The arrangement of water molecules around a hydrated electron has eluded explanation for more than 40 years. Here we report sharp vibrational bands for small gas-phase water cluster anions, (H2O)(4-6)- and (D2O)(4-6)-. Analysis of these bands reveals a detailed picture of the diffuse electron-binding site. The electron is closely associated with a single water molecule attached to the supporting network through a double H-bond acceptor motif. The local OH stretching bands of this molecule are dramatically distorted in the pentamer and smaller clusters because the excited vibrational levels are strongly coupled to the electron continuum. The vibration-to-electronic energy transfer rates, as revealed by line shape analysis, are mode-specific and remarkably fast, with the symmetric stretching mode surviving for less than 10 vibrational periods [50 fs in (H2O)4-].

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We report the mid-infrared (3200-3800 cm -1 ) vibrational predissociation spectra of the Br -‚W n and I -‚W n (W ) H 2 O; 1 e n e 6) clusters, as well as several mixed solvent cases, I -‚W‚M [M ) Ar, CH 3 I, (CH 3 I) 2 ], involving the iodide monohydrate. While the spectra of the pure monomers and dimers are quite dependent on the halogen, the envelopes become similar by the trimers, with the larger clusters displaying a very wide, unresolved band reminiscent of the bulk water spectrum. There is a general blue shift of the band maxima with increasing solvation in both systems, an effect consistent with the strengthening of the inter-water hydrogen-bonding network at the expense of the ionic hydrogen bonds.

We elucidate the interplay between the ion−water and water−water interactions in determining the structures of halide ion−water clusters using infrared spectroscopy, interpreted with ab initio theory. Vibrational predissociation spectra of the X-·(H2O)2·Ar m (X = F, Cl, Br, I) clusters in the OH stretching region (2300−3800 cm-1) reveal a strongly halide-dependent pattern of bands. These spectra encode the incremental weakening of the interaction between the water molecules with the lighter halides, finally leading to their complete dissociation in the fluoride complex. A consequence of this is that the F-·(H2O)2 cluster is likely to be a floppy system with high amplitude zero point motion, in contrast to the pseudo-rigid behavior of the other halide hydrates.

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We present mid-IR argon predissociation spectra for a series of complexes, M-·H2O (M = CS2 -, OCS-, SO2 -, CH3NO2 -, CH3CO2 -, and NO2 -), chosen to explore how changes in the triatomic binding site affect the H-bonding configuration of the attached water molecule. With the exception of NO2 -, the calculated global minima on the potential surfaces of all of the complexes occur in a configuration where both OH groups are attached to the anion. The observed spectra, on the other hand, fall into three distinct categories. Simple spectra characteristic of the double ionic H-bonding arrangement are observed for the monohydrates of SO2 -, OCS-, and CS2 -, whereas the CH3NO2 -·H2O and CH3CO2 -·H2O spectra are complicated, displaying a progression of closely spaced bands with a broad, bell-shaped envelope beginning several hundred wavenumbers below the calculated fundamentals. Although the spectrum of the NO2 -·H2O complex is the most red-shifted, it is again simple, reflecting the expected asymmetric (single ionic H-bonded) motif. These data indicate that the transition from single to double ionic H-bonding occurs at a critical domain length of about 2.2 Å. We explore the potential surfaces governing the interconversion between the two forms with density functional calculations and construct vibrationally adiabatic potential surfaces to assess the cause of the spectral complexity displayed by the methylated anion hydrates.

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

The matrix-isolated molecular complexes CO /X F (X= Cl,Br,I ) and the molecular structure of FC(O)Br Gas phase infrared spectroscopy of cluster anions as a function of size: The effect of solvation on hydrogenbonding in Br − (HBr) 1,2,3 clusters A comparative study of anharmonicity and matrix effects on the complexes XH:NH 3 , X=F , Cl, and Br An infrared study of the competition between hydrogen-bond networking and ionic solvation: Halide-dependent distortions of the water trimer in the X ؊ -"H 2 O… 3 , "X‫؍‬Cl, Br, I… systemsVibrational spectra of the water trimers solvating the halide anions (Cl Ϫ , Br Ϫ , I Ϫ ) have been acquired in the OH stretching region by predissociation spectroscopy of the X Ϫ •͑H 2 O͒ 3 •Ar 3 complexes. These ''wet'' ions display two groups of bands assigned to normal modes of the (C 3 ) pyramidal structure. We interpret the evolution of the spectra down the halogens in the context of the rings closing up toward the structure of the bare (H 2 O) 3 neutral. This trend is discussed in terms of the disruptive effect of the ionic H bonds on the water network.