Size dependence of the electronic and magnetic properties of fullerenes (C60-C240)

We use the Gutzwiller method for studying various characteristic energies of fullerences in the free state (first and second ionization energy, electron affinity and singlet and triplet excitation energies). The stability of spin density wave (SDW) in fullerene has been investigated. The critical value of (U/β) (U intraatomic correlation energy, β transfer energy to stabilize SDW) is equal to 3 in C₆₀ and 4.25 in C₂₄₀ (icosahedrally symmetric C₂₄₀).     

Structural and optical properties of highly hydroxylated fullerenes: stability of molecular domains on the C60 surface

The excitation spectra and the structural properties of highly hydroxylated C(60)(OH)(x) fullerenes (so-called fullerenols) are analyzed by comparing optical absorption experiments on dilute fullerenol-water solutions with semiempirical and density functional theory electronic structure calculations. The optical spectrum of fullerenol molecules with 24-28 OH attached to the carbon surface is characterized by the existence of broad bands with reduced intensities near the ultraviolet region (below approximately 500 nm) together with a complete absence of optical transitions in the visible part of the spectra, contrasting with the intense absorption observed in C(60) solutions. Our theoretical calculations of the absorption spectra, performed within the framework of the semiempirical Zerner intermediate neglect of diatomic differential overlap method [Reviews in Computational Chemistry II, edited by K. B. Lipkowitz and D. B. Boyd (VCH, Weinheim, 1991), Chap. 8, pp. 313-316] for various gas-phase-like C(60)(OH)(26) isomers, reveal that the excitation spectra of fullerenol molecules strongly depend on the degree of surface functionalization, the precise distribution of the OH groups on the carbon structure, and the presence of impurities in the samples. Interestingly, we have surprisingly found that low energy atomic configurations are obtained when the OH groups segregate on the C(60) surface forming molecular domains of different sizes. This patchy behavior for the hydroxyl molecules on the carbon surface leads in general to the formation of fullerene compounds with closed electronic shells, large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, and existence of an excitation spectrum that accounts for the main qualitative features observed in the experimental data.     

Bond orders and atomic properties of the highly deformed halogenated fullerenes C60F18 and C60Cl30 derived from their charge densities

The experimental charge densities of the halogenated C(60) fullerenes C(60)F(18) and C(60)Cl(30) were determined from high-resolution X-ray data sets measured with conventional Mo(Kalpha) radiation at 20 K for C(60)Cl(30) and with synchrotron radiation at 92 K for the fluorine compound. Bond topological and atomic properties were analyzed by using Bader's AIM theory. For the different C--C bonds, which vary in lengths between 1.35 and 1.70 A bond orders n between n=2 and significantly below n=1 were calculated from the bond topological properties at the bond critical points (BCP's). The low bond orders are seen for 5/6 bonds with each contributing carbon carrying a halogen atom. By integration over Bader's zero flux basins in the electron density gradient vector field atomic properties were also obtained. In contrast to free C(60), in which all carbon atoms have a uniform volume of 11 A(3) and zero charge, atomic volumes vary roughly between 5 and 10 A(3) in the halogenated compounds. Almost zero atomic charges are also found in the Cl derivative but a charge separation up to +/-0.8 e exists between C and F in C(60)F(18) due to the higher fluorine electronegativity, which is also seen in the electrostatic potential for which the electronegativity difference between carbon and fluorine, and the addition to one hemisphere of the fullerene cage leads to a strong potential gradient along the C(60)F(18) molecule. From the summation over all atomic volumes it follows that the halogen addition does not only lead to a dramatic distortion of the C(60) cage but also to a significant shrinkage of its volume.     

Geometric and electronic structures of metal-substitutional fullerene C59Sm and metal-exohedral fullerenes C60Sm

The geometric and electronic structures of metal-substituted fullerene C59Sm and exohedral fullerenes C60Sm are studied using the density-functional theory. The geometric optimization shows that the replacement of a C atom with a Sm in C60 yields a stable substitutionally doped fullerene C59Sm, and among the five possible optimized geometries for C60Sm, the most favorable exohedral sites are above the center of a hexagon and a pentagon ring. The calculations for electronic structures show that the magnetic moment of Sm is preserved for all the stable structures as tiny hybridization takes place between the orbitals of the Sm atom and those of their neighboring carbons. Because of the small energy gaps and the half occupation of the highest occupied molecular orbitals, all the stable C60Sm isomers are inferred to be conductors.     

Synthesis of C59Hx and C58Hx fullerenes stabilized by hydrogen

Prolonged hydrogenation of C(60) molecules by reaction with H(2) at elevated temperature and pressure results in fragmentation and collapse of the fullerene cage structure. However, fragments can be preserved by immediate termination of dangling bonds by hydrogen. Here we demonstrate that not only fullerene fragments but also hydrogenated fragmented fullerenes (e.g., C(58)H(40) and C(59)H(40)) can be synthesized in bulk amount by high-temperature hydrogenation of C(60). We confirm successful synthesis of these species by matrix-assisted laser desorption ionization time-of-flight mass spectrometry and complete speciation of the resultant complex fullerene mixtures by high-resolution field desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry.     

The electrochemical reduction of fullerenes, C60 and C70

The hexaanion of fullerene, C(60)(6-), was obtained in 1:5 (v/v) acetonitrile-toluene mixture with a mercury hemispherical ultramicroelectrode as a working electrode at a temperature of up to 30 degrees C. The C(70)(6-) ion also can be observed under the same conditions. The differences between the redox potentials of C(60) relative to C(70) indicate that it is easier to add electrons to C(70) and its anions compared to the counterparts of C(60). The results show that the mercury electrode is very suitable for investigation of the properties of the electrochemical reduction for the fullerenes, particularly C(60), at room temperature.     

Technology for manufacture of pure fullerenes C60, C70 and a concentrate of higher fullerenes

An integrated technology for manufacture of fullerenes was developed. It includes the following stages: synthesis of a fullerene black, extraction of a mixture of fullerenes from the black, preliminary separation of the mixture into concentrates enriched in C₆₀ and C₇₀ fullerenes, and production of C₆₀ and C₇₀ fullerenes of purity exceeding 99.5 and 98.0 wt %, respectively, from the concentrates.     

Preparation and study of hydrides of fullerenes C60 and C70

Fullerene hydrides were prepared by hydrogenation of fullerences C₆₀ and C₇₀ using proton transfer from 9,10-dihydroanthracene to fullerene and were studied by mass spectrometry (electron impact, field desorption), IR, UV, and¹H and¹³C NMR spectroscopy. The main product of the hydrogenation of C₆₀ is C₆₀H₃₆, which is sufficiently stable. Hydrogenation of fullerene C₇₀ gives a series of polyhydrides C₇₀H _n (n=36–46), and the main product is C₇₀H₃₆. The dehydrogenation of C₆₀H₃₆ by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is not quantitative and results in the formation of fullerene derivatives along with C₆₀. The comparison of the IR and¹H and¹³C NMR spectral data for solid C₆₀H₃₆ with the theoretical calculations suggests that the fullerene hydride has aT-symmetric structure and contains four isolated benzenoid rings located at tetrahedral positions on the surface of the closed skeleton of the molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 4, pp. 671–678, April, 1997.     

Time-dependent density functional theoretical study of the absorption properties of BN-substituted C60 fullerenes

Time-dependent density functional theoretical calculations using the B3LYP functional and 6-31G* basis set for a series of BN-substituted C60 fullerenes reveal that, unlike C60, these molecules would absorb in the visible region and that the optical and electronic properties of fullerenes can be fine-tuned with proper BN substitution.     

C(60)-based triads with improved electron-acceptor properties: pyrazolylpyrazolino[60]fullerenes.

A series of triad pyrazolylpyrazolino[60]fullerenes has been prepared in one pot from suitably functionalized hydrazones by 1,3-dipolar cycloaddition reactions under microwave irradiation. The electrochemical properties of the compounds obtained were investigated by cyclic voltammmetry, and they show better electron acceptor character than the parent C(60) in all cases. Fluorescence experiments and time-resolved transition spectroscopy indicate the existence of photoinduced charge-transfer processes with the C(60) triplet acting as the acceptor.     

Charge transfer collisions between fullerenes: C 60 3+ + C60

Charge transfer collisions between C ₆₀ ³⁺ and C₆₀ are studied for collision energies between 400 and 3600 eV. Single and double electron transfers are observed, both occuring under single collision conditions. Absolute charge transfer cross sections are determined as a function of collision energy. The cross section for single electron capture of approx. 300 Ų is about two times larger than that for double electron transfer. For both processes the cross section increases slightly with increasing collision energy.     

Theoretical study of the stability of multiply charged C70 fullerenes

We have calculated the electronic energies and optimum geometries of C(70) (q+) and C(68) (q+) fullerenes (q=0-14) by means of density functional theory. The ionization energies for C(70) and C(68) fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C(60) and C(58) [S. Diaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C(70) (q+)-->C(68) (q+)+C(2), C(70) (q+)-->C(68) ((q-1)+)+C(2) (+), C(70) (q+)-->C(68) ((q-2)+)+C(+)+C(+), C(70) (q+)-->C(68) ((q-3)+)+C(2+)+C(+), and C(70) (q+)-->C(68) ((q-4)+)+C(2+)+C(2+) decay channels show that C(70) (q+) (like C(60) (q+)) is thermodynamically unstable for q>or=6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C(60) (q+) fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C(70) (q+).     

Ionization and excitation of C60, C70, and C80 fullerenes

Good agreement of E_SCF results for the ionization potentials with the corresponding one-electron levels in C₆₀, C₇₀, and C₈₀ fullerenes, as well as with generalizations of the Koopmans theorem to cases considering various one-electron transitions in ions, was observed. Both are in good agreement with the available experimental data. An explanation is given both for the agreement and for the existing deviations, according to which the dispersions of the results for the ionization potentials obtained in a number of studies of the Koopmans theorem should be ascribed to differences in the parametrization and methods of construction of the semiempirical Fockian for acceptable methods of calculation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 6, pp. 501–513, November–December, 1993.     

Structure and stability of the defect fullerene clusters of C60: C59, C58, and C57

We have performed density functional calculations for the structures and stabilities of various isomers of the defect fullerene clusters of C(60): C(59), C(58), and C(57). The C(59_)5-8, C(58_)5-5-7, and C(57_)4-5-9 clusters were calculated to be the most stable isomers of the C(59), C(58), and C(57) clusters, respectively. There are obvious relationships between structure and stability of the defect fullerene clusters. First, an unsaturated carbon atom favors being located at a 6-membered ring rather than a 5-membered ring. Second, the most stable isomers prefer to have newly formed 5-membered rings, rather than newly formed 4-membered rings.     

Structure and electronic properties of TI8C12 cluster

Structure and electronic properties of recently discovered Ti₈C₁₂ molecule are studied using the discrete variational method within the framework of local density approximation. The geometry of this novel cluster is optimized to be a distorted dodecahedron by varing the bond lengths of Ti-C and C-C while maintaining theT _h symmetry, and the electronic states and the chemical bonding are discussed.     

Reactions of excited fullerenes C60 and C70 studied by mass spectrometry

The transformation of the mass spectra of the laser-desorbed C₆₀ and C₇₀ samples with a successive increase in the laser power, resulting in an increase in the degree of excitation of C₆₀ (C₇₀) and in the number of the particles in the laser plume, was studied. Unusual metastable clusters (C₆₀ + C₂) and (C₇₀ + C₂) are formed even at a minimum laser power and begin to dissociate after 0.5 s following a short (3 ns) laser pulse. An increase in the laser power results in the appearance of peaks of metastable clusters C₆₂ (C₇₂) with the statistically normal lifetime without a delay of dissociation. A further increase in the laser power produces metastable clusters C_60k–2n and C_70k–2n (k = 2, 3) formed without a lag from the dimers and trimers of C₆₀ (C₇₀) by the ejection of a number of C₂ required for the stabilization of the C₂ molecules. The peak of C₇₀ appears simultaneously with the appearance of the (C₆₀)_2–2n peaks upon the laser desorption of pure C₆₀. These findings provide evidence for the growth of the excited fullerene clusters by coalescence and subsequent stabilization due to the ejection of a small fragment rather than by the implantation of C₂ into the fullerene framework. This mechanism of cluster growth should be taken into consideration in modeling fullerene formation in an electric arc reactor, because the clusters formed under these conditions have a substantial excess internal energy.     

Non-aqueous capillary electrophoresis separation of fullerenes and C60 fullerene derivatives

As the interest in the use of fullerene compounds in biomedical and cosmetic applications increases, so too does the need to develop methods for their determination and quantitation in such complex matrices. In this work, we studied the behavior of C60 and C70 fullerenes in non-aqueous capillary electrophoresis, as well as two C60 fullerene derivatives not previously reported by any electrophoretic method, N-methyl-fulleropyrrolidine and (1,2-methanofullerene C60)-61-carboxylic acid. The separation was performed using fused-silica capillaries with an I.D. of 50?μm and tetraalkylammonium salts, namely tetra-n-decylammonium bromide (200?mM) and tetraethylammonium bromide (40?mM), in a solvent mixture containing 6?% methanol and 10?% acetic acid in acetonitrile/chlorobenzene (1:1?v/v) as the background electrolyte. Detection limits, based on a signal-to-noise ratio of 3:1, were calculated, and values between 1 and 3.7?mg/L were obtained. Good run-to-run and day-to-day precisions on concentration were achieved with relative standard deviation lower than 15?%. For the first time, an electrophoretic technique has been applied for the analysis of C60 fullerene in a commercial cosmetic cream. A standard addition method was used for quantitation, and the result was compared with that obtained by analyzing the same cream by liquid chromatography coupled to mass spectrometry.     

DFT study on electronic properties of single- and double-shell icosahedral fullerenes

Multi-shell fullerenes are widely studied for their interesting properties although comparative studies on single- and multi-shell structures remain scarce. In this work, important electronic features of single- and double-shell icosahedral fullerenes as a function of their sizes were calculated in the framework of the density functional theory. Fully optimized structures were used to get the gap between the highest occupied molecular and the lowest unoccupied molecular orbital (H-L gap), electronegativity, softness and density of the electronic states. This work shows that the H-L gap of the single-shell fullerenes decreases nonlinearly as the nanoparticles size increases, whereas for the double-shell fullerenes an opposite trend is obtained. A decrease of the H-L gap is found going from single- to double-shell fullerenes with similar external sizes, up to a diameter of 3.13 nm. The electron density of states revealed that isolated peaks give way to more dense electronic states for nanoparticles with diameters above 2 nm.