Potential energy surface and orientational disorder in solid fullerene C60

The polymorphism and molecular disorder in crystalline C₆₀ have been studied by modelling the optimum packing of fullerene molecules by the atom-atomic potential method. The study includes the calculation of minima and saddle points of the potential energy surface with sorting out of the most common space symmetry groups. Two models of intermolecular potential for C₆₀ have been checked, one of which assumes effective charges at the centers of C-C bonds. It has been found that the calculated barrier of reorientations is much lower in the case where the concerted character of rotations of different molecules is taken into account. The model of orientational disorder in the face-centered cubic phase is suggested, which is based on consideration of symmetrically arranged equivalent minima separated by low potential barriers.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1466–1469, August, 1995.The work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08895).     

The rotation and the phase transition in solid C60

The interaction between C60's in solid C60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C60 's were discussed. In order to describe the disordered degree of C60 rotation, an equivalent M is introduced. The phase transitions at the ~260 K and at the ~90 K are studied from the viewpoint of C60 rotation. The potential barriers of the ordered rotation below the ~260 K and the disordered rotation above the ~260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.     

Hyperpolarizability of the C60 fullerene cluster

Motivated by a discrepancy of five orders of magnitude between three different hyperpolarizability measurements on the C₆₀ fullerene, we calculated the optical response of this cluster using a tight-binding Hamiltonian and compared the results to those for a benzene molecule. Our Hamiltonian reproduces the linear polarizability and hyperpolarizability of benzene reasonably well. For C₆₀, our calculations of the bare polarizability agree only with two of the optical response measurements and indicate that the corresponding linear and nonlinear response of C₆₀ is much larger than that of C₆H₆. We find that screening effects decrease this difference strongly, and also reduce the calculated hyperpolarizability of C₆₀ to a value which is two orders of magnitude below the favored measurements.     

Electrochemical polymerization of fullerene C60

Cyclic voltammetry data show that fullerene C₆₀ films on the Pt electrode surface in MeCN are electropolymerized to form polymeric C₆₀. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 863–865, April, 1997.     

The use of solid phase synthesis for the preparation of monoadducts of fullerene C60

The method of solid phase synthesis was proposed for the preparation of monoadducts of fullerene C₆₀ using 3′H-cyclopropa[1,9](C₆₀-I _h )[5,6]fullerene-3′-carboxylic acid as an example.     

Characteristics of fullerene C60-doped polymers

The results of studying the characteristics of various polymers that possess different structures and are used for various applications after their modification by minor amounts of fullerene C₆₀ are summarized. The character of bonds that arise in fullerene-doped polymer compositions is considered. Mechanical, physicochemical, and physical characteristics of fullerene-containing polymers that show promise for modern technologies are described. The reasons for improving the performance of fullerene-containing polymers are analyzed.     

Thermochemistry of C60 and C70 fullerene solvates

In an effort to improve understanding of dissolution behaviour of fullerenes and their simple chemical derivatives the binary systems of C₆₀, C₇₀ and the piperazine monoadduct of [60] fullerene C₆₀ N₂C₄H₈ with a series of aromatic solvents have been studied by means of DSC. In certain systems solid solvates have been found to be the thermodynamically stable phases relative to saturated solution at room temperature. Identified solid solvates were characterized by their compositions, temperatures and enthalpies of incongruent melting transitions. The regularities in thermodynamic stability of the solvated crystals have been discussed along with dissolution properties of fullerenes and the derivative. Certain correlations have been observed.     

Adsorption of ascorbic acid on the C60 fullerene

Adsorption of ascorbic acid (AsA) on C60 is investigated using classical molecular mechanics and density functional theory (DFT). Classical annealing was performed to explore the space of molecular configurations of ascorbic acid adsorbed on C60, searching for optimal geometries. From the structure with the smallest total energy, 10 initial configurations were prepared by applying rotations of 90 degrees about three orthogonal axes. Each one of these configurations was optimized using DFT (for both LDA and GGA exchange-correlation functionals), and an estimate of their total and adsorption energies was found. Different configurations have minimal adsorption energies (defined here as the total energy of the adsorbate minus the total energy of the separate molecules) from -0.54 to -0.10 eV, with distinct optimal distances between the AsA and C60 centers of mass. According to a Hirshfeld population analysis, AsA is, in general, an acceptor of electrons from C60. Our results demonstrate the feasibility of noncovalent functionalization of C60 with AsA and provide minimal energy values for the several different configurations investigated. These results should be considered in reactions as a possible way to prevent against the oxidative damage and toxicity of C60. The beneficial effects of using AsA-C60 includes its action when administered together with levodopa, against the neurotoxicity generated by levodopa isolated, which opens new strategies for the Parkinson's disease treatment.     

Solvent Effects on the Solubility of C60 fullerene

The solubility of C₆₀ fullerene in organic liquids is not directly related to any of their physicohemical characteristics, but can be quantitatively described by linear multiparameter equations. The solubility increases with increasing solvent polarizability and decreases with increasing solvent polarity and cohesive energy density. Various types of multiparameter equations proposed in the literature were compared to show that they all are suitable but Koppel-Palm's equation is preferred.     

Polyadducts of fullerene C60 with tert-butyl groups

New soluble fullerene derivatives Bu^t _n C₆₀ were synthesized by the reaction of C₆₀M _n (THF) _x (M = Li, Na; n = 4, 6, 8, 10, 12) with tert-butyl bromide ButBr. The thus obtained compounds were characterized by spectroscopic methods (IR and NMR spectroscopy), mass spectrometry (MALDI), elemental analysis, and quantum chemical calculations. HPLC was used for chromatographic separation of the products of the reaction of C₆₀Na _n (THF) _x (n = 4, 6, 8, 12) with Bu^tBr. It was found that Bu^t ₄C₆₀ and Bu_t ⁶C₆₀ decompose on the column. A complex of physicochemical methods of analysis was used for establishing compositions of individual fractions of chromatographic separation of Bu^t ₄C₆₀. The structure of one of the components was determined as 1-tert-butyl-1,2-dihydrofullerene 1,2-Bu^tC₆₀H.     

Star-shaped polymers with the fullerene C60 branching center

The anionic methods for the synthesis of homo- and heteroarm (hybrid) star-shaped polymers using fullerene C₆₀ aPre considered. The possibilities of fullerene C₆₀ as an agent of combination of living polymer chains and the procedures of transformation of polymer derivatives of C₆₀ (hexaadducts) into polyfunctional macroinitiators of anionic polymerization of vinyl monomers are shown. The methods for functionalization of polymer fullerene derivatives and their combinations into structures of complex controlled architecture are presented. The structural features and initiating properties of the living polymer fullerene derivatives and their role in the formation of heteroarm star-shaped macromolecules with the controlled number of branches and predetermined molecular weight characteristics of the arms are discussed. The hydrodynamic properties of the star-shaped fullerene-containing polymers are considered. The data on the small-angle neutron scattering study of self-organization of the stars in solutions are presented.     

Catalytic cycloaddition of diazoalkanes to fullerene C60

Cycloaddition to fullerene C₆₀ of monosubstituted diazomethanes generated in situ by oxidation of aldehyde hydrazones in the presence of Pd(acac)₂-2 PPh₃-4 Et₃Al as catalytic system resulted in selective formation of homofullerenes in which the alkyl substituent is located above the plane of the five-membered ring in C₆₀. Under analogous conditions, unsymmetrical disubstituted diazomethanes generated from the corresponding ketone hydrazones gave rise to mixtures of stereoisomeric 5,6-open adducts.     

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.     

Phase equilibria in the system fullerene C60-hexane-o-xylene-dimethylformamide

Phase diagrams in ternary liquid systems hexane o-xylene DMF were studied at 298.15 333.15 K. Critical points in these systems were determined. The fullerene C₆₀ distribution in the ternary liquid systems with constant and variable compositions of two organic phases was studied at variable fullerene concentration.     

Endohedral chemistry of C60-based fullerene cages

Fullerenes have unique chemistry owing to their cage structure, their richness in pi-electrons, and their large polarizabilities. They can trap atoms and small molecules to generate endohedral complexes as superconductors, drug carriers, molecular reactors, and ferroelectric materials. An important goal is to develop effective methods that can affect the behavior of the atoms and small molecules trapped inside the cage. In this paper, the quantum chemical density functional theory was employed to demonstrate that the stability and position of a guest molecule inside the C60 cage can be changed, and its orientation controlled, by modifying the C60 cage shell. The outside attachment of two hydrogen atoms to two adjacent carbon atoms located between two six-membered rings of the C60 cage affects the orientation of the LiF molecule inside and increases the stability of LiF inside the cage by 45%. In contrast, when 60 hydrogen atoms were attached to the outside surface of the C60 cage, thus transforming all C=C double bonds into single bonds, the stability of the LiF inside was reduced by 34%. If two adjacent carbon atoms were removed from C60, the stability of LiF inside this defect C60 was reduced by 41%.     

Resonance hyper-Raman scattering of fullerene C60 microcrystals

The hyper-Raman spectrum of buckminsterfullerene C60 was observed at room temperature. The spectrum clearly showed infrared-active modes with t(1u) symmetry and the silent modes with t(2u), g(u), and h(u) as a result of electronic resonances via the Herzberg-Teller mechanism. Moreover, Raman-active modes with a(g) and h(g) were also detected, although these were hyper-Raman-forbidden in the electric dipole approximation, suggesting a contribution of higher-order nonlinear processes such as magnetic dipole transitions. These results suggest that hyper-Raman spectroscopy and microscopy are indeed widely applicable in the field of molecular science.     

Friction and adhesion between C60 single crystal surfaces and AFM tips: effects of the orientational phase transition

We have investigated the nanotribological properties of C60 single crystal (111) and (100) surfaces around its orientational order-disorder phase transition temperature, approximately 260 K, by atomic force microscopy and frictional force microscopy (AFM/FFM) in high vacuum. Results show that for both surfaces across the phase transition temperature, the friction force and the adhesive force between a C60 coated AFM tip and the C60 crystal surfaces exhibit discontinuous behavior. The friction force within the applied external load range in the low temperature phase is significantly larger than that in the high temperature phase, with no obvious change in the slope of the friction force curves (the friction coefficient) in the low and high temperature phases. The abrupt change in friction was found to be caused mainly by the abrupt change in adhesion, which, in turn, can be qualitatively understood through changes in the van der Waals interaction and the short-range Coulomb interaction associated with the structural changes across the phase transition. Compared to most other degrees of freedom, the rotation of C60 molecules was found to have little effect on friction and is an ineffective energy dissipation channel.     

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.     

Kinetics of oxidation of fullerene C60 with dimethyldioxirane

The kinetics of the reaction of dimethyldioxirane with fullerene C₆₀ was studied, and the activation parameters logk = (8.3±0.8) – (14.2±0.9)/, ( = 2.3RT kcal mol^–1) (20—45°C) were determined. The formation of paramagnetic particles was detected.