Yuan Lee, Nobel laureate chemist with LBL's Materials and Chemical Sciences Division, is leading an effort to crack Nature's version of an enigma wrapped inside a riddle -- the structure of water.
Water is weird. It's a liquid when it ought to be a gas, it expands when it should contract, and it dissolves just about everything it touches -- given enough time. Yet without water's weirdness, Earth would be just another lifeless snowball in space.
Theorists have offered numerous predictions about water's structure to explain its unusual properties, but nothing has been experimentally confirmed. The difficulty arises from the presence in all water of the hydronium ion, a water molecule with an extra hydrogen atom. Hydronium's hydrogen atoms bond with the oxygen atoms of water molecules to create ionized molecular "clusters." The "hydrogen bonds" that hold these clusters of water molecules together are so weak (about 10 percent the strength of the average ionic or covalent bond) that the clusters are in a constant state of flux, forming and reforming about once every ten billionth of a second. Nonetheless, ionic clusters are largely responsible for water's uniqueness. Without them, water would be a gas at room temperature.
X-ray diffraction patterns of ice first suggested the existence of water cluster ions but attempts to study their structure proved inconclusive. Working with MCSD chemists James Myers and John Price, and L. Yeh and M. Okumura, who were then chemists at UC Berkeley, Lee analyzed the structure of water cluster ions in a gas by measuring their absorption of infrared light.
"The study of the infrared spectroscopy of ionic clusters is a challenge," Lee says. "The difficulty is caused largely by the very low ion densities obtained. To overcome this limitation, we have used consequence spectroscopy, where the consequence of absorbing an infrared photon is an observable event."
Lee and his colleagues mixed hydrogen ions with trace amounts of water in a molecular beam to form ionic clusters containing one, two, or three water molecules attached to each ion of hydronium. The beam was then expanded into a vacuum to freeze the unstable clusters into formation. Light from a tunable infrared laser was used to selectively excite the hydrogen bond vibrations of whatever type of cluster the researchers wished to study. A mass spectrometer fed the excited clusters into a radio frequency ion trap where a second laser beam was used to break them up through the "dissociation" of their bonds. The resulting fragment ions were then detected and counted in a second mass spectrometer to obtain an infrared spectrum that could be compared to theoretical predictions.
"The combination of a tandem mass spectrometer system and a radio frequency ion trap is ideal for the detection of dissociation products," says Lee. "There is little or no background at the fragment ion mass and every fragment ion can be detected with nearly perfect efficiency."
When they compared their data with theoretical models, Lee and his colleagues found that "theory is not reliable yet" for predicting the stable structures of cluster ions. Because of the weakness of the hydrogen bonds that form these clusters, it only takes a slight increase in energy for the structures of the clusters to be completely rearranged.