Experimental Observation of Hydrocarbon Growth by Resonance-Stabilized Radical-Radical Chain Reaction

Unconventional pathway for the gas-phase formation of 14π-PAHs via self-reaction of the resonantly stabilized radical fulvenallenyl (C7H5˙)

Resonantly stabilized free radicals (RSFRs) are contemplated to be the reactive intermediates in molecular mass-growth processes leading to polycyclic aromatic hydrocarbons (PAHs), which are prevalent in deep space and on Earth. The self-reaction routes of two RSFRs have been recognized as fundamental but more-efficient pathways to form fused benzenoid rings. The present experiment, which exploits a chemical microreactor in combination with an isomer-selective identification technique through fragment-free photoionization utilizing a tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high-resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), provides compelling evidence for the formation of phenanthrene and a minor amount of anthracene in the presence of fulvenallenyl (C7H5˙). Further theoretical calculations of the potential energy surfaces of C14H10 and C14H11 reveal that phenanthrene and anthracene can be efficiently produced via a hydrogen-assisted multi-step mechanism [C7H5˙ + C7H5˙ → i3, i3 = (3,4-di(cyclopenta-2,4-dien-1-ylidene)cyclobut-1-ene); i3 + H → phenanthrene + H/anthracene + H or i3 + H → i8 + H → phenanthrene + H/anthracene + H, i8 = (1-(cyclopenta-2,4-dien-1-ylidene)indene)] at low pressures, rather than through the one-step recombination-isomerization of fulvenallenyl radicals. This study provides a novel growth mechanism for tricyclic PAHs, especially in hydrogen-rich environments such as combustion and interstellar environments, which advances the knowledge of PAH propagation and even the formation mechanisms of carbonaceous nanoparticles in our universe.

Bimetallic Effect of Hybrid Flame Retardant on Flame Retardant Performance in Polypropylene

Polypropylene composites are widely used for their excellent properties, but poor flame retardance is a major concern. The phosphorus content of the material in intumescent flame retardant (IFR) has a direct effect on retarding smoke and dehydrating to carbon during combustion. This study presents a hybrid flame retardant (MPA@ZrP-Zn) synthesized by noncovalent bonding between melamine and phytic acid molecules, containing a large number of phosphorus atoms in the system. Theoretical calculations confirm that the structure formed by zinc ions and melamine phytate (MPA) is more stable, and the overall electrophilicity index is at the level of a moderate Lewis acid. In addition, the Zr-Zn bimetallic effect significantly improves the drip resistance of IFR and reduces smoke emissions during combustion. UL-94 and LOI tests have shown that PP composites with MPA@ZrP-Zn (PPZn30) can achieve a V-0 rating and 33.5% of the LOI value. The THR and PHRR were 15.1% and 54.2% lower, respectively, for PPZn30 (compared to pure PP), and the ID/IG curves further indicate a higher degree of graphitization of the carbon layer in the combustion residue, which should be ascribed to the bimetallic effect. Therefore, this work puts forward a strategy to achieve high-performance PP composites with excellent flame retardant properties, which exhibit huge applications in electrical appliances and energy vehicles.

Identification of Dihydropentalenes as Products of the Molecular-Weight Growth Reaction of Cyclopentadienyl Plus Propargyl

The resonance-stabilized cyclopentadienyl (C5H5) and propargyl (C3H3) radicals are important precursors for polycyclic aromatic hydrocarbons (PAHs) and thus play a significant role in molecular-weight growth and soot formation processes under combustion conditions. In this work, we describe an experimental and theoretical investigation of the C5H5 + C3H3 reaction. Experimentally, we studied this reaction in a resistively heated microtubular SiC reactor at a controlled temperature of ∼1150 K and a pressure of 10-20 mbar. The reactants C5H5 and C3H3 were pyrolytically generated from anisole (C6H5OCH3) and propargyl bromide (C3H3Br). We identified the reactants and the C8H8 products isomer-selectively utilizing photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). The experimentally observed predominant formation of dihydropentalenes over the ring-enlargement reaction to styrene is consistent with our theoretical predictions of the kinetics on the newly calculated C8H8 potential energy surface. This work highlights dihydropentalenes as reactants in molecular-weight growth reactions and as potential building blocks in versatile routes for the formation of curved PAHs.

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.

Direct Observation of Covalently Bound Clusters in Resonantly Stabilized Radical Reactions and Implications for Carbonaceous Particle Growth

Based on quantum mechanically guided experiments that observed elusive intermediates in the domain of inception that lies between large molecules and soot particles, we provide a new mechanism for the formation of carbonaceous particles from gas-phase molecular precursors. We investigated the clustering behavior of resonantly stabilized radicals (RSRs) and their interactions with unsaturated hydrocarbons through a combination of gas-phase reaction experiments and theoretical calculations. Our research directly observed a sequence of covalently bound clusters (CBCs) as key intermediates in the evolution from small RSRs, such as benzyl (C7H7), indenyl (C9H7), 1-methylnaphthyl (1-C11H9), and 2-methylnaphthyl (2-C11H9), to large polycyclic aromatic hydrocarbons (PAHs) consisting of 28 to 55 carbons. We found that hydrogen abstraction and RSR addition drive the formation and growth of CBCs, leading to progressive H-losses, the generation of large PAHs and PAH radicals, and the formation of white smoke (incipient carbonaceous particles). This mechanism of progressive H-losses from CBCs (PHLCBC) elucidates the crucial relationship among RSRs, CBCs, and PAHs, and this study provides an unprecedentedly seamless path of observed assembly from small RSRs to large nanoparticles. Understanding the PHLCBC mechanism over a wide temperature range may enhance the accuracy of multiscale models of soot formation, guide the synthesis of carbonaceous nanomaterials, and deepen our understanding of the origin and evolution of carbon within our galaxy.

Gas-phase preparation of azulene (C10H8) and naphthalene (C10H8) via the reaction of the resonantly stabilized fulvenallenyl and propargyl radicals

Synthetic routes to the 10π Hückel aromatic azulene (C10H8) molecule, the simplest polycyclic aromatic hydrocarbon carrying an adjacent five- and seven-membered ring, have been of fundamental importance due to the role of azulene - a structural isomer of naphthalene - as an essential molecular building block of saddle-shaped carbonaceous nanostructures such as curved nanographenes and nanoribbons. Here, we report on the very first gas phase preparation of azulene by probing the gas-phase reaction between two resonantly stabilized radicals, fulvenallenyl and propargyl , in a molecular beam through isomer-resolved vacuum ultraviolet photoionization mass spectrometry. Augmented by electronic structure calculations, the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism affords a versatile concept for introducing the azulene moiety into polycyclic aromatic systems thus facilitating an understanding of barrierless molecular mass growth processes of saddle-shaped aromatics and eventually carbonaceous nanoparticles (soot, interstellar grains) in our universe.

Elucidating the toluene formation mechanism in the reaction of propargyl radical with 1,3-butadiene

Toluene is one of the simplest mono-substituted benzene derivatives and an important precursor to form polycyclic aromatic hydrocarbons (PAHs) and soot. However, there is a lack of critical understanding of the formation mechanisms of the toluene molecule. In this work, we explore high-temperature reactions of propargyl radical addition to 1,3-butadiene in a tubular flow microreactor. We obtain experimental evidence for the distinct formations of three C₇H₈ isomers consisting of toluene, 1,3,5-cycloheptatriene, and 5-methylene-1,3-cyclohexadiene discriminated by synchrotron VUV photoionization efficiency curves. Toluene is identified as the dominant product, which shows strong contrast with the calculated results of the system. By performing theoretical calculations and kinetic simulations, we found that 5-methylene-1,3-cyclohexadiene is a key product of the primary reaction, and toluene formation is enhanced by unavoidable secondary reactions, such as unimolecular isomerization and/or H-assisted isomerization reactions in the SiC microreactor. The current work provides competitive pathways for the enhanced formation of toluene, and may further help disentangle the toluene-promoted molecular growth mechanism of PAHs in combustion environments.

Unconventional Pathway in the Gas-Phase Synthesis of 9H-Fluorene (C13 H10 ) via the Radical-Radical Reaction of Benzyl (C7 H7 ) with Phenyl (C6 H5 )

The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five-membered ring-9H-fluorene (C₁₃ H₁₀ )-is produced isomer-specifically in the gas phase by reacting benzyl (C₇ H₇ ⋅) with phenyl (C₆ H₅ ⋅) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpected 1-(6-methylenecyclohexa-2,4-dienyl)benzene intermediates (C₁₃ H₁₂ ) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical-radical reactions are initiated through recombination at their radical centers. The structure of 9H-fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings.

A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical-radical reactions

Exploiting a chemical microreactor in combination with an isomer-selective product identification through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), the present investigation reveals molecular mass growth processes to four distinct polycyclic aromatic hydrocarbons carrying two six- and one five-membered ring (C₁₃H₁₀): 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene, 1H-cyclopenta[a]naphthalene, and fluorene in the gas phase. Temperatures of 973 and 1023 K simulating conditions in combustion settings along with circumstellar envelopes of carbon-rich stars and planetary nebulae. These reactions highlight the importance of methyl-substituted aromatic reactants (biphenyl, naphthalene) which can be converted to the methylene (-CH₂˙) motive by hydrogen abstraction or photolysis. Upon reaction with acetylene, methylene-substituted aromatics carrying a hydrogen atom at the ortho position of the ring can be then converted to cyclopentadiene-annulated aromatics thus providing a versatile pathway to five-membered ring aromatics at elevated temperatures.

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.

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.