Sequential dissociation of ionized benzonitrile: New pathways to reactive interstellar ions and neutrals

¹ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
² CEFITEC, Departamento de Física, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
³ Institute of Chemistry, Hybrid Nanostructures, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476, Potsdam, Germany
⁴ Instituto de Física, Universidade de São Paulo, Rua do Matão 1731, 05508-090, São Paulo, Brazil

^★ Corresponding author; sam.eden@open.ac.uk

Received: 29 February 2024
Accepted: 6 December 2024

Abstract

Since benzonitrile’s discovery in the interstellar medium (ISM) in 2018, several studies have explored the strongest unimolecular dissociations of its radical cation (C₆H₅CN^•+). However, sequential dissociation processes, which become important when ionization occurs with significant excess energy transfer, have received almost no attention to date. The present metastable dissociative ionization experiments reveal 14 different dissociations, of which 11 have never been observed before. Nine of these new reactions involve the dissociation of a fragment ion. A notable result shows that C₄H₂^•+ production (the second most intense fragment ion in conventional mass spectra without metastable dissociation analysis) derives from sequential dissociation via C₆H₄^•+, as well as from the previously reported unimolecular dissociation of C₆H₅CN^•+. Furthermore, our experiments demonstrate new pathways that produce astrochemically important neutrals including HCN/CNH and CN^•, as well as revealing CH^• and C₃H^• production from ionized benzonitrile for the first time. In addition to the metastable dissociation experiments, we applied density functional theory to calculate two sequential dissociation routes and report the results of our detailed analysis of the peak shapes in a conventional mass spectrum of benzonitrile. The latter enabled the dominant ion to be identified in peaks with nearest-integer m/z values that match two conceivable ions. The present identification of C₆H₂N⁺ production using this approach allows its presence in the ISM to be inferred for the first time. This paper extends our understanding of how the dissociative ionization of benzonitrile can contribute to the abundances of radicals and other reactive species in interstellar environments.