THE SEARCH FOR A POSSIBLE TWIST PHASE-TRANSITION IN A BIPHENYL ANALOG- HEAT-CAPACITY OF CRYSTALLINE TOLANE

The heat capacity of crystalline tolane (diphenylacetylene) has been m easured by adiabatic calorimetry between 9 and 303 K in order to see a ny phase transitions like those found in a Similar compound, biphenyl. No thermal anomaly has, been detected. The phonon dispersion relation of the compound is calculated using a simple model and compared with that of biphenyl. It is shown that the absence of a phase transition i n crystalline tolane is consistent with the calculated dispersion rela tion. A certain height of the potential barrier at the planar conforma tion in the intramolecular potential for twisting motion is essential for the lattice instability of crystalline biphenyl. Some smoothed the rmodynamic quantities are tabulated.