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Water and Ice

Researchers now understand how H2O's unique properties result from its bonds


It is no exaggeration that Earth has been called the "water planet." This simple molecule consisting of one oxygen atom and two hydrogen atoms covers nearly three quarters of the Earth's surface; it accounts for more than 70 percent of the weight of living organisms.

Water exhibits an array of unique properties that make it the stabilizing influence on global climate and the key to life itself. The oceans can store vast amounts of heat because it takes a large amount of heat to raise water temperature one degree. Unlike other liquids which contract when they are cooled, water expands below four degrees C. so ice is less dense and floats on top of liquid water--acting an insulating layer and providing a favorable habitat for life below. Water's enormous heat-carrying capacity allows the atmosphere and ocean currents to balance global temperatures. Its polarity and electrical conductivity make it an ideal solvent for conducting the chemical reactions that are the basis of metabolism.

Yet the basis of these remarkable characteristics has remained a mystery. It turns out that the answer lies in the interaction between the bonds that hold the atoms in the water molecule together and the much weaker bonds, known as hydrogen bonds, that are the glue holding groups of water molecules together. Although relatively feeble, hydrogen bonds are so plentiful in water that they play a large role in determining its properties.

In a paper published in the January 18 issue of Physical Review Letters, an international physics collaboration demonstrated that both types of bonds play by the same rules--quantum mechanics, the strange state in which matter exists as particles and waves at the same time. They proved that the weak hydrogen bonds in water partially get their identity from stronger covalent bonds in the H2O molecule.

Ice crystal
PROVING PAULING

Hydrogen bonds were once thought to be distinctly different from the strong bonds. While the strong sigma or covalent bonds were explained by the new theories of quantum mechanics, hydrogen bonds were seen as nothing more than an electrostatic attraction between charged particles and were explained according the principles of classical physics.

But, in the 1930s, the famous chemist and Nobel laureate Linus Pauling created a long-lived controversy when he proposed that the hydrogen bonds between water molecules would be affected by the sigma bonds within the water molecules and partially assume the identity of these bonds. Pauling's idea was that electron waves on the sigma and hydrogen bonding sites would overlap somewhat so these electrons would become indistinguishable. As a result, the hydrogen bonds could not be completely be described as electrostatic bonds.

Pauling's theory was not tested until the group of researchers--comprised of Eric Isaacs, Donald Hamann and Phil Platzman of Bell Labs; Bernardo Barbiellini, now at Northeastern University; Abhay Shukla of the European Synchrotron Radiation Facility (ESRF); and Christopher A. Tulk of National Research Council of Canada--devised a clever experiment. Working at the ESRF in Grenoble, France, they bombarded ice with powerful x-rays and studied the scattering patterns when the x-ray photons ricocheted off the electrons in the hydrogen bonds.

The researchers' results confirmed Pauling's notion. The findings will allow investigators to improve theories of water and the many biological structures such as DNA which possess hydrogen bonds. They may also help nanotechnologists design more advanced self-assembling materials, many of which rely heavily on hydrogen bonds to put themselves together correctly. The new experimental technique may also help other researchers plumb the properties of materials lacking hydrogen bonds, such as superconductors and semiconductors, where they hope to manipulate the quantum mechanical behavior of advanced materials.

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