Resonance-stabilized Radicals Research Articles

Rotational Spectrum of the Phenoxy Radical.

We report the hyperfine-resolved rotational spectrum of the gas-phase phenoxy radical in the 8-25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates that are yet undetected at radio wavelengths are discussed.

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.

Direct Probes of π-Delocalization in Prototypical Resonance-Stabilized Radicals: Hyperfine-Resolved Microwave Spectroscopy of Isotopic Propargyl and Cyanomethyl.

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CH2CCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of 13C- and 2H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron. Our results highlight the importance of considering both spin-polarization and orbital-following contributions when interpreting the isotropic hyperfine coupling constants of π radicals. These physical insights are strengthened by a parallel analysis of the isoelectronic species cyanomethyl, CH2CN, using new 13C measurements also reported in this work. A detailed comparison of the structure and electronic properties of propargyl, cyanomethyl, and other closely related species allows us to correlate trends in their chemical bonding and electronic structure with critical changes in their reactivity and thermochemistry.

Experimental and kinetic modeling study of C5 cyclic hydrocarbons pyrolysis at elevated pressures: Effect of unsaturation on the reactivity and PAH formation

To further explore the decomposition behaviors of C5 cyclic hydrocarbons, important components in the transportation fuels and pivotal intermediates in high-density liquid fuels, the pyrolysis of cyclopentane, cyclopentene and cyclopentadiene with different unsaturation degrees were investigated. The experiments of cyclopentane and cyclopentene were performed at 1.0, 30.0 atm and 773–1173 K in a flow tube reactor. The mole fraction profiles of pyrolysis species, involving C1-C5 small hydrocarbons and monocyclic, bicyclic as well as polycyclic aromatic hydrocarbons (MAHs, BAHs and PAHs), were provided. A universal kinetic model for three C5 cyclic fuels pyrolysis chemistry was proposed and reasonably predicted the speciation profiles newly reported in this work and literature, where rate coefficients of the crucial reactions were compared and upgraded. The differences among three fuels specific pathways were evaluated and the effects of unsaturation on the pyrolysis characteristics of intrinsic reactivity and aromatics formation were revealed. Cyclopentene has the highest pyrolysis reactivity, followed by cyclopentane and cyclopentadiene. Simultaneously formed fuel radicals and key intermediates in three C5 cyclic fuels influence the formation channels of small products, MAHs and BAHs. The saturated cyclopentane is easily occur the ring-opening reaction to generate more C2 and C3 species, whereas the increase in unsaturation promotes MAHs and BAHs formation in cyclopentene and cyclopentadiene pyrolysis, especially the toluene, styrene, indene and naphthalene. The rate of production analysis shows the formations of these MAHs and BAHs are strongly dependent on the concentration levels of cyclopentadienyl resonance stabilized radical (RSR). Differently, the dominated growth pathways of three- and four-ring PAHs are barely influenced by the specific fuel chemistry and indenyl plays a crucial role via the typical RSR chain reactions at high pressures.

Astrochemical Modeling of Propargyl Radical Chemistry in TMC-1

Recent detections of aromatic species in dark molecular clouds suggest that formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC-1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by 2 orders of magnitude compared to its observed value. Predicted abundances for the chemically related C4H3N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in polycyclic aromatic hydrocarbon formation.

Importance of resonance-stabilized radicals in soot formation mechanism of diphenyl ether pyrolysis

Soot formation during fuel pyrolysis can pose environmental and human health hazards. As a typical coal-related model compound, diphenyl ether (DPE) represents the specific structures in coal. In this study, soot formation mechanism of DPE pyrolysis was systematically investigated from theoretical principle. The molecular dynamics (MD) simulations show that the variety and number of DPE pyrolysis products increase with the raising temperature. In addition, the higher temperature also promotes the soot formation. Quantum mechanics (QM) calculations indicate that the energy barrier of CO removal from the phenoxy radical is lower than that of the phenyl isomerization reaction, leading to the easier depletion of the phenoxy radical. It is consistent with the result observed from the MD simulation that the phenoxy is consumed more rapidly than phenyl. The main pathways for initial ring formation are the isomerization of polyyne-like chains and the decomposition of DPE molecules which form the ring molecules of C6H5 and C5H5. C2H2 molecule and resonance-stabilized radical (RSR) species like C3H3, C6H5 and C5H5 are precursors for the growth of polycyclic aromatic hydrocarbons (PAHs), which have an important role in the growth process of soot. Subsequently, larger PAHs aggregate with each other to form incipient soot particles which will undergo surface growth and graphitization processes. We hope this work can provide worthy insights into the mechanism of soot formation in DPE and the suppression of carbon soot.

Investigation of resonance-stabilized radicals associated with soot particle inception using advanced electron paramagnetic resonance techniques

In order to tackle the climate emergency, it is imperative to advance cleaner technologies to reduce pollutant emission as soot particles. However, there is still a lack of complete understanding of the mechanisms responsible for their formation. In this work, we performed an investigation devoted to the study of persistent radicals potentially involved in the formation of soot particles, by continuous wave and pulsed electron paramagnetic resonance. This work provides experimental evidence of the presence in nascent soot of highly branched, resonance-stabilized aromatic radicals bearing aliphatic groups, linked together by short carbon chains, and reinforced by non-covalent π-π interactions. These radicals appear to be highly specific of nascent soot and quickly disappear with the increasing soot maturity. Their presence in nascent soot could represent an underestimated health risk factor in addition to the already well documented effect of the high specific surface and the presence of harmful adsorbates.

C7 reaction mechanism and its self-imitation in the kinetic modeling of PAH formation

Polycyclic aromatic hydrocarbons (PAHs), serving as critical soot precursors, are formed via complicated chemical processes, such as the first aromatic ring formation and the aromatic ring growth, from small molecules. Benzene/phenyl (“C6”) is commonly considered as the critical first aromatic ring formed, which has attracted many studies on its formation and its further mass growth. Reactions of cyclopentadienyl (“C5”) are also recognized as PAH formation pathways without involving benzene. Compared to “C5” and “C6”, less attention has been paid to “C7” aromatics, such as vinyl-cyclopentadienyl, tropyl and fulvenallenyl, for their roles in PAH formation. Previous PAH models included very few reactions of “C7” sub-mechanism, and somewhat ignored the self-consistency of the reaction networks of aromatic species with characteristic molecular structures. In this study, we upgraded “C7” sub-mechanism and revealed the self-imitation between the reaction network of monocyclic and polycyclic aromatic hydrocarbons. Model validation with literature experimental data shows that: a) benzyl, vinyl-cyclopentadienyl, tropyl, and fulvenallenyl are crucial intermediates in “C7” chemistry; b) “C7” resonance stabilized radicals (RSRs) are mainly contributed by different entrance channels from small aromatic precursors; c) the reactions of “C7” species provide unique contribution to PAH formation according to their specific molecular moieties. This study also reveals the basic aromatic classes, “C5”, “C6”, “C7”, and “fC7”, for the hierarchical reaction network, and the self-imitation between the hierarchical reaction networks of monocyclic and polycyclic aromatic hydrocarbons. Future work will build a kinetic model up to very large PAHs, close to soot nanoparticle, using the self-imitation of the hierarchical reaction network proposed in this study.

A Metastable Ketenyl Radical-Water Complex from UV Photolysis of the Carboxymethyl Radical.

The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical-molecule (and especially radical-water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C-H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C-H···O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C-O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products.

Isolated 2-phenylallyl radicals (2-PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2-PA is a resonance-stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas-phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self-reaction as well as reactions with unimolecular decomposition products of 2-PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2-PA is compared with the one of the related 2-methylallyl and phenylpropargyl radicals.

Pyrolysis and oxidation of benzene and cyclopentadiene by NOx: a ReaxFF molecular dynamics study.

Benzene (C6H6) and 1,3-cyclopentadiene (c-C5H6) are critical intermediate species in the combustion of fossil fuel and the formation of polycyclic aromatic hydrocarbons (PAHs). This study investigates the underlying mechanisms of pyrolysis and oxidation of C6H6 and c-C5H6 in the presence of O2, NO and NO2, respectively, under combustion conditions via ReaxFF molecular dynamics simulations. The size growth in the pyrolysis system is accompanied by an amorphous nature as well as an increase in the C/H ratio. In the oxidation sytems, NO2 is the most effective in the oxidation of both C6H6 and c-C5H6, followed by NO and O2. In the presence of NOx, O and N radicals generated in the high-temperature decomposition reactions of NO and NO2 are actively involved in the addition and H-abstraction reactions of C6H6 and c-C5H6. Remarkably, the decomposition of NO2 dramatically increases the number of O radicals in the system, which significantly accelerates the ring-opening of C6H6 and c-C5H6 by O-addition and forms linear-C6H6O and C5H6O species, respectively. Afterwards, the formation of -CH2- by H-transfer plays an essential role in the decomposition of linear-C6H6O and -C5H6O. Reaction pathways of O and N radicals with C6H6 and c-C5H6 are reported in detail. The O and N-addition of C6H6 facilitate the decomposition to resonance-stabilized cyclopentadienyl radicals after the restructuring of the C-C bond.

Polycyclic aromatic hydrocarbons production from the supercritical pyrolysis of n-decane, ethylcyclohexane, and n-decane/ethylcyclohexane blends

To investigate the effects of fuel composition on the production and growth of polycyclic aromatic hydrocarbons (PAH) at conditions relevant to the pre-combustion environment of fuels in future high-speed aircraft, we have conducted supercritical pyrolysis experiments in an isothermal, silica-lined stainless-steel flow reactor at 568 °C, 94.6 atm, and 133 s, with the model fuels n-decane and ethylcyclohexane—as well as n-decane/ethylcyclohexane blends in which the fraction of fuel carbon coming from ethylcyclohexane is 0.25, 0.50, and 0.75. High-pressure liquid-chromatographic analyses of the reaction products have led to the isomer-specific identification and quantification of 169 three- to nine-ring PAH—159 of which have never before been reported as products of a cyclic-alkane fuel. Quantification of the aliphatic products by gas chromatographic techniques reveals that n-decane produces mostly 1-alkenes and n-alkanes, whose radicals generate more radicals; whereas ethylcyclohexane produces mostly C5-ring and C6-ring alkanes and alkenes, whose radicals show a propensity for ring dehydrogenation. These contrasts in the aliphatic-product distributions have major impacts on PAH growth, which, in this reaction environment, occurs chiefly through the reactions of resonance-stabilized arylmethyl radicals with the C2-C4 1-alkenes. For fuel compositions rich in n-decane, all the ingredients necessary for these PAH-growth reactions to thrive are in place: methyl-substituted PAH, as sources of arylmethyl radicals; a radical-rich environment that fosters H abstraction and arylmethyl-radical formation; and C2-C4 1-alkenes, as growth species. However, an increase in the level of ethylcyclohexane in the fuel brings about substantial reductions in the levels of H-abstracting radicals and C2-C4 1-alkenes as well as an increase in the supply of “donated” hydrogen for stabilizing radicals. These factors combine to bring about marked reductions in PAH growth at fuel compositions rich in ethylcyclohexane, resulting in much lower production of the high-ring-number PAH that are the precursors to solids in the supercritical fuel-pyrolysis environment.

Exploring low-temperature oxidation chemistry of 2- and 3-pentanone

The low-temperature oxidation chemistry of 2- and 3-pentanone was investigated over a wide range of conditions using a jet-stirred reactor (JSR) and a rapid compression machine (RCM). The JSR oxidation experiment was performed at the pressure of 93.3 kPa over a temperature range of 600-1000 K. Detailed speciation information was obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. Ignition delay times (IDTs) of 2- and 3-pentanone were measured in an RCM from 640 to 820 K at pressures of 15 and 25 bar and an equivalence ratio of 1.0. The two C5 ketones showed NTC behavior and two-stage ignition phenomena. Mole fraction time histories of intermediate species during the two-stage ignition process of both ketones were obtained using a fast-sampling system coupled with gas chromatography. There are distinct differences between 2- and 3-pentanone in species concentration profiles and IDTs. A kinetic mechanism for the low-temperature oxidation of 2- and 3-pentanone was developed, which can satisfactorily predict all available measurements. The reaction path analyses indicate that the intramolecular hydrogen migration reaction of ROO radicals tends to produce resonance-stabilized QOOH radical structures. The secondary oxygen addition reaction of resonance-stabilized QOOH radicals thus is the most important source of OH radicals in the low-temperature oxidation of ketone fuels. The intramolecular hydrogen migration reactions are slowed down in the presence of the carbonyl functional group, which makes the low-temperature reactivity of the two C5 ketones lower than that of n-pentane. The position of the carbonyl functional group affects the species pools during the oxidation of the two ketones to a great extent. Larger production of CH4, C3H6, CH3COCH3, and C2H5CHO were observed in 2-pentanone oxidation, while the production of CH3CHO was favored during 3-pentanone oxidation both in the JSR and RCM experiments. The different lengths of the carbon chain on both sides of the carbonyl group in 2- and 3-pentanone resulted in the difference in the species distribution.

Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene.

The reaction of the OH radical with cyclopentadiene (C5H6) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H2O2 or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C5H6OH) is the main observed reaction product. Abstraction products (C5H5) were not detected. The C5H6OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.

Investigating the kinetic effect of prenol on iso-octane auto-ignition by means of an experimental and modeling study

• An experimental study of iso -octane/prenol blends is performed inside a RCM. • Validated kinetic models of the oxidation of the neat fuels are merged. • IDTs reflect the strong inhibiting behavior of prenol on low-temperature reactivity. • This effect is suggested to originate from combined gas-phase and catalytic effects. A detailed experimental and kinetic modeling study was dedicated to understand the reported octane hyperboosting effect of prenol, by means of the measurement of the ignition delay times of its blends with iso -octane, and measurement of the mole fraction profiles of the fuels and intermediates inside the ULille rapid compression machine. These results show that prenol addition leads to a reduction of the first-stage ignition phenomena and negative temperature coefficient behavior, which is only qualitatively captured by the model and is consistent with knock resistance improvement. It is suggested that this behavior is caused by two different factors. The first originates from gas-phase reactivity of prenol, and spans from the formation of unreactive unsaturated species through resonance-stabilized radicals, thereby constituting a competitive pathway for the radical pool generated by iso -octane. The second is of catalytic nature and cannot be captured by means of gas-phase kinetic modeling, but could also play an important role in the behavior of prenol in internal combustion engines.

Gas-Phase Optical Spectra of the Indenyl Radical and Its Quantitative Detection in a Jet-Stirred Reactor

Resonance-stabilized radicals (RSRs), such as the indenyl radical (C9H7), are proposed to be initiator radicals in soot inception and growth in hydrocarbon combustion processes, but spectroscopic data for many RSRs are still lacking. In this work, the gas-phase optical absorption spectra of the B̃2A2-X̃2A2 electronic transition of indenyl were identified in a supersonic indene/argon plasma jet. Spectroscopic parameters, including the transition energy, rotational constants, and upper-state lifetime broadening, were obtained from analysis of the experimental spectra. The results were readily applied to the quantitative detection of indenyl produced from high-temperature reactions in a jet-stirred reactor. This study now makes indenyl optically accessible in further reaction kinetics studies and in situ spectroscopic diagnostics of hydrocarbon combustion processes.

Gas-Phase Infrared Spectra of the C7H5 Radical and Its Bimolecular Reaction Products.

Resonance-stabilized radicals are considered as possible intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) in interstellar space. Here, we investigate the fulvenallenyl radical, the most stable C7H5 isomer by IR/UV ion dip spectroscopy employing free electron laser radiation in the mid-infrared region between 550 and 1750 cm-1. The radical is generated by pyrolysis from phthalide. Various jet-cooled reaction products are identified by their mass-selective IR spectra in the fingerprint region, based on a comparison with computed spectra. Interestingly, benzyl is present as a second resonance-stabilized radical. It is connected to fulvenallenyl by a sequence of two H atom losses or additions. Among the identified aromatic hydrocarbons are toluene and styrene, as well as polycyclic molecules, such as indene, naphthalene, fluorene and phenanthrene. Mechanisms for the formation of PAH from C7H5 have already been suggested in previous computational work. In particular, the radical/radical reaction of two fulvenallenyl radicals provides an efficient route to phenanthrene in one bimolecular step and might be relevant for PAH formation under astrochemical conditions.

Assessment of anti-inflammatory and antioxidant activity of quercetin-rutin blend (SophorOx™)- an invitro cell based assay.

Quercetin & Rutin, are bioactive compounds that are widely used for various therapeutic properties. There's been growing interest in the biological activities of these polyphenols belonging to the class of flavonoids known to have various health benefits. Quercetin is now popularly recognized as a phytochemical remedy for a plethora of disease groups such as metabolic syndrome (more specifically diabetes), obesity/weight management and mood disorders. Due to its unique chemical structure, the most prominent property of Quercetin is probably itsantioxidant capability. It acts as a free radical scavenger to form resonance-stabilized phenoxyl radicals. Certain in vitro studies have also shown quercetin to have anti-viral, anti-carcinogenic and platelet aggregation properties. Rutin has also been shown to exert diverse biological effects such as anti-tumor activities, reduction of inflammatory cytokines and antimicrobial activities. The current study was designed to further confirm the antioxidant and anti-inflammatory property of a Quercetin-Rutin blend (SophorOx™). The analysis was performed in a cell-based assay using RAW 264.7 macrophage cell line. SophorOx™ was screened for cytotoxicity using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to obtain optimum concentrations for experimental assays. SophorOx™ was measured for pro-inflammatory cytokine levels (TNF-α & IL-6) and nitric oxide (NO) levels. Additionally, ROS (reactive oxygen species) levels in RAW cells were estimated using a cell-permeant reagent 2'7'-dichlorofluorescein diacetate (DCFH-DA). SophorOx™ at 10µM concentration, exhibited an anti-inflammatory property with significant inhibitory levels of TNF-α (∼28.25%) and IL-6 (∼32.25%). SophorOx™ at similar concentrations reduced nitric oxide levels to 70.55% in lipopolysaccharide (LPS) stimulated RAW 264.7 cells. Raw 264.7 cells stimulated with LPS exhibited a significant increase in intracellular ROS and this was significantly reduced (78.2% reduction) at lower concentrations (0.3 µM) of SophorOx™. The anti-inflammatory effects of SophorOx™ were investigated in LPS stimulated RAW 264.7 macrophages. Data suggests, that SophorOx™ reduced levels of nitric oxide, intracellular ROS and pro-inflammatory cytokines (TNF-α & IL-6) at low concentrations without affecting the viability of RAW cells. Present invitro trial suggests that SophorOx™ is a potent antioxidant and anti-inflammatory agent and displays a prominent ability to block mediators of oxidative stress and inflammation.

Reaction kinetics of phenyl + phenylacetylene at combustion-relevant intermediate temperatures

The reaction between phenyl radical and phenylacetylene is a prototype for the reactions of aryl radicals and alkynyl peri‑condensed aromatic hydrocarbons (PCAHs), which may help explain the dimerization of PCAHs by covalent bond with acetylene assistance. In this work, we have experimentally and theoretically investigated reaction kinetics of phenyl radical and phenylacetylene. Gas chromatography-mass spectrometry (GC–MS) is applied to separate and identify molecular structures of reaction products in a jet-stirred reactor (JSR). Diphenylacetylene is observed as the major product of this reaction, which shows dominant concentration near 1000 K. Formation of phenanthrene increases with temperature, while 9-methylene-fluorene is a minor product, and 2-ethynyl-biphenyl is negligible. The potential energy surface and rate coefficients of phenyl and phenylacetylene reaction are calculated by quantum chemistry and transition state theory. 1,2-Diphenylvinyl is the only resonance-stabilized radical (RSR) among the adducts formed via all six possible addition reaction channels. Its formation reaction channel has a much lower energy barrier and deeper potential well and, therefore, it wins the competition in the reaction of phenyl and phenylacetylene at combustion-relevant intermediate temperatures.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products.

The resonance-stabilized 2-methylallyl radical, 2-MA, is considered as a possible intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs) in combustion processes. In this work, we report on its contribution to molecular growth in a high-temperature microreactor and provide mass-selective IR/UV ion dip spectra of the radical, as well as the various jet-cooled reaction products, employing free electron laser radiation in the mid-infrared region. Small (aromatic) hydrocarbons such as fulvene, benzene, styrene, or para-xylene, as well as polycyclic molecules, like (methylated) naphthalene, were identified with the aid of ab initio DFT computations. Several reaction products differ by one or more methyl groups, suggesting that molecular growth is dominated by (de)methylation in the reactor.