Revisiting CN ⁻ Formation Mechanisms in Electron Collisions with Benzonitrile

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.