Three-coordinated boron-11 chemical shifts in borates

Area of Science:

• Solid-state chemistry
• Materials science
• Nuclear magnetic resonance spectroscopy

Background:

• Borate glasses are crucial in various applications, but their structure-property relationships require detailed understanding.
• Nuclear magnetic resonance (NMR) is vital for characterizing borate glass structures.
• A clear correlation between (11)B NMR parameters and the coordination environment of boron, specifically the number of nonbridging oxygens, has been lacking.

Purpose of the Study:

• To establish a quantitative correlation between (11)B NMR chemical shifts and the number of nonbridging oxygens in three-coordinate boron atoms.
• To investigate the structural implications of these correlations in anhydrous polycrystalline binary borates.
• To explore the physical basis for observed NMR trends using theoretical calculations.

Main Methods:

• Utilized high-resolution (11)B magic-angle spinning (MAS) NMR spectroscopy.
• Precisely measured isotropic chemical shifts for three-coordinate boron in various binary borates.
• Employed bond valence sum calculations to analyze second-neighbor effects influencing boron chemical shifts.
• Conducted high-field NMR experiments to probe the anisotropy of boron shielding.

Main Results:

• A clear trend was observed: isotropic chemical shifts of three-coordinate boron increase with the number of nonbridging oxygens, ranging from 14.6 ppm in B(2)O(3) to 22.5 ppm in magnesium orthoborate.
• Bond valence sum calculations satisfactorily explained the observed chemical shift trends, highlighting the role of second-neighbor interactions.
• NMR data support a structural model for B(2)O(3) glass, suggesting approximately 72% of boron atoms are incorporated into rings.
• Boron shielding was found to be anisotropic, with higher anisotropy observed for boron atoms with one or two nonbridging oxygens compared to those with zero or three.

Conclusions:

• Established a robust method for correlating (11)B NMR chemical shifts with the number of nonbridging oxygens in borate glasses.
• Demonstrated the utility of bond valence sum calculations in understanding the electronic structure and local coordination of boron.
• Provided insights into the structural characteristics of borate glasses, including ring formation in B(2)O(3) glass.
• Highlighted the anisotropic nature of boron shielding in MAS NMR studies of borates.