C₆₀ solid state rotational dynamics and production and EPR spectroscopy of fullerenes containing metal atoms

Abstract

The narrow single-line NMR spectrum of solid C60 at ambient temperature demonstrates that the molecules rapidly reorient within the crystal lattice. The rate of this motion can be quantitatively probed using solid-state NMR by measuring the relaxation rate, 1/T1. The dominant contribution to this rate at ambient temperature arises from the combination of rotation and chemical shift anisotropy (CSA). The contribution, 1/T1CSA, is isolated by measuring the magnetic field dependence of T 1, allowing the molecular reorientational correlation time, tau , to be determined from T1CSA and the CSA tensor components. At 283 K tau =9.1 ps, only three times longer than the calculated tau for free rotation, and shorter than the value measured for C60 in solution (15.5 ps). This tau corresponds to a rotational diffusion constant D=1.8*1010 s-1. Below 260 K a second phase with a much longer reorientation time is observed, consistent with reports of an orientational phase transition in solid C60. In both phases tau shows Arrhenius behaviour with apparent activation energies of 1.4 and 4.2 kcal mol-1 for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The authors succeeded in producing quantities of fullerene cages containing metal atoms sufficient to allow the first characterization of such a species by electron spin resonance spectroscopy. The EPR spectrum of La@C82 shows an octet of lines centered in the region characteristic of fullerene anion radicals, with a small hyperfine splitting which indicates that the spin is predominantly associated with the carbon shell, leaving the lanthanum atom in its preferred +3 oxidation state.