Physical Chemistry

D.A. Neumann, J.R.D. Copley, R.L. Cappelletti, W.A. Kamitakahara, et al., "Coherent quasielastic neutron scattering study of the rotational dynamics of C60 in the orientationally disordered phase," Physical Review Letters, 67:3808, 1991. Dan Neumann (National Institute of Standards and Technology, Gaithersburg, Md.): "One of the most compelling features of the molecule C60 is its beautiful symmetric structure. On first reflection, however, there is an apparent paradox, since the molecular sha

Dec 7, 1992
The Scientist Staff

D.A. Neumann, J.R.D. Copley, R.L. Cappelletti, W.A. Kamitakahara, et al., "Coherent quasielastic neutron scattering study of the rotational dynamics of C60 in the orientationally disordered phase," Physical Review Letters, 67:3808, 1991.

Dan Neumann (National Institute of Standards and Technology, Gaithersburg, Md.): "One of the most compelling features of the molecule C60 is its beautiful symmetric structure. On first reflection, however, there is an apparent paradox, since the molecular shape, that of a truncated icosahedron, contains five- fold symmetry axes that cannot be ideally accommodated within a crystal structure. X-ray and neutron powder diffraction measurements, nevertheless, show that at room temperature solid C60 adopts a face-centered cubic structure.

"NMR results (C.S. Yannoni, et al., Journal of Physical Chemistry, 95:9, 1991; and R. Tycko, et al., J. Phys. Chem., 95:518, 1991) suggested the resolution of this difficulty by demonstrating that dynamic disorder in the orientations of the molecules exists at room temperature. We became intrigued with the question of exactly how these molecular reorientations occur. Neutron scattering provides an exceptional tool for addressing this problem because thermal neutrons have wavelengths comparable to interatomic spacings and energies comparable to typical energies of the excitations in condensed matter. This combination allows one to determine both the geometry of molecular reorientations and correlations between molecules.

"Our results show that at room temperature the reorientations of C60 can be described as `rotational diffusion,' that is, the molecules tumble randomly through a continuum of orientations rather than executing jumps among some discrete set of molecular orientations. Thus, the molecules fit into the crystal structure by becoming essentially spherical. This work also demonstrates that the rotations of adjacent C60s are, to a good approximation, uncorrelated. Therefore, one can regard the room-temperature phase of solid C60 as an `orientational liquid' in which the centers of the molecules are fixed."