Comment on: “The ν3 infrared spectrum of the HeNH4+ complex”

The preference for the proton to form a linear bridge in HeNH4+ is explained in terms of the angle-dependent induction and dispersion energies. These arise from the non-sphericity of the charge distribution and polarisability of NH4+. The long-range energy of interaction, correct to terms in the inverse eighth power of the separation, favours the vertex-bound structure over the face-bound geometry by 19 cm−1. Application to the series RgNH4+, where Rg is an inert gas atom, suggests that the face-bound structure is the global minimum for XeNH4+. The uncharged Ar-SiH4 complex is predicted to have a face-bound global minimum.