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By P. Weiss 
Life owes its existence to a relatively weak connection called the hydrogen bond, which joins molecules or regions within a molecule. Without it, liquid water would be scarce on Earth and biological machinery involving DNA and proteins would halt. Despite intense scrutiny, the bond has remained mysterious in many ways (SN: 7/20/96, p. 37). A new study of ice now shows experimentally that the frail hydrogen bond between water molecules taps into a molecule's internal covalent bonds, formed when atoms share electrons. The late Nobel laureate Linus Pauling first suggested this might be the case in 1935. Although scientists have long assumed that hydrogen bonds are partly covalent, the experimental proof ranks as a major milestone, some hydrogen-bond experts say. Demonstrated in water, the findings apply to all hydrogen bonds, they add. The results may help investigators better understand properties of the bonds, such as why they are strongest in a certain direction, and improve models of their behavior. A report on the experiment in the Jan. 18 Physical Review Letters is "certainly a very, very important new paper," comments Jose Teixeira of the Saclay research center of France's Atomic Energy Commission. In water, hydrogen bonds forge links between hydrogen and oxygen atoms in adjacent molecules. Such a bond's character derives mostly from attraction between unlike electric charges that the two types of atoms acquire. However, the new findings show that an electron contributing to that charge separation spends roughly 10 percent of its time mingling with an electron covalently binding the hydrogen and oxygen atoms within the adjacent molecule. "That's what Pauling said, and it's consistent with our data," says Eric D. Isaacs, the leader of the new study and one of three scientists at Lucent Technologies' Bell Labs in Murray Hill, N.J., who took part in the work. The specific 10 percent estimate has not yet been published, he says. In the new experiment, Isaacs' team, which also included scientists at Northeastern University in Boston, the European Synchrotron Radiation Facility in Grenoble, France, and the Canadian National Research Council in Ottawa, Ontario, shone X rays at millimeter-thick crystals of ultrapure ice. X rays lose some energy and change direction as they strike electrons in the crystal. Their transformations reveal the spatial distribution of the ice's electrons—considered waves, according to quantum mechanics. By studying X rays bounced off various planes in the crystal with different numbers of hydrogen bonds, the team highlighted features of the bonds. In the portrait of the electron waves that emerged, the team found fluctuations like those observed when overlapping light waves interfere with each other—their crests and troughs adding and canceling. The researchers deduce that the electron wave in each hydrogen bond is interfering with the wave in an adjacent covalent bond. Consequently, the electrons in both bonds must overlap to some degree, indicating that the electron in the hydrogen bond is circulating around two linked atoms—the hallmark of covalency. |
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From Science News, Vol. 155, No. 4, January 23, 1999, p. 52. Copyright © 1999, Science Service. |
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