The structure and vibrational frequencies of the ArH3+ ion and its isotopomers

There is much experimental interest in weakly bound cluster ions. Here we present the first ab initio study of ArH₃⁺, ArD₃⁺, ArHD₂⁺, and ArDH₂⁺. Structures and harmonic frequencies are reported at the MP2 level of theory using large basis sets. SCF anharmonic corrections are also reported. CISD and CPF calculations with the smallest of the basis sets used in this study are used to refine further the MP2 harmonic frequencies. The most intense infrared vibrations are the ν₂ (a₁, H₃⁺ bend) and ν₃ (a₁, intermolecular Ar…H stretch), for which our best predictions of the fundamental vibrations obtained by combining MP2 harmonic and SCF anharmonic corrections are 1819 and 395 cm⁻¹.