Infrared spectrum of the Ar-NH ₂ ⁺ ionic complex

Rotationally resolved infrared spectra of the ν ₁ and ν ₃ N-H stretching vibrations of the Ar-NH ₂ ⁺ radical ionic complex have been observed by means of photodissociation spectroscopy. The analysis of the rotational structure shows that the complex has a ³ Σ ^- ground 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 ν ₁ and ν ₃ bands were determined as 2803.65(2) and 3287.36(2) cm ^-1 , and the frequency of the intermolecular stretch vibration, ν _s , 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 NH ₂ ⁺ in its electronic ground state are not changed upon Ar complexation. The calculated properties of the intermolecular bond of the complex (D _e = 1773 cm ^-1 , R ^Ar -H ∼ 2.01 Å, ν _s ∼ 185 cm ^-1 ) and the predicted complexation induced frequency shifts for ν ₁ and ν ₃ are in good agreement with the experimental results.