Thermodynamic Properties of Galvinoxyl Radical and Its Phenol Derivative; Mechanism of the Phase Transition

Measurements of heat capacity of Galvinoxyl radical and its phenol derivative were carried out in the temperature region from 12 to 300°K. Galvinoxyl radical exhibits a phase transition of first-order nature at 81.5°K which corresponds to the susceptibility anomaly already measured. The enthalpy and entropy of transition amount to 1504.6 J/mol and 18.67 J/(°K·mol), respectively. The entropy change, which corresponds formally to R In 9.45, cannot be explained by taking only the magnetic contribution. On the other hand, its phenol derivative, the corresponding diamagnetic substance, has not thermal anomaly in the above temperature region. X-Ray diffraction patterns at room temperature for both substances agree well with each other in their diffraction angles. From these experimental findings, it seems to be plausible that a kind of transformation of the crystal structure of Galvinoxyl radical takes place at the transition point which may be induced by a pairing of magnetic spins between neighboring molecules.