Synthesis and Single-Crystal X-ray Structure of a Highly Symmetrical C60 Derivative, C60Br24

C(60) and liquid bromine react to form C(60)Br(24), a crystalline compound isolated as a bromine solvate, C(60)Br(24)(Br(2))(x), The x-ray crystal structure defines a new pattern of addition to the carbon skeleton that imparts a rare high symmetry. The parent C(60) framework is recognizable in C(60)Br(24), but sp(3) carbons at sites of bromination distort the surface, affecting conformations of all of the hexagonal and pentagonal rings. Twenty-four bromine atoms envelop the carbon core, shielding the 18 remaining double bonds from addition. At 150 degrees to 200 degrees C there is effectively quantitative reversion of C(60)Br(24) to C(60) and Br(2).     

Radical reactions of c60

Photochemically generated benzyl radicals react with C(60) producing radical and nonradical adducts Rn C(60) (R = C(6)H(5)CH(2)) with n = 1 to at least 15. The radical adducts with n = 3 and 5 are stable above 50 degrees C and have been identified by electron spin resonance (ESR) spectroscopy as the allylic R(3)C(60)(.) (3) and cyclopentadienyl R(5)C(60)(.) (5) radicals. The unpaired electrons are highly localized on the C(60) surface. The extraordinary stability of these radicals can be attributed to the steric protection of the surface radical sites by the surrounding benzyl substituents. Photochemically generated methyl radicals also add readily to C(60). Mass spectrometric analyses show the formation of (CH(3))nC(60) with n = 1 to at least 34.     

13C NMR Spectroscopy of KxC60: Phase Separation, Molecular Dynamics, and Metallic Properties

The results of (13)C nuclear magnetic resonance (NMR) measurements on alkali fullerides KxC(60) are reported. The NMR spectra demonstrate that material with 0 < x < 3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C(3-)(60) ions rotate rapidly in the K(3)C(60) phase at 300 K, while C(6)-(60) ions in the insulating K(6)C(60) phase are static on the time scale of the lineshape measurement. The temperature dependence of the (13)C spin-lattice relaxation rate in the normal state of K(3)C(60) is found to be characteristic of a metal, indicating the important role of the C(3-)(60) ions in the conductivity. From the relaxation measurements, an estimate of the density of electronic states at the Fermi level is derived.     

Orientational Disorder of C60 in Li2CsC60

The x-ray diffraction of the nonsuperconducting ternary fulleride Li(2)CsC(60) reveals at room temperature a face-centered-cubic (Fm3m) disordered structure that persists to a temperature of 13 Kelvin. The crystal structure is best modeled as containing quasispherical [radius of 3.556(4) angstroms] C(60)(3-) ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K(3)C(60) (merohedral disorder) and primitive cubic Na(2)CsC(60) (orientational order). The orientational disorder of the carbon atoms on the C(60)(3-) sphere was analyzed with symmetry-adapted spherical-harmonic functions. Excess atomic density is evident in the <111> directions, indicating strong bonding Li(+)-C interactions, not encountered before in any of the superconducting alkali fullerides. The intercalate-carbon interactions and the orientational state of the fullerenes have evidently affected the superconducting pair-binding mechanism in this material.     

Taming superacids: stabilization of the fullerene cations HC60+ and C60.+

A new superacid, H(CB11H6X6) (where X = chlorine or bromine), whose conjugate base is the exceptionally inert CB11H6X6- carborane anion, separates Bronsted acidity from oxidizing capacity and anion nucleophilicity in a manner not previously achieved. Reaction of this superacid with C60 gives HC60+ as a stable ion in solution and in the solid state. In a separate experiment, an oxidant was developed such that the long-sought C60.+ ion can be synthesized in solution. The preparation of these two fullerene carbocations is a notable departure from the prevalent chemistry of C60, which is dominated by the formation of anions or the addition of nucleophiles. The H(CB11H6X6) superacid overcomes the major limitations of presently known superacids and has potentially wide application.     

C_(60)-D-氨基葡萄糖衍生物的制备及其抗氧化性的研究

在非均相碱性体系中,合成了C60-D-氨基葡萄糖衍生物,用FT-IR和1HNMR对产物的结构进行了表征。考察了C60-D-氨基葡萄糖衍生物的超氧阴离子清除能力、羟基自由基清除能力、还原能力以及金属螯合能力。结果表明:C60-氨基葡萄糖衍生物对超氧阴离子自由基O2-.和羟基自由基.OH的半抑制浓度分别是0.42mg/mL和0.26mg/mL;在还原能力体系中当吸光度达到0.5时,所需C60-D-氨基葡萄糖衍生物的浓度是2.44mg/mL;在金属螯合能力体系中,C60-D-氨基葡萄糖衍生物对亚铁离子的半螯合浓度是0.013mg/mL;即表明C60-D-氨基葡萄糖衍生物在4个抗氧化体系中都表现出明显的抗氧化性。并且D-氨基葡萄糖盐酸盐在4个体系中表现出的抗氧化能力都不强,说明C60-D-氨基葡萄糖衍生物的抗氧化能力可能主要来自于C60上保留下来的双键。     

C_(60)-D-氨基葡萄糖衍生物的制备及其抗氧化性的研究

在非均相碱性体系中,合成了C60-D-氨基葡萄糖衍生物,用FT-IR和1HNMR对产物的结构进行了表征。考察了C60-D-氨基葡萄糖衍生物的超氧阴离子清除能力、羟基自由基清除能力、还原能力以及金属螯合能力。结果表明:C60-氨基葡萄糖衍生物对超氧阴离子自由基O2-.和羟基自由基.OH的半抑制浓度分别是0.42mg/mL和0.26mg/mL;在还原能力体系中当吸光度达到0.5时,所需C60-D-氨基葡萄糖衍生物的浓度是2.44mg/mL;在金属螯合能力体系中,C60-D-氨基葡萄糖衍生物对亚铁离子的半螯合浓度是0.013mg/mL;即表明C60-D-氨基葡萄糖衍生物在4个抗氧化体系中都表现出明显的抗氧化性。并且D-氨基葡萄糖盐酸盐在4个体系中表现出的抗氧化能力都不强,说明C60-D-氨基葡萄糖衍生物的抗氧化能力可能主要来自于C60上保留下来的双键。     

Electrical Resistivity and Stoichiometry of CaxC60 and SrxC60 Films

The temperature- and concentration-dependent resistivities of annealed CaxC(60) and SrxC(60) films were measured near room temperature. Resistivity minima were observed at x = 2 and 5. The resistivities of these films were rho(min) approximately 1 ohm-centimeter for x = 2 and rho(min) approximately 10(-2) ohm-centimeter for x = 5. This latter value is comparable to the resistivities found in similar experiments on K(3)C(60) films. There is a maximum in the resistivity between x = 2 and 3, and another at x approximately 7. The conductivity is activated over the whole range of compositions, and the activation energy scales with the logarithm of the resistivity. The results suggest that the conductivity and superconductivity observed in Ca(5)C(60) are associated with the population of bands derived from the t(1g) level of C(6O).     

C60 rotation in the solid state: dynamics of a faceted spherical top

The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.     

Reversible, metal-free hydrogen activation

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100 degrees C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25 degrees C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.     

不同加成数C_(60)-β-丙氨酸衍生物的还原能力及金属螯合能力

为研究水溶性富勒烯衍生物抗氧化性能,通过控制反应配比,合成三种具有不同加成数目的水溶性C60-β-丙氨酸衍生物,C6(0NHCH2CH2COONa)nHn。采用红外、紫外、核磁共振、元素分析等手段对其进行结构表征。元素分析表明,三种不同加成数目的水溶性C60-β-丙氨酸衍生物A、B、C的加成数分别为2、4、8。同时考察三种衍生物的还原能力及金属螯合能力。结果表明,在还原能力体系中,当三种衍生物A、B、C浓度为2.5μmol.mL-1时,吸光度分别为0.424、0.612和1.040;在金属螯合能力体系中,三种衍生物A、B、C的半螯合浓度(当螯合率达到50%时,所需螯合物的浓度)分别为0.348、0.261和0.135μmol.mL-1。随着加成数目的增多,C60-β-丙氨酸衍生物的还原能力及金属螯合能力增强。     

甲酸诺卜酯和诺卜甲基醚的~(13)C化学位移分析

由β-蒎烯合成诺卜醇,再用诺卜醇与甲酸合成甲酸诺卜酯,GC纯度为96.9%产物的产率为88.3%;用相转移催化法由诺卜醇与碘甲烷合成诺卜甲基醚,GC纯度为93.1%产物的产率为89.8%,诺卜醇的这2个衍生物都具有较好的香气特性,它们的结构均用IR,MS,1H NMR及13C NMR分析进行了表征.对2个衍生物13C化学位移的分析表明:它们的C1--C2--C3--C4--C5-部分处在同一个平面上,整个6,6-二甲基双环[3.1.1]庚-2-烯部分呈"Y"形,它们可被看作阿朴蒎烯在C2-发生取代的衍生物,取代基对13C化学位移的影响主要集中在C2-,C1-和C3-,对其它碳原子的影响比较小.     

Tunneling Spectroscopy of M3C60 Superconductors: The Energy Gap, Strong Coupling, and Superconductivity

Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (triangle up) for K(3)C(60) and Rb(3)C(60). At low temperature the reduced energy gap, 2triangle upkappaT(c) (where T(c) is the transition temperature) has a value of 5.3 +/- 0.2 and 5.2 +/- 0.3 for K(3)C(60) and Rb(3)C(60), respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by Bardeen-Cooper-Schrieffer theory. Hence, these results show that the pair-coupling interaction is strong in the M(3)C(60) superconductors. In addition, measurements of triangle up(T) for both K(3)C(60) and Rb(3)C(60) exhibit a similar mean-field temperature dependence. The characterization of triangle up and triangle up(T) for K(3)C(60) and Rb(3)C(60) provides essential constraints for theories evolving to describe superconductivity in the M(3)C(60) materials.     

New phases of c60 synthesized at high pressure

The fullerene C(60) can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C(60) on reheating to 300 degrees C at ambient pressure. For synthesis temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degrees to 800 degrees C at the same pressure, C(60) transforms into a rhombohedral structure with hexagonal lattice parameters of a(o) = 9.22 angstroms and c(o) = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C(60) molecules are linked.     

Isolation of two seven-membered ring C58 fullerene derivatives: C58F17CF3 and C58F18

Fluorination of C60 at 550 degrees C leads to milligram quantities of two stable fullerene derivatives with 58-carbon cage structures: C58F18 and C58F17CF3. The compounds were characterized by mass spectrometry and fluorine nuclear magnetic resonance spectroscopy, and the data support a heptagonal ring in the framework. The resulting strain, which has hindered past attempts to prepare these smaller quasi-fullerenes, is mitigated here by hybridization change of some of the carbons in the pentagons from sp2 to sp3 because of fluorine addition. The loss of carbon from C60 is believed to occur via sequential fluorine addition to a CC single bond and an adjacent CC bond, followed by loss of a:CF2 carbene.     

Charge Donation by Calcium into the t1g Band of C60

Photoemission spectra of compounds prepared by the reaction of C(60) films with calcium show two distinct metallic phases, whereas alkali-doped C(60) films have only one. In the first phase the bulk t(1u) band, derived from the lowest unoccupied molecular orbital of C(60), is partially occupied. This is followed by an insulating phase that has the composition Ca(3)C(60) in which the t(1u) band is filled and has properties analogous to those of K(6)C(60). Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t(1g) band. The superconductivity of Ca(5)C(60) is associated with the t(1g) band.     

Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride

The crystal structure of superconducting Na(2)CsC(60) was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C(60) molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.     

A rational chemical synthesis of C60

Isolable quantities of C60, the smallest stable fullerene, have been synthesized in 12 steps from commercially available starting materials by rational chemical methods. A molecular polycyclic aromatic precursor bearing chlorine substituents at key positions forms C60 when subjected to flash vacuum pyrolysis at 1100 degrees C. No other fullerenes are formed as by-products. The methods we have developed for the target-specific synthesis of fullerenes, applied here to a synthesis of C60, should make possible the directed laboratory preparation of other fullerenes as well, including those not accessible by graphite vaporization.     

Buckminsterfullerane: the inside story

The stereoisomers of fully reduced buckminsterfullerene C(60)H(60) have been investigated with the molecular mechanics program MM3. Although it might be expected that chemical reduction would deliver all of the hydrogens to the outside, the symmetric structure produced in this way is predicted to be highly strained. Moving just one hydrogen to the inside is predicted to decrease the energy by 53 kilocalories (kcal) per mole. Putting additional hydrogens on the inside further lowers the predicted energy (depending on the steric relations between them). The miniimum energy isomer is predicted to have ten hydrogens inside with C(1) symmetry and to have an energy 400 kcal/mol less than that of the all-outside isomer. These results suggest that a process which could achieve isomerization would produce a mixture of isomers, most of which with ten hydrogens on the inside.     

Superconductivity at 45 k in rb/tl codoped c60 and c60/c70 mixtures

The appearance of superconductivity at relatively high temperatures in alkali metal-doped C(60) fullerene provides the challenge to both understand the nature and origin of the superconductivity and to determine the upper limit of the superconducting transition temperature (T(c)). Towards the latter goal, it is shown that doping with potassium-thallium and rubidium-thallium alloys in the 400 to 430 degrees C temperature range increases the T(c) of C(60)/C(70) mixtures to 25.6 K and above 45 K, respectively. Similar increases in T(c) were also observed upon analogous doping of pure C(60). Partial substitution of potassium with thallium in interstitial sites between C(60) molecules is suggested by larger observed unit cell parameters than for the K(3)C(60) and K(4)C(60) phases. Contrary to previous results for C(60) doped with different alkali metals, such expansion does not alone account for the changes in critical temperature.