Studies of silicon-29 chemical shifts in methylsilyl carboxylates by heteronuclear magnetic double resonance

Abstract

The ¹H and ²⁹Si chemical shifts in Me_nSi(O₂CR)_4–n(n= 1–4) correlate linearly with the electron-withdrawing ability of the carboxylate group as measured by the pK_a of the acid RCO₂H. The relationships indicate that the ¹H shielding depends upon the effective charge on silicon whereas the ²⁹Si shielding varies with the electron imbalance in the Si–O bonds. It is not necessary to invoke pπ–dπ bonding to account for the results, and it is probable that silicon chemical shifts cover a wider overall range than do carbon shieldings.