Magnetic endohedral metallofullerenes with floppy interiors

It is shown that ${\mathrm{Gd}}_3\mathrm{N}@{\mathrm{C}}_{80}$ is a highly magnetic and very stable motif that allows enhanced contrast magnetic resonance imaging and electric dipole moment with potential for cancer treatment. Using a synergistic approach combining Stern-Gerlach experiments in beams and first-principles electronic structure studies, it is demonstrated that an isolated ${\mathrm{Gd}}_3\mathrm{N}$ has a ground state spin moment of $23{\mu }_B$ followed by a noncollinear state of $17.2{\mu }_B$ only $88\mathrm{meV}$ above the ground state. The large moment is largely due to localized $f$ electrons. As a ${\mathrm{Gd}}_3\mathrm{N}$ is embedded inside a ${\mathrm{C}}_{80}$ cage, the localized $f$ electrons maintain the magnetic character while the hybridization between the $s$, $d$ states of isolated ${\mathrm{Gd}}_3\mathrm{N}$ and $p$ states of ${\mathrm{C}}_{80}$ leads to a strongly bound motif with an interaction energy of $13.63\mathrm{eV}$ and a large highest-occupied-molecular-orbital–lowest-unoccupied-molecular-orbital gap of $1.48\mathrm{eV}$. ${\mathrm{Gd}}_3\mathrm{N}@{\mathrm{C}}_{80}$ is further shown to possess two isomers corresponding to the location of the N atom on either side of the ${\mathrm{Gd}}_3$ triangle with an appreciable electric dipole moment and a low barrier of $91\mathrm{meV}$ for transition between them offering potential for a fluctuating dipole.

Figure 1

(Color online) TOF mass spectra of gadolinium clusters. Top trace: pure helium carrier gas; bottom trace: 0.05% ${\mathrm{N}}_2$ added to carrier gas. The small peaks to the high mass side of the ${\mathrm{Gd}}_n$ mass peaks in the top trace are due to the cluster monoxides, ${\mathrm{Gd}}_n\mathrm{O}$.

Figure 2

(Color online) Position-sensitive time-of-flight spectra of ${\mathrm{Gd}}_3\mathrm{N}$. Solid trace: Deflection magnet off ($B=0\mathrm{T}$, $\partial B∕\partial z=0\mathrm{T}{\mathrm{m}}^{-1}$). Dashed trace: Deflection field on ($B=0.98\mathrm{T}$, $\partial B∕\partial z=192\mathrm{T}{\mathrm{m}}^{-1}$).

Figure 3

(Color online) The equilibrium geometries of the Gadolinium clusters for the collinear ferromagnetic configurations and noncollinear magnetic configuration. The total spin magnetic moment is also given (in unit of ${\mu }_B$).

Figure 4

(Color online) One electron energy levels for the fullerene ${\mathrm{C}}_{80}$. The three LUMO orbitals are indicated by red lines and their charge density surrounding the cage is shown by the green isosurface with the value of $0.03\text{electrons}\text{per}{\mathrm{\AA }}^3$.

Figure 5

(Color online) The isosurface of the Bader charge volume of the molecule with the value of $0.7\text{electrons}\text{per}{\AA }^3$ as the molecule ${\mathrm{Gd}}_3\mathrm{N}$ is embedded in the fullerene ${\mathrm{C}}_{80}$ cage. (b) One electron energy levels in spin majority and minority channels for the ${\mathrm{Gd}}_3\mathrm{N}$ embedded in the ${\mathrm{C}}_{80}$ cage. The densities of states are projected on the N and Gd atomic sites.

Figure 6

(Color online) Inside the ${\mathrm{C}}_{80}$ cage, the molecule ${\mathrm{Gd}}_3\mathrm{N}$ has the energy barrier for the N atom moving through the ${\mathrm{Gd}}_3$ plane. The labels correspond to the configurations along the path. A1 and A2 are two mirror configurations and S is the transition state.