Calculation of the 29Si NMR shielding tensor in forsterite

Abstract

29Si NMR studies on synthetic single crystal forsterite have shown an isotropic NMR shift of −63.2 ppm relative to tetramethyl silane and shift tensor components of −24.4, −7.9 and +32.2 ppm relative to this isotropic value. The most shielded component (+32.2 ppm) lies close to the Si-O1 vector, where Si-O1 is the shortest bond, R(Si-O1)=1.614A (Weiden and Rager, Z. Naturforsch 40a, 126 (1985)). Ab initio self consistent field molecular orbital calculations using the Random Phase Approximation Localized-Orbital Local-Origin (RPA LORG) method and a polarized split valence basis set yield shift tensor components of −28.9, +2.3 and +26.6 ppm relative to the isotropic value, with the most shielded component 17.6° from the Si-O1 axis. This good agreement is obtained for a SiO 4 4− cluster with forsterite local geometry stabilized by four +1 point charges, PC, with the <Si-O-PC set equal to < Si-Oi-Mg's observed in forsterite. The shielding anisotropy calculated for free SiO 4 4− with Si-O distances and < O-Si-O set equal to those observed in forsterite is much smaller. Likewise, conventional common origin coupled Hartree-Fock (CHF) calculations give shielding anisotropies which are somewhat too small even for the SiO4PC4 model. These results indicate that 29Si NMR shielding tensors in silicates can be explained by simple cluster calculations incorporating only nearest neighbors and simple models for second nearest neighbors.