The exchange coupling between the valence electrons of the fullerene cage and the electrons of the N atoms in N@C60^{-1,3}

MCSCF calculations are performed in order to determine the exchange coupling between the 2p electrons of the N atom and the LUMOs of the fullerene cage in the case of mono- and tri-anions of N@C60. The exchange coupling resulted by our calculations is large compared to the hyperfine interaction. The strong coupling can explain the missing EPR signal of the nitrogen in paramagnetic anions.