromatic core around which the structure grows. These results are very good evidence of HACA processes in benzene-doped systems. E. Application of a Probabilistic Reaction Tree for the Modeling of Mixed DCE and TCE Pyrolysis Systems. While explicit addition of trichlorobenzene appears to drive chemistry consistent with an HACA mechanism, its generality still requires confirmation. The mixed DCE/TCE experiments outlined in Experimental Results of Chloroethylene/Trichlorobenzene (Co-)Pyrolyses provide a convenient test case. Addressing phenylacetylenes first, we find that the model correctly describes the dominant homologue family formed in 12215

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produced in particularly high quantities in the trichlorobenzene-doping systems. Despite this lack of prior testing in simple systems, we still find a very good fit is noted with these compounds in the more general chloroethylene systems, with an SI value of 0.9152. These isomers, and their fitted theoretical yields, are shown in Figure 8. Again, some unmodeled peaks are contaminants.

each experiment (these data are included in the Supporting Information). However, as mentioned in the preceding section, lack of published data regarding the identities of phenylacetylene congeners makes analysis difficult. In spite of this, some peculiarities may still be explained. C8H3Cl3 is formed abundantly in both DCE and DCE/ trichlorobenzene systems. The model predicts congeners in both systems form via C2HCl addition to trichlorophenyl radicals, with Cl-loss from the acetylene. Consequently, the identities of C8H3Cl3 isomers formed in all experiments should be similar, all of the type Cl3-ϕ-CCH. Experimental results corroborate this; i.e., all experiments, trichlorobenzene-doped or not, form the same set of C8H3Cl3 isomers, differing only in their relative yields. On the other hand, 25:75 TCE:DCE and TCE:trichlorobenzene systems both produce C8H2Cl4 abundantly. In the latter, C2Cl2 addition to trichlorophenyl radicals (forming Cl3-ϕ-CCCl isomers exclusively) is the only process modeled. In mixed ethylene systems, however, both C2Cl2 addition to C6H2Cl3 and C2HCl addition to C6HCl4 radicals (yielding Cl4-ϕ-CCH C8H2Cl4 isomers) are possible. However, with closer scrutiny, our model suggests Cl4-ϕ-C CH should dominate; if growth occurs via successive additions of three C2HCl and one C2Cl2 molecules, there is only a one in four chance that C2Cl2 will add last to form Cl3-ϕ-CCCl. Consequently, two distinctly different sets of C8H2Cl4 isomers are predicted; Cl3-ϕ-CCCl isomers during TCE/trichlorobenzene pyrolyses, and Cl4-ϕ-CCH isomers during chloroethylene-only pyrolyses. A set of C8H2Cl4 isomers different from those shown in Figure 4 is indeed observed during ethylene-only pyrolyses, in good agreement with theory; these chromatograms are given in the Supporting Information. Chloronaphthalene yields are similarly well described across all experiments. Neat-DCE pyrolyses are seen to yield the highest abundances of tetrachloronaphthalene as predicted. The isomers formed are shown in Figure 7. Despite the greater

Figure 8. C10H3Cl5 isomers observed (black line) and predicted (red boxes) during 0.75:0.25 DCE:TCE pyrolyses.

The 0.5:0.5 DCE:TCE experiments are the most instructive for examining hexachloronaphthalene yields. Isomers are well modeled with SI = 0.9473see Figure 9. Unlike tetrachlor-

Figure 9. C10H2Cl6 isomers observed (black line) and predicted (red boxes) during 0.5:0.5 DCE:TCE pyrolyses.

onaphthalene yields, which share similarities with 1,2,4trichlorobenzene-doping experiments, hexachloronaphthalene yields in chloroethylene-only and trichlorobenzene-doped chloroethylene systems are quite dissimilar. This is understandable in the HACA framework as, given low trichlorobenzene abundances during 0.5:0.5 DCE:TCE pyrolyses, naphthalene growth is far more likely to occur around a tetraor pentachlorobenzene nucleus. Despite this, the model still describes all three unique hexachloronaphthalene profiles well. Product yields in the final two systems, 0.25:0.75 and 0:1 DCE:TCE, are rather more straightforward. In the former, hepta- and octachloronaphthalene are predicted to form in quite high abundances, in agreement with observation. The earlier eluting 1,2,3,4,5,6,7-C10HCl7 isomer is predicted to form in higher yield than 1,2,3,4,5,6,8-C10HCl7, and the SI value of fit for these isomers is very high (0.9536). Neat-TCE pyrolyses are predicted, and observed, to yield C10Cl8 in very high abundances. Additional non-naphthalenic C10H8−xClx isomers are observed only when higher ratios of TCE are employed (≤50%). This too is in agreement with our model; these isomers are predicted to be chlorinated phenylvinylacetylenes, with a C4Cl3 vinylacetylenic moiety exclusively, which requires C2Cl2-rich environments. Neat-TCE pyrolyses should form Cl5-ϕ-C CCClCCl2 abundantly according to our model; an early

Figure 7. C10H4Cl4 isomers observed (black line) and predicted (red boxes) during 1:0 DCE:TCE pyrolyses.

generality of this system, we see quite a good fit is obtained. The SI value is 0.8970, and although the lowest value we obtain for any fit performed in this work, it is still much higher than the threshold for an acceptable degree of similarity. The SI value is lowered partly as a consequence of interfering coelution with other products. The profile is quite similar to the 1,2,4trichlorobenzene/DCE experiments (Figure 3) in agreement with HACA processes as 1,2,4- trichlorobenzene is the most abundantly formed triCB isomer formed during DCE pyrolysis.59,86 The 0.75:0.25 DCE:TCE experiments produce the highest pentachloronaphthalene abundances; this too is well modeled. These congeners have so far gone unexamined as they are not 12216

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This casts doubt on the reliability of previous models, which require C8Cl7 to persist long enough to undergo bimolecular growth reactions. We have made similar arguments for C2Cl3 radicals in C4 formation,57 and C4Cl3/C4Cl5 radicals in aromatic formation.58 The experimental and modeling studies in this work support our hypothesis, highlighting an important difference between chlorinated and non-chlorinated pyrolysis systems: due to the weak C−Cl bond strength, pertinent radicals may not persist in chlorinated intermediates. Aromatic radicals are an important exception as Cl-loss to form benzynes (or similar) will be particularly high energy (and probably relatively unstable). Therefore, radicals have no viable product to unimolecularly decompose to. Due to the rapid dimerization of C2Cl2, however, C4Cl4 accumulates in significant quantities and their addition to chlorinated phenyl radicals appears to the be the dominant route to naphthalene formation in heavily chlorinated systems. Steric interactions appear to favor acetylenic addition, as opposed to vinylic addition; this mechanism is supported both by quantum chemical estimates (part 137) and explicit congener modeling in this study. Further, unimolecular Cl-loss from the acetylene addition adduct, leading to chlorinated phenylvinylacetylene congeners as the chlorine content is increased, is also important, and these non-naphthalene isomers are observed in numbers and quantities consistent with the proposed C4Cl4 addition to phenyl radicals. In summary, in the systems we have studied, chloronaphthalenes appear to form via a novel modified Bittner−Howard route. H-/Cl-loss from chlorinated 2-phenylvinyl radicals competing with these channels leads to chlorophenylacetylene formation. This is particularly rapid as the chlorine content of the precursors increases. However, increased yields of C4Cl4 leads to an increased probability of phenyl-C4 additions, and these eventually become the dominant naphthalene formation routes. Phenylvinylacetylenes form as byproducts. This highlights the caution that must be exercised when extending the chemistry of hydrocarbon systems to that of their chlorinated analogues.

eluting C10Cl8 isomer approximately one-third the area of OCN is observed. Similarly, during 0.25:0.75 DCE:TCE co-pyrolyses, a cluster of minor C10HCl7 isomers is observed with a combined area roughly 10% of the total heptaCN yield. There are three isomers with retention times of 49.15, 49.25, and 49.68 min (on the HP5-MS column) in an approximate ratio of 1:1.75:1.5. According to our model these form through C4Cl4 addition to tetrachlorobenzene-based phenyl radicals, in ∼10% yields, and in a ratio of about 1:5:4. Qualitative yield agreement is achieved; however, the low abundances of these C10HCl7 isomers will make quantitative studies somewhat unreliable but are still good evidence for the qualitative features of our model.

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CONCLUSION We have developed what we believe is the first congenerspecific mechanistic study into C8 and C10 growth in chlorohydrocarbon pyrolysis systems. A simplified kinetic model capable of describing the relative yields of chlorinated phenylacetylene, phenylvinylacetylene, and naphthalene isomers in pyrolysis systems has been constructed. To simplify the modeling, the branching reactions have been viewed as a dendrites of a reaction “tree”, and kinetic parameters are therefore able to be expressed as branching probabilities. Model parameters have been derived through computational studies of the previously unexplored energy surfaces of chlorinated phenyl/acetylene association reactions, considered largely in our preceding work (part 137). The model system has been compared for C8 − C10 yields in an extensive range of chlorinated ethylene and benzene (co)-pyrolyses. Congener profiles for simple case studies (in particular, the reaction of 1,2,3-trichlorobenzene with C2Cl2) afforded an initial screening of potential reactions dramatically reducing the size of an exploratory probability tree model. In our IR LPHP experiments, this ruled out thermodynamically controlled reactions, electrophilic substitutions, and the role of essentially all extensions of conventional HACA routes. However, a novel amendment to the Bittner−Howard route in which the δcarbon of the C4 moiety abstracts the ring ortho-substituent prior to ring closure (required as the conventional channel predicts a more highly chlorinated product than that observed) has performed exceptionally well in the current testing. Simple kinetic arguments suggest that this mechanism should in fact be slightly faster than the conventional Bittner−Howard route (shown by the higher associated branching probabilities, which are in turn based on relative rates of competing reactions). Although this has yet to be tested explicitly, there appears to be no reason why this modified Bittner−Howard mechanism should not be operative, perhaps even dominant, even in nonchlorinated systems. This example also demonstrates the power of making kinetic deductions from partially chlorinated systems as the chlorine can effectively label each carbon atom. This information is simply not as readily obtainable in either the non-chlorinated or fully chlorinated systems that have so far been the only systems to have been studied extensively. Studies of heavily chlorinated systems have also led to some particularly unexpected results. Our preceding paper37 has called into question the kinetic parameters associated with Clloss, leading to phenylacetylene production, in α-chlorinated 2phenylvinyl radicals. Early studies suggested dissociation barriers of 123.0 kJ mol−1 from C8Cl7, a downward revision of a mere ∼30 kJ mol−1 from analogous values utilized in nonchlorinated systems.51,52 However, in part 137 we noted a downward revision of ∼70 kJ mol−1 was perhaps more realistic.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing isomer identification with retention times, Cartesian coordinates of stationary points considered in quantum chemical calculations, and Cl, 4Cl, and 3Cl parameters giving reaction probability and other fixed parameters in probabilistic modeling and figures showing linear relationships betweeen experimental polychloronaphthalene retention times and the literature data set, probability reaction trees for all processes, chlorophenylacetylene homologue yields, and chromatograms of C8H3Cl3 and C8H2Cl4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +64 (0) 9 373 7599. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We 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 12217

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G.J.M. through a Guaranteed Doctoral Scholarship. Finally, we are also very grateful for the support regarding computational facilities provided by NESI and the Centre for eResearch, The University of Auckland.

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ABBREVIATIONS (Cl)-PAH, (chlorinated)-polycyclic aromatic hydrocarbon; CTST, Classical Transition State Theory; DCE, dichloroethylene; HACA, hydrogen abstraction acetylene addition; IR LPHP, infrared laser powered homogeneous pyrolysis; MEP, minimum energy path; MWI, municipal waste incinerator; PCN, polychlorinated naphthalene; TCE, trichloroethylene; ZPE, zero-point energy

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