Li-Hua Gan

Nonclassical fullerenes with a heptagon violating the pentagon adjacency penalty rule

Nonclassical fullerenes with heptagon(s) and their derivatives have attracted increasing attention, and the studies on them are performing to enrich the chemistry of carbon. Density functional theory calculations are performed on nonclassical fullerenes Cn (n = 46, 48, 50, and 52) to give insight into their structures and stability. The calculated results demonstrate that the classical isomers generally satisfy the pentagon adjacency penalty rule. However, the nonclassical isomers with a heptagon are more energetically favorable than the classical ones with the same number of pentagon–pentagon bonds (B55 bonds), and many of them are even more stable than some classical isomers with fewer B55 bonds. The nonclassical isomers with the lowest energy are higher in energy than the classical ones with the lowest energy, because they have more B55 bonds. Generally, the HOMO–LUMO gaps of the former are larger than those of the latter. The sphericity and asphericity are unable to rationalize the unique stability of the nonclassical fullerenes with a heptagon. The pyramidization angles of the vertices shared by two pentagons and one heptagon are smaller than those of the vertices shared by two pentagons and one hexagon. It is concluded that the strain in the fused pentagons can be released by the adjacent heptagons partly, and consequently, it is a common phenomenon for nonclassical fullerenes to violate the pentagon adjacent penalty rule. These findings are heuristic and conducive to search energetically favorable isomers of Cn, especially as n is 62, 64, 66, and 68, respectively.

Geometrical and electronic rules in fullerene-based compounds.

The discovery of buckminsterfullerene C(60) opened up a new scientific area and stimulated the development of nanoscience and nanotechnology directly. Fullerene science has since emerged to include fullerenes, endohedral fullerenes (mainly metallofullerenes), exofullerenes, and carbon nanotubes as well. Herein, we look back at the development of fullerene science from the perspective of epistemology by highlighting the proposed main rules or criteria for understanding and predicting the structures and stability of fullerene-based compounds. We also point out that a rule or criterion may contribute significantly to the corresponding discipline and suggest that two unsolved issues in fullerene science are the addition patterns of fullerene derivatives and the structures and stability of nonclassical fullerenes.

A computational study on Sc2S@C68 and Sc2O2@C68

In order to predict the structures of the detected and assumed endohedral metallofullerene Sc2S@C68, and Sc2O2@C68, and provide insights into their structures and properties, we have studied all of the isomers of C68 and tens of candidate isomers of Sc2S@C68 and Sc2O2@C68. The results show that Sc2S@C68 shares the same parent cage as Sc2C2@C68:6073, however Sc2O2 is ready to be encaged inside C68:6094. The calculations demonstrate that the transferred electrons from the encaged Sc2S and Sc2O2 clusters stabilize the active cages. Sc2S is V-shaped inside the cage whereas Sc2O2 is square-like inside the cage. Interestingly, the two Sc atoms of Sc2O2@C68 have no tendency to bond with the two fused pentagons of C68:6094 and unusually form a Sc–Sc single bond. The calculations show that encagement of Sc2O2 in C68:6094 is more favourable than that of Sc2S inside C68:6073, and that the HOMO–LUMO gap of Sc2O2@C68 is evidently broader than that of Sc2S@C68. These results suggest that there is a distinct possibility that the detected compound is Sc2O2@C68 and at least Sc2O2@C68 is much easier to synthesize under similar experimental conditions. The MS and UV spectra are provided to help their structural identification in the future.

Theoretical Study on Experimentally Detected Sc2S@C84

Sc(2)S@C(84) has recently been detected but not structurally characterized.1 Density functional theory calculations on C(84) and Sc(2)S@C(84) show that the favored isomer of Sc(2)S@C84 shares the same parent cage as Sc(2)C2@C(84), whereas Sc(2)S@C(84):51383, which violates the isolated-pentagon rule, is the second lowest energy isomer with the widest HOMO-LUMO gap and shows high kinetic stability. The analysis shows that Sc(2)S@C(84):51575 is favored when the temperature exceeds 2,800 K and it can transform into the most favorable isomer Sc(2)S@C(84):51591. Molecular orbital analysis indicates that both Sc(2)S and Sc(2)C(2) formally transfer four electrons to the cage, and quantum theory of atoms in molecules analysis demonstrates that there is a covalent interaction between Sc(2)S and C(84):51591. The IR spectra of Sc(2)S@C(84) are provided to aid future structural identification.

The structures and stability of BnNn clusters with octagon(s)

The structures and stability of (BN)n clusters with alternate B and N atoms containing squares, hexagons and octagons ((BN)n-F4F6F8) are investigated by using density functional theory. The results demonstrate that the isomers of (BN)n-F4F6F8 clusters generally satisfy the isolated-square rule (ISR) and the square adjacency penalty rule (SAPR). The energetically favorable isomers generally have fewer square–square bonds, larger HOMO–LUMO gaps, lower sphericity and asphericity, as well as lower pyramidalization of B and N atoms than other structures. As a whole, the stability of (BN)n-F4F6F8 clusters decreases with the number of octagons. However, four isomers containing one or two octagons in four isomeric clusters (i.e. (BN)n-F4F6F8 (n = 19, 20, 23, and 24) are more thermodynamically stable than their (BN)n-F4F6 counterparts. Further structural analysis demonstrates that octagon(s) of (BN)n-F4F6F8 clusters can release the strain energy by decreasing the pyramidalization angles of the corresponding vertex. Finally, the entropy effect is examined to evaluate the relative stability of (BN)n-F4F6F8 (n = 19, 20, 23, and 24) clusters at high temperatures.

A Potential Superhard Material m-BCN

We here propose a new superhard material m-BCN with comparable Vickers hardness to cBN by the use of first-principles calculations. The calculations show that the mentioned m-BCN is a thermodynamically and kinetically stable semiconductor. Hydrostatic calculation shows that it is anisotropic and its incompressibility is very close to cBN. Structural analysis shows that its excellent mechanical property and thermodynamically stability are inherited from diamond and cBN. These results provide a new clue to find new superhard phase.