Infrared Spectroscopy of Radical-Cation Clusters (NH ₃ ) _{<i>n</i> =4–6} ⁺

Radical-cation clusters of ammonia ((NH₃)_<i>n</i>=4-6⁺) were produced inside helium droplets and studied via infrared laser spectroscopy in the NH stretching region. The clusters have multiple isomers, whose structures were studied with quantum chemical calculations. The comparison of the measured and calculated spectra of the free NH stretches shows that each cluster has an NH₄⁺ core, solvated by NH₃ molecules and an NH₂ radical through hydrogen bonds, as their respective global minima states. The spectra of (NH₃)_<i>n</i>=4-6⁺ contain the arrays of strong bands in the frequency range of 2300-3200 cm^-1, which stem from the interaction between the hydrogen bond bridge vibrations of the ammonium core NH₄⁺ and the overtones of its bending modes. The spectra of (NH₃)₅⁺ and (NH₃)₆⁺ indicate the presence of some additional higher-energy isomers, which have a free NH stretch of the NH₄⁺ core.