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

Abstract

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.

Keywords

• Quadrupolar spin-relaxation
• Glassy crystals
• Second-order quadrupole interaction
• 11B NMR