Large Polycyclic Aromatic Hydrocarbons Research Articles

Pyrolytic decomposition of methanol, ethanol, and butanol at various temperatures and residence times in a high-temperature flow reactor

The potential use of low-carbon renewable fuel molecules, such as short chain alcohols, as a substitute or extender for fossil fuels, offers an opportunity to address pollutant emissions from transport and the sustainability of energy sources. Moreover, it may help reduce pollutants detrimental to air quality and human health, of which particulate matter (PM) is a significant contributor. To this end, the pyrolysis of three renewable alcohol fuels, methanol, ethanol, and butanol, has been studied in a laminar flow reactor at a sample inlet temperature range of 869 0C to 1120 0C. All the tested fuels were injected into a nitrogen carrier gas stream at a fixed concentration of 10,000 ppm on a carbon atom basis. A range of pyrolysis temperatures and residence times were selected so as to gain insight into the pyrolytic decomposition of short chain alcohol fuels to intermediate species important to the subsequent formation of benzene rings, larger polycyclic aromatic hydrocarbons (PAH), and particulate matter. In this paper, the presence of ten intermediary species that may aid in the formation of benzene rings and the growth of PAHs were quantified, with an emphasis on probable routes of first ring formation from the alcohols of varying chain length. Gaseous samples were collected from the pyrolyser at different temperatures and residence times using a novel high-temperature air-cooled ceramic sampling probe. The identification and quantification of intermediate gaseous species were undertaken by Gas Chromatography-Flame Ionization Detection (GC-FID). During methanol pyrolysis, only a tenth of the supplied carbon was detected as hydrocarbon gaseous species (mainly methane), with no benzene found, highlighting that methanol pyrolysis does not readily result in polyaromatic hydrocarbons and soot. Ethanol and butanol pyrolysis produced substantial amounts of ethylene and acetylene, suggesting a role in benzene and soot formation. Uniquely, pyrolysis of the longest chain alcohol, butanol, produced appreciable levels of C3 and C4 hydrocarbons, in addition to C1 and C2 hydrocarbons, suggesting additional reaction pathways for benzene formation and growth in comparison with ethanol.

Measurement report: Brown carbon aerosol in polluted urban air of the North China Plain – day–night differences in the chromophores and optical properties

Abstract. Brown carbon (BrC) aerosol is light-absorbing organic carbon that affects radiative forcing and atmospheric photochemistry. The BrC chromophoric composition and its linkage to optical properties at the molecular level, however, are still not well characterized. In this study, we investigate the day–night differences in the chromophoric composition (38 species) and optical properties of water-soluble and water-insoluble BrC fractions (WS-BrC and WIS-BrC) in aerosol samples collected in Shijiazhuang, one of the most polluted cities in China. We found that the light absorption contribution of WS-BrC to total BrC at 365 nm was higher during the day (62±8 %) than during the night (47±26 %), which is in line with the difference in chromophoric polarity between daytime (more polar nitrated aromatics) and nighttime (more less-polar polycyclic aromatic hydrocarbons, PAHs). The high polarity and water solubility of BrC in the daytime suggests the enhanced contribution of secondary formation to BrC during the day. There was a decrease in the mass absorption efficiency of BrC from nighttime to daytime (2.88±0.24 vs. 2.58±0.14 for WS-BrC and 1.43±0.83 vs. 1.02±0.49 m2 g C−1 for WIS-BrC, respectively). Large polycyclic aromatic hydrocarbons (PAHs) with four- to six-ring PAHs and nitrophenols contributed to 76.7 % of the total light absorption between 300–420 nm at nighttime, while nitrocatechols and two- to three-ring oxygenated PAHs accounted for 52.6 % of the total light absorption during the day. The total mass concentrations of the identified chromophores showed larger day–night difference during the low-pollution period (day-to-night ratio of 4.3) than during the high-pollution period (day-to-night ratio of 1.8). The large day–night difference in BrC composition and absorption, therefore, should be considered when estimating the sources, atmospheric processes, and impacts of BrC.

Oxidation of the 1‐naphthyl radical C10H7• with oxygen: Thermochemistry, kinetics, and possible reaction pathways

AbstractThe reaction of the 1‐naphthyl radical C10H7• (A2•) with molecular (3O2) and atomic oxygen, as part of the oxidation reactions of naphthalene, is examined using ab‐initio and DFT quantum chemistry calculations. The study focuses on pathways that produce the intermediate final products CO, phenyl and C2H2, which may constitute a repetitive reaction sequence for the successive diminution of six‐membered rings also in larger polycyclic aromatic hydrocarbons. The primary attack of 3O2 on the 1‐naphthyl radical leads to a peroxy radical C10H7OO• (A2OO•), which undergoes further propagation and/or chain branching reactions. The thermochemistry of intermediates and transition state structures is investigated as well as the identification of all plausible reaction pathways for the A2• + O2 / A2• + O systems. Structures and enthalpies of formation for the involved species are reported along with transition state barriers and reaction pathways. Standard enthalpies of formation are calculated using ab initio (CBS‐QB3) and DFT calculations (B3LYP, M06, APFD). The reaction of A2• with 3O2 opens six main consecutive reaction channels with new ones not currently considered in oxidation mechanisms. The reaction paths comprise important exothermic chain branching reactions and the formation of unsaturated oxygenated hydrocarbon intermediates. The primary attack of 3O2 at the A2• radical has a well depth of some 50 kcal mol−1 while the six consecutive channels exhibit energy barriers below the energy of the A2• radical. The kinetic parameters of each path are determined using chemical activation analysis based on the canonical transition state theory calculations. The investigated reactions could serve as part of a comprehensive mechanism for the oxidation of naphthalene. The principal result from this study is that the consecutive reactions of the A2• radical, viz. the channels conducting to a phenyl radical C6H5•, CO2, CO (which oxidized to CO2) and C2H2 are by orders of magnitude faster than the activation of naphthalene by oxygen (A2 + O2 → A2• + HO2).

Investigation of Rod‐Shaped Single‐Graphene Quantum Dot

In recent years, there has been a resurgence of interest in graphene quantum dots (i.e., large polycyclic aromatic hydrocarbons) in particular due to their interesting properties as single‐quantum emitters. Herein, a thorough study of a graphene quantum dot made of 96 carbon atoms in a D2h symmetry is reported. In particular, experiments at the single‐molecule level reveal that this graphene quantum dot structure is a bright and photostable single‐photon emitter at room temperature. Finally, a statistical study of the emission wavelength as a function of the graphene quantum dot concentration highlights the high purity and degree of control on the samples.

(Invited) Synthetic Exploration of Functional Nanographenes for Photonic Applications

Nanographenes, or large polycyclic aromatic hydrocarbons (PAHs), have been attracting renewed attentions for their unique optical, electronic, and magnetic properties [1]. Nanographenes can also be considered as atomically precise graphene quantum dots (GQDs), which are promising as an environmentally friendly alternative of semiconductor quantum dots, especially for photonic applications ranging from light-emitting devices to bioimaging. To this end, we have been exploring the synthesis and functionalization of dibenzo[hi,st]ovalene (DBOV) as a nanographene with zigzag and armchair edges, which demonstrated high stability, strong red emission, and optical gain properties [1,2]. For example, the functionalization of DBOV with two fluoranthene imide (FAI) groups induced red-shift of the absorption and emission bands, increase of the Stokes shift, and enhancement of the stimulated emission (SE) signals with significantly reduced excited state absorption, allowing the efficient lasing at 720 nm [3]. More recently, we have synthesized dibenzo[a,m]dinaphtho[3,2,1-ef:1',2',3'-hi]coronene (DBDNC) with armchair, zigzag, and fjord edges, which also showed high stability and red emission [4]. Notably, DBDNC has a nonplanar structure around the fjord edges, as revealed by the single-crystal X-ray analysis, and displayed a SE signal at 710 nm with a longer lifetime than that of DBOV, presumably due to the suppression of intermolecular interactions. These results provide a further insight into the relationship between the PAH structures and their photophysical properties, paving the way toward their photonic applications.[1] Paternò, G. M.; Goudappagouda, Chen, Q.; Lanzani, G.; Scotognella, F; Narita A., Adv. Optical Mater. 2021, 9, 2100508.[2] Paternò, G. M.; Chen, Q.; Wang, X.-Y.; Liu, J.; Motti, S. G.; Petrozza, A.; Feng, X.; Lanzani, G.; Müllen, K.; Narita, A.; Scotognella, F., Angew. Chem. Int. Ed. 2017, 56, 6753–6757.[3] Paternò, G. M.; Chen, Q.; Muñoz-Mármol, R.; Guizzardi, M.; Bonal, V.; Kabe, R.; Barker, A. J.; Boj, P. G.; Chatterjee, S.; Ie, Y.; Villalvilla, J. M.; Quintana, J. A.; Scotognella, F.; Müllen, K.; Díaz-García, M. A.; Narita, A.; Lanzani, G., Mater. Horiz. 2022,9, 393–402.[4] Xu, X.; Serra, G.; Villa, A.; Muñoz-Mármol, R.; Vasylevskyi, S.; Gadea, M.; Lucotti, A.; Lin, Z.; Boj, P. G.; Kabe, R.; Tommasini, M.; Díaz-García, M. Á.; Scotognella, F.; Paternò, G. M.; Narita, A., Chem. Sci. 2022, 13, 13040–13045

Quantum Tunneling Facilitates Water Motion across the Surface of Phenanthrene.

Quantum tunneling is a fundamental phenomenon that plays a pivotal role in the motion and interaction of atoms and molecules. In particular, its influence in the interaction between water molecules and carbon surfaces can have significant implications for a multitude of fields ranging from atmospheric chemistry to separation technologies. Here, we unveil at the molecular level the complex motion dynamics of a single water molecule on the planar surface of the polycyclic aromatic hydrocarbon phenanthrene, which was used as a small-scale carbon surface-like model. In this system, the water molecule interacts with the substrate through weak O-H···π hydrogen bonds, in which phenanthrene acts as the hydrogen-bond acceptor via the high electron density of its aromatic cloud. The rotational spectrum, which was recorded using chirped-pulse Fourier transform microwave spectroscopy, exhibits characteristic line splittings as dynamical features. The nature of the internal dynamics was elucidated in great detail with the investigation of the isotope-substitution effect on the line splittings in the rotational spectra of the H218O, D2O, and HDO isotopologues of the phenanthrene-H2O complex. The spectral analysis revealed a complex internal dynamic showing a concerted tunneling motion of water involving its internal rotation and its translation between the two equivalent peripheral rings of phenanthrene. This high-resolution spectroscopy study presents the observation of a tunneling motion exhibited by the water monomer when interacting with a planar carbon surface with an unprecedented level of detail. This can serve as a small-scale analogue for water motions on large aromatic surfaces, i.e., large polycyclic aromatic hydrocarbons and graphene.

Sensitivity comparison of gas chromatography-mass spectrometry with Cold EI and standard EI.

Gas chromatography-mass spectrometry (GC-MS) with Cold EI is based on interfacing GC and MS with supersonic molecular beams (SMBs) along with electron ionization of vibrationally cold sample compounds in SMB in a fly-through ion source (hence the name Cold EI). Cold EI improves all the central performance aspects of GC-MS, and in this paper, we focus on its improvement of signal-to-noise ratio (S/N) and limits of detection (LODs). We found that the harder the compound for analysis with standard EI, the greater the Cold EI gain in S/N and LOD. The lower LOD and higher S/N of Cold EI emerge from a few reasons: (a) similar ionization yield as standard EI, (b) enhanced abundance of molecular ions, (c) elimination of vacuum background noise, (d) elimination of ion source-related peak tailing and degradation, (e) ability to lower the elution temperatures via the use of high column flow rates, and (f) greater range of thermally labile and low-volatility compounds that can be analyzed. We demonstrate the superior S/N and lower LOD of Cold EI versus standard EI in a range of compounds, from the simple-to-analyze octafluoronaphthalene all the way to reserpine and an organo-metallic compound that cannot be analyzed by standard EI. These compounds include methyl stearate, cholesterol, n-C32 H66 , large polycyclic aromatic hydrocarbons, dioctyl phthalates, diundecyl phthalate, pentachlorophenol, benzidine, lambda-cyhalothrin, and methidathion. The significantly lower Cold EI LODs that can be over 1000 times better than in standard EI further result in far superior response linearity and greater measurement dynamic range.

Turbulent processing of PAHs in protoplanetary discs

Context. Polycyclic aromatic hydrocarbons (PAHs) have been detected in numerous circumstellar discs. Despite the correlation between stellar temperature and low PAH detections rates, the diversity of PAH detections and non-detections at similar stellar properties is not well understood. Aims. We propose the continuous processing of PAHs through clustering, adsorption on dust grains, and their reverse-processes as key mechanisms to reduce the emission-capable PAH abundance in protoplanetary discs. This cycle of processing is driven by vertical turbulence in the disc mixing PAHs between the disc midplane and the photosphere. Methods. We used a theoretical Monte Carlo model for photodesorption in the photosphere and a coagulation code in the disc midplane to estimate the relevance and timescale of these processes in a Herbig Ae/Be disc environment. By combining these components in a 1D vertical model, we calculated the gas-phase depletion of PAHs that stick as clusters on dust grains. Results. Our results show that the clustering of gas-phase PAHs is very efficient, and that clusters with more than 100 monomers can grow for years before they are able to freeze out in the disc midplane. Once a PAH cluster is frozen on the dust grain surface, the large heat capacity of these clusters prevents them from evaporating off the grains in UV-rich environments such as the photosphere. Therefore, the clustering of PAHs followed by freeze-out can lead to a depletion of gas-phase PAHs in protoplanetary discs. We find that this mechanism is more efficient when the PAH species has fewer carbon atoms. In contrast, PAH monomers and very small clusters consisting of a few monomers can easily detach from the grain by absorption of a single UV photon. Evaluated over the lifetime of protoplanetary discs, we find a depletion of PAHs by a factor that ranges between 50 and 1000 compared to the standard ISM abundance of PAHs in the inner disc through turbulent processing. Conclusions. Through these processes, we favour PAHs smaller than circumovalene (C66H20) as the major gas-phase emitters of the disc photosphere as larger PAH monomers cannot photodesorb from the grain surface. These gas-phase PAHs co-exist with large PAH clusters sticking on dust grains. We find a close relation between the amount of PAHs frozen out on dust grains and the dust population, as well as the strength of the vertical turbulence.

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.

SAR-AD Method to Characterize Eight SARA Fractions in Various Vacuum Residues and Follow Their Transformations Occurring during Hydrocracking and Pyrolysis

Model compounds were used to provide some chemical boundaries for the eight-fraction SAR-ADTM characterization method for heavy oils. It was found that the Saturates fraction consists of linear and highly cyclic alkanes; the Aro-1 fraction consists of molecules with a single aromatic ring; the Aro-2 fraction consists of mostly 2 and 3-ring fused aromatic molecules, the pericondensed 4-ring molecule pyrene, and molecules with 3–5 rings that are not fused; and the Aro-3 fraction consists of 4-membered linear and catacondensed aromatics, larger pericondensed aromatics, and large polycyclic aromatic hydrocarbons. The Resins fraction consists of mostly fused aromatic ring systems containing polar functional groups and metallated polar vanadium oxide porphyrin compounds, and the Asphaltene fraction consists of both island- and archipelago-type structures with a broad range of molecular weight variation, aromaticity, and heteroatom contents. The behavior of the eight SAR-ADTM fractions during hydrocracking and pyrolysis was investigated, and quantitative relations were established. Intercriteria analysis and evaluation of SAR-ADTM data of hydrocracked vacuum residue and sediment formation rate in commercial ebullated bed vacuum residue hydrocracking were performed. It showed that total asphaltene content, toluene-soluble asphaltenes, and colloidal instability index contribute to sediment formation, while Resins and Cyclohexane-soluble asphaltenes had no statistically meaningful relation to sediment formation for the studied range of operation conditions.

Potential of derivation of a series of C6–C14 1-alkene skeletal oxidation mechanisms on the basis of reaction rate rules

Olefins, accounting for one-fifth proportion of gasoline fuel, are also essential intermediates in the combustion of large hydrocarbons and vital precursors of large polycyclic aromatic hydrocarbons and soot. The unsaturated C = C bond of alkenes renders a more complex low-temperature reaction scheme compared with their corresponding n-alkanes. Thus, it is of great necessity to obtain a thorough knowledge of the chemical kinetics of alkenes. However, the studies on the chemical kinetics of long-chain olefins are far from sufficient. In this work, a collection of skeletal oxidation mechanisms for 1-alkenes from 1-hexene (C6H12-1) to 1-tetradecene (C14H28-1) were constructed in light of recent advances in skeletal mechanism construction for fuels sharing similar structures based on reaction rate rules. For each 1-alkene, the skeletal mechanism includes ∼54 species and ∼216 reactions. Meanwhile, the effect of the C = C double bond was evaluated by conducting a bond dissociation energy calculation to assist the skeletal mechanism construction and derivation. It is discovered that the allylic position is preferred for H-atom abstraction reactions owing to its lowest bond dissociation energy among different locations for all heavy 1-alkenes. Furthermore, the reactions of OH addition to the C = C bond become important at low temperatures, which competes with the H-atom abstraction reactions. This results in the lower reactivity of 1-alkenes compared with corresponding n-alkanes at low temperatures. The acquired skeletal mechanisms were validated against extensive experimental data including laminar flame speeds, ignition delay times in shock tubes, and key species concentrations in jet-stirred reactors, flow reactors, and premixed laminar flames. The good agreements between experiments and simulations demonstrate the prediction capability of all the skeletal mechanisms of 1-alkenes although minor discrepancies exist in the concentration predictions for C2–C3 species, which might stem from a highly reduced C2–C3 sub-mechanism being utilized in the present mechanism.

Spectral variations among different scenarios of PAH processing or formation

ABSTRACT We examine the variations in the spectral characteristics and intensities of polycyclic aromatic hydrocarbons (PAHs) in two different scenarios of PAH processing (or formation): (1) small PAHs are being destroyed (or equivalently large PAHs are being formed, referred to as SPR, i.e. small PAHs removed), and (2) large PAHs are being destroyed (or equivalently small PAHs are being formed, referred to as LPR, i.e. large PAHs removed). PAH emission was measured considering both the presence and absence of plateau components. The variation in the PAH band intensities as a function of the average number of carbon atoms 〈NC〉 has the highest dynamic range in the SPR case, suggesting that smaller PAHs have higher impact on the PAH band strengths. The plateaus show overall declining emission with 〈NC〉, and their higher dynamic range in the SPR case also suggests that smaller PAHs are mainly contributing to the plateau emission. The 7.7/(11.0 + 11.2) $\mu$m PAH band ratio presents the least amount of variance with the lowest dynamic range, rendering this ratio as the better choice for tracing PAH charge. The 3.3/(11.2 + 11.0) $\mu$m PAH band ratio is the only ratio that has both a monotonic variance and fully separated values among the SPR and LPR scenarios, highlighting its efficiency as PAH size tracer but also allowing the characterization of the dominant scenario of processing or formation in a given region or source. We present new PAH charge–size diagnostic diagrams, which can provide insights into the average, maximum, or minimum NC within astrophysical sources.

A Plausible Model for the Galactic Extended Red Emission: Graphene Exposed to Far-ultraviolet Light

Extended red emission (ERE) is a broad feature in the spectral region of 500–900 nm commonly observed in a wide range of circumstellar and interstellar environments. Although the observational constraints for ERE are well established, definitive identifications of the carriers and associated processes complying with these constraints remain unanswered. We report a plausible two-step model involving far-ultraviolet (UV)-irradiated single-layer graphene (SLG), considered as large polycyclic aromatic hydrocarbons, to meet these constraints and supported by laboratory experiments. The far-UV-treated SLG, producing structural defects and graphene quantum dots, showed photoluminescence excitation spectrum extending from the far-UV to UV–visible region, hence meeting the requirements of far-UV light and high photon conversion efficiency. Furthermore, a photoluminescence band shifted from ∼585 to ∼750 nm for high-dose-exposed SLG agrees with the observed redshift of the ERE band in regions under a greater far-UV radiation density.

Origins of Offset-Stacking in Porous Frameworks

Parallel-displaced π-stacking in the benzene dimer and larger polycyclic aromatic hydrocarbons is driven by competition between dispersion and exchange-repulsion interactions. The present work examines whether the same is true in porous frameworks that exhibit stacking interactions, including the [18]annulene dimer, porphyrin dimer, and several models of the covalent organic framework known as COF-1. Interaction energies and their components are computed using extended symmetry-adapted perturbation theory along two-dimensional scans representing slip-stacking. As in the polycyclic aromatic hydrocarbons studied previously, we find that the van der Waals interaction potential (defined as the sum of dispersion and Pauli repulsion) drives the system into a slip-stacked geometry. Electrostatics is a relatively small component of the total interaction energy. In the case of COF-1, the van der Waals potential drives the conformational preference whether or not a solvent molecule intercalates into the framework, although the presence of the guest (mesitylene) molecule substantially limits the low-energy slip-stacking configurations that are available. Even when the COF-1 pore is empty, a modest lateral offset of ≲1.5 Å is preferred, which is small compared to the pore size.

Aryne Atropisomers: Chiral Arynes for the Enantiospecific Synthesis of Atropisomers and Nanographene Atropisomers

The basics about arynes and their applications in synthetic organic chemistry are briefly presented, and the concept of atropisomerism is defined, highlighting that it is a time-dependent form of isomerism and chirality. It is remembered that racemization is a macroscopic and statistical irreversible process, while enantiomerization is a nanoscopic reversible process that occurs at the molecular scale, with racemization being twice as fast as enantiomerization. The concept of aryne atropisomers is introduced with a naive question: Can synthetically useful nonracemic aryne atropisomers having a triple bond ortho to the stereogenic single bond exist in solution? It was found that such aryne atropisomers can be generated in solution from easily available ortho-iodoaryl triflate precursors and excess trimethylsilylmethylmagnesium chloride. Analysis of the barriers to enantiomerization of some aryne atropisomers by computational modeling revealed the key contribution to the configurational stability of the H atom in tris-ortho-substituted biphenyl-based atropisomers. Using a specially designed prototype of aryne atropisomer, for which the barrier to enantiomerization was accurately evaluated by advanced computational modeling, the kinetic parameters of its reaction with furan were experimentally determined. From these measurements, it was concluded that any aryne atropisomer with a barrier to enantiomerization ΔGenant⧧ equal to or higher than 50 kJ mol-1 would lead to fully enantiospecific reactions. The synthetic applications of two structurally distinct aryne atropisomers built on a 1-phenylnaphthalene platform are described: one has the aryne triple bond embedded in the naphthyl moiety, and the other has the aryne triple bond embedded in the phenyl moiety. Both aryne atropisomers allowed for the fully enantiospecific, and possibly overall enantioselective, syntheses of original atropisomers based on standard aryne chemistry. For instance, reactions with anthracene and perylene afforded triptycene and nanographene atropisomers, respectively, in high enantiomeric excesses. A bis(aryne) atropisomer synthetic equivalent prepared from either enantiomer of BINOL is described for 3D bidirectional reactions with a single handedness. Its 2-fold reactions with anthracene and perylene afforded the corresponding severely congested bis(benzotriptycene) (99% ee) nanocarbon atropisomer and bis(anthra[1,2,3,4-ghi]perylene) (98% ee) nanographene atropisomer, respectively. This allowed the discovery of bis(twistacene) atropisomers as a new class of polycyclic aromatic hydrocarbons (PAH) with multiple stereogenicities. Cross reactions with the bis(aryne) atropisomer synthetic equivalent and two different arynophiles proved feasible, providing a nanographene atropisomer with a benzotriptycene unit and an anthra[1,2,3,4-ghi]perylene unit assembled around a stereogenic axis as a unique chiral PAH (99% ee). Overall, because the concept is simple and its implementation is easy, aryne atropisomers is an attractive approach to the synthesis of atropisomers in a broad meaning. Applications to the synthesis of large PAH atropisomers with single handedness are particularly promising.

Size distribution of polycyclic aromatic hydrocarbons in space: an old new light on the 11.2/3.3 μm intensity ratio.

The intensity ratio of the 11.2/3.3 μm emission bands is considered to be a reliable tracer of the size distribution of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). This paper describes the validation of the calculated intrinsic infrared (IR) spectra of PAHs that underlie the interpretation of the observed ratio. The comparison of harmonic calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra reveals a consistent underestimation of the 11.2/3.3 μm intensity ratio by 34%. IR spectra based on higher level anharmonic calculations, on the other hand, are in very good agreement with the experiments. While there are indications that the 11.2/3.3 μm ratio increases systematically for PAHs in the relevant size range when using a larger basis set, it is unfortunately not yet possible to reliably calculate anharmonic spectra for large PAHs. Based on these considerations, we have adjusted the intrinsic ratio of these modes and incorporated this in an interstellar PAH emission model. This corrected model implies that typical PAH sizes in reflection nebulae such as NGC 7023 - previously inferred to be in the range of 50 to 70 carbon atoms per PAH are actually in the range of 40 to 55 carbon atoms. The higher limit of this range is close to the size of the C60 fullerene (also detected in reflection nebulae), which would be in line with the hypothesis that, under appropriate conditions, large PAHs are converted into the more stable fullerenes in the ISM.

Laboratory hydrogenation of the photo-fragments of PAH cations: Co-evolution interstellar chemistry

To investigate co-evolution interstellar chemistry, we studied the gas-phase hydrogenation processes of possible photo-fragments of large polycyclic aromatic hydrocarbon (PAH) cations. Our experimental results show that hydrogenated photo-fragments of hexa-peri-hexabenzocoronene (HBC, C42H18) cations are efficiently formed. The predominance of even-mass fragments (C42H2n+, n = [0–9]) is observed in the photo-fragmentation experiments, while no even-odd hydrogenated mass patterns are observed in the hydrogenation experiments. We investigated the structure of these newly formed hydrogenated photo-fragments and the bonding energies for the reaction pathways with quantum chemistry calculations. We used a molecular kinetic reaction model to simulate the hydrogenation processes of the photo-fragments (e.g. C42H12+) as a function of the reaction time under the experimental conditions. We obtain the possible structure distribution of the newly formed hydrogenated fragments of C42H18+ and the infrared (IR) spectra of these possible molecules. We infer that the hydrogenation and photo-dehydrogenation channels are not reversible reaction channels. Hydrogenation tends to be more random and disorderly, with no restrictions or requirements for the carbon reaction sites of PAH species. As a result, under the co-evolution interstellar chemistry network, there is little chance that PAH compounds return to their initial state through hydrogenation processes after photo-dehydrogenation. Consequently, the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium (ISM).

A ReaxFF molecular dynamics study of polycyclic aromatic hydrocarbon oxidation assisted by nitrogen oxides

Fossil fuel-derived soot poses a persistent problem. A joint reduction is conducted via the reaction between NOx and soot. The underlying reaction mechanisms of large polycyclic aromatic hydrocarbons (PAHs), as well as the interactions between O2, NO and NO2, have been extensively investigated by ReaxFF molecular dynamics simulations for the first time. The pyrolysis and oxidation of coronene are conducted at 2500 K and 50 atm. Coronene in the pyrolysis system first experiences dehydrogenation reactions and subsequently undergoes recombination reactions, leading to the formation of large PAHs. Among the three oxidizers, NO2 is the strongest in coronene oxidation, followed by NO and O2. Meanwhile, the O radical is identified as the key species for PAH oxidation. In the presence of O2, the formation of O radicals requires the assistance of H radicals that are formed by the dehydrogenation of PAHs, which retards the oxidation process. In the presence of NO, O radicals can be directly formed via reactions of NO → N+O and N + NO ⇌ N2 + O. NO2 first undergoes a decomposition reaction: NO2 → NO + O, followed by NO decomposition. O-addition and N-addition, as well as subsequent fragmentation reactions including the generation of COx and (H)CN are important routes in PAH oxidation. Furthermore, compared to NO, NO2 provides more O radicals that significantly accelerate PAH oxidation. This study provides fundamental insight into PAH oxidation that may help to design strategies to inhibit soot and NOx emissions at the source.

Reparameterized semi-empirical methods for computing anharmonic vibrational frequencies of multiply-bonded hydrocarbons

Abstract Reparameterized semi-empirical methods can reproduce gas-phase experimental vibrational frequencies to within 24 cm−1 or better for a 100-fold decrease in computational cost in the anharmonic fundamental vibrational frequencies. To achieve such accuracy and efficiency, the default parameters in the PM6 semi-empirical model are herein optimized to reproduce the experimental and high-level theoretical vibrational spectra of three small hydrocarbon molecules, C2H2, c-C3H2, and C2H4, with the hope that these same parameters will be applicable to large polycyclic aromatic hydrocarbons (PAHs). This massive cost reduction allows for the computation of explicit anharmonic frequencies and the inclusion of resonance corrections that have been shown to be essential for accurate predictions of anharmonic frequencies. Such accurate predictions are necessary to help to disentangle the heretofore unidentified infrared spectral features observed around diverse astronomical bodies and hypothesized to be caused by PAHs, especially with the upcoming influx of observational data from the James Webb Space Telescope. The optimized PM6 parameters presented herein represent a substantial step in this direction with those obtained for ethylene (C2H4) yielding a 37% reduction in the mean absolute error of the fundamental frequencies compared to the default PM6 parameters.

Benchmark Thermochemistry of Polycyclic Aromatic Hydrocarbons.

The DLPNO-CCSD(T1)/CBS method in conjunction with isodesmic-type reactions was used to calculate the enthalpies of formation of 30 polycyclic aromatic hydrocarbons (PAHs). The small reference species with well-determined experimental enthalpies of formation were used in these reactions. From comparison with available experimental data and other recently published high-level quantum chemical calculations, the reference values of the standard enthalpy of formation are recommended for 13 PAHs. The difference between the calculated and experimental values for these compounds does not exceed 2 kJ/mol and their experimental data can be considered as benchmark values for use in calculation of enthalpies of formation of various large PAHs and testing the accuracy of theoretical methods. The use of isodesmic-type reactions involving reference PAHs conserves the structural features of these compounds on both sides of the reaction which leads to a better error compensation. As a result, as shown in the example of coronene, a less-expensive DLPNO-CCSD(T)/CBS method can be used in this case for accurate prediction of enthalpies of formation of large PAHs.