Mid-infrared characterization of the NH4+•(H2O)n clusters in the neighborhood of the n=20 "magic" number -: art. no. 164309

Vibrational predissociation spectra are reported for size-selected NH(4)(+)center dot(H2O)(n) clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH(4)(+)center dot(H2O)(n) evolve from a quartet at small sizes (n < 7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H(+)center dot(H2O)(n) clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.(c) 2005 American Institute of Physics. (c) 2005 American Institute of Physics.