Theoretical Studies On The Structures And Properties Of (4,6)-Fullerenes

Fullerene is a joint name of the closed hollow cages composed of carbon atoms. Fullerenes usually can be divided into two categories. One is the classical fullerenes like C60, which are made up of only five and six membered rings. The other one is the nonclassical fullerenes in whose structures there are not only five membered rings (5-MRs) and six membered rings (6-MRs), but also some other polygon rings. The nonclassical fullerenes are indispensable members of the fullerene family and play an important role in the increase and fracture of fullerenes and the preparation of embedded fullerene derivatives. Recently, the topological rules of (3,6)-and (4,6)-fullerenes had been systematically deduced by Ashrafi and Mehranian. The two-dimensional carbon material of 6-MRs and 4-MRs had been theoretically proposed, from which the (4,6)-fullerenes can be formed by rolling up. In experiment, the benzene-like antikekekulene had been nearly synthesized, which consists of six 6-MRs connected by six 4-MRs. From the point of topology,3-MRs,4-MRs even 7-MRs,10-MRs and so on can be combined to form a closed three-dimensional geometrical structure. On the basis of the results of these studies, in this paper we will expect to construct some nonclassical fullerenes with different polygonal rings as the fragments and try to find some stable nonclassical fullerenes.Based on the structure design program, we have constructed a series of (4,6)-fullerenes and researched their geometric structure, bonding characteristics, thermodynamic stability, aromaticity and other properties. The main contents and conclusions of this paper are as followings:1. In this paper, we designed a structure generating program based on the point group symmetry parameters. Using the program, we generated a series of (4,6)-fullerenes, including the Ih C120, C180, C240, C300, C360, C360’, C480 and the fullerenes C24, C48, C72, C96, C120, C144, whose initial symmetry are Oh. These fullerenes can be classified into two types:with the isolated 4-and 6-member rings and with the connected 4-and/or 6-member rings.2. Using the semi-empirical PM6, DFTB and the density functional theory(DFT) methods respectively to optimize the initial structures, we found the symmetry for Ih initial symmetry (4,6)-fullerenes have not changed, but for Oh initial fullerenes sometimes reduced to D4h, such as C48, C96, C120, C144. In addition, our calculated experience shows that it is difficult to calculate bigger fullerenes, such as C240, using density functional theory method.3. The energy research of (4,6)-fullerenes shows that in spite of these Ih and Oh (4,6)-fullerenes are about 0.13-0.32 eV/atom and 0.17-0.76 eV/atom less stable than the Ih C60 fullerene, it is possible to synthesize them from their element unit, antikekulene, which was nearly synthesized in experiment. (4,6)-fullerene Ih C360’and Oh C72 may be stable structures as high atomic binding energy and big HOMO-LUMO gap.4. Aromaticity studies shows that though the (4,6)-fullerenes are mainly spherical antiaromatic, the aromaticity of those fullerenes will be increase after losing some electrons. This indicates that the (4,6)-fullerenes should become more stable when losing some electrons by chemical modification, such as halogenations. In addition, the local aromatic indexes shows that all of the planar-conjugated rings are local aromatic in fullerenes and the proportion of six-member rings in (4,6)-fullerenes is higher, the more possible (4,6)-fullerenes have aromaticity, which provides a reasonable delocalization view consistent with our common chemical sense.