Issue 42, 2023

Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile

Abstract

The threshold photoionization and dissociative ionization of benzonitrile (C6H5CN) were studied using double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source (SLS). The threshold photoelectron spectrum was recorded from 9.6 to 12.7 eV and Franck–Condon simulations of ionization into the ionic ground state, [X with combining tilde]+, as well as the [B with combining tilde]+ and [C with combining tilde]+ states were performed to assign the observed vibronic structures. The adiabatic ionization energies of the [X with combining tilde]+, [B with combining tilde]+ and [C with combining tilde]+ states are determined to be (9.72 ± 0.02), (11.85 ± 0.03) and, tentatively, (12.07 ± 0.04) eV, respectively. Threshold ionization mass spectra were recorded from 13.75 to 19.75 eV and the breakdown diagram was constructed by plotting the fractional abundances of the parent ion and ionic dissociation products as a function of photon energy. The seven lowest energy dissociative photoionization channels of benzonitrile were found to yield CN˙ + c-C6H5+, HCN + C6H4˙+, C2H4 + HC5+, HC3N + C4H4˙+, H2C3N˙ + C4H3+, CH2CHCN + C4H2˙+ and H2C4N˙ + c-C3H3+. HCN loss from the benzonitrile cation is the dominant dissociation channel from the dissociation onset of up to 18.1 eV and CH2CHCN loss becomes dominant from 18.1 eV and up. We present extensive potential energy surface calculations on the C6H5CN˙+ surface to rationalize the detected products. The breakdown diagram and time-of-flight mass spectra are fitted using a Rice–Ramsperger–Kassel–Marcus statistical model. Anchoring the fit to the CBS-QB3 result (3.42 eV) for the barrier to HCN loss, we obtained experimental dissociation barriers for the products of 4.30 eV (CN loss), 5.53 eV (C2H4 loss), 4.33 eV (HC3N loss), 5.15 eV (H2C3N loss), 4.93 eV (CH2CHCN loss) and 4.41 eV (H2C4N loss). We compare our work to studies of the electron-induced dissociative ionization of benzonitrile and isoelectronic phenylacetylene (C8H6), as well as the VUV-induced dissociation of protonated benzonitrile (C6H5CNH+). Also, we discuss the potential role of barrierless association reactions found for some of the identified fragments as a source of benzonitrile(˙+) in interstellar chemistry and in Titan's atmosphere.

Graphical abstract: Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile

Supplementary files

Article information

Article type
Paper
Submitted
18 Aug 2023
Accepted
09 Oct 2023
First published
11 Oct 2023

Phys. Chem. Chem. Phys., 2023,25, 29070-29079

Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile

J. Kamer, D. Schleier, M. Donker, P. Hemberger, A. Bodi and J. Bouwman, Phys. Chem. Chem. Phys., 2023, 25, 29070 DOI: 10.1039/D3CP03977C

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