The equilibrium αβ transition temperature, T αβ, between the α crystalline phase and the β crystalline phase of resorcinol has been determined when both supercooling and superheating effects vanished. The latent volume and dT αβ/dP have been determined by use of a precision pycnometer, and the thermodynamic characteristics of the αβ transition are presented. The P − V − T relations of each phase have also been obtained. All of the thermal expansion coefficient, the isothermal compressibility, and the internal pressure for the higher-temperature, higher-density polymorph, the β crystal, are much larger than those for the lower-temperature, lower-density polymorph, the α crystal. Contributions of the hydrogen bonds and the van der Waals energy to the internal energy have semiquantitatively been analyzed by use of a simple potential model. When the α crystal transforms to the β crystal, the energy of hydrogen bonds decreases due to the breakdown of the hydrogen bonds, whereas the van der Waals energy increases with the contraction in volume. Since the latter cancels the most of the former, we observe the small latent heat. The breakdown of the hydrogen bonds induces a significant change in the potential depths of the covalent structure O−H···O and ionic, proton-transferred structure O-···H−O+ in each hydrogen bond. In consequence, the distribution of the protons between the covalent structure and the ionic structure changes with the transition. The increase in the entropy produced from the redistribution of the protons is the same order of magnitude as the latent entropy.

Hydroquinone crystals have been found to undergo first-order phase transition to a denser polymorph with an enthalpy increment, as does resorcinol. The phase stable at room temperature (called R) transforms to the new phase (designated δ) only under elevated pressures. P-V-T relations of the new phase have been measured, and characteristics of the transitions and the triple point among the R phase, the δ phase, and the liquid have been estimated from the thermodynamic consistency. The thermal expansion coefficient, the internal pressure, and the heat capacities of the δ phase are significantly larger than those of the R phase. Rigid-body vibrations and/or librations may play an important role in the Rδ transition. Contributions of hydrogen bonds and the structure of the δ phase are discussed.