Volume 20. number 6 CHEMICAL PHYSICS LETTERS 15 Ju!y 1973 zyxwvutsr PHONON RAMAN SPECTRA, MOLECULAR MOTIONS, AND PHASE TRANSITIONS OF DLMETHYLACETYLENE CRYSTAL* Received 8 March 1973 The phonon Raman spectra of crystals of dimethy~~tylene, pcrdeyrero-cjimetfiylacetylene and their n%ures have been investigated down to 20°K. The high temperature phase shows only one Iine and the Iow temperature phase only two. Neither of these is related to the R, (metbyi) rotation, which seems to remain pmctidiy free. How- ever, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here fist, displayins 5-6 phonon bands. The well-known A-point transition appears little related to methyl rotations. The in- ternal rotation of d~~:hylacetyiene seems to stay practi&y free down to 20°K in the stable phases. 1. Introduction In view of the fact that the phonon states are en- tirely due to solid state interactions and since their transitions are very sensitive to the unit cell symmetry, one would expect the phase transition to manifest it- self as a change in the phonon spectra. The phonon spectra, however, have not been wideIy used to in- vestigate the phase transition phenomena in molecular solids. On the other hand, a temperature dependent study of the heat capacity has conventionally been used to detect and investigate phase transitions in &ding that of d~e~ylacetylene [I]. In this paper we present a Rarnan phonon study of the phase transi- tion in dimethylacetylene (DMA) crystal which, in our opinion, provides a clear example of how sensitive the phonon spectrnm is to the phase transition. Also, with the heIp of the phonon spectra we have dis- covered a new metastable phase of the D?viA crystal which has been missed before by all other methods, including a very recent NMR study [2]. The reason is that this phase does not appear if the crystal is cooled slowly. The Raman phonon spectra provide us with. some insight into the nature.of the phase transition at the molecular level, though the investigation at this * #pported b$ NIH Cranl NSD8116. stage is only fragmentary. A compIete kvestigation the phase transition in D&fAwiti be reported Iarcr. 2. Experimental of DMA-h6, DMAdc and their various concentration mixtures were sealed in a capillary under vacuum. The ilow of nitrogen vapor over the capillary provided a temperature bath from room temperature to 100°K. To get a lower temperature (down to 7,O” K) helium vapor was used. The Raman spectra were taken on a Ramalog Sp.ex double spectrometer model 1401 at a resolution of 1 cm-r. The excitation Iine was the 4880 .k laser emission from a Coherent Radiation Laboratory argon ion laser. 3. Results and discttssion Fig. I shows the Raman phonon spectra of DMA-hg in the three different crystalline phases. Phase I is a stable phase which persists down to lSS*K when cooled slowIy. When cooied’fast with nitrogen vapor, this phase can very often be su?ercooied [3 ] . In this phase we see one band at 71 cm-’ (200°K value) Ghich shifts to 80 cm’ I in the supercooled state at 513