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

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