NMR, Calorimetric, and Diffraction Study of Molecular Motion in Crystalline Carboranes

Abstract

Molecular motion of two nearly icosahedral molecules, ortho- and meta-carborane, in the solid state has been studied by x-ray diffraction, calorimetric, and NMR techniques. At 25°C the powder diffraction patterns for both crystals are consistent with a fcc unit cell of dimension 9.86 Å containing four molecules. For both isomers the corresponding molecular site symmetry is higher than the molecular symmetry, implying molecular orientational disorder at this temperature. That the orientational disorder is of a kinetic nature is confirmed by NMR measurements. The proton resonance line second moment at 25°C is 0.711 ± 0.051 G2 for o-carborane and 0.82 ± 0.10 G2 for m-carborane. This can be compared with a line second moment of 0.630 G2 calculated assuming rapid isotropic molecular reorientation and a line second moment of 34 G2 calculated for the rigid lattice. For both isomers the observed second moment is much smaller than calculated assuming rotation about the molecular twofold axis, implying rapid reorientation between thermodynamically indistinguishable orientations. Measurements above and below room temperature were made only for the ortho isomer. A phase transition was observed at 4.0 ± 3.0°C both by calorimetry and x-ray diffraction. The associated transition enthalpy and entropy were respectively 0.84±0.05 kcal/mol and 3.0 e.u. The proton resonance line second moment observed at − 0.6°C is 1.13 ± 0.1 G2, implying a rather general molecular reorientation between indistinguishable molecular orientations at this temperature in the low temperature phase. With decreasing temperature the line second moment rapidly increases in the vicinity of − 80°C. At − 98°C, the lowest temperature measurement, molecular reorientation is still rapid enough to significantly narrow the resonance line. No high entropy phase transition was found by calorimetric measurements from below the 4°C transition to − 130°C, indicating either that an order–disorder transition lies below − 130°C or that for kinetic reasons the ordered phase is not attained, necessitating a nonzero entropy at 0°K. On increasing temperature above 25°C, no change was observed in line second moment until about 200°C. Above this temperature the linewidth and second moment decrease due to rapid molecular translational diffusion. Apparent activation parameters for molecular rotation and for translational diffusion have been obtained from linewidth measurements.