ON REASONS OF SI-29 NMR CHEMICAL SHIFT STRUCTURE RELATIONS FOR SILICON-OXIDES, NITRIDES, AND CARBIDES - AN INDIVIDUAL-GAUGE-FOR-LOCALIZED-ORBITALS STUDY/

For alpha-quartz, monoclinic ZSM-5, alpha-and beta-Si3N4 and SiC-6H po lytype, the silicon chemical shifts have been calculated using the IGL O (individual gauge for localized orbitals) method and models of diffe rent size in real crystal geometry. The result is a theoretical chemic al shift scale, which is very similar to the corresponding experimenta l scale from Si-29 MAS NMR experiments. It is shown that the assignmen t of isotropic silicon chemical shifts of crystallized solids based on theory is a method of practical applicability, also in cases where ex perimental methods or empirical relations fail. The two NMR spectral l ines of alpha-Si3N4 are for the first time assigned to the crystallogr aphic positions. The partition of the silicon chemical shifts into loc alized 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. Published by Elsevier Science B.V.