Fine structure in the solution state 13C-NMR spectrum of C60 and its endofullerene derivatives

The C-13 NMR spectrum of fullerene C-60 in solution displays two small "side peaks" on the shielding side of the main C-13 peak, with integrated intensities of 1.63% and 0.81% of the main peak. The two side peaks are shifted by -12.6 ppb and -20.0 ppb with respect to the main peak. The side peaks are also observed in the C-13 NMR spectra of endofullerenes, but with slightly different shifts relative to the main peak. We ascribe the small additional peaks to minor isotopomers of C-60 containing two adjacent C-13 nuclei. The shifts of the additional peaks are due to a secondary isotope shift of the C-13 resonance caused by the substitution of a C-12 neighbour by C-13. Two peaks are observed since the C-60 structure contains two different classes of carbon-carbon bonds with different vibrational characteristics. The 2 : 1 ratio of the side peak intensities is consistent with the known structure of C-60. The origin and intensities of the C-13 side peaks are discussed, together with an analysis of the C-13 solution NMR spectrum of a C-13-enriched sample of C-60, which displays a relatively broad C-13 NMR peak due to a statistical distribution of C-13 isotopes. The spectrum of C-13-enriched C-60 is analyzed by a Monte Carlo simulation technique, using a theorem for the second moment of the NMR spectrum generated by J-coupled spin clusters.