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.