Calculation of the structural and spectral properties of boroxol ring and non-ring B sites in B2O3 glass

Abstract

Abstract To help in the interpretation of recent dynamic angle spinning nuclear magnetic resonance (NMR) studies of multiple B sites in B 2 O 3 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 ] 2 O, a model for non-ring corner-sharing BO 3 units; (2) B 3 O 3 (OH) 3 , a model for the boroxol rings; (3) B 3 O 6 3− , a model for the metaborate ring; and (4) B 3 O 3 (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) [B 3 O 3 (OH) 2 ]OB(OH) 2 and (6) [B 3 O 3 (OH) 2 ] 2 O. The non-ring B site in [B(OH) 2 ] 2 O 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 B 3 O 3 (OH) 3 , in good agreement with NQR and NMR data. Quadrupole parameters and NMR shieldings calculated for three-coordinated ring B atoms in B 3 O 6 3− , B 3 O 3 (OH) 4 − and [B 3 O 3 (OH) 2 ] 2 have values very much like those in B 3 O 3 (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 BO 3 groups have intermediate quadrupole coupling values. The O breathing frequency in B 3 O 3 (OH) 3 is calculated to be 796 cm −1 , in good agreement with the value of 808 cm −1 found in glassy B 2 O 3 while the B-O-B symmetric stretching mode of [B(OH) 2 ] 2 O is calculated as 1407 cm −1 , near the 1325 and 1475 cm −1 features attributed to BO 3 chains in high- T B 2 O 3 glass. The addition of water to B 3 O 3 (OH) 3 is calculated to be less exothermic than that for addition of water to [B(OH) 2 ] 2 O.