C60 and carbon: a postbuckminsterfullerene perspective

The discovery of the fullerenes and nanotubes has completely changed our perspective on various aspects of carbon chemistry and materials science in quite fundamental ways. The experiments, which uncovered C₆₀, occurred between 1985 and 1990 and there are lessons to be learned of various kinds over the way scientific advances occur and more importantly the way misconceptions can propagate. For instance much of our received wisdom over the behaviour of carbon, in particular graphite on a microscopic scale, was really quite ill-conceived and certainly misleading. Questions might be asked as to why it took almost till the end of the 20th century for the fact to be uncovered that the elegant C₆₀ molecule had been lurking in the dark shadows of soot chemistry all the time. After all, mass spectrometric techniques were sufficiently advanced for the discovery to have been made in the 1960’s—perhaps even earlier. Some of these issues are addressed here and the discussion gives an insight into the curiously unpredictable way fundamental scientific advances sometimes occur and also highlights the limitations of applied research in this case.     

Thermal Plasma Decomposition of Tetrachloroethylene

Tetrachloroethylene (C₂Cl₄) has been used widely as a solvent and dry cleaning agent, but was later specified as possible human carcinogen. As a result, its safe treatment became a priority. In this paper, we report on its decomposition in an atmospheric radiofrequency thermal plasma reactor. Main components of the exhaust gases were determined by Fourier transform infrared spectroscopy. We found that complete decomposition can be achieved in either oxidative or reductive conditions but not in neutral one. The solid soot product was characterised by transmission electron microscopy and specific surface area measurement. Organic compounds adsorbed on the surface of the soot were extracted by toluene and comprised, based on gas chromatography mass spectrometry, of various perchlorinated aliphatic (for example hexachlorocyclopentadiene) and aromatic compounds (like hexachlorobenzene, octachloronaphthalene or octachloroacenaphthylene). Several nitrogen containing molecules were also identified whose presence are rare during thermal plasma treatments. Further investigation of the extract by mass spectrometry revealed various higher molar mass chlorinated carbon clusters and two types of fullerenes (C₆₀ and C₇₀).     

Single-stage plasma-arc synthesis of metallo-endofullerences

Single-stage plasma-arc synthesis of metallo-endofullerences of the types C₆₀Pd, C₆₀Ni, and C₆₀Cr, whose content in a mixture of extracted fullerenes was 0.05 to 0.15 wt %, was performed. The effect of introduction of these metals into the reaction plasma on the total yield of fullerenes and on the fraction composition of the fullerene mixture was studied. The fullerene mixtures were analyzed by mass spectrometry and liquid chromatography.     

Studies of the Formation of Carbon Clusters

Experimental and theoretical studies focusing on the formation of carbon clusters are described. In the experiment on discharge in liquid chloroform, a series of perchlorinated fragments of C₆₀ was synthesized and a scarce amount of C₆₀ was detected. In the laser vaporization experiments, it was found that the production of C ₆₀ ⁺ and other fullerene ions could be promoted by doping chlorine-containing compounds into carbon targets. Chlorine atoms were found to play key roles of not only tying up the dangling bonds of the polycyclic carbon clusters, but also catalyzing the formation of fullerenes. The results showed that C₆₀ and other fullerenes are formed from growth of small carbon species and supported the Pentagon Road scheme of the fullerene formation mechanism. On the other hand, ab initio calculations were carried out on formation reactions of C₆₀ from its various perchlorinated fragments, C_60–2mCl₁₀. The monotonically decreasing calculated energies of reactions with growing size of the fragments confirm that the formation reaction is energetically favorable.     

2‐Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity

Extraction with 2‐aminoethanol is an inexpensive method for removing empty cage fullerenes from the soluble extract from electric‐arc‐generated fullerene soot that contains endohedral metallofullerenes of the type Sc₃N@C_2n (n=34, 39, 40). Our method of separation exploits the fact that C₆₀, C₇₀, and other larger, empty cage fullerenes are more susceptible to nucleophilic attack than endohedral fullerenes and that these adducts can be readily extracted into 2‐aminoethanol. This methodology has also been employed to examine the reactivity of the mixture of soluble endohedral fullerenes that result from doping graphite rods used in the Krätschmer–Huffman electric‐arc generator with the oxides of Y, Lu, Dy, Tb, and Gd. For example, with Y₂O₃, we were able to detect by mass spectrometry several new families of endohedral fullerenes, namely Y₃C₁₀₈ to Y₃C₁₂₆, Y₃C₁₀₇ to Y₃C₁₂₅, Y₄C₁₂₈ to Y₄C₁₄₆, that resisted reactivity with 2‐aminoethanol more than the empty cage fullerenes and the mono‐ and dimetallo fullerenes. The discovery of the family Y₃C₁₀₇ to Y₃C₁₂₅ with odd numbers of carbon atoms is remarkable, since fullerene cages must involve even numbers of carbon atoms. The newly discovered families of endohedral fullerenes with the composition M₄C_2n (M=Y, Lu, Dy, Tb, and Gd) are unusually resistant to reaction with 2‐aminoethanol. Additionally, the individual endohedrals, Y₃C₁₁₂ and M₃C₁₀₂ (M=Lu, Dy, Tb and Gd), were remarkably less reactive toward 2‐aminoethanol.     

Isomerization kinetics of carbon clusters 1. Potential of interatomic interaction

Formation of fullerenes — spherical carbon clusters C₆₀ and clusters of other sizes — during condensation of carbon vapors has not yet received theoretical explanation. Recent experimental works concerned with cluster formation in carbon vapors have established that during condensation carbon atoms form rings and then polycyclic clusters, which are precursors of fullerenes. Theoretical investigation of the spontaneous formation of fullerenes from polycyclic rings calls for a simple model of the potential of interatomic interaction of carbon, which would allow fast calculations of bond energies and statistical sums of the clusters. We use the modified Brenner potential, which was developed for hydrocarbon molecules. The parameters of the potential are refitted according to the results of quantum chemical calculations. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No.4, pp. 664–670, July–August, 1996. Translated by I. Izvekova     

Quantum molecular dynamics simulations of fullerenes and graphitic microtubules

We describe the results of extensiveab initio molecular dynamics calculations of the properties of fullerenes and microtubules. Our finite temperature quantum MD simulations for solid C₆₀ are in excellent agreement with NMR, photoemission and neutron scattering data. The C₆₀ isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C₆₀, but the transformation between these two structures is hindered by a 5.4 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C₂₀ isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C₆₀. The corannulene structure is a natural precursor for the formation of C₆₀. Simulations of reactions show that C₂ can insert into C₅₈, perfect C₆₀, and defect C₆₀ fullerenes without an activation barrier, while C₃ attaches only to their surfaces. Evaporative fragmentation of carbon clusters during annealing is unlikely, but atom and fragment exchange during collision favor "locally" most stable structures, such as C₆₀. These results may explain the large increase in the abundance of C₆₀ and C₇₀ when carbon clusters are annealed at high density. We have also carried out calculations for paradigmatic microtubules, both reflection-symmetric and chiral. We find that the optimized geometries of the tubules are close to the ideal ones. It is possible to fabricate tubules with direct band gaps away from the Γ point by exploiting the similarities between the projected band structure of graphite and that of the tubule. The semiconducting tubules can be doped n- and p-type by substitutional N and B, respectively.     

Tandem mass spectrometry of aminated fullerene derivatives

The products of the reaction between fullerenes (C₆₀/C₇₀) and dimethylamine were investigated by fast atom bombardment (FAB) mass spectrometry and tandem mass spectrometry (MS/MS). The FAB mass spectrum shows peaks corresponding to the addition of up to eight dimethylamine species, exclusively to C₇₀. MS/MS reveals an unusual fragmentation pattern. The mass spectrum of the reaction products, together with a number of tandem mass spectra, are shown.     

Synthetic Approaches toward Molecular and Polymeric Carbon Allotropes

Life on earth is based on compounds that have carbon frames and backbones. Today, chemists have added to the world of biomolecules and biopolymers approximately 10⁷ different synthetic molecules and polymers, the structures of which also depend on the formation of strong, stable carbon–carbon bonds. Although the stability of carbon–carbon bonds has been recognized for more than a century, the two natural modifications graphite and diamond were, until recently, the only allotropic forms of carbon on earth that were available in macroscopic quantities and were structurally well characterized. With the synthesis of macroscopic quantities of buckminsterfullerene (C₆₀) and the higher fullerenes (C₇₀, C₇₆, C₇₈, etc.) and the exploration of the fascinating properties of these all-carbon spheres, this situation has completely changed. In the coming decades, the design, preparation, and study of novel molecular and polymeric allotropic forms of carbon will be a central topic in chemistry. Research in this area will dramatically advance the fundamental knowledge on carbon-based matter and, as already illustrated by the ongoing work on C₆₀, generate unprecedented technological perspectives. This review surveys synthetic organic-chemical approaches toward the preparation and study of all-carbon molecules and polymers that differ from the familiar networks of graphite and diamond as well as from the fullerenes. We will also discuss the ongoing research on fullerenes with a particular focus on the synthetic approaches to these all-carbon spheres and their transition metal complexes.     

Mass spectrometric investigation of the aggregation and coalescence of fullerenes in C60-modified poly(N-vinylcarbazole) by UV-laser ablation

The phenomena of aggregation and coalescence of fullerenes in the UV-laser ablation time-of-flight mass spectrometric investigation of C₆₀-modified poly(N-vinylcarbazole) both in the positive and in the negative ion channels have been observed. The results indicate that in C₆₀ chemically modified PVK (C₆₀–PVK) copolymer the nascent fullerene fragments ruptured from main chain can easily coalesce into large fullerenes through collisions, whereas in the C₆₀-doped PVK the aggregation and coalescence of C₆₀ were relative weak due to nonbounding action and incomplete charge transfer behavior between C₆₀ and PVK. Furthermore, the photoinduced electron transfer behavior between C₆₀ and carbazole units in the C₆₀ chemically modified poly(N-vinylcarbazole) in benzonitrile by laser flash photolysis at 355 nm has also been investigated. Efficiency of the anion radical of C₆₀ in copolymer at 1080 nm is higher than that of the C₆₀-doped poly(N-vinylcarbazole) polymers. The formation of a C₆₀ radical anion may be ascribed to photoinduced electron transfer between C₆₀ pendanted on the main chain backbone and the inter-, and intrachain carbazole units in the copolymer. © 1997 John Wiley & Sons, Inc. 35 : 1185–1190, 1997     

Some aspects of the symmetry and topology of possible carbon allotrope structures

Elemental carbon has recently been shown to form molecular polyhedral allotropes known as fullerenes in addition to the familiar graphite and diamond known since antiquity. Such fullerenes contain polyhedral carbon cages in which all vertices have degree 3 and all faces are either pentagons or hexagons. All known fullerenes are found to satisfy the isolated pentagon rule (IPR) in which all pentagonal faces are completely surrounded by hexagons so that no two pentagonal faces share an edge. The smallest fullerene structures satisfying the IPR are the known truncated icosahedral C₆₀ of I _h symmetry and ellipsoidal C₇₀ of D _5h symmetry. The multiple IPR isomers of families of larger fullerenes such as C₇₆, C₇₈, C₈₂ and C₈₄ can be classified into families related by the so-called pyracylene transformation based on the motion of two carbon atoms in a pyracylene unit containing two linked pentagons separated by two hexagons. Larger fullerenes with 3ν vertices can be generated from smaller fullerenes with ν vertices through a so‐called leapfrog transformation consisting of omnicapping followed by dualization. The energy levels of the bonding molecular orbitals of fullerenes having icosahedral symmetry and 60n ² carbon atoms can be approximated by spherical harmonics. If fullerenes are regarded as constructed from carbon networks of positive curvature, the corresponding carbon allotropes constructed from carbon networks of negative curvature are the polymeric schwarzites. The negative curvature in schwarzites is introduced through heptagons or octagons of carbon atoms and the schwarzites are constructed by placing such carbon networks on minimal surfaces with negative Gaussian curvature, particularly the so-called P and D surfaces with local cubic symmetry. The smallest unit cell of a viable schwarzite structure having only hexagons and heptagons contains 168 carbon atoms and is constructed by applying a leapfrog transformation to a genus 3 figure containing 24 heptagons and 56 vertices described by the German mathematician Klein in the 19th century analogous to the construction of the C₆₀ fullerene truncated icosahedron by applying a leapfrog transformation to the regular dodecahedron. Although this C₁₆₈ schwarzite unit cell has local O _h point group symmetry based on the cubic lattice of the D or P surface, its larger permutational symmetry group is the PSL(2,7) group of order 168 analogous to the icosahedral pure rotation group, I, of order 60 of the C₆₀ fullerene considered as the isomorphous PSL(2,5) group. The schwarzites, which are still unknown experimentally, are predicted to be unusually low density forms of elemental carbon because of the pores generated by the infinite periodicity in three dimensions of the underlying minimal surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Versatile fullerenes as sensor materials

The last century outstanding discovery of fullerenes (or C₆₀), as they are popularly called ‘buckyball’ structured molecules with icosahedral spherical structure, consists of 60 sp^2-hybridized carbon atoms. These fullerenes have created immense applications in various fields, such as catalysts, sensors, photocatalysts, energy production, and storage materials. Fullerenes because of their improved conductivity, charge transfer, and photophysical properties have gained considerable attention, particularly in sensor area. The activity of sensors depends upon the interactions between fullerene and the sensing material. Among all the types of fullerenes, C₆₀ has been extensively used. This review is an attempt to cover different aspects of fullerene-based sensing devices, wherein fullerenes act as important component (s) of the sensor device because of their electron-accepting properties. We will discuss the fullerene-based sensors for diverse applications as strain/gas sensors, electrochemical sensors, and optical sensors as much effort has been recently made to detect different analytes such as gases, volatile organic compounds, metal ions, anions, and biomolecules.     

A general method for quantitative measurement of molecular mass distribution by mass spectrometry

A method is presented to test whether the conversion of the mass spectrum of a polydisperse analyte to its molecular mass distribution is quantitative. Mixtures of samples with different average molecular masses, coupled with a Taylor’s expansion mathematical formalism, were used to ascertain the reliability of molecular mass distributions derived from mass spectra. Additionally, the method describes how the molecular mass distributions may be corrected if the degree of mass bias is within certain defined limits. This method was demonstrated on polydisperse samples of C₆₀ fullerenes functionalized with ethylpyrrolidine groups measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; however, it is applicable to any polydisperse analyte and mass spectrometric method as long as spectrum resolution allows individual oligomers to be identified. Mass spectra of the derivatized fullerenes taken in positive ion mode were shown to give an accurate measurement of the molecular mass distribution while those taken in negative ion mode were not. Differences in the mechanisms for ion formation are used to explain the discrepancy.     

High performance liquid chromatographic–mass spectrometric detection of giant fullerenes

Fullerene-rich soot generated by resistive heating of graphite has been gently extracted with toluene, in order to remove some C₆₀ and C₇₀ compounds, followed by extraction with boiling 1,2,4-trichlorobenzene at 214°C. After filtration and removal of the solvent, the residue was re-dissolved in dichloromethane and characterized by non-aqueous reversed phase liquid chromatography coupled on-line with atmospheric pressure chemical ionization mass spectrometry. Several novel fullerenes were detected, including C₇₂, C₈₀, C₈₆, and C₈₈ as well as other fullerenes up to C₁₀₈ and higher. The results indicate that chromatographic separation of large fullerene molecules can be achieved with low boiling point solvents and conventional liquid chromatographic techniques.     

A study on mass spectrometry of methylated [60] fullerenes using the “in-beam” electron impact technique

Mass spectra of the methylated [60]fullerenes were obtained by EI mass spectrometry using “desorption” or “in-beam” technique. The mass spectra of the methylated fullerenes, C₆₀Me_n, have the molecular ion peak M⁺ indicating that the product is stable under the MS (EI) conditions. The appearance of an intense peak at m/z 360 was assigned to the formation of fullerene dication C₆₀⁺⁺. The remaining peaks were assigned to successive loss of methyl groups from molecular monocation and dication.     

A MNDO study of carbon clusters with specifically fitted parameters

Summary A MNDO method with new parameters for carbon clusters is presented. The parameters in the new sets are specifically tuned to fit the properties of small carbon clusters, C₂, C₃, C₅ and C₇–C₁₀, and buckminsterfullerene, C₆₀. The validity of these MNDO parameters is verified by experimental data. The calculated (with new parameters) IR spectra of C₆₀ and the heat of formation, geometry and IR spectra of C₇₀ agree satisfactorily with observed data. Heats of formation of other fullerenes, from C₂₀ to C₈₄, and C₆₀O are evaluated. The resulting heats of formation of the isomers of C₇₆ and C₈₄ are reliable and their relative stability is in excellent agreement with other reports. The predicted IR spectra of several fullerenes, C₂₄(C_6v ), C₂₈(T _d ), C₃₂(D₃), C₃₆(D_6h ), C₅₀(C_5h ) and C₈₀(D_5d ) are provided to aid assignments of experimental spectra.     

Homolytic reactive mass spectrometry of fullerenes: interaction of C60 and C70 with ketones in the electron impact ion source of a mass spectrometer and the comparison of results with those of photochemical reactions of C60 with several ketones in solution

Our previous investigations showed that homolytic reactions of C₆₀ with a number of perfluoroorganic and organomercury(II) compounds occurring under electron impact (EI) in the ionization chamber (IC) of a mass spectrometer could predict the reactivity of C₆₀ towards these compounds in solution or solid state. To expand the scope of this statement, C₆₀ and C₇₀ have been reacted with ketones RCOR₁, where R and R₁ are alkyl, aryl, benzyl, and CF₃, in an IC under EI to yield products of the addition of R^· and R₁^· radicals to the fullerenes, paramagnetic ones being stabilized by hydrogen addition and loss. Experimental evidence in support of a mechanism involving homolytic dissociation of ketone molecules via superexcited states to afford these radicals that react with the fullerenes at the IC surface has been obtained. As anticipated, the reactions between C₆₀ and several ketones conducted in solution under UV irradiation have afforded Me-, Ph-, and CF₃-derivatives of C₆₀. However, some other products have been identified by mass spectrometry and their formation is reasonably explained. When decalin has been employed as a solvent, decalinyl derivatives of the fullerene have been found among the products and the (9-decalinyl)fullerenyl radical has been registered by EPR. Thus, incomplete but reasonable conformity of the results of the reactions of fullerenes with ketones in an IC under EI with those of the reactions of the same reagents in solution under UV irradiation has been demonstrated, and the former results can predict the latter ones to a reasonable extent.     

Study of structural and thermodynamic parameters of adsorption layers using density functional theory. Local density and adsorption isotherms on spherical surfaces

Local density profiles in adsorption layers of Lennard-Jones fluids on two-dimensional adsorbents with spherical geometry and isotherms of excess (Gibbs) adsorption have been calculated using the classical density functional theory (approximations with weighting coefficients). The local density profiles have been found in hydrogen adsorption layers on C₆₀, C₂₄₀, and C₅₄₀ fullerene molecules. The calculations have been performed for both subcritical and supercritical temperature ranges. It has been shown that, at a pressure of 10 MPa and a temperature of 77 K, the gravimetric (mass) hydrogen density on C₆₀ fullerene is 7.6 wt %, which is in good agreement with the results of molecular dynamics simulation and experimental data. It has also been established that the gravimetric hydrogen density on C₆₀ fullerene is higher than that on C₂₄₀ and C₅₄₀ fullerenes, being comparable with its value in a slitlike pore of a carbon adsorbent.