Three-coordinated boron-11 chemical shifts in borates

Despite the importance of B-11 nuclear magnetic resonance (NMR) in structural studies of borate glasses, no clear means of correlating NMR parameters with the number of nonbridging oxygens on three-coordinate boron has been demonstrated. In this work, a series of anhydrous, polycrystalline, binary-borates has been examined-by B-11 magic-angle spinning (MAS) NMR to obtain precise measurements of their three-coordinate. boron isotropic chemical shifts. The shifts generally increase with the replacement of bridging oxygens. by nonbridging oxygens, ranging from 14.6 ppm in crystalline B2O3 to 22.5 ppm in magnesium orthoborate. The underlying physical basis for this trend is satisfactorily accounted for by considering second neighbor effects using bond valence sums. These data are supportive of a structural model for B2O3 glass in which 72% of the boron atoms are in rings. High-field MAS NMR experiments (B-0 = 18.8 T) indicate that the boron shielding is anisotropic, with greater anisotropy measured for three-coordinate borons possessing one or two nonbridging oxygens, than for those with zero or three nonbridging oxygens.