Ultraviolet and vacuum ultraviolet photo-processing of protonated benzonitrile (C₆H₅CNH⁺)

A plausible pathway to larger interstellar aromatics

¹ Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405 Orsay, France
e-mail: ugo.jacovella@universite-paris-saclay.fr
² CNRS, Aix-Marseille Université, Laboratoire PIIM, Marseille, France
e-mail: jennifer.noble@univ-amu.fr
³ Synchrotron SOLEIL, L’Orme des Merisiers, 91192 Saint Aubin, Gif-sur-Yvette, France
⁴ INRAE, UAR1008, Transform Department, Rue de la Géraudière, BP 71627, 44316 Nantes, France
⁵ School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
⁶ Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia

Received: 13 September 2021
Accepted: 24 November 2021

Abstract

Context. The recent detection in pre-stellar sources of cyano-substituted and pure hydrocarbon cycles has emphasized the importance of aromatic chemistry in the earliest stages of star formation. Ultraviolet (UV) and vacuum-UV (VUV) radiation is ubiquitous in space and thus the photo-processing of small cyclic ions may open a window onto rich chemical networks and lead to the formation of larger aromatics in space.

Aims. The aim is to investigate the fate of protonated benzonitrile species after UV and VUV photoexcitation and the subsequent potential impact on stellar and interstellar chemistry.

Methods. Protonated benzonitrile was isolated in a linear ion trap prior to irradiation with UV and VUV radiation (4.5–13.6 eV) from the DESIRS beamline at synchrotron SOLEIL. The study was extended down to 3.5 eV using a cryogenic Paul ion trap coupled to an OPO laser at the PIIM laboratory. Photodissociation action spectra were obtained by monitoring the photofragment yields as a function of photon energy.

Results. The UV/VUV photodissociation action spectra of protonated benzonitrile show structured bands from 3.8 to 9 eV. The primary dissociation channel of protonated benzonitrile corresponds to HCN/HNC loss and formation of the phenylium cation (C₆H₅⁺); whereas at high energies, a minor channel is observed that correlates with HC₃N loss and formation of C₄H₅⁺.

Conclusions. The UV and VUV photodestruction of protonated benzonitrile leads to the formation of a highly reactive cationic species, C₆H₅⁺, predicted to be an important precursor of larger aromatic molecules in space, such as polycyclic aromatic hydrocarbons. The inclusion of C₆H₅⁺ – a precursor of benzene and, by extension, of benzonitrile – as the result of formation via the photodissociation of protonated benzonitrile in current astrochemical models could improve the predicted abundance of benzonitrile, which is currently underestimated.