CALCULATION OF THE STRUCTURAL AND SPECTRAL PROPERTIES OF BOROXOL RING AND NON-RING-B SITES IN B2O3 GLASS

To help in the interpretation of recent dynamic angle spinning nuclear magnetic resonance (NMR) studies of multiple B sites in B2O3 glass, equilibrium structures, B and O quadrupole coupling constants, B and O NMR shieldings and vibrational spectral energies have been calculated for: (1) [B(OH)(2)]O-2, a model for non-ring corner-sharing BO3 units; (2) B3O3(OH)(3), a model for the boroxol rings; (3) B3O63-, a model for the metaborate ring; and (4) B3O3(OH)(4)(-), a model for a ring with two three-coordinated and one four-coordinated B. All properties except the NMR shieldings have also been calculated for the larger molecules: (5) [B3O3(OH)(2)]OB(OH)(2) and (6) [B3O3(OH)(2)]O-2. The non-ring B site in [B(OH)(2)]O-2 has a coupling constant product lower by about 3% and a NMR shift more negative by about 5 ppm compared with the ring site in B3O3(OH)(3), in good agreement with NQR and NMR data. Quadrupole parameters and NMR shieldings calculated for three-coordinated ring B atoms in B3O63-, B3O3(OH)(4)(-) and [B3O3(OH)(2)]O-2 have values very much like those in B3O3(OH)(3). Non-ring O atoms are calculated to have larger quadrupole couplings than boroxol ring O atoms, in agreement with NMR data, as well as having larger NMR shieldings. O atoms bridging between boroxol rings and free BO3 groups have intermediate quadrupole coupling values. The O breathing frequency in B3O3(OH)(3) is calculated to be 796 cm(-1), in good agreement with the value of 808 cm(-1) found in glassy B2O3 while the B-O-B symmetric stretching mode of [B(OH)(2)]O-2 is calculated as 1407 cm(-1), near the 1325 and 1475 cm(-1) features attributed to BO3 chains in high-T B2O3 glass. The addition of water to B3O3(OH)(3) is calculated to be less exothermic than that for addition of water to [B(OH)(2)]O-2.