Mass-Spectrometric Study of Ions at Near-Atmospheric Pressure. II. Ammonium Ions Produced by the Alpha Radiolysis of Ammonia and Their Solvation in the Gas Phase by Ammonia and Water Molecules

Abstract

A new mass-spectrometric method for the study of solvation of ions and of ion complexes is applied to the ammonium ion. The major ion in the α-particle radiolysis of NH3 is the clustered ammonium ion NH4+·nNH3. A study of the distribution of these clusters is made at room temperature and in the pressure range 0.05 to 200 Torr ammonia. Clusters up to n=20 can be observed. The intensity distribution of the ions shows that the inner solvation shell contains four molecules. Approximate free-energy changes for inner- and outer-shell solvation are obtained. Studies of competitive solvation of the NH4+ ion by water and ammonia molecules confirm the inner four shell. Ammonia is preferentially taken up in the inner shell and water in the outer shell. The higher stability of the ammoniated inner shell requires a very small effective NH4+ radius. This requirement leads to an examination of the structure of the NH4+·4NH3 ion.