Potassium beryllium sulfate dihydrate, K 2Be(SO 4) 2·2H 2O: Crystal structure and infrared spectroscopy

Abstract

The crystal and molecular structure of the title compound, K 2Be(SO 4) 2·2H 2O, as determined by single crystal X-ray diffraction and infrared spectroscopy is reported and discussed. K 2Be(SO 4) 2·2H 2O crystallizes in the monoclinic space group P2 1/ c ( a=11.143(2), b=11.515(2), c=7.314(1) Å, β=95.08(1)°, V=934.8 Å 3, Z=4, R1=0.034 for 3524 F o>4 σ( F o) and 153 variables). The structure is characterized by tetrahedral chain fragments consisting of a BeO 2(H 2O) 2 group sharing corners with two SO 4 tetrahedra. These [Be(SO 4) 2(H 2O) 2] 2− units are interlinked by irregular KO 7 polyhedra and hydrogen bonds. Generally, the observed bond lengths and angles comply well with crystal chemical experience. Most of the infrared and Raman bands corresponding to the internal vibrations of the SO 4 2 − ions appear as doublets, thus reflecting the existence of two crystallographically different [SO 4]-tetrahedra in agreement with the structural data. The strengths of the hydrogen bonds as deduced from the infrared wavenumbers of the uncoupled OD modes of matrix-isolated HDO molecules (bands at 2278, 2352 and 2388 cm −1, liquid nitrogen temperature) are discussed in terms of the the O w⋯O bond distances, the different hydrogen bond acceptor capabilities of the oxygen atoms and the Be–OH 2 interactions. The intramolecular OH bond lengths are derived from the ν OD vs. r OH correlation curve [J. Mol Struct. 404 (1997) 63]. The water librations couple intensively with the translatory modes of the Be 2+ ions (BeO 4 skeleton vibrations) and the normal modes of SO 4 2 − ions, thus producing small isotopic shifts. The SO 4 2 − guest ions matrix-isolated in the isostructural host lattice K 2Be(SeO 4) 2·2H 2O exhibit doublet bands in the region of the stretching modes (the bands corresponding to the bending modes could not be recognized well), thus indicating a statistical distribution of the guest ions over the two available crystallographic positions. The distortion of the SO 4 2 − guest ions in the host selenate lattice is weaker than that of the same ions in the neat sulfate owing to the larger unit-cell volume of the host compound, i.e. to the smaller repulsion potential of the host lattice.