STRUCTURAL CHARACTERIZATION OF C-60 AND C-70 FULLERENES BY SMALL-ANGLE NEUTRON-SCATTERING

Small-angle neutron scattering (SANS) is a proven tool for examining t he structure and interactions of particles in solution, though the dim ensions of carbon-cage molecules are close to the lower resolution lim it of the technique. Deuterated solvents (toluene-d8 and benzene-d6) h ave virtually no scattering contrast with carbon, and the high incoher ent cross section of protonated (hydrogen containing) solvents severel y limits the path length of solutions by reducing the sample transmiss ion. We have circumvented these difficulties by using CS2 as a solvent which has good contrast with carbon, and a low incoherent cross secti on which allows the use of long sample path lengths (up to approximate ly 10 cm). In addition, CS2 has good solubility for fullerenes and the se properties permit the measurement of the radii of gyration (R(g)) o f both C60 (R(g) = 3.82 +/- 0.05 angstrom) and C70 (R(g) = 4.13 +/- 0. 05 angstrom). These dimensions are similar to those calculated from th e atomic coordinates after allowing for a solvent exclusion volume. Cl ose agreement between the measured absolute scattering cross sections and the values calculated from the particle and solvent scattering len gth densities serves as a cross check on the validity of this methodol ogy. To our knowledge, this represents the first successful applicatio n of SANS for the characterization of fullerenes. SANS makes it possib le to study the size and shapes of modified buckyballs, solute/solvent interactions, crystal growth from saturated solutions, and temperatur e dependent transitions in solutions.