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Science 21 May 2004:
Vol. 304. no. 5674, pp. 1137 - 1140
DOI: 10.1126/science.1096466
Infrared Signature of Structures Associated with the H+(H2O)n (n = 6 to 27) Clusters
J.-W. Shin,1 N. I. Hammer,1 E. G. Diken,1 M. A. Johnson,1* R. S. Walters,2 T. D. Jaeger,2 M. A. Duncan,2* R. A. Christie,3 K. D. Jordan3*
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.
1 Sterling Chemistry Laboratory, Yale University, Post Office Box 208107, New Haven, CT 06520, USA.
2 Department of Chemistry, University of Georgia, Athens, GA 30602, USA.
3 Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.
-Excerpt:
The nature of the proton in water is one of the most fundamental aspects of aqueous chemistry, and one important aspect of the aqueous proton is its anomalously high mobility (1, 2). This phenomenon immediately introduces the crucial role of H3O+ and H5O+2, the so-called Eigen (3) and Zundel (4) forms of the cation, respectively. Fluctuations between these species (1, 2) are thought to mediate the Grotthuss mechanism (5) for proton transport, and accurate simulations of this process require quantum treatment of the hydrogen motion in the complex network environment of bulk water.
A powerful way to test the validity of various theoretical methods is through the use of the cluster ions (6), H+(H2O)n, which can be prepared and isolated in the laboratory. Here, we report size-selected vibrational spectra of the H+(H2O)n clusters in the intermediate size regime, 6 n 27, chosen to explore the putative role (7–12) of dodecahedral clathrate structures in the region around n = 21. The resulting spectra are analyzed with the aid of calculated structures and vibrational frequencies of selected isomers for the n = 20 and n = 21 clusters.
Protonated water clusters have been studied for decades (3, 4, 7–18), and in the small size regime (n
vibrational spectra have been reported and interpreted with ab initio theory (17). H3O+ itself is C3v pyramidal (13), but adding a second water molecule leads to a symmetrical sharing of the proton in the H2O···H+···OH2 Zundel arrangement (4, 18). Larger protonated water clusters possess multiple low-energy isomers with both Eigen and Zundel forms of the cation, and the complexity of the observed spectra indicate that several isomers are present under experimental conditions. One of the most curious aspects of the H+(H2O)n clusters is Searcy and Fenn's (7) report in 1974 of the discontinuity in the cluster ion intensity distribution or "magic number" at n = 21 (Fig. 1). There has been much speculation about the structure of the magic number cluster, especially because water clathrates are known to trap methane and other gases in water cages composed of water dodecahedrons (19). Indeed, Searcy and Fenn (7) suggested that H+(H2O)21 is also derived from the pentagonal dodecahedron motif, with one water molecule in the cage and the H3O+ ion on the surface.
end of exceprt.
Statistics: Posted by decisive — Mon Jul 24, 2006 1:41 pm
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http://www.psc.edu/science/2005/jordan/
Excerpt:
Better understanding of the “magic number” cluster may help to unleash a major source of untapped energy
Resolving questions about the magic-number cluster, notes Jordan, has implications for a major source of untapped energy. Methane hydrates — a structure that includes dodecahedral cages of water enclosing molecules of methane, aka natural gas. Research in the last decade has found that huge deposits of methane hydrate lie on the ocean floor, and there are major research efforts underway to find how to harness this methane. Jordan’s group is working with four laboratory groups in California looking at relationships between water clusters and the similarly structured gas hydrates.
In 2005, as a follow-up to their 2004 report on the magic-number cluster, Jordan and his collaborators at Georgia and Yale published exciting results involving an extra proton in smaller water clusters, often referred to as the “hydrated proton.” Research over many years identified two competing arrangements. “One,” says Jordan, “is where the proton is associated with a single water molecule, and that gives H3O+ — often called the Eigen form, for the Nobel scientist who proposed it. The other form is with the proton equally shared between two waters — H5O2+, called Zundel.”
With innovative spectroscopic techniques and Jordan’s calculations to interpret the data, the researchers for the first time identified clear spectral signatures for the two structures. By adding water molecules one by one, they found striking shifts in the frequencies — indications of movement back and forth between the Eigen and Zundel forms as well as the importance of structures that are intermediate between these two limiting forms.
“It gives us a handle,” says Jordan, “on how sensitive the spectra are to the environment.” The extra proton is fundamental to the chemistry of acids and this finding, which no one expected, has wide implications. What especially intrigues Jordan is that when water reveals secrets it seems to hint at even deeper ones. “To me that’s the most interesting science — when you discover hidden questions you didn’t anticipate when you started.”
Statistics: Posted by decisive — Mon Jul 24, 2006 1:30 pm
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