High-resolution 17O double-rotation NMR characterization of ring and non-ring oxygen in vitreous B2O3

The application of double rotation (DOR) NMR to crystalline materials (both inorganic and organic) has made tremendous strides in providing site-specific information about materials in recent years. However 17O DOR has yet to demonstrate its potential in disordered materials such as glasses. In the present study, we have successfully recorded high resolution 17O DOR spectra of vitreous B2O3 (v-B2O3), a highly effective glass-forming oxide of considerable technological importance. Two distinct oxygen sites are resolved and a complete set of 17O NMR parameters were determined from the DOR spectra. These were assigned to oxygen atoms in the planar boroxol ring [B3O6] and in the non-boroxol [BO3] groups which share oxygen with the ring boron atoms. This assignment was based on the similarity of all of their 17O parameters with those found by DFT calculation for caesium enneaborate, Cs2O·9B2O3, which has two boroxol rings in its structure. The boroxol ring oxygens have a more positive chemical shift, a larger shift anisotropy and a smaller electric field gradient than non ring oxygens (OR: δiso = 100 ± 1 ppm, span = 180 ± 20 ppm, skew = −0.4 ± 0.1, Pq = 5.0 ± 0.2 MHz; ONR: δiso = 86 ± 1 ppm, span = 100 ± 20 ppm, skew = 0.1 ± 0.1, Pq = 5.7 ± 0.2 MHz). The relative proportions of the two sites in v-B2O3 are ∼1 : 1, as expected if all three boron atoms in the boroxol ring are each connected to one oxygen in a linking [BO3] group and there are very few [BO3]–[BO3] linkages. We see no evidence for a third oxygen site such as has been reported in an earlier study of v-B2O3. This work demonstrates the potential of 17O DOR to provide site-specific information in disordered materials.