Vibrational spectroscopy of the ammoniated ammonium ions NH4+(NH3)n (n=1-10)

The gas-phase vibration-internal rotation spectra of mass-selected ammoniated ammonium ions, NH{sub 4}{sup +}(NH{sub 3}){sub n} (for n = 1-10), have been observed from 2600 to 4000 cm{sup {minus}1}. The spectra show vibrational features that have been assigned to modes involving both the ion core species, NH{sub 4}{sup +}, and the first shell NH{sub 3} solvent molecules. Nearly free internal rotation of the solvent molecules about their local C{sub 3} axes in the first solvation shell has been observed in the smaller clusters (n = 1-6). For the larger clusters studied (n = 7-10) the spectra converge, with little difference between clusters differing by one solvent molecule. For these clusters, the spectrum in the 3200-3500 cm{sup {minus}1} region is quite similar to that of liquid ammonia, and the entire region of 2600-3500 cm{sup {minus}1} also bears considerable resemblance to the spectra of ammonium salts dissolved in liquid ammonia under some chemical conditions. This indicates the onset of a liquidlike environment for the ion core and first shell solvent molecules in clusters as small as NH{sub 4}{sup +}(NH{sub 3}){sub 8}.