Dimers Of Polycyclic Aromatic Hydrocarbons Research Articles

Soot nucleation in diffusion flames and the role of aromatic hydrocarbons

We perform spatially resolved measurements of light scattering of soot in atmospheric pressure counterflow diffusion flames to complement previously reported data on soot pyrometry, temperature and gaseous species up to three-ring polycyclic aromatic hydrocarbons (PAHs). We compare two flames: a baseline ethylene flame and a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of pre-vaporized toluene. The goal is twofold: directly adding an aromatic fuel to bypass the formation of the first aromatic ring, widely regarded as the main bottleneck to soot formation from aliphatic fuels, and assessing the impact of a common component of surrogates of transportation fuels on soot formation. The composition of the fuel and oxidizer streams are adjusted to ensure invariance of the temperature-time history, thereby decoupling the chemical effects of the fuel substitution from other factors. The doping approach enables the comparison of very similar flames with respect to combustion products, radicals and critical precursors to aromatic formation (C2–C5 species), in addition to the temperature-time history. Doping with toluene boosts the aromatic content and soot volume fraction relative to the baseline ethylene flame, but, surprisingly, the soot number density and nucleation rate are affected modestly. As a result, the observed difference in volume fraction in the toluene-doped flame is reflective of larger initial particles at the onset of soot nucleation. The nucleation rate when soot first appears near the flame is of the same order as the dimerization rate of single-ring aromatics, in contrast with the expectation that the dimerization of larger PAHs initiates the process. Even though in and of itself nucleation contributes modestly to the overall soot loading, nucleation conditions the overall soot loading by affecting the size of the initial particle, which ultimately affects subsequent growth.

Dimerization of dehydrogenated polycyclic aromatic hydrocarbons on graphene.

Dimerization of polycyclic aromatic hydrocarbons (PAHs) is an important, yet poorly understood, step in the on-surface synthesis of graphene (nanoribbon), soot formation, and growth of carbonaceous dust grains in the interstellar medium (ISM). The on-surface synthesis of graphene and the growth of carbonaceous dust grains in the ISM require the chemical dimerization in which chemical bonds are formed between PAH monomers. An accurate and cheap method of exploring structure rearrangements is needed to reveal the mechanism of chemical dimerization on surfaces. This work has investigated the chemical dimerization of two dehydrogenated PAHs (coronene and pentacene) on graphene via an evolutionary algorithm augmented by machine learning surrogate potentials and a set of customized structure operators. Different dimer structures on surfaces have been successfully located by our structure search methods. Their binding energies are within the experimental errors of temperature programmed desorption measurements. The mechanism of coronene dimer formation on graphene is further studied and discussed.

Simulating yield and morphology of carbonaceous nanoparticles during fuel pyrolysis in laminar flow reactors enabled by reactive inception and aromatic adsorption

Carbon black and soot are synthesized by gas-phase pyrolysis of hydrocarbons. Inception, surface growth, and coagulation control process yield and agglomerate morphology. Here, we present a sectional population balance model (SPBM) coupled with chemistry to simulate carbon black formation by ethylene pyrolysis in laminar flow reactors. The SPBM tracks agglomerate size distribution and morphology. New particles are formed by the dimerization of polycyclic aromatic hydrocarbons (PAHs) and grow with hydrogen abstraction acetylene addition as well as the addition of PAHs on their surface. Three different dimerization models for inception and PAH adsorption are compared: irreversible, reversible, and reactive. With irreversible PAH dimerization and surface growth, more than 90% of PAH mole fraction converts to particles even at low-temperatures (T<1200 K) causing an overestimation of the primary particle number density due to non-stop new particle formation. With reversible dimerization, the mass yield is underestimated by more than a factor of 3. With reversible dimerization and surface growth followed by a chemical bond formation step (chemical dimerization/reactive model), particles form and grow mostly at high temperatures (T>1200 K); the particle size distribution, as well as the average primary particle and mobility diameters are predicted within 30% of measurements. The reactive model enhances the agglomerate morphology prediction in the low-temperature regions of the reactor and appears to be more consistent with the latest understanding of inception and surface growth.

Evidence on the formation of dimers of polycyclic aromatic hydrocarbons in a laminar diffusion flame

The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo- and heterodimers stabilized by the formation of covalent bonds (second group).

Nucleation of soot: experimental assessment of the role of polycyclic aromatic hydrocarbon (PAH) dimers

Abstract The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs) – typically pyrene (C16H10) dimerization – is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z = 404.157 (C32H20) and m/z = 452.157 (C36H20) were detected and identified as dimers of closed-shell C16H10 and C18H10 monomers, respectively. A complex between a C13H9 radical and a C24H12 closed-shell PAH was observed at m/z = 465.164 (C37H21). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.

Study on soot evolution under different hydrogen addition conditions at high temperature by ReaxFF molecular dynamics

The physical and chemical properties evolution of soot particles was studied by ReaxFF MD simulation. The dimerization of PAHs, nucleation and coagulation, and graphitization from hydrocarbons to the final soot nanoparticle were investigated, and the chemical effects of the hydrogen addition on the formation mechanism of primary soot nanoparticle was explored by considering important polycyclic aromatic hydrocarbons (PAHs) in ethylene flames under high temperatures. The results obtained reveal that, in the first stage, the activated aliphatic hydrocarbons mainly grow into a large ring through the chain growth reactions, and then gradually form PAH-like molecules with inner ring bridging instead of directly generating the five or six-carbon ring structures. Hydrogenation effectively inhibited the soot nanoparticle nucleation at all stages. Due to a large number of vinyl and acryl groups attacked by the hydrogen to decarbonize and produce methane, a rapid decrease of the formation and growth rate of the PAHs was observed. With the increase of hydrogen, the nucleation and surface growth rate of soot particles are slowed down due to lose activity.

From Benzene to Graphene: Exploring the Electronic Structure of Single-Layer and Bilayer Graphene Using Polycyclic Aromatic Hydrocarbons

In this work, two exercises are described that are designed to teach students about the evolution and behavior of the electronic bands of graphene and bilayer graphene. These exercises involve performing extended Huckel molecular orbital theory calculations on polyacenes and polycyclic aromatic hydrocarbons. In the first exercise, students investigate how the molecular orbitals of polyacenes converge into bands as polyacene size increases. Further, students learn that long-range interactions cause frontier-orbital crossing as the size of the polyacene increases. In the second exercise, the concepts of band structures, band crossing, and k-space are explored using the results of frontier orbital calculations on π-stacked dimers of polycyclic aromatic hydrocarbons, which represent molecular analogues of layered 2D materials. The results of these calculations show how the geometry and layer–layer offset of the dimer system can affect its electronic structure, and the results can be extrapolated to provide a framework for understanding why subtle changes in the relative orientations of the layers in bilayer graphene can produce qualitative changes in the electronic properties. These calculations are easily implemented with Python or with widely available algebraic manipulation software such as Maple or Mathematica. These exercises are accessible to students who have experience with extended Huckel molecular orbital theory and are suitable for inclusion in the physical chemistry curriculum at the upper-level undergraduate or introductory graduate level.

Quantum chemical exploration of new π-electron systems: Capsule-formed dimers of polycyclic aromatic hydrocarbons

Abstract We have found capsule-formed dimers of polycyclic aromatic hydrocarbons (cap-di-PAH) for coronene, ovalene, and circumcoronene by quantum chemical calculations. Two monomers are connected at the edge that gives a capsule form, holding the aromatic characters in the inner six-membered rings. The aromatic π-electron networks at the upper and lower parts of the dimers are not merged each other. Those cap-di-PAHs are located in sufficiently deep potential wells surrounded by high energy barriers, indicating the high possibility of existence.

Leveraging Fluorescent Emission to Unitary Yield: Dimerization of Polycyclic Aromatic Hydrocarbons

AbstractWe report on the synthesis and characterization of novel substituted 1,1′‐biperylene‐2,2′‐diols in which the dihedral angle between the two polycyclic aromatic hydrocarbon (PAH) units is tailored from ca. 60° to ca. 90° in the solid state by introduction of cyclo‐etheric straps or sterically hindered groups such as the triisopropylsilyl (TIPS) group. Depending on the type of substitution, we lock the dihedral angle between the perylenyl moieties enabling fine‐tuning of the molecular optoelectronic properties, with the molecules displaying the smallest angles acting as exceptionally strong emitters with unitary quantum yields.

Dimers of polycyclic aromatic hydrocarbons: the missing pieces in the soot formation process.

The soot nucleation process, defined as the transition from molecular precursors to condensed matter, is the less understood step in the whole soot formation process. The possibility that polycyclic aromatic hydrocarbon (PAH) dimers, especially those containing moderate-sized PAHs, can play a major role in soot nucleation is a very controversial issue. Although PAH dimers have often been considered as potential soot precursors, their formation is not thermodynamically favored at a typical flame temperature, their binding energies being considered too weak to allow them to survive in this environment. Hereby, we report experimental evidence supporting the existence of PAH dimers in the proximity of the soot nucleation region of a methane laminar diffusion flame that gives strong evidence for the nucleation process to be kinetically rather than thermodynamically controlled.

Atomistic insights into the dynamics of binary collisions between gaseous molecules and polycyclic aromatic hydrocarbon dimers.

Polycyclic aromatic hydrocarbon (PAH) dimers are important intermediates in combustion and soot formation. The scattering dynamics of gaseous molecules colliding with PAH dimers and the subsequent PAH dimer stability are investigated by performing molecular dynamics (MD) simulations. Effects of properties of the surrounding gaseous molecules and PAH dimers as well as temperature are investigated in this study. Depending on the residence time of N2 molecules trapped by the PAH dimers, two scattering types, that is, specular scattering and inelastic scattering, have been observed, which is correlated to the temperature and the type of the PAH dimer. Specifically, specular scattering preferentially takes place at high temperatures on small PAH dimers, while inelastic scattering tends to happen at low temperatures on large PAH dimers. During collision, energy transfer between the gaseous molecule and the PAH dimer changes the thermodynamic stability of the PAH dimer. Statistical analysis indicates that the decomposition rate of a PAH dimer to PAH monomers is sensitive to temperature and the PAH dimer type. Furthermore, effects of the gaseous molecule type on the PAH dimer stability are considered. The molecular mass of the colliding gaseous molecule is a key factor in determining the PAH dimer stability, as heavier gaseous molecules are more effective in promoting the PAH dimer decomposition. Results from this study indicate that collisions with gaseous molecules decrease the PAH dimer stability, while increasing the PAH dimer size and decreasing the collision temperature both decrease the decomposition rate of the PAH dimer.

Dynamics and kinetics of reversible homo-molecular dimerization of polycyclic aromatic hydrocarbons

Physical dimerization of polycyclic aromatic hydrocarbons (PAHs) has been investigated via molecular dynamics (MD) simulation with the ReaxFF reactive force field that is developed to bridge the gap between the quantum mechanism and classical MD. Dynamics and kinetics of homo-molecular PAH collision under different temperatures, impact parameters, and orientations are studied at an atomic level, which is of great value to understand and model the PAH dimerization. In the collision process, the enhancement factors of homo-molecular dimerizations are quantified and found to be larger at lower temperatures or with smaller PAH instead of size independent. Within the capture radius, the lifetime of the formed PAH dimer decreases as the impact parameter increases. Temperature and PAH characteristic dependent forward and reverse rate constants of homo-molecular PAH dimerization are derived from MD simulations, on the basis of which a reversible model is developed. This model can predict the tendency of PAH dimerization as validated by pyrene dimerization experiments [H. Sabbah et al., J. Phys. Chem. Lett. 1(19), 2962 (2010)]. Results from this study indicate that the physical dimerization cannot be an important source under the typical flame temperatures and PAH concentrations, which implies a more significant role played by the chemical route.

Towards a predictive model for polycyclic aromatic hydrocarbon dimerization propensity

Soot particles are a significant pollutant formed as the result of incomplete combustion. Particle nucleation significantly impacts the formation and morphology of soot particles, yet remains a key knowledge gap. To elucidate the process of nucleation, we have investigated the thermodynamic stability of dimers of polycyclic aromatic hydrocarbons (PAHs), towards developing a more comprehensive model for PAH clustering behavior. Using a computational methodology based on molecular dynamics and well-tempered Metadynamics, we quantified the impact of morphological parameters on homo-molecular dimerization, as well as the relative size of monomers on the stability of hetero-molecular dimers. The results illustrated the substantial impact of PAH mass and geometry on the stability of homo-molecular and hetero-molecular dimers at flame temperatures. In particular, dimer stability was found to depend most strongly on monomer mass, followed by solvent-accessible surface area. Additionally, hetero-molecular dimer stability was found to be largely determined by the size of the smallest monomer. Identifying relationships between PAH morphology and thermodynamic stability is a significant step towards a more comprehensive understanding of the physical interactions between PAHs. Altogether, this work presents a framework for elucidating the clustering behavior of arbitrary PAHs and will greatly impact understanding and modeling of particle nucleation and growth.

Thermodynamics of poly-aromatic hydrocarbon clustering and the effects of substituted aliphatic chains

Over the last few decades, the understanding of the processes related to the formation of soot has progressed considerably. However, the mechanisms that are responsible for the nucleation of soot are still unclear. While there is consensus that the formation of soot nuclei can be related to two classes of mechanisms (physical and chemical growth), their relative importance is still under debate. In particular, the aggregation of polycyclic aromatic hydrocarbons (PAHs), especially pyrene, has been proposed as key step for soot formation but strong experimental or computational proofs are still missing. To shed light on this issue, we conducted a thermodynamic analysis of the physical growth of poly-aromatic hydrocarbons using atomistic models. Free energy profiles of dimerization and trimerization processes of several PAHs are computed using molecular dynamics simulations in conjunction with advanced sampling techniques. Our study focuses not only on the potential energy of the clustering processes, but it also addresses the entropic contributions that affect the dimerization and trimerization of PAHs. The results of these simulations show that even at 1000K, only the formation of dimers of relative big species, such as ovalene or bigger, are favored over their corresponding free monomers, ruling out the simple stacking of pyrene as the main step for soot formation. The shapes of the free energy profiles also illustrate that there are no barriers in the exploration of the phase space and therefore, that the dimerization process itself is not kinetically controlled. Geometrical factors, such as the symmetry of the monomers, as well as the presence of substituted aliphatic chains play an important role in the physical agglomeration of PAH that can eventually lead to soot nuclei.

Exploring the Role of PAHs in the Formation of Soot: Pyrene Dimerization

A critical step in currently accepted models for soot formation in combustion is the dimerization of polycyclic aromatic hydrocarbons as small as pyrene, which is necessary within these models to reproduce correctly the soot particle size distribution. We present experimental measurements on the kinetics of pyrene dimerization performed in low-temperature supersonic flows with photoionization mass spectrometric detection, coupled with theoretical results based on careful consideration of the intermolecular interaction energies, binding energy, equilibrium constant, and intermolecular dynamics. These results demonstrate that the equilibrium of the reaction strongly favors the dissociation of the pyrene dimer at high temperature and that physical dimerization (involving van der Waals forces) of pyrene cannot be a key step in carbon particle formation in hot environments such as flames and circumstellar shells.

Physical origins of interactions in dimers of polycyclic aromatic hydrocarbons

Dimers of several polycyclic aromatic hydrocarbons (PAHs): naphthalene, anthracene and pyrene have been investigated by symmetry-adapted perturbation theory based on the density functional description of the monomers [SAPT(DFT)]. Calculations have been performed for a number of radial cross-sections of selected stacked and T-shaped configurations. The interaction energies at the minima of stacked configurations of the benzene, naphthalene, anthracene and pyrene dimers increase with the number of rings, but -- in contrast to literature findings -- the rate of increase is somewhat irregular. In particular, the anthracene molecules interact slightly stronger than indicated by the number of rings and the pyrene molecules interact significantly weaker (although the latter do interact stronger than the former). These trends can be partly rationalized by the physical decomposition of the interaction energies given by SAPT. We find the stacked structures to be significantly more stable than the T-shaped ones, with the relative stability of the former structures increasing as the size of the system increases. This observation extends to the benzene dimer where the two structures are nearly isoenergetic and a tilted T-shaped structure actually becomes the global minimum. For the naphthalene dimer, the greater stability of the stacked configuration than of the T-shaped one is in disagreement with recent experiments observing only the latter structure. For the anthracene dimer, theory is in agreement with experiments.

Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

The species responsible for the broad extended red emission (ERE), discovered in 1975 and now known to be widespread throughout the Galaxy, still is unidentified. Spanning the range from approximately 540 to 900 nm, the ERE is a photoluminescent process associated with a wide variety of different interstellar environments. Over the years, a number of plausible candidates have been suggested, but subsequent observations ruled them out. The objects that present the ERE also emit the infrared features attributed to free polycyclic aromatic hydrocarbon (PAH) molecules, suggesting that closely related materials are plausible ERE carriers. Here, we show that the peculiar spectra and unique properties of closed-shell cationic PAH dimers satisfy the existing observational constraints and suggest that emission from mixtures of charged PAH clusters accounts for much of the ERE. This work provides a view into the structures, stabilities, abundances, and ionization balance of PAH-related species in the emission zones, which, in turn, reflects physical conditions in the emission zones and sheds fundamental light on the nanoscale processes involved in carbon-particle nucleation and growth and carbonaceous dust evolution in the interstellar medium.

Electron affinities of polycyclic aromatic hydrocarbons determined by the kinetic method

AbstractThe kinetic method has been applied in the determination of electron affinities (EA) of polycyclic aromatic hydrocarbons (PAHs) using electron attachment desorption chemical ionization mass spectrometry (DCI‐MS) and triple quadrupole tandem mass spectrometry. Electron‐bound dimers of PAHs (viz. solvated radical anions) are generated in the ion source and they fragment competitively upon collisional activation to yield the intact monomeric molecular radical anions. The ratio of the abundances of these two ions reflects the difference in electron affinities of the PAHs. Relative electron affinities are ordered as phenanthrene &lt; perylene &lt; 1,2‐benzanthracene &lt; benzo [ghi] perylene &lt; anthracene &lt; pyrene &lt; fluoranthene &lt; biphenylene on the basis of multiple pairwise measurements. On the assumption that the effective temperature of the electron‐bound dimers is constant, under fixed desorption conditions, and using phenanthrene (EA = 0.307 eV), anthracene (EA = 0.556 eV), pyrene (EA = 0.591 eV) and fluoranthene (EA = 0.63 eV) as reference compounds, the electron affinities of several PAHs are estimated as perylene (0.35 eV), 1,2‐benzanthracene (0.39 eV), benzo [ghi] perylene (0.42 eV) and biphenylene (0.89 eV). The calculated effective temperature using the known EAs of the reference compounds, phenanthrene, anthracene, pyrene and fluoranthene, is 1251 K. The high effective temperature in the electron‐bound dimer is ascribed to both the weak binding in the electron‐bound dimer and to the choice of ionization conditions. The same methodology was applied to estimate the electron affinities of substituted PAHs. For alkyl‐substituted PAHs, e.g. 1‐methylanthracene, the estimated electron affinity (0.55 ± 0.10 eV) is consistent with electron transfer equilibrium studies. However, for halogenated PAHs, the ratio of fragment ion abundances does not correlate with the known electron affinity difference when a PAH and a halogenated PAH comprise the electron‐bound dimer. This is suggested to be due to differences in the structures of the cluster ions generated upon halogen substitution. The expected affinity results are obtained when the electron‐bound dimer is comprised of two halogenated PAHs. Estimated electron affinity values for 1‐bromopyrene (0.72 ± 0.10 eV) and 9‐bromoanthracene (0.61 ± 0.10 eV) are slightly higher than the electron affinities of pyrene and anthracene, respectively.

A theoretical study of the structure and energetics of stacked dimers of polycyclic aromatic hydrocarbons. Application of INDO 1/S method to singlet excimers of naphthalene and phenanthrene

We show here that, with the incorporation of intermolecular distance dependence of π—σ interactions, the INDO 1/S method promises to be a very useful tool for investigating the structure and energetics of singlet excimers of polycyclic aromatic hydrocarbons. Application of the method to naphthalene yields binding energies and vertical transition energies (in absorption and in emission) of the singlet excimer which are in good agreement with experiment. The method also provides a possible explanation for the failure of phenanthrene to exhibit excimer fluorescence or the self-quenching of the monomer fluorescence.