Nuclear Magnetic Resonance Spectrum of B11 in Lesserite

Abstract

The quadrupolar splitting of the B11 nuclear magnetic resonance signal in a single crystal of lesserite, the monoclinic dimorph of Mg2B6O11·15H2O, has been investigated in a magnetic field of 6345 gauss. The spectra obtained as the crystal was rotated in turn about three mutually perpendicular axes held normal to the magnetic field were interpreted on the basis that the 12 B11 sites per unit cell belong to three distinct types with the four sites of each type related by the symmetry of point group 2/m. The electric quadrupole coupling constants, eqQ/h, and asymmetry parameters η of the electric field gradient tensors at the three unique B11 sites were determined to be eqQ/h406±4 kc/sec270±4 kc/sec2529±10 kc/secη0.397±0.0020.927±0.0020.075±0.003.It has been inferred from the foregoing values that two of the boron atoms in the asymmetric unit of lesserite have tetrahedral coordinations whereas the third has triangular coordination and that the two BO4 groups form with the BO3 group a closed-ring polyion similar to that found in colemanite. The orientations of the principal axes of the electric field gradient tensors at the B11 sites have also been determined.