11B Quadrupole Coupling Tensors in the Hydrated Borates. II. The Pentaborate Polyanion

Abstract

The quadrupolar splitting of the 11B nuclear magnetic resonance signal has been investigated in potassium pentaborate tetrahydrate, KB5O6(OH)4·2H2O, and ulexite, NaCaB5O6(OH)6·5H2O. In the case of the former material, a good single crystal was available, permitting a complete determination of the quadrupole coupling tensors at the single unique tetrahedral site and at the two unique triangular boron sites. A complete determination of the tensors was not possible in the case of ulexite because of the fibrous nature of the material but, by combining single-crystal and powder techniques, it was possible to identify unambiguously three tetrahedral and two triangular boron sites and to set upper and lower limits on the quadrupole coupling constants. The electric quadrupole coupling constants eqQ/h and asymmetry parameters η of the electric-field gradient tensors at the boron sites were determined to be eqQ/hCoordination of(kc/sec)ηboron sitePotassium pentaborate tetrahydrate234±10.093±0.009Tetrahedral2536±30.212±0.004Triangular2521±30.195±0.005TriangularUlexite50<eqQ/h<600Tetrahedral2530±50TriangularThe orientations of the principal axes of the electric-field gradient tensors have also been determined for potassium pentaborate tetrahydrate. The NMR results are in complete agreement with the crystal structures of both compounds as to numbers and coordinations of the boron atoms and as to the nature of the boron—oxygen polyanions.