29Si and 13C NMR spectra of some alkyldiphenylchlorosilanes, alkyldiphenylsilanoles and bis(alkyldiphenylsilyl)chromates

Abstract

29Si and 13C NMR spectra of alkyldiphenylchlorosilanes, alkyldiphenylsilanoles, and bis(alkyldiphenylsilyl)chromates (C6H5)2Si(R)X have been studied. The chemical shifts δ(29Si) are most affected by nature of the electronegative substituent X, i.e. Cl, OH or CrO3OSi(R). (C6H5)2. The δ(29Si) shifts due to effects of the substituents R can be correlated successfully with values of the Taft constants σ. Slopes of the correlation dependences have negative signs for all the groups of the studied compounds (increasing electron-acceptor ability of substituent R causes upfield shifts of δ(29Si), and their values decrease with increasing electronegativity of the substituent X (sensitivity of the δ(29Si) shifts due to sustituent R increases with increasing electronegativity of the substituent X). Methyl groups at γ-position of the chain of substituent R cause a diamagnetic shift of δ(29Si) by 0.6 to 3.3 ppm. The δ(13C) shifts depend little on nature of the substituent X. Chemical shifts δ(13C) of carbon atoms at α-, β-, and γ-positions of the substituent R with respect to silicon show a good linear correlation with the δ(13C) shifts of the corresponding carbon atoms in aliphatic hydrocarbons CH3R. Slopes of the correlation dependences of all the groups of studied compounds are close to 1, the intercepts for α- and β-positions are negative because of induction effect. Positive value of the intercepts for γ-positions reflects the influence of sterical γ-effect.