Formation of the acenaphthylene cation as a common C2H2-loss fragment in dissociative ionization of the PAH isomers anthracene and phenanthrene

Exploring the Scaling Factors for Infrared Modes of PANHs - A Case Study on Cationic 3-Azafluoranthene⋅+ and Protonated 3-Azafluoranthene

Infrared (IR) emission bands by interstellar polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocycles (PANHs) are observed towards a large variety of interstellar objects and offer detailed insights into the chemistry and physics of the interstellar medium. The analysis of the emission bands, and thus the interpretation of the molecular characteristics of the carriers, heavily relies on the use of density functional theory (DFT) calculated IR spectra. However, there are significant challenges in accurately predicting the experimental IR band positions, particularly for PANH emission vibrational modes around 6 μm. In this work, we present gas-phase mid-infrared (mid-IR) spectra of cationic 3-azafluoranthene (3AF⋅+) and protonated 3-azafluoranthene (3AFH+) to investigate their experimental IR band positions in relation to DFT calculated bands. The experimental spectra are compared to DFT simulated spectra, where different approaches were followed to correct for anharmonicities. The best agreement is achieved by scaling frequencies of modes with large nitrogen displacements with a different factor. Even though our findings might be limited to a small number of PANH structures, they indicate, that nitrogen atom incorporation needs to be accounted for by carefully adjusting the corresponding scaling factors while computing IR spectra of PANHs on DFT level.

Photofragmentation of corannulene (C20H10) and sumanene (C21H12) cations in the gas phase and their astrophysical implications

Aromatic infrared bands (AIBs) dominate the mid-infrared spectra of many galactic and extragalactic sources. These AIBs are generally attributed to fluorescence emission from aromatic molecules. Unified efforts from experimentalists and theoreticians to assign these AIB features have recently received additional impetus with the launch of the James Webb Space Telescope (JWST) as the Mid-InfraRed Instrument (MIRI) delivers a mid-IR spectrum with greatly increased sensitivity and spectral resolution. PAHs in space can exist in either neutral or ionic form, absorb UV photons and undergo fragmentation, becoming a rich source of small hydrocarbons. This top-down mechanism of larger PAHs fragmenting into smaller species is of utmost importance in photo-dissociation regions (PDR) in space. In this work, we experimentally and theoretically investigate the photo-fragmentation pathways of two astronomically significant PAH cations - corannulene (C20H10) and sumanene (C21H12), which are structural motifs of fullerene C60, to understand their sequential fragmentation pathways. The photo-fragmentation experiments exhibit channels that are significantly different from planar PAHs. The breakdown of the carbon skeleton is found to follow different pathways for C20H10 and C21H12 because of the number and positioning of pentagon rings, yet the most abundant low mass cations produced by these two species are found to be similar. The low mass cations showcased in this work could be of interest due to their possible astronomical detections. For completeness, the qualitative photofragmentation behaviour of dicationic corannulene and sumanene has also been investigated, but the potential energy surface of these dications is beyond the scope of this paper.

Laser-induced fragmentation of coronene cations

Polycyclic aromatic hydrocarbons are an important component of the interstellar medium of galaxies and photochemistry plays a key role in the evolution of these species in space. Here, we explore the photofragmentation behaviour of the coronene cation (C24H12˙+) using time-of-flight mass spectrometry. The experiments show photodissociation fragmentation channels including the formation of bare carbon clusters (Cn˙+) and hydrocarbon chains (CnHx+). The mass spectrum of coronene is dominated by peaks from C11˙+ and C7H+. Density functional theory was used to calculate relative energies, potential dissociation pathways, and possible structures for relevant species. We identify 6-6 → 5-7 ring isomerisation as a key step in the formation of both the bare carbon clusters and the hydrocarbon chains observed in this study. We present the dissociation mechanism outlined here as a potential formation route for C60 and other astrochemically relevant species.

Photoprocessing of cationic triazacoronene: dissociation characteristics of polycyclic aromatic nitrogen heterocycles in interstellar environments

Polycyclic aromatic nitrogen heterocycles (PANHs) are present in various astronomical environments where they are subjected to intense radiation. Their photodissociation pathways give crucial insights into the cycle of matter in the universe, yet so far only the dissociation characteristics of few PANHs have been investigated. Moreover, most experiments use single photon techniques that only reveal the initial dissociation step, and are thus unsuited to replicate astronomical environments and timescales. In this work, we use the Instrument for the Photodynamics of PAHs (i-PoP) at the Laboratory for Astrophysics to simulate the interstellar photodissociation of a model PANH, cationic triazacoronene (TAC˙+, C21H9N3). Comparing the observed fragments to similar PAHs such as the isoelectronic coronene can give mechanistic insight into PAH dissociation. For coronene the major photodissociation products were found to be C9H+, C10+, and C11+. In contrast, fragmentation in TAC˙+ is initiated by up to three HCN losses often in combination with H- or H2 losses. In the lower mass region, the fragments show similarities to comparable PAHs like coronene, but for TAC˙+ the inclusion of nitrogen atoms into the ionic fragments in the form of e.g. (di)cyanopolyynes is also observed. These nitrogen-containing species may be important tracers of regions in interstellar space where interstellar PANHs are being photodissociated.

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

Ionic fragmentation products of benzonitrile as important intermediates in the growth of polycyclic aromatic hydrocarbons

In various astronomical environments such as the interstellar medium or (exo)planetary atmospheres, an interplay of bottom-up growth and top-down destruction processes of (polycyclic) aromatic hydrocarbons (PAHs) takes place. To get more insight into the interplay of both processes, we disentangle the fragmentation and formation processes that take place upon dissociative ionization of benzonitrile. We build on previous spectroscopic detections of the ionic fragmentation products of benzonitrile and use these as reactants for low-temperature bottom-up ion-molecule reactions with acetylene. By combining kinetics and infrared action spectroscopy, we reveal exothermic pathways to various (polycyclic) aromatic molecules, including the pentalene and phenylacetylene radical cations. We determine the reaction rate coefficients and unambiguously assign the structures of the reaction products. The data is supplemented by potential energy surface calculations and the analysis of non-covalent interactions. This study shows the unexpected formation of a linked four- and six-membered ring structure (phenylcyclobutadiene radical cation) with molecular formula C10H8˙+, and not the commonly observed isomer naphthalene˙+. All observed reactions proceed via radiative association processes and are relevant for the chemistry in (cold) astrochemical environments.

Fingerprinting fragments of fragile interstellar molecules: dissociation chemistry of pyridine and benzonitrile revealed by infrared spectroscopy and theory

The cationic fragmentation products in the dissociative ionization of pyridine and benzonitrile have been studied by infrared action spectroscopy in a cryogenic ion trap instrument at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. A comparison of the experimental vibrational fingerprints of the dominant cationic fragments with those from quantum chemical calculations revealed a diversity of molecular fragment structures. The loss of HCN/HNC is shown to be the major fragmentation channel for both pyridine and benzonitrile. Using the determined structures of the cationic fragments, potential energy surfaces have been calculated to elucidate the nature of the neutral fragment partner. In the fragmentation chemistry of pyridine, multiple non-cyclic structures are formed, whereas the fragmentation of benzonitrile dominantly leads to the formation of cyclic structures. Among the fragments are linear cyano-(di)acetylene˙+, methylene-cyclopropene˙+ and o- and m-benzyne˙+ structures, the latter possible building blocks in interstellar polycyclic aromatic hydrocarbon (PAH) formation chemistry. Molecular dynamics simulations using density functional based tight binding (MD/DFTB) were performed and used to benchmark and elucidate the different fragmentation pathways based on the experimentally determined structures. The implications of the difference in fragments observed for pyridine and benzonitrile are discussed in an astrochemical context.

Cooling dynamics of energized naphthalene and azulene radical cations

Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az+) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording "molecular cloud in a box" conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ ⇌ Az+ quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az+ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D0 ← D2 transition, efficiently quenches dissociation for vibrational energies up to ≈1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation.