Conformation of cyclic polysilane in the solid state as studied by variable-temperature 29Si CP/ MAS NMR

Abstract

The 29Si cross-polarization/magic-angle spinning (CP/MAS) spectra of a cyclic polysilane, decaphenylpentasilane (DPPS), which has a five-membered Si ring puckered in an envelope form ( C s symmetry) and a half-chair form ( C 2 symmetry), were measured in the solid state over a wide temperature range, in order to obtain information about their conformational behaviour. In addition, the quantum-chemical calculation of the 29Si shielding constant was done in order to understand clearly the observed 29Si-NMR behaviour and to be able to discuss the conformational behaviour. From these results, it was found that DPPS mainly takes an envelope form at low temperature (ca. 153 K) and takes a half-chair form at high temperature (ca. 393 K). The effect of the ring current due to the phenyl group is discussed.