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

Abstract

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 La@ C 60 ͑OH͒ 32 +q and La@ C 82 ͑OH͒ 20 +q ͑q =0, 1, 2, 4, 6, and 8͒ 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 detachment͒ of our here-considered La@ C 60 ͑OH͒ 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 C 60 cavity ͑made of nine-membered rings͒, we obtain that the encapsulated La atom was unable to escape from the cage. In contrast, in the polyhydroxylated La@ C 82 ͑OH͒ 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.

Most stable structures of polyhydroxylated endohedral metallofullerene Gd@C82(OH)x (x=1–24) from density function theory

“…Generally speaking, the Gd atom slightly shifts towards the outer carbon cage as the number of OH groups increases, that is, the nearest Gd-C distance reduces from 2.468 Å for Gd@C 82 to 2.427 Å for Gd@C 82 (OH) 12 and then to 2.391 Å for Gd@C 82 (OH) 2 4. A Similar phenomenon was found in the case of La@C 82 (OH) n [20]. We also examined the shift of Gd's spatial location from Gd@C 82 to Gd@C 82 (OH) 24 (see Table 1).…”

“…2i) with hydroxyl groups separated on two opposite sides of cage are 3.225 eV higher in energy than the most stable one. In previous studies of the polyhydroxylated carbon fullerenes such as C 60 (OH) 32 [19] and La@ 82 (OH) 20 [20], the hydroxyl groups also distributed on the surface homogeneously. This is partially consistent with our results on Gd@C 82 (OH) 24 .…”

“…Previously, there were only few calculations on the structures and stabilities of a few other water-soluble endohedral fullerenes like C 60 (OH) n and La@C 82 (OH) n [19][20][21][22][23]. Currently, even the most stable atomic structures of Gd@C 82 (OH) x are still unknown.…”

“…Meanwhile, 12 hydroxyl groups are separated into two equal parts symmetrically and two carbon hexagons were enclosed by the hydroxylated carbon atoms. Previously, Rodriguez-Zavala [19] determined the most stable structures of C 60 (OH) n (n = 9-12) and found that the absorbed OH groups prefer to aggregate on the two sides of C 60 cage, forming islands with up to seven hydroxyl groups. Obviously, the most stable configuration of C 60 (OH) n is different from that of Gd@C 82 (OH) 12 .…”

Reactivity Indexes and Structure of Fullerenes

“…As our work group has studied fullerenes and their reactivity [7,16,[62][63][64][65] and it has been proposed that C 60 might be an ideal candidate for trapping nitrogen by a polymer [53,54], the intention now is to show that C 70 might also be an alternative way of storing polynitrogenous species, contributing to the development of new materials in the future.…”