Molecular mechanics on bonding and non-bonding interactions in (atom@C60)

The interactions between the embedded atom X (X = Li, Na, K, Rb, Cs; F, Cl, Br, I) andC60 cage in the endohedral-form complexes (X@C₆₀) are calculated and discussed according to molecular mechanics from the point of view of the bonding and non-bonding. It is found from the computational results that for atoms with radii larger than Li’s, their locations with the minimum interaction in (X@C₆₀) are at the cage center, while atom Li has an off-center location with the minimum interaction deviation of ~0.05 nm, and the cage-environment in C₆₀ can be regarded as syhero-symmetry in the region with radiusr of ~0.2 nm. It is shown that the interaction between X and C₆₀ cage is of non-bonding characteristic, and this non-bonding interaction is not purely electrostatic. The repulsion and dispersion in non-bonding interactions should not be neglected, which make important contribution to the location with minimum interaction of X, at center or off center. Some rules about the variations of interactions with atomic radii have been obtained. Project supportt:d by the National Natural Science Foundation of China.     

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.     

Energy relationship and the polarization of C_(60) cage in the endohedral complexes (X@C_(60))

In this paper, we carry out the calculation on the system (X@C60)(X=Li, Na, K, Kb, Cs; F, Cl, Br, I), where the position of X changes along 5 typical symmetry directions. For the calculation of quantum chemistry we use EHMO/ASED method, for the calculation of molecular mechanics we use Buckingham potential (exp-6-1) function, and for the calculation of thermo-chemical cycle we use individually isolating the processes such as the structure variation, charge transfer and charge distribution, and their interactions etc. The calculation results show that (1) In the region of radius r≈0.2 nm of the Ceo cage, the potential field is nearly spherical; (2) Except for Li and Na, the systems are the most stable with minimum energies at the center of C60 cage. For Li and Na, the systems are the most stable with minimum energies at r≈0.16 nm and r≈0.13 nm, respectively. In view of the interactive region of chemical bonds, the interactions between X and the C60 cage do not belong to the classical chemical bonds; (     

Interaction Between C60 Fullerene and Alkali Metals Demonstrating Superconductivity

A discret summation method [1] has been used to calculate the van der Waals dispersion interactions between an alkali metal atom and individual C₆₀ fullerene molecules, as well as between an intercalated alkali metal atom and the face-centred-cubic lattice of solid fullerite. It is known [2, 3] that the conductivities observed in the doped C₆₀ films vary considerably for different alkali atoms. Our interest was to investigate any correlation between these conductivities and the long range potential field behaviour. In the present study, we have obtained interaction potential curves of the C₆₀ fullerene molecule including within its cage, as well the potential field topography within the overall C₆₀ fullerite solid unit cell. We have found that for all intercalated alkali metals, except Cs, there are voids within the unit-cell of fullerite where the dopant experiences attractive interactions. Whereas on the other hand, inside the fullerene cage only Li and Na experience attractive forces. Importantly, it has been shown that the localization of the crystallographic sites [4] of doped alkali metals in fullerite coincide with the potential energy minima of long range van der Waals forces.     

Theoretical investigation on icosahedral C60(FeCp)12: A hybrid of C60 and ferrocene

A perfect hybrid complex C₆₀(FeCp)₁₂ is predicted using density functional theory method_. This fullerene derivative could be view as a C₆₀ cage of which each C₅ ring coordinates a (FeCp) ligand. Theoretical calculation reveals that it has a large lowest unoccupied molecular orbital–highest unoccupied molecular orbital gap (2.53 eV) and keeps the I_h symmetry of C₆₀. But the C? C bond length of its inner C₆₀ cage trends to be uniform, which is quite different from the bonding character of C₆₀ fullerene. Further investigation reveals that the chemical bonding, TDOS and the aromaticity of the (C₅FeCp) unit in C₆₀(FeCp)₁₂ are similar as those of ferrocene molecule, which indicates the similarity of their electronic properties. So, this compound could be viewed as the combination of ferrocene molecules. Thus, its unconventional formation process from 12 Fe(Cp)₂ is proposed and the reaction energy is calculated. As the C₆₀(FeCp)₁₂ compound has the geometry framework as C₆₀ and the electronic characters as ferrocene, it would inherit the outstanding properties from both two molecules and have wild potential applications in nanochemistry. We hope our study could give some references for the further investigation and experimental synthesis research of the C₆₀(FeCp)₁₂ compound. © 2015 Wiley Periodicals, Inc.     

Electronic spectra and electron structure of fullerene complex (η2-C60)Fe(CO)4

The electronic spectrum of the C₆₀Fe(CO)₄ complex was studied in a toluene solution. The more intense absorption of C₆₀Fe(CO)₄ in the visible region, relative to the free C₆₀, can be attributed to the effect of lower symmetry of the C₆₀ fullerene cage in C₆₀Fe(CO)₄ and, thus, relaxation of selection rules for forbidden internal electronic transitions of C₆₀. No bands of the charge transfer from 3d(Fe) to C₆₀ orbitals were observed in the visible region of the complex spectrum. Assignment of the bands was confirmed by semiempirical calculations of the electronic spectrum.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1453–1458, June, 1996     

Structure,energy, and spin characteristics of La@C<Subscript>60</Subscript>-Lu@C<Subscript>60</Subscript> lanthanide endofullerenes

Calculations of the equilibrium geometric and electronic structure of lanthanide endofullerenes are presented. Two types of the Ln@C₆₀ structure are found. For endofullerenes of the first type (La@C₆₀-Dy@C₆₀), the stable position of the lanthanide atom is achieved at a distance of 0.67R from the center of fullerene (R is the fullerene radius); in endofullerenes of the second type (Ho@C₆₀-Lu@C₆₀), the character of the interaction between the lanthanide atom and fullerene changes because of the transfer of unpaired electrons from the corresponding atom to fullerene. It is found that in endofullerenes of the second type, metal atom mobility increases, and two minima of the potential energy appear, which corresponds to the Ln position in the center and at a distance of 0.5R from the center. Based on the obtained spin density distribution for La@C₆₀-Lu@C₆₀ endofullerenes, we conclude that there is spin leakage.     

Encapsulation Chalcogen Anions in Perfluorinated Silicon Fullerene: X2−@Si20F20 (X=O,S, Se)

The structures and stabilities of cage Si₂₀F₂₀ and its endohedral complexes X²⁻@Si₂₀F₂₀ (X=O, S, Se) were determined at the B3LYP/6‐31G(d) levels of density functional theory (DFT). It is found that the adiabatic electron affinity (EA_ad) of host cage Si₂₀F₂₀ (I_h) is higher than that of isolated O atom (4.24 vs. 1.46 eV). This suggests the Si₂₀F₂₀ cage can selectively trap and stabilize the capsulated spherical anions. The calculations predict that X=S and Se are nearly located at the center of the cage, and O dramatically deviates from the center in C_3v symmetry. Moreover, the corresponding X²⁻@Si₂₀F₂₀ complexes have more negative inclusion energies (ΔE_inc) and thermodynamic parameters (ΔZ) than X²⁻@C₂₀F₂₀. The amount of charge that is being transferred from the encapsulated anions to the cage increases with the atomic radius, i.e., from O²⁻ (ca. 45%), S²⁻ (ca. 51%) to Se²⁻ (ca. 59%), and such a novel model of cage may have practical uses as potential and electrical building units of nanoscale materials.     

Theoretical study on the structures and electron spectra of C60M12 (M=Li,Na, Be)

Intermediate neglect of differential overlap (INDO) method was used to study the structures and the electronic spectra of C₆₀M₁₂ (M=Li, Na, Be). The calculations indicate that in the minimal energy configuration of C₆₀M₁₂ (M=Li, Na) the C₆₀ cage still retains I_h symmetry and the 12 Li or Na atoms are symmetrically located above the pentagons of the C₆₀ cage, whereas the difference between the double and single bonds has been significantly reduced. In contrast, because six electrons are filled in the fivefold‐degenerated h_g orbital of C₆₀, the C_s structure of C₆₀Be₁₂ has illustrated the occurrence of Jahn‐Teller distortion. Based on the optimized geometries, the electronic absorption spectra were calculated and the nature of red shift was discussed. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 505–509, 1999     

Interaction and superconducting transition in alkali-doped C_(60)

The intermolecular interactions in alkali-doped C60 solids, A3-x-yA'xA"yC60(A, A', A" =alkali; x y ≤3), have been calculated, and their correlations with the structures and the superconducting transition temperaturesTc have also been investigated. It is found that the variations of the lattice constants a0 of A3-x-yA'x A"yC60 solids and the superconducting transition temperatures Tc can be satisfactorily explained from the viewpoint of interaction. It is shown that the relationship of Tc with interaction is much closer than that of Tc with lattice constant a0. Therefore, we can say it is the physical factors such as interaction, other than the geometrical structure factors such as a0, that should be reasonably taken as the primitive cause of the sudden change of superconductivity from pristine C60 to its alkali-doped compounds and of the variation of Tc among superconductors of A3-x-yA'xA'yC60.     

Isolation and Crystallographic Characterization of the Labile Isomer of Y@C82 Cocrystallized with Ni(OEP): Unprecedented Dimerization of Pristine Metallofullerenes

Although the major isomers of M@C₈₂ (namely M@C_2v(9)‐C₈₂, where M is a trivalent rare‐earth metal) have been intensively investigated, the lability of the minor isomers has meant that they have been little studied. Herein, the first isolation and crystallographic characterization of the minor Y@C₈₂ isomer, unambiguously assigned as Y@C_s(6)‐C₈₂ by cocrystallization with Ni(octaethylporphyrin), is reported. Unexpectedly, a regioselective dimerization is observed in the crystalline state of Y@C_s(6)‐C₈₂. In sharp contrast, no dimerization occurs for the major isomer Y@C_2v(9)‐C₈₂ under the same conditions, indicating a cage‐symmetry‐induced dimerization process. Further experimental and theoretical results disclose that the regioselective dimer formation is a consequence of the localization of high spin density on a special cage‐carbon atom of Y@C_s(6)‐C₈₂ which is caused by the steady displacement of the Y atom inside the C_s(6)‐C₈₂ cage.     

Mass spectrometric investigation of polyfluorinated C60. Evidence for the existence of C60F2n (n = 0–30)

Electron-capture negative ion chemical ionization (EC-NICI) and field desorption (FD) mass spectrometric techniques were utilized to examine polyfluorinated C₆₀. Two different samples from the same preparation, one prior to sublimation and the other sublimed material, were investigated. From the raw non-sublimed product in EC-NCI six series of ions corresponding to different numbers of attached oxygen atoms were obtained, which are represented by the formula [C₆₀F_2nO_m]^?, where n ranged from 0 to 30 and m from 0 to 5. The sublimed material in EC-NICI produced the same six series of ions with up to 48 fluorine atoms attached to C₆₀. The field desorption of the same sample produced similar results, but the signal-to-noise ratios of the spectra were low. Both samples, in the two different techniques examined, yielded C₆₀F₆₀ ions with only an even number of fluorine atoms attached. The present investigation, for the first time, provides direct experimental evidence for the existence of higher fluorinated C₆₀ up to C₆₀F₆₀ and multiple oxides of polyfluoro-C₆₀ with up to five oxygen atoms attached.     

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 (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.     

Fused-Pentagon Isomers of C60 Fullerene Isolated as Chloro and Trifluoromethyl Derivatives

The carbon cage of buckminsterfullerene I_h-C₆₀, which obeys the Isolated-Pentagon Rule (IPR), can be transformed to non-IPR cages in the course of high-temperature chlorination of C₆₀ or C₆₀Cl₃₀ with SbCl₅. The non-IPR chloro derivatives were isolated chromatographically (HPLC) and characterized crystallographically as ¹⁸⁰⁹C₆₀Cl₁₆, ¹⁸¹⁰C₆₀Cl₂₄, and ¹⁸⁰⁵C₆₀Cl₂₄, which contain, respectively two, four, and four pairs of fused pentagons in the carbon cage. High-temperature trifluoromethylation of the chlorination products with CF₃I afforded a non-IPR CF₃ derivative, ¹⁸⁰⁷C₆₀(CF₃)₁₂, which contains four pairs of fused pentagons in the carbon cage. Addition patterns of non-IPR chloro and CF₃ derivatives were compared and discussed in terms of the formation of stabilizing local substructures on fullerene cages. A detailed scheme of the experimentally confirmed non-IPR C₆₀ isomers obtained by Stone–Wales cage transformations is presented.     

Endohedral Complex of Fullerene C<Subscript>60</Subscript> with Tetrahedral N<Subscript>4</Subscript>, N<Subscript>4</Subscript>@C<Subscript>60</Subscript>

B3LYP/6-31G(d) hybrid HF/DFT and BLYP/6-31G(d, p) DFT calculations were carried out to determine the structural and electronic properties of the endohedral complex of C₆₀ with Tetrahedral N₄ (T_d N₄), N₄@C₆₀. It was demonstrated that N₄ was seated in the center of the fullerene cage and the tetrahedral structure of N₄ is remained in the cage. The formation of this complex is endothermic with inclusion energy of 37.92 kcal/mol. N₄ endohedral doping perturbs the molecular orbitals of C₆₀ not so much, the calculated HOMO–LUMO gaps, the electron affinity (EA) and the ionizational potential (IP) of N₄@C₆₀ are similar to that of C₆₀.     

A theoretical approach to the complexation and decomplexation processes in the reaction C60 + He ⇌ (He@C60)

To theoretically investigate the complexation and decomplexation processes in the reaction C₆₀ + He ? (He@C₆₀), four possible reaction paths are assumed, so that while He approaches and penetrates the C₆₀ cage (a) a pentagon, (b) a hexagon, (c) a short bond, or (d) a long bond will be expanded on its original sphere or plane to form a window. The computation is performed by the quantum chemical method EHMO/ASED. The results show that the probability of completing this reaction in terms of the tunnel effect can be neglected and the reaction is completed by overcoming a potential barrier of the reaction. It is easily completed by opening a planar window than by opening a shperical window. The probability through reaction path (b) with a barrier of 1247.94 kJ/mol is larger than that through reaction path (a) with a barrier of 1438.26 kJ/mol. The probability through reaction path (d) is the largest with a barrier of 1076.78 kJ/mol, when the planar expansion forming a window of a 9-membered ring is at the optimized value of 0.40 å. When He deviates the center and approaches the C₆₀ cage, there will be a charge distribution on C₆₀ with changing in size and even in sign along the longitude corresponding to the symmetry axis, but the absolute value gradually decreases. This situation is similar to the charge transfer on carbons in sraight-chain polyene. © 1995 John Wiley & Sons, Inc.     

Multiple Adducts of C60 by Tether-Directed Remote Functionalization and synthesis of soluble derivatives of new carbon allotropes Cn(60+5)

A comprehensive series of multiple adducts of C₆₀ was prepared by tether-directed remote functionalization. When the tether-reactive-group conjugates 2 and 10 were attached to methano[60]fullerenecarboxylic acid ( = cyclopropafullerene-C₆₀-I_h-carboxylic acid) and C₆₀, respectively, the e-bis-adducts 4 and 9 (Schemes 1 and 2) were obtained with complete regioselectivity as predicted by semi-empirical PM3 calculations (Fig. 2). Attachment of the anchor-tether-reactive-group conjugate 13 to C₆₀ by Bingel reaction, followed by double intramolecular Diels-Alder cycloaddition afforded the tris-adduct 12 (Scheme 3). Starting from 12 , a series of selective e-additions led to the tetrakis-adducts 16 and 19 (Scheme 4), pentakis-adducts 20 – 23 (Scheme 5), and, ultimately, to hexakis-adducts 24 and 25 (Scheme 6), and 29 and 30 (Scheme 7) with a pseudo-octahedral addition pattern on the fullerene core. Oxidative cyclization of diethynylmethanofullerene 30 under Eglinton-Glaser conditions afforded the trimeric and tetrameric acetylenic macrocycles 26 , with three, and 27 , with four appended C₆₀ moieties, respectively (Scheme 8). These multinanometer-sized compounds are the first soluble derivatives of C₁₉₅ and C₂₆₀, two members of a new class of fullerene-acetylene hybrid C-allotropes with the general formula C_{n(60 + 5)}. The matrix-assisted laser-desorption time-of-flight mass spectra of 26 and 27 showed a remarkable fragmentation; the sequential loss of fullerene spheres led to the formation of ions corresponding to mono-fullerene adducts of the cyclocarbons cyclo-C₁₅ and cyclo-C₂₀ (Fig. 4). Large solvent effects were observed in the Bingel addition of 2-bromomalonates to higher adducts of C₆₀, with the use of polar solvents enhancing the reaction rate without loss of regioselectivity. Experimental evidence for the enhanced reactivity of e_face over e_edge bonds was obtained, which had previously been predicted in computational studies. The correlated series of mono- to hexakis-adducts of C₆₀ allowed identification of the changes in reactivity and physical properties that occur, when the conjugated π-electron chromophore of the fullerene is reduced as a result of increasing functionalization; this analysis is the subject of the directly following paper.     

Synthesis,Structure, and Properties of the Fullerene C60 Salt of Crystal Violet, (CV+)(C60.−)⋅0.5 C6H4Cl2, which Contained Closely Packed Zigzagged C60.− Chains

The reduction of fullerene C₆₀ by zinc dust in the presence of crystal violet cations (CV⁺) yielded a deep‐blue solution, from which crystals of (CV⁺)(C₆₀^.?) ? 0.5 C₆H₄Cl₂ ( 1 ) were obtained by slow mixing with n‐hexane. The salt contained isolated, closely packed zigzagged chains that were composed of C₆₀^.? radical anions with a uniform interfullerene center‐to‐center distance of 9.98 Å. In spite of the close proximity of the fullerenes, they did not dimerize, owing to spatial separation by the phenyl substituents of CV⁺. The room‐temperature conductivity of compound 1 was 3×10^?2 S cm^?1 along the fullerene chains. The salt exhibited semiconducting behavior, with an activation energy of E_a=167 meV. Spins localized on C₆₀^.? were antiferromagnetically coupled within the fullerene chains, with a Weiss temperature of ?19 K without long‐range magnetic ordering down to 1.9 K.     

A computational study of the endohedral fullerene GeH4@C60

The structures, stabilities, and electronic properties of the endohedral fullerene GeH₄@C₆₀ have been systematically studied by using the hybrid DFT-B3PW91 functional in conjunction with 6-31G(d) basis sets. Our calculated results show that the GeH₄ molecule is more compact in the center of the C₆₀ cage and exists in molecular form inside the fullerene. The Zero-Point and BSSE corrected binding energy of GeH₄@C₆₀ is −1.77 eV. The calculated HOMO–LUMO energy gap, the vertical ionization potentials (VIP) and vertical electron affinities (VEA) are similar to that of C₆₀ cage. It is indicated that GeH₄@C₆₀ also seems to be very stable species. Natural population analysis on the GeH₄@C₆₀ reveals that the central GeH₄ only gain −0.06 charges from the C₆₀ cage. Additionally, the vibrational frequencies and active infrared intensities of GeH₄@C₆₀ are also discussed.     

Molecular mechanics on bonding and non-bonding interactions in (atom@C_(60))

The interactions between the embedded atom X (X=Li,Na,K,Rb,Cs; F,Cl,Br,I) and C60cage in the endohedral-form complexes (X@C60) are calculated and discussed according to molecular mechanics from the point of view of the bonding and non-bonding.It is found from the computational results that for atoms with radii larger than Li's,their locations with the minimum interaction in (X@C60) are at the cage center,while atom Li has an off-center location with the minimum interaction deviation of-0.05 nm,and the cage-environment in C60 can be regarded as sphero-symmetry in the region with radius r of ~0.2 nm.It is shown that the interaction between X and C60 cage is of non-bonding characteristic,and this non-bonding interaction is not purely electrostatic.The repulsion and dispersion in non-bonding interactions should not be neglected,which make important contribution to the location with minimum interaction of X,at center or off center.Some rules about the variations of interactions with atomic radii have been ob