NEUTRON-SCATTERING STUDY OF THE LATTICE MODES OF SOLID CUBANE

We use neutron time-of-flight scattering techniques to measure the low-frequency vibrational density of states of solid cubane at 100 and 300 K. Debye-like behavior is observed up to similar to 60 cm(-1) at both temperatures. At 100 K, the density of states exhibits three distinct peaks at 78, 94, and 114 cm(-1) before dropping to zero around 150 cm(-1). We identify the 114 cm(-1) peak with the E(g) librational mode of cubane based on the marked temperature dependence of its frequency and a comparison to Raman measurements. In similar fashion we identify the 94 cm(-1) peak with the A(g) librational mode. At 300 K, the librational modes have softened to the extent that the density of states exhibits a plateau-like feature between 60 and 100 cm(-1). This is consistent with reports of two Raman-active lattice modes at 63 and 85 cm(-1) having A(g) and E(g) symmetry, respectively. The energy scale associated with low-amplitude Librational excitations about an axis of 4-fold symmetry, calculated from the NMR-measured activation energy for reorientation E(a) = 58 +/- 6 kJ/mol, is of order 94 +/- 5 cm(-1), which is consistent with our observations. The heat capacity we calculate from the measured density of states agrees well with recent calorimetric data.