ENDOR Evidence of Electron–H₂ Interaction in a Fulleride Embedding H₂

An endofulleropyrrolidine, with H₂ as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H₂ has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H₂ guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H₂ guest. The ortho-H₂/para-H₂ interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic ortho-H₂ to the diamagnetic para-H₂ causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride.