Ab initio calculation of ²⁹Si NMR chemical shifts for the clusters of Si (OH)₄, Si (OH)₅⁻ and Si(OH)₆⁻²

Abstract

In order to identify five-coordinated Si(Si^v) species in crystalline and amorphous silicates by NMR spectroscopy, ²⁹Si NMR chemical shifts for the clusters of Si(CH₃)₄ (TMS), Si(OH)₄, Si(OH)₅⁻ and Si(OH)₆⁻² were calculated using ab initio molecular orbital theory. The calculated shifts relative to TMS cluster were −71.7 and −179.0 ppm for Si(OH)₄ and Si(OH)₆⁻², respectively, and are in good agreement with observed shifts. The calculated shift for Si(OH)₅⁻ was −127.4 ppm, falls just between the shifts of Si(OH)₄ and Si(OH)₆⁻². The chemical shift for Si^v species in polymerized silicates is expected to appear at about −150 ppm, if the effect of polymerization is considered. Present calculation thus supports the assignment of −150 ppm peak as Si^v species, which has been found in the alkali-silicate glasses quenched from high pressures.