NH(4)(+) Resides Inside the Water 20-mer Cage As Opposed to H(3)O(+), Which Resides on the Surface: A First Principles Molecular Dynamics Simulation Study

Abstract

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 NR4+(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 NR4+(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+.