Infrared spectrum and ab initio calculations of the He–HNH+ open-shell ionic complex

The infrared photodissociation spectrum of the asymmetric N–H stretch vibration (ν3) of the He–HNH+ open-shell ionic complex has been recorded. The complexation-induced frequency red shift, Δν3=7.0 cm−1, and the rotational structure of the transition are consistent with a quasi-linear geometry of the complex in the electronic ground state, with a vibrationally averaged intermolecular bond length of 1.90 Å. Ab initio calculations performed at the UMP2 level confirm that the quasi-linearity and diradical character of the nitrenium ion in its 3Σ−g electronic ground state are not changed upon He complexation. The calculated properties of the intermolecular bond (De=273 cm−1, Re=1.82 Å) and the frequency shift Δν3=13.5 cm−1 are in good agreement with experiment.