Magnetic interaction in diluted N@C60

Nitrogen in C60 is a paramagnetic atom with a magnetic moment corresponding to the S=3/2 electronic spin. The dipolar interaction of two N@C60 at closest distance, i.e. 1 nm apart, reaches a maximum value of 1.85 mT or 52 MHz, more than three times larger than the hyperfine interaction with the nuclear spin. For diluted N@C60 in a C60 matrix, the distribution of interactions leads to a line broadening in EPR experiments. We report here on a EPR line width measurement for N@C60 in C60 with concentrations varying by a factor of 1000. For the first time a concentration in the percent range was reached by repeated HPLC enrichment. A surprising and completely unexpected result is that sharp lines appear superimposed on the broad lines. We attribute the sharp line to motional narrowing of N@C60 diffusing on the surface of C60 grains. It can be assumed that the diffusion seen here is a general phenomenon of C60, the doped fullerene being just the indicator.