Assignments of the out-of-plane normal vibrations of [1H5]- and [2H5]pyridines were reexamined based on the polarization behavior of the Raman bands and also through the normal coordinate calculation.

The effect of pressure on the Raman active inter- and intramolecular vibrations of the hexachlorobenzene crystal was studied under hydrostatic pressure up to 5.5 GPa. The Raman frequencies of the inter- and intramolecular vibrations increased monotonically and some bands due to the degenerate intramolecular vibrations were resolved into doublets with increasing pressure. The pressure-induced frequency shift and factor group splitting of the bands due to the intramolecular vibrations were calculated using an intermolecular potential of the atom–atom type. These results show that (1) no phase transition takes place in the crystal under pressure up to 5.5 GPa, (2) the observed pressure-induced frequency shift is mainly caused by the repulsive force between chlorine atoms belonging to the adjacent molecules, and (3) the observed splitting of the bands due to the degenerate intramolecular vibrations is caused by the different intermolecular force acting to the a and b vibrational modes.

The effect of pressure on the Raman active inter- and intramolecular vibrations of the pyrazine crystal was studied under hydrostatic pressure up to 5 GPa. The frequency shift of the intermolecular vibrations and the discontinuous variation of the bandwidth of the ν2 and ν1 intramolecular vibrations induced by pressure at constant temperature indicate that the pyrazine crystal undergoes phase transition (change of the molecular orientation in the crystal) under about 1 GPa. The pressure-induced frequency shift of the intramolecular vibrations was calculated using an intermolecular potential of the atom–atom type. The calculation also suggests the occurrence of the phase transition of the crystal.

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.

Low-frequency Raman bands ot the [1H4]- and [2H4]pyrazine crystals were studied at various temperatures between 4.2 and 300 K. All six rotational lattice vibrations were identified based on the isotopic factor (the ratio of the vibrational frequency of the [1H4]- and [2H4]pyrazine crystals) of the individual vibrations. Classification of the rotational lattice vibrations into symmetry species was made through the polarization measurement of the Raman bands in single crystals.