Infrared fingerprints of cationic benzonitrile·+ and its astrophysically important fragmentation pathway to ortho-benzyne·+
The radio-astronomical detection of nitrogen containing aromatic molecules such as neutral benzonitrile and cyano-naphthalene isomers in the Interstellar Medium (ISM) has sparked the interest in the investigation of both fragmentation and formation processes of these (polycyclic) aromatic hetero molecules [1–4]. Using a cryogenic ion trap apparatus coupled to an infrared free electron laser, we are able to structurally identify key ions of astrophysical relevance that take part in these processes [5,6]. By measuring the infrared fingerprint spectra using an action spectroscopic method involving rare-gas tagging, the structural characteristics of the ions can be determined by comparison with calculated infrared frequencies from quantum chemical computations. Here, we present the experimental infrared fingerprint of the benzonitrile radical cation and its major fragment C6H4·+ upon HCN loss. Quantum chemical computations of the infrared spectra of different C6H4·+ isomers enable us to assign the experimental structure to the non-planar ortho-benzyne radical cation. The attached rare-gas tag is shown to have a negligible effect on the majority of the vibrational band positions. The obtained structural information of benzonitrile·+ and its fragment ortho-benzyne·+ will extend the knowledge, and allows to speculate on the variety of structures that are likely to exist in the ISM besides the already detected neutral cyano-substituted cyclic molecules. Moreover, the accurate vibrational band positions can aid infrared observations in relevant astronomical environments such as photodissociation regions (PDRs) and planetary atmospheres.
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