13Cand23NaNMR studies ofNa2C60andNa6C60fullerides

We report on ${}^{13}\mathrm{C}$ and ${}^{23}\mathrm{Na}$ NMR measurements on ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ and ${\mathrm{Na}}_6{\mathrm{C}}_{60}$ compounds. The room-temperature ${}^{13}\mathrm{C}$ NMR spectra of ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ and ${\mathrm{Na}}_6{\mathrm{C}}_{60}$ samples present a narrow isotropic line at 172 and 176 ppm, respectively. The ${\mathrm{Na}}_6{\mathrm{C}}_{60}$ resonance is shifted 20 ppm more down field than the resonances of $A_6{\mathrm{C}}_{60}$ compounds with heavier alkalis, indicating a partial charge transfer to the threefold degenerate $t_{1u}$ level which is totally filled in the latter compounds. The ${}^{23}\mathrm{Na}$ NMR spectrum of $A_2{\mathrm{C}}_{60}$ shows one line at 73 ppm and the one of $A_6{\mathrm{C}}_{60}$ presents two lines at 73 and 147 ppm. The intensity ratio of the latter lines is about 2:1. According to previously reported x-ray data we attribute the line at 147 ppm to the Na tetramers in the octahedral sites and the line at 73 ppm to the Na cations in the tetrahedral ones which are singly occupied.