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Low temperature Raman study of dimethylacetylene

On the basis of low temperature Raman spectra from 10 to 170 K of polycrystalline dimethylacetylene (DMA), two stable and two metastable crystalline phases were detected. One can be identified with a previously reported metastable phase [N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513-516] and can be observed from 10 to 75 K, when it transforms into another metastable phase. This one, in turn, transforms at 115 It into the stable C2/m phase. To enable better assignment of the observed internal modes, especially those involving methyl group, matrix isolation of DMA at 10 K was performed. In the recorded Raman spectra at matrix ratios of 1:100, 1:50 and 1:25, the strongest bands observed are of a(1s) symmetry. Besides them, we were able to detect bands at 218 and 1040 cm(-1) of e(1d) symmetry, which are only infrared active when no intermolecular interactions are present. The Raman spectrum of DMA as a thin solid film was also recorded. The bands' positions are found not to deviate significantly in comparison with the spectra of the matrix isolated sample, and thanks to the better signal to noise ratio, several additional bands were detected. Among those is the nu(8) at 1147 cm(-1) of a(4s) symmetry.

dimethylacetylene; Raman spectroscopy; phase transitions; matrix isolation

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