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Nuclear magnetic resonance investigations of organosilicon compounds Hunter, Brian K.

Abstract

The nuclear magnetic resonance spectra of a number of organosilieon compounds are examined. The 7.95 Mcs. silicon-29 spectra are obtained for these compounds and the chemical shifts are considered in detail. It is proposed that the trends observed are the result of a competition between two opposing effects; a paramagnetic shift influenced by changes in the effective nuclear charge on silicon and a diamagnetic shift influenced by (p→d)π bonding. The concept of the competitive shift mechanism rationalizes the chemical shifts observed for series of the type MexSiX₄₋x (where: X = 0Me,0Et, 0Ac, Cl, F and C₆H₅) but does not explain the shifts of silicon hydrides. This concept also rationalizes the trends observed in ¹¹⁹Sn chemical shifts but, logically, does not apply to ¹³C chemical shifts where the d-orbital contribution must vanish. None of the observed coupling constants correlate with the ²⁹Si chemical shifts. Available theories are found to be inadequate to describe the coupling constants in these molecules. A new long range coupling is observed between the methyl groups on silicon attached to oxygen and chlorine with J[subscript]HCSiCH of about 0.35 c.p.s.

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