Low temperature Raman study of dimethylacetylene

On the basis of low temperature Raman spectra from 10 to 170 K of polycryst alline dimethylacetylene (DMA), two stable and two metastable crystalline p hases were detected. One can be identified with a previously reported metas table phase [N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513-516] a nd can be observed from 10 to 75 K, when it transforms into another metasta ble phase. This one, in turn, transforms at 115 It into the stable C2/m pha se. 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, t he strongest bands observed are of a(1s) symmetry. Besides them, we were ab le 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' positio ns 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. (C) 1999 Elsevier Science B.V. All rights reserve d.