Abstract

Infrared and Raman spectra have been recorded at room temperature for the following four cyclohexasilanes: dodecamethylcyclohexasilane, (SiMe<sub>2</sub>)<sub>6</sub> (I); all-<i>trans</i>-hexabenzylcyclohexasilane, [Si(H)CH<sub>2</sub>Ph]<sub>6</sub> (II); all-<i>trans</i>-hexaphenylcyclohexasilane, [Si(H)Ph]<sub>6</sub>(III); and dodecaphenylcyclohexasilane, (SiPh<sub>2</sub>)<sub>6</sub> (IV). Vibrational assignments have been proposed on the basis of the local symmetries of the silicon ring skeletons (<i>D</i><sub>3<i>d</i></sub>) and the substituent groups (Me, <i>C</i><sub>3<i>v</i></sub>; Ph and CH<sub>2</sub>Ph, <i>C</i><sub>2<i>v</i></sub>). The four polysilane oligomers can be readily distinguished from one another by their vibrational spectra. For example, characteristic ν<sub>SiH</sub> and δ<sub>SiH</sub> Raman bands of II and III were observed in the 2150–2075 and 790–600 cm<sup>−1</sup> regions, respectively, where the vibrations of III were higher in energy than were those of II. All four oligomers exhibited distinct differences in the ν<sub>SiC</sub>, ν<sub>SiSi</sub>, and δ<sub>SiC</sub> regions located below 800 cm<sup>−1</sup>. Finally, from the observed vibrational band shifts, it appears that the σ-π hyperconjugation between the Ph groups and Si atoms in the >SiR<sub>1</sub>R<sub>2</sub> fragments decreases in the order: IV (R<sub>1</sub> = R<sub>1</sub> = R<sub>2</sub> = Ph) > III (R<sub>1</sub> = H, R<sub>2</sub> = Ph) > II (R<sub>1</sub> = H, R<sub>2</sub> = CH<sub>2</sub>Ph).

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