The melting diagram of the quasibinary system D2O— DF was determined by low-temperature DTA and X-ray powder diffraction. It was found to be largely similar to that of the corresponding non-deuterated system H2O—HF with the striking exception of an additional phase with a composition of its own, 2D2O· 3 DF, and stable between ca. —78 and —71°C. Its structure, determined from single-crystal MoKā diffractometer data at —150°C, is rhombohedral (space group R3c, Z = 6, a = 769.9, c = 1242.1 pm) and strongly related to that of NH, · 4 HF or NH4[F(HF)3] with also seven H (as to be compared to D) and five non-H (non-D) atoms per formula unit. But with the O atom involved in four hydrogen bonds, one O · · · O and three O · · · F. at distances of 273.9 and 259.5 pm, respectively, the compound appears to be a molecular adduct rather than an oxonium salt. The D atoms in the hydrogen bonds are distributed over two positions each. — The remaining intermediary phases of the deuterated system, i. e. D2O · DF, D2O· 2 DF and D2O · 4 DF, are isotypic to their protonated counterparts of known crystal structure. For D2O · DF and D2O-2 DF these results from powder patterns were confirmed by two more single-crystal studies. The ionic structures — D3OF and D3O[F(DF)], respectively — show no distinctive effect of the H/D substitution even on details of the interatomic geometries.
Cover Picture: Protonation versus Oxonium Salt Formation: Basicity and Stability Tuning of Cyanoborate Anions (Angew. Chem. Int. Ed. 10/2017)
