Modifying the endohedral La-ion location in both neutral and positively charged polyhydroxylated metallofullerenes

We present extensive ab initio density-functional theory calculations in order to analyze the structure and energetics of both neutral and positively charged highly hydroxylated $\text{La}@{\text{C}}_{60}\left(\text{OH}\right)_{32}{}^{+q}$ and $\text{La}@{\text{C}}_{82}\left(\text{OH}\right)_{20}{}^{+q}$ ($q=0$, 1, 2, 4, 6, and 8) metallofullerenes. Interestingly we obtain, for the neutral hydroxylated structures, that the location of the encapsulated La atom strongly depends on the precise distribution of the OH groups on the carbon surface. Actually, we found atomic configurations in which the La ion is attached to different regions of the inner surface as well as atomic arrays in which weakly bonded endohedral species are obtained, being located near the center of the structure. The previous strong variations in the endohedral position of the lanthanum atom leads to sizable modifications in its valence state as well as to notable changes in the electronic spectra and in the lowest-energy spin configuration of the molecules, which are all facts that could be used to better identify the atomic structure of these highly relevant metallofullerenes. The positive charging (by means of simply electron detachment) of our here-considered $\text{La}@{\text{C}}_{60}{\left(\text{OH}\right)}_{32}$ compounds reveals that the rupture of various C-C bonds of the carbon network can be achieved. However, even if the previous bond breakage leads to the formation of sizable holes in the ${\text{C}}_{60}$ cavity (made of nine-membered rings), we obtain that the encapsulated La atom was unable to escape from the cage. In contrast, in the polyhydroxylated $\text{La}@{\text{C}}_{82}{\left(\text{OH}\right)}_{20}$ fullerene, no C-C bond breakage is induced with increasing charge state of the cage, defining thus a more stable molecular compound. The previous results are expected to be of fundamental importance in medical and biological applications where the permanent encapsulation of these highly toxic atomic species is required.

Figure 1

(Color online) Calculated lowest energy atomic configurations for (a) ${\text{C}}_{60}$ fullerene, (b) $\text{La}@{\text{C}}_{60}$ compound, and for [(c) and (d)] two isomers of the ${\text{C}}_{60}{\left(\text{OH}\right)}_2$ fullerene diol. In the left (right) column we show results obtained within the PWSCF (GAUSSIAN03) methodology.

Figure 2

(Color online) Illustration of the PWSCF lowest energy atomic configurations for three representative ${\text{C}}_{60}{\left(\text{OH}\right)}_{32}$ isomers (see text).

Figure 3

(Color online) Illustration of the PWSCF lowest energy atomic configurations for our considered $\text{La}@{\text{C}}_{60}{\left(\text{OH}\right)}_{32}$ compounds (see text).

Figure 4

Comparison of the eigenvalue spectra (obtained with the hybrid PWSCF/GAUSSIAN03 approach) for the empty and La-doped ${\text{C}}_{60}{\left(\text{OH}\right)}_{32}$ fullerenes shown in Figs. 2a, 3a, respectively. In the figure, we show results for both (a) spin-up and (b) spin-down distributions. We only include the 11 occupied molecular orbitals with the highest energies and the 11 unoccupied molecular orbitals with the lowest energies. The energy location of the HOMO for the empty (endohedrally-doped) molecular compound is marked with a horizontal doted (dashed) line.

Figure 5

(Color online) Illustration of the PWSCF lowest energy atomic configurations for the $\text{La}@{\text{C}}_{60}\left(\text{OH}\right)_{32}{}^{+q}$ molecular compound shown in Fig. 3a having different charge states $q$. To the right of each one of the equilibrium configurations obtained we show in addition the structure of the underlying carbon network (by removing all the OH groups from the surface) in order to appreciate the structural deformations induced on the fullerene cavity.

Figure 6

(Color online) Illustration of the PWSCF lowest energy atomic configurations for three representative ${\text{C}}_{82}{\left(\text{OH}\right)}_{20}$ isomers (see text).

Figure 7

(Color online) Illustration of the PWSCF lowest energy atomic configurations for our considered $\text{La}@{\text{C}}_{82}{\left(\text{OH}\right)}_{20}$ compounds (see text).