Three-Coordinated Boron-11 Chemical Shifts in Borates

Abstract

Despite the importance of (11)B 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 (11)B 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 B(2)O(3) 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 B(2)O(3) 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.

HP‐KB3O5 Highlights the Structural Diversity of Borates: Corner‐Sharing BO3/BO4 Groups in Combination with Edge‐Sharing BO4 Tetrahedra

“…[35][36][37][38] While BO 3 units typically are observed between 14 and 23 ppm (δ iso ) and exhibit large C Q values (2.4 to 2.9 MHz), tetrahedrally coordinated boron atoms usually range from -1 to 3.5 ppm and 0.2 to 0.5 MHz for δ iso and C Q , respectively. The asymmetry parameter η largely depends on local distortions and connectivities and can vary between 0 and 0.8 for both units.…”

Contribution of first-principles calculations to multinuclear NMR analysis of borosilicate glasses

“…However a limited number of clusters with a single (optimised) geometry were used and left some pending questions. For example the S162 A. Soleilhavoup et al [48] span = 15(±2) ppm and skew = 1, GIPAW span = 3 ppm and skew = 0.8. Table 4.…”

“…The number of CSA values reported in literature for borates [48,71] is so far insufficient for a systematic assessment of the GIPAW method. Additionally, at the magnetic field used in this work (11.72T), a homonuclear 11 B-11 B dipolar broadening of the same order of MAS and static spectra of boron atoms in three-membered rings ( [3] B ring species) are also displayed in dashed lines.…”

Network structure and thermal stability study of high temperature seal glass

“…It is also interesting to note that the 11 B chemical shift for the BO 3 site is located around 11 ppm, indicating that these sites contain at least one nonbridging oxygen atom forming a T 2 boron site ͑where T stands for "ternary" by analogy to the Q n nomenclature for quaternary silicon͒. 23 In addition, the 11 B chemical shift for the BO 4 sites shows systematic increase from −0.50 to 0.44 ppm with increasing B 2 O 3 content from SABS-5 to SABS-35. This result means that increasing 11 B chemical shift for the BO 4 sites causes less sharing of oxygen between BO 4 and SiO 4 tetrahedra from SABS-5 to SABS-35.…”