Nitrogen-containing Polycyclic Aromatic Hydrocarbons Research Articles

Unraveling the impact of acrylonitrile on naphthyl radical during the growth process of polycyclic aromatic hydrocarbons

The introduction of ammonia effectively inhibits soot formation through its chemical interactions, partly attributable to the effects exerted by CN species. This study aims to investigate the influence of larger CN species, specifically acrylonitrile (C3H3N), identified in NH3-doped flames, on the growth process of polycyclic aromatic hydrocarbons (PAHs). The potential energy surfaces (PESs) governing the reaction between C3H3N and the naphthyl radical were explored utilizing G3(MP2, CC)//B3LYP/6-311G++(d,p) level theory. The formed C3H3N adducts exhibit the potential to generate PAHs featuring CH=CHCN side chain or nitrogen-containing PAHs (N-PAHs). The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was employed to calculate temperature- and pressure-dependent rate coefficients, enabling the assessment of yield distribution across the temperature range of 300–2500 K. In the 1-naphthyl + C3H3N reaction at 1 atm, the preferential formation of acenaphthylene-1-carbonitrile occurs at 300–1600 K, shifting towards PAHs with CH=CHCN side chains at temperatures exceeding 1600 K. The predominant production of PAHs with CH=CHCN side chains occurs in the 2-naphthyl + C3H3N reaction across all temperature ranges. These findings enhance our understanding of the mechanisms involving CN species and PAHs, significantly advancing the study of how intricate mechanisms influence PAH growth in NH3-doped flames.

Machine learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons

ABSTRACT Machine learning feature importance calculations are used to determine the molecular substructures that are responsible for mid- and far-infrared (IR) emission features of neutral polycyclic aromatic hydrocarbons (PAHs). Using the extended-connectivity fingerprint as a descriptor of chemical structure, a random forest model is trained on the spectra of 14 124 PAHs to evaluate the importance of 10 632 molecular fragments for each band within the range of 2.761 to $1172.745\, \mu$m. The accuracy of the results is confirmed by comparing them with previously studied unidentified infrared emission (UIE) bands. The results are summarized in two tables available as Supplementary Data, which can be used as a reference for assessing possible UIE carriers. We demonstrate that the tables can be used to explore the relation between the PAH structure and the spectra by discussing about the IR features of nitrogen-containing PAHs and superhydrogenated PAHs.

Low-temperature nitrogen-bearing polycyclic aromatic hydrocarbon formation routes validated by infrared spectroscopy

Polycyclic aromatic hydrocarbons (PAHs) are abundant in many regions of the Universe, representing a major reservoir for cosmic carbon. However, their formation pathways in cold regions of space remain elusive. Recent astronomical detections show that current astrochemical models drastically underestimate the abundance of aromatic molecules and suggest that additional formation pathways such as ion–molecule reactions need to be considered. Here we reveal efficient low-temperature formation pathways towards nitrogen-containing PAHs via exothermic pyridine+ and acetylene ion–molecule reactions. The experimental approach combines kinetics with spectroscopic probing and unambiguously identifies key reaction intermediates and the final nitrogen-containing PAH product quinolizinium+, a structure that is thought to contribute to the 6.2 μm interstellar emission feature. This study not only provides competing formation pathways relevant in the chemistry of the interstellar medium and Titan’s atmosphere, but also delivers information to verify in-silico potential energy surfaces, astrochemical models and infrared observations.

Rotational and Vibrational Spectra of the Pyridyl Radicals: A Coupled-Cluster Study.

Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm-1 were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron-spin, spin-spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.

Effects of ammonia addition on soot formation in ethylene laminar premixed flames

This paper presents particle size distributions and chemical compositions of soot measured using a Scanning Mobility Particle Sizer (SMPS) and a Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS), respectively, in flames of ethylene-oxygen and ethylene-oxygen doped with ammonia. The inhibitory effect of ammonia addition on soot formation in ethylene flames, as well as the influence on soot particle size are discussed. The results of particle size distributions showed that doping with ammonia decreases soot particle size and the volume fraction of soot particles. In this work, the flames were carefully studied under similar temperature conditions, so all observed changes could be attributed to chemical effects. The results of FTICR-MS analysis showed that polycyclic aromatic hydrocarbons (PAHs) were the main compositions in both pure ethylene and ammonia doped ethylene flames. Nitrogen-containing PAHs (N-PAHs) did not contribute significantly to soot nucleation or mass growth processes. Chemical kinetic analysis revealed that CN species generated in ethylene-ammonia flames removed carbon from participating in soot precursor formation, thereby reducing soot formation.

Synthesis and Self-Assembly Behavior of Double Ullazine-Based Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic azomethine ylides (PAMY, 1) are versatile building blocks for the bottom-up synthesis of nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs). Although the chemistry of PAMY was already established few years ago, the cycloaddition of a double PAMY building block has not been reported so far. In this work, we demonstrate the first cycloaddition of a PAMY-dimer (6), which opens the access to three different alkyl ester-substituted N-PAHs with a laterally extended double ullazine scaffold (DU-1, DU-2 and DU-3). Interestingly, the cyclic voltammetry of DU-1–3 exhibited three reversible oxidation waves, which confirmed the electron-rich nature of the double ullazine scaffold. Furthermore, in situ spectroelectrochemistry study of ethylhexyl ester-substituted DU-3 revealed the formation of different cationic species with new absorption bands up to 1689 nm. Additionally, the influence of the attached substituents on the film formation and supramolecular organization in the thin films was investigated by polarized optical microscopy and grazing incidence wide-angle X-ray scattering.

Robust Cobalt Catalyst for Nitrile/Alkyne [2+2+2] Cycloaddition: Synthesis of Polyarylpyridines and Their Mechanochemical Cyclodehydrogenation to Nitrogen-Containing Polyaromatics*.

The transition-metal-catalyzed [2+2+2] cycloaddition of nitriles and alkynes is an established synthetic approach to pyridines; however, these cycloadditions often rely on the use of tethered diynes or cyanoalkynes as one of the reactants. Thus, examples of efficient, fully intermolecular catalytic [2+2+2] pyridine synthesis, especially those employing unactivated nitriles and internal alkynes leading to pentasubstituted pyridines, remain scarce. Herein, we report on simple and inexpensive catalytic systems based on cobalt(II) iodide, 1,3-bis(diphenylphosphino)propane, and Zn that promote [2+2+2] cycloaddition of various nitriles and diarylacetylenes for the synthesis of a broad range of polyarylated pyridines. DFT studies support a reaction pathway involving oxidative coupling of two alkynes, insertion of the nitrile into a cobaltacyclopentadiene, and C-N reductive elimination. The resulting tetra- and pentaarylpyridines serve as precursors to hitherto unprecedented nitrogen-containing polycyclic aromatic hydrocarbons via mechanochemically assisted multifold reductive cyclodehydrogenation.

Hydrophobic Organic Components of Ambient Fine Particulate Matter (PM2.5) Associated with Inflammatory Cellular Response.

Nowadays, knowledge regarding component-specific inflammatory effect of fine particulate matter (PM2.5) is limited. In this study, an omics approach based on time-of-flight mass spectrometry was established to identify the key hydrophobic components of PM2.5 associated with pro-inflammatory cytokines released by macrophages after in vitro exposure. Of 764 compounds, 62 components were robustly screened with firmly identified 37 specific chemicals. In addition to polycyclic aromatic hydrocarbons (PAHs) and their methylated congeners, novel oxygen- and nitrogen-containing PAHs and, especially, oxygenated PAHs (Oxy-PAHs) were identified. Interleukin (IL)-6 was associated with Oxy-PAHs of 1,8-naphthalic anhydride, xanthone, and benzo[h]quinolone, especially, whereas IL-1β and tumor necrosis factor (TNF)-α were associated with most species. Most species were related to IL-1β, which was significantly higher in the heating season, with a monotonic dose-response pattern mainly for Oxy-PAHs and a U-shaped dose-response pattern for primary species. On the basis of the identified components, four sources of pollution (coal combustion, traffic emissions, biomass burning, and secondary formation, traced by Oxy-PAHs such as 1,8-naphthalic anhydride and quinones) were resolved by the positive matrix factorization model. TNF-α was associated with primary sources, whereas IL-1β and IL-6 were associated with both primary and secondary sources, suggesting different inflammatory effects between primary and secondary sources when assessing the toxicity-driven disparities of known and unknown PM2.5 components.

Use of in vitro bioassays to facilitate read-across assessment of nitrogen substituted heterocycle analogues of polycyclic aromatic hydrocarbons

Nitrogen-containing polycyclic aromatic hydrocarbons (PANHs or azaarenes) are compounds structurally similar to PAHs (carbon substituted by a nitrogen) reported to occur at low levels in food. Although limited, literature may suggest possible higher toxicity than for PAHs. Using a battery of in vitro assays, the toxicological properties of uncharacterized PANHs of increasing ring number were compared to those of characterized structural PAH analogues. The parameters measured covered key events relevant to the AOP developed for Benzo(a)pyrene: AhR activation, mutagenicity and DNA-damage with and without metabolic activation and endocrine receptors activation/inhibition. There was a strong correlation between the chemical structure and the biological activities of the compounds. AhR activation was the most sensitive parameter with a direct correlation between potency and ring number. The most potent genotoxic chemicals were found amongst the ones with the highest number of ring, and under metabolic activation. Such an approach allowed designing sub-groups based on biological properties in addition to structural similarities. Within a sub-group, toxicological data of tested chemicals may be used to characterize hazard of biologically similar but toxicologically uncharacterized substances. This indicates that in addition to structural properties, in vitro biological data may be useful to conduct read-across.

Polycyclic Aromatic Hydrocarbons Containing A Pyrrolopyridazine Core.

Polycyclic aromatic azomethine ylides (PAMYs) are versatile building blocks for the bottom-up construction of unprecedented nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs). Here, we demonstrate the 1,3-dipolar cycloaddition between PAMY and 1,4-diphenylbut-2-yne-1,4-dione and the subsequent condensation reaction with hydrazine, which led to unique N-PAHs with a phenyl-substituted pyrrolopyridazine core (PP-1 and PP-2). The molecular structures of pristine PP-1 and tert-butyl-substituted PP-2 were verified by NMR spectroscopy and mass spectrometry. Moreover, the structure of PP-2 was unambiguously elucidated by X-ray single crystal analysis. The optoelectronic properties were investigated by solvent-dependent UV-Vis absorption and fluorescence emission spectroscopy as well as cyclic voltammetry. Additionally, density functional theory (DFT) calculations showed that PP-1 and PP-2 exhibit push-pull behavior. Furthermore, in situ EPR/UV-Vis-NIR spectroelectrochemistry allowed the detailed insight into the spectroscopic properties and spin distribution of radical cation species of PP-2.

Synthesis of functionalized nitrogen-containing polycyclic aromatic hydrocarbons and other prebiotic compounds in impacting glycine solutions.

Proteinogenic amino acids can be produced on or delivered to a planet via impacting abiotic sources and consequently were likely present before the emergence of life on Earth. However, the role that these materials played in prebiotic scenarios remains an open question, in part because little is known about the survivability and reactivity of astrophysical organic compounds upon impact with a planetary surface. To this end, we use a force-matched semi-empirical quantum simulation method to study impacts of aqueous proteinogenic amino acids at conditions reaching 48GPa and 3000 K. Here, we probe a relatively unstudied mechanism for prebiotic synthesis where sudden heating and pressurization causes condensation of complex carbon-rich structures from mixtures of glycine, the simplest protein-forming amino acid. These carbon-containing clusters are stable on short timescales and undergo a fundamental structural transition upon expansion and cooling from predominantly sp3-bonded tetrahedral-like moieties to those that are more sp2-bonded and planar. The recovered sp2-bonded structures include large nitrogen containing polycyclic aromatic hydrocarbons (NPAHs) with a number of different functional groups and embedded bonded regions akin to oligo-peptides. A number of small organic molecules with prebiotic relevance are also predicted to form. This work presents an alternate route to gas-phase synthesis for the formation of NPAHs of high complexity and highlights the significance of both the thermodynamic path and local chemical self-assembly in forming prebiotic species during shock synthesis. Our results help determine the role of comets and other celestial bodies in both the delivery and synthesis of potentially significant life building compounds on early Earth.

Helical Ullazine-Quinoxaline-Based Polycyclic Aromatic Hydrocarbons.

Polycyclic aromatic azomethine ylides (PAMYs) are powerful building blocks in the bottom-up synthesis of internally nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) through 1,3-cycloaddition reactions. In this work, the cycloaddition reaction of PAMYs to asymmetric ortho-quinones is presented, which, in contrast to the addition to symmetric para-quinones, facilitates subsequent condensation reactions and allows the synthesis of three helical N-PAHs with ullazine-quinoxaline (UQ-1-3) backbones. UQ-1 and UQ-2 possess two helical centers; however, single-crystal X-ray analysis together with the computational modeling of UQ-3 elucidate the formation of only the thermodynamically most stable geometry with four helical centers in a (P,P,M,M) configuration. For the series UQ-1-3, the number of redox steps is directly correlated with the number of ullazine or quinoxaline units incorporated into the targeted molecular backbones. A detailed investigation of the spectroscopic and magnetic properties of the radical cation and anion as well as the dication and dianion species by in situ EPR/UV/Vis-NIR spectroelectrochemistry is provided. The excellent optical and redox properties combined with helical geometries render them possibly applicable as chiral emitter or ambipolar charge transport material in organic electronics.

Infrared spectra of 3-hydroxy-(1H)-pyridinium cation and 3-hydroxy-(1H)-pyridinyl radical isolated in solid para-hydrogen.

As pyridine and its derivatives are regarded as building blocks of nitrogen-containing polycyclic aromatic hydrocarbons, spectral identifications of their protonated and hydrogenated species are important. The infrared (IR) absorption spectra of the 3-hydroxy-(1H)-pyridinium cation, 3-C5H4(OH)NH+, and the 3-hydroxy-(1H)-pyridinyl radical, 3-C5H4(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-C5H4(OH)N, and para-H2 to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-C5H4(OH)NH+ at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm-1 and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-C5H4(OH)NH at 3646.2, 3493.4, 3488.7, 1546.7, 1349.6, 1244.1, 1209.1, 1177.3, 979.8, and 685.2 cm-1 and nine weaker ones increased. The intensities of lines of trans-3-C5H4(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-C5H4(OH)NH+ remained unchanged upon irradiation at 370, 450, and 520 nm. Observed vibrational wavenumbers and relative intensities of these species agree satisfactorily with the scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. The observed 3-C5H4(OH)NH+ cation and 3-C5H4(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

Titanocene-Mediated Dinitrile Coupling: A Divergent Route to Nitrogen-Containing Polycyclic Aromatic Hydrocarbons.

A general synthetic strategy for the construction of large, nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) is reported. The strategy involves two key steps: (1) a titanocene-mediated reductive cyclization of an oligo(dinitrile) precursor to form a PAH appended with di(aza)titanacyclopentadiene functionality; (2) a divergent titanocene transfer reaction, which allows final-step installation of one or more o-quinone, diazole, or pyrazine units into the PAH framework. The new methodology enables rational, late-stage control of HOMO and LUMO energy levels and thus photophysical and electrochemical properties, as revealed by UV/vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. More generally, this contribution presents the first productive use of di(aza)metallacyclopentadiene intermediates in organic synthesis, including the first formal [2 + 2 + 2] reaction to form a pyrazine ring.

Impact of nitrogen-polycyclic aromatic hydrocarbons on phenanthrene and benzo[a]pyrene mineralisation in soil

When aromatic hydrocarbons are present in contaminated soils, they often occur in mixtures. The impact of four different (3-ring) nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) on 12/14C-phenanthrene and 12/14C-benzo[a]pyrene (B[a]P) mineralisation in soil was investigated over a 90 d incubation period. The results revealed that both 12/14C-phenanthrene and 12/14C-benzo[a]pyrene showed no significant mineralisation in soils amended with 10mgkg –1 and 100mgkg –1 N-PAHs (p>0.05). However, increases in lag-phases and decreases in the rates and extents of mineralisation were observed, over time. Among the N-PAHs, benzo[h]quinoline impacted 14C-phenanthrene mineralisation with extended and diauxic lag phases. Furthermore,12/14C-B[a]P and 14C-benzo[a]pyrene–nitrogen-containing polycyclic aromatic hydrocarbons (14C-B[a]P-N-PAHs) amended soils showed extensive lag phases (> 21 d); with some 14C-B[a]P-N-PAH mineralisation recording <1% in both concentrations (10mgkg –1 and 100mgkg –1), over time. This study suggests that the presence of N-PAHs in contaminated soil may impact the microbial degradation of polycyclic aromatic hydrocarbons (PAHs) and the impact was most likely the result of limited success in achieving absolute biodegradation of some PAHs in soil.

Evaluation of toxic activities of nitrogen-containing polycyclic aromatic hydrocarbons PANH compared with their PAH analogues using in vitro cell-based bioassays

Simplification of highly complex mixtures such as crude oil by using chromatographic methods makes it possible to get more detailed information about the composition of the analyte. Separation by argentation chromatography can be achieved based on the interaction of different strength between the silver ions (Ag+) immobilized through a spacer on the silica gel surface and the π-bonds of the analytes. Heavy crude oils contain compounds with a high number of heteroatoms (N, O, S) and a high degree of unsaturation thus making them the perfect analyte for argentation chromatography. The direct coupling of argentation chromatography and ultrahigh-resolution mass spectrometry allows to continuously tracking the separation of the many different compounds by retention time and allows sensitive detection on a molecular level. Direct injection of a heavy crude oil into a ultrahigh-resolution mass spectrometer showed components with DBE of up to 25, whereas analytes with DBE of up to 35 could be detected only after separation with argentation chromatography. The reduced complexity achieved by the separation helps increasing the information depth.

Effects of Single, Binary and Quinary Mixtures of Phenanthrene and Its N-PAHs on Eisenia fetida in Soil

It is now acknowledged that aromatic hydrocarbons present in contaminated soils occur in mixtures. The effect of single, binary and quinary mixtures of phenanthrene and selected nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) were investigated on the survival, growth and behavioural index of earthworms (Eisenia fetida) over a 21-day incubation in soil. The results showed that the LC50 values ranged from (not detected) ND–329.3 mg kg−1 (single mixture), ND–219.8 mg kg−1 (binary mixtures) to 148.4 mg kg−1 (quinary mixture), while the EC50 values (based on weight loss) ranged from 13.3–148.4 mg kg−1 (single mixture), 63.8–148.4 mg kg−1 (binary mixture) to 24.2 mg kg−1 (quinary mixture). Greater impacts were recorded where N-PAHs are present with phenanthrene. Further, behavioural index of E. fetida was affected after 24-h exposure to N-PAH-amended soils. Among the N-PAHs however, benzo[h]quinoline recorded the greatest impact on the survival, growth and behavioural index of E. fetida in soil. Findings from this study showed that three ring-N-PAHs are more toxic than phenanthrene as expected from their physico-chemical properties. The binary and quinary mixtures of phenanthrene and N-PAHs in soil intensified toxicity, suggesting that PAHs-N-PAHs mixtures represent greater risk to soil biota.

Quinoline Inhibiting of Hydrodesulfurization and Hydrogenation Reactions over Co(Ni)PMo(W)/Al2O3 Catalysts: Influence of the Active Phase Composition on the Stability during Hydrotreatment of Model and Oil Feedstock

Co(Ni)-PMo(W)/Al 2 O 3 catalysts were prepared using Keggin heteropoly acids H 3 PMo(W) 12 O 40 and cobalt (nickel) citrate. Physicochemicalproperties of the catalysts were characterized using low-temperature nitrogen adsorption, XPS and HRTEM techniques. Catalytic properties were studied during hydrotreatment of a model feedstock containing dibenzothiophene, naphthalene and quinoline in different proportions (up to 1000 ppm of nitrogen), as well as during hydrotreatment of straight-run diesel fraction and vacuum gasoil. The composition of Со(Ni)-PMo(W)/Al 2 O 3 catalysts was shown to play a crucial role in the hydrotreatment of complex hydrocarbon feedstock. The Ni-PW/Al 2 O 3 catalyst was more stable to organonitrogen inhibitors than Ni(Co)-PMo/Al 2 O 3 bearing more reactive active centers. The highest conversions of sulfur- and nitrogen-containing compounds and polycyclic aromatic hydrocarbons during hydrotreatment of vacuum gasoil were observed with the Ni-PW/Al 2 O 3 catalyst.

Polycyclic heteroaromatic hydrocarbons containing a benzoisoindole core

By the combination of 9a-azaphenalene and a perpendicularly oriented acene, we have synthesized three derivatives of a series of novel, fully-conjugated nitrogen-containing polycyclic aromatic hydrocarbons (PAHs).

The Influence of Metal Ion Binding on the IR Spectra of Nitrogen-Containing PAHs.

Astronomical IR emission spectra form the basis for the now widely accepted abundant presence of polycyclic aromatic hydrocarbons (PAHs) in inter- and circumstellar environments. A small but consistent frequency mismatch is found between the astronomically observed emission band near 6.2 μm and typical CC-stretching vibrations of PAHs measured in laboratory spectra near 6.3-6.4 μm. The shift of the band has been tentatively attributed to a variety of effects, among which the inclusion of heteroatoms, in particular nitrogen, in the PAH skeleton (PANH) as well as to metal ion binding to the PAH molecule. Here we experimentally investigate the combined effect of nitrogen-inclusion and metal ion binding on the IR spectra. In particular, infrared multiple-photon dissociation (IRMPD) spectra are recorded for coordination complexes of Cu+ with one or two quinoline, isoquinoline, and acridine ligands; complexes of the form Cu+(PANH) (MeCN), where the MeCN (acetonitrile) ligand acts as a relatively weakly bound "messenger" are also recorded to qualitatively verify that potential frequency shifts induced by IRMPD are minimal. The experimental IR spectra document the accuracy of IR spectral predictions by density functional theory calculations performed at the B3LYP/6-311+G(2df,2p) level and confirm that a σ-bond is formed between the copper ion and the exoskeletal N atom. The experimental spectra suggest that the CNC stretching mode undergoes a small red shift of up to 20 cm-1, with respect to the band position in the uncomplexed PANH molecule, away from the 6.2 μm interstellar position.