The crystal and molecular structure of the title compound, BeSeO 4 $4H 2 O, as determined by single crystal X-ray diffraction and infrared spectroscopy is discussed. BeSeO 4 $4H 2 O 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(H 2 O) 4 and SeO 4 tetrahedra which are interconnected by strong hydrogen bonds with O w /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 BeSO 4 $4H 2 O, but differs in pronounced polyhedral rotations and a partial rearrangement of the hydrogen bonding scheme.
The infrared spectrum of BeSeO 4 $4H 2 O 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 Be 2C ions. The appearance of two bands only (2327 and 2239 cm K1 ) corresponding to OD stretches instead of four expected is owing to both the similar O w /O hydrogen bond distances and the strong Be-OH 2 and O w /OSeO 3 interactions. The water librations which couple intensively with the translatory modes of the Be 2C ions (BeO 4 skeleton vibrations) are also discussed. Matrix-isolated SO 2K 4 guest ions in BeSeO 4 $4H 2 O display a higher local symmetry (close to T d ) than the crystallographic one (C 2 (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.