Calculations on endohedral C74 complexes

While empty C74 is still a rather elusive fullerene, its endohedral derivatives are well established. The report surveys computations for three of C74-based endohedrals--Ca@C74, Ba@C74, and Yb@C74. A set of six C74 cages is considered, namely one cage with isolated pentagons, three isomers with a pentagon-pentagon junction, two structures with one pair of connected pentagons, and one heptagon. Special interest is paid to the Gibbs-energy evaluations for estimations of the relative populations.     

CHROMATOGRAPHIC PURIFICATION OF Kr@C60

Unusual compounds have been reported to form by the incorporation of atoms into fullerene cages. The compounds in which a noble gas atom is trapped into a C₆₀ cage are very stable and only van der Waals interactions occur between the noble gas and the fullerene cage. The study of these compounds requires efficient chromatographic (or other) separations. We performed the isolation of Kr@C₆₀ and a study of its chromatographic separations on several HPLC columns using toluene : hexane mixtures as solvent. We found that the separation factor is small (1.09 to 1.12) with the highest separation factor on a Buckyprep column.     

Sm@C74: Computed Relative Isomeric Populations

Density-functional theory (DFT) calculations are reported for Sm@C₇₄, namely for Sm encapsulation in the IPR (isolated pentagon rule) C₇₄ cage and in two C₇₄ cages with a pentagon-pentagon junction. Their relative thermodynamic production yields are evaluated using the DFT calculated terms and it is shown that the IPR-based Sm@C₇₄ endohedral prevails at relevant temperatures in agreement with observation, however, isolation of a minor isomer could be possible.     

Ab initio calculation on fullerene C32 and Its Derivative

In this paper, we studied the equilibrium geometries, the electronic structures and the binding energies of fullerene C₃₂ as well as its hydrogenates and fluorides interactions by the SCF Hartree-Fock calculations using the latest version of GAMESS of ab initio programs. The fullerene C₃₂ is found to have a ¹A₂' closed-shell ground state in D_3d symmetry or a ¹E_U singlet state in D_3h symmetry. It is predicted to be easy to add some hydrogen or fluorine atoms to the C₃₂ cage and to produce a series of derivatives C₃₂H_2n,C₃₂F_2n (n=l, 3, 6), in which C₃₂H₂,C₃₂F₂ (D_3d), C₃₂H₆^a,C₃₂F₆^c(D_3h) are more stable. We suggest that this implies that the vertex of triplet pentagons of the C₃₂ cage is an activation site for addition.     

Isomers of Endohedral Fullerene Sc2@C84

Density functional calculations are carried out to predict the distance between two Sc atoms, that are encapsulated into C₈₄ fullerene cage, and structure of the endohedral fullerene isomers. It is found that the C_s symmetry isomer is originated from cage 21 of C₈₄ fullerene. The new structure of the endohedral fullerene with C_2v symmetry is found in which the distance between two encapsulated Sc atoms is larger than that previously reported.     

Molecular orbital studies on the rings composed of D2d C36 cages

The structures and electronic properties of the rings composed of D_2d C₃₆ cages are investigated using the semi-empirical AM1 molecular orbital method with full geometric optimization. It is found that most of the converged structures are more stable after the rings are formed. Strain plays an important role in the stability of the fullerene rings. Other factors influencing the stability are also discussed, such as the type of the bonded carbon atoms and the size of retained aromatic domains. Comparison of the structures and electronic properties are made between the C₃₆-rings and those composed of D_6h C₃₆ isomers.     

Fullerane, the Hydrogenated C60 Fullerene: Properties and Astrochemical Considerations

Hydrogenated C₆₀ fullerene, C₆₀H₃₆ was prepared in different solvents using Zn/HCl as reducing agents. The structure of C₆₀H₃₆ was confirmed both by electronic and FT-IR spectroscopy and the purity of the reaction product was checked by HPLC analysis. It has been confirmed that C₆₀H₃₆ is not stable in air, especially in presence of light which enhances the oxidation. The oxidation of C₆₀H₃₆ was studied by FT-IR spectroscopy and by differential scanning calorimetry (DSC) in air; the formation of hydroxyl groups on the fullerene cage and ketonic groups (involving cage breakdown) have been detected. Furthermore, the action of O₃ on C₆₀H₃₆ was investigated and it has been found that O₃ exerts practically the same effect of air but causing an enhanced cage breakdown. The thermal stability of C₆₀H₃₆ was checked by a thermogravimetric analysis (TGA) coupled with a differential thermal analysis (DTA) under N₂ flow. The vaporization of C₆₀H₃₆ occurs at very high temperature: the DTA trace has shown an endothermic peak at 540°C (at a heating rate of 20°C/min). C₆₀H₃₆ shows an electronic absorption spectrum with a maximum at about 217 nm and it is able to match both in position and in half width the peak at 217.5 nm observed in the spectrum of the interstellar extinction of light which was attributed to hydrogenated, radiation processed and thermally annealed carbon dust. Similarly, the absorption spectrum of C₆₀H₃₆ is able to match several infrared emission bands (called UIBs) detected from certain astrophysical objects like the protoplanetary nebulae (PPNe). It is proposed that hydrogenated fullerenes can be used as model compounds in the laboratory simulation studies of interstellar carbon dust.     

GENERATION OF HIGHER FULLERENES FROM LASER ABLATION OF CARBYNE AND C60 PHOTOPOLYMER ASTROCHEMICAL IMPLICATIONS

Laser ablation of targets of carbonaceous matter containing carbyne nanodomains (the sp hybridised carbon chains) or targets of C₆₀ photopolymer produced carbon clusters which have been detected by FT-ICR (Fourier-Transform Ion Cyclotron Resonance) mass spectrometer. When the carbonaceous matter containing carbyne has been employed as laser target, no C₆₀ has been generated but only fullerene cages from C₇₄ up to C₁₂₄. Larger cages were also obtained but with odd number. Starting from C₆₀ photopolymer, laser ablation regenerates free C₆₀ and creates a sequence of C₆₀ superior homologues all possessing even number and each member of the series is separated from the preceding and the following member by the loss or by the addition respectively of a C₂ unit. Fullerenes up to C₁₆₂ have been recorded. The implications about the presence of free C₆₀ fullerene in the interstellar and circumstellar space, its formation from carbyne chains and its stability towards its photopolymerization tendency and its regeneration from the photopolymer together with its superior homologues have been discussed thoroughly in the present paper.     

C60 - An Abiotic Terpenoid?

It is proposed that the formation of the C₆₀ cage, named buckminsterfullerene, proceeds via the modified isoprene mechanism. Hence, it is postulated that this molecule is an abiotic terpenoid. Similar mechanism is also proposed for the formation of the C₅₉N cage. Since the thermodynamic stabilities of C₆₀ and C₅₉N cages are comparable, the reason why the latter cage is not yet observed in the laboratory conditions is probably related to its greater kinetic instability than that of buckminsterfullerene.     

Carbon Atoms Catalyse Fullerene Growth

The fullerene road to C_6o must proceed through isomers which do not obey the Isolated Pentagon Rule (IPR). Stone Wales (SW) transformations are required to redistribute the pentagons to obtain the almost exclusively observed IPR icosa-hedral C₆₀. We find a very high barrier for these transformations (over 6 eV), which make their occurrence rare during the annealing phase of C₆₀ formation. Earlier we reported that the presence of a loosely bound single carbon atom reduces this barrier to 3.9 eV, making frequent pentagon rearrangements possible.     

Computations of metal-covered C60 and C70

Semiempirical quantum-chemical PM3 calculations are reported for a new class of exohedral metallo-fullerenes – metal-coated or metal-covered fullerenes: C₆₀M_n and C₇₀M_n. The exohedral species have been observed in gas phase, however, their geometrical and electronic structures are not known yet. Relatively-even metal-atom distributions over the fullerene rings are considered – such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C₆₀M₁₂/C₇₀M₁₂ (metal atoms above all pentagons), C₆₀M₂₀/C₇₀M₂₅ (metal atoms above all hexagons), and C₆₀M₃₂/C₇₀M₃₇ (metal atoms above all rings). If an odd number of electrons should result (Al), the related cation is computed, or one metal atom added or removed. This interesting arrangement above the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational technique used – the PM3 semiempirical method is parametrized only for some selected metals. Metal-layer atomization heats are suggested as a stability measure. Structural characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to Jahn–Teller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in the nanotube synthesis and could eventually lead to new interesting materials.     

Localized Molecular Orbital Studies of Fullerenes: C60 and C70

Localized molecular orbitals (LMOs) have been calculated for C₆₀ and C₇₀ by using the CNDO/2 approximation and the energy-localization scheme of Edmiston-Ruedenberg. It is found that the π-bonding on the surfaces of the spherical carbon cages is localized into two-centered two-electron (2c2e) and three-centered two-electron (3c2e) bonds with additional charge distributions delocalized in the neighboring atoms. The localized electronic structures, the hybridized states of carbon atoms, the spherical aromaticity, and the reactivity for forming metal complexes of C₆₀ and C₇₀ are discussed.     

C32: Computations of Low-Energy Cages with Four-Membered Rings

C₃₂ cages built from four-, five-, six-, and seven-membered rings are computed. The computations are primarily performed with semiempirical quantum-chemical methods (AM1, PM3, SAM1), and altogether 199 cages are optimized. The energetics is further checked through ab initio HF SCF computations with the standard 3-21G basis set, and also by density functional theory at the B3LYP level in the standard 6-31G* basis set. All five levels of theory suggest a D_4d cage (two four-membered rings, eight pentagons, eight hexagons) as the lowest-energy structure. Temperature effects are treated in the terms of partition functions so that the entropy contributions are considered accordingly. The thermodynamic treatment points out five cages significantly populated at high temperatures. At very high temperatures the structure lowest in energy is not the most abundant isomer. There are just six conventional fullerenes C₃₂, built exclusively from pentagons and hexagons, however, only two of them show significant populations at high temperatures. The remaining three relatively stable cages contain at least one four-membered ring. No structure with a heptagon shows a non-negligible concentration at high temperatures. The study suggests that in the non-IPR region the quasi-fullerene cages with four-membered rings can in some cases be more important than the conventional fullerenes built from pentagons and hexagons only.     

Stone-Wales Rearrangement Pathways from the Hinge-Opened [2+2] C60 Dimer to Ipr C120 Fullerenes. Vibrational Analysis of Intermediates

Possibility of the previously proposed hinge-opened product 2 of [2+2] C₆₀, dimer 1 transforming itself into IPR C₁₂₀ fullerenes by a series of generalized Stone-Wales (GSW) rearrangements has been tested by seeking all topologically acceptable pathways with the help of a graphical search program. the first IPR isomer 4 appeared after 20 GSW steps from the wide-bridged dumb-bell shaped precursor 3. More than 1,000 C₁₂₀ fullerene structures were generated during subsequent ten GSW steps, but the outstanding T_dC₁₂₀ global minimum 5 was not reached. Semiempirical vibrational calculations predict characteristic transition in the vibrational spectra in the course of rearrangement pathway.     

Covalently-Bonded “Onion Type” Double Fullerenic Carbon Cages

Three “onion type” double carbon cages (consisting of an inner sp³-hybridized C₆₀ buckminsterfullerene skeleton and an outer larger carbon cage with icosahedral symmetry, with covalent bonds between the carbon atoms of the 5-membered rings) are described and their strain energies are discussed: (i) when the outer cage is a C₁₂₀ cage derived from buckminsterfullerene by replacing each bond between two six-membered rings by an acetylenic group, the strain is smallest.; (ii) when the same derivation concerns each bond between 5- and 6-membered rings leading to a C₁₈₀ outer cage, the strain is largest.; (iii) when the outer cage is C₁₈₀ with benzenoid rings, the strain is intermediate.     

STRUCTURE OF HEXA-SULFOBUTYL FULLERENES: A COMPUTATIONAL STUDY

Quantum-chemical computations are reported for hexa-sulfobutyl fullerenes C₆₀((CH₂)₄SO₃H)₆ (FC4S). FC4S represent novel water-soluble fullerene derivatives with an enhanced radical scavenging activity and also possible potential for materials science. The computations have been carried out with the standard version of the PM3 semiempirical quantum-chemical method. It is found out that the lowest among the computed eight FC4S species has the structural pattern already observed for C₆₀Cl₆. However, a larger-scale search is at present prevented by the demands on computational resources.     

Fullerene Research in Poland

Main results of the investigations of fullerene and its derivatives are briefly reviewed. Such topics as plasma spectroscopy, fullerenes and nanotubes formation, C₆₀ carbyne knots, fullerene reduction and doping, charge transfer states and electroabsorption of C₆₀, electrical conductivity, superconductivity, ESR properties, fullerene clathrates, C₆₀/C₇₀ complexes with organic donors, fullerene adducts, hydrogenated fullerenes, metallofullerenes and carbon nanotubes are discussed.     

Synthesis of Fullerenols from Halofullerenes

Halogenated fullerenes were used as reagents to prepare fullerenols. Depending on reaction conditions two types of substances (complex mixtures of products with average compositions C₆₀O_nH_m, n=10-26, m=14-30 and C₆₀O_nH_mM_k, M=K, Na; n=17-24, m=16-28, k=3-8) possessing essentially different water solubility were obtained. Highly soluble metal-containing species are most likely ionic compounds similar to metal derivatives of alcohols-alcoholates. An electron withdrawing effect of the carbon cage could make alkali metal fullerenolates (C₆₀(OH)_x(OM)_y) inert towards hydrolysis. Halogens were selectively substituted when fullerene bromides and chlorides were treated with silver trifluoroacetate. Fullerenol esters C₆₀(OOCCF₃)_n formed were then hydrolyzed to form C₆₀(OOCCF₃)_x(OH)_y. Attempts to cleave all ester groups in C₆₀(OOCCF₃)_x(OH)_y failed in acidic media, while alkaline hydrolysis was accompanied by unselective polyhydroxylation of the fullerene cage.     

B3LYP/6-31G*//SAM1 Calculations of C36 Fullerene and quasi-Fullerene Cages

The recently isolated fullerene C₃₆ is computed by the SAM1 method with energetics refined at the B3LYP/6-31G* level. Twelve low-energy cages are considered in detail, exhibiting both fullerene (pentagons and hexagons) and quasi-fullerene (also squares and heptagons) pattern. In contrast to other fullerene systems, the SAM1 and B3LYP/6-31G* methods do produce somewhat different energetics. Consequently, the equilibrium isomeric composition at high temperatures is different, too. In the best affordable B3LYP/6-31G* approach the fullerene cage with the topological D_6d symmetry represents the most populated species at high temperatures. Hence, an agreement with the solid-state NMR experiment is achieved at the B3LYP/6-31G* theoretical level.     

Electronic structure and electrical transport characteristics of C60, 2C60 and 4C60 fullerene molecules

The extended H?ckel method and the Green s function method were used to calculate the electronic structure and electrical transport of Au electrode-C₆₀, 2C₆₀ or 4C₆₀ fullerene-Au electrode systems. Furthermore, their electronic structure and electrical transport characteristics were compared and analyzed. The results show that (i) owing to the contact with the Au electrodes, the C₆₀, 2C₆₀ and 4C₆₀ molecules change in their electronic structures significantly, and their energy gaps between LUMO and HOMO are narrow; (ii) the bonding between C₆₀, 2C₆₀ or 4C₆₀ fullerene and Au electrodes is partially covalent and partially electrovalent; and (iii) the conductance of the three fullerenes conforms to the order of C₆₀>2C₆₀>4C₆₀.