Beryllium selenate tetrahydrate, BeSeO4$4H2O: crystal structure and infrared spectroscopy

The crystal and molecular structure of the title compound, BeSeO4$4H2O, as determined by single crystal X-ray diffraction and infrared spectroscopy is discussed. BeSeO4$4H2O crystallizes in the orthorhombic space group Cmca (at 295 K: aZ11.920(1), bZ 11.449(1), cZ10.764(1) A ° , VZ1468.8 A ° 3, ZZ2, wR2Z0.057 for 2035 reflections and 68 variables, R1Z0.022). The crystal structure is composed of isolated Be(H2O)4 and SeO4 tetrahedra which are interconnected by strong hydrogen bonds with Ow/O lengths between 2.619 and 2.661 A ° . Contrary to expectation, the cell volume and the mean Be–O and Se–O distances increase with decreasing temperature. The structure is closely related with that of acentric tetragonal BeSO4$4H2O, but differs in pronounced polyhedral rotations and a partial rearrangement of the hydrogen bonding scheme. The infrared spectrum of BeSeO4$4H2O in the region of the uncoupled OD modes (matrix-isolated HDO molecules) reveals that very strong hydrogen bonds are formed in the title compound, analogical to other hydrated beryllium salts due to the strong synergetic effect of the Be2C ions. The appearance of two bands only (2327 and 2239 cmK1) corresponding to OD stretches instead of four expected is owing to both the similar Ow/O hydrogen bond distances and the strong Be–OH2 and Ow/OSeO3 interactions. The water librations which couple intensively with the translatory modes of the Be2C ions (BeO4 skeleton vibrations) are also discussed. Matrix-isolated SO2K 4 guest ions in BeSeO4$4H2O display a higher local symmetry (close to Td) than the crystallographic one (C2(x), in correlation with the host ion distortion and the large unit-cell volume of the host (repulsion potential of the lattice) if the S–O stretching modes are considered. q 2004 Elsevier B.V. All rights reserved