Journal of Physical Chemistry A, Vol.108, No.4, 586-594, 2004
B-11 chemical shift anisotropies in borates from B-11 MAS, MQMAS, and single-crystal NMR spectroscopy
B-11 chemical shift anisotropies (CSAs) have been obtained for tetrahedral and trigonal boron sites in tetraphenyl borates, datolite (CaBSiO4(OH)), danburite (CaB2Si2O8), colemanite (CaB3O4(OH)(3)(H2O)-H-.), borax (Na2B4O7. 10H(2)O), and Li2B4O7 from solid-state B-11 NMR spectra recorded at 14.1 T. These parameters along with B-11 quadrupole couplings and the relative orientation of the quadrupole coupling and CSA tensors have been determined from either the manifold of spinning sidebands observed for the satellite transitions in B-11 MAS NMR spectra or single-crystal NMR spectra of the satellite transitions. Furthermore, the potential of the MQMAS experiment for determination of small B-11 CSAs is illustrated for the trigonal boron site in Li2B4O7. The B-11 single-crystal NMR spectra of the satellite transitions are strongly dominated by the first-order quadrupolar interaction, which may prevent a direct determination of small CSAs. However, an improved precision of the CSA parameters, and the Euler angles, describing the relative orientation of the CSA and quadrupole coupling tensors, are achieved from analysis of rotation plots for the sum frequencies of the m = 1/2 <----> m = 3/2 and m = - 1/2 <----> m = -3/2 transitions, which are influenced only by the first-order CSA and the second-order quadrupolar interactions. The B-11 CSA parameters determined in this work show that tetrahedrally coordinated boron in borates possess shift anisotropies (delta(sigma) = delta(iso) - delta(zz)) of magnitude \delta(sigma)\ less than 10 ppm.