Infrared Spectroscopy of Radical Cation Clusters (NH₃)₂⁺ and (NH₃)₃⁺

The ionization of protic molecules such as H₂O and NH₃ in the condensed phase initiates ion-molecule reactions, which remain poorly understood. Studies of the structure and reactivity of small ionic clusters in molecular beams have yielded a wealth of information on protonated clusters. However, unprotonated radical cation clusters have a low concentration in a typical experiment and thus remain challenging. Here we report the infrared spectra of the (NH₃)₂⁺ and (NH₃)₃⁺ radical cations solvated in helium nanodroplets. Radical cation clusters often have several isomers with different ionic cores, including proton-transferred and hemibonded structures. Infrared spectra of the cations obtained in this work indicate that the formation of the ammonia dimer ((NH₃)₂⁺) and trimer ((NH₃)₃⁺) cations yields the proton-transferred structures, which correspond to the respective global minima of the calculated structures. Spectral assignments are corroborated by density functional theory calculations. The spectra also indicate that the NH₄⁺ and NH₃ moieties within the clusters undergo internal rotation with rotational constants close to those in the gas phase.