The Origin And Structural Progress Of Fullerenes In Arc-discharge

In the past three decades,great enthusiasms and efforts have been devoted to the formation mechahism of fullerenes,and several possible mechanism models have been proposed.However,there is still no models can explain all the experimental phenomena due to the extremely violent synthesis conditions and complex system.It is well known that the further understand:ing of the formation mechanisms plays an essential role in the development of fullerenes and even all carbon clusters.Therefore,I arrt deeply convinced that a breakthrough in the origin of fiullerenes means a deeper understand:ing of fullerenes and a ftuther step toward truth.The original purpose of this paper is to study the formation and growth mechanisms of fullerenes in arc-discharge.Based on the view of structural progress of fullerenes,this work focuses the two following aspects:On the one hand,we focus whether fullerenes self-assemble by the mechanism of"bottom.up" or "top-down" in the arc macroseopieally starting from the idea to search for the origin of ffillerenes.The smallest carbon clusters C1-C5 were captured through introducing H2 into the arc reactor and unambiguously characterized by X-ray single crystal diffraction.Further controlled experiments made it clear that there are abundant small carbon clusters in the process of arc-discharge,and they will finally grow to fullerenes-For the first time,this work proves:in molecular level that fullerenes self-assemble in the arc in the "bottom-up" approach,which states fullerenes start from carbon atoms or small carbon clusters,and ultimately grow to cage-like structures.On the one hand,we focus how the fullerenes grow microscopically after they are caged starting from the idea of structural progress of fullereries.According to the well-known "Isolated Pentagon Rule",there is at least one an IPR-violated fullerene abbreviated as C2n+2 in the growth process from an IPR-obeyed fullerenes C2n to another IPR-obeyed fullerenes C2n+2.This means the structural process of fullerenes will be clarified in molecular level if there are enough non-IPR fullerenes captured.Based on this idea,we starts with the endohedral fullerenes Sc2C2@C2n,of which ten structures have been characterized and published in the past eighteen years.However,most of them are IPR-obeyed structures.In terms of current situation,I believe the important non-IPR intermediates are still distributed in the soot containing Sc2C2@C2n,Therefore,I carefully search for these intermediates in the chromatography and finally isolated eleven new structures.Although all products are very low in content and it is estimated to be only tens to hundreds of micrograls,occupying only one in a million to one in ten millions in soot,seven samples are lucky enough to be characterized unambiguously by X-ray crystallography so far.As expected,there are six new structures are IPR-violated.According to the topological analysis of these non-IPR structures,we make a surprising conclusion that the formation mechanisms of empty fullerenes and endohedral fullerenes are different because of the charge transfer from the endo-units,resulting in different relative stability of carbon cages and even different structural progress.Furthermore,endohedral fullerenes with different endo-units show different structural progress based on the topological comparison of Sc2C2@C2n and M3N@C2n-In summary,this work proves firstly in molecular level that the growth mechanism of endohedral fullerenes are driven by the endo-units,and then proposed a new growth mechanism,namely "endo-units driven growth".In addition,an important deduction can be concluded that the endo-units are encapsulated by the fullerenes before they are caged.The endo-metal could be coordinated with a bowl-shaped carbon cluster,which will continuously grow by absorbing small carbon clusters such as C1 and C2 to form closed cage.At the meantime,the system of endo-metal and bowl-shaped carbon clusters could absorb bridged unit such as N,S,O or C2 etc.to form clusterfullerenes,or fuse together to form dimetaliofullerenes and trimetallofullerenes,resulting in various endohedral fullerenesIn addition,based on the analysis of the new structures,we found the structures of endo-units depends not only on the size and shape of carbon cage and the location of the endo-metals.but also on the different arrangement of the local structure of the carbon cage.This result shows a unique "local aromaticity" of endohedral fullerenes providing a valuable case for thoroughly understanding the detailed interaction among core-shell structures.Furthermore,an endo-metal-metal bond was also found by comparing the ordered crystal data of Sc2C2@C3v(8)-C82 and Sc2@C3v(8)-C82.The unambiguous determination of the endo-metal-metal bond and the accurate measurement of the bond length give us new insights in atomic level resolution to further understanding the bond formation of metal-metal bond,especially in unique environments.