By Sean Henahan, Access Excellence

Purdue University researchers cooled water molecules in the gas phase to very low temperatures and studied the resulting molecular topology using sophisticated techniques including resonant two-photon ionization and resonant ion-dip infrared spectroscopy. These studies revealed that minimal configurations containing eight water molecules naturally arranged themselves into small cubic structures. These molecular icecubes took one of two forms, both of which had the same mass and structure, but differed in the arrangement of the hydrogen bonds within the cubes.

Caption: Going Through a Phase-  The D2 structure at the top shows hydrogen bonds in the top andbottom layers oriented in opposite directions. The S4 structure shows the bonds in the top and bottom layers oriented in the same direction. (Purdue graphic by Timothy Zwier)

"These findings verify what theorists have predicted for years; namely, that the eight water molecules referentially form a cubic structure. It also provides the first evidence that even in very small water clusters, water has the capacity to arrange its hydrogen bonds in several distinct orientations, much as it does in forming the many   different solid phases of ice," reports Purdue chemist Timothy Zwier.

This is the first report of ice cubes at such a small scale. The study provides new information on the unique ability of  water to hydrogen bond to itself to form large networks. This ability gives water many of its unique properties, including the unusual capacity of solid ice to float on liquid water.

"Our understanding of water is so important because all biological processes, including those occurring in the human body, take place in a water-based solution", Zwier says.

The hydrogen bonds that form between water molecules and other molecules are part of what makes water unique. Each V-shaped molecule of water contains one oxygen atom centered between two hydrogen atoms. The molecule is held together by chemical bonds that create a slightly negative charge on the oxygen atom and a small positive charge on each of the hydrogen atoms.  This unequal charge distribution makes the water molecule extremely "sociable", eager to bond with other water molecules, and gives water its unequaled ability to dissolve compounds, he explained.

Zwier and colleagues used a high-pressure gas expansion to cool water molecules in the gas phase to  temperatures as low as 1 degree Kelvin, the equivalent of -457 degrees Fahrenheit. As the water cooled and condensed into solid clusters, some of the clusters incorporated a single benzene molecule on their surface. The benzene molecule allowed the various clusters to be identified by size using lasers to "weigh" the clusters.

The researchers then applied an infrared laser to excite the clusters, causing the hydrogen bonds in the tiny cubes to stretch and contract. By analyzing the wavelengths of this spectrum, they were able to determine the molecular arrangement of the hydrogen bonds within the cubes.

As theorists had predicted, the cubes stacked up in layers of  four molecules of water. While the hydrogen bonds in the top layer of each cube were oriented in the same manner, the hydrogen bonds in the bottom layers of the cubes took one of two possible arrangements, with the bonds facing either the same direction or opposite direction as the bonds in the top layer of the cube.

"Since the two structures are virtually identical in energy, the orientation that a particular cluster takes depends on the specific collisions the cluster undergoes while it is being made. It is interesting that already with only eight water molecules, water makes up two different 'phases' which differ only in the orientations of the hydrogen bonds," he says. "This is the beginnings of what we know to be true in the solid phase. Water has more solid phases--nine total--than any other known pure substance because it can form phases which differ only in the orientations of the hydrogen bonds." 

The study appears in the June 13, 1997 issue of the journal Science.