Novel Products from C6H5 + C6H6/C6H5 Reactions

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.

Ring Growth Mechanism in the Reaction between Fulvenallenyl and Cyclopentadienyl Radicals

Recombination between resonance-stabilized hydrocarbon radicals is an important class of reactions that contribute to molecular growth chemistry in combustion. In the present study, the ring growth mechanism in the reaction between fulvenallenyl (C7H5) and cyclopentadienyl (C5H5) radicals is investigated computationally. The reaction pathways are explored by quantum chemical calculations, and the phenomenological and steady-state rate constants are determined by solving the multiple-well master equations. The primary reaction routes following the recombination between the two radicals are found to be as follows: formation of the adducts, isomerization by hydrogen shift reactions, cyclization to form tricyclic compounds, and their isomerization and dissociation reactions, leading to the formation of acenaphthylene. The overall process can be approximately represented as C7H5 + C5H5 → acenaphthylene + 2H with the bimolecular rate constant of about 4 × 10-12 cm3 molecule-1 s-1. A reaction mechanism consisting of 20 reactions, including the formation, isomerization, and dissociation processes of major intermediate species, is proposed for use in kinetic modeling.

Laser-based sensing in the long-wavelength mid-infrared: chemical kinetics and environmental monitoring applications

We present chemical kinetics and environmental monitoring applications in the long-wavelength mid-infrared (LW-MIR) region using a new diagnostic that exploits a widely tunable light source emitting in the LW-MIR. The custom-designed laser source is based on a difference-frequency generation (DFG) process in a nonlinear orientation-patterned GaAs crystal. The pump laser, an external-cavity quantum cascade laser, is tuned in a continuous-wave (cw) mode, while the signal laser, a C O ₂ gas laser, is operated in a pulsed mode with a kilohertz repetition rate. The idler wavelength can be tuned between 11.58 (863.56c m ^-1) and 15.00 µm (666.67c m ^-1) in a quasi-cw manner. We discuss the unique prospective applications offered by probing the LW-MIR region for chemical kinetics and environment-monitoring applications. We showcase the potential of the DFG laser source by some representative applications.

Simulating X-ray photoelectron spectra with strong electron correlation using multireference algebraic diagrammatic construction theory

A new theoretical approach for the simulations of X-ray photoelectron spectra of strongly correlated molecular systems that combines multireference algebraic diagrammatic construction theory (MR-ADC) with a core–valence separation (CVS) technique.

Formation of polycyclic aromatic hydrocarbons, benzofuran, and dibenzofuran in fuel-rich oxidation of toluene using a flow reactor

Recently, polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) have been attracting considerable attention owing to their high toxicity. Understanding their formation mechanism during combustion processes is important to control their emission. However, there are few studies that have quantitatively investigated OPAH formation in the fuel-rich oxidation of hydrocarbons, despite the availability of several studies on PAH formation. In this study, benzofuran and dibenzofuran as OPAHs were quantified in the fuel-rich oxidation of toluene using a flow reactor at atmospheric pressure in a temperature range of 1050-1350 K at equivalence ratios from 3.0 to 12.0 and residence times from 0.2 to 1.5 s. In addition to benzofuran and dibenzofuran, 4 types of monocyclic aromatic hydrocarbons and 19 types of PAHs were also evaluated. The experimental data obtained in this study were compared with those of the ethylene oxidation performed in our previous study. The existing kinetic model for PAH growth was modified based on several theoretical studies to predict the behavior of OPAHs with furan structures. The modified model showed significant improvements in the prediction of benzofuran and dibenzofuran formation. Based on the rate of production and sensitivity analysis using the modified model, the dominant reaction pathways of benzofuran and dibenzofuran were investigated.

Alteration in formation behaviors of chloroaromatic precursors of PCDD/Fs: An experimental study on the effect of extrinsic and intrinsic oxygen on chlorination

Due to the promotion on Cl radical generation by enhanced oxidation, chlorination of hydrocarbon intermediates becomes a potential formation path for chloroaromatic precursors of PCDD/Fs (polychlorinated dibenzo-p-dioxins and dibenzofurans) in both MSW (municipal solid waste) incineration and gasification-combustion processes, in which intrinsic oxygen might have a significant effect on the competition between oxidation and chlorination. Thus, chlorination of benzene and phenol was experimentally studied on a homogeneous flow reaction system. Effects of temperature and ER (equivalence ratio) were assessed, and comparison was carried out to clarify the alteration in formation behaviors of chloroaromatics by extrinsic and intrinsic oxygen. At 600 °C, chlorobenzenes were already largely formed in benzene chlorination, and the addition of extrinsic oxygen barely affected it. On the contrary, with intrinsic oxygen, phenol tended to decompose to light compounds. With rising temperature, oxidation was promoted and extrinsic oxygen strongly inhibited the formation of chloroaromatics in benzene chlorination at 900 °C and higher temperature. For phenol chlorination, chlorobenzenes were still rarely generated. However, high proportions of octachloronaphthalene and octachlorodibenzofuran were observed, due to the enhancement in polymerization by high temperature. When increasing ER, oxidative decomposition was also promoted in both the chlorination of benzene and phenol. Extra extrinsic oxygen led to a further reduction of chloroaromatics during benzene chlorination, and till ER = 1.0 at 1000 °C, comparable performance to intrinsic oxygen could be achieved in the control of chloroaromatics. Based on these results, formation pathways of the major chloroaromatics from chlorination, oxidation and polymerization were summarized, and the roles of extrinsic and intrinsic oxygen in altering their formation behaviors were revealed.

The thermal transformation mechanism of chlorinated paraffins: An experimental and density functional theory study

The increasing production and usage of chlorinated paraffins (CPs) correspondently increase the amount of CPs that experience thermal processes. Our previous study revealed that a significant amount of medium-chain chlorinated paraffins (MCCPs), short-chain chlorinated paraffins (SCCPs) as well as aromatic and chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) were formed synergistically during the thermal decomposition of CP-52 (a class of CP products). However, the transformation mechanisms of CP-52 to these compounds are still not very clear. This article presents a mechanistic analysis on the decomposition of CP-52 experimentally and theoretically. It was found that CP-52 initially undergoes dehydrochlorination and carbon chain cleavage and it transformed into chlorinated and unsaturated hydrocarbons. Cyclization and aromatization were the most accessible pathways at low temperatures (200-400°C), both of which produce mostly aromatic hydrocarbons. As the temperature exceeds 400°C, the hydrocarbons could decompose into small molecules, and the subsequent radical-induced reactions become the predominant pathways, leading to the formation of Cl-PAHs. The decomposition of CP-52 was investigated by using density functional theory and calculations demonstrating the feasibility and rationality of PCB and PCN formation from chlorobenzene. The results improve the understanding of the transformation processes from CP-52 to SCCPs and Cl-PAHs as well as provide data for reducing their emissions during thermal-related processes.

On the formation of nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) in circumstellar and interstellar environments

The chemical evolution of extraterrestrial environments leads to the formation of nitrogen substituted polycyclic aromatic hydrocarbons (NPAHs) via gas phase radical mediated aromatization reactions.

Benzo- and Naphthopentalenes: Syntheses, Structures, and Properties

Benzo- and naphthopentalene derivatives were synthesized, and the effects of structural variations on their antiaromaticity and optoelectronic and electrochemical properties were examined experimentally and theoretically in detail. The results unveiled that with increasing the bond order of the carbon-carbon bond ([5,6] junction) shared by the pentalene and aromatic moieties, the 8π antiaromatic character of pentalene is enhanced and the HOMO-LUMO gap is decreased, which accompanies both the elevation of the HOMO level and the lowering of the LUMO level. The ethynylene units between the pentalene skeleton and the phenyl groups proved to extend π-conjugation sufficiently.

Unexpected chemistry from the reaction of naphthyl and acetylene at combustion-like temperatures

The hydrogen abstraction/acetylene addition (HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons (PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third six-membered ring. Here, by probing the products formed in the reaction of 1- and 2-naphthyl radicals in excess acetylene under combustion-like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1- and 2-ethynylnaphthalenes (C12 H8 ), acenaphthylene (C12 H8 ) and diethynylnaphthalenes (C14 H8 ). Importantly, neither phenanthrene nor anthracene (C14 H10 ) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.

o-Benzyne fragmentation and isomerization pathways: a CASPT2 study

The mechanism for the thermal fragmentation of o-benzyne to C₄H₂ + C₂H₂ and C₆H₂.

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.

Chemistry of polycyclic aromatic hydrocarbons formation from phenyl radical pyrolysis and reaction of phenyl and acetylene

An experimental investigation of phenyl radical pyrolysis and the phenyl radical + acetylene reaction has been performed to clarify the role of different reaction mechanisms involved in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) serving as precursors for soot formation. Experiments were conducted using GC/GC-MS diagnostics coupled to the high-pressure single-pulse shock tube present at the University of Illinois at Chicago. For the first time, comprehensive speciation of the major stable products, including small hydrocarbons and large PAH intermediates, was obtained over a wide range of pressures (25-60 atm) and temperatures (900-1800 K) which encompass the typical conditions in modern combustion devices. The experimental results were used to validate a comprehensive chemical kinetic model which provides relevant information on the chemistry associated with the formation of PAH compounds. In particular, the modeling results indicate that the o-benzyne chemistry is a key factor in the formation of multi-ring intermediates in phenyl radical pyrolysis. On the other hand, the PAHs from the phenyl + acetylene reaction are formed mainly through recombination between single-ring aromatics and through the hydrogen abstraction/acetylene addition mechanism. Polymerization is the common dominant process at high temperature conditions.

Radical/π-bond addition between o-benzyne and cyclic C5 hydrocarbons

Recent theoretical investigations of the radical/π-bond addition between single-ring aromatic hydrocarbons highlight the importance of this category of reactions for the formation of PAH intermediates and soot. The present investigation extends the theory of the radical/π-bond addition reactions to the o-benzyne + cyclic C(5) hydrocarbons systems. The calculations, performed using the uB3LYP/6-311+G(d,p) method, have addressed the possible role of the reaction between o-benzyne and cyclopentadiene in the formation of indene through the fragmentation of the bicyclo intermediate benzonorbornadiene. The complex potential energy surface for the reaction between o-benzyne and cyclopentadienyl radical was also investigated. In this case, the formation of the bicyclo benzonorbornadienyl radical and its subsequent fragmentation to indenyl radical and acetylene is not the main reaction pathway, although it could be relevant at relatively high temperatures. At lower temperatures, the isomerization reactions, which lead to the formation of a variety of multiring compounds, are dominant.