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

Abstract

Thermodynamically stable La@C_2v(24 107)-C₇₈, La@D_3h(24 109)-C₇₈, La@C₁(22 595)-C₇₈, Ce@D_3h(24 109)-C₇₈, Sm@C_2v(24 107)-C₇₈, and Sm@D_3h(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 C_2v(24 107)-C₇₈ are unpaired. Thus, the surface of the endohedral metallofullerene, Sm@C_2v(24 107)-C₇₈, firstly displays diradical characteristics. Notably, the spin states of the two electrons transferred from Sm to D_3h(24 109)-C₇₈ are different from those on C_2v(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.