Temperature-induced phase transitions in 1,3,5-trimethylbenzene crystal

The temperature effect on the Raman- active inter- and intramolecular vibra tions of 1,3,5 -trimethylbenze ne crystal was studied. The spectral structu re due to the intermolecular vibrations changed as the temperature was vari ed and the vibrational frequencies shifted discontinuously at about 95 and 195 K when the temperature rose from 77 K. For the intramolecular vibration s, a discontinuous shift of the vibrational frequencies of the nu(1) and nu (12) modes was observed at about 95 K. Thermograms obtained with a differen tial scanning calorimetry for the crystal show two endothermic peaks at 91 and 188 K as well as three endothermic peaks at 220, 222, and 227 K around the melting point. These observations indicate that the first-order phase t ransitions rake place at 91 and 188 K under 1 arm in a 1,3,5-trimethylbenze ne crystal, and that the crystal has three different crystal structures aro und the melting point between 220 and 227 K.