Infrared Spectrum of the Ar - NH 2 + Ionic Complex

Rotationally resolved infrared spectra of the ν1 and ν3 N-H stretching vibrations of the Ar-NH2 radical ionic complex have been observed by means of photodissociation spectroscopy. The analysis of the rotational structure shows that the complex has a 3Σground electronic state with a linear or quasi-linear proton-bound structure Ar-H-N-H+ characterized by an intermolecular center of mass separation of 3.085 Å. The origins of the ν1 and ν3 bands were determined as 2803.65(2) and 3287.36(2) cm-1, and the frequency of the intermolecular stretch vibration, νs, as 170.4(6) cm-1. Ab initio calculations performed at the UMP2 level of theory confirm that the quasi-linearity and the diradical character of NH2 in its electronic ground state are not changed upon Ar complexation. The calculated properties of the intermolecular bond of the complex (De ) 1773 cm-1, RAr-H ∼ 2.01 Å, νs ∼ 185 cm-1) and the predicted complexation induced frequency shifts for ν1 and ν3 are in good agreement with the experimental results.