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. 1. A Quantum Chemical Investigation 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 addition of chloroacetylene or tetrachlorovinylacetylene to 2,4,5-trichlorophenyl radicals, leading to the formation of tetra-, penta-, and hexachloronaphthalene congeners, has been explored at the M06-2X/6311+G(3df,3p)//B3LYP/6-31G(d) level of theory. The accuracy of this method was justified by comparing the barriers of several pertinent reactions against energies from single point calculations at the B3LYP/cc-pVDZ, CCSD(T)/631G(d), and G2MS levels. Bittner−Howard and Frenklach hydrogen abstraction acetylene addition mechanisms were developed, as was a channel based on acetylene additions to chlorinated [4.2.0]octa-1,3,5-trien-7-yl congeners. While the latter channel exhibits relatively high C2HCl addition barriers and may be a minor growth channel at best, both the Bittner−Howard and Frenklach sequences appear facile. In all mechanisms, the additions of C2HCl leading to a βchlorinated adduct is favored by ∼15 kJ mol−1 relative to the α-chlorinated analogue, and the addition products typically access a variety of facile cyclization channels. The α-chlorinated product of C2HCl addition to 2,4,5-trichlorophenyl, however, undergoes a particularly rapid Cl-loss leading to 1-ethynyl-2,4,5-trichlorobenzene, effectively shutting down further growth. Generalization implies that α-chlorinated C6H5CHCH congeners do not participate in growth reactions. Addition of 2,4,5-trichlorophenyl to the CC bond of tetrachlorovinylacetylene and subsequent cyclization is found to be a facile route to hexachloronaphthalene formation and may be operative in fully chlorinated systems where the C6Cl5CClCCl congeners cannot participate in the major growth processes.

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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are associated with soot-producing combustion processes and represent a common class of air pollutant. As such, an understanding of PAH growth mechanisms has generated much interest. Naphthalene formation has been well studied in this regard; as the smallest aromatic fused ring system possible, C10H8 is a convenient prototype system, particularly for computational explorations. Similarly, polychlorinated naphthalenes (PCNs), commonly produced in municipal waste incinerators,1−3 have been identified in a number of environmental samples1,4−9 and exhibit mutagenic properties similar to those of non-chlorinated PAHs, and therefore deserve study in their own right. As such, the limited study into PCN growth in combustion and pyrolysis systems is rather surprising and is presumably a consequence of the significant additional complexity introduced if the growth of all 76 congeners is to be described. Several mechanisms of C10H8 formation have been proposed. It has been suggested that growth is initiated by propargyl (C3H3) addition to benzyl (ϕ-CH2, ϕ denotes a phenyl radical). Proposed, and later rejected, by Bittner and Howard,10 there has since been evidence in support of these schemes with certain fuels, and particularly toluene-doped flames.11,12 The dimerization of C5 hydrocarbons has also been proposed, © 2014 American Chemical Society

commonly either by recombination of cyclopentadienyl radicals (C5H5) or C5H5 addition to cyclopentadiene. Both routes appear feasible, and considerable work has been undertaken exploring cyclopentadienyl-based growth.13−20 Various hydrogen abstraction/acetylene addition (HACA) processes21−23 have also been very widely studied. In these sequences a fused ring structure develops as a consequence of successive acetylene additions to an aromatic radical. There are two commonly explored HACA sequences. Considering C10H8 growth, both mechanisms are initiated by phenyl addition to acetylene yielding a 2-phenylvinyl radical. The earliest sequence proposed was the Bittner−Howard mechanism,10 in which the second acetylene adds to the first. This is followed by cyclization and H-loss. This process requires that the 2phenylvinyl radical formed in step 1 be stabilized against bond homolysis leading to phenylacetylene.10,24 A second ring closure mechanism, due to Frenklach and co-workers,25,26 postulates that such stabilization may be achieved by an internal H-shift to give a 2-vinylphenyl radical. The second acetylene unit then adds directly to the ring, and ring closure with H-loss Received: September 4, 2014 Revised: November 9, 2014 Published: November 24, 2014 12192

dx.doi.org/10.1021/jp508979u | J. Phys. Chem. A 2014, 118, 12192−12204

The Journal of Physical Chemistry A

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Richter et al.30 [3.3.0]-Bicyclic species were predicted to form in negligible quantities. The addition of vinylacetylene to phenyl radicals as a means of naphthalene formation has also been scrutinized in several computational studies. Bittner and Howard developed reaction b,10and found the heat of formation for C6H5HCC

follows. These reactions are depicted in Scheme 1 and are often generalized to describe the formation of larger PAHs.27,28 Scheme 1. (Top to Bottom) Phenylacetylene Formation and Bittner−Howard and Frenklach Mechanisms of Naphthalene Formation, Depicting Pertinent Intermediates

CHCH2 to be 61 kJ mol−1 more exothermic than the analogous acetylene addition to phenyl and an effective source of naphthalene. However, a later B3LYP/6-311G(d,p) study by Moriarty and Frenklach argued that the necessary rotation step prior to ring closure, with a barrier of 188 kJ mol−1, is in fact prohibitively high.32 A novel sequence initiated by the addition of phenyl to the double bond of C4H4 was also developed, but this too was found to require a high energy isomerization step. Unexpectedly, this new addition mode was found to exhibit both a lower barrier and greater exothermicity than reaction b. In fact, the recent study of Parker et al.33 re-addressing these two mechanisms strongly suggests that the vinylic addition pathway is favorable over the acetylenic addition mode. To address isomerization barriers, Moriarty and Frenklach suggested that re-addition of hydrogen to the C6H5−C4H3 adduct, if occurring with the correct geometry, might yield a C6H5−C4H4 isomer which bypasses the rotation step.32 Experimentally, 1-phenyl-1-buten-3-yne does indeed readily isomerize to naphthalene when pyrolyzed, giving credence to this hypothesis.34 The problem of the rotation step in reaction b, without requiring bimolecular addition of H atoms, was circumvented by Aguilera-Iparraguirre et al.35 These authors introduced cyclization to vinylbenzocyclobutadiene which exhibits free rotation of the vinyl moietysee reaction c. The total barrier found was 140 kJ mol−1 at the B3LYP/TZVP level of theory, leading to much more efficient naphthalene formation.

Bauschlicher and Ricca provide one of the first computational studies comparing the various HACA sequences.29 The addition of C2H2 to phenyl radicals was found to be very facile, with a barrier of 10.4 kJ mol−1 and H-loss from the resultant 2phenylvinyl radicals endothermic by 145.6 kJ mol−1. The competing Frenklach internal H-shift, yielding 2-vinylphenyl, was found to have a lower barrier (119.3 kJ mol−1) and is endothermic by only 4.7 kJ mol−1. The small energy difference between 2-phenylvinyl and 2-vinylphenyl led the authors to conclude that both Bittner−Howard and Frenklach mechanisms are possible in combustion and pyrolysis systems. Richter et al.30 later studied the C8H7 potential energy surface (PES) with density functional theory (DFT). Facile C 2 H 2 addition was again found, with a barrier and exothermicity of 13.5 and 175.5 kJ mol−1 respectively. H-loss was again found to have a substantially higher barrier than the H-shift leading to 2-vinylphenyl, which was again observed to be slightly higher in energy than 2-phenylvinyl. Novel rearrangements leading to various [4.2.0]-bicyclic structures (reaction a) were also explored, with the pictured [4.2.0]octa-

Despite extensive work on the C10H8 PES, to our knowledge there appears to be no rigorous computational studies explicitly exploring the formation of any PCN congener. Taylor and coworkers have, however, compiled and refined an extensive kinetic model for the formation of C10Cl8. This has been applied with considerable success to high temperature trichloroethylene,36 tetrachloroethylene,37 hexachloropropene,38 and hexachloro-1,3-butadiene39 pyrolysis systems. El Mejdoub et al.40 has since applied the model to the pyrolysis of C6Cl6. The Bittner−Howard HACA sequence and C4Cl4 addition channels have been included in various versions of the mechanism. However, despite its successes, some pertinent energy barriers and thermodynamic parameters may be unreliable as most, out of necessity, were derived via far less sophisticated methods than those now available. Our previous work has already argued that energy values obtained from these works lead to an unrealistically high lifetime of pertinent chlorinated radicals. This consequently shifts the dominant predicted growth reactions from a class of radical-based

1,3,5-trieny-7-yl radical found to be the global minimum of the C8H7 PES. Rice−Ramsperger−Kassel−Marcus (RRKM) analyses suggested that Bittner−Howard and Frenklach routes are both viable; however, despite being the global minimum, [4.2.0]octa-1,3,5-trieny-7-yl radicals were not expected to accumulate in appreciable quantities. Tokmakov and Lin subsequently reconsidered these processes at the G2M level.31 Pertinent energies compared well with those of previous computational studies. Ring opening of the [4.2.0]-bicyclic intermediates to form cyclooctatetraen-1-yl radicals, and their subsequent rearrangement to [3.3.0]-bicyclic species, was also examined. Accompanying RRKM analyses suggested significant accumulation of both 2phenylvinyl and 2-vinylphenyl, in agreement with previous works. However, [4.2.0]octa-1,3,5-trieny-7-yl radicals were also predicted to form abundantly, under certain conditions and relatively long reaction times, contrary to the findings of 12193



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Figure 1. Reaction network for chloroacetylene addition to 2,4,5-trichlorophenyl radicals. Route 1 is initiated by β-chlorinated, and route 2 by αchlorinated, C2HCl addition. Barriers of each step are provided, as are heats of reaction (in parentheses). Energies in plain text are from the B3LYP/ cc-pVDZ level; bold values are from M06-2X/6-311+G(3df,3p) SPEs; and bold, italicized values are from G2MS calculations.

channels to nonradical processes.41−44 Clearly, a detailed exploration of PCN formation channels is long overdue. In this work we present, to our knowledge, the first explicit computational studies of HACA and vinylacetylene addition sequences in chlorinated systems. The Frenklach and Bittner− Howard processes are developed upon comparison with hydrocarbon systems; a minor amendment to the mechanism of ring closure in the latter is also considered. A novel channel of naphthalene growth involving acetylene addition to bicyclo[4.2.0]octa-1,3,5-trien-7-yl-based C8H3Cl7 intermediates is also explored. While our current study focuses exclusively on the addition of chloroacetylene or C4Cl4 to 2,4,5-trichlorophenyl radicals, an accompanying work (henceforth referred to as part 245) will explore generalizations of the important channels identified in this work by means of a combined experimental and kinetic modeling study.

single imaginary frequency). All stationary points and vibrational frequencies were obtained with the Spartan ’08 quantum chemistry package.49 Several stationary points have been reoptimized from the DFT/B3LYP/6-31G(d) geometry with Dunning’s correlation consistent cc-pVDZ basis set. This not only affords additional accuracy but also enables a direct comparison with analogous reactions on the C6H5 + C2H2 energy surface as considered by Richter et al.30 ZPE corrections from B3LYP/6-31G(d) computations are employed. We have also calculated M062X/6-311+G(3df,3p) single point energies (SPEs) on all stationary points in this work. This functional has been recently developed by Zhao and Truhlar50 and recommended for barrier heights and reaction kinetics, among other applications, and a survey of the gas-phase enthalpies of isomerization of a number of hydrocarbons (including thermodynamics associated with chlorinated hydrocarbons) has shown that M06-2X/6-311+G(3df,3p) SPE calculations are of comparable accuracy to expensive high level methods such as CBS-QB3 and G4MP2.51 We have also shown previously that M06-2X/6-311+G(3df,3p) SPEs perform very well in benchmarking calculations relevant to the chemistry described in this work.43 The G2MS composite method,52,53 developed as an approximation to the high level CCSD(T)/6-311+G(2df,2p) method for systems with a number of heavy atoms, has also been employed. The G2MS energy is calculated as follows:

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COMPUTATIONAL METHODOLOGY The geometries of all intermediates and transition states were initially optimized with the B3LYP hybrid density functional46,47 utilizing a 6-31G(d) basis set. The harmonic frequencies of all stationary points were computed at the same level of theory following optimization to account for zeropoint energy (ZPE) corrections to heats of reaction and barrier heights. ZPE corrections were scaled by 0.9806.48 Harmonic frequencies also allowed the characterization of these structures as minima (no imaginary frequencies) or transition states (a 12194



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1.72, respectively, with 0.75 expected for a doublet species. Our previous experience with MP2 calculations on chlorinated organic radicals leads us to believe that this contamination typically has a particularly adverse effect on the energies derived. All other levels predict addition barriers within 10 kJ mol−1 of one another (Table 1), relative to the zero of energy, and therefore show reasonable agreement with regard to the C2HCl addition barriers. Consequently, MP2-based energies will be excluded from discussion. On the other hand, Table 1 suggests that the relatively low cost B3LYP/6-31G(d) and M06-2X methods should describe most energies adequately. Finally, we note that β-Cl addition is found to have negative (almost zero) barriers. Negative energy barriers are clearly unphysical, but there are several possible reasons for this. First, there may be inadequacies in the description of the change from closed- to open-shell spin systems in C2HCl upon radical addition (due to the use of a single reference wave function) or additionally, in the case of SPE calculations, deviations of the fully optimized geometries from the approximate geometry used in the SPE scheme. However, there is the possibility of the existence of a weakly bound phenyl radical−acetylene association complex; thus the transition state located does not truly directly link the infinitely separated reactants and the addition adduct. Internal reaction coordinate scans, at the ZPEuncorrected DFT/B3LYP/6-31G(d) level of theory, of the αand β-chlorinated C2HCl/2,4,5-trichlorophenyl radicals addition routes are given in Figure 2; these scans indeed reveal the formation of a weakly bonded complex. These complexes have energies of ∼−0.5 and −4 kJ mol−1 lower than the zero of energy for the α- and β-chlorinated channels, respectively; this indicates energy barriers of ∼2 and 15 kJ mol−1 (using ZPEuncorrected values from the IRC), respectively, which differ negligibly from those given in Figure 1 where we ignore the possibility of this complex. This will therefore be unlikely to influence the ultimate kinetics, especially in high temperature systems, but is a very interesting feature of the fundamentals of the reaction pathways. Contrasting the C2HCl addition steps with the reaction of acetylene with phenyl shows that α-chlorinated addition (TS6) proceeds through a comparable barrier. Previous computational works29−31 put the C2H2−C6H5 addition barrier at 10.4−15.5 kJ mol−1, which matches closely the range of 8.2−17.2 kJ mol−1 computed here. Chlorination of the β-carbon of the vinyl moiety in TS2, however, leads to a barrierless, or nearbarrierless, addition. Spin density maps of the addition products (Figure 3) show that the unpaired spin is localized largely on the β-carbon. Consequently, both the lower barrier and greater exothermicity of the C2HCl addition reaction (by 19.4−33.6 kJ mol−1) upon chlorination of the β-carbon is probably a consequence of enhanced radical stabilization by chlorine. Additional stabilization due to chlorination of the ring may also be evident as the addition products I2 and I6 are both considerably lower in energy than the analogous C8H7 species which are only 162−176.6 kJ mol−1 lower than phenyl + C2H2.29−31 Formation of chlorinated phenylacetylenes is feasible from the 2-phenylvinyl congeners I2 and I6. The most direct route involves unimolecular loss of the α-substituent from the vinylic moiety. H-loss from I2, leading to I5 + H, is assumed barrierless. There is a discrepancy of ∼30−40 kJ mol−1 between B3LYP energies (with barriers of 193.9 and 192.7 kJ mol−1 when using 6-31G(d) and cc-pVDZ basis sets, respectively) and those predicted with coupled cluster and G2MS approaches.

E(G2MS) = E[CCSD(T)/6‐31G(d)] + E[MP2/6311+G(2df,2p)] − E[MP2/6‐31G(d)] + HLCG2MS + ZPE DFT

All energies are obtained from single point corrections on DFT/B3LYP/6-31G(d) geometries, with the (scaled)48 ZPEDFT coming from the same calculations. HCLG2MS is an empirically determined high level correction factor, which is commonly deemed negligibly small. This is a far less expensive approach than the G2 method but also attains near-chemical accuracy.54,55 Despite this, it is still a very computationally expensive method and will be employed sparingly. All single point energy calculations were performed with the Gaussian 09 suite.56

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RESULTS AND DISCUSSION A. C2HCl Addition to 2,4,5-Trichlorophenyl Radicals. The addition of the first C2HCl unit to a 2,4,5-trichlorophenyl radical is explored in Figure 1; energy barriers and heats of reaction from the B3LYP/cc-pVDZ and G2MS levels of theory are also given. Energies of all stationary points calculated at various levels of theory, relative to C 2 HCl + 2,4,5trichlorophenyl, are presented in Table 1. Two initiation Table 1. Computed Energies/(kJ mol−1) of C8H3Cl7 Stationary Points Considered in Figure 1a B3LYP631G(d)

B3LYPccpVDZ

M062X

CCSD(T)631G(d)

G2MS

−0.9 −225.4 −108.7 −206.7 −102.6 −245.9 −31.5

0.3 −222.0 −111.2 −200.0 −95.7 −235.7 −29.3

−0.3 −209.3 −82.0 −189.8 −65.6 −222.6 −36.0

0.4 −229.4 −100.1 −210.8 −85.3 −242.6 −75.8

−3.6 −229.9 −112.7 −210.4 −72.9 −235.2 −64.9

route 1 TS2 I2 TS3 I3 TS4 I4 I5 route 2 TS6 I6 TS7 I7 TS8 I8 I9

14.5 17.2 15.5 12.5 8.2 −193.2 −188.4 −185.7 −209.0 −210.5 −83.9 −87.2 −64.1 −81.3 −97.0 −183.9 −177.2 −176.4 −186.3 −182.2 −90.9 −84.2 −64.1 −76.7 −65.3 −259.5 −248.9 −236.9 −255.6 −246.8 −137.4 −133.4 −114.8 −173.7 −160.4 Point-to-Point Deviations (Relative to G2MS) av 12.6 11.8 16.8 7.2 max 33.4 35.6 45.6 15.7 Deviations in Barriers/Heats of Reaction (See Figure 1) Relative To G2MS av 12.3 13.5 10.9 7.2 max 33.4 33.2 24.8 14.8 a All values are relative to C2HCl + 2,4,5-trichlorophenyl. CCSD(T) and G2MS values are single point energies based on B3LYP/6-31G(d) geometries, employing scaled ZPEs also from this level of theory.

channels are considered involving either β-chlorinated or αchlorinated C2HCl addition (routes 1 and 2, respectively). Both bimolecular addition transition states, TS2 and TS6, are early with forming C−C bonds of ∼2.5 Å, and consequently possess low energies. The energy of TS6 is 10−17 kJ mol−1 higher than that of TS2. MP2/6-311+G(2df,2p) level SPEs were also computed but showed significant disagreement with other values. However, the MP2 calculations are heavily spincontaminated; TS2 and TS6 have ⟨S2⟩ values of 1.60 and 12195



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Figure 2. ZPE-uncorrected DFT/B3LYP/6-31G(d) IRCs for the α- and β-chlorinated addition of C2HCl to 2,4,5-trichlorophenyl radicals.

manipulations and constraints. A number of other phenyl/ acetylene pairings were also trialed but were also unsuccessful. Both H-abstraction transition states, as shown in Figure 1, have similar barriers when computed at the same level of theory, suggesting chlorine has little influence on this barrier. The range of computed barriers on the C8H7 PES (114.6−116.7 kJ mol−1)29,31 is comparable to the barriers encountered from I2 and I6 which further supports this claim; although the M06-2X calculations predict values that are somewhat higher than this range. This reaction is 4.7−5.0 kJ mol−1 endothermic on the C8H7 PES.29,31 Kinetic modeling has suggested that Bittner− Howard mechanisms, initiated by intermediates analogous to I2 and I6, outcompete Frenklach channels (through I3 and I7, for example) in temperature and pressure regimes where the 2phenylvinyl radicals are collisionally stabilized against bond homolysis. The higher energies, and thus lower concentrations, of 2-vinylphenyl relative to 2-phenylvinyl are probably responsible. Our results suggest that chlorinated 2-phenylvinyl radicals are significantly more stable than 2-vinylphenyl congeners again. As such, PCN formation via the Frenklach mechanism may be less likely than comparable reactions on the C8H7 PES. While 2-vinylphenyl congeners I3 and I7 are the least stable C8H3Cl7 congeners considered in this work, four-membered ring cyclization between the β-carbon of the vinyl moiety and the vacant ortho-ring position yields I4 and I8, respectively, which are the most stable species along routes 1 and 2. Cyclization barriers are similar to those encountered in the Hshift transition states TS3 and TS7. DFT underestimates cyclization barriers by 20−30 kJ mol−1 relative to G2MS values, with M06-2X somewhat intermediate between the two; exothermicity, however, is comparable. G2MS barriers and heats of reaction along route 1 are comparable to those found by Tokmakov and Lin31 on the C8H7 PES, who found 130.1 and −25.5 kJ mol−1, respectively, at their highest level of theory. In their kinetic study, these authors found that these bicyclo[4.2.0]octa-1,3,5-trien-7-yl species may eventually accumulate in significant quantities; however, their role as a base for further growth was not explored. The congener I8 formed along route 2 encounters a far lower barrier and greater exothermicity (116.9 and −64.6 kJ mol−1, respectively, with G2MS values) than either the formation of I4 or bicyclo[4.2.0]-

Figure 3. (Left to right) Spin density maps from the B3LYP/6-31G(d) level of C2HCl + 2,4,5-trichlorophenyl addition adducts I2 and I6, respectively.

The dissociation energies calculated by the CCSD(T) and G2MS approaches, at 153.6 and 165.0 kJ mol−1, respectively, compare favorably with energies and barriers derived in the decomposition of 2-phenylvinyl to C8H6 + H (145.6−166 kJ mol−1).29−31 Consequently, these methods are arguably better suited to describing bond homolysis than our B3LYP calculations. The M06-2X calculations, on the other hand, coincide with the upper range of these values at 173.3 kJ mol−1. This provides a useful benchmark for the decomposition of I6 in route 2 via Clloss for which there have been, to our knowledge, no attempts to rigorously ascertain such barriers. DFT and G2MS methods differ only by around 5 kJ mol−1, with Cl-loss from the αcarbon proceeding with a dissociation energy of only 50.1 kJ mol−1 at our highest level of theory. CCSD(T) predicts a lower, and M06-2X a higher, barrier at 35.3 and 70.9 kJ mol−1, respectively. However, all estimates are substantially lower than the estimated activation energies for Cl-loss from C8Cl7 utilized in kinetic models developed by Taylor and co-workers,36,37 who quote values of 123.0 kJ mol−1. Assuming Cl-dissociation energies calculated here for α-chlorinated C8H3Cl4 species are comparable to those in C8Cl7, this may have severe implications for the applicability of these kinetic models. 2-Phenylvinyl congeners may be stabilized with respect to bond homolysis via the intramolecular H-shift proposed by Frenklach et al.25,57 Transition states for this reaction, TS3 and TS7, were located in routes 1 and 2, respectively. No analogous Cl-abstraction could be located despite exhaustive geometry 12196



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Figure 4. Reaction network for Bittner−Howard PCN formation routes in the reaction of 2,4,5-trichlorophenyl radicals with C2HCl. Barriers and heats of reaction (in parentheses) in kilojoules per mole are given. All energies are based on M06-2X/6-311+G(3df,3p) SPEs.

octa-1,3,5-trien-7-yl on the C8H7 PES.31 Consequently, the role of [4.2.0]-bicyclic species in chlorinated systems may play a significant role in PCN growth. We will explore the role of these pathways experimentally in C2H2Cl2/C2HCl3/trichlorobenzene pyrolysis systems in part 2.45 B. Bittner−Howard Ring Closure and Modifications. The most direct pathway to PCN formation is via a Bittner− Howard ring closure mechanism. The M06-2X/6-311+G(3df,3p)//B3LYP/6-31G(d) barriers and heats of reaction for this complex network are shown in Figure 4. This level of theory should be sufficient given the generally good agreement found between B3LYP, M06-2X, and G2MS calculations of the reactions of C8H3Cl4 for pertinent reactions while remaining relatively computationally inexpensive. This accuracy of M062X can be seen in the average and maximum absolute energy deviations relative to our highest, and presumably most accurate, level of theory, G2MS. Point-to-point, M06-2X performs the most poorly; however, this appears to be due to

an overstabilization of the reactants, affecting the zero of energy and leading to an artifact that shifts all of the subsequent points along the PES relative to the G2MS values. A more physically reasonable metric is to use the energy deviations related to the actual energy barriers and heats of reaction encountered for each elementary step, i.e., the energies provided in Figure 1; here the effect of reactant overstabilization is only encountered in a few instances and is therefore not overcounted. These values indicate that CCSD(T) provides, unsurprisingly, thermochemistry most similar to the G2MS method; M06-2X is the next most accurate but is significantly less expensive, and hence its adoption throughout as the method of choice for the larger C10H4Cl5 moieties encountered in the remaining PESs studied throughout this work. C2HCl adds to the radical site of intermediate I2 or I6, localized on the β-carbon of the vinylic moiety. This may proceed with either H or Cl on the β-carbon of the adding C2HCl, leading to four distinct pathways to growth: α- and β-chlorinated C2HCl addition to I2 via TS10 12197



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Figure 5. M06-2X/6-311+G(3df,3p) SPE PES of a modified Bittner−Howard PCN ring closure mechanism operative at H-substituted orthopositions of the phenyl ring. All energies are relative to 2,4,5-trichlorophenyl + 2C2HCl.

to the chlorinated o-carbon, via TS12, TS15, TS22, and TS25, belong to the second set of cyclization routes. These reactions are more direct than cyclization at the H-substituted site as the loss of the o-Cl atom now occurs in concert with cyclization. C10H4Cl4 isomers are produced in this sequence. The barriers of cyclization are higher at the chlorinated position, presumably due to steric factors, although the formation of C10H4Cl4 + Cl is more thermodynamically favorable than C10H3Cl5 + H, and, in general, in the C10H4Cl5 intermediates I13, I16, I20, and I23 the barrier to ring opening (the reverse reaction) is generally lower than that of H-loss. An amendment to the conventional Bittner−Howard mechanism has also been developed here. We have considered the role of a Frenklach-like internal H-shift, now by the δcarbon of a C4 moiety, immediately prior to cyclization (we note that, again, Cl-shifts could not be located). The M06-2X/ 6-311+G(3df,3d) PES of this reaction from I10, the lowest energy C6H2Cl3−C4H2Cl2 isomer considered in Figure 4, is shown in Figure 5. The defining transition state (TS26) is approximately 30 kJ mol−1 higher in energy than direct cyclization (TS13). We find, on the B3LYP/6-31G(d) PES, a lower barrier of ∼20 kJ mol−1 (see Figure S.2 in the Supporting Information). In Figure 1, we find that B3LYP values of the Frenklach H-migration (TS3 and TS7) underestimate the presumably accurate G2MS values, with M06-2X overestimating. The differences between the true barriers are therefore likely ∼25 kJ mol−1. We will explore the role of this energy difference on the efficacy of these two routes in our following work and will demonstrate that once preexponential factors are accounted for, this new channel becomes extremely important. Once I26 is formed, cyclization and Cl-loss (now leading to C10H4Cl4, as opposed to C10H3Cl5, congeners) appears feasible. H-loss from I27, leading to C10H3Cl5, has not been considered explicitly but is unlikely to be competitive given the typically higher barriers to H-loss relative to C−Cl bond homolysis. C. Frenklach Ring Closure Mechanism. In higher temperature and/or lower pressure regimes, 2-vinylphenyl radicals may accumulate in higher concentrations than 2phenylvinyl radicals, as they are stabilized with respect to H-

and TS11, respectively, and the analogous additions to I6 via TS18 and TS19, respectively. Incidentally, IRC searches based on the I2 to I10 presumed elementary step suggest there may also be a weakly bonded C8H3Cl4−C2HCl complex (analogous to that seen in Figure 2). In this pair, a fully optimized adduct possesses an energy of only 3.6 kJ mol−1 lower in energy than the zero of energy and will therefore only have a minor effect on kinetics. The second C2HCl moiety adds more easily if the β-carbon is chlorinated (TS10 and TS18). In fact, both transition states are ∼15 kJ mol−1 lower than their respective α-chlorinated addition analogous TS11 and TS19. This is comparable to the energy difference between the first two addition routes leading to C8H3Cl4, TS2 and TS6. It is also apparent that the two addition modes to the β-chlorinated C8H3Cl4 adduct I2 (leading to I10 or I11) experience higher barriers (by ∼7−8 kJ mol−1) and smaller exothermicities (by ∼20 kJ mol−1) than comparable additions in I6 (leading to I18 or I19). This reflects the greater instability of α-chlorinated I6. Barriers and heats of reaction are consistent with published B3LYP/4-31G values for C10H9 isomers (17.7 and −165.7 kJ mol−1, respectively).29 Each addition product, I10, I11, I18, and I19, may undergo two distinct cyclization reactions as the ortho-sites on the phenyl ring possess different substituents. The first set, via TS13, TS16, TS20, and TS23, respectively, involve cyclization at the hydrogenated site. Barriers are variable given the differing stabilities of the starting intermediate; however, none appear to exceed 30 kJ mol−1. Consequently, at typical pyrolytic temperatures, none of these steps should be inaccessible. The resulting intermediates are particularly stable and require H-loss to form PCNs; these products are C10H3Cl5 isomers (I14, I17, I21, and I24, respectively). The dissociation energies (barrierless loss is presumed) are generally only a little higher than the cyclization barriers reaching approximately 50 kJ mol−1. B3LYP/6-31G(d) barriers are also less than 50 kJ mol−1 (see the Supporting Information, Figure S1), in agreement with the values here despite the higher barriers to H-loss predicted in I5. This suggests that the barriers found here are likely quite reasonable. Additions of the δ-carbon of the C4H2Cl2 fragment 12198



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Figure 6. Reaction network for Frenklach PCN formation routes in the reaction of 2,4,5-trichlorophenyl radicals with C2HCl. Barriers and heats of reaction (in parentheses) in kilojoules per mole are given. All energies are based on M06-2X/6-311+G(3df,3p) SPEs.

and will be worthwhile investigating in later, more refined, studies. Ring closure occurs through the transition states TS33, TS35, TS37, and TS39. Forming C−C bond lengths are typically 2.4 Å. Reaction passing through TS35 and TS39, where the second C2HCl added is α-chlorinated, have barriers of ∼20 kJ mol−1, somewhat higher than the Frenklach cyclization barrier of C10H9, 7.5 kJ mol−1, at the G3(MP2,CC) level.58 β-Chlorination of the second C2HCl moiety leads to low barriers to cyclization, TS33 and TS37; these steps are, in fact, found to be barrierless at the B3LYP/6-31G(d) level. Cyclization is very exothermic in all instances, and these reactions are also therefore reasonably facile in light of the low barriers. The H- and Cl-losses required to complete naphthalene formation are endothermic and, Cl-losses particularly, of similar magnitude to G2MS estimates of analogous losses from tetrachlorinated 2-phenylvinyl radicals I2 and I6 (Figure 1). This comparability indicates that the M06-2X/6-311+G(3df,3p) level probably provides an adequate description of this system. It is important to note that this sequence predicts a set of products that are different from those associated with conventional or modified Bittner−Howard sequence despite involving the same reagents. D. PCN Formation via C2HCl Addition to C8H3Cl4 Bicyclo[4.2.0]octa-1,3,5-trien-7-yl Radicals. Although the work of Tokmakov and Lin suggests that [4.2.0]-bicyclic C8

loss. Consequently, the Frenklach mechanism arguably outcompetes the Bittner−Howard sequence to become the major route of naphthalene formation. Although we have argued that these sequences may be less likely with chlorinated congeners, we have nonetheless explored this pathway in detail. The extension of this cyclization scheme to the production of various C10H4Cl5 isomers is considered in Figure 6, starting from 2-vinylphenyl congeners I3 and I7. The barrier and reaction energy of C2H2 addition to 2vinylphenyl has been computed to be 16.3 and −157.3 kJ mol−1, respectively, at the G3(MP2,CC) level.58 These values compare reasonably well with the α-chlorinated addition of C2HCl to I3 and I7 via TS30 and TS32, respectively. βChlorinated addition steps, via TS29 and TS31, overcome barriers ∼15 kJ mol−1 lower than those through TS30 and TS32, respectively. Similarly, these reactions liberate ∼25 kJ mol−1 more heat than the α-chlorinated addition steps. This barrier lowering with β-chlorinated additions has been observed in all C2HCl reactions studied thus far and may have a significant impact on PCN growth via chloroacetylene additions. Again, we note that IRC searches reveal the presence of a prereaction complex; at 7.4 kJ mol−1 below the zero of energy this will have a minor impact on the chemistry in high temperature systems, with this energy difference influencing rate constants at 1000 K by a factor of ∼2.4. However, these considerations are likely to be important at lower temperatures 12199



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Figure 7. Example tetrachloronaphthalene formation routes via bicyclo[4.2.0]octa-1,3,5-trien-7-yl congeners in the reaction of 2,4,5-trichlorophenyl radicals with C2HCl. Barriers and heats of reaction (in parentheses) in kilojoules per mole are given. All energies are based on M06-2X/6311+G(3df,3p) SPEs.

Figure 8. M06-2X/6-311+G(3df,3p) SPE based PES of a PCN ring closure mechanism initiated through C4Cl4 addition to a phenyl ring. The example shown considers 2,4,5-trichlorophenyl radicals as the nucleus. All energies are relative to 2,4,5-trichlorophenyl + C4Cl4.

considered as this intermediate should form in higher abundances than the more stable isomer I8 if these systems are kinetically controlled. Additionally, reaction may be less sterically hindered as the addition site is not occupied by bulky chlorine atoms, as is the case in I8. Reaction barriers and energies computed at the M06-2X/6-311+G(3df,3p)//B3LYP/ 6-31G(d) level of theory are depicted in Figure 7. For brevity, only a single addition channel has been considered. In both Bittner−Howard and Frenklach mecha-

species may reach nonnegligible concentration in combustion/ pyrolysis systems,31 the explicit role of such species in growth reactions does not appear to have been considered. Table 1 indicates that bicyclo[4.2.0]octa-1,3,5-trien-7-yl congeners are the lowest minima on the C8H3Cl7 PES and their accumulation in chlorinated systems may also be important. This does assume, however, that the energy of the associated 2vinylphenyl radicals, through which these channels must pass, is not prohibitively high. C2HCl addition to I4 has been 12200



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Figure 9. ZPE-uncorrected DFT/B3LYP/6-31G(d) IRCs for the vinylic and acetylenic addition modes of C4Cl4 to 2,4,5-trichlorophenyl radicals.

nisms, β-chlorinated C2HCl addition has been found to encounter significantly lower barriers than the analogous αchlorinated channels. Consequently, the only addition mode considered from I4, passing through TS41, is the β-chlorinated attack process. Examination of Figure 7 indicates that addition requires only a slight barrier (30.0 kJ mol−1; we find 31.9 kJ mol−1 directly from B3LYP/6-31G(d)), and it is reasonably exothermic. However, comparison with other pathways considered in earlier figures shows that barriers of addition to I4 are considerably higher than other β-chlorinated C2HCl additions, which are typically almost barrierless. Similarly, formation of I41 is the least thermodynamically favored channel concerning the addition of a C2HCl unit. Consequently, while bicyclo[4.2.0]octa-1,3,5-trien-7-yl congeners may accumulate in significant quantities, the appreciably larger barriers and lower exothermicity associated with further growth potentially renders these channels largely inoperative. This will be explored in detail in our subsequent experimental/kinetic modeling study in part 2.45 Assuming addition does proceed, however, two distinct routes to closure have been developed. Reaction branching occurs through either TS42 or TS45 which are based on Frenklach and Bittner−Howard channels, respectively. Both resulting adducts pass through fused Dewar-benzene intermediates (I43 and I45, respectively) which undergo facile ringopening reactions via TS44 and TS46, respectively, to give naphthalene-based C10H4Cl5 radicals. Figures 4 and 5 both indicate that Cl-loss from analogous intermediates, yielding tetrachloronaphthalenes, requires only a small input of energy and is therefore not explicitly considered. While ring closure via TS42 proceeds through a lower barrier than the pathway initiated through TS45, the latter requires fewer steps and encounters lower subsequent barriers. As a consequence, it is difficult to conclude which of these pathways is dominant if channels through bicyclo[4.2.0]octa-1,3,5-trien-7-yl congeners are operative. However, both cyclization models again predict distinctive C10H4Cl4 isomers which will be exploited in our experimental studies discussed in our following work.45 E. C4Cl4 Addition and Ring Closure Routes. While the energy barriers of the bimolecular addition reactions explored thus far are relatively low and will therefore lead to high rate constants, the ultimate rate of these reactions is also heavily

dependent upon the concentrations of the precursors. C2Cl2 undergoes rapid dimerization to yield perchlorovinylacetylene;41,42,44,59 consequently, not only are C2Cl2 yields typically low but also C4Cl4 accumulates in significant quantities in its place. (This reaction is very important for C2Cl2 but appears to be increasingly less so for C2HCl and C2H2).41,42 As such, C4Cl4 may also be a candidate precursor in PCN growth reactions. The final channel explored in this work is therefore not initiated by sequential acetylene additions but an analogous single C4Cl4 addition. The M06-2X/6-311+G(3df,3p)//DFT/ B3LYP/6-31G(d) PES of the addition reaction between 2,4,5trichlorophenyl radicals and C4Cl4 is shown in Figure 8. The addition step, via TS47, occurs barrierlessly on the M062X PES; this is possibly an artifact of differences in the B3LYP and “true” underlying M06-2X geometries (a barrier of 7.0 kJ mol−1 is found from the B3LYP/6-31G(d) PES itself). This is comparable to the barriers found with acetylene additions suggesting that it is also a facile route. Further, the B3LYP energy is comparable to the barrier found by Moriarty and Frenklach for phenyl addition at the vinylacetylene triple bond (8.4 kJ mol−1).32 Although not shown in Figure 7, the analogous addition to the C4Cl4 double bond has also been considered; the ZPE-uncorrected IRC scans in Figure 9 address this point, however, as well as confirming the (potential) formation of weakly bonded complexes prior to covalent bond formation. While this mode proceeds with a lower barrier than addition to the CC site on the C10H9 PES,32,34 the addition barrier on the ZPE-corrected 2,4,5-trichlorophenyl/C4Cl4 B3LYP/6-31G(d) PES is appreciably higher at 23.9 kJ mol−1 (relative to the zero of energy). Addition to the C4Cl4 double bond also liberates less heat (ΔrH = −135.9 kJ mol−1 relative to I1 + C4Cl4 compared with addition to the CC bond, forming I47, with ΔrH −238.7 kJ mol−1 on the ZPE-corrected B3LYP PES). At temperatures of ∼1000 K applicable to pyrolytic conditions (and particularly systems such as those under study in our following work, part 245) then a kinetically controlled process under the assumption that both vinylic and acetylenic addition pathways possess the same preexponential factors, should proceed with a branching ratio of 85% in favor of an acetylenic-based addition process. In hydrocarbon systems, with an energy barrier of ∼5 kJ mol−1 (as advocated in the recent study by Parker et al.33) for the acetylenic addition pathway, 12201



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C4Cl4 additions. Additionally, all rearrangement steps are much lower than re-dissociation to phenyl radicals and vinylacetylene.

but barrierless vinylic addition process, the same kinetic arguments suggest only 35% of product formation proceeds through the acetylenic pathway. The higher barrier and lower thermodynamic drive of addition to the C4Cl4 double bond, presumably arising due to steric interactions between chlorine atoms, will render these pathways appreciably slower, and it should be safe to ignore them, at least within the focus of the current studies. Closer scrutiny on these points will be required in future studies; and indeed, our following work (part 245) presents experimental evidence in which we compare predicted to experimental congener distributions. These results support our findings here, with acetylenic addition channels describing product congener distributions very well while the vinylic addition pathway plays at best a minor role being unable to correctly predict even the identity of the major product congeners (see part 245 for details). This example again demonstrates that care must be exercised when extrapolating hydrocarbon pyrolysis mechanisms to chlorinated systems. Two reaction pathways for the bimolecular adduct I47 have been considered. The first is the presumed barrierless Cldissociation step leading to phenylvinylacetylene congeners. Formation of I48 + Cl from I47 is endothermic by 119.0 kJ mol−1. Comparison with the analogous Cl-loss from I6 to yield phenylacetylene congener I9 (Figure 1), with an energy of 70.9 kJ mol−1 at the same level of theory, suggests phenylacetylenes are far more readily formed than phenylvinylacetylenes. Our following experimental study, detailed in part 2,45 will demonstrate that this is one of several important factors necessitating the inclusion of C4Cl4 addition pathways in a successful kinetic model of PCN growth. The competing channel is a cyclization sequence. Cyclization is now unimolecularly initiated in C4Cl4 addition reactions. This presents a further significant advantage for PCN growth via C 4Cl 4 addition channels when compared with HACA sequences. In the latter, growth must be initiated through statistically less favorable bimolecular channels. However, significant rearrangement barriers from intermediates such as I47, due to the stiff partially sp-hybridized C4 backbone and the absence of a ring-based radical site for cyclization must be addressed for efficient PCN formation. The internal H-shift originally proposed by Moriarty and Frenklach,32 TS49, proceeds with a barrier of 157.8 kJ mol−1. This is somewhat higher than analogous shifts in C8H3Cl7 intermediates (Table 1) but in reasonable agreement with previous estimates for C6H5−C4H4 systems (117.2−164 kJ mol−1).32,35 The formation of the adduct, I49, is endothermic, also in agreement with computed results on the C10H9 PES. Based on the results of Aguilera-Iparraguirre and Klopper,35 cyclization to a [4.2.0]bicyclic intermediate such as I50 should provide the most facile means of rearranging the C4 backbone such that it is much more amenable to PCN formation. Cyclization through TS50, with a barrier of 99.6 kJ mol−1, is substantially more facile than the analogous process on the C10H9 PES (140 kJ mol−1).35 Once formed, relatively free rotation of the vinyl moiety is assumed, and hence production of I51 is shown without further rearrangement steps. Although this elementary step (via TS51) has a higher barrier than the reverse reaction (via TS50), the low barrier associated with the subsequent cyclization process, proceeding through TS52, and thermodynamic drive of hexachloronaphthalene formation (in fact, it appears as though ring closure and Cl-loss appear largely in concert on the M062X PES) should lead to feasible PCN production through

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CONCLUSION This work describes, to our knowledge, the first detailed computational studies into the growth of PCN congeners in high temperature systems. Several traditional growth sequences have been extended from hydrocarbon systems, and it is found that the majority of these reactions differ little upon chlorination. In particular, the relative energies of congeners of 2-phenylvinyl and [4.2.0]octa-1,3,5-trieny-7-yl radicals appear similar irrespective of the degree of chlorination, although 2-vinylphenyl is a little less stable after the inclusion of chlorine. Similarly, the barriers of the internal H-shift driving 2-phenylvinyl ↔ 2-vinylphenyl and the ring closure responsible for 2-vinylphenyl ↔ [4.2.0]octa-1,3,5-trieny-7-yl are comparable on both the C8H7 and C8H3Cl7 PESs. Consequently, several features from the kinetic studies of hydrocarbon systems should be readily applicable to chlorinated congeners. Most importantly, 2-phenylvinyl congeners should dominate in high pressure/low temperature regimes, and the Bittner− Howard mechanism of PCN growth should therefore be the fastest under these circumstances. Otherwise, Frenklach mechanisms, where 2-vinylphenyl provides the nucleus for growth, should explain PCN yields. [4.2.0]Octa-1,3,5-trieny-7yl congeners are the global C8H3Cl7 minima and should accumulate at longer reaction times;31 however, high barriers to C2HCl addition and the possibility that acetylene sources may be exhausted by the time these radicals accumulate sufficiently may render these channels largely inactive. Barriers of α-chlorinated C2HCl addition to various pertinent radicals along both the Bittner−Howard and Frenklach pathways also appear comparable to analogous additions in hydrocarbon-based routes, as are the heats of reaction of Hlosses from β-chlorinated 2-phenylvinyl congeners at ∼170 kJ mol−1. Consequently, the reaction kinetics of a number of pertinent processes should be similar in hydrocarbon and chlorohydrocarbon systems. These observations ostensibly justify the assumption that the direct extension of hydrocarbon growth mechanisms to chlorinated systems, with only minor revision of rate constants, is valid. This assumption is inherent in the kinetic models of Taylor and co-workers,36−39 who have compiled the only current mechanism of PCN formation. However, there are several pertinent reactions which differ considerably upon chlorination and will likely significantly impact the chemistry of PCN growth. First, all C2HCl additions considered are found to depend heavily on whether the acetylene adds in a α- or β-chlorinated geometry, with the barrier of the latter being 15 kJ mol−1 lower. This will have a substantial impact on the congener distribution produced, which may be exploited (as we will do in a subsequent work, part 245) to differentiate between models to find the active growth sequences. Second, the reaction heats of Cl-loss from 2phenylvinyl congeners, producing chlorinated phenylacetylenes, are substantially lower than previous estimates,36−39 which we revise down from ∼120 to ∼60 kJ mol−1. This will undoubtedly have a major impact on, and call into question, the kinetic models of Taylor and co-workers36−39 as the lifetimes of αchlorinated C8H7 congeners may be far too short to accommodate growth. It is likely that a novel non-HACA sequence, such as C4Cl4 addition to chlorinated phenyl radicals, will become active in place of acetylene additions. These routes are found to be facile, and addition modes differ substantially 12202



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thalenes in Soil, Sediment, and Biota Collected near the Site of a Former Chlor-Alkali Plant. Environ. Sci. Technol. 1998, 32, 2507−2514. (7) Hayward, D. Identification of Bioaccumulating Polychlorinated Naphthalenes and Their Toxicological Significance. Environ. Res. 1998, 76, 1−18. (8) Coelhan, M.; Reil, I.; Rimkus, G.; Parlar, H. Peak Patterns of Chlorostyrenes in Fish and Fish Oils from the North Atlantic. Environ. Sci. Technol. 2000, 34, 4695−4700. (9) Ishaq, R.; Naf, C.; Zebuhr, Y.; Broman, D.; Jarnberg, U. PCBs, PCNs, PCDD/Fs, PAHs and Cl-PAHs in Air and Water Particulate SamplesPatterns and Variations. Chemosphere 2003, 50, 1131− 1150. (10) Bittner, J. D.; Howard, J. B., Composition Profiles and Reaction Mechanisms in a Near-Sooting Premixed Benzene/Oxygen/Argon Flame. In Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, USA, 1981; pp 1105−1116. (11) McEnally, C. S.; Pfefferle, L. D. Experimental Assessment of Naphthalene Formation Mechanisms in Non-Premixed Flames. Combust. Sci. Technol. 1997, 128, 257−278. (12) McEnally, C. S.; Pfefferle, L. D. The Use of Carbon-13-Labeled Fuel Dopants for Identifying Naphthalene Formation Pathways in Non-Premixed Flames. Proc. Combust. Inst. 2000, 28, 2569−2576. (13) Melius, C. F.; Colvin, M. E.; Marinov, N. M.; Pit, W. J.; Senkan, S. M., Reaction Mechanisms in Aromatic Hydrocarbon Formation Involving the C5H5 Cyclopentadienyl Moiety In Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, USA, 1996; Vol. 26, pp 685−692. (14) Kislov, V. V.; Mebel, A. M. The Formation of Naphthalene, Azulene, and Fulvalene from Cyclic C5 species in Combustion: An ab Initio/RRKM Study of 9-H-Fulvalenyl (C5H5−C5H4) Radical Rearrangements. J. Phys. Chem. A 2007, 111, 9532−9543. (15) Wang, D.; Violi, A.; Kim, D. H.; Mullholland, J. A. Formation of Naphthalene, Indene, and Benzene from Cyclopentadiene Pyrolysis: A DFT Study. J. Phys. Chem. A 2006, 110, 4719−4725. (16) Mebel, A. M.; Kislov, V. V. Can the C5H5 + C5H5 → C10H10 → C10H9 + H/C10H8 + H2 Reaction Produce Naphthalene? An ab Initio/ RRKM Study. J. Phys. Chem. A 2009, 113, 9825−9833. (17) Lu, M.; Mulholland, J. A. PAH Growth From the Pyrolysis of CPD, Indene and Naphthalene Mixture. Chemosphere 2004, 55, 605− 610. (18) Murakami, Y.; Saejung, T.; Ohashi, C.; Fujii, N. Investigation of a New Pathway Forming Naphthalene by the Recombination Reaction of Cyclopentadienyl Radicals. Chem. Lett. 2003, 32, 1112−1113. (19) Ikeda, E.; Tranter, R. S.; Kiefer, J. H.; Kern, R. D.; Singh, H. J.; Zhang, Q. The Pyrolysis of Methylcyclopentadiene: Isomerization and Formation of Aromatics. Proc. Combust. Inst. 2000, 28, 1725−1732. (20) Hore, N. R.; Russell, D. K. The Thermal Decomposition of 5Membered Rings: A Laser Pyrolysis Study. New J. Chem. 2004, 28, 606−613. (21) Bockhorn, H.; Fetting, F.; Wenz, H. W. Investigations of the Formation of High Molecular Hydrocarbons and Soot in Premixed Hydrocarbon-Oxygen Flames. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1067−1073. (22) Frenklach, M.; Clary, D. W., ; Gardiner, W. C., Jr.; Stein, S. E. Detailed Kinetic Modeling of Soot Formation in Shock-Tube Pyrolysis of Acetylene. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, USA, 1984; pp 887−901. (23) Frenklach, M.; Warnatz, J. Detailed Modeling of PAH Profiles in a Sooting Low-Pressure Acetylene Flame. Combust. Sci. Technol. 1987, 51, 265−283. (24) Gu, X.; Zhang, F.; Guo, Y.; Kaiser, R. I. Crossed-MolecularBeam Study on the Formation of Phenylacetylene from Phenyl Radicals and Acetylene. Angew. Chem., Int. Ed. 2007, 43, 6866−6869. (25) Appel, J.; Bockhorn, H.; Frenklach, M. Kinetic Modeling of Soot Formation with Detailed Chemistry and Physics: Laminar Premixed Flames of C2 Hydrocarbons. Combust. Flame 2000, 121, 122−136. (26) Moriarty, N. W.; Brown, N. J.; Frenklach, M. Hydrogen Migration in the Phenylethen-2-yl Radical. J. Phys. Chem. A 1999, 103, 7127−7135.

from their non-chlorinated analogues. In particular, only the addition to the vinylacetylenic CC bond appears energetically feasible enough to facilitate growth. Finally, it is apparent that electronic and steric effects inherent in chlorinated hydrocarbon systems makes the elucidation of their high temperature reactions a nontrivial task, and great caution must be exercised when extending the results of hydrocarbon chemistry to chlorinated analogues. The feasibility of these models is explored in a recent study uses a probability-tree-based approach60 to understand the general kinetics of these systems and derive the expected product yields applying the models considered here to a number of mixed chlorinated ethylene/trichlorobenzene systems, and comparing expected to observed congener yields provides a powerful means to assess the relative activities of each of these reaction sequences.

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ASSOCIATED CONTENT

* Supporting Information S

Table listing Cartesian coordinates of all structures and figures showing all energy diagrams and PESs reproduced using DFT/ B3LYP/6-31G(d) values. 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 towards equipment. We also gratefully acknowledge the University of Auckland for financial support of 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 HACA, hydrogen abstraction acetylene addition; IRC, intrinsic reaction coordinate; PAH, polycyclic aromatic hydrocarbon; PCN, polychlorinated naphthalene; PES, potential energy surface; tri(/tet/p/h)CN, tri(/tetra/penta/hexa)chloronaphthalene; ZPE, zero-point energy

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REFERENCES

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