Crystal and molecular structure of ammonium beryllium sulfate dihydrate, (NH4)2Be(SO4)2·2H2O

In this paper we report the crystal and molecular structure of the title compound, (NH4)2Be(SO4)2·2H2O, as determined by single-crystal X-ray diffraction and vibrational spectroscopy data. Be(SO4)2·2H2O crystallizes in the monoclinic space group P21/c (a = 11.448(2), b = 11.876(2), c = 7.439(1) Å, β = 96.64(1)°, V = 1004.6 Å3, Z = 4, R1 = 0.036 for 3753 Fo > 4σ(Fo) and 185 variables), and is isotypic with the respective potassium and rubidium sulfates and the potassium selenate. The crystal structure is built from three-membered tetrahedral chain fragments, consisting of a central BeO2(H2O)2 group and two adjacent SO4 tetrahedra. These building blocks are linked by rather strong hydrogen bonds of the water molecules (O⋯O = 2.66–2.77 Å) and by moderate to weak hydrogen bonds of the NH4 groups (N⋯O ≥ 2.78 Å) to a three-dimensional framework structure. frared bands corresponding to the stretching modes ν3 and ν1 as well as the Raman band corresponding to ν1 of the SO 4 2 - ions are observed in the spectra as doublets, thus reflecting the existence of two crystallographically different sulfate ions. The spectroscopic experiments reveal that the sulfate tetrahedra exhibit a remarkably large extent of energetic distortion as deduced from the values of Δν3 (site group splitting) and Δνmax (the difference between the highest and the lowest wavenumbered component of the stretching modes) (111 and 183.5 cm−1, respectively), which could not be predicted from the structural data. The comparison of the spectral region widths of the stretching and bending modes of the SO 4 2 - ions allows us to assume that these ions undergo a stronger energetic distortion with respect to the SO bond lengths than that with respect to the OSO bond angles. The water librations couple intensively with both the translatory modes of the Be2+ cations (BeO4 skeleton vibrations) and the normal modes of the sulfate ions, thus producing small isotopic shifts (for example, the ratio νr(H2O)/νr(D2O) has value of 1.12). The strength of the hydrogen bonds formed by the water molecules in the title compound is also commented in comparison with that of the hydrogen bonds in the respective isomorphous potassium and rubidium compounds.