Ab initio calculation of ~(29)Si NMR chemical shifts for the clusters of Si (OH)_4, Si (OH)_5~- and Si (OH)_6~(-2)

摘要

In order to identify five-coordinated Si(Si~V) species in crystalline and amorphous silicates by NMR spectroscopy, ~(29)Si NMR chemical shifts for the clusters of Si(CH_3)_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.