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-trioxadecyl) adducts, the monoadduct and the trans 1-bisadduct have been studied in 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 constants 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 distribution on the fullerene sphere obtained by restricted Hartree-Fock half-electron approximation (RHF-HE), and the hyperfine coupling constants obtained with the density functional method. The calculation results show a reasonable agreement with the experimental data. The g tensors have been determined by HF-EPR in frozen matrix at low temperature, the monoadduct showing a rhombic tensor and the bisadduct an axial one. The temperature dependence of the EPR line widths and of the electron spin-lattice relaxation times measured by Inversion Recovery of the magnetization have been studied. The radical anion of the bisadduct shows line widths decreasing on increasing the temperature, as usually expected, whereas the radical anion of the monoadduct shows the opposite behavior. The electron spin-lattice relaxation times at room temperature are respectively T-1 = 2.8 mus and T-1 = 200 ns. The relaxation behavior has been discussed and compared with that of the C-60 monoanion and of other fulleropyrrolidine adducts, taking into account the time modulation of the parameters of the spin Hamiltonian due to the reorientation in solution, and the coupling between rotational and electron spin magnetic moments. This spin-rotational mechanism gives rise to the short T1 and anomalous temperature dependence of the line widths in the monoadduct, whereas the larger stiffness of the bisadduct reduces this effect.