The Raman active inter- and intramolecular vibrations of tetramethylpyrazine and trihydrated tetramethylpyrazine crystals were studied under various pressures between 1 atm and 5 GPa. The pressure effect on the Raman frequency due to the intermolecular vibrations indicates that a tetramethylpyrazine crystal undergoes phase transitions under about 1.2 and 2.2 GPa and a trihydrated tetramethylpyrazine crystal undergoes phase transition under about 3.5 GPa. The pressure effect on the Raman frequency due to the intramolecular vibrations indicates that the pressure-induced frequency shift for the skeletal vibrations of trihydrated tetramethylpyrazine is larger than the corresponding shift of unhydrated tetramethylpyrazine, while the shift for the characteristic vibrations of the methyl groups of trihydrated tetramethylpyrazine is smaller than the corresponding shift of unhydrated tetramethylpyrazine. These observations suggest that the attractive force induced by the hydration plays a considerable role in the intermolecular interaction under high pressure in the molecular crystal, in addition to the repulsive force. The change of electron distribution, which strengthens the chemical bonds of the pyrazine ring more strongly than the bonds in the methyl groups, takes place with increasing pressure.

ABSTRACT:The effects of pressure on the frequencies and band structures of the Raman active inter-and intramolecular vibrations of 1,2,4,5-tetrafluorobenzene crystal were studied under hydrostatic pressures from 1 atm to 4 GPa at 300 K in a gasketed diamond anvil cell. Changes of the Raman frequencies and band behaviors of both the inter-and intramolecular vibrations due to the application of pressure indicate that the phase transitions take place under about 0.3 and 3 GPa.