The effect of pressure on the structure of light and heavy water

X-ray diffraction experiments were carried out on light and heavy water to pressures of 6 kbar. The results are in qualitative agreement with the molecular dynamics studies of Impey, Klein and McDonald. However, there are appreciable quantitative differences. The intramolecular O-H and O-D linewidths exhibit a different dependence on pressure, and this is interpreted in terms of the differing dynamical properties of the two liquids. The nearest neighbour intermolecular hydrogen bonding O- H correlation is at a separation of ∼2·0 Å and there is a 15 per cent increase of coordination number over the pressure range of investigation. For pressures of 1 bar and 5 kbar the nearest neighbour oxygen-oxygen separation changes from 2·89 Å to 2·84 Å in light water and from 2·89 Å to 2·84 Å in heavy water. This result is a factor of 6 larger than that predicted by molecular dynamics and about a factor of 3 less than that inferred from N.M.R. studies. There is also an increase in coordination number of about 10 per cent. Other longer range O-O correlations are observed and a qualitative explanation of their pressure dependence is presented.