Observations of magic numbers in gas phase hydrates of alkylammonium ions

Hydration of alkylammonium ions under nonanalytical electrospray ionization conditions has been found to yield cluster ions with more than 20 water molecules associated with the central ion. These cluster ion species are taken to be an approximation of the conditions in liquid water. Many of the alkylammonium cation mass spectra exhibit water cluster numbers that appear to be particularly favorable, i.e., ''magic number clusters'' (MNC). We have found MNC in hydrates of mono-and tetra-alkyl ammonium ions, NH3(CmH2m+1)(+)(H2O)(n), m = 1-8 and N(CmH2m+1)(4)(+) (H2O)(n), m = 2-8. In contrast, NH2(CH3)(2)(+)(H2O)(n), NH(CH3)(3)(+)(H2O)(n), and N(CH3)(4)(+)(H2O)(n) do not exhibit any MNC. We conjecture that the structures of these magic number clusters correspond to exohedral structures in which the ion is situated on the surface of the water cage in contrast to the widely accepted caged ion structures of H3O+(H2O)(n) and NH4+(H2O)(n).