13C NMR and Theoretical Calculations of Silyl Ketones and Thioketones

Abstract MNDO calculations of (Me3C)2C=X, (Me3C)(Me3Si)C=X, (Me3Si)2C=X, where X=O,S, are reported. It is shown that the large downfield shifts of the carbonyl and thiocarbonyl carbons observed upon silylation correspond to a remarkable increase of negative charge in the same atoms. For acylsilanes a linear correlation between δC=O and λmax is found; this indicates that the mean molecular excitation energy component of the paramagnetic shielding contributes significantly to the chemical shift. The unexpectedly low δC=S and λmax values reported for bis(trimethylsilyl)thioketone are discussed.