Quantum Differences between Heavy and Light Water

The structures of heavy and light water at ambient conditions are investigated with the combined techniques of x-ray diffraction, neutron diffraction, and computer simulation. It is found that heavy water is a more structured liquid than light water. We find the OH bond length in ${\mathrm{H}}_2\mathrm{O}$ is $\sim 3\%$ longer than the OD bond length in ${\mathrm{D}}_2\mathrm{O}$. This is a much larger change than current predictions. Corresponding to this, the hydrogen bond in light water is $\sim 4\%$ shorter than in heavy water, while the intermolecular HH distance is $\sim 2\%$ longer.

Figure 1

EPSR fits to the neutron [(a) and (c)] and x-ray [(b) and (d)] diffraction data for heavy [(a) and (b)] and light [(c) and (d)] water. The fits are shown as the solid lines while the difference between data and fit is shown by the dots. Some systematic misfit is found for both neutron and x-ray data. In addition, truncation oscillations are found for the heavy water fits caused by finite size effects from the simulations.

Figure 2

Top: EPSR fits (solid lines) to the high $Q$ region of the neutron diffraction data (dots) for heavy (a) and light (b) water. Also shown is the fit to the ${\mathrm{H}}_2\mathrm{O}$ data assuming the same OH bond length, $d_{\mathrm{OH}}=0.98\AA$ as for ${\mathrm{D}}_2\mathrm{O}$ (dashed line). Bottom: EPSR fits (solid lines) to the high $Q$ region of the neutron diffraction data (dots) for ${\mathrm{C}}_6{\mathrm{D}}_6$ (a), ${\mathrm{C}}_6{\mathrm{D}}_6:{\mathrm{C}}_6{\mathrm{H}}_6$ (b), and ${\mathrm{C}}_6{\mathrm{H}}_6$ (c). The benzene data have been analyzed with an assumed C-H distance of 1.06 Å. Unlike water, there is no indication of a longer C-H bond distance in ${\mathrm{C}}_6{\mathrm{H}}_6$ compared to ${\mathrm{C}}_6{\mathrm{D}}_6$.

Figure 3

Radial distribution functions for heavy (solid line) and light (dashed line) water. Also shown are the RDFs found in a previous study [21] in which both heavy and light water data were refined against the same box of molecules. The results are shown in the order HH (a), OH (b), and OO (c) and are shifted for clarity. The inset shows the second OO peak in more detail.

Figure 4

Distribution of triplet angles, ${\theta }_{\mathrm{OOO}}$ for triplets of water molecules, two of which are not more than 3.18 Å from a third. Heavy water is the solid line while light water is the dashed line. The results have been normalized to the $\sin ({\theta }_{\mathrm{OOO}})$ dependence that would occur if the triple angles were randomly distributed in space. The broad peak near 100° indicates water is broadly tetrahedral in its local neighborhood, but there are wide variations from molecule to molecule.