Synthesis and EPR studies of N@C60 and N@C70 radical anions

Dilute solutions of anion radicals produced from enriched endohedral N@C60 and N@C70 samples were prepared by reduction with lithium in tetrahydrofuran (THF). EPR spectra were measured at room temperature with a Bruker X-Band spectrometer. Although EPR signals from mono- to penta-anions of empty fullerenes gave characteristic signals, no signals could be detected for these reduction states originating from endohedral nitrogen. We believe that increased spin relaxation rates of states of higher spin multiplicity cause a line broadening impeding detection. Continuing reduction finally resulted in the diamagnetic hexaanions, detected by vanishing EPR signals of empty fullerenes and a reappearance of the characteristic three line spectrum of endofullerenes. The chemical shift was quite similar to those measured by the point-like 3He.