11B Chemical Shift Anisotropies in Borates from 11B MAS, MQMAS, and Single-Crystal NMR Spectroscopy

Abstract

11B 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), borax (Na2B4O7·10H2O), and Li2B4O7 from solid-state 11B NMR spectra recorded at 14.1 T. These parameters along with 11B 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 11B MAS NMR spectra or single-crystal NMR spectra of the satellite transitions. Furthermore, the potential of the MQMAS experiment for determination of small 11B CSAs is illustrated for the trigonal boron site in Li2B4O7. The 11B 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 11B CSA parameters determined in this work show that tetrahedrally coordinated boron in borates possess shift anisotropies (δσ = δiso − δzz) of magnitude |δσ| less than 10 ppm.