High-resolution O-17 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 O-17 DOR has yet to demonstrate its potential in disordered materials such as glasses. In the present study, we have successfully recorded high resolution O-17 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 O-17 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 O-17 parameters with those found by DFT calculation for caesium enneaborate, Cs2O center dot 9B(2)O(3), 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 (O-R: delta(iso) = 100 +/- 1 ppm, span = 180 +/- 20 ppm, skew = -0.4 +/- 0.1, P-q = 5.0 +/- 0.2 MHz; O-NR: delta(iso) = 86 +/- 1 ppm, span = 100 +/- 20 ppm, skew = 0.1 +/- 0.1, P-q = 5.7 +/- 0.2 MHz). The relative proportions of the two sites in v-B2O3 are similar to 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 O-17 DOR to provide site-specific information in disordered materials