A theoretical study on the structure of acetonitrile (CH3CN) and its anion CH3CN− Original Research Article

The electronic and geometrical structures of the CH3CN molecule and its anion CH3CN− are calculated at the UHF, UMP2, UMP4, CASSCF and CASPT2 levels of theory. The anion CH3CN− is confirmed to possess a dipole-bound state. The electron affinity (EA) of CH3CN is computed to be ≈ 6 meV. The potential energy surfaces (PES) of both the neutral and anionic species are computed as functions of the distance between carbon atoms of the CH3 and CN constituents. It is found that the neutral molecule possesses the avoiding crossing between two lowest singlet states having different dissociation limits, namely CH3(doublet) + CN(doublet) and CH3+(singlet) + CN−(singlet). The anionic PES is always lower than the neutral one and possesses an appreciable barrier of ≈ 0.8 eV at the distance range of 2.0–2.5 Å between the CH3 and CN groups. This barrier separates the dipole-bounded and “normal” bound states of the CH3CN− anion. After passing the barrier there is a local minimum at the anion PES with respect to which the adiabatic EA of the neutral CH3CN molecule is negative of ≈ −1.6 eV.