Vibrational Study of 13C-enriched C60 Crystals
  The infrared (IR) spectrum of solid C60 exhibits many weak vibrational modes. Symmetry breaking due to 13C isotopes provides a possible route for optically activating IR-silent vibrational modes. Experimental spectra and a semi-empirical theory on natural abundance and 13C-enriched single crystals of C60 are presented. By comparing the experimental results with the theoretical results, we exclude this isotopic activation mechanism from the explanation for weakly active fundamentals in the spectra. 
Michael C. Martin, J. Fabian, J. Godard, P. Bernier, J.M. Lambert, and   L. Mihaly (State Univ of New York at Stony Brook, U...