Anion radicals of mono- and bisfulleropyrrolidines: g tensors, spin density distribution and spin-lattice relaxation

The radical anions of two C-60 Nteg-fulleropyrrolidine (teg = 3,6,9-trioxad ecyl) adducts, the monoadduct and the trans 1-bisadduct have been studied i n liquid solution and glassy matrix of 2-MeTHF by continuous wave (cw) and pulsed X-band EPR and cw high-frequency EPR. The hyperfine coupling constan ts of the N-14 nuclei and of the C-13 nuclei in natural abundance have been determined and discussed, also in relation with the spin density distribut ion on the fullerene sphere obtained by restricted Hartree-Fock half-electr on approximation (RHF-HE), and the hyperfine coupling constants obtained wi th the density functional method. The calculation results show a reasonable agreement with the experimental data. The g tensors have been determined b y HF-EPR in frozen matrix at low temperature, the monoadduct showing a rhom bic tensor and the bisadduct an axial one. The temperature dependence of th e EPR line widths and of the electron spin-lattice relaxation times measure d by Inversion Recovery of the magnetization have been studied. The radical anion of the bisadduct shows line widths decreasing on increasing the temp erature, as usually expected, whereas the radical anion of the monoadduct s hows the opposite behavior. The electron spin-lattice relaxation times at r oom temperature are respectively T-1 = 2.8 mus and T-1 = 200 ns. The relaxa tion behavior has been discussed and compared with that of the C-60 monoani on and of other fulleropyrrolidine adducts, taking into account the time mo dulation of the parameters of the spin Hamiltonian due to the reorientation in solution, and the coupling between rotational and electron spin magneti c moments. This spin-rotational mechanism gives rise to the short T1 and an omalous temperature dependence of the line widths in the monoadduct, wherea s the larger stiffness of the bisadduct reduces this effect.