Band structure calculations for some binary mixed alkali-metal doped C60

In this paper we present the three dimensional EHMO crystal orbital calculations for crystalline Li₂CsC₆₀, Na₂CsC₆₀, K₂CsC₆₀ and Rb₂CsC₆₀. For all these four dopants with a fcc structure, our calculated band structures and density of state unequivocally suggest a metallic conducting phase with each a set of three half-filled band across the Fermi level which is just located close to a peak of density of state. A monotonic relationship is found between our calculated Fermi-level density of state N and experimental transition temperature T_c which is in excellent agreement with some theoretical studies and experimental results. With our other calculation results, such as the overlap populations between A (where A represents an alkali-metal) and C₆₀ molecules, net atomic and orbital charges, crystal orbital vectors and various projected DOS, we could probably shed some light on the mechanism of the superconductivity for this new marvelous kind of materials from a chemical point of view.     

BaxC60 fullerides: π Electronic peculiarities of the C60 molecule and their consequences for the solid state

The electronic structure of Ba_xC₆₀ fullerides was studied theoretically under special consideration of π electronic effects in the C₆₀ molecule. Band structure data were derived by an intermediate neglect of differential overlap (INDO) crystal orbital (CO) approach. Different electronic configuration were evaluated in the Ba-doped C₆₀ fullerides. Ba_xC₆₀ solids with x=0, 3, 4, 6 are insulators. For a Ba₅C₆₀ model extrapolated from the crystal structure of Ba₆C₆₀, a finite band gap is also predicted. For a Ca₅C₆₀-like structure of Ba₅C₆₀, a quasi-degeneracy between a metallic configuration and an insulating Mott-like state was found. With an increasing Ba-to-C₆₀ charge transfer (CT), sizable changes in the π system of C₆₀ occur. In the neural molecule and for not too high an electron count, the π electrons form more or less electronically isolated hexagon–hexagon (6–6) “double” bonds with only minor hexagon–pentagon (6–5) “double-bond” admixtures. In the vicinity of C₆₀¹²⁻, the 6–6 bonds have lost most of their double-bond character while it is enhanced for the 6–5 bonds. In highly charged anions, the π electron system of the soccer ball approaches a configuration with 12 decoupled 6π electron pentagons. For electron numbers between C₆₀ and C₆₀¹²⁻, the net π bonding is not weakened. The INDO CO results of the Ba_xC₆₀ solids are supplemented by INDO MO and ab initio (3-21 G* split-valence basis) calculations of molecular C₆₀ and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C₆₀ with short 6–6 and long 6–5 bonds is inverted in C¹²⁻₆₀. The high acceptor capability of C₆₀ is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C₆₀ and C₆₀¹²⁻, the influence of the Pauli antisymmetry principle (PAP) is minimized. The quantum statistics of (π) electron ensembles with a deactivated PAP is of the so-called hard-core bosonic (hcb) type. In these ensembles, the on-site interaction is fermionic while the intersite interaction is bosonic. Energetic consequences of the quantum statistical peculiarities of π systems are explained with the aid of simple model systems; we selected annulenes and polyenes. Computational tools in this step are Green's function quantum Monte Carlo (GF QMC) and full configuration interaction (CI) calculations for the π electrons of the model systems. These many-body techniques were combined with a Pariser–Parr–Pople (PPP) Hamiltonian. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 333–373, 1997     

Darstellung und Charakterisierung der Fulleren-Kokristallisate C60 · 12 C6H12, C70 · 12 C6H12, C60 · 12 CCl4, C60 · 2 CHBr3, C60 · 2 CHCl3, C60 · 2 H2CCl2

Synthesis and Characterization of the Fullerene Co-Crystals C₆₀ · 12 C₆H₁₂, C₇₀ · 12 C₆H₁₂, C₆₀ · 12 CCl₄, C₆₀ · 2CHBr₃, C₆₀ · 2CHCl₃, C₆₀ · 2H₂CCl₂ By crystallization of fullerenes from non-polar solvents (C₆H₁₂, CCl₄, CHBr₃, CHCl₃, H₂CCl₂) compounds of the following compositions were obtained: C₆₀ · 12C₆H₁₂, C₇₀ · 12C₆H₁₂, C₆₀ · 12CCl₄, C₆₀ · 2CHCl₃, C₆₀ · 2CHBr₃ and C₆₀ · 2H₂CCl₂. Lattice parameters have been determined by X-ray diffraction of powder samples; according to single-crystal examinations on C₆₀ · 12C₆H₁₂, C₆₀ · 12CCl₄ and C₆₀ · 2CHBr₃ the fullerene is orientationally disordered. C₆₀ · 12C₆H₁₂, cubic, a = 28.167(1) Å; C₇₀ · 12C₆H₁₂, cubic, a = 28.608(2) Å; C₆₀ · 12CCl₄, cubic, a = 27.42(1) Å; C₆₀ · 2CHBr₃, hexagonal, a = 10.212(1), c = 10.209(1) Å; C₆₀ · 2CHCl₃, hexagonal, a = 10.08(1), c = 10.11(2) Å; C₆₀ · 2H₂CCl₂, tetragonal, a = 16.400(1) Å, c = 11.645(7) Å.     

Theoretical predictions of the structures and electronic spectra of C60NH with comparisons with the isoelectronic molecules C60O and C60CH2

Intermediate neglect of differential overlap (INDO ) calculations were used to study two structures of C₆₀NH: one of C_2ν, geometry with a bridging NH across the bond between two fused six-membered rings in C₆₀ and the other of C_s geometry with a bridging NH across the bond between a five- and a six-membered ring. We calculated the most stable isomer of C₆₀NH to be of C_2ν, symmetry. It was found that the C_2ν isomer has a protonated aziridine structure with a bridging C? C bond length of 0.1520 nm. The electronic spectra of both isomers of C₆₀NH were calculated. Comparisons were made with the isoelectronic molecules C₆₀O and C₆₀CH₂, cases in which the calculated electronic spectra for the most stable isomers C₆₀O (C_2ν) and C₆₀CH₂ (C_2ν) are in good agreement with recent experimental results. © 1995 John Wiley & Sons, Inc.     

Semiempirical vibrational and electronic structures of C60 and C70

The electronic and vibrational structures of C₆₀ and C₇₀ have been calculated at the PM3 semiempirical level. C₆₀ has a partially delocalized structure, while C₇₀ has both a localized set and a delocalized set of MOs. As with AM1 and MNDO, PM3 predicts the heat of formation of C₇₀ to be greater than that of C₆₀, and that C₇₀ is the thermodynamically more stable species. Calculation of the normal modes was accelerated over 40 times by limited use of symmetry theory.     

The electronic structure of two novel carbides,Ca3Cl2C3 and Sc3C4, containing C3 units

The triatomic C₃ unit that is known to exist in Mg₂C₃ has recently been found in the new compounds Ca₃Cl₂C₃ and Sc₃C₄. The electronic structure of these compounds is analyzed with the aid of extended Hückel Calculation. A fragment molecular Orbital analysis (FMO) is used to study the bonding characteristic of the C₃ unit in the ionic Ca₃Cl₂C₃, and in Sc₃C₄, the latter Containing C₂ unit and single C atoms as well. Sc₃C₄ Contain partially filled Sc (d) and C₂ bands leading to Metallic conductivity and Pauli Paramagnetism. The C? C bond distance in the diatomic C₂ units is significantly increased (d_{c? c}= 125 pm) relative to C^2?₂ or acetylene, because antibonding π*_g orbitals are partially filled. The unusual bending of the C₃ unit (d_{c? c}= 134 pm) in Sc₃C₄ (175,8°) and in Ca₃Cl₂C₃ (169,0°) is likely to be a result of the packing arrangement in these structures.     

3:1 Molecular Complex of Hydroquinone and C60

The preparation of a 3:1 complex of hydroquinone (HQ) and C₆₀, (HQ)₃C₆₀, is reported and its X-ray crystal structure described. The HQ host builds up a single H-bonded super-polonium network with super-cubes as building blocks, which accommodate the large C₆₀ guest molecules. The enclathration of C₆₀ is rather tight (high density of the complex) owing, in part, to favorable charge-transfer host-guest interactions (π-donor-acceptor complex). Nonetheless, the C₆₀ molecules arc orientationally disordered, and no individual bond lengths could be measured. In contrast, the HQ host network is essentially ordered and well-defined with normal intramolecular geometry. No appreciable (powder) conductivity could be observed for (HQ)₃C₆₀, yet ESR spectra of single crystals showed interesting narrow signals.     

NMR Properties of Polylithiated C60

Endohedral, ¹³C, ⁷Li, and nucleus‐independent (NICS) chemical shifts are reported for selected Li_nC₆₀ isomers (n = 6, 12, 18) at the GIAO (gauge‐including atomic orbitals)‐SCF/DZP//BP86/3–21G level. Li₆C₆₀ closely resembles C₆₀^6– in terms of NMR criteria for aromaticity, as evidenced by an exceptionally high endohedral shielding. In contrast, nonaromaticity is indicated for Li₁₂C₆₀, based on a positive endohedral chemical shift. NICS and δ(endo) values very similar to those of Li₁₂C₆₀ are obtained for Li₁₈C₆₀. According to population analysis, indeed the same number of electrons are transferred to the fullerene cage in both cases. Endohedral chemical shifts, accessible via ³He NMR of the corresponding endohedral helium compounds, could thus be a valuable indicator for the extent of reduction of the C₆₀ molecule. Energetic estimates suggest that in the bulk, Li₁₂C₆₀ should be unstable with respect to decomposition into Li₆C₆₀ and lithium metal.     

Crystal solvate of C60 with tricloroethylene,C60 · C2HCl3

Extraction of fullerenes from carbon soot by trichloroethylene has been studied. We have found that C₆₀ forms a solvate with trichloroethylene (C₆₀ · C₂HCl₃:a=31.31(1);b= 10.156(4);c=10.146(4) Å;V=3228.6 ų,Z=4,d _calc=1.752 g cm^–3, orthorhombic symmetry). Its thermal stability has been studied using TG and DSC. A phase transition of the first order at 167 K has been detected.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1248–1250, July, 1994.The authors are grateful to V. P. Bubnov and I. S. Krainskii for providing them with the samples of fulle-rene-containing carbon soot, and to M. G. Kaplunov and A. V. Zvarykina for assistance in the work.This work was carried out with the financial support of the Russian Foundation for Basic Research, Project Nos. 93-03-18705 and 93-03-5650.     

Induced Absorption of C60 and a Water-Soluble C60-Derivative in SiO2 Sol-Gel Matrices

Porous sol-gel glasses, either impregnated with pure C₆₀ or doped with a methanofullerene derivative, have been studied and induced absorption or reverse saturable absorption (RSA) has been observed in both types of solid materials. The samples impregnated by pure C₆₀ mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C₆₀, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C₆₀-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics that are influenced by solid state effects. We observe an important decrease of transmission at high fluences for both kinds of samples, a shortened singlet-state lifetime to that observed in solution, but nonetheless, a triplet yield, that cannot be considered as negligible. In the case of pure C₆₀ in a sol-gel matrix, we can explain the faster de-excitation dynamics, relative to behavior in solution, mainly by the absence of stabilizing aromatic solvents and also by the interaction of the amorphous environment with the molecules. Concerning the methanofullerene samples, the acceleration of the de-excitation dynamics can be principally attributed to solid-state effects due to the micellar aggregation.     

Computer modeling of C2 cluster addition to fullerene C60

The reaction between C₂ cluster and C₆₀ fullerene resulting in C₂ insertion to C₆₀ with formation of closed C₆₂ cage (reaction of C₂ ingestion by C₆₀) was investigated by the semiempirical MNDO‐PM3 method. The geometries and energies of extremal points on the C₆₂ potential energy surface were calculated. Several reaction pathways leading to the formation of three different closed C₆₂ fullerenes were investigated. All insertion reactions proceed stepwise through intermediate adducts of different structures. The main reaction pathways were found to be addition of C₂ by its one side to the 6,6‐ or 5,6‐bond of C₆₀ with formation of primary unclosed C₆₂ adducts of “ball‐with‐fork” structures, lying in deep potential wells. Back reaction of C₂ detachment from primary adducts can compete with that of their transformation to the closed C₆₂ cages inasmuch as calculated activation barriers of the both reactions are comparable. Model calculations at the B3LYP/6‐31G* level, using C₃₂H₁₂ semisphere instead of C₆₀, confirmed the conclusion about two competitive pathways of the primary adducts transformation, C₂ detachment, and C₂ ingestion. The concerted insertion of C₂ to C₆₀ was realized only in the case of severe restrictions on starting geometry of the C₂ + C₆₀ system. The results of calculations explain recent experimental data on the formation of metastable adducts upon addition of C₂ to C₆₀, obtained using the time‐of‐flight mass spectrometer with laser desorption. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Triptycene‐Derived Calix[6]arenes: Synthesis,Structures, and Their Complexation with Fullerenes C60 and C70

Two pairs of novel triptycene‐derived calix[6]arenes 4 a , b and 5 a , b have been efficiently synthesized through both one‐pot and two‐step fragment‐coupling strategies starting from 2,7‐bis(hydroxymethyl)‐1,8‐dimethoxytriptycene 1 . Subsequent demethylation of 4 a , b and 5 a , b with BBr₃ in dry dichloromethane gave the macrocyclic compounds 6 a , b and 7 a , b . Treatment of either 4 a or 6 a with AlCl₃ resulted in the same debutylated product 8 , while 9 was similarly obtained from either 5 a or 7 a . Structural studies revealed that all of the macrocycles have well‐defined structures with fixed conformations both in solution and in the solid state owing to the introduction of the triptycene moiety with a rigid three‐dimensional (3D) structure, making them very different from their classical calix[6]arene counterparts. As a consequence, it was found that all of these the triptycene‐derived calix[6]arenes could encapsulate small neutral molecules in their cavities in the solid state. Moreover, it was also found that the macrocycles 4 b and 5 b showed highly efficient complexation abilities toward fullerenes C₆₀ and C₇₀, forming 1:1 complexes with association constants ranging from (5.22±0.20)×10⁴ to (8.68±0.30)×10⁴ M ^?1.     

Charge transfer in complexes of C60 and C70 in solutions and solid state

Electronic absorption spectra of complexes of C₆₀ and C₇₀ fullerenes with donors, tetrathiafulvalene and pyranylidene derivatives, were studied in solutions and in the solid state. Charge transfer bands were found in the 680–1300 nm range. The charge transfer energies (hv _ct) for the C₆₀ and C₇₀ complexes in solutions are close and almost independent of the solvent polarity. For the C₆₀ complexes in the solid state, the dependence ofhv _ct on the ionization potential (IP) of donors was found to behv _ct=0.82IP–3.93 eV. In the C₆₀ complexes in the solid state, thehv _ct values are 0.15–0.20 eV lower than those in the solution. The linear dependences ofhv _ct onIP of donors for the C₆₀ complexes lie 0.6–0.7 eV higher than those in the complexes with tetracyanoethylene (TCNE). This is associated with lower values of the electron affinity of C₆₀ and the energy of the electrostatic interaction in the fullerene complexes as compared to those of the TCNE complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 478–483, March, 1999.     

Positron annihilation in C60 and K6C60

The positron density distributions in C₆₀ and K₆C₆₀ have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C₆₀, positrons are distributed in the interstitial sites between the C₆₀ molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K₆C₆₀, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K₆C₆₀. This component is considered to reflect the positron annihilation inside a C₆₀ molecule.     

Molecular Complexes of Fullerenes C60 and C70 with Saturated Amines

New molecular complexes of fullerenes C₆₀ and C₇₀ with leuco crystal violet (LCV, 1-3); leucomalachite green (LMG, 4-6); crystal violet lactone (CVL, 7); N,N,N′,N′-tetrabenzyl-p-phenylenediamine (TBPDA, 8, and 9); N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA, 10, and 11); triphenylamine (TPA, 12, and 13); and substituted phenotellurazines (EPTA and TMPTA, 14, and 15) have been synthesized. Crystal structures have been solved for C₆₀ complexes with LMG (5, 6) TBPDA (8), TMPDA (10), and TPA (12). The C₆₀ molecules form closely packed double layers in 5 and 6, hexagonal layers in 10 and quasi-three-dimensional layers in 8 and 12. The substitution of disordered solvent molecules in the complexes with LMG (4, 5) by naphthalene ones results in the ordering of the C₆₀ molecules. According to IR-, UV-visible-NIR and ESR-spectroscopy the complexes have a neutral ground state. The spectra of 1-8, and 10 show intense charge transfer bands in the visible and NIR-range. On photoexcitation by white light (light-induced ESR (LESR) spectroscopy), 1 and 10 were shown to have an excited ionic state. The LESR signals were generated at light energies <2.25 eV indicating that the excited states in the complexes are realized mainly by direct charge transfer from donor to the C₆₀ molecule.     

Marriage of Heterobuckybowls with Triptycene: Molecular Waterwheels for Separating C60 and C70

Exploration of π-conjugated polycycles, particularly those have π-frameworks spread over the three-dimensional space, is essential in materials science and synthetic chemistry as these chemical entities possess featured optoelectronic properties and supramolecular assembly. Herein, the bowl-shaped trichalcogenasumanenes are fused onto three branches of triptycene through pyrazine units, affording waterwheel-like three-dimensional polycycles 4 a/4 b . Because the three branches on 4 a/4 b are chemically equal, the molecular orbitals of 4 a/4 b show degenerate feature that results in the strong UV-Vis absorbance at steady state. 4 a/4 b exhibit photo-induced charge-separation and subsequent charge-redistribution at transient state, leading to excited state absorption in NIR-II window (1165–1400 nm). 4 a/4 b are excellent fullerene receptors, and they form 1 : 1 host-guest complexes with C₆₀/C₇₀ as proved by spectroscopic titrations and single crystal structure analysis. Moreover, 4 a/4 b show much stronger affinity toward C₇₀ than C₆₀. Consequently, 4 a/4 b are able to separate C₆₀ and C₇₀ from their mixture, giving the purity of C₆₀ up to 99.5 %.     

Ultrafast Photoinduced Charge Separation Leading to High‐Energy Radical Ion‐Pairs in Directly Linked Corrole–C60 and Triphenylamine–Corrole‐C60 Donor–Acceptor Conjugates

Closely positioned donor–acceptor pairs facilitate electron‐ and energy‐transfer events, relevant to light energy conversion. Here, a triad system TPACor‐C₆₀ , possessing a free‐base corrole as central unit that linked the energy donor triphenylamine ( TPA ) at the meso position and an electron acceptor fullerene (C₆₀) at the β‐pyrrole position was newly synthesized, as were the component dyads TPA‐Cor and Cor‐C₆₀ . Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady‐state fluorescence studies showed efficient energy transfer from ¹ TPA* to the corrole and subsequent electron transfer from ¹corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron‐transfer products, the corrole radical cation ( Cor^?+ in Cor‐C₆₀ and TPA‐Cor^?+ in TPACor‐C₆₀ ) and fullerene radical anion (C₆₀^??), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, k_CS, was found to be about 10¹¹ s^?1, suggesting the occurrence of an ultrafast charge‐separation process. Interestingly, although an order of magnitude slower than k_CS, the rate of charge recombination, k_CR, was also found to be rapid (k_CR≈10¹⁰ s^?1), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge‐separated species relaxed directly to the ground state in polar solvents while in toluene, formation of ³corrole* was observed, thus implying that the energy of the charge‐separated state in a nonpolar solvent is higher than the energy of ³corrole* being about 1.52 eV. That is, ultrafast formation of a high‐energy charge‐separated state in toluene has been achieved in these closely spaced corrole–fullerene donor–acceptor conjugates.     

Redox-induced hydrogen transfer from hydrofullerene C60H36 to fullerene C60

The possibility of hydrogen transfer from hydrofullerene C₆₀H₃₆ to electrogenerated radical anion C₆₀ ^.− or dianion C₆₀ ²⁻ in propylene carbonate-toluence (3∶2, v/v) was demonstrated by cyclic voltammetry. The process affords C₆₀H₂ as the product. The reaction found is the typical redox-induced process. Translated fromIzvestiya Akodemii Nauk. Seriya Khimicheskaya, No. 6, pp. 1136–1139, June, 1998.     

Zur Kenntnis von Ca3IrCuO6

On Ca₃IrCuO₆ Single crystals of Ca₃IrCuO₆ were prepared by a flux technique and investigated by single crystal X-ray work. It crystallizes with monoclinic symmetry, space group C—C12/c1; a = 9.032, b = 9.295, c = 6.466 Å, β = 91.35°, Z = 4. Ca₃IrCuO₆ is isotypic to Sr₃IrCuO₆. The square planare CuO₄ polygones show probably a slightly deficit accompanied by an adequate part of iridium in the oxydation state Ir⁵⁺.     

Reverse Saturable Optical Absorption of C60, Soluble Methanofullerenes, and Fullerodendrimers in Sol-Gel Mesoporous Silica Host Matrices

Porous sol-gel glasses with various pore size distributions are prepared and either impregnated with pure C₆₀ or soaked with methanofullerenes or fullerodendrimers derivative solution. Induced absorption or reverse saturable absorption (RSA) has been studied in both types of solid materials. The samples impregnated by pure C₆₀ mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C₆₀, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C₆₀-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics, which are influenced by solid state effects. Fullerodendrimers derivatives lead to the highest quantum yield.