“Unexpected” 29Si NMR Chemical Shifts in Heteroatom-Substituted Silyllithium Compounds: A Quantum-Chemical Analysis
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Abstract
Previous 29Si NMR spectroscopic investigations of various heteroatom-substituted silyllithium compounds revealed “unexpectedly” high silicon chemical shifts. To find explanations for these observations, the 29Si chemical shift tensors of various methoxy-, dimethylamino-, and methylthio-substituted chloro and lithiosilanes have been evaluated by quantum-chemical calculations. Substituent effects on shielding have been analyzed by IGLO-DFT calculations. Popular notions on the influence of atomic charge and substituent electronegativity on the central-atom shifts are not applicable. The combination of very electronegative and electropositive substituents leads to large deshielding contributions in heteroatom-substituted silyllithium compounds. As shown by analyses of localized molecular orbital (LMO) contributions to shielding, this is due to a coupling of bonding and antibonding orbitals with large silicon character and small energy differences. After a comparison of IGLO-LMO results and GIAO-based natural chemical shielding analyses, the LMO interpretations have been augmented further by an inspection of natural bond orbitals, to provide a consistent interpretation of the substituent effects on chemical shifts. The presented results should be transferable also to other heteroatom-substituted p-block main-group compounds.
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