a553
J. Phys. Chem. 1993,97, 8553-8556
Rotational Dynamics of Solid C,o Monitored by Positive Muon Spin Labels T.John S. Dennis and Kosmas hassides' School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, UK
Emil Roduner Physikalisch- Chemisches Institut, Universitiit Ziirich, CH-8057 Ziirich, Switzerland
Luigi Cristofolini and Roberto DeRenzi Department of Physics, University of Parma, I-431 00 Parma, Italy Received: May 10, 1993
We report the results of an avoided-level-crossing muon spin resonance study of a muonated radical of C70 between 73 and 310 K. The temperature-dependent averaging of the anisotropic hyperfine interaction of the polarized muon spin label is interpreted in terms of the reorientational dynamics of the fullerene. Uniaxial rotation about the long molecular axis appears likely even a t 73 K. Reorientational disorder then sets in gradually at 160 K with a small anisotropy persisting across the phase transition in the vicinity of 270 K into the plastic phase.
Following the successful isolation of crystalline fullerene samples, early research focused on understanding their structural and dynamic properties.'-5 Solid Cm is a prototypical plastic crystal with the molecules performing rapid isotropic reorientations. Below 260 K, the rotational dynamics change and the molecules shuffle between nearly degenerate orientations. Then at 85 K, thereis aneffectivefreezingofthemotionanda transition to an orientational glassy state. The understanding of the rotational dynamics of the ellipsoidal (Dsh) fullerene C70 is, however, still in its infancy. Molecular dynamics calculations6 starting with a facecentered-cubic ( f a ) high-temperature phase7 reveal a complicated structural behavior, associated with the presence of strong anisotropy in the molecule. Inelastic neutron scattering measurements show that at high temperatures the molecular motion can be adequately described by an isotropic diffusion model$ below the orientational ordering transition9 in the vicinity of 270 K, well-defined librational modes appear.*JO Experimentally, pSR (muon spin relaxation/rotation/resonance) measurements can be used toprobe the rotational dynamics of molecules in condensed phases." Positive muons may bind an electron to form a muonium atom (Mu p+e-), the light isotope of hydrogen. Mu addition to unsaturated systems may lead to radical formation. The muon spin label can then act as a sensitive probe of the dynamic behavior of the radicals.11-13 A zero-field (ZF) pSRstudy of theendohedral Mu@C70complex~~ has already provided insight into the rotational dynamics of the C70 molecules and revealed the presence of an axial anisotropy below 270 K. In this letter, we present the results of an avoided-level-crossing muon spin resonance ( A L C - 6 R ) study of the dynamics of an exohedral muonated C70 radical between 73 and 310 K. This experimental technique has revealed small anisotropies in the plastic crystal norbornene.l5 In c70, it clearly indicates the persistence of a dynamical anisotropy even at room temperature, with evidence of uniaxial rotations present below 160 K. The C70 powder sample (>98.5% purity, -500 mg) was prepared as described previously.14J6 Sublimation at 650 OC followed by extended annealing at 250 OC (22 days) leads to a fcc highly crystalline material. The sample was characterized by powder X-ray, high-resolution neutron diffraction, and 13C Author to whom correspondence should be addressed.
NMR spectroscopy. Prompt y-ray neutron activation analysis showsa hydrogen content of 0.046(7)% by weight. ALC-$3R1lJ7 experiments were performed using muons with a momentum of 29 MeV/c at the r E 3 port of the accelerator at the Paul Scherrer Institut, Villigen, Switzerland. The 100% spin polarized p+ are stopped in the sample and placed in a variable magnetic field parallel to the muon spin direction. Muon decay positrons, which are emitted preferentially along the instantaneous muon spin direction,are counted in the directions"forward" and "backward" to the muon polarization of the incoming beam, and the decay anisotropy is plotted as a function of field. Avoided crossings of magnetic energy levels can allow for the inversion of the muon spin, which leads to a resonance in the observed signal. The resonance monitored here is a muon spin flip transition (Figure 1) which is made allowed by the anisotropic (dipolar) part of the muon-electron hyperfine interaction, indicated by 4, the anisotropy of axial symmetry in frequency units. The line width of the powder pattern scales with Dl, and the resonance disappears when isotropic reorientational motion sets in on a time scale
