Raman scattering study of C120, a C60 dimer

Mapping intermolecular bonding in C₆₀

The formation of intermolecular bonds in C₆₀ has been investigated in detail at pressures below 2.2 GPa and up to 750 K. Fullerene samples were heated in a temperature gradient to obtain data on the formation of dimers and low-dimensional polymers along isobars. Intermolecular bonding was analyzed ex situ by Raman scattering, using both intramolecular modes and intermolecular stretching modes. Semi-quantitative reaction maps are given for the formation of dimers and chains. The activation energy for dimer formation decreases by 0.2 meV pm(-1) when intermolecular distances decrease and dimer formation is noticeably affected by the rotational state of molecules. Above 400-450 K larger oligomers are formed; below 1.4 GPa most of these are disordered, with small domains of linear chains, but above this the appearance of stretching modes indicates the existence of ordered one-dimensional polymers. At the highest pressures and temperatures two-dimensional polymers are also observed.

Mechanochemistry of fullerenes and related materials

The low or lack of solubility of fullerenes, carbon nanotubes and graphene/graphite in organic solvents and water severely hampers the study of their chemical functionalizations and practical applications. Covalent and noncovalent functionalizations of fullerenes and related materials via mechanochemistry seem appealing to tackle these problems. In this review article, we provide a comprehensive coverage on the mechanochemical reactions of fullerenes, carbon nanotubes and graphite, including dimerizations and trimerizations, nucleophilic additions, 1,3-dipolar cycloadditions, Diels-Alder reactions, [2 + 1] cycloadditions of carbenes and nitrenes, radical additions, oxidations, etc. It is intriguing to find that some reactions of fullerenes can only proceed under solvent-free conditions or undergo different reaction pathways from those of the liquid-phase counterparts to generate completely different products. We also present the application of the mechanical milling technique to complex formation, nanocomposite formation and enhanced hydrogen storage of carbon-related materials.

PERIODIC FULLEROIDS

Construction of periodic fulleroids (including covering polygons other than the classical pentagons and hexagons) is achieved by several coupling procedures. A constitutive typing enumeration for topologically periodic fulleroids is given. π-electronic periodicity of several series of tubulenes is rationalized in terms of some "rules of thumb". The strain energy of these nonplanar molecules was evaluated from the pyramidalization angles of the sp2 carbon atoms by the POAV1 procedure. Semiempirical PM3 calculations support the presented spectral data.

Formation and reinforcement of clusters composed of C60 molecules

We carry out two experiments: (1) the formation of clusters composed of C60 molecules via self-assembly and (2) the reinforcement of the clusters. Firstly, clusters such as fibres and helices composed of C60 molecules are produced via self-assembly in supercritical carbon dioxide. However, C60 molecules are so weakly bonded to each other in the clusters that the clusters are broken by the irradiation of electron beams during scanning electron microscope observation. Secondly, UV photons are irradiated inside a chamber in which air is filled at 1 atm and the above clusters are placed, and it was found that the clusters are reinforced; that is, they are not broken by electron beams any more. C60 molecules located at the surface of the clusters are oxidised, i.e. C60On molecules, where n = 1, 2, 3 and 4, are produced according to time-of-flight mass spectroscopy. It is supposed that oxidised C60 molecules at the surface of the clusters may have an important role for the reinforcement, but the actual mechanism of the reinforcement of the clusters has not yet been clearly understood and therefore is an open question.

Raman spectra of C60 dimer and C60 polymer confined inside a (10, 10) single-walled carbon nanotube

A new set of C-C interball force constant was developed in order to reproduce the low wavenumber density of states measured by neutron scattering and the Raman spectra of the C(60) dimer and C(60) polymer chain. The nonresonant Raman spectra of the C(60) dimer and C(60) polymer confined inside a (10, 10) single-walled carbon nanotube were calculated in the framework of the bond-polarization theory by using the spectral moments method. The main changes of the Raman spectrum as a function of the organization of the C(60) molecules inside the nanotubes were identified. We found that the radial breathing modes of a (10, 10) single-walled carbon nanotube are more sensitive on the structure of the C(60) molecules than the G-modes. These predictions are useful to interpret the experimental Raman spectrum of fullerene peapods.

A DFT study on the dimerization of C62 , H2 C62 , and F2 C62

On the basis of calculations using the density functional theory, we show that C(62), a recently synthesized nonclassical fullerene, will presumably undergo dimerization with various isomers at elevated temperatures. This is shown by calculating the dimerization energy and the activation barrier of the dimerization. Eight possible isomers of the dimer were identified, all of which are more stable than the two isolated monomers. The relative stability of various isomers depends upon the kind of C=C bonds within the four-membered carbon ring involved in the dimerization. In addition, similar calculations were performed for the monomers and dimers of H(2)-C(62) and F(2)-C(62). Six isomers were identified for each of the dimers. Although less pronounced than the case of the C(62) dimer, all isomers of the H(2)-C(62) dimer are appreciably more stable than the individual monomers. Although a large steric repulsion due to F atoms significantly reduces the stability of F(2)-C(62) dimer, its two isomers are still more stable than separate monomers.

Electrochemical nanostructuring of fullerene films—spectroscopic evidence for C60 polymer formation and hydrogenation

Electrochemical reduction of ordered C60 fullerene films in aqueous solution was studied by AFM, FTIR and Raman spectroscopy, mass spectrometry and elastic recoil detection analysis. During the irreversible reduction process the film morphology changed from a heteroepitaxial (111) surface to a nanostructured array with clusters of 20 to 50 nm lateral size on average. On the molecular level the initial C60 underwent electrochemical reactions to form C60 polymers and hydrogenated C60. Chemical follow-up reactions of electrochemically formed C60- with water are responsible for the different reduction behaviour of C60 films in aqueous solution compared to C60 reduction in organic solvents and to C60 doping with alkali metals. Based on the spectroscopic analysis a reaction scheme accounting for the chemical processes at the C60 / aqueous electrolyte interface is presented.

Raman spectroscopy of fullerenes and fullerene-nanotube composites

The discovery of fullerenes in 1985 opened a completely new field of materials research. Together with the single-wall carbon nanotubes (SWCNTs) discovered later, these curved carbon networks are a playground for pure as well as applied science. We present a review of Raman spectroscopy of fullerenes, SWCNTs and composite materials. Beginning with pristine C(60), we discuss intercalated C(60) compounds and polymerized C(60), as well as higher and endohedral fullerenes. Concerning SWCNTs, we show how the diameter distribution can be obtained from the Raman spectra and how doping modifies the spectra. Finally, the Raman response of C(60) encapsulated into SWCNTs (C(60) peapods) is discussed.

Theoretical studies of structures and stabilities of a new odd-numbered fullerene dimer: C141

The possible isomers of a newly synthesized C(141) molecule are calculated using MNDO, AM1, PM3, B3LYP/3-21G, and B3LYP/6-31G(d) methods. The geometry optimizations showed that the isomer 8-8 has the lowest total energy in all 64 possible structures of C(141). Unlike those of C(130), C(140), etc., the C(141) 8-8 shows a new structure: two C(70) side cages open [6.6] ring junctions located at the equator (instead of cap) area to create new chemical bonds for the bridge atom. Theoretical measurements of the average length of the long and short axes of C(70) side cages in the C(141) molecule reveal that when two C(70) cages are connected with each other at the equators, their geometric shapes become more spherical compared with the pristine C(70); this leads to a reduction of the molecular polarizability. Analysis of the local and global strain indicates that the global strain of C(70) monomer in the C(141) 8-8 is greatly reduced compared to the pristine C(70). The stable C(70) derivatives that are formed with reacted C-C bonds in the equator area may put new insights into fullerene chemistry, in particular, for C(70) to react with a large molecule. The results are discussed together with the experimental data.

Reversible Dimerization of [5,6]-C60O

The recently discovered [5,6]-open isomer of C(60)O has been found to undergo facile dimerization to form a new C(2) symmetry isomer of C(120)O(2), which can be photodissociated with relatively high efficiency to regenerate monomeric [5,6]-C(60)O. High yield dimerization of [5,6]-C(60)O proceeds spontaneously in toluene solution near room temperature. On the basis of (13)C NMR spectroscopy, ab initio quantum computations, and HPLC retention patterns, the resulting C(120)O(2) product has been deduced to be a nonpolar dimer of C(2) symmetry in which the C(60)O moieties are linked by two single bonds between sp(3)-hybridized carbon atoms adjacent to oxygen atoms. Photophysical properties of this dimer have also been measured and compared to those of C(120), the [2 + 2]-dimer of C(60). The ground-state absorption spectrum of C(120)O(2) in toluene is slightly red-shifted relative to that of C(120), with a distinctive peak at 329 nm and an S(1)-S(0) origin band at 704 nm. Its fluorescence spectrum shows two major peaks at 718 and 793 nm. In room-temperature toluene, the measured triplet state intrinsic lifetime of this C(120)O(2) isomer is 34 +/- 2 micros, a value somewhat shorter than that of C(120) (44 micros). C(120)O(2) undergoes photodissociation from its triplet state to regenerate monomeric [5,6]-C(60)O with quantum yields of 2.5% at 24 degrees C and 43% at 70 degrees C. It can therefore serve as a stable reactant for photolytic production of [5,6]-C(60)O. As a simple fullerene adduct that reacts under mild conditions, [5,6]-C(60)O may prove useful in special synthetic applications. Solutions of [5,6]-C(60)O are also unique because they can provide mixtures of a fullerene monomer and its dimer in a dynamic balance controllable by adjustment of concentration, temperature, and optical irradiation.

Specific Raman signatures of a dimetallofullerene peapod

We report on the Raman spectroscopy of a dimetallofullerene peapod, (La(2)@C(80))(m)@SWNT. Drastic changes are observed with respect to pristine nanotubes. A sharp intense line at 142 cm(-1) is interpreted as a signature of polymerization of the encapsulated metallofullerenes. Additional strong signatures appear at about 400, 520, and 640 cm(-1), respectively. Their intensity suggests the existence of an enhancement effect. The stiffening and the up-shift of the G-band modes appear to imply that a charge transfer process between the nanotube and the peas occurs.