Covalent interactions depend on the distances between metals and fullerenes for thermodynamically stable M@C78 (M = La, Ce, and Sm)

Thermodynamically stable La@C2v(24 107)-C78, La@D3h(24 109)-C78, La@C1(22 595)-C78, Ce@D3h(24 109)-C78, Sm@C2v(24 107)-C78, and Sm@D3h(24 109)-C78 based on density functional theory and statistical thermodynamic analysis are studied in theory. C1(22 595)-C78, violating the isolated pentagon rule, is a second novel isomer stabilized by encaging a La atom. In addition, three- and two-electron transfers occur in M@C78 (M = La and Ce) and Sm@C78, respectively. Although there are two electrons transferred from Sm to C78, these electrons transferred to the surface of C2v(24 107)-C78 are unpaired. Thus, the surface of the endohedral metallofullerene, Sm@C2v(24 107)-C78, firstly displays diradical characteristics. Notably, the spin states of the two electrons transferred from Sm to D3h(24 109)-C78 are different from those on C2v(24 107)-C78, leading to different spin ground states. Furthermore, the natural bond orders and bond critical point analyses on thermodynamically stable M@C78 (M = La, Ce, and Sm) reveal that the distance between the metal and carbon atom plays an important role in the covalent interaction between the inner metal atoms and C78. Because of the strong ionic interaction, the studies on the magnetic character of M@C78 (M = La, Ce, and Sm) show that Sm@C78 is a promising candidate for single-molecule magnets with high isotropic susceptibility. The infrared spectra were simulated to facilitate further experimental study on the stable M@C78 (M = La, Ce, and Sm). We believe that this work will provide good guidance and assistance for the further study of mono-metallofullerenes and coordination compounds in both experiment and theory.