Douglas K. Russell

The thermal decomposition of 5-membered rings: a laser pyrolysis study

The mechanisms of pyrolysis of cyclopentadiene, furan, pyrrole and thiophene have been investigated using a combination of IR laser powered homogeneous pyrolysis, chemical and physical trapping of radical intermediates, and use of precursors specifically designed to generate selected radical intermediates. The results confirm the central role played by free radicals in the cases of cyclopentadiene and thiophene, and the dominant step of 1,2-H shifts in the cases of furan and pyrrole. The experimental results may be interpreted according to the high level ab initio calculations recently reported in the literature.

Radical pathways in the thermal decomposition of pyridine and diazines: a laser pyrolysis and semi-empirical study

The mechanisms of thermal decomposition of pyridine and the three isomeric diazines have been investigated by IR laser pyrolysis in conjunction with stable end-product analysis by FTIR, NMR and GC–MS, and radical detection by EPR spectroscopy. Calculations at semi-empirical and ab initio levels have provided confirmation of potential reaction pathways. For pyridine, reaction is initiated by formation of pyridyl radicals, as indicated by extensive isotope exchange with added deuterium. Experiments with bromopyridines show that open chain radicals arising from ring opening of 2-pyridyl and 3-pyridyl radicals each lead to stable gaseous products, while 4-pyridyl radicals produce solid deposits, and may be the principal agents in soot formation. The evidence suggests that 1,2-diazine decomposes via a molecular route leading to stoichiometric production of HCN and C2H2, while 1,3- and 1,4-diazine follow a pattern of H loss and ring radical opening analogous to that of pyridine.

Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons: formation of benzene rings. 1. Quantum chemical studies.

Analogues of important aromatic growth mechanisms in hydrocarbon pyrolysis and combustion systems are extended to chlorinated systems. We consider the addition of C2Cl2 to both C4Cl3 and C4Cl5 radicals at the M06-2X/6-311+G(3df,3p)//B3LYP/6-31G(d) level of theory, and we demonstrate that these reaction systems have much in common with those of nonchlorinated species. In particular, we find that these radicals appear to lead preferentially to fulvenes, and not to the observed aromatic products, as is found in nonchlorinated systems. We have therefore also considered nonradical C4/C2 channels by way of Diels-Alder cyclization of C4Cl4/C2Cl2 and C4H2Cl2/C2HCl pairs to describe aromatic formation. While the latter pair readily leads to the formation of partially chlorinated benzenes, the fully chlorinated congeners are sterically prohibited from ring closing directly; this leads to a series of novel rearrangement processes which predict the formation of hexachloro-1,5-diene-3-yne, in addition to hexachlorobenzene, in good agreement with experiment. This suggests, for the first time, that facile nonradical routes to aromatic formation are operative in partially and fully chlorinated pyrolysis and combustion systems.

Role of hydrogen abstraction acetylene addition mechanisms in the formation of chlorinated naphthalenes. 2. Kinetic modeling and the detailed mechanism of ring closure.

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.

Laser pyrolysis studies of the thermal decomposition of chlorinated organic compounds. Part 1—acyl chlorides

The thermal decomposition of chloroacetyl chloride, propanoyl chloride, 3-chloropropanoyl chloride, acryloyl chloride, E-2-butenoyl chloride, methacryloyl chloride, and cyclopropanecarbonyl chloride has been investigated using infrared laser-powered homogeneous pyrolysis, together with product analysis using IR spectroscopy, gas chromatography/mass spectrometry, matrix isolation spectroscopy and tuneable diode laser spectroscopy. Decomposition is usually initiated by 1,2-HCl elimination (where possible), followed by further decomposition of ketene products. Reaction pathways have been validated in many cases by the ab initio calculation of activation energies.

Tunable diode laser study of the ν3 band of oxirane

Abstract We have measured and fitted over 600 well-resolved lines in the ν3 ring breathing band of oxirane. The spectrum is accurately reproduced by previously determined rotational and centrifugal distortion constants for the ground state, together with newly determined rotational, quartic and some sextic distortion constants for the upper state. The magnitudes of the distortion constants reveal some evidence of Coriolis interactions with nearby states. The band centre was determined as 1270.370468(88) cm −1 .

Experimental and theoretical studies into the formation of C4-C6 products in partially chlorinated hydrocarbon pyrolysis systems: a probabilistic approach to congener-specific yield predictions.

This work presents a study of the pyrolytic formation of vinylacetylene and benzene congeners formed from chlorinated hydrocarbon precursors, a complex, multipath polymerization system formed in a monomer-rich environment. (Co-)pyrolyses of dichloro- and trichloroethylene yield a rich array of products, and assuming a single dominant underlying growth mechanism, this (on comparing expected and observed products) allows a number of potentially competing channels to C4 and C6 products to be ruled out. Poor congener/isomer descriptions rule out even-carbon radical routes, and the absence of C3 and C5 products rule out odd-carbon processes. Vinylidenes appear unable to describe the increased reactivity of acetylenes with chlorination noted in our experiments, leaving molecular acetylene dimerization processes and, in C6 systems, the closely related Diels-Alder cyclization as the likely reaction mechanism. The feasibility of these routes is further supported by ab initio calculations. However, some of the most persuasive evidence is provided by congener-specific yield predictions enabled by the construction of a probability tree analogue of kinetic modeling. This approach is relatively quick to construct, provides surprisingly accurate predictions, and may be a very useful tool in screening for important reaction channels in poorly understood congener- or isomer-rich reaction systems.

Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons: formation of benzene rings. 2. Experimental and kinetic modeling studies.

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

Role of hydrogen abstraction acetylene addition mechanisms in the formation of chlorinated naphthalenes. 1. A quantum chemical investigation.

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/6-311+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)/6-31G(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 β-chlorinated 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.

Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons

Laser-induced pyrolysis of small chlorinated hydrocarbons at temperatures where radical processes are likely to be unimportant has provided clear evidence for direct low-energy molecular reactions between acetylenes and chlorinated acetylenes or ethylenes. High-level ab initio calculations of possible pathways have shown that these reactions proceed via two routes. Direct bimolecular reaction accompanied by HCl elimination leads to diacetylenes or vinylacetylenes, respectively, while carbene molecular adducts, followed by Cl or H migrations to yield three- or four-membered rings and ring-opening, lead to vinylacetylenes or butadienes, respectively. Analogues of these processes are likely to be central to the growth of larger molecules in such systems.