Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

¹³C and ²⁹Si NMR chemical shifts are reported for several 1-trimethylsiloxycycloalkenes and 1,2-bis(trimethylsiloxy)cycloalkenes, (CH₃)₃SiO)_xC_nH_2n-2-x (x = 1,2), dissolved in hexadeuterioacetone. Several correlations of the chemical shifts with structural parameters (ring size, interatomic distances etc.) are noted and an attempt is made to explain the found strong dependence of the ²⁹Si chemical shifts on the ring size. Steric crowding with the nearest CH₂ group drives the trimethylsilyl group out of the double bond plane and thus causes steric inhibition of the resonance of unshered oxygen electron pairs with electrons of the double bond. Since the crowding becomes more accute with increasing ring size, the ring size is in this way projected into the chemical shifts of silicon and olefinic carbons. This mechanism provides a united interpretation of all observed NMR chemical shifts.