Winning Water Cluster Structure

The lowest energy, and hence most stable, structure for six-molecule water clusters is a “cage,” according to a study in Science (DOI: 10.1126/science.1220574). The hexamer is the smallest water cluster with a three-dimensional hydrogen-bonded network and is key to understanding the structure of liquid water. The cage structure is one of several hexamer clusters that researchers have previously seen. But they have never observed all the differently shaped clusters at the same time. Brooks H. Pate of the University of Virginia; Zbigniew Kisiel of the Polish Academy of Sciences, in Warsaw; George C. Shields of Bucknell University; and coworkers now report observing coexisting “cage,” “prism,” and “book” isomers at low temperatures. The researchers used microwave spectroscopy to measure the rotational spectra of ¹⁸O-substituted versions of the clusters and then used the data to construct the clusters’ oxygen framework. They recorded separate spectra using argon, neon, and helium as the carrier gas. When using argon, the researchers observed the cage hexamer but not the others, suggesting that the cage is the lowest energy structure.