by Robert Sanders
Like a mason adding one stone at a time, chemist Richard Saykally is constructing a detailed model of water one molecule at a time, in a decade-long attempt to understand the microscopic chemistry of water and ice.
He and his students have created isolated clusters of two, three, four and five water molecules, then hit them with a precision infrared laser to see how they tumble and vibrate. From these studies they have been able to show that the molecules link together in a ring and dance about on the scale of a trillionth of a second.
In the June 6 issue of the journal Nature, Saykally and his colleagues report their observations on clusters of six water molecules, which, unlike smaller clusters, appear to curl up into a three-dimensional cage structure.
"Because the six-molecule cluster represents the transition between a two-dimensional ring structure and a three-dimensional geometry, studies of this cluster are of special importance in understanding the structure of solid and liquid water," he says. "We are really starting to address questions of major importance to life on this planet and one of the great problems of chemical physics."
The point of these experiments is to build a coherent picture of how water molecules act when they get together in large numbers, such as in a glass of water or an ice cube. He and other chemists hope to use information from these smaller clusters to predict how molecules interact in the bulk fluid and how water molecules interact with other chemicals.
One field where this will be of use is atmospheric chemistry, which deals with the reactions airborne chemicals undergo, such as the formation of acid rain when sulfur oxides react with water.
"Water is involved in almost everything," Saykally says. "Even in drug design you need to know how water interacts with a drug, since most function in water. So you would like your model of water to be as good as possible so you can go as far as possible with computer simulation."
He and his group have shown that three, four and five water molecules form a basically flat ring reminiscent of skydivers holding hands in freefall.
In the Jan. 5 issue of Science, Saykally and graduate students Kun Liu, Jeff Cruzan and Mac Brown reported the first structural information on both four- and five-membered rings made with their new technique, terahertz laser vibration-rotation-tunneling spectroscopy.
Now, in the June 6 issue of Nature, Saykally and colleagues report that six molecules fold into a three-dimensional cage.
His coauthors on the Nature paper are graduate students Liu and Brown, J. K. Gregory from Cambridge; D. C. Clary of University College, London; and high school teacher Clifford Carter from College High School in Pleasant Hill, Calif., supported through a "Partners in Science" award from the Research Corporation. The work is supported by the National Science Foundation.