Molecular orbital calculation of the Si NMR chemical shift in borosilicates: The effect of boron coordination to SiO₄ units

Borosilicate cluster models were constructed, and the ²⁹Si NMR chemical shift was estimated by molecular orbital calculations. For Q ⁴ species (an SiO₄ unit consisting of four bridging oxygen atoms), a higher frequency shift was confirmed due to the replacement of the surrounding SiO₄ with BO₄ units, and for Q⁴ species associated with more than one BO₄ unit, the chemical shifts were nearly identical to those for Q³ species (an SiO₄ unit including one nonbridging oxygen) in alkali silicates. The chemical shifts of Q⁴ species in borosilicates were interpreted in terms of the change in bond angle of Si-O-(Si,B4) bridges. A change in chemical shift anisotropy of Q⁴ species was also found. The Q⁴ species associated with two BO₄ units indicated an anisotropy maximum, which was smaller than the anisotropy of Q³ species.