Endofullerenes: size effects on structure and energy

The geometric and energy characteristics of endohedrals X@C _n (X = He, Ne, Ar; n = 20, 24, 30, 32, 40, 50, 60) were calculated by the density functional theory. The insertion of the helium atom leads to only a slight change in the geometry of the fullerene cage of the endohedrals. As the size of the trapped atom increases, the average C—C bond length increases in proportion to the radii of these atoms (by 0.05 for Ne@C₂₀ and 0.12 for Ar@C₂₀). The inclusion energies of endofullerenes and the pressure of the cage exerted on the endo atom were calculated for all the above-mentioned endohedrals.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.     

Metallofullerenes Encaging Mixed‐Metal Clusters: Synthesis and Structural Studies of GdxHo3−xN@C80 and GdxLu3−xN@C80

Metallofullerenes of Gd_xHo_3?xN@C₈₀ and Gd_xLu_3?xN@C₈₀ encapsulating mixed‐metal nitride clusters were synthesized. Spectroscopic characterization of Gd_xHo_3?xN@C₈₀ and Gd_xLu_3?xN@C₈₀ was employed to reveal their structural and vibrational properties. The structural properties of these species were analyzed by using theoretical calculations. The studies of Gd_xHo_3?xN@C₈₀ and Gd_xLu_3?xN@C₈₀ laid the foundations for these species to be used as multifunctional molecules.     

Quantum chemical calculations of N@Cn endofullerenes (n ≤ 60)

The geometric parameters and energy characteristics of small endofullerenes N@C_n (n = 20, 24, 30, 32, 40, 50) and N@C₆₀ in the quartet ground state were calculated by the B3LYP/6-31G* method. The N atom is located at the center of the carbon cage in all molecules except N@C₃₀, where it is bound to the cage wall. Encapsulation of nitrogen atom has little effect on the fullerene cage geometry for n = 40, 50, and 60. No significant charge transfer from the N endo-atom to the cage was revealed for all the N@C_n endofullerenes studied. The calculated spin density on the nitrogen endo-atom increases as the size (n) of the carbon cage increases. The relative stabilities of C_n fullerenes and corresponding endofullerenes N@C_n are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 15–20, January, 2006.     

Influence of caged noble-gas atom on the superatomic and valence states of C60−

Using high-level ab initio coupled cluster methods, we have conducted a detailed study of two endofullerene anions, He@C^?₆₀ and Ne@C^?₆₀. The main focus was on elucidating the effect of the noble-gas atom on the energy and electronic structure of the stable anion states of the parent C^?₆₀. Our study revealed that the noble-gas atom has no influence on the valence-like states of C^?₆₀, but strongly affects the superatomic-like ²A_g state. The latter was found to undergo pronounced destabilisation in both the He@C^?₆₀ and Ne@C^?₆₀ species, losing more than 70% of its original electron binding energy. The observed destabilisation is due to perturbation of the excess electron density of the ²A_g state inside the C₆₀ cage by the caged noble-gas atom. We show how the results of our analysis can be used to predict the behaviour of the ²A_g state in other closed-shell endohedral complexes. Some possible implications of the observed features of the ²A_g state are discussed.     

Energy of compressed endoatoms and the energy capacity of small endohedral rare-gas fullerenes

The excess energies of the endoatoms within endofullerenes X@C_n (X = He, Ne, Ar; n = 20–50) as compared with the energies of the free atoms X were estimated using the simplest Thomas-Fermi model and density functional calculations at the B3LYP/6-311G* level. The energy capacities of the endofullerenes under study are primarily determined by the contributions of the compressed electronic systems of the endoatoms rather than the stretched fullerene cages. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1939–1942, September, 2005.     

CN bond orientation in metal carbonitride endofullerenes:A density functional theory study

The geometric and electronic structures of scandium carbonitride endofullerene Sc_3CN@C_(2n)(2 n = 68, 78, 80, 82,and 84) and Sc(Y)NC@C_(76) have been systematically investigated to identify the preferred position of internal C and N atoms by density functional theory(DFT) calculations combined with statistical mechanics treatments. The CN bond orientation can generally be inferred from the molecule stability and electronic configuration. It is found that Sc_3CN@C_(2n) molecules have the most stable structure with C atom locating at the center of Sc3 CN cluster. The CN bond has trivalent form of [CN]~(3-)and connects with adjacent three Sc atoms tightly. However, in Sc(Y)NC@C_(76) with [NC]~-, the N atom always resides in the center of the whole molecule. In addition, the stability of Sc_3CN@C_(2n)has been further compared in terms of the organization of the corresponding molecular energy level. The structural differences between Sc_3CN@C_(2n) and Sc_3NC@C_(2n)are highlighted by their respected infrared spectra.