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, NH4+(NH3)n (for n = 1-10), have been observed from 2600 to 4000 cm-1. The spectra show vibrational features that have been assigned to modes involving both the ion core species, NH4+, and the first shell NH3 solvent molecules. Nearly free internal rotation of the solvent molecules about their local C3 axes in the first solvation shell has been observed int he smaller clusters (n = 1-6). For the largest 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-1 region is quite similar to that of liquid ammonia, and the entire region of 2600-3500 cm-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 NH4+(NH3)8.