ORIENTATIONAL DISORDER IN SOLID CUBANE - A THERMODYNAMIC AND C-13 NMR-STUDY

Through a combination of adiabatic calorimetry, differential scanning calorimetry, and C-13 NMR spectroscopy, cubane (C8H8, pentacyclo[4.2.0.0(2,5)0(3,8).0(4,7)]octane) has been found to exhibit rapid reorientational motion in the solid state at room temperature. The molecules rotate more rapidly as the temperature is increased, and this motion culminates in a transition (T(tr) = 394.02 +/- 0.04 K, DELTA(tr)H = 5940 +/- 20 J mol-1, DELTA(tr)S = (1.849 +/- 0.006)R) to an orientationally disordered solid phase that exists for a short temperature range, 11 K, prior to melting at 405 K. The entropy change at the melting point (DELTA(fus)S = (2.7 +/- 0.1)R) is typical for a solid composed of globular-shaped molecules with "plastic crystalline" characteristics.