Vibrational Spectra of Cyclic Polysilanes: Dodecamethylcyclohexasilane,All- trans -Hexabenzylcyclohexasilane,All- trans -Hexaphenylcyclohexasilane,and Dodecaphenylcyclohexasilane

Abstract

Infrared and Raman spectra have been recorded at room temperature for the following four cyclohexasilanes: dodecamethylcyclohexasilane, (SiMe₂)₆ (I); all-trans-hexabenzylcyclohexasilane, [Si(H)CH₂Ph]₆ (II); all-trans-hexaphenylcyclohexasilane, [Si(H)Ph]₆(III); and dodecaphenylcyclohexasilane, (SiPh₂)₆ (IV). Vibrational assignments have been proposed on the basis of the local symmetries of the silicon ring skeletons (D_3d) and the substituent groups (Me, C_3v; Ph and CH₂Ph, C_2v). The four polysilane oligomers can be readily distinguished from one another by their vibrational spectra. For example, characteristic ν_SiH and δ_SiH Raman bands of II and III were observed in the 2150–2075 and 790–600 cm⁻¹ 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_SiC, ν_SiS, and δ_SiC regions located below 800 cm⁻¹. Finally, from the observed vibrational band shifts, it appears that the σ-φ hyperconjugation between the Ph groups and Si atoms in the > SiR₁R₂ fragments decreases in the order: IV (R₁ = R₂ = Ph) > III (R₁ = H, R₂ = Ph) > II (R₁ = H, R₂ = CH₂Ph).