NH4+ Resides Inside the Water 20-mer Cage As Opposed to H3O+, Which Resides on the Surface: A First Principles Molecular Dynamics Simulation Study

Experimental vibrational predissociation spectra of the magic NH4+(H2O)20 clusters are close to those of the magic H3O+(H2O)20 clusters. It has been assumed that the geometric features of NH4+(H2O)20 clusters might be close to those of H3O+(H2O)20 clusters, in which H3O+ resides on the surface. Car–Parrinello molecular dynamics simulations in conjunction with density functional theory calculations are performed to generate the infrared spectra of the magic NH4+(H2O)20 clusters. In comparison with the experimental vibrational predissociation spectra of NH4+(H2O)20, we find that NH4+ is inside the cage structure of NH4+(H2O)20 as opposed to on the surface structure. This shows a clear distinction between the structures of NH4+(H2O)20 and H3O+(H2O)20 as well as between the hydration phenomena of NH4+ and H3O+.