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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.