HACA's Heritage: A Free‐Radical Pathway to Phenanthrene in Circumstellar Envelopes of Asymptotic Giant Branch Stars

The hydrogen‐abstraction/acetylene‐addition (HACA) mechanism has been central for the last decades in attempting to rationalize the formation of polycyclic aromatic hydrocarbons (PAHs) as detected in carbonaceous meteorites such as in Murchison. Nevertheless, the basic reaction mechanisms leading to the formation of even the simplest tricyclic PAHs like anthracene and phenanthrene are still elusive. Here, by exploring the previously unknown chemistry of the ortho ‐biphenylyl radical with acetylene, we deliver compelling evidence on the efficient synthesis of phenanthrene in carbon‐rich circumstellar environments. However, the lack of formation of the anthracene isomer implies that HACA alone cannot be responsible for the formation of PAHs in extreme environments. Considering the overall picture, alternative pathways such as vinylacetylene‐mediated reactions are required to play a crucial role in the synthesis of complex PAHs in circumstellar envelopes of dying carbon‐rich stars.     

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.     

适用于汽油参比燃料TRF的多环芳香烃生成机理

构造了一个包括287种组分和1569个反应的汽油参比燃料TRF(toluene reference fuel)燃烧过程中多环芳香烃(PAHs)生成机理的详细化学反应动力学模型,引入四种PAH生长路径将多环芳香烃的生成机理发展到芘A4(C20H12)水平,并通过对PAH产率的分析,指出乙炔(C2H2)、丙炔(C3H3)、乙烯基乙炔(C4H4)以及含有奇数碳原子的环戊二烯自由基(C5H5)和茚基(C9H7)等物质对PAHs生成和生长起到重要作用.该机理可以较准确计算基础燃料(PRF)和TRF火焰的着火延迟期、燃烧火焰中小分子(PAH前驱体C2H2、C3H4等)和PAHs的物质浓度.通过与实验数据的比较表明,该机理在不同温度、压力、化学计量比下具有较好的性能.由此分析,该机理对碳烟前驱物PAHs的预测性能是可靠的.     

Formation of complex organics from acetylene catalyzed by ionized benzene

We present direct evidence for low temperature associative charge transfer (ACT) reactions of acetylene onto the benzene cation that catalyze the conversion of acetylene molecules into polymerized cations and for high temperature addition/elimination reactions that lead to the generation of naphthalene-type ions. At low temperatures acetylene molecules bind noncovalently to the benzene cation, where partial charge transfer from the ion activates an acetylene molecule for addition polymerization with other associated acetylene molecules, thus amounting to catalytic cyclization/polymerization of the acetylene molecules. At high temperatures the barrier of the covalent addition of acetylene to the benzene cation to form a styrene-type ion is measured as 3.5 kcal/mol. The second acetylene addition followed by H elimination to form a naphthalene-type ion is calculated to be highly exothermic and without a barrier. These reactions can explain the formation of complex organics by gas phase ion-molecule reactions under a wide range of temperatures and pressures in astrochemical environments.     

Synthesis of Polycyclic Aromatic Hydrocarbons by Phenyl Addition–Dehydrocyclization: The Third Way

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.     

In situ direct sampling mass spectrometric study on formation of polycyclic aromatic hydrocarbons in toluene pyrolysis

The gas-phase reaction products of toluene pyrolysis with and without acetylene addition produced in a flow tube reactor at pressures of 8.15-15.11 Torr and temperatures of 1136-1507 K with constant residence time (0.56 s) have been detected in an in situ direct sampling mass spectrometric study by using a vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry technique. Those products range from methyl radical to large polycyclic aromatic hydrocarbons (PAHs) of mass 522 amu (C(42)H(18)) including smaller species, radicals, polyynes, and PAHs, together with ethynyl, methyl, and phenyl PAHs. On the basis of observed mass spectra, the chemical kinetic mechanisms of the formation of products are discussed. Especially, acetylene is mixed with toluene to understand the effect of the hydrogen abstraction and acetylene addition (HACA) mechanism on the formation pathways of products in toluene pyrolysis. The most prominent outputs of this work are the direct detection of large PAHs and new reaction pathways for the formation of PAHs with the major role of cyclopenta-fused radicals. The basis of this new reaction route is the appearance of different sequences of mass spectra that well explain the major role of aromatic radicals mainly cyclopenta fused radicals of PAHs resulting from their corresponding methyl PAHs, with active participation of c-C(5)H(5), C(6)H(5), C(6)H(5)CH(2) ,and C(9)H(7) in the formation of large PAHs. The role of the HACA only seemed important for the formation of stable condensed PAHs from unstable primary PAHs with zigzag structure (having triple fusing sites) in one step by ring growth with two carbon atoms.     

汽油替代燃料燃烧过程中多环芳烃生成的化学动力学模型

提出了一个包含103组分和395个基元反应, 能够较好描述多组分汽油替代燃料多环芳烃(PAHs)生成的化学动力学机理模型. 计算结果与实验数据的对比表明, 该机理能够准确地计算乙烯、甲苯、正庚烷预混火焰和正庚烷部分预混对冲火焰中PAHs及其前驱物组分分布. 虽然本文机理目前还无法直接应用于汽油燃烧过程的PAHs多维数值模拟, 但与现有的汽油替代燃料PAHs机理相比, 本文提出的机理规模更小, 距离实际应用的目标更近.     

Ring conserved isodesmic reactions: A new method for estimating the heats of formation of aromatics and PAHs

Density functional theory (DFT) has been used along with isodesmic reaction schemes to estimate heats of formation for aromatics and polynuclear aromatic hydrocarbons (PAHs). Calculations have been performed for 42 molecules, 12 of which have uncertain or unknown experimental values, using the B3-LYP functional with the small 6-31G(d) basis set. Heats of formation for the group of test molecules were estimated using both conventional bond separation (BS) isodesmic reactions as well as a new technique of ring conserved (RC) isodesmic reactions which is able to correct systematic errors in B3-LYP calculations. When a ring conserved isodesmic reaction based on delocalization energies is used, the estimated heat of formation is more accurate than that obtained by the bond separation technique. The methodology for creating and using appropriate ring conserved isodesmic reactions is discussed. The present scheme also compares favorably against a recently developed bond centered group additivity scheme that was tested against a large number of PAH molecules.     

Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma 相似文献   

Interstellar chemistry: a strategy for detecting polycyclic aromatic hydrocarbons in space

Polycyclic aromatic hydrocarbons (PAHs) have long been postulated as constituents of the interstellar gas and circumstellar disks. Observational infrared emission spectra have been plausibly interpreted in support of this hypothesis, but the small (or zero) dipole moments of planar, unsubstituted PAHs preclude their definitive radio astronomical identification. Polar PAHs, such as corannulene, thus represent important targets for radio astronomy because they offer the possibilities of confirming the existence of PAHs in space and revealing new insight into the chemistry of the interstellar medium. Toward this objective, the high-resolution rotational spectrum of corannulene has been obtained by Fourier transform microwave spectroscopy, and the dipole moment (2.07 D) of this exceptionally polar PAH has been measured by exploiting the Stark effect.     

Hydrogen‐Abstraction/Acetylene‐Addition Exposed

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent in the interstellar medium (ISM) and also in carbonaceous meteorites (CM) such as Murchison. However, the basic reaction routes leading to the formation of even the simplest PAH—naphthalene (C₁₀H₈)—via the hydrogen‐abstraction/acetylene‐addition (HACA) mechanism still remain ambiguous. Here, by revealing the uncharted fundamental chemistry of the styrenyl (C₈H₇) and the ortho‐vinylphenyl radicals (C₈H₇)—key transient species of the HACA mechanism—with acetylene (C₂H₂), we provide the first solid experimental evidence on the facile formation of naphthalene in a simulated combustion environment validating the previously postulated HACA mechanism for these two radicals. This study highlights, at the molecular level spanning combustion and astrochemistry, the importance of the HACA mechanism to the formation of the prototype PAH naphthalene.     

A review of monitoring of airborne polycyclic aromatic hydrocarbons: An African perspective

In this review, we focus on the status of the monitoring of polycyclic aromatic hydrocarbons (PAHs) in ambient air as well as in living (indoor) and working environments in Africa from 2000 to 2018. This is important as PAHs are ubiquitous in the environment and are known to be potentially carcinogenic. Aspects of sampling such as collection media for particle bound and gaseous PAHs are discussed. The efficiency and basic quality assurance data of commonly employed extraction techniques for separating target PAHs from sampling media using conventional solvent-based and emerging solvent-free approaches were also evaluated. Polyurethane foam and quartz fiber filters are generally the most commonly used collection media for gaseous and particle bound PAHs, respectively. A wide range of total PAH concentrations in ambient air has been reported across the continent of Africa, with the highest levels found at sampling sites close to high density traffic and industrial areas. A rapidly increasing population, commercial and industrial development, poor urban transportation infrastructure and the use of low quality oil products were the main causes of high total gas and particulate PAH concentrations (1.6–103 μg/m³) in West African port cities such as Cotonou, Benin. With regards to indoor environments, gas phase PAHs were detected at the highest total concentrations in rural areas ranging from 1 to 43 μg/m³ in Burundi with naphthalene being the most prevalent. Firewood burning was the major emission source in most developing countries and resulted in benzo[a]pyrene concentrations above the European permissible risk level of 1 ng/m³.     

Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five‐ and six‐ membered aromatic rings, such as indene (C₉H₈), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five‐ and six‐membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free‐radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C₇H₇) with acetylene (C₂H₂) under combustion‐like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five‐ and six‐membered PAH.     

The Molecular Composition of Soot

Soot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low‐fluence laser desorption ionization, coupled with high‐resolution mass spectrometry imaging. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6‐membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae limit the observed isomers to those which are nearly circular with high aromatic conjugation lengths for a given aromatic ring count. These results stand in contrast to recent reports that suggest higher aliphatic composition of primary particles.     

The Molecular Composition of Soot

Soot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low-fluence laser desorption ionization, coupled with high-resolution mass spectrometry imaging. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6-membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae limit the observed isomers to those which are nearly circular with high aromatic conjugation lengths for a given aromatic ring count. These results stand in contrast to recent reports that suggest higher aliphatic composition of primary particles.     

Hydrogenation of polycyclic aromatic hydrocarbons as a factor affecting the cosmic 6.2 micron emission band

While many of the characteristics of the cosmic unidentified infrared (UIR) emission bands observed for interstellar and circumstellar sources within the Milky Way and other galaxies, can be best attributed to vibrational modes of the variants of the molecular family known as polycyclic aromatic hydrocarbons (PAH), there are open questions that need to be resolved. Among them is the observed strength of the 6.2 micron (1600 cm(-1)) band relative to other strong bands, and the generally low strength for measurements in the laboratory of the 1600 cm(-1) skeletal vibration band of many specific neutral PAH molecules. Also, experiments involving laser excitation of some gas phase neutral PAH species while producing long lifetime state emission in the 3.3 micron (3000 cm(-1)) spectral region, do not result in significant 6.2 micron (1600 cm(-1)) emission. A potentially important variant of the neutral PAH species, namely hydrogenated-PAH (H(N)-PAH) which exhibit intriguing spectral correlation with interstellar and circumstellar infrared emission and the 2175 A extinction feature, may be a factor affecting the strength of 6.2 micron emission. These species are hybrids of aromatic and cycloalkane structures. Laboratory infrared absorption spectroscopy augmented by density function theory (DFT) computations of selected partially hydrogenated-PAH molecules, demonstrates enhanced 6.2 micron (1600 cm(-1)) region skeletal vibration mode strength for these molecules relative to the normal PAH form. This along with other factors such as ionization or the incorporation of nitrogen or oxygen atoms could be a reason for the strength of the cosmic 6.2 micron (1600 cm(-1)) feature.     

Chemical dynamics of the formation of the 1,3-butadiynyl radical (C4H(X2Sigma+)) and its isotopomers

The reaction of dicarbon molecules in their electronic ground, C2(X1Sigma(g)+), and first excited state, C2(a3Pi(u)), with acetylene, C2H2(X1Sigma(g)+), to synthesize the 1,3-butadiynyl radical, C4H(X2Sigma+), plus a hydrogen atom was investigated at six different collision energies between 10.6 and 47.5 kJ mol(-1) under single collision conditions. These studies were contemplated by crossed molecular beam experiments of dicarbon with three acetylene isotopomers C2D2(X1Sigma(g)+), C2HD (X1Sigma+), and 13C2H2(X1Sigma(g)+) to elucidate the role of intersystem crossing (ISC) and of the symmetry of the reaction intermediate(s) on the center-of-mass functions. On the singlet surface, dicarbon was found to react with acetylene through an indirect reaction mechanism involving a diacetylene intermediate. The latter fragmented via a loose exit transition state via an emission of a hydrogen atom to form the 1,3-butadiynyl radical C4H(X2Sigma+). The D(infinity)(h) symmetry of the decomposing diacetylene intermediate results in collision-energy invariant, isotropic (flat) center-of-mass angular distributions of this microchannel. Isotopic substitution experiments suggested that at least at a collision energy of 29 kJ mol(-1), the diacetylene isotopomers are long-lived with respect to their rotational periods. On the triplet surface, the reaction involved three feasible addition complexes located in shallower potential energy wells as compared to singlet diacetylene. The involvement of the triplet surface accounted for the asymmetry of the center-of-mass angular distributions. The detection of the 1,3-butadiynyl radical, C4H(X2Sigma+), in the crossed beam reaction of dicarbon molecules with acetylene presents compelling evidence that the 1,3-butadiynyl radical can be formed via bimolecular reactions involving carbon clusters in extreme environments such as circumstellar envelopes of dying carbon stars and combustion flames.     

Computational Studies on the Reactivity of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widely present in the environment as toxic pollutants. In this study, quantum chemistry methods are used to study reactions of PAHs in both particle and gas phases. Seven theoretical methods are exploited to predict the reactive sites of 15 PAHs in the particle phase. Among these methods, the performance of the condensed Fukui function (CFF) is optimum. The gas-phase reactions of eight PAHs are also investigated. Except for fluorene, CFF predicts correctly the gas-phase mono-nitro products for seven systems. The products of fluorene predicted by CFF are 1-nitrofluorene and 3-nitrofluorene, which is however inconsistent with the experimental results. Transition state theory is then used to investigate the reaction mechanism of fluorene. Calculated rate constants for 3-nitrofluorene and 2-nitrofluorene formation are much bigger than that for 1-nitrofluorene formation, which is in agreement with the experimental results.     

Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyrene

Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H(2) formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H(2) is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H(2). The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion-molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H(2) formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H(2) formation because deuteration lowers the energies of polyatomic PAH cations appreciably.