Endohedral Molecules Research Articles

(Invited) Metallofullertubes or Metallofullerenes: Which Have We Isolated?

In our labs, we are isolating several novel carbon-metal endohedral molecules of unknown structures with rare-earth metals. In this presentation, we will discuss the feasibility of using chemical reactivity differences (Round 1) to obtain samples enriched in their presence. Next, using a multiple column HPLC approach for further resolution (Round 2), we have isolated and begun a preliminary characterization (e.g., UV-vis) for these pristine samples. We know their chemical formulas, but we do not yet know the structural arrangement of metal and carbon atoms. When sufficient quantities are obtained, we will pursue X-ray crystallography to address the following big question. Namely, do we have metals in tubes (metallofullertubes) or do we have metals in big balls (metallofullerenes). An objective of this presentation is to update and share with the fellow scientific community our most recent advances and progress in the research area of larger carbon cage structural housings (e.g., C96-C120).

Структура молекулы изомера 17894 (С1) фуллерена C76: причины её нестабильности и вероятные пути стабилизации

It is well-known that non-IPR fullerenes are highly unstable. For this reason, they cannot be obtained as pristine fullerenes; however, some of them become stable as derivatives (exohedral or endohedral). In this article, we attempted to elucidate in detail molecular structure for such a non-IPR fullerene. Using theoretical approach supported by DFT calculations, the features of molecular structure of isomer 17894 (C1) of fullerene C76 with data about distribution of single, double and delocalized π-bonds as well its structural formula has been determined for the first time. The instability of the studied fullerene molecule caused by its open-shell structure and significant local overstrains related to the high folding angle value of pentagons in pentalene fragment. The supposed synthesis of the endohedral molecule starts with the ionic pair formation, i.e. anionic fragment of fullerene cage and metal cation electrostatically bound with it. It would lead to closing of open electron shell of fullerene and local overstrain release at pentalene fragment. As to the exohedral derivatives the probable positions of addends are discussed. Both methods in their own demonstrate the possibilities to stabilize the molecule of the C76 isomer 17894. The elucidation and analysis of structural features along with electronic characteristics of non-IPR fullerene molecules appear to be useful for predicting the possibility of their synthesis as derivatives and will assist in determination of their reactivity. This will ensure the targeted production of fullerenes and their derivatives for the needs of medicine, electronics and other industries. The fundamental knowledge of the properties of nanoobjects, namely fullerenes, is actually developing as the independent direction with a long-term perspective.

First Synthesis and Characterization of CH4@C60

AbstractThe endohedral fullerene CH4@C60, in which each C60 fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The 1H spin‐lattice relaxation times (T1) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C60 cage. The synthesis of CH4@C60 opens a route to endofullerenes incorporating large guest molecules and atoms.

First Synthesis and Characterization of CH4 @C60.

The endohedral fullerene CH4@C60, in which each C60 fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The 1H spin‐lattice relaxation times (T 1) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C60 cage. The synthesis of CH4@C60 opens a route to endofullerenes incorporating large guest molecules and atoms.

Ab initio scrutiny of endohedral C20 fullerenes implanted in between gold electrodes.

Using the smallest non-classical fullerene, we investigate the impact of endohedral fullerene molecules on the quantum transport through molecular junctions, and then compared this with the pure C20-based molecular junction. By employing the density functional theory combined with the non-equilibrium Green's function, we contemplated different electronic parameters, namely, density of states, transmission coefficient, energy levels, molecular orbitals, conduction gaps, electron density and their charge transfer. A knowledge of these physical parameters is necessary in order to calculate current and conductance computed using Landauer-Büttiker formalism. The molecule-electrode coupling influenced by endohedral molecules affects junction devices in a unique manner. We observe that the highest quantum transport is possible in an Au-N@C20-Au and Au-O@C20-Au junction device, and is even higher than that of the intrinsic C20 fullerene junction. Another notable observation is that the F@C20 molecule exhibits the least conducting nature, being even lower than that of the endohedral molecule formed by inserting the noble element, neon. Graphical abstract Electrical characteristics of Endohedral fullerene junctions.

NMR of molecular endofullerenes dissolved in a nematic liquid crystal.

We report the NMR of the molecular endofullerenes H2@C60, H2O@C60 and HF@C60 dissolved in the nematic liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA). The 1H NMR lines of H2 and H2O display a doublet splitting due to residual dipole-dipole coupling. The dipolar splitting depends on temperature in the nematic phase and vanishes above the nematic-isotropic phase transition. The 19F spectrum of HF@C60 dissolved in MBBA displays a doublet splitting in the nematic phase, with contributions from the 1H-19F dipole-dipole coupling and J-coupling. The phenomena are analyzed using a model in which the fullerene cages acquire a geometrical distortion, either through interaction with the liquid crystal environment, or through their interaction with the endohedral molecules. The distorted cages become partially oriented with respect to the liquid crystal director, and the endohedral molecules become partially oriented with respect to the distorted cages.

Trifluoromethyl Derivatives of a Monometallic Cyanide Cluster Fullerene, YCN@C82(6)(CF3)16/18.

Recently discovered monometallic cyanide cluster fullerenes are a novel family of endohedral molecules, whose chemical reactivity has not yet been probed. High-temperature trifluoromethylation of the yttrium cyanide cluster fullerene YCN@C82(6), followed by high-performance liquid chromatography separation and an X-ray diffraction study of the two crystallized fractions, resulted in the structural characterization of YCN@C82(6)(CF3)16/18. In both molecules, exohedrally attached CF3 groups delimit the spherical π system into localized double bonds, benzenoid rings, larger aromatic assemblies, and a conjugated fragment with the only intact pentagon that is involved in coordination to the interior Y atom. We also present theoretical results on charge distributions in the compounds reported.

An optimised scalable synthesis of H2O@C60 and a new synthesis of H2@C60.

New high-yielding synthetic routes to the small-molecule endofullerenes H2O@C60, D2O@C60 and H2@C60 are described. The use of high temperatures and pressures for the endohedral molecule incorporation are avoided. A new partial closure step using PPh3, and final suturing using a novel Diels-Alder/retro-Diels-Alder sequence are amongst the advances reported.

Generalized oscillator strength of endohedral molecules

AbstractWe calculate here the main characteristic of the fast electron scattering on endohedrals, namely their generalized oscillator strength (GOS), total and differential in the angle of atomic electron emission. Endohedrals are atoms A staffed inside the fullerene CN and denoted as A@CN. We take into account two major effects of CN on ionization of the atom A: the scattering of the emitted electron by the static potential of the fullerenes shell and the modification of the interaction between the fast incoming and atomic electrons due to polarization of the fullerenes shell by the incoming electron beam. We substitute the complex fullerenes shell CNby a static zero thickness potential, and express its deformation under incoming electron action via CNpolarizabilities. We perform all consideration in the frame of the random phase approximation with exchange that gave reliable results for GOSes of isolated atoms. We limit ourselves with the dipole and the biggest nondipole contributions to the differential in angle GOSes. We compare differential in angle GOSes with differential in angle photoionization cross‐section as a source of information on the target electronic structure and emphasize some advantages of the fast electron scattering. The concrete objects of calculations are noble gas endohedrals Ar@C60 and Xe@C60, and the results are presented for two transferred momentum q values, q = 0.1 and 1.0. Even for small q = 0.1, in the optical limit, the deviation from photoionization case is prominent and instructive. We demonstrate impressive new features of GOSes that appear due to fullerenes shell actions—regular oscillations and very big maxima. As an interesting and very specific object, we study onion‐type endohedrals A@CN1@CN2, in which the construction A@CN1 is stuffed inside CN2 with N2 >> N1. The consideration is similar to that developed in our recent article on total and differential in angle GOSes of isolated atoms (Amusia et al., Phys. Rev. 2012, A 85, 042722). © 2012 Wiley Periodicals, Inc.

Proton Transfer in the Complex H3N⋅⋅⋅HCl Catalyzed by Encapsulation into a C60 Cage

We report proton transfer in the complex H(3)N...HCl to form the ion pair NH(4)(+)Cl(-), which is favored inside the C(60) cage according to quantum chemical calculations. The results show that the NH(4)(+)Cl(-)@C(60) is stable with an interaction energy of -2.78 kcal mol(-1). Compared with the complex H(3)N...HCl without proton transfer, it is found that the C(60) cage plays the role of a catalyst for proton transfer. In NH(4)(+)Cl(-)@C(60) a negative charge area in the C(60) cage is near the cation NH(4)(+) whereas a positive charge area is near the anion Cl(-). Also, a confinement effect of the C(60) cage is noticed, as the endohedral structure of NH(4)(+)Cl(-) is more compact than the structure of NH(4)(+)Cl(-) in the gas-phase complex. These findings indicate that the catalysis by the C(60) cage comes from two effects: 1) electrostatic inducement between the C(60) cage and endohedral molecules and 2) the confinement effect that compresses endohedral molecular structures inside the C(60) cage. In the infrared spectrum, it is found that the confinement effect of the cage can cause large blue shifts of the N-H stretching vibrations in NH(4)(+)Cl(-)@C(60) compared with those in the NH(4)(+)Cl(-)...H(2)O complex.

H 2 , HD, and D2 inside C60: Coupled translation-rotation eigenstates of the endohedral molecules from quantum five-dimensional calculations

We have performed rigorous quantum five-dimensional (5D) calculations of the translation-rotation (T-R) energy levels and wave functions of H(2), HD, and D(2) inside C(60). This work is an extension of our earlier investigation of the quantum T-R dynamics of H(2)@C(60) [M. Xu et al., J. Chem. Phys. 128, 011101 (2008)] and uses the same computational methodology. Two 5D intermolecular potential energy surfaces (PESs) were employed, differing considerably in their well depths and the degree of confinement of the hydrogen molecule. Our calculations revealed pronounced sensitivity of the endohedral T-R dynamics to the differences in the interaction potentials, and to the large variations in the masses and the rotational constants of H(2), HD, and D(2). The T-R levels vary significantly in their energies and ordering on the two PESs, as well as from one isotopomer to another. Nevertheless, they all display the same distinctive patterns of degeneracies, which can be qualitatively understood and assigned in terms the model which combines the isotropic three-dimensional harmonic oscillator, the rigid rotor, and the coupling between the orbital and the rotational angular momenta of H(2)/HD/D(2). The quantum number j associated with the rotation of H(2), HD, and D(2) was found to be a good quantum number for H(2) and D(2) on both PESs, while most of the T-R levels of HD exhibit strong mixing of two or more rotational basis functions with different j values.

Fullerene C60 as an Endohedral Molecule within an Inorganic Supramolecule

C60⊂[Cu26Cl26L2{Cp*Fe(η5-P5)}13(CH3CN)9] (L = H2O) represents a first example of a complete encapsulation of C60 itself by an inorganic fullerene-like system via molecular recognition of the five- and sixfold-symmetric structural motif of C60. The product exhibits an inorganic cover consisting of 99 inorganic core atoms from pentaphosphaferrocene and Cu(I) chloride building blocks. This is the first instance of a hollow structure that completely encapsulates C60. The overall spherical molecule possesses an outer diameter of 2.3 nm and shows a remarkable electronic interaction between C60 and the encapsulating giant molecule.

Endohedral Encapsulation of Lithium in C<sub>70</sub>

A new diatomic molecule-containing endohedral fullerenes were prepared by ion implantation technique (introducing negatively charged C70 into a low temperature lithium plasma column by a strong exial magnetic field).The species prepared were Li@C70 and Li2@C70 and ionized by surface ionization. Their existance was demonstrated through high sensitivity, magnetic mass spectrometer. The corresponding endohedral ions undergo the Rice shrinkwrap mechanism: a mass –analyzed ion spectrum demonstrates the loss of a C2 unit from the cage. The observed ionization energies of endohedral molecules were 5.1 " 0.1 eV and 5.3 " 0.1 eV, respectively.

First-principles molecular dynamics study of the stretching frequencies of hydrogenmolecules in carbon nanotubes

We present a study of the vibrational frequencies of hydrogen molecules adsorbed in bundles ofsingle wall carbon nanotubes. The frequencies are extracted from the analysis of first-principlesmolecular dynamics trajectories. We study the case of molecules inside the nanotubes(endohedral) and in the interstitial pores between the nanotubes (exohedral). We find that,in both configurations, the stretching frequencies are redshifted with respect to the freeH2 molecule. However, whereas all the exohedral molecules exhibit a redshift of around45 cm−1,the endohedral molecules show two frequencies, one close to thatof the exohedral molecules and the other similar to that of freeH2.

Orientation and constraints of endohedral lanthanum inLa@C82molecules adsorbed on Cu(111)

The orientation of the endohedral La atom in $\mathrm{La}@{\mathrm{C}}_{82}$ adsorbed on Cu(111) has been determined using the normal-incidence x-ray standing wave (NIXSW) technique. The NIXSW measurements were carried out using both the normal 〈111〉 and 70.5\ifmmode^\circ\else\textdegree\fi{} off-normal $〈1\ifmmode\bar\else\textasciimacron\fi{}11〉$ Bragg reflections. The La $〈1\ifmmode\bar\else\textasciimacron\fi{}11〉$ profiles of the $\mathrm{La}@{\mathrm{C}}_{82}$ monolayer and submonolayers are found to be similar to the 〈111〉 profile of the multilayer, while the 〈111〉 profiles of the monolayer and submonolayers are distinctly different with much lower peak height. This indicates significant ordering of the La atoms in the monolayers with respect to height above the Cu surface but little lateral ordering. The results of curve fits to the monolayer profiles show that the La atoms are regionally located at 1.62 (\ifmmode\pm\else\textpm\fi{}0.77) \AA{} above an extended scattering plane from the crystal. Cooling the monolayers to a temperature of 170 K increases ordering of the La atoms with respect to height above the surface. The NIXSW results demonstrate that the endohedral molecules adopt a restricted orientation upon adsorbing on the surface and the La atom resides on either the upper or lower half of the cage. Theoretical calculations show that the most stable La positions are located in the top half of the fullerene cage, which supports the La upper configuration. The results are explained in terms of electrostatic interactions between the endohedral atom, the fullerene cage, and the substrate.

Electron affinity of some trimetallic nitride and conventional metallofullerenes

The electron affinity of a series of discandium endohedral fullerenes and new trimetallic nitride endohedral molecules Sc xEr 3-xN@C 80 was determined by means of the Knudsen cell mass spectrometry—ion-molecular equilibria method. Some considerations were made concerning the influence of the charge transfer from the endohedral atoms to the carbon cage on the electron affinity of the endohedral molecule.

Some new diatomic molecule containing endohedral fullerenes

Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C60 or C70 under high pressures of the corresponding gases. The species prepared are N2@C60, N2@C70, 13CO@C60, and 3He22Ne@C70. Their existence was demonstrated through high sensitivity, wide dynamic range mass spectrometry. Out of two thousand C60 molecules about one is observed to incorporate N2. The nitrogen molecule containing endohedral molecules are stable and the mass spectrometric signal is not lost even after several hours heating at 500 K. The corresponding endohedral ions undergo the Rice shrink-wrap mechanism; a mass-analyzed ion kinetic energy spectrum demonstrates the loss of a C2 unit from the cage. The observation of 13CO@C60 by mass spectrometry opens up the possibility for future NMR studies of this molecule. The observation of 3He22Ne@C70 is in accordance with the “promoter” mechanism of Thiel and co-worker [J. Am. Chem. Soc. 118 (1996) 7164; Helv. Chim. Acta 80 (1997) 495], whereby the singly and doubly occupied fullerenes are in equilibrium but the empty and filled fullerenes are not.