title: Mid-infrared characterization of the NH4 +(H2O)n clusters in the neighborhood of the n=20 "magic" number.
abstract: Vibrational predissociation spectra are reported for size-selected NH4+ (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 NH4+ (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+(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.
authors.affiliation: Sterling Chemistry Laboratory, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, USA.
authors.forename: Eric G
authors.initials: EG
authors.lastname: Diken
authors.forename: Nathan I
authors.initials: NI
authors.lastname: Hammer
authors.forename: Mark A
authors.initials: MA
authors.lastname: Johnson
authors.forename: Richard A
authors.initials: RA
authors.lastname: Christie
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Jordan