Nonclassical Fullerene Research Articles

C84Cl30, A non-classical fullerene chloride featuring local aromatic motifs. Evaluation of 13C NMR and aromatic patterns from DFT calculations

Non-classical fullerene exploits the rich structural diversity in fullerenes. Here, we evaluate theoretically C84Cl30, as a representative low-symmetric non-classical fullerene involving a consecutive trail of thirty chlorinated sp3-C atoms. Our results offer an evaluation of both 13C NMR and induced magnetic field, accounting for the magnetic properties and the overall non-aromatic behavior as a consequence of the sp3-C trail, with local aromaticity ascribed to the isolated sp2-motifs. In addition, the formation of plausible σ-holes is depicted. Thus, the introduction of chlorinated sites, serve to envisage asymmetric molecular units with particular local aromatic motifs towards enriching π-surface diversity in fullerenes.

M@C78 (M = U, Th): Inherent Topological Connectivity Existed in Thermodynamically Stable Isomers and the Possibility of an Endohedral Fullerene Containing One Heptagon Ring.

The density functional theory combined with statistical thermodynamic analyses of M@C78 (M = U and Th) demonstrated that four isomers, M@D3h(24109)-C78, M@C2v(24107)-C78, M@C1(22595)-C78, and M@C1(23349)-C78, and a nonclassical isomer, M@C1(id7)-C78, containing one heptagon ring possess outstanding thermodynamic stabilities in the two M@C78 series. Especially, the M@C1(id7)-C78 isomer is the first nonclassical C78 fullerene that can exist stably. Importantly, these five fullerene cages are found to be related in the form of Stone-Wales (SW) transformations. Geometric analyses disclosed that, unlike lanthanide metals, actinide metals are more likely to bond with sumanene-type hexagonal rings when they are encapsulated in IPR C78 cages. Frontier molecular orbital analysis showed that both U and Th atoms donate four electrons to the C78 carbon cages.

Study of fullerenes C126 and C156 with seven heptagon rings

We consider fullerenes C126 and C156, both of them with seven heptagon rings. The first of them is C126 with 19 pentagons, and 39 hexagons. It is also considered C156 with 19 pentagons, and 54 hexagons. From previous work, a couple of fullerenes, C130 and C134, are mentioned for comparative reasons. These two fullerenes also contain seven heptagon rings. Fullerene C130 is constituted of 19 pentagons, and 41 hexagons. On the other hand, fullerene C134 has 19 pentagons, and 43 hexagons. Therefore, the four fullerenes considered, with seven heptagon rings, also share the property that, the number of pentagons in each of them, is 19. This fact is a consequence of a couple of equations that relates the number of pentagons, hexagons, and heptagons contained in a fullerene. Classical fullerenes contain only pentagons and hexagons, and the more familiar example is C60 with 20 hexagons, and 12 pentagons. Actually, all classical fullerenes have 12 pentagons, regardless of how many hexagons are involved. Besides this type of molecules, there exists nonclassical fullerenes, where another type of ring is presented, like squares, or heptagons. In this paper, fullerenes with 126, 130, 134, and 156 carbons are considered, with seven heptagons, and as a consequence, all them contain 19 pentagons, and they have 39, 41, 43, and 54 hexagons, respectively. Future work would focus on finding fullerenes with a different number of carbons, and hexagons, but with 7 heptagons, and 19 pentagons. Thus, fullerene C128 of this type, should contain 40 hexagons.

Total Irregularity Strengths of an Arbitrary Disjoint Union of (3,6)- Fullerenes.

A fullerene graph is a mathematical model of a fullerene molecule. A fullerene molecule, or simply a fullerene, is a polyhedral molecule made entirely of carbon atoms other than graphite and diamond. Chemical graph theory is a combination of chemistry and graph theory, where theoretical graph concepts are used to study the physical properties of mathematically modeled chemical compounds. Graph labeling is a vital area of graph theory that has application not only within mathematics but also in computer science, coding theory, medicine, communication networking, chemistry, among other fields. For example, in chemistry, vertex labeling is being used in the constitution of valence isomers and transition labeling to study chemical reaction networks Method: In terms of graphs, vertices represent atoms while edges stand for bonds between the atoms. By tvs (tes) we mean the least positive integer for which a graph has a vertex (edge) irregular total labeling such that no two vertices (edges) have the same weights. A (3,6)-fullerene graph is a non-classical fullerene whose faces are triangles and hexagons Results: Here, we study the total vertex (edge) irregularity strength of an arbitrary disjoint union of (3,6)-fullerene graphs and provide their exact values. The lower bound for tvs (tes) depends on the number of vertices. Minimum and maximum degree of a graph exist in literature, while to get different weights, one can use sufficiently large numbers, but it is of no interest. Here, by proving that the lower bound is the upper bound, we close the case for (3,6)-fullerene graphs.

Capturing nonclassical C70 with double heptagons in low-pressure combustion.

A double-heptagon-containing C70H6 (dihept-C70H6) was isolated and unambiguously characterized in the soot of low-pressure combustion, which shares the identical heptagonal cage as dihept-C70Cl6 previously identified in the products of carbon arc, and thus represents the first nonclassical fullerene isolable in both carbon arc and combustion.

Structural, Electronic, and Nonlinear Optical Properties of C66H4 and C70Cl6 Encapsulating Li and F Atoms.

Recently, nonclassical fullerene derivatives C66H4 and C70Cl6, which both contain two negatively curved moieties of heptagons, have been successfully synthesized. Inspired by these experimental achievements, the structural and electronic properties of C66H4, C70Cl6, Li@C66H4, F@C66H4, Li@C70Cl6, and F@C70Cl6 were systematical studied through density functional theory calculations in this work. Our results show that the reduction of the front molecular orbital gap of fullerene derivatives occurs with the introduction of Li and F atoms. After quantitative analysis of back-donations of charge between an encapsulated atom and an external carbon cage, it is found that C66H4 and C70Cl6 prefer to act as electron acceptors. It is interesting to note that the strong covalent nature of the interactions between a F atom and a carbon cage is observed, whereas the weak covalent and strong ionic interactions occur between a Li atom and a carbon cage. On the other hand, according to the first hyperpolarizability results, the encapsulation of the Li atom enhances the nonlinear optical response of fullerene derivatives. This work provides a strategy to improve nonlinear optical properties of C66H4 and C70Cl6, reveals the internal mechanism of the contribution from Li and F atoms to endohedral fullerene derivatives, and will contribute to the designation of endohedral fullerene derivative devices.

BN-Substituted non-classical fullerenes containing square rings

We performed density functional calculations to investigate the electronic and magnetic properties of BN-substituted non-classical fullerenes. The substitutional structures, binding energies, energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), ionisation potentials, electron affinities, vibrational frequencies and nucleus-independent chemical shifts (NICS) were systematically investigated. The binding energies of the BN-substituted non-classical fullerenes were found to be slightly smaller than those obtained for pure non-classical fullerenes. While the reverse trend was observed for BN-substituted $$\hbox {C}_{46}$$ and $$\hbox {C}_{32}$$ fullerenes, it was found that the BN-substituted $$\hbox {C}_{62}$$ fullerene has bigger ionisation potentials (IP) and smaller electron affinities (EA) than that of their parents. Because of low concentration of BN impurity, the IR spectra in the BN-substituted fullerenes are very similar to those of their parents, which can be considered as two separate regions: a low-frequency region at 200–1000 $$\text {cm}^{-1}$$ corresponding to the out-of-plane bending and breathing modes and a high-frequency region at 1000–1800 $$\text {cm}^{-1}$$ derived from the stretching of C–B, C–N, B–N and C–C bonds. It was shown that diatropic and paratropic ring currents of hexagons and pentagons together with the harshly antiaromatic character of the four-membered ring combine to produce a relatively small NICS at the centre of the $$\hbox {C}_{62}$$ and $$\hbox {C}_{46}$$ fullerene cages. The decrease in the antiaromatic and aromatic character of the $$\hbox {B}_{2}\hbox {N}_{2}$$ ring and the adjacent hexagons affects the aromaticity character of the BN-substituted fullerenes.

Structural Studies of Giant Empty and Endohedral Fullerenes.

Structure elucidations of giant fullerenes composed of 100 or more carbon atoms are severely hampered by their extremely low yield, poor solubility and huge numbers of possible cage isomers. High-temperature exohedral chlorination followed by X-ray single crystal diffraction studies of the chloro derivatives offers a practical solution for structure elucidations of giant fullerenes. Various isomers of giant fullerenes have been determined by this method, specially, non-classical giant fullerenes containing heptagons generated by the skeletal transformations of carbon cages. Alternatively, giant fullerenes can be also stabilized by encapsulating metal atoms or clusters through intramolecular electron transfer from the encapsulated species to the outer fullerene cage. In this review, we present a comprehensive overview on synthesis, separation and structural elucidation of giant fullerenes. The isomer structures, chlorination patterns of a series of giant fullerenes C2n (2n = 100-108) and heptagon-containing non-classical fullerenes derived from giant fullerenes are summarized. On the other hand, giant endohedral fullerenes bearing different endohedral species are also discussed. At the end, we propose an outlook on the future development of giant fullerenes.

How to Stabilize a Heptagon-Containing C80 Cage by Endohedral Derivation.

Nonclassical fullerene is a new member of the fullerene family. In the present work, a systematic investigation on LaxSc3-xN@C80 (x = 0-3) covering both classical and nonclassical C80 cages was performed utilizing density functional theory combined with statistical mechanics. At absolute zero, LaSc2N@Hept(6)-Cs(2)-C80 with a heptagon-containing nonclassical carbon is the second most stable isomer, whereas at the temperature range of endohedral metallofullerene (EMF) formation, due to the large vibrational frequencies, LaSc2N@Hept(6)-Cs(2)-C80 is the third most abundant isomer, and its mole fraction is very low, accounting for the low experimental yield of LaSc2N@Hept(6)-Cs(2)-C80; La2ScN@Hept(6)-Cs(2)-C80, and La3N@Hept(6)-Cs(2)-C80 are the overwhelming isomers of the corresponding series, but compared with the cases of Sc3N@C80 and LaSc2N@C80, La2ScN and La3N clusters suffer much larger constraints from the C80 cages, perhaps preventing the synthesis of La2ScN@C80 and La3N@C80 species. Because of the large mole fractions and large electron donation and back-donation of La2ScN@Hept(6)-Cs(2)-C80 and La3N@Hept(6)-Cs(2)-C80, it can be inferred that La2ScN and La3N clusters may be used to stabilize some other larger nonclassical fullerene cages. This work will provide useful insights into the origins of stabilization of nonclassical fullerene cages by endohedral derivation and guidelines for synthesis EMF with nonclassical cages.

Theoretical Investigation of Seven Membered Ring C120X6 (X = H2, F2, Cl2, Br2, O, O2, and CH2) Fullerene Derivatives

Based on a shorter (6,6) carbon nanotube, a eleven layers D6h C108, a novel nonclassical hept-C120 fullerene fabricated by inserting symmetrically six C2 around the circumference of D6h C108 and its derivatives C120X6 (X = H2, F2, Cl2, Br2, O, O2, and CH2) have been reported here. The geometrical structures and electronic properties of them are studied using the B3LYP/6-31(d,p) level of theory. Present calculations show that all the investigated nonclassical fullerene derivatives are thermodynamically stable. Moreover, the HOMO–LUMO energy gaps of C120X6 are higher than the precursor, implying substantial kinetic stability. In addition, vibrational frequencies and the nucleus-independent chemical shift have also been investigated to further analyze the stability of these molecules.

Chlorination-Promoted Skeletal Transformations of Fullerenes.

Classical fullerenes are built of pentagonal and hexagonal rings, and the conventional syntheses produce only those isomers that obey the isolated-pentagon rule (IPR), where all pentagonal rings are separated from each other by hexagonal rings. Upon exohedral derivatization, the IPR fullerene cages normally retain their connectivity pattern. However, it has been discovered that high-temperature chlorination of fullerenes with SbCl5 or VCl4 can induce skeletal transformations that alter the carbon cage topology, as directly evidenced by single crystal X-ray diffraction studies of the chlorinated products of a series of fullerenes in the broad range of C60 to C102. Two general types of transformations have been identified: (i) the Stone-Wales rearrangement (SWR) that consists of a rotation of a C-C bond by 90°, and (ii) the removal of a C-C bond, i.e., C2 loss (C2L). Single- or multistep SWR and/or C2L transformations afford either classical non-IPR fullerenes bearing fused pentagons (highlighted in red in the TOC picture) or nonclassical (NCx) fullerenes with x = 1-3 heptagonal rings (highlighted in blue in the TOC picture) often flanked by fused pentagons. Several subtypes of the SWR and C2L processes can be further discerned depending on the local topology of the transformed region of the cage. Under the chlorination conditions, the non-IPR and NC carbon cages that would be energetically unfavorable and mostly labile in their pristine state are instantaneously stabilized by chlorination of the pentagon-pentagon junctions and by delimitation of the original spherical π-system into smaller favorable aromatic fragments. The significance of the chlorination-promoted skeletal transformations within the realm of fullerene chemistry is demonstrated by the growing body of examples. To date, these include single- and multistep SWRs in the buckminsterfullerene C60 and in the higher fullerenes C76(1), C78(2), C82(3), and C102(19), single and multistep C2Ls (i.e., cage shrinkage) in C86(16), C88(33), C90(28), C92(50), C96(80), C96(114), and C102(19), and multistep combinations of SWRs and C2Ls in C88(3), C88(33), and C100(18), (IPR isomer numbering in parentheses is according to the spiral algorithm). Remarkably, an IPR precursor can give rise to versatile transformed chlorinated fullerene cages formed via branched pathways. The products can be recovered either in their initial chlorinated form or as more soluble CF3/F derivatives obtained by an additional trifluoromethylation workup. Reconstruction of the skeletal transformation pathways is often complicated due to the lack of the isolable intermediate products in the multistep cases. Therefore, it is usually based on the principle of selecting the shortest pathways between the starting and the final cage. The quantum-chemical calculations illustrate the detailed mechanisms of the SWR and C2L transformations and the thermodynamic driving forces behind them. A particularly important aspect is the interplay between the chlorination patterns and the regiochemistry of the skeletal transformations.

Theoretical Studies on the Structural and Spectral Properties of Several Classical and Nonclassical C78 Isomers and Its Novel Chlorinated Derivative C78Cl24

X-ray photoelectron spectra (XPS), near-edge X-ray absorption fine structure spectra (NEXAFS), and X-ray emission spectroscopy (XES), as well as the ground-state electronic/geometrical structures about the significant classical isolated-pentagon rule (IPR) isomer D3h-#24109C78, the non-IPR isomers C2-#22010C78 and C1-#23863C78, and the nonclassical isomer C2-C78(NC2) with its chlorinated derivative C2-C78(NC2)Cl24, which are newly obtained in the experiment, have been calculated at the density functional theory (DFT) level. Significant differences have been observed in the electronic structure and the X-ray spectra. All X-ray spectra have shown strong isomer dependence; consequently, the “fingerprint” in X-ray spectra shows a very effective way to isolate the fullerene isomers above. As a result, this work indicates that X-ray spectroscopy can provide valuable identification for classical and nonclassical fullerenes as well as their derivatives on experimental and theoretical studies.

(Invited) Cage Skeletal Transformation of Fullerene Via Chlorination

A classical fullerene is composed of hexagons and pentagons only, and its stability is generally determined by the Isolated-Pentagon-Rule (IPR). In 2005 Taylor et al. reported the first cage skeletal transformation of fullerene C60 via fluorination at 550 0C, affording two stable non-classical fullerene derivatives containing heptagons. It is then intriguing to investigate whether cage skeletal transformation of fullerene can be fulfilled under less harsh conditions. Herein, we present our recent studies on cage skeletal transformations of classical higher fullerenes in the course of high-temperature chlorination. Chlorination-promoted cage skeletal transformations occur by Stone-Wales rearrangements (SWRs, i.e., a rotation of a C−C bond by ca. 90°) and/or losses of C2 units (C2Ls). In particular, the C2L realized by the removal of a 5:6 C-C bond from the cage generates a cage heptagon, thus resulting in the formation of a non-classical cage. We show several examples of chlorination-promoted one-, two-, and multi-step cage transformations in different higher fullerenes.

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.

Electronic structures and stabilities of the defective nanotube-like fullerenes C58+10n and their derivatives C58+10nCl8

The stability and electronic properties of the defective nanotube-like fullerenes C58+10n obtained by removing a 5–6 bond closest to the poles of C60+10n and their derivatives C58+10nCl8 (n=1, …, 10) have been investigated by using the density functional theory method. For each system, two isomer structures (the nonclassical fullerene with a heptagon and non-IPR fullerene with two pentagon pairs) and two electronic states (singlet and triplet) were examined. The results show that the non-IPR fullerenes C58+10n with two pentagon pairs are more stable than their nonclassical isomer with a heptagon. Both the relative energies and the gap energies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) reveal that the ground state for non-IPR Cs-C68, Cs-C98, Cs-C128, and Cs-C158 is the triplet state, whereas the singlet state is the ground state for non-IPR Cs-C78, Cs-C88, Cs-C108, Cs-C118 Cs-C138, and Cs-C148. Further chlorination on non-IPR fullerenes enhances the stability of chlorofullerenes. Especially, Cs-C68Cl8, Cs-C98Cl8, Cs-C128Cl8, and Cs-C158Cl8 have the most enhance stability both thermodynamically and kinetically exhibiting high negative reaction energies and large HOMO-LUMO gap energies. In addition, the nucleus-independent chemical shift (NICS) analysis shows that the triplet spin state is more aromatic than the corresponding singlet spin state for non-IPR Cs-C68, Cs-C98, Cs-C128, and Cs-C158. All derivatives C58+10nCl8 are also aromatic.

The DFT-NEGF scrutiny of doped fullerene junctions

Using the smallest non-classical fullerene, we investigate the impact of doping at the molecule-electrode interface on the electron transport of molecular junctions. This is accomplished by employing the density functional theory combined with the non-equilibrium Green's function. We contemplate different electronic parameters, namely, density of states, transmission coefficient, energy levels, molecular orbitals, conduction gaps, electron density, and their charge transfer. The relevance of these physical parameters is obtained to calculate their electrical parameters, current, and conductance, computed from Landauer-Büttiker formalism. The molecule-electrode coupling is influenced by the nature of doping atoms and affects the junction devices in a unique course. A particular aftermath is noticed in Au-C18O2-Au device with highest ballistic transport despite the electro-negative nature of oxygen atoms. Moreover, an interesting feature is observed in Au-C18Be2-Au device with double-barrier transmission resonance and corresponding oscillating conductance. Graphical abstract The doped C20 fullerene in molecular and device mode.

(Invited) Extending the Spiral Algorithm to Non-Classical Isomers and Metallofullerenes

The face-spiral algorithm proposed by Manolopoulos et al.1 has been extensively used to construct classical fullerenes, 2 and the corresponding numbering of isomers has been recommended as a systematic nomenclature. The spiral algorithm is an important help in clarifying the structures of fullerenes, endohedral fullerenes and fullerene derivatives. Recent and accumulating evidence shows that non-classical fullerene cages with heptagon(s) or square(s) may be competitive with their classical counterparts in some situations; non-classical fullerene cages may be favored parent cages of metallofullerenes and fullerene derivatives. It is desirable to make a systematic examination of the effect of heptagon(s) and/or square(s) on the structures and properties of bare fullerenes, fullerene ions, metallofullerenes and fullerene derivatives. The original spiral algorithm does not generate non-classical fullerene isomers, but can be adapted to do so. An extended spiral algorithm can selectively generate non-classical fullerenes with one or two heptagon(s) or square(s) in the size range of interest, and has been used [3-5] to identify favored non-classical fullerenes and metallofullerenes. Energetic arguments suggest that non-classical fullerene cages may play a significant role in the formation of fullerenes and metallofullerenes.

Ab-initio molecular characterization of nonclassical fullerenes cluster using two probe approach

Abstract

Zigzag Sc2C2 Carbide Cluster inside a [88]Fullerene Cage with One Heptagon, Sc2C2@Cs(hept)-C88: A Kinetically Trapped Fullerene Formed by C2 Insertion?

A non-isolated pentagon rule metallic carbide clusterfullerene containing a heptagonal ring, Sc2C2@Cs(hept)-C88, was isolated from the raw soot obtained by electric arc vaporization of graphite rods packed with Sc2O3 and graphite powder under a helium atmosphere. The Sc2C2@Cs(hept)-C88 was purified by multistage high-performance liquid chromatography (HPLC), cocrystallized with Ni-(octaethylporphyrin), and characterized by single-crystal X-ray diffraction. The diffraction data revealed a zigzag Sc2C2 unit inside an unprecedented Cs(hept)-C88 carbon cage containing 13 pentagons, 32 hexagons, and 1 heptagon. Calculations suggest that the observed nonclassical fullerene could be a kinetically trapped species derived from the recently reported Sc2C2@C2v(9)-C86 via a direct C2 insertion.

General rules for predicting the local aromaticity of carbon polyhedra

The aromaticity of classical and nonclassical C24 fullerene isomers and their anions were studied using the topological resonance energy (TRE) method. Local aromaticity was studied using the bond resonance energy (BRE) method. On the basis of BRE values, the contributions of different types of chemical bonds to the molecular global aromaticity were analyzed and general rules for predicting the local aromaticity of fullerenes are proposed. It was found that pentagons, heptagons, and squares preferred hexagons as neighbors rather than squares as neighbors. In the hexaanionic state, pentagon pair sites exhibit larger local aromaticity than other places in the same molecule.