Infrared signature of structures associated with the [H.sup.+][ clusters

We report the OH stretching vibrational spectra of size-selected [H.sup.+][([H.sub.2]O).sub.n] clusters through the region of the pronounced "magic number" at n = 21 in the duster distribution. Sharp feature8 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 duster 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 dusters 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.