On the Limitation of Net Algorithm for Enumerating Isomeric Structures of Giant Fullerenes

Application of our net algorithm to the generation of all possible IPR isomers for giant fullerenes C_n, n=102 to 120, missed nine out of 39,621 possible structures. Analysis revealed that the omission of too small and too large cap triangles was the reason. Within the range of fullerenes studied, the missed structures are of high-energy and do not affect the distribution of significantly low-energy isomers.     

Doubly Bonded C60 Dimers and Congeners: Computational Studies of Structures, Bond Energies and Transformations

Initial steps of thermal transformation from doubly bonded [2+2] (1) and [4+4] (2) dimers of C₆₀ have been analyzed on the basis of computed structural features and Pople's energy partition scheme. Completely conjugated C₁₂₀ structures 3 and 4 are found to be considerably stable and proposed to be important intermediates. The linkage patterns in 3 and 4 are also likely to appear in the repeating units of the metastable dimer and polymer phases of A₁ crystals.     

Thermal Transformation of Ring-Opened [2 2] C60 Dimer into a Wide-Bridged C120 Isomer. A Computational Evaluation of Fulvalene-Naphthalene Rearrangements

As a continuation of the studies on thermal transformation of the [2+2] C₆₀ dimer (1), the consequence of the pyracylene-rearrangement-like valence isomerization of the fulvalene partial structure at the bridge of the ring-opened product from 1, namely 2, was searched by dynamic reaction coordinate /AM1 semiempirical MO calculations. It is predicted that the fulvalene bridge of 2 rearranges into naphthalene partial structure by the concerted 'in-plane' mechanism to give a wide-bridged C₁₂₀ intermediate having twenty five-membered rings and two ten-membered rings (3). The computed energy of activation (145 kcal/mol) is 40 kcal/mol lower than those computed for pyracylene rearrangements. In contrast, the recently reported analogous rearrangement of indigo (13) to dibenzonaphthyridindione (14) is computed to occur by the stepwise 'sp³' mechanism.     

Detection of Buckminsterfullerene in Usual Soots and Commercial Charcoals

Abstract

It was confirmed that soot produced by free burning of small hydrocarbons like benzene and cyclohexane generally contains less than 0.1 ppm of C₆₀. Small, but measurable amounts of C₆₀ were found in two samples of charcoal, providing the first indication of fullerene formation in the solid state.     

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

Abstract

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 _d C₁₂₀ global minimum 5 was not reached. Semiempirical vibrational calculations predict characteristic transition in the vibrational spectra in the course of rearrangement pathway.     

Thermodynamic Properties of He@C60

Contributions to thermodynamic properties of the endohedral He@C₆₀, resulting from motion of He inside the cage, are calculated. The contribution to C_v shows a maximum at low temperatures. The maxima for ³He@C₆₀ and ⁴He@C₆₀ are at about 41 K and 30 K, respectively.     

Natural Fullerenes-Will they Offer a Hint to the Selective Synthesis of Fullerenes?

In addition to the known laboratory methods of preparing fullerenes, which generally start from carbon vapor, studies of naturally occurring C₆₀/C₇₀ suggest a catalytic process of fullerene formation that proceeds in solid phase under mild condition. Recent observation of C₆₀/C₇₀ in Yunnan coal and Kalerian C-rich rock is reviewed in the light of the above perspective.     

Metal-Coated Fullerenes C60Mn: Calculations for M = Be,Mg, Al AND n = 12, 20, 32

Abstract

Semiempirical quantum-chemical PM3 calculations are reported for a relatively new class of exohedral metallo-fullerenes - metal-coated or metal-covered fullerenes: C₆₀M_n. The exohedral species were recently observed, however, their geometrical and electronic structure is not known yet. In this paper, relatively-even metal-atom distributions over the fullerene rings are considered - such regular forms are computed for M= Be, Mg, Al. Three selected stoichiometrics are treated: C₆₀M₁₂, C₆₀M₂₀, and C₆₀M₃₂. The stoichiometrics correspond to the location of the metal atoms above the twelve pentagons, above the twenty hexagons, and above each of the thirty two rings of C₆₀ This interesting arrangement over 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 methods - the computations are performed with the PM3 semiempirical method and metal-layer atomization heats are used as a stability measure. Structural characteristics are presented, too. Considerable reductions of the cage symmetry are reported and their relationships to Jahn-Teller effect are discussed, too (no case of the icosahedral symmetry is found).     

Computations of Low-Energy Non-Icosahedral Structures of C6− 60

Abstract

Owing to the three-fold degeneracy of the LUMO in C₆₀, its hexa-anion is not subjected to Jahn-Teller distortions. In contrast to the accepted presumptions, however, computations at the MNDO, AMI, PM3, SAM1, HF/STO-3G, HF/3-21G, and HF/4-31G levels show that the completely relaxed, non-icosahedral cage of C^6? ₆₀is lower in energy. The computed energy gain varies between 60 and 150 kJ/mol and thus, it is consistently significant (the most sophisticated approach, HF/4-31G, yields 93 kJ/mol). The longest C-C bond in the relaxed structures is at most computed 0.05 Å longer compared to the 5/6 bond in the icosahedral C^6? ₆₀The symmetry of C^6? ₆₀is relaxed to D₂ A C_2v, isomer is discussed, too.     

Stone-Wales Rearrangement as the Double Olefin-Carbene 1,2-CC Bond Shift. Denied Role of Triplet Biradicals

Abstract

Stone-Wales rearrangement may be considered to consist of two contiguous steps of olefin-carbene 1,2-C-C bond shift. Computational study on this mechanism led to high activation energies comparable to the bond dissociation energies of C-C bonds in fullerenes and their precursors. the possibility of passing through triplet transition state is denied on computational grounds.