Gas-phase infrared spectroscopy of the rubicene cation (C₂₆H₁₄^•+)

Abstract

Infrared bands at 3.3, 6.2, 7.6, 7.8, 8.6, and 11.2 μm have been attributed to polycyclic aromatic hydrocarbons (PAHs) and are observed toward a large number of galactic and extragalactic sources. Some interstellar PAHs possibly contain five-membered rings in their honeycomb carbon structure. The inclusion of such pentagon defects can occur during PAH formation, or as large PAHs are eroded by photo-dissociation to ultimately yield fullerenes. Pentagon formation is a process that is associated with the bowling of the PAH plane, that is, the ability to identify PAH pentagons in space holds the potential to directly link PAHs to cage and fullerene structures. It has been hypothesized that infrared (IR) activity around 1100 cm−1 may be a spectral marker for interstellar pentagons. We present an experimentally measured gas-phase IR absorption spectrum of the pentagon-containing rubicene cation (C26H14•+) to investigate if this band is present. The NASA Ames PAH IR Spectroscopic Database is scrutinized to see whether other rubicene-like species show IR activity in this wavelength range. We find that a specific molecular characteristic is responsible for this IR band. Namely, the vibrational motion attributed to this IR activity involves pentagon-containing harbors. An attempt to find this specific mode in Spitzer observations is undertaken and tentative detections around 9.3 μm are made toward the reflection nebula NGC 7023 and the H II-region IRAS 12063-6259. Simulated emission spectra are used to derive upper limits for the contributions of rubicene-like pentagonal PAH species to the IR band at 6.2 μm toward these sources.

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

“…Our study validates this proposal for physical conditions relevant to PDRs and for larger PAHs. It is therefore interesting to study if these species have specific bands that would help identifying them in space (Galué 2014;Bouwman et al 2020). We also remark that both the pyrene and coronene derivatives carrying pentagonal rings that we studied here, show an overall IR activity systematically stronger than their respective parent species, in theoretical vibrational spectra, while they do not show large systematic differences in their calculated photoabsorption spectra at ∼10.5 eV.…”

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

“…We also note that in the AcN˙ + and BP˙ + spectra the bands around 1000 to 1150 cm −1 (8.7–10 μm) correspond to the in-plane C–H bending vibration involving the pentagonal ring. Features in this range were also observed in the IR spectra of rubicene˙ + , 62 corannulene˙ + , 63 and diindenoperylene˙ + 64 and were found to involve the pentagonal structures. At the era of the James Webb Space Telescope, these spectral features can serve as probes for detecting pentagonal carbon-ring containing PAHs in the interstellar medium (ISM).…”

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

“…Our study validates this proposal for physical conditions relevant to PDRs and for larger PAHs. It is therefore interesting to study if these species have specific bands that would help identifying them in space (Galué 2014; Bouwman et al 2020). We also remark that both the pyrene and coronene derivatives carrying pentagonal rings that we studied here, show an overall IR activity systematically stronger than their respective parent species, in theoretical vibrational spectra, while they do not show large systematic differences in their calculated photo-absorption spectra at ∼10.5 eV.…”