Mid-infrared characterization of the NH <inf>4</inf>⁺ · (H <inf>2</inf> O)n clusters in the neighborhood of the n=20 "magic" number

Vibrational predissociation spectra are reported for size-selected N H4+ middot; (H2 O)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 N H4+ middot; (H2 O)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+ middot; (H2 O)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 N H4+ ion that lies considerably lower in energy than isomers with the N H4+ in the interior. © 2005 American Institute of Physics

Infrared signature of structures associated with the H⁺(H <inf>2</inf>O)<inf>n</inf> (n = 6 to 27) clusters

We report the OH stretching vibrational spectra of size-selected H +(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 ≤ n ≤ 27. A multiplet of such bands appears at small cluster sizes. This pattern simplifies to a doublet at n = 11, with the doublet persisting up to n = 20, but then collapsing to a single line in the n = 21 and n = 22 clusters and reemerging at n = 23. This spectral simplification provides direct evidence that, for the magic number cluster, all the dangling OH groups arise from water molecules in similar binding sites

Chemistry: Spectral signatures of hydrated proton vibrations in water clusters

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment

The vibrational predissociation spectra of the H <inf>5</inf>O <inf>2</inf>⁺·RG <inf>n</inf>(RG=Ar,Ne) clusters: Correlation of the solvent perturbations in the free OH and shared proton transitions of the Zundel ion

Predissociation spectra of the H5 O2+ ·R Gn (RG=Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350-3850 cm-1) and shared proton (850-1950 cm-1) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H5 O2+ [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical H5 O2+ "Zundel" ion remains largely intact in H5 O2+ ·Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H5 O2+ ·Ar, and is dominated by two sharp transitions at 928 and 1047 cm-1, with a weaker feature at 1763 cm-1. The H5 O2+ · Arn, n=1-5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H5 O2+ rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H5 O2+. These results are consistent with assignment of the strong low-energy bands in the H5 O2+ ·Ne spectrum to the vibration of the shared proton mostly along the O-O axis, with the 1763 cm-1 band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules. © 2005 American Institute of Physics