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 (nu(3)) of the He-HNH+ open-shell ionic complex has been recorded. The complexation-induced frequency red shift, Delta nu(3) = 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 Angstrom. Ab initio calculations performed at the UMP2 level confirm that the quasi-linearity and diradical character of the nitrenium ion in its (3)Sigma(g)(-), electronic ground state are not changed upon He complexation. The calculated properties of the intermolecular bond (D-e = 273 cm(-1), R-e = 1.82 Angstrom) and the frequency shift Delta nu(3) = 13.5 cm(-1) are in good agreement with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.