The Recombination of Propargyl Radicals and Other Reactions on a C6H6 Potential

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.

Absolute photoionization cross-section of the propargyl radical

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Product branching ratios in photodissociation of phenyl radical: a theoretical ab initio/Rice-Ramsperger-Kassel-Marcus study

Ab initio CCSD(T)/CBS//B3LYP/6-311G** calculations of the potential energy surface for possible dissociation channels of the phenyl radical are combined with microcanonical Rice-Ramsperger-Kassel-Marcus calculations of reaction rate constants in order to predict statistical product branching ratios in photodissociation of c-C(6)H(5) at various wavelengths. The results indicate that at 248 nm the photodissociation process is dominated by the production of ortho-benzyne via direct elimination of a hydrogen atom from the phenyl radical. At 193 nm, the statistical branching ratios are computed to be 63.4%, 21.1%, and 14.4% for the o-C(6)H(4) + H, l-C(6)H(4) ((Z)-hexa-3-ene-1,5-diyne) + H, and n-C(4)H(3) + C(2)H(2) products, respectively, in a contradiction with recent experimental measurements, which showed C(4)H(3) + C(2)H(2) as the major product. Although two lower energy pathways to the i-C(4)H(3) + C(2)H(2) products are identified, they appeared to be kinetically unfavorable and the computed statistical branching ratio of i-C(4)H(3) + C(2)H(2) does not exceed 1%. To explain the disagreement with experiment, we optimized conical intersections between the ground and the first excited electronic states of C(6)H(5) and, based on their structures and energies, suggested the following photodissociation mechanism at 193 nm: c-C(6)H(5) 1 → absorption of a photon → electronically excited 1 → internal conversion to the lowest excited state → conversion to the ground electronic state via conical intersections at CI-2 or CI-3 → non-statistical decay of the vibrationally excited radical favoring the formation of the n-C(4)H(3) + C(2)H(2) products. This scenario can be attained if the intramolecular vibrational redistribution in the CI-2 or CI-3 structures in the ground electronic state is slower than their dissociation to n-C(4)H(3) + C(2)H(2) driven by the dynamical preference.

Reactions of o-benzyne with propargyl and benzyl radicals: potential sources of polycyclic aromatic hydrocarbons in combustion

The kinetics and mechanisms of the reactions of o-benzyne with propargyl and benzyl radicals have been investigated computationally. The possible reaction pathways have been explored by quantum chemical calculations at the M06-2X/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level and the mechanisms have been investigated by the Rice-Ramsperger-Kassel-Marcus theory/master-equation calculations. It was found that the o-benzyne associates with the propargyl and benzyl radicals without pronounced barriers and the activated adducts easily isomerize to five-membered ring species. Indenyl radical and fluorene + H were predicted to be dominantly produced by the reactions of o-benzyne with propargyl and benzyl radicals, respectively, with the rate constants close to the high-pressure limits at temperatures below 2000 K. The related reactions on the two potential energy surfaces, namely, the reaction between fulvenallenyl radical and acetylene and the decomposition reactions of indenyl and α-phenylbenzyl radicals were also investigated. The high reactivity of o-benzyne toward the resonance stabilized radicals suggested a potential role of o-benzyne as a precursor of polycyclic aromatic hydrocarbons in combustion.

Theoretical study of the formation of naphthalene from the radical/π-bond addition between single-ring aromatic hydrocarbons

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.

On the formation of resonantly stabilized C5H3 radicals--a crossed beam and ab initio study of the reaction of ground state carbon atoms with vinylacetylene

The formation of polycyclic aromatic hydrocarbons in combustion environments is linked to resonance stabilized free radicals. Here, we investigated the reaction dynamics of ground state carbon atoms, C((3)P(j)), with vinylacetylene at two collision energies of 18.8 kJ mol(-1) and 26.4 kJ mol(-1) employing the crossed molecular beam technique leading to two resonantly stabilized free radicals. The reaction was found to be governed by indirect scattering dynamics and to proceed without an entrance barrier through a long-lived collision complex to reach the products, n- and i-C(5)H(3) isomers via tight exit transition states. The reaction pathway taken is dependent on whether the carbon atom attacks the π electron density of the double or triple bond, both routes have been compared to the reactions of atomic carbon with ethylene and acetylene. Electronic structure/statistical theory calculations determined the product branching ratio to be 2:3 between the n- and i-C(5)H(3) isomers.

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C(2)H + H(2)CCHCHCH(2) → C(6)H(6) + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

Experimental and theoretical investigation of the self-reaction of phenyl radicals

A combination of experiment and theory is applied to the self-reaction kinetics of phenyl radicals. The dissociation of phenyl iodide is observed with both time-of-flight mass spectrometry, TOF-MS, and laser schlieren, LS, diagnostics coupled to a diaphragmless shock tube for temperatures ranging from 1276 to 1853 K. The LS experiments were performed at pressures of 22 +/- 2, 54 +/- 7, and 122 +/- 6 Torr, and the TOF-MS experiments were performed at pressures in the range 500-700 Torr. These observations are sensitive to both the dissociation of phenyl iodide and to the subsequent self-reaction of the phenyl radicals. The experimental observations indicate that both these reactions are more complicated than previously assumed. The phenyl iodide dissociation yields approximately 6% C(6)H(4) + HI in addition to the major and commonly assumed C(6)H(5) + I channel. The self-reaction of phenyl radicals does not proceed solely by recombination, but also through disproportionation to benzene + o-/m-/p-benzynes, with comparable rate coefficients for both. The various channels in the self-reaction of phenyl radicals are studied with ab initio transition state theory based master equation calculations. These calculations elucidate the complex nature of the C(6)H(5) self-reaction and are consistent with the experimental observations. The theoretical predictions are used as a guide in the development of a model for the phenyl iodide pyrolysis that accurately reproduces the observed laser schlieren profiles over the full range of the observations.