Water Cluster Structure Probed

For more than a quarter century, researchers have been studying protonated water clusters in the gas phase in hopes of better understanding atmospheric processes, hydrogen bonding, and other water interactions. But the structure of some protonated water clusters--particularly larger ones with 10 or more water molecules--has been elusive.

Now, the first infrared spectra of such clusters reveal important new details about these intriguing species. The work was carried out by two independent groups: Asuka Fujii, Naohiko Mikami, and coworkers at Tohoku University, Sendai, Japan; and Mark A. Johnson of Yale University, Michael A. Duncan of the University of Georgia, Kenneth D. Jordan of the University of Pittsburgh, and coworkers [Science, published online April 29, http://www.sciencemag.org/cgi/content/abstract/1096037v1 and 1096466v1].

Protonated water clusters with a magic number of water molecules (21) are unusually stable, and they have been a particular focus of interest. They've been thought to be roughly spherical structures with 20 water molecules forming the sphere and one molecule inside. But it's been uncertain whether the proton was bound to one water molecule (Eigen model) or two (Zundel model) and whether the proton was located on the surface or interior.

Data from the two studies are remarkably consistent. They suggest that water clusters are chain structures at small sizes, two-dimensional nets at intermediate sizes, and cages at large sizes (21 or more molecules). But the studies couldn't determine conclusively whether the Eigen or Zundel model is correct or whether the proton is on the outside or inside of larger clusters.

"The final word on this problem is still forthcoming," Johnson says, "but you can bet a lot of groups, both theoretical and experimental, will be feverishly working on this in the coming year."