On reasons of Si NMR chemical shift/structure relations for silicon oxides, nitrides, and carbides: An individual-gauge-for-localized-orbitals study

For -quartz, monoclinic ZSM-5, -and β-Si₃N₄ and SiC---6H polytype, the silicon chemical shifts have been calculated using the IGLO (individual gauge for localized orbitals) method and models of different size in real crystal geometry. The result is a theoretical chemical shift scale, which is very similar to the corresponding experimental scale from ²⁹Si MAS NMR experiments. It is shown that the assignment of isotropic silicon chemical shifts of crystallized solids based on theory is a method of practical applicability, also in cases where experimental methods or empirical relations fail. The two NMR spectral lines of -Si₃N₄ are for the first time assigned to the crystallographic positions. The partition of the silicon chemical shifts into localized contributions from different parts of the model allows insight into the interactions around the resonance nucleus due to substituent and geometry variations leading to silicon chemical shifts.