The Quest to Uncover the Nature of Benzonitrile Anion

Anionic states of benzonitrile are investigated by high-level electronic structure methods. The calculations using equation-of-motion coupled-cluster theory for electron-attached states confirm earlier conclusions drawn from the photodetachment experiments that the ground state of the anion is the valence ²B₁ state, while the dipole bound state lies adiabatically ~0.1 eV above. Inclusion of triple excitations and zero-point vibrational energies is important for recovering relative state ordering. The computed Franck–Condon factors and photodetachment cross-sections further confirm that the observed photodetachment spectrum originates from the valence anion. The valence anion is electronically bound at its equilibrium geometry, but is metastable at the equilibrium geometry of the neutral. The dipole-bound state, which is the only bound anionic state at the neutral geometry, may serve as a gateway state for capturing the electron. Thus, the emerging mechanistic picture entails electron capture via dipole bound state, followed by non-adiabatic relaxation forming valence anion.