Split hydrogen bond allows water to flow.

Hydrogen bonds -- strong connections between hydrogen atoms and atoms belonging to other molecules -- keep anti-freeze from boiling away in overheated radiators and stabilize the kinks and folds of proteins. One might expect water, with two hydrogen atoms and one oxygen in each molecule, to be quite stiff because it contains so many of these bonds. Yet it manages to flow as easily as substances lacking such tight intermolecular ties.

Three researchers have now demonstrated that defect in this network of bonds may explain the apparent paradox. Usually, hydrogen bonds cause every water molecule to link up with four others. But every so often, a fifth squeezes in, and this crowding defect allows molecules to shift around, the team reports in the Nov. 21 NATURE. Physicists Francesco Sciortino and H. Eugene Stanley of Boston University conducted the work with chemist Alfons M. Geiger of the University of Dortmund, Germany.

The researchers view water as an ephemeral gel, with hydrogen bonds forming "a network, like a fisherman's net," says Stanley. Last year, he and Sciortino discovered that in computer simulations of water as a gel, some hydrogen bonds last longer than others. This contradicted the long-held idea that hydrogen bonds take a characteristic amount of energy -- and time -- to break (SN:4/14/90, p.231).

From the new work, the team concludes that every so often, five water molecules get together. In order for this five-way alliance to form, one hydrogen bond splits -- in an energy sense-- and holds on to the fifth water molecule, leading to two weak bonds instead of one strong bond. This change "allows the network to come apart and for one molecule to go to another place," Stanley says.

He and his colleagues simulated water molecules with varying numbers of neighbors. The more crowded molecules tended to be more mobile and to rotate more easily, they found. The team also showed that in "stretched" water -- with increased spacing between molecules -- the liquid's mobility decreased.

In experiments to be described in an upcoming JOURNAL OF MOLECULAR LIQUIDS, a separate group of German chemists observed similar effects in mixtures of organic molecules and water.

"When water is diluted by other molecules, in some ways it makes the same effect: There are fewer water neighbors," Geiger explains. "You have a new mechanism that explains several things that are not related." This mechanism can also explain why water flows faster under pressure, which forces molecules closer together, he adds.

Earlier this year, geologists at Stanford University proposed that the temporary formation of a fifth bond between silicon and neighboring oxygen atoms might explain the flow of modern rock (SN: 6/29/91, p.404).