A theoretical interpretation of the chemical shift of 29Si NMR peaks in alkali borosilicate glasses

Abstract

Abstract−−In29 Si-NMR, it has so far been accepted that the chemical shifts of Q n species (SiO 4 units containing n bridging oxygens) were equivalent between alkali borosilicate and boron-free alkali silicate glasses. In the sodium borosilicate glasses with low sodium content, however, a contradiction was confirmed in the estimation of alkali distribution; 11 B NMR suggested that Na ions were entirely distributed to borate groups to form BO 4 units, whereas a −90 ppm component was also observed in 29 Si-NMR spectra, which has been attributed to Q 3 species associated with a non-bridging oxygen (NBO). Then, cluster molecular orbital calculations were performed to interpret the −90 ppm component in the borosilicate glasses. It was found that a silicon atom which had two tetrahedral borons (B4) as its second nearest neighbors was similar in atomic charge and Si2p energy to the Q 3 species in boron-free alkali silicates. Unequal distribution of electrons in Si−O−B4 bridging bonds was also found, where much electrons were localized on the Si−O bonds. It was finally concluded that the Si−O−B4 bridges with narrow bond angle were responsible for the −90 ppm 29 Si component in the borosilicate glasses. There still remained another interpretation; the Q 3 species were actually present in the glasses, and NBOs in the Q 3 species were derived from the tricluster groups, such as (O 3 Si)O(BO 3 ) 2 . In the glasses with low sodium content, however, it was concluded that the tricluster groups were not so abundant to contribute to the −90 ppm component.2

Effect of thermally induced structural disorder on the chemical durability of International Simple Glass

“…It is generally expected that substitution of one bonded silicon for one aluminum impact the isotropic chemical shift of the central silicon by an increase of about 4-5 ppm. 34 Similar increase has been reported for Si-O-B linkages, 35,36 but shown to depend on the boron speciation 33 (no change is observed for Si-O-B III in contrast to Si-O-B IV ). Because of the glass composition complexity, these data are therefore inconclusive as to possible differences in the degree of polymerization in the as-received and annealed fibers.…”

Contribution of first-principles calculations to multinuclear NMR analysis of borosilicate glasses

“…[59] In aluminosilicates it is established that 29 Si isotropic chemical shift δ iso can be well correlated to the bond angles in Si-O-T bondings (T Si Al). Similar angular dependencies have been recognised in borosilicates using molecular orbital (MO) calculations by Nanba et al [18,19] They observed changes in the 29 Si chemical shifts when surrounding SiO 4 units are replaced by BO 3 (or BO 4 ) units. It was first noticed that an increase in the number of surrounding BO 3 units not only shifts 29 Si isotropic chemical shift δ iso to higher values but also leads to a decrease of the average bond angle Si-O-(Si,B).…”

“…A global structural analysis of the models is presented in Table 4 and structural parameters compare well with literature data. [18] The CASTEP outputs were analysed and NMR spectra calculated with the fpNMR package. [32] As described in previous works, [30] the reference isotropic shielding σ ref and nuclear quadrupole moment Q (here for 11 B) were adjusted using GIPAW calculations on reference crystalline systems (see Table 3).…”

“…[8,10 -17] Only recently some attempts at understanding the effect of boron coordination on 29 Si chemical shifts have been made using molecular orbital calculations. [18,19] However such calculations were performed on a limited diversity of local geometries and small cluster sizes. Molecular dynamics (MD) simulations allow to take into account the inherent longer range structural constraints of the vitreous state.…”

Structural studies of ThO2 containing barium borosilicate glasses

“…Deconvolution of this peak assuming a Guassian line shape resulted in two peaks around À95 and À86 ppm. Based on the 29 Si MAS NMR studies of borosilicate glasses [12][13][14][15][16][17][18][19], the peak around À95 ppm, has been attributed to the Q 3 structural units of silicon and that around À86 ppm to Q 2 structural units (where Q n represents silicon structural units having 'n' bridging oxygen atoms). With increase in ThO 2 concentration in the glass, the peak maxima and line shapes for the 29 Si MAS NMR patterns remained same within experimental errors.…”