Multifrequency EPR Study of Metallofullerenes: Eu@C82 and Eu@C74

Abstract

In this work, the cage structure of a family of europium metallofullerenes, Eu@C74 and three isomers of Eu@C82, were investigated. The analogy of the electronic states of three isomers of Eu@C82 with those of three isomers of Ca@C82 was confirmed by UV−vis−near-IR absorption spectra. The symmetries Cs, C2, and C2v were assigned for three isomers of Eu@C82 by comparing each corresponding isomer of Ca@C82. The D3h cage structure of Eu@C74, which is expected only for the C74 from the isolated pentagon rule, was also confirmed by comparing its photoabsorption spectrum with that of Ca@C74. Multifrequency electron paramagnetic resonance (EPR) spectroscopy was employed to determine the zero-field splitting (ZFS) parameters of the metallofullerenes, which are closely related to the surrounding cage structures. The experimental X- and W-band EPR spectra were completely reproduced by computer simulations based on a spin Hamiltonian considering the ZFS terms up to fourth order. The introduction of the fourth-order ZFS terms into the Hamiltonian resulted in a precise determination of the second-order ZFS terms. A nonvanishing rhombicity parameter E of Eu@C74 demonstrated the reduction of the symmetry from D3h to C2v due to the positioning of Eu2+ ion at the off-center of C74 cage. Moreover, in this work, the semiphenomenological superposition-exchange model (SPEM) was invoked to examine a relationship between the molecular structures and ZFS parameters. All of the experimental ZFS parameters were satisfactory reproduced in terms of the SPEM, leading to confirmation of the molecular symmetries. The semiphenomenological analysis also enabled us to select the most appropriate cage for Eu@C82 with C2 symmetry from among three possible isomers.