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

Abstract In this paper we report the crystal and molecular structure of the title compound, (NH 4 ) 2 Be(SO 4 ) 2 ·2H 2 O, as determined by single-crystal X-ray diffraction and vibrational spectroscopy data. (NH 4 ) 2 Be(SO 4 ) 2 ·2H 2 O crystallizes in the monoclinic space group P 2 1 / c ( a  = 11.448(2), b  = 11.876(2), c  = 7.439(1) A, β  = 96.64(1)°, V  = 1004.6 A 3 , Z  = 4, R1 = 0.036 for 3753 F o  > 4 σ ( F o ) 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 BeO 2 (H 2 O) 2 group and two adjacent SO 4 tetrahedra. These building blocks are linked by rather strong hydrogen bonds of the water molecules (O⋯O = 2.66–2.77 A) and by moderate to weak hydrogen bonds of the NH 4 groups (N⋯O ≥ 2.78 A) to a three-dimensional framework structure. The infrared bands corresponding to the stretching modes ν 3 and ν 1 as well as the Raman band corresponding to ν 1 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 Δ ν 3 (site group splitting) and Δ ν max (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 S O bond lengths than that with respect to the O S O bond angles. The water librations couple intensively with both the translatory modes of the Be 2+ cations (BeO 4 skeleton vibrations) and the normal modes of the sulfate ions, thus producing small isotopic shifts (for example, the ratio ν r (H 2 O)/ ν r (D 2 O) 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.