Silicon-29 and carbon-13 NMR spectra of model alkylsilanes of the type (CH3)3-nXnSi(CH2)mH

Abstract

29Si and 13C NMR chemical shifts are presented for the compounds of the type (CH3)3-nXnSi(CH2)mH with various combinations of substituent X (X = C2H5, OC2H5, OC(O)CH3, and Cl), their number n, and alkyl chain length m (m = 1-5). Silicon-29 chemical shifts are shown to follow general trends of the dependence on the total charge or on the number and nature of the substituents on the silicon atom, the alkyl chain length playing only a minor role. It is demonstrated that the silicon upfield shift due to a γ-methyl group varies between is -0.9 and -1.8 ppm in the compounds, the silicon atom of which bears only one secondary carbon atom. The 13C chemical shifts also follow the general dependence on the total charge. In the trimethylsilyl derivatives methylene carbon chemical shifts correlate with the corresponding shifts in carbon analogues and with those in hydrocarbons in which the trimethylsilyl group is replaced by a methyl group. For the short alkyls (m = 1-3) the substituent chemical shifts of various silyl groups depend on the alkyl chain length, α and β effects seem to depend on the number and nature of the substituents on the silicon atom.