Chlorination of C86 to C84Cl32 with Nonclassical Heptagon-Containing Fullerene Cage Formed by Cage Shrinkage

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Double Negatively Curved C70 Growth through a Heptagon-Involving Pathway

All previously reported C₇₀ isomers have positive curvature and contain 12 pentagons in addition to hexagons. Herein, we report a new C₇₀ species with two negatively curved heptagon moieties and 14 pentagons. This unconventional heptafullerene[70] containing two symmetric heptagons, referred to as dihept-C₇₀ , grows in the carbon arc by a theoretically supported pathway in which the carbon cluster of a previously reported C₆₆ species undergoes successive C₂ insertion via a known heptafullerene[68] intermediate with low energy barriers. As identified by X-ray crystallography, the occurrence of heptagons facilitates a reduction in the angle of the π-orbital axis vector in the fused pentagons to stabilize dihept-C₇₀ . Chlorination at the intersection of a heptagon and two adjacent pentagons can greatly enlarge the HOMO-LUMO gap, which makes dihept-C₇₀ Cl₆ isolable by chromatography. The synthesis of dihept-C₇₀ Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon-clustering process.

Hybrid ADFT Study of the C104 and C106 IPR Isomers

This work presents a hybrid auxiliary density functional theory (ADFT) study of the neutral and hexaanionic C₁₀₄ and C₁₀₆ fullerenes with the aim to determine their ground state structures. To this end, all C₁₀₄ and C₁₀₆ fullerene structures that obey the isolated pentagon rule (IPR) were optimized with the Perdew-Burke-Ernzerhof generalized gradient approximation followed by a single-point energy calculation with the PBE0 hybrid functional. Our studies show that this composite approach yields relative energies of giant fullerenes that are accurate to around 1 kcal/mol. As a result, the ground states of C₁₀₄, C₁₀₄^6-, and C₁₀₆^6- can be assigned to the isomers 234:C_s, 821:D₂, and 891:C_s, respectively. On the other hand, the energetically lowest lying IPR isomers of C₁₀₆, 331:C_s, 1194:C₂, 534:C₁ are separated by less than 1 kcal/mol which makes an unequivocal ground state assignment by hybrid DFT methods impossible. To guide future experiments, we also report the simulated IR and Raman spectra of the most stable neutral and hexaanionic C₁₀₄ and C₁₀₆ fullerenes.

Salvaging Reactive Fullerenes from Soot by Exohedral Derivatization

The awesome allotropy of carbon yields innumerable topologically possible cage structures of molecular carbon. This field is also related to endohedral metallofullerenes constructed by metal-atom encapsulation. Stable and soluble empty fullerenes and endohedral metallofullerenes are available in pure form in macroscopic amounts from carbon arc production or other physical processes followed by extraction and subsequent chromatographic separation. However, many other unidentified fullerene species, which must be reactive and insoluble in their pristine forms, remain in soot. These "missing" species must have extremely small HOMO-LUMO gaps and may have unconventional cage structures. Recent progress in this field has demonstrated that reactive fullerenes can be salvaged by exohedral derivatization, which can stabilize the reactive carbon cages. This concept provides a means of preparing macroscopic amounts of unconventional fullerenes as their derivatives.

Fabrication of a Contamination-Free Interface between Graphene and TiO2 Single Crystals

The uniform and seamless interface between graphene and semiconductors, that is, without adsorbates or contamination in between, is of importance for optimizing the electronic and catalytic performances of the combined system. In this work, we try to synthesize graphene directly over atomically flat TiO₂ single-crystal surfaces using chemical vapor deposition (CVD) with acetylene as the carbon source. The facile synthetic conditions facilitate the formation of ultrathin polycrystalline graphene films with nanosize domains, while reasonably maintaining the terrace-and-step morphologies of the TiO₂ surfaces. The established recipe can thus lead to the construction of a contamination-free interface between graphene and reducible oxides and also provide a well-defined platform for further investigations into the physicochemical properties of the graphene-oxide complex system from an atomic/molecular level.

C100 is Converted into C94 Cl22 by Three Chlorination-Promoted C2 Losses under Formation and Elimination of Cage Heptagons

Chlorination of the C100 (18) fullerene with a mixture of VCl4 and SbCl5 gives rise to branched skeletal transformations affording non-classical (NC) C94 (NC1)Cl22 with one heptagon in the carbon cage together with the previously reported C96 (NC3)Cl20 with three heptagons. The three-step pathway to C94 (NC1)Cl22 starts with two successive C2 losses of 5:6 CC bonds to give two cage heptagons, whereas the third C2 loss of the 5:5 CC bond from a pentalene fragment eliminates one of the heptagons. Quantum-chemical calculations demonstrate that the two unusual skeletal transformations-creation of a heptagon in C96 (NC3)Cl20 through a Stone-Wales rearrangement and the presently reported elimination of a heptagon through C2 loss-are both characterized by relatively low activation energy.

Structures of chlorinated fullerenes, IPR C₉₆Cl₂₀ and non-classical C₉₄Cl₂₈ and C₉₂Cl₃₂: evidence of the existence of three new isomers of C₉₆

Chlorination of various HPLC fractions of C96 with a mixture of VCl4 and SbCl5 at 340-360 °C and single-crystal X-ray diffraction study of the products led to the identification of three new IPR isomers of C96. The C96(175) isomer forms a stable chloride, C96(175)Cl20, while chlorides of two other new isomers, C96(114) and C96(80), undergo cage shrinkage yielding C94(NC1)Cl28 and C96(NC2)Cl32 with non-classical (NC) cages. These two NC chlorides contain, respectively, one and two heptagons flanked by pairs of fused pentagons and are stabilized by chlorine attachment to the emerging pentagon-pentagon junctions. Thus, the number of the experimentally confirmed C96 isomers has reached nine, which corroborates the empirical rule that the C(6n) fullerenes exhibit particularly rich isomerism.

Chlorination of IPR C100 fullerene affords unconventional C96 Cl20 with a nonclassical cage containing three heptagons

Chlorination of C100 fullerene with a mixture of VCl4 and SbCl5 afforded C96Cl20 with a strongly unconventional structure. In contrast to the classical fullerenes containing only hexagonal and pentagonal rings, the C96 cage contains three heptagonal rings and, therefore, should be classified as a fullerene with a nonclassical cage (NCC). There are several types of pentagon fusions in the C96 cage including pentagon pairs and pentagon triples. The three-step pathway from isolated-pentagon-rule (IPR) C100 to C96(NCC-3hp) includes two C2 losses, which create two cage heptagons, and one Stone-Wales rotation under formation of the third heptagon. Structural reconstruction established C100 isomer no. 18 from 450 topologically possible IPR isomers as the starting C100 fullerene. Until now, no pristine C100 isomers have been confirmed based on the experimental results.

Capturing the most-stable C56 fullerene cage by in situ chlorination

The most-stable (#916)C(56) carbon cage has been captured by in situ chlorination during the radio frequency furnace process. The resulting exohedral (#916)C(56)Cl(12) was separated and unambiguously characterized by single crystal X-ray structure determination. The discovery of (#916)C(56) provides evidence for a thermodynamically controlled mechanism of fullerene formation, and on the other hand shows that the in situ chlorination does not remarkably influence the fullerene formation itself but just results in the capture of preformed cages. A detailed analysis of the chlorination pattern of (#916)C(56)Cl(12) reveals the main factors controlling the reactivity of non-IPR fullerenes. A high degree of aromatization was observed in the remaining π-system by considering geometric criteria and nucleus-independent chemical-shift analysis (NICS). Along with the well-known stabilization of pentagon-pentagon junctions during chlorination, the formation of aromatic islands plays an important role in the stabilization of the fullerene cage and also in the determination of the chlorination pattern. Based on these empirical rules, the preferable addition patterns for non-IPR fullerene cages can be easily predicted.