Structural and magnetic properties of Tc$_n$@C$_{60}$ endohedral metallofullerenes: An {\em ab initio} study*

We use {\em ab initio} spin density functional calculations to study the equilibrium structure and magnetic properties of Tc$_n$@C$_{60}$ endohedral metallofullerenes. The radionuclide $^{99{\rm m}}$Tc is well established in biomedicine as a potent {\em in vivo} diagnostic radiopharmaceutical; its encapsulation in the inert C$_{60}$ shell is expected to limit possible cytotoxicity of radiometal nanoparticles catabolized by the biological host. We find that C$_{60}$ can endohedrally accommodate Tc$_n$ clusters with up to $n=7$. The encapsulation does not change significantly the structure of the enclosed clusters, but reduces the magnetic moment due to a stronger Tc--C hybridization for the larger clusters.