A theoretical interpretation of ¹⁷O NMR spectra in borosilicate glasses

Local structure around oxygen is directly elucidated by ¹⁷O nuclear magnetic resonance (NMR). However, the ¹⁷O NMR spectra in borosilicate glasses are quite complicated, and hence the assignments of the peaks have been empirically done. In the present study, ¹⁷O NMR parameters for the various oxygen sites in the borosilicate system were obtained from the ab-initio molecular orbital calculations using the Gaussian03 programs. As for the oxygen atoms in SiOSi, SiOB[3], and B[3]OB[3] bridging bonds (B[n]: n-fold coordinated boron), the isotropic chemical shift δ_i^O decreased with increasing the bond angle. The oxygen atoms coordinated by B[4] as SiOB[4], B[3]OB[4], and B[4]OB[4] bridges showed weak dependency in δ_i^O against the bond angle, and B[4]OB[4] showed a maximum in δ_i^O at 150°. The quadrupole coupling constant C_q of all the bridging oxygens increased with increasing the bond angle. When the oxygen atoms in SiOB[4], B[3]OB[4] bridges were coordinated by Na ion, δ_i ^O decreased by 5 ∼ 10 ppm. The oxygen atoms in trimeric ring clusters showed different δ_i^O from dimeric clusters, indicating that δ_i^O was also influenced by the second neighboring cations. The order of δ_i^O was expected as SiOSi < SiOB[4] < SiOB[3] < B[4]OB[4] < B[3]OB[4] < B[3]OB[3], which was in accordance with the order of bond covalency.