Synthesis, Isolation And Structural Characterization Of Large Fullerenes And Ca-containing Large Endohedral Fullerenes

Synthesis, extraction, isolation and structure resolution of large empty fullerenes and Ca-containing large endohedral fullerenes are explored systematically. A series of above mentioned samples isomer free with high purity are obtained. Optical absorption and mass spectrum characterizations, single crystal X-ray diffraction measurement together with density functional theory calculation were performed to enlighten possible structure properties of these large cage empty fullerenes and the endohedral ones.General synthetic method for fullerene - direct current arc (DC) discharge method was tried optimization experimentally. High temperature precalcination and opposite direction discharge were adopted, which increased the yields effectively. Raw soot are extracted in o-dichlorobenzene solvent repeatedly, and it is found to have special effect for the enrichment of large fullerene and endohedral cages with this treatment. During multi-step high-performance liquid chromatography (HPLC) isolation, different separation methods were tried. Chlorobenzene was used as mobile phase at the first stage on the Buckyprep M column to remove C60, C70, C84 and other small cage fullerenes for the first time reported, which greatly increases the isolation and enrichment efficiency of large cage comparing with conventional isolation method with toluene flow. Several kinds of columns cooperating with various kinds of mobile solvents xylene, toluene, benzene as HPLC conditions, large cage Ca-containing endohedral fullerenes such as Ca@C82, Ca@C84, Ca@C88, Ca@C90(â… ,â…¡), Ca@C92, Ca@C94(â… ,â…¡) and Ca@C96 are completely isolated isomer free. Among those, Ca@C92, Ca@C94(I, II) and Ca@C96 are the largest Ca-containing endohedral fullerene till now, which were isolated for the first time without ever reporting. Ca@C92 and Ca@C94(I) gave similar ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra with those of Yb@C92 and Yb@C94, respectively, indicating that they might possess very similar cage structures. However, Ca-containing endohedral fullerenes have distinctly different absorption spectra with those of+3 oxidation state endohedral fullerenes such as Er and Dy. As Ca possesses constant+2 oxidation state in Ca-metallofullerenes, the above similar property suggests that Yb probably possess +2 oxidation state too. So Ca could be an indicator of charge state for other rare earth endohedral fullerene encapsulated, and it is very meaningful for us to estimate the oxidation state of other metals incased in their endohedral fullerene cages by comparing their UV-Vis-NIR absorption spectra with those of the corresponding Ca-containing endohedral fullerenes.We also obtained a series of large empty fullerene and their isomers such as C86, C88, C90(â… ,â…¡), C92, C94(â… ,â…¡), C96(â… ,â…¡,â…¢,â…£), C100 and C102. It is for the first time the C96(â… ,â…¡,â…¢), C100 and C102, which are the largest cage, were isolated up to now. Among the four isomers of C96, C96(â… ) shows especially short retention time on Buckyprep M column, while C96(â…¢) and C96(â…£) show higher stability and abundance. These four C96 isomers show different colors as well as spectroscopic characteristics. Most Ca-containing large endohedral fullerenes and large empty fullerenes synthesized in our experiment belong to small band gap series according to their optical absorption onset values.Theoretical calculations were carried out simultaneously for the four C96 isomers. We firstly performed semi-empirical calculations (AM1 and PM3) on C96 IPR cages, selecting 20 low-lying energy isomer cages, and then further calculations using density function theory (DFT) with high precision level on those low-lying energy cages were thoroughly done. At B3LYP/6-31G* level, we calculated the total energy and the band gap values of HOMO-LUMO for each of the above 20 cages. As a result, both thermodynamically and kinetically stable cages are found. Possible structures of the four C96i somers obtained experimentally were tried assigning by the calculation results jointing with their HPLC behavior characteristics.181:C2, the most stable cages by calculation results, was tentatively assigned to C96(â…£), which showed much higher abundance experimentally. C96(â… ) showed much shorter chromatographically retention time, so it seems reasonable to be associated with 183:D2 cage, possessing relative zero dipole value and being round shape. The possible structures for C96(â…¡) and C96(â…¢) are likely to corresponding to 146:Cs and 182:C2cage symmetries in our primary conjecture, respectively. On the other hand, Ca@C94(I) successfully co-crystallized with Niâ…¡(octa-ethylporphyrin) in toluene solvent. Single crystal of the largest cage metallofullerene at present Ca@C3v-C94Ni(OEP)2(CH3C6H5) was obtained and structure determined by single crystal X-ray diffraction measurement, followed by structure resolution. Crystal structure indicated that, the cage possessed of 134:C3v cage symmetry. Although there was somewhat disorder of the cage position, but it has relatively higher order level than most other crystal structures of endohedral fullerene presented. The Ca ion located below one of the 6:6 ring junctions that radiated from the C3 axis close to fullerene cage surface. The closest Ca-C distance is 2.358 A. This particular short Ca-C distance suggests that there was strong covalent function between Ca ion and the carbon cage. DFT calculations further revealed that Ca@C94-134:C3v was the most stable one among all those IPR C94 cage Ca endohedral fullerenes.