The ν3 infrared spectrum of the HeNH4+ complex

The mid-infrared spectrum of the ionic complex HeNH4+ has been recorded in the vicinity of the triply degenerate ν3 (t2) vibration of the free ammonium ion. Apart from a small blue shift (≈ 0.7 cm−1), the spectrum of the complex closely resembles that of the monomer. Ab initio calculations predict a vertex-bound minimum structure with an intermolecular well depth De ≈ 150 cm−1, a center-of-mass separation of Re ≈ 3.17 Å and barriers for internal rotation less than 30 cm−1.