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Asymmetric profile of the totally symmetric stretching band in Raman spectra of bis(trimethylsilyl)acetylene

The asymmetric band profile of the triple stretching C=C Raman band in bis(trimethylsilyl)acetylene was decomposed into a higher and a lower wavenumber part, each described as one half of a Lorentzian bandshape with its own halfwidth (Gamma_high and Gamma_low), but with common band centre and maximum point. Gamma_high and Gamma_low were fitted at seventeen different temperatures ranging from 110 K to 323 K (the melting point occurring roughly at 290 K).It was found that lower wavenumber side of the band was considerably wider than the higher wavenumber side, throughout the whole temperature interval chosen. This unexpected crystal band asymmetry present together with the "normal" phonon spectrum may indicate freezing of orientational disorder of methyl groups, while molecular centres of mass remain positioned on the Bravais lattice - like in the case of so called glassy crystals. The asymmetry of the band above 290 K suggests the existence of a nonisotropic liquid state.

Raman; asymmetric bandshape; glassy crystal

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