Everyone knows that heavy water (water with its hydrogen replaced by the heavier hydrogen isotope deuterium) is a clear, colourless liquid that flows easily. It appears and behaves like ordinary water. Yet it freezes at 4°C, has a temperature of maximum density of 11°C, and if consumed in large enough quantities is poisonous. In a classical world the structure of D2O should be identical to H2O, but quantum mechanics dictates this cannot be, due to the mass difference between H and D. Here we explore those differences using a combination of neutron and x-ray diffraction, and computer simulation. The result? We find the OH bond in H2O is ~3% longer than the OD bond in D2O, while the hydrogen bond is shorter and more distorted in H2O compared to D2O. The effects are much larger in the liquid than are found in the isolated molecules, suggesting that hydrogen-bonding and quantum mechanics work together to give water its unique properties, and make heavy water different in structure to light water.
AK Soper (ISIS), CJ Benmore, (Argonne National Laboratory)