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

The ¹³C NMR spectrum of fullerene C₆₀ in solution displays two small "side peaks" on the shielding side of the main ¹³C 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 ¹³C NMR spectra of endofullerenes, but with slightly different shifts relative to the main peak. We ascribe the small additional peaks to minor isotopomers of C₆₀ containing two adjacent ¹³C nuclei. The shifts of the additional peaks are due to a secondary isotope shift of the ¹³C resonance caused by the substitution of a ¹²C neighbour by ¹³C. Two peaks are observed since the C₆₀ 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 C₆₀. The origin and intensities of the ¹³C side peaks are discussed, together with an analysis of the ¹³C solution NMR spectrum of a ¹³C-enriched sample of C₆₀, which displays a relatively broad ¹³C NMR peak due to a statistical distribution of ¹³C isotopes. The spectrum of ¹³C-enriched C₆₀ 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.