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.     

Band structure studies on crystalline C60, Ca3C60, and Ca5C60

The three-dimensional EHMO crystal orbital calculations for crystalline C₆₀, Ca₃C₆₀, and Ca₅C₆₀ are reported. The ground states of both undoped solid C₆₀ and partially doped Ca₃C₆₀ are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca₅C₆₀ forms a metallic conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca₃C₆₀ and Ca₅C₆₀ is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C₆₀ molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.     

Contrasting photodynamics between C60-dithiapyrene and C60-pyrene dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.     

Radioiodination of C60 Derivative C60(OH)xOy

In this paper water-soluble fullerene derivative C₆₀(OH)_xO_y was radioiodinated with the iodogen method. The labeling yield was determined by radio-TLC. The effects of pH value, reaction time, temperature and amount of the iodogen on the labeling yield were studied. The labeled product was purified by Sephadex G-25 column chromatography and then the stability of ¹²⁵I-C₆₀(OH)_xO_y was examined . The results showed that the radiochemical purity of ¹²⁵I-C₆₀(OH)_xO_y solution with benzylalcohol remained 82.7% after 43 hours.     

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.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.     

Theoretical studies of C60/C70 fullerene derivatives: C60O and C70O

A semiempirical (AM1) calculation on the structures and stabilities of isomers of the fullerene derivatives C₆₀O and C₇₀O is carried out. The ozonolysis reaction mechanism and the thermodynamics of the compounds are studied. The two isomers of C₆₀O (56 bond and 66 bond) formed by an oxygen atom bridging across a C-C bond have an epoxide-like or an annulene-like structure. According to the ozonolysis reaction mechanism and kinetic factor analysis, the possible products of this ozonolysis reaction are C₆₀O with oxygen bridging over the 66 bond (C_2v) as an epoxide-like isomer and that with oxygen bridging over the 56 bond (C_s) as an annulene-like isomer. Further, the sixteen isomers of C₇₀O (both epoxide-like and annulene-like structures) have been studied with respect to the same reaction mechanism. The most possible product in this ozonolysis reaction contains oxygen bridging across in the upper part (66 bond in C₇₀O-2 or C₇₀O-4) as an epoxide-like structure. The other possible product is C₇₀O-8 (annulene-like structure), in which oxygen bridges across an broken equatorial CC bond in C₇₀ (D_5h). The vibrational frequency analysis and the electronic structure of the selected C₆₀O and C₇₀O isomers are generated for experimental characterisation. The experimental results indicate that C₆₀O and C₇₀O may decompose into the odd number fullerenes C₅₉ and C₆₉. We therefore studied the structures of C₅₉ and C₆₉ also.     

Incorporating C2 into C60 films

The material formed by depositing C(2)(-) anions onto/into thin C(60) films (on graphite) at room temperature has been studied by means of thermal desorption mass spectroscopy, ultraviolet photoionization spectroscopy, atomic force microscopy (AFM), and surface enhanced Raman spectroscopy. As-prepared, C(2)/C(60) films manifest thermal desorption behaviour which differs significantly from pure C(60) films. Whereas the latter can be fully sublimed, we observe decomposition of C(2)/C(60) films to a high-temperature-stable material while predominantly C(60), C(62), and C(64) are desorbed in parallel. Deposition of C(2)(-) also leads to significantly modified electronic and vibrational properties. Based on DFT model calculations of the Raman spectra, we suggest that as-prepared C(2)/C(60) films contain appreciable amounts of polymeric networks comprising -C(2)-C(60)-C(2)-C(60)- chains. Detection of sublimed C(62) and C(64) upon heating implies that thermal decomposition of C(2)/C(60) films involves addition/uptake of C(2) units into individual fullerene cages. Correspondingly, annealing films up to various intermediate temperatures results in significant modifications to valence-band UP spectra as well as to surface topographies as imaged by AFM. The novel carbonaceous material obtained by heating to T > 950 K has a finite density of states at the Fermi level in contrast to as-prepared C(2)/C(60). It comprises fused fullerene cages.     

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.     

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.     

Rotation dynamics of C60 molecules in organic superconductor K3C60

The rotation dynamics of C₆₀ molecules in organic superconductor K₃C₆₀ has been investigated from the viewpoint of intramolecular interaction. It is determined that the rotation of C₆₀ at mom temperature has been frozen up within a small region of rotation angle (0°–50°), and pointed out that the reason for the freeze is the physical interaction rather than the geometrical hindrance. The computations of the interactions for alkali-doped compounds A_3-x A′ _x C₆₀ (x = 1, 2, 3; A, A′ = K, Rb, Cs) other than K₃C₆₀ have also been camed out. Fmm the obtained results, it is seen that the superconducting transition temperatures T, are strongly connected with the interactions in them, and this observation is consistent with the discovery of the correlation between T_c, and lattice constants a. Project supported by the National Natural Science Foundation of China.     

Synthesis of C60H2 by rhodium-catalyzed hydrogenation of C60.

Reduction of C60 with rhodium(0) on alumina and hydrogen in deuterated benzene (C6D6) at ambient temperature and pressure yields a mixture of hydrogenated compounds; C60H2 has been characterized as the major product in 14% yield based on 1H NMR.     

Energy transfer in oligofluorene-C60 and C60-oligofluorene-C60 donor-acceptor conjugates

Two complementary series of C(60)-(Fl)(n) and C(60)-(Fl)(n)-C60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C(60)-(Fl)(n)-C(60) series, each reduction wave is a two-electron process with no observable interaction between the C(60) units. Two or, in some cases, three oxidation waves--most of them irreversible--are ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.     

The former "C60F16" is actually a double-caged adduct: (C60F16)(C60)

X-ray diffraction study of the substance originally believed to be C(60)F(16) reveals a double-caged structure, (C(60)F(16))(C(60)); MALDI mass spectra, 19F NMR spectral data and reasons for stability are discussed.     

Tube and cage C(60)H(60): a comparison with C(60)F(60)

Tube C(60)H(60) (5) with fused five-membered rings is more stable than the cage isomer (1) with isolated five-membered rings. Introduction of endo C-H bonds into structure 5 results in further stabilization, but the most stable tube structure with four endo C-H bonds (7) is higher in energy than the most stable cage structure with ten endo C-H bonds (3) by 74.2 kcal/mol. A comprehensive comparison of C(60)H(60) with C(60)F(60) has been made.     

Hydrosilylation of fullerence C60

The hydrosilylation of C₆₀ by diphenylsilane was studied. Silicon-containing derivatives C₆₀H _n (HSiPh₂) _n (n=2, 4, 6) were obtained. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1697–1699, September, 1997.