Icosahedral C60 Research Articles

Formation of the icosahedral C60 fullerene via migration of single sp atoms and annihilation of sp-atom pairs.

The disappearance of sp2 structural defects during abundant fullerene isomer formation is considered within the framework of the atomistic mechanism with participation of carbon atoms with sp hybridization. The study is carried out using the example of the icosahedral C60-Ih fullerene formation from the appropriate C58-C2v fullerene with a 7-ring. In this case the studied atomistic mechanism includes the following stages: (1) insertion of single carbon atoms into the fullerene from carbon vapor as an sp-atom instead of or above a bond, (2) directional migration of the sp-atom positions towards the 7-ring with decrease of energy, and (3) meeting of two sp atoms near the 7-ring with annihilation of the sp-atom pair and formation of the sp2 structure of the C60-Ih fullerene. The probabilities of all possible sp-atom positions on the appropriate C58-C2v fullerene shell are estimated as a function of temperature using the total energies of these positions obtained by spin-polarized density functional theory calculations using the PBE functional. Based on these estimations, it is shown that formation of the C60-Ih isomer is the most probable within the framework of the considered mechanism relative to other C60 isomers. The energetics of sp-atom pair annihilation in the formation of the C60-Ih isomer is also studied via DFT calculations. The advantages of the considered atomistic mechanism of the abundant fullerene isomer formation are discussed.

Evaluating the performance of ReaxFF potentials for sp2 carbon systems (graphene, carbon nanotubes, fullerenes) and a new ReaxFF potential.

We study the performance of eleven reactive force fields (ReaxFF), which can be used to study sp2 carbon systems. Among them a new hybrid ReaxFF is proposed combining two others and introducing two different types of C atoms. The advantages of that potential are discussed. We analyze the behavior of ReaxFFs with respect to 1) the structural and mechanical properties of graphene, its response to strain and phonon dispersion relation; 2) the energetics of (n, 0) and (n, n) carbon nanotubes (CNTs), their mechanical properties and response to strain up to fracture; 3) the energetics of the icosahedral C60 fullerene and the 40 C40 fullerene isomers. Seven of them provide not very realistic predictions for graphene, which made us focusing on the remaining, which provide reasonable results for 1) the structure, energy and phonon band structure of graphene, 2) the energetics of CNTs versus their diameter and 3) the energy of C60 and the trend of the energy of the C40 fullerene isomers versus their pentagon adjacencies, in accordance with density functional theory (DFT) calculations and/or experimental data. Moreover, the predicted fracture strain, ultimate tensile strength and strain values of CNTs are inside the range of experimental values, although overestimated with respect to DFT. However, they underestimate the Young’s modulus, overestimate the Poisson’s ratio of both graphene and CNTs and they display anomalous behavior of the stress - strain and Poisson’s ratio - strain curves, whose origin needs further investigation.

Selective extraction of fullerene C60 by in situ interweaved AgNO3 complex encapsulated C60 molecular cages

With an in situ synthesized AgNO3 complex molecular cage, icosahedral C60 can be selectively extracted from fullerene mixture by a facial liquid–liquid interfacial precipitation, resulting in exclusive C60 organometallic microcrystal (C60/C70 = 6/1). By exploiting its intrinsic insolubility against nonpolar solvent, the C60 can be separated from the homologues, C70, which contrarily is assembled through merely hydrophobic interaction and π-π stacking. The AgNO3 complex encaged C60 crystal was morphologically and structurally characterized by SEM, Raman and XRD spectra to revealing the underlying separation mechanism. The selective extraction efficiency (89.7%) was semiquantitatively characterized by combined optical spectroscopy and mass spectrometry, indicating a decent specificity of spontaneously formed molecular cage toward C60 extraction. Thus, this discovery of a template-specific molecular cavity would pave the way to advance molecular imprinting technology using supramolecular array for fullerene separation, prevailing over the conventional “design-synthesis-performance” strategy by substantially reducing the laborious work and economic cost.

Electronic Structure and Optical Absorption Spectrum of Icosahedral Golden Fullerene Au32

Abstract—The electronic structure of golden fullerene Au32 has been studied using quantum field theory methods in the framework of the Hubbard model. Expressions for the Fourier transforms of a Green’s function, the poles of which determine the energy spectrum of the nanosystem under consideration, have been derived. The energy spectrum of Au32, studied in comparison with the spectrum of icosahedral carbon fullerene C60, indicates the semiconductor state of golden fullerene Au32. The peaks of the density of states correspond to Van Hove singularities. The optical absorption spectra of the neutral and negatively charged fullerene Au32 are presented. The energy of the first direct optical transition of the negatively charged ion of golden fullerene $${\text{Au}}_{{32}}^{ - }$$ is 1.26 eV.

Optical Absorption Spectrum of Metal–Nitride Complexes of С80 Fullerene as Strongly Correlated Systems

Within the framework of Hubbard’s model using an approximation of static fluctuations, the energy spectrum of the π-electron subsystem of an icosahedral C80 fullerene isomer is obtained. Based on the energy spectrum, an optical absorption spectrum of metal-nitride complexes M3N@C80 (M = Gd, Tm, Dy) is simulated. It is quite consistent with the respective experimental spectrum.

(Invited) Growth of Endohedral Metallofullerenes: The Transformation of Sc3n@D 3-C68 into Sc3n@C80

Carbon cages that entrap metals were first observed just days after the landmark discovery of C60. The synthesis of Sc3N@C80, which is comprised of a planar Sc3N cluster encapsulated within an icosahedral C80 cage, transformed significantly the field of endohedral metallofullerenes (EMFs). Recently, efficient routes toward arc discharge plasma synthesis of EMFs by vaporization of graphite, metal oxide, and solid nitrogen sources, such as guanidine thocyanate and guanidinium salts, have been developed. However, the origin of EMFs produced by plasma synthesis techniques and any associated formation processes that take place remains still unclear. Here, we report some recent results on the gas-phase synthesis of Sc3N@C2n and particularly on the transformation of the pentalene-containing Sc3N@D 3-C68 compound, which transform through a bottom-up mechanism into Sc3N@C80. We will discuss on potential pathways for the growth of Sc3N@D 3-C68 when it is exposed to an atmosphere of carbon plasma produced from graphite, and on the potential role that heptagonal rings may play.

Key Structural Motifs To Predict the Cage Topology in Endohedral Metallofullerenes

We show that the relative isomer stability of fullerene anions is essentially governed by a few simple structural motifs, requiring only the connectivity information between atoms. Relative energies of a large number of isomers of fullerene anions, C(2n)(q) (2n = 68-104; q = -2, -4, -6), can be satisfactorily reproduced by merely counting the numbers of seven kinds of hexagon-based motifs. The dependence of stability on these motifs varies with the charge state, which reflects the fact that the isomeric form of the carbon cage in endohedral metallofullerenes (EMFs) often differs from that in neutral empty fullerenes. The chemical origin of the stabilization differences between motifs is discussed on the basis of electronic and strain effects as well as aromaticity. On the basis of this simple model, the extraordinary abundance of the icosahedral C80 cage in EMFs can be easily understood. We also provide an explanation for why the well-known isolated pentagon rule is often violated in smaller EMFs. Finally, simple topological indices are proposed for quantitatively predicting the relative stability of fullerene anions, allowing a rapid determination of suitable hosting cages in EMFs by just counting three simple structural motifs.

Resonances in inner-shell photoemission from caged molecules

Resonances in X-ray absorption and inner-shell photoemission from encapsulated molecules are examined in the framework of the double barrier optical potential model. Special emphasis is put on resonances in S 1s and 2p spectra of SF6 confined in icosahedral fullerenes C60, C240 and C540. In addition to the shape and confinement resonances we predict appearance of new resonance features such as transparency windows. The transparency windows are originated from interference of the outgoing primary photoelectron wave and waves scattered within the fluorine and carbon cages and assigned with the quasibound states localized in the interstitial region of the endohedral systems.

Scandium carbide/cyanide alloyed cluster inside fullerene cage: synthesis and structural studies of Sc3(μ3-C2)(μ3-CN)@Ih-C80.

A new Sc3(C2)(CN)@Ih-C80 metallofullerene encaging a scandium carbide/cyanide alloyed cluster was prepared and investigated. Sc3(C2)(CN)@Ih-C80 was synthesized by the arc-discharging method and isolated by HPLC. Its experimental (13)C NMR spectrum with two signals clearly confirms an icosahedral C80 cage, and theoretically calculated (13)C NMR peaks agree well with the experimental results. Further, theoretical calculations disclosed that the endohedral μ3-C2 and μ3-CN moieties are situated on each side of the triangular shaped Sc3 unit to form a scandium carbide/cyanide alloyed cluster. Kohn-Sham molecular orbitals reveals the electronic structure of (Sc(3+))3(C2)(2-)(CN)(-)@C80(6-), in which two anions, μ3-C2(2-) and μ3-(CN)(-), construct and stabilize this special molecule together. The FTIR and Raman spectra of Sc3(C2)(CN)@Ih-C80 were analyzed by comparison of the experimental and calculated results to further confirm its structure and to uncover cluster-based vibrational modes.

C60+ and B80+: A Comparative Study of the Jahn-Teller Effect

The ground state wave function of the neutral icosahedral C60 and B80 belong to the totally symmetric representation, where the HOMOs are fivefold degenerate and form the basis of the Hu representation of the Ih point group. Hence both C60+, and B80+ are prone to a molecular distortion of the Jahn-Teller type. Density Functional Theory calculation is applied and revealed that a minimum energy configuration in D5d point group is obtained for C60+; whereas a slight S6 distortion of a D3d nuclear configuration is obtained for B80+. Thus the vibronic coupling between the 2Hu electronic states of both systems with the degenerate normal modes in the Ih point group are analysed and presented here in a comparative point of view. Moreover, a simple and efficient procedure, which is fully non-empirical, based on the harmonic approximation, is presented in order to calculate the Jahn-Teller parameters and the first order vibronic coupling coefficient.

Theoretical study of N20, C20, and B20 clusters “squeezed” inside icosahedral C80 and He80 cages

The equilibrium geometries and spectroscopic and energetic characteristics of model endohedral M20@C 80 n− clusters, in which the guest clusters M20 = N20, C20, and B20 are squeezed inside the fullerene C 80 n− cages (n = 0, 2, 4, and 6), have been calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. Analogous calculations with partial geometry optimization have been performed for their congeners M20@He 80 n− with a fixed icosahedral helium cage He80. According to the calculations, all the structures of the N20@C80, C20@C80, and B20@C80 series correspond to local minima of the potential energy surface (all vibrational frequencies are real). In the first cluster, the N20 guest has a structure of a dodecahedron with a diameter of ∼4.0 A. The alternative 10N2@C80 structure containing ten separated endohedral N2 molecules is considerably less favorable and transforms without a barrier to the dodecahedral N20@C80 isomer upon geometry optimization. It has been suggested that, under extreme supercompresison conditions, molecular nitrogen can be associated without barriers into highly endothermal chemically bound clusters of the N20 type. In the helium analogues, the relative position of the N20@He80 and 10N20@He80 structures on the energy scale is determined by the degree of compression and can change its sign with a change in the diameter of the external cage D(He80). The mechanism of gradual assembly of the N20 dodecahedron from the 10N2 set has been traced with a decrease in the diameter D(He80) in the range 7.5–8.6 A. In the C20@C80 cluster, the C20 guest has a structure of a distorted dodecahedron bound to the C80 cage through four “inner” (endohedral) bonds. In the B20@C80 cluster, the B20 guest is severely squeezed along the C5 axis. Its equatorial atoms form ten endohedral B-C bonds to C80 cage atoms. In similar systems, the division of the endoclusters into the internal guest and external cage becomes uncertain. Calculations predict that the isolated “salt” molecules of the L n C20 and L n B20 type in which the C20 and B20 clusters function as anions surrounded by the L atoms of alkali metals (n = 1–6) should be stable to stepwise dissociation accompanied by elimination of separate L atoms and L2 molecules.

A multi-state single-molecule switch actuated by rotation of an encapsulated cluster within a fullerene cage

We demonstrate a single-molecule switch based on tunneling electron-driven rotation of a triangular Sc3N cluster within an icosahedral C80 fullerene cage among three pairs of enantiomorphic configurations. Scanning tunneling microscopy imaging of switching within single molecules and electronic structure theory identify the conformational isomers and their isomerization pathways. Bias-dependent action spectra and modeling identify the antisymmetric stretch vibration of Sc3N cluster to be the gateway for energy transfer from the tunneling electrons to the cluster rotation. Hierarchical switching of conductivity through the internal cluster motion among multiple stationary states while maintaining a constant shape, is advantageous for the integration of endohedral fullerene-based single-molecule memory and logic devices into parallel molecular computing architectures.

Dissociation of water on Ti-decorated fullerene clusters

Spin-polarized density functional theory calculations have been applied to investigate water dissociation catalyzed by Ti adsorbed on icosahedral C20, C60 and C80 fullerene clusters, in order to elucidate the roles that cluster size and Ti-cluster interaction play in the proposed hydrogen generation reaction. We find that two water molecules can be dissociated consecutively by overcoming moderate energy barriers of a few tenths of eV, accompanied by the generation of a H2 molecule for all three clusters. Depending on the cluster size, the fullerene clusters may participate directly in water splitting or indirectly through stereochemical control of the Ti adsorption sites. Our results suggest that fullerene clusters can serve as a flexible platform for rational design of nanostructured catalysts for hydrogen generation.

Size‐Determining Dependencies in Supramolecular Organometallic Host–Guest Chemistry

Treatment of the pentaphosphaferrocene [Cp*Fe(η(5)-P(5))] with Cu(I) halides in the presence of different templates leads to novel fullerene-like spherical molecules that serve as hosts for the templates. If ferrocene is used as the template the 80-vertex ball [Cp(2)Fe]@[{Cp*Fe(η(5)-P(5))}(12){CuCl}(20)] (4), with an overall icosahedral C(80) topological symmetry, is obtained. This result shows the ability of ferrocene to compete successfully with the internal template of the reaction system [Cp*Fe(η(5)-P(5))], although the 90-vertex ball [{Cp*Fe(η(5):η(1):η(1):η(1):η(1):η(1)-P(5))}(12)(CuCl)(10)(Cu(2)Cl(3))(5){Cu(CH(3)CN)(2)}(5)] (2 a) containing pentaphosphaferrocene as a guest is also formed as a byproduct. With use of the triple-decker sandwich complex [(CpCr)(2)(μ,η(5)-As(5))] as a template the reaction between [Cp*Fe(η(5)-P(5))] and CuBr leads to the 90-vertex ball [(CpCr)(2)(μ,η(5)-As(5))]@[{Cp*Fe(η(5)-P(5))}(12){CuBr}(10){Cu(2)Br(3)}(5){Cu(CH(3) CN)(2)}(5)] (6), in which the complete molecule acts as a template. However, if the corresponding reaction is instead carried out with CuCl, cleavage of the triple-decker complex is found and the 80-vertex ball [CpCr(η(5)-As(5))]@[{Cp*Fe(η(5)-P(5))}(12){CuCl}(20)] (5) is obtained. This accommodates as its guest [CpCr(η(5)-As(5))], which has only 16 valence electrons in a triplet ground state and is not known as a free molecule. The triple-decker sandwich complex [(CpCr)(2)(μ,η(5)-As(5))] requires 53.1 kcal mol(-1) to undergo cleavage (as calculated by DFT methods) and therefore this reaction is clearly endothermic. All new products have been characterized by single-crystal X-ray crystallography. A favoured orientation of the guest molecules inside the host cages has been identified, which shows π⋅⋅⋅π stacking of the five-membered rings (Cp and cyclo-As(5)) of the guests and the cyclo-P(5) rings of the nanoballs of the hosts.

A Molecular Switch Based on Current-Driven Rotation of an Encapsulated Cluster within a Fullerene Cage

By scanning tunneling microscopy imaging and electronic structure theory, we investigate a single-molecule switch based on tunneling electron-driven rotation of a triangular Sc3N cluster within an icosahedral C80 fullerene cage among three pairs of enantiomorphic configurations. Bias-dependent action spectra and modeling implicate the antisymmetric stretch vibration of Sc3N cluster as the gateway for energy transfer from the tunneling electrons into the cluster rotation. Hierarchical switching of conductivity among multiple stationary states while maintaining a constant molecular shape, offers an advantage for the integration of endohedral fullerene-based single-molecule switches into multiple logic state molecular devices.

Structural, electronic and magnetic properties of Fen@C60 and Fen@C80 (n=2–7) endohedral metallofullerene nano-cages: First principles study

We studied the structural, electronic and magnetic properties of small Fen clusters (n=2–7) endohedrally doped in icosahedral C60 and C80 fullerenes using first principles calculations based on the density functional theory. It is found that the encapsulated Fen clusters inside icosahedral C80 are energetically favorable while Fen@C60 metallofullerene nano-cages are not. The binding energies of the Fen encapsulated in C60 are positive and increase with the number of iron atoms (n) while those of the Fen@C80 are negative and their absolute values increase up to n=6. The encapsulation does not significantly change the enclosed cluster structure, but the total magnetic moment of the larger clusters reduces due to a stronger Fe–C hybridization.

14N and 45Sc NMR study of trimetallic nitride cluster (M3N)6+ dynamics inside a icosahedral C80 cage

The dynamics of the trimetallic nitride (M(3)N)(6+) (M = Sc, Y and Lu) clusters in the I(h)-(C(80))(6-) cage have been studied by (14)N and (45)Sc nuclear magnetic resonance. These NMR studies suggest that the motional barrier of (M(3)N)(6+) is related to the cluster size and increases in the series (Sc, Y, and Lu).

Structural and Electronic Stability of Russian-Doll-Structured Sc4C2@C80

Recent experimental work reported the synthesis, isolation, and characterization of a “Russian-doll”-style endohedral fullerene, encompassing a carbon dimer within a scandium tetrahedron, all encased by a C80 cage. We have investigated the equilibrium conformations and the associated charge transfer of the endohedral fullerene Sc4C2@C80 based on first-principles density-functional calculations. Our results show that a distorted tetrahedron Sc4 cluster enfolding around the C2 dimer has the desired electronic structure that leads to efficient charge transfer to the open-shell icosahedral C80. A detailed analysis of the charge transfer between the Sc4C2 configurations and the icosahedral C80 cage indicate that the structural stability of the Russian-doll-structured Sc4C2@C80 can be attributed to the donor−acceptor effects.

A combination of Clar number and Kekulé count as an indicator of relative stability of fullerene isomers of C60

Kekule count is not as useful in predicting the thermodynamic stability of fullerenes as it is for benzenoid hydrocarbons. For example, the Kekule count of the icosahedral C60, the most stable fullerene molecule, is surpassed by its 20 fullerene isomers (Austin et al. in Chem Phys Lett 228:478–484, 1994). This article investigates the role of Clar number in predicting the stability of fullerenes from Clar’s ideas in benzenoids. We find that the experimentally characterized fullerenes attain the maximum Clar numbers among their fullerene isomers. Our computations show that among the 18 fullerene isomers of C60 achieving the maximum Clar number (8), the icosahedral C60 has the largest Kekule count. Hence, for fullerene isomers of C60, a combination of Clar number and Kekule count predicts the most stable isomer.

ChemInform Abstract: Principles of Fullerene Reactivity

The fullerenes have been established as new and versatile building blocks in organic chemistry. A large number of fascinating fullerene derivatives, especially of the icosahedral buck-minsterfullerene C60, have been synthesized. The chemistry of C60 continues to be good for many surprises. However, based on present knowledge a series of reactivity principles can be deduced which makes derivatization of this all carbon cluster more and more predictable. In this article first the geometric and electronic properties of the parent molecule are analyzed. The bent structure of the carbon network C60 and the filling of its molecular orbital with 60 π-electrons dictate the chemical reactivity. A very important aspect that was introduced with the investigation of fullerene chemistry is the shape dependence of reactivity.