Eiji Ōsawa

Molecular Mechanics Calculations of Giant- and Hyperfullerenes with Eicosahedral Symmetry

Abstract Structures and energies of the first few members of giant- and hyperfullerenes with Ih symmetry have been calculated by using MM3.

C36 fullerenes and quasi-fullerenes: computational search through 598 cages

C36 is computed at the SAM1 level and partially also at the HF/4-31G and B3LYP/6-31G∗ levels. Altogether 598 cages are generated by a topological Stone–Wales treatment. Three cages contribute by more than 10% to the high-temperature equilibrium isomeric mixture – two conventional fullerenes with D2d and C2v symmetries and a Csquasi-fullerene containing one four-membered ring.

Computational studies of calix[4]arene homologs : influence of 5,11,17,23- and 25,26,27,28-substituents on the relative stability of four conformers

Abstract The effects of 5,11,17,23- and 25,26,27,28-substitutents on the relative stability of four conformers of calix[4]arene are discussed on the basis of a computational method using molecular mechanics (MM3) calculations. It is shown that a substituent in the lower rim site has a decisive role in the determination of the stability order among four conformers of calix[4]arene, while a substituent in the upper rim site only slightly affects the energy differences. It is also shown that in [1 4 ]metacyclophanes with no substituent in the lower rim the structure of 1,2-alternate conformers is very different from the typical 1,2-alternate structure common to calix[4]arenes.

How many conformers are there for small n-alkanes? Consequences of asymmetric deformation in GG′ segment

Abstract Conformers have been generated exhaustively or nearly so for n-heptane, n-octane, n-nonane and n-decane using a new conformational space search program in combination with MM2 as the minimizer. When the duplications due to symmetry are ignored, they number 109, 347, 1101 and 3263, respectively. These numbers are greater than 3 n (n is the number of internal C-C bonds), mainly due to the fact that a GG′ sequence in the backbone chain deforms asymmetrically and approximately doubles the number of high-energy conformers. Based on these results, the aspects of conformer distribution in linear molecules are discussed. The proportion of high-energy conformers to the total number is predicted to increase rapidly with the increase in the chain length, whereas low-energy conformers having no GG′ sequence increases only by 2.4 n .

Computational modeling of the elemental catalysis in the Stone–Wales fullerene rearrangements

Abstract Catalytic effects on the kinetics of the Stone–Wales fullerene transformation are studied computationally. The catalytic agents are represented by free elements, neutral or charged. The computations are performed at semiempirical (PM3) and DFT (B3LYP/6-31G*//PM3) levels on a model bowl-shaped fragment C 34 H 12 . The semiempirical and DFT activation energies agree reasonably well. In all computed cases, the activation barrier is lowered compared with that of the uncatalyzed reaction. The kinetic barriers for the catalyzed rearrangements increase in the following order: N, H, O, P, S, B, Cl, C, F, Li, Se, Fe, Hg, Zn, Si, Sn, Ge, Mg, and Al. Nitrogen atoms are pointed out as especially potent catalytic agents. At the PM3 computational level, the isomerization kinetic barrier is reduced to 193, 110, and 342 kJ mol −1 for the N + , N, and N − species, respectively. If the activation barriers are re-computed at the B3LYP/6-31G*//PM3 level, they are changed to 76, 105, and 323 kJ mol −1 for the N + , N, and N − species, respectively. As small amounts of nitrogen (as well as other elements) are available in virtually any kind of fullerene synthesis, the study offers a computational support for kinetic feasibility of the Stone–Wales fullerene transformation.

C90 temperature effects on relative stabilities of the IPR isomers

Abstract The complete set of 46 isolated-pentagon-rule (IPR) isomers of C 90 is treated by the SAMI quantum-chemical method (Semi-Ab-Initio Model 1), and their energetics is also checked by ab initio SCF computations (HF/3-21G, HF/4-31G) and the AM1 and PM3 semiempirical methods. All the methods point out a C 2 species as the system state (with an exception at the HF/3-21G level). However, the energetics itself is not able to produce a good agreement with recent observations (one C 2 v , three C 2 , and one C 1 , separeted into three HPLC fractions). The symmetries of the five SAM1 lowest-energy structures are: C 2 , C 2 v , C s , D 5 h , C 1 . In order to respect temperature effects on relative stabilities, entropy contributions are also computed and significant changes are found. The symmetries of the five structures most populated in a high-temperature region are according to the SAM1 computations: two times C 2 , C s , C 2 v , C 1 . One of the recently reported HPLC fractions shows 70 lines in the 13 C NMR spectrum, assigned to a C 2 species (45 lines) and C 2 v species (25 lines, 5 weaker). There is however an alternative interpretation of the spectrum: a C s (46 lines, 2 weaker) and a C 2 v species (24 lines, 3 weaker). The last two structures are indeed present in the SAM1 high-temperature stability set so that agreement between observations and calculations can be achieved.

Viewpoint 11 — approaches to the global minimum problem

Abstract Algorithms of conformational space search, namely, methods of ranking conformational isomers of flexible molecules, are discussed with emphasis on the search of low-energy regions. Perspectives on future developments in the saddle point search and the conformational interconversion paths are also mentioned.

Further developments in the algorithm for generating cyclic conformers. Test with cycloheptadecane

Abstract Program CONFLEX reached an unprecedented level of performance as shown by its ability to find all of the 262 conformers of cycloheptadecane known to exist within 3 kcal/mol from the global energy minimum within reasonable computer time.

Shape and Fantasy of Fullerenes

One of the many wonders that fullerenes have brought to us during the past few years is the variety of their shapes. When the elusive C 60 finally showed up in 1990, the perfect symmetry and astounding beauty of its molecular structure touched the hearts of scientists before they could consider the molecules vast technical possibilities. Already much has been said about the unique shape of C 60 and its potentialities. C 70 and higher fullerenes have simultaneously been found in the same soot that produced C 60 and were quickly revealed to be shaped like rugby balls or oblong eggs. Hence we were aware that there had to be an extensive series of roundish polyhedral clusters of carbon atoms. Then, in the following year, multilayered tubular fullerenes (Figures 1a and 1b) were discovered by Iijima and were named buckytubes (see the article by Iijima in this issue). Iijima also observed similarly huge and multilayered carbon balls, before C 60 was discovered. Soon after, buckyonions were recognized as an important class of fullerene (Figure 1c, see article by Ugarte in this issue). So, in the early days of fullerene research, we already knew three forms of fullerene: sphere, tube, and particle. At that time, however, nobody anticipated that this was only the beginning of a big show of stunning variations in the shapes of fullerenes. This article introduces current developments in the study of these fullerene styles.

Computations on nineteen isolated-pentagon-rule isomers of C86

Abstract The complete set of nineteen isolated-pentagon-rule isomers of C 86 is described by the SAM1 (semi-ab-initio model 1) quantum-chemical method, and their energetics are checked by ab initio SCF computations. Considerable temperature effects on the relative stabilities in the system are found. The ground state structure is a C 2 isomer but at elevated temperatures other structures also become significant.