Collect. Czech. Chem. Commun. 1981, 46, 377-390

The role of electronic and steric effects in 29Si-NMR spectra of compounds with Si-O-C group

Jan Schramla, Václav Chvalovskýa, Märt Mägib and Endel Lippmaab

a Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6-Suchdol
b Institute of Cybernetics, Estonian Academy of Sciences, Tallin 200 001, USSR


29Si-NMR chemical shifts are presented for several classes of compounds of general formula (CH3)4-n-mClnSi(OR)m (for n =0 m = 1-4 and for n = 1-3, m = 1). In most of the classes the shifts correlate linearly with polar constants of the substituents R or with Del Re net atomic charges on the silicon. The slopes in these correlations are all positive (i.e. the shielding decreases with electron withdrawal) and decrease with increasing n or m (i.e. the sensitivity of silicon shielding to substituent effects decreases with increasing number of electronegative substituents bonded to the silicon). This finding contradicts the available quantitative theories but can be accounted for by the qualitative model which considers back-bonding to silicon. Each methyl group in the γ position causes' a diamagnetic shift of 2 to 3 ppm, depending on the values of m and n, the effects are additive. Compounds with some heteroatoms in the γ position (R = CH2Cl, CH2Si(CH3)3, but not R = CH2Ge(CH3)3) behave anomalously, the origin of the anomalous shifts could not be ascertained.