Structural characterization of C60 and C70 fullerenes by small-angle neutron scattering

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Abstract
Small‐angle neutron scattering (SANS) is a proven tool for examining the structure and interactions of particles in solution, though the dimensions of carbon‐cage molecules are close to the lower resolution limit of the technique. Deuterated solvents (toluene‐d8 and benzene‐d6) have virtually no scattering contrast with carbon, and the high incoherent cross section of protonated (hydrogen containing) solvents severely limits the path length of solutions by reducing the sample transmission. We have circumvented these difficulties by using CS2 as a solvent which has good contrast with carbon, and a low incoherent cross section which allows the use of long sample path lengths (up to ∼10 cm). In addition, CS2 has good solubility for fullerenes and these properties permit the measurement of the radii of gyration (Rg) of both C60 (Rg=3.82±0.05 Å) and C70 (Rg=4.13±0.05 Å). These dimensions are similar to those calculated from the atomic coordinates after allowing for a solvent exclusion volume. Close agreement between the measured absolute scattering cross sections and the values calculated from the particle and solvent scattering length densities serves as a cross check on the validity of this methodology. To our knowledge, this represents the first successful application of SANS for the characterization of fullerenes. SANS makes it possible to study the size and shapes of modified buckyballs, solute/solvent interactions, crystal growth from saturated solutions, and temperature dependent transitions in solutions.