Quantum Chemical Modelling of the Scandium Sub-Group Metal Endofullerenes

Endofullerenes with one or several metal atoms inside the carbon cage (metallofullerenes) are of considerable practical interest as promising basic materials for creating highly effective contrasting agents for magnetic resonance imaging (MRI) as well as antioxidant and anticancer drugs. These compounds can also be used in spintronics to build nanoscale electronic devices. In the framework of the density functional theory, this work presents a calculation of the structural, electronic, and thermodynamic characteristics of scandium sub-group metal endofullerenes with the number of encapsulated atoms from one to seven in the gaseous phase. The stable structures with symmetries Cs, C2, C3, and Ci, were described. They correspond to the positions of the metal atoms inside the fullerene cage. The theoretical limit for the number of metal atoms at which the endofullerene structure remains stable is six atoms for scandium, four for yttrium, and three for lanthanum. The calculations showed that the most stable structures are the ones with two and three encapsulated atoms. The relationship between the number of encapsulated atoms and the nature of electron density distribution were described. The total charge on the encapsulated metal cluster is positive for Me@C60 – Me3@C60 compounds, weakly positive for Me4@C60 (some of the atoms have negative charge), and negative for Me5C60 – Me6@C60 compounds. The spin leakage effect was described for the structures with a doublet spin state. As for the endofullerenes with three and more encapsulated atoms, this effect is insignificant, which makes the creation of contrasting agents for MRI based on them impractical.