The Products of the Reactions of o-Benzyne with Ethene, Propene, and Acetylene: A Combined Mass Spectrometric and Quantum Chemical Study

Experimental observation of molecular-weight growth by the reactions of o-benzyne with benzyl radicals

The chemistry of ortho-benzyne (o-C6H4) is of fundamental importance due to its role as an essential molecular building block in molecular-weight growth reactions. Here, we report on an experimental investigation of the reaction of o-C6H4 with benzyl (C7H7) radicals in a well-controlled flash pyrolysis experiment using a resistively heated SiC microtubular reactor at temperatures of 800-1600 K and pressures near 30  torr. To this end, the reactants o-C6H4 and C7H7 were pyrolytically generated from 1,2-diiodobenzene and benzyl bromide, respectively. Using molecular-beam time-of-flight mass spectrometry, we found that o-C6H4 associates with the benzyl to form C13H11 radicals, which decompose at higher temperatures via H-loss to form closed-shell C13H10 molecules. Our experimental results agree with earlier theoretical calculations by Matsugi and Miyoshi [Phys. Chem. Chem. Phys., 2012, 14, 9722-9728], who predicted the formation of fluorene (C13H10) + H to be the dominant reaction channel. At temperatures above 1400 K, we also observed the formation of C13H9 radicals, most likely the resonance-stabilized fluorenyl π-radical. Our study confirms that molecular-mass growth via the o-C6H4 + C7H7 reaction provides a versatile pathway for introducing five-membered rings, and hence curved structures, into polycyclic aromatic hydrocarbons.

Mechanism and the Origins of Periselectivity in Cycloaddition Reactions of Benzyne with Dienes

Density functional theory calculations have been used to explore the reaction mechanism of (4 + 2) and (2 + 2) cycloadditions of benzyne with classical dienes. The results indicate the following: (1) (4 + 2) products arise via concerted pathways, (2) (2 + 2) products arise via stepwise pathways with diradical intermediates, and (3) these diradical intermediates are formed via isomerization of carbene intermediates. The origins of periselectivity in these reactions are analyzed using distortion/interaction analysis for the key steps, and they indicate that the tiny distortion in the very early [4 + 2] transition structure, coupled with an entropic favorability, controls selective (4 + 2) cycloaddition.

Formation of phenylacetylene and benzocyclobutadiene in the ortho-benzyne + acetylene reaction

Ortho-benzyne is a potentially important precursor for polycyclic aromatic hydrocarbon formation, but much is still unknown about its chemistry. In this work, we report on a combined experimental and theoretical study of the o-benzyne + acetylene reaction and employ double imaging threshold photoelectron photoion coincidence spectroscopy to investigate the reaction products with isomer specificity. Based on photoion mass-selected threshold photoelectron spectra, Franck-Condon simulations, and ionization cross section calculations, we conclude that phenylacetylene and benzocyclobutadiene (PA : BCBdiene) are formed at a non-equilibrium ratio of 2 : 1, respectively, in a pyrolysis microreactor at a temperature of 1050 K and a pressure of ∼20 mbar. The C₈H₆ potential energy surface (PES) is explored to rationalize the formation of the reaction products. Previously unidentified pathways have been found by considering the open-shell singlet (OSS) character of various C₈H₆ reactive intermediates. Based on the PES data, a kinetic model is constructed to estimate equilibrium abundances of the two products. New insights into the reaction mechanism - with a focus on the OSS intermediates - and the products formed in the o-benzyne + acetylene reaction provide a greater level of understanding of the o-benzyne reactivity during the formation of aromatic hydrocarbons in combustion environments as well as in outflows of carbon-rich stars.

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C₄ H₄ to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals. The o-C₆ H₄ +C₄ H₄ reaction is highly exothermic, by ∼143 kcal/mol to form naphthalene and by 31-32 kcal mol^-1 to produce naphthyl radicals plus H, but features relatively high entrance barriers of 9-11 kcal mol^-1 . Although the reaction is rather slow, much slower than the reaction of phenyl radical with vinylacetylene, it forms naphthalene and 1- and 2-naphthyl radicals directly, with their relative yields controlled by the temperature and pressure, and thus represents a viable source of the naphthalene core under conditions where ortho-benzyne and vinylacetylene are available.

Initiation reactions in the high temperature decomposition of styrene

The thermal decomposition of styrene was investigated in a combined experimental, theory and modeling study with particular emphasis placed on the initial dissociation reactions. Two sets of shock tube/time-of-flight mass spectrometry (TOF-MS) experiments were performed to identify reaction products and their order of appearance. One set of experiments was conducted with a miniature high repetition rate shock tube at the Advanced Light Source at Lawrence Berkeley National Laboratory using synchrotron vacuum ultraviolet photoionization. The other set of experiments was performed in a diaphragmless shock tube (DFST) using electron impact ionization. The datasets span 1660–2260 K and 0.5–12 atm. The results show a marked transition from aromatic products at low temperatures to polyacetylenes, up to C₈H₂, at high temperatures. The TOF-MS experiments were complemented by DFST/LS (laser schlieren densitometry) experiments covering 1800–2250 K and 60–240 Torr. These were particularly sensitive to the initial dissociation reactions. These reactions were investigated theoretically and revealed the dissociation of styrene to be a complex multichannel process with strong pressure and temperature dependencies that were evaluated with multi-well master equation simulations. Simulations of the LS data with a mechanism developed in this work are in excellent agreement with the experimental data. From these simulations, rate coefficients for the dissociation of styrene were obtained that are in good agreement with the theoretical predictions. The simulation results also provide fair predictions of the temperature and pressure dependencies of the products observed in the TOF-MS studies. Prior experimental studies of styrene pyrolysis concluded that the main products were benzene and acetylene. In contrast, this study finds that the majority of styrene dissociates to create five styryl radical isomers. Of these, α-styryl accounts for about 50% with the other isomers consuming approximately 20%. It was also found that C–C bond scission to phenyl and vinyl radicals consumes up to 25% of styrene. Finally the dissociation of styrene to benzene and vinylidene accounts for roughly 5% of styrene consumption. Comments are made on the apparent differences between the results of this work and prior literature.

A combined theoretical and experimental study showing styrene primarily decomposes to styryl radicals + H.

Gas-phase synthesis of benzene via the propargyl radical self-reaction

Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (C₃H₃) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and o-benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.

Self-Reaction of ortho-Benzyne at High Temperatures Investigated by Infrared and Photoelectron Spectroscopy

ortho-Benzyne, a Kekulé-type biradical is considered to be a key intermediate in the formation of polycyclic aromatic hydrocarbons (PAH) and soot. In the present work we study the ortho-benzyne self-reactions in a hot microreactor and identify the high-temperature products by IR/UV spectroscopy and by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) in a free jet. Ms-TPES confirms formation of ortho-benzyne as generated from benzocyclobutenedione, as well as benzene, biphenylene, diacetylene, and acetylene, originating from the reaction o-C₆H₄ → HCC-CCH + C₂H₂, and CH₃. PAH molecules like naphthalene, 2-ethynylnaphthalene, fluorene, phenanthrene, and triphenylene are identified based on their IR/UV spectra. By comparison with recent computations their formation starting from o-benzyne can be readily understood and supports the importance of the biradical addition (1,4-cycloaddition followed by fragmentation) pathway to PAH molecules, recently proposed by Comandini et al.

The ortho-benzyne cation is not planar

A recent review on the photoionisation of the C₆H₄ isomer ortho-benzyne suggests that bands reported in earlier photoelectron spectra might be due to side products or contaminations, while computations raise doubts, whether the cation has a planar geometry. We therefore reinvestigate the photoionisation of ortho-benzyne, generated by pyrolysis from benzocyclobutenedione, by photoion mass-selected threshold photoelectron (ms-TPE) spectroscopy using synchrotron radiation. The experiments are accompanied by a theoretical study that investigates the structure of the ortho-benzyne cation systematically as a function of the computational method, up to CASPT2(11,14) ab initio computations. Our study leads to a re-evaluation of the ionisation energy of ortho-benzyne. It reveals that the ortho-benzyne cation has indeed a twisted C₂ geometry rather than a C_2v structure. A vertical ionisation energy IE_vert of 9.77 eV and an adiabatic ionisation energy of IE_ad = 9.56 eV are computed for ortho-benzyne. A Franck-Condon simulation of the photoelectron spectrum based on the CASPT2 results and including three electronic states of the cation is in agreement with the experiment and yields IE_ad = 9.51 eV (+50 meV/-100 meV). Since this value is in contrast with previous work, the ionisation energy has to be revised based on our study. Computational methods based on density functional theory give a reasonable description of the cationic ground state, but fail for the corresponding excited electronic states that are indispensible for a proper assignment of the photoelectron spectrum.

Is it possible to achieve a complete desaturation of cycloalkanes promoted by o-benzyne?

The competence between the possible reaction pathways in the reaction of different cycloalkanes and o-benzyne has been computationally explored. Systems able to undergo complete desaturation by consecutive double hydrogen atom transfer reactions are suggested.

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.

Bimolecular rate constant and product branching ratio measurements for the reaction of C2H with ethene and propene at 79 K

The reactions of the ethynyl radical (C(2)H) with ethene (C(2)H(4)) and propene (C(3)H(6)) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C(2)H(2)) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C(2)H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C(2)H and propene results in (85 ± 10)% C(4)H(4) (m/z = 52) via CH(3)-loss and (15 ± 10)% C(5)H(6) (m/z = 66) by H-loss. The C(4)H(4) channel is found to consist of 100% vinylacetylene. For the C(5)H(6) channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis- and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne.