Solid-state dynamics in the closo- carboranes: A 11b MAS NMR and molecular dynamics study

Abstract

This work explores the dynamic behavior of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS line widths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (Ic) calculated by molecular dynamics are on the picosecond time scale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from dynamic shift measurements using 11B MQMAS NMR spectroscopy. The behavior of PQeff as a function of temperature for p-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures, and this was supported by the molecular dynamics simulations.