The heat capacity and thermodynamic functions of crystalline and liquid triptycene

Abstract

The heat capacity of the propeller-shaped molecule triptycene (C20H14) was measured from 5 to 550 K. No anomaly other than melting was apparent, and the sample (99.999 per cent pure, as determined by analysis of the melting curve) melted at 527.18 K ([Delta]mS = 13.73 cal mol-1 K-1). The crystal density, determined from X-ray measurements, was 1.227 g cm-3. A comparison of the heat capacity of triptycene with that of bicyclo[2.2.2]octane showed that the two were simply related at low temperatures, but that the comparison was not valid beyond 164.25 K where bicyclo-octane has a transition to a restricted-rotor phase. The values of Cp, So (Ho - H0o)/T, and -(Go - H0o)/T for triptycene at 298.15 K were found to be 67.56, 65.48, 33.23, and -32.25 cal mol-1 K-1.