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Optically clear crystals ofresorcinal [C 6 H 4 (OHh] were grown and employed in the Raman and calorimetric studies of the phase transitions. The enthalpy and entropy change for the solid-solid (a-{3) phase transition at 369 ± 6 K are 1.370 ± 0.007 kJ mol-I and 3.71 ± 0.05 J K-I mol-I, respectively. The slope (dp/dn of the a-{3phase boundary at 1 atm is negative ( -16.7 atm K -I ). The characteristic Raman features of the a-/3 transition are the disappearance of the librational mode at 102 cm -1 and one of the O-H stretching modes at 3060 cm -I (298 K and 1 atm), while other vibrations remain continuous in both phases. The pressure induced a-{3 phase transition at room temperature was observed in a diamond anvil cell at 5 k.bar. The enthalpy of fusion of tJ-resorcinal at 382.8 K is 20.89 ± 0.07 kJ mol-, and the entropy change is 54.59 ± 0.18 J K -I mol-'.

COOD)2.2D20 are reported. The 0 -H stretch frequencies correlate well to the changes in the hydrogen bond length and the bond angle. The band width of the stretch frequencies, however, is not related to the hydrogen bond strength. The a phase of oxalic acid dihydrate is the more stable, low temperature form and the p crystals were observed to transform to the (Y phase.

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