29Si NMR spectra of methylphenylcyclotri‐and ‐tetrasilazanes

Abstract

²⁹Si NMR spectra of various methylphenyl‐substituted cyclotri‐ and ‐tetrasilazanes, composed of different combinations of the unitsd= Me₂SiNH,d^ph= MePhSiNH andd= Ph₂SiNH, have been investigated. For most of the compounds having more than one asymmetric centre, the spectra of both the individual isomers and their mixtures have been studied, and the signals of the individual isomers have been assigned. The effect of a different arrangement of phenyl groups in the rings, and the influence of the stereochemistry of the isomers on the²⁹Si chemical shifts in cyclotri‐ and ‐tetrasilazanes have been studied. The²⁹Si chemical shifts in cyclotrisilazanes are additive and can be represented as the sum of increments corresponding to the number and positions of the phenyl groups in the rings. In these compounds, the spatial structure of the isomers does not exert any noticeable effect on the spectra. On the contrary, chemical shifts are not simply additive in cyclotetrasilazanes: a pronounced stereochemical dependence is apparent.