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

Volume 47, Number 10 (Oct. 1993) Page 1571-1576

Li, Hongqi; Butler, Ian S.; Harrod, John F.


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 (D3d) and the substituent groups (Me, C3v; Ph and CH2Ph, C2v). The four polysilane oligomers can be readily distinguished from one another by their vibrational spectra. For example, characteristic νSiH and δSiH 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 νSiC, νSiSi, and δSiC regions located below 800 cm−1. Finally, from the observed vibrational band shifts, it appears that the σ-π hyperconjugation between the Ph groups and Si atoms in the >SiR1R2 fragments decreases in the order: IV (R1 = R1 = R2 = Ph) > III (R1 = H, R2 = Ph) > II (R1 = H, R2 = CH2Ph).