Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

NMR spectra of a few 4-substituted 1,2-bis(trimethylsiloxy)benzenes and positional isomers of bis(trimethylsiloxy)benzene were measured under standard conditions that approach those of infinite dilution. Under these conditions somewhat smaller values of steric or proximity deshielding effects on ²⁹Si chemical shifts are found but the trends established earlier are confirmed; the deshielding effect of the ìsecondî ortho positioned trimethylsiloxy group in a tris(trimethylsiloxy)benzene derivative on ²⁹Si chemical shifts is somewhat smaller. Also, overcrowding of three trimethylsiloxy groups probably drives the outer groups to assume, at least for a part of their time, coplanar conformation which is not encountered in bis(trimethylsiloxy) derivatives. The sensitivity of ²⁹Si chemical shifts to para substitution is in 1,2-bis(trimethylsiloxy)benzenes equal to that in 2-methoxytrimethylsiloxybenzenes.