Hyperfine Interactions in La@C82 Studied by W-Band Electron Paramagnetic Resonance and Electron Nuclear Double Resonance

The analysis of dipolar and quadrupolar lanthanum hyperfine data measured with electron paramagnetic resonance (EPR) and electron nuclear double resonance reveals that at low temperatures no significant change occurs at the internal binding site of the endohedral complex. We interpret this result as indicative of freezing of the large-scale motion of the encased ion, which is observed at room temperature. Averaging of hyperfine interactions is fast on the time scale of the EPR experiment, preventing drastic changes of dipolar and quadrupolar hyperfine interaction (hfi), providing that the equilibrium position is unchanged. The detection of hfi in disordered samples was possible by invoking orientation selection in the 94-GHz EPR spectrum. Quadrupolar hfi could be directly measured for the first time in a metallo-endohedral fullerene complex.