Low-temperature phase transitions and reorientational dynamics studied by 11B NMR in glassy crystal ortho-carborane

Ortho-carborane condenses into a plastically crystalline state in which the icosahedrally shaped molecules perform various kinds of motion. Using ¹¹B nuclear magnetic resonance (NMR), the molecular motions and the phase transitions occurring in solid ortho-carborane are revisited. The motional narrowing of the ¹¹B spectra and the spin-relaxation times are monitored over wider temperature ranges than accessed previously. The spin-relaxation times are successfully described using an approach that takes second-order quadrupolar effects and a distribution of correlation times explicitly into account. Our work resolves a discrepancy previously noted for ortho-carborane when comparing activation energies from dielectric spectroscopy with those from NMR studies. In the temperature range between about 160 and 210 K a fast contribution to the longitudinal spin-lattice relaxation is observed which we interpret as a reporter of phase transitions occurring in ortho-carborane. Quantum chemical calculations are performed, not the least to assess possible effects of cross-correlated relaxation phenomena and to check whether or not the ¹¹B quadrupolar coupling constants display significant intermolecular contributions.