Ordered Proton Configuration in Ice II, from Single-Crystal Neutron Diffraction

D₂O ice II has a fully deuteron‐ordered structure in which two crystallographically independent types of water molecule form a tetrahedrally linked network of H bonds, with the deuterons lying near but not on the O···O centerlines. Orientation of the molecules is nearly, but not exactly, symmetrical in the donor O···O···O angles. The D–O–D angles (average, 105.4°) do not differ significantly from the angle in D₂O vapor (104.5°) although the corresponding O···O···O angles are smaller (average, 93.8°). Agreement between D–O–D and O···O···O angles is improved about 6° (on average) by a distortion of the bond network, from symmetry $\text{R3̄c}$ to $\text{R3̄}$ . The approach to a match between D–O–D and O···O···O angles is only partly responsible for the existence of proton ordering in ice II. Twinning of $\text{R3̄}$ individuals is established by variations in certain diffraction intensities from crystal to crystal. Mean O–D bond length (0.98 Å) is slightly longer than in the vapor (0.957 Å). Variations among the individual O–D distances (range 0.95–0.01 Å) do not correlate with the H‐bonded O···O distances (range 2.77–2.84 Å) and hence are probably not significant.