August 28, 2008
Quantum Effects Make H2O Weird
By Castelvecchi, Davide
Bond lengths are different in heavy, light water molecules Heavy water is not just heavier. Swapping each H in H2O with a D- hydrogen's isotope deuterium - changes water's properties. The deuterium version is mildly poisonous, and its freezing point is 4[degrees] Celsius, instead of 0[degrees]C Such differences reveal that quantum effects, which aren't usually manifest to the naked eye, rule in ordinary water, researchers suggest.
Alan Soper of the Rutherford Appleton Laboratory in Didcot, England, and Chris Benmore of Argonne National Laboratory in Illinois found that, in the liquid state, the distance between oxygen and deuterium nuclei in a D^sub 2^O molecule is 3 percent shorter than the distance between oxygen and hydrogen in an H^sub 2^O molecule. Conversely, hydrogen bonds - relatively weak bonds connecting the oxygen in one molecule with the hydrogen or the deuterium in another-are 4 percent longer in heavy water than in light, the team reports in an upcoming Physical Review Letters. These differences are less than 1 percent in water vapor, where molecules are isolated.
"A 4 percent change in bond length is quite a bit," says Michael Rubhausen of the University of Hamburg in Germany.
The deuterium nucleus, which contains a neutron in addition to the usual single proton, is heavier than the hydrogen nucleus. That makes deuterium nuclei behave more like classical objects, so the positions of the deuterium nuclei in space suffer less from the quantum uncertainty that "smears out" a proton's location. These nuclei stick closer to the oxygen nuclei within the heavy water molecule, and an oxygen atom from a nearby heavy water molecule exerts a weaker pull.
The susceptibility of ordinary water to quantum effects may explain some of its unusual features, such as its high surface tension and the fact that its density decreases when it freezes, Soper suggests. "Probably all the properties of water are affected by the hydrogen bond length," he says.
Rubhausen says the difference could also help explain some surprising results he and his collaborators reported last year. They compared RNA made with ordinary organic molecules with RNA made of those molecules' mirror images to understand why life always uses one type and not the other. Chemically, the molecules and their mirror images should be identical. But the researchers found small differences in the energy it took to excite electrons in the two types of RNA when the RNA molecules were in ordinary water. The differences disappeared in heavy water.
Different bond lengths in D^sub 2^O could somehow mask or enhance the energy differences in the two types of RNA, Rubhausen speculates.
Bond lengths within and between molecules of water (top) differ from those In heavy water (bottom) due to quantum effects, researchers find.
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