A comparative study of anharmonicity and matrix effects on the complexes XH:NH3, X=F, Cl, and Br

Ab initio calculations have been performed to investigate the structures and infrared spectra of the complexes FH:NH3, ClH:NH3, and BrH:NH3, and the effects of the presence of inert gas atoms on structures and spectra. Two-dimensional MP2/6-31+G(d,p) potential energy surfaces were constructed for the complexes XH:NH3, and model two-dimensional Schrödinger equations were solved for the proton stretching and dimer (heavy-atom) stretching modes. Although all complexes have equilibrium structures characterized by traditional hydrogen bonds, their infrared spectra differ significantly. In FH:NH3 both the ground (v=0) and first excited state for the proton stretching mode (v=1) are confined to the potential well describing the equilibrium structure. In this case the harmonic approximation is appropriate, and matrix effects are unimportant. In ClH:NH3 the v=1 proton stretching vibration accesses the more polar, proton-shared region of the potential surface. Here the harmonic treatment leads to a significant overestimation of the experimental proton-stretching frequency. Significant improvement results from an anharmonic treatment, which shows some coupling between proton and dimer modes. The presence of rare gas atoms preferentially stabilizes the proton-shared region of the surface, lowering the energy of the v=1 state. Further improvement results if the MP2/aug’-cc-pVDZ potential energy surface is used. In BrH:NH3 the proton-shared region of the potential surface is accessible in both v=0 and v=1 vibrational states, and an anharmonic treatment is required to obtain reasonable agreement with experiment. In BrH:NH3 proton stretching and dimer stretching modes are highly coupled, and rare gas atoms have structural and spectral effects.