Molecular reorientation and self-diffusion in solid cubane by deuterium and proton NMR

Deuterium and proton NMR line shape and relaxation measurements are reported for the sym-cubane-d(2) isotopomer (D-3d symmetry) in the temperature range from 200 K to the melting point of the solid. The results confirm the polymorphic phase sequence: [GRAPHICS] reported earlier by White et al. The deuterium spectrum below 215 K exhibits a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a coupling constant of Q(c) = 178 kHz. Above this temperature the spectrum develops features characteristic of the onset of dynamic processes leading to a single line at about room temperature. Longitudinal relaxation time measurements show a T-1 minimum at 344 K. On transition to solid I there is a discontinuous narrowing of the line and a concomitant increase in TI. These results are quantitatively interpreted in terms of reorientational jumps of the cubane molecules between their various equivalent orientations. The jump rate k in solid II follows an Arrhenius behavior over a range of almost seven decades with an activation energy E = 62 kJ mol(-1). Transition to solid I results in a discontinuous increase of k. Proton line width measurements in solid II show two motional narrowing steps. The first, at around 240 K, is due to the cubic jumps while the second, at around 375 K, corresponds to molecular selfdiffusion between the lattice sites. The activation energy for this process is E(d) = 83 kJ mol(-1). On transition to solid I there is also a discontinuous increase in the rate of this process. The proton T-1 values are predominantly affected by the reorientation process and are consistent with the deuteron data. At temperatures above 340 K, where the deuterium NMR spectrum is expected to be a single Lorentzian with a width of less than 400 Hz, it actually exhibits, in both phases II and I, a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a very small coupling constant Q'(c) = 0.38 kHz. This indicates that the reorientation process is not perfectly cubic. Possible reasons for this surprising effect are discussed.