Eiji sawa

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.

Fullerenes in Solid Bitumens from Pillow Lavas of Precambrian Age (MÍTOV, Bohemian Massif)

Abstract At Mitov solid bitumens occur in pillow lavas cutting black shale formation of Precambrian age. They represent non-graphitised carbons and contain about 0.2 ppm of C60.

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.

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

Abstract 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 D4d 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.

Analysis of the Growth Mechanism of Carbon Nanotubes by C2 Ingestion

Abstract Cap structures capable of sustaining infinite growth by ingestion of C2 fragments to give a carbon nanotube were screened systematically. For this purpose a total of 23,296 IPR cap structures having characteristic tubule vectors of (5∼20, 0∼10) were generated by the use of net algorithm. Eighteen recurring patterns were found which involve 50 cap structures having tubule vectors (6∼11, 5).

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

Abstract 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.

A Computational Treatment of 35 IPR Isomers of C88

Abstract The complete sets of 35 isolated-pentagon-rule (IPR) isomers of C88 is described by the SAM1 (Semi-Ab-initio Model 1) quantum-chemical method. The separation energetics is also computed at the HF/STO-3G, HF/3-21G, and HF/4-31G levels. The SAM1 and HF/4-31G data mostly agree within a few kJ/mol. As the SAM1 energetics does not reproduce the recent NMR observations, entropy contributions are included, too, being based on the harmonic-oscillator and rigid-rotator model. Considerable temperature effects on the relative stabilities in the system are found. The ground-state structure of C88 is a C 3 isomer, however, with an increase of temperature a C 2 structure becomes important. At still higher temperatures a near C 2 species is dominant. The results can be viewed as a good agreement with the available observations, and they further expand the family of the IPR sets where the thermodynamic equilibrium treatment allows for a satisfactory support of observations.

Natural fullerenes : Will they offer a hint to the selective synthesis of fullerenes ?

Abstract 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.

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.

Numerical Generation of Nanotube Caps

Abstract A new method for the numerical generation of nanotube caps based on unfolding of caps onto honeycomb lattice is developed. Expressions connecting the components of the tubular vector and coordinates of pentagonal defects on the honeycomb lattice are derived. Numerical solutions of the Diophantine equations give a full set of all possible nanotube caps. We find seven caps with isolated pentagons for the (10,0) nanotube (instead of the known three), and one cap for the (9,1) nanotube.