# Revisiting CN− Formation Mechanisms in Electron Collisions with Benzonitrile

## TL;DR

This study investigates how CN− ions form when electrons collide with benzonitrile, revealing key energy thresholds and chemical pathways important for interstellar chemistry.

## Contribution

The paper provides the first G4(MP2) threshold energies and CASPT2 calculations for CN− formation in benzonitrile, revealing resonance coupling mechanisms.

## Key findings

- CN− formation occurs at 3.0 eV and 7–10 eV via π4* and σ* resonance coupling.
- Threshold energies for dissociation channels are reported at the G4(MP2) level for the first time.
- Core excited π4* and σ* resonances contribute strongly to CN− formation in the 7–10 eV range.

## Abstract

Radiation‐induced processes in the aromatic cyano compound benzonitrile have attracted renewed interest since its detection in the interstellar medium in 2018, and recent studies have elucidated dissociative ionization pathways leading to species such as CN• and HCN, which can play important roles in interstellar chemistry. This work explores negative ion formation from benzonitrile upon electron attachment with mass spectrometry experiments and the most extensive theoretical study to date of the underlying negative ion states and their respective dissociative relaxation pathways. The measurements confirm the previously reported CN− formation at a collision energy of 3.0 eV as well as formation of the dehydrogenated parent anion and phenyl anion and CN− formation in the 7–10 eV energy range. Threshold energies for these dissociation channels are reported at the G4(MP2) level of theory for the first time. Furthermore, by using both scattering calculations and bound state techniques, CN− formation at around 3.0 eV may proceed from a 2B1, π4* shape resonance through nonadiabatic coupling with the σ*, C—CN state. In the 7–10 eV range, complete active space plus second‐order perturbation (CASPT2) calculations suggest strong contributions from core excited π4* and σ* resonances.

This study explores negative ion formation via electron attachment, revealing CN− formation at 3.0 and 7–10 eV. Dissociation pathways are analyzed using G4(MP2) and CASPT2 calculations, highlighting the coupling between the π4* and σ* resonances critical for interstellar chemistry.© 2026 WILEY‐VCH GmbH