Browsing by Author "Simeonov, Kalin"

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Higher fullerenes : isolation, halogenation and structural studies
(2009) Simeonov, Kalin; Jansen, Martin (Prof. Dr. Dr. h.c.)
The present work might formally be subdivided into two sections. Within the frames of the first section, the chromatographic methods, systems and techniques used in obtaining of isomerically pure fullerene species in preparative amounts are widely discussed. The following eleven fullerene isomers have been obtained in bulk amounts: D2-C76(1), D3-C78(1), C2v-C78(2), C2v-C78(3), D3h-C78(5), D2-C80(2), a C2-C82, a C2-C84, Cs-C84(14), a D2-C84 and a D2d-C84. Among these, D3h-C78(5) has been isolated for the first time and its formation during the inductive heating of graphite confirmed. Newly developed and improved chromatographic systems are introduced in order to facilitate the laborious and resource-consuming isolation procedures. The inability of the specially designed for fullerene separation stationary phases to provide reasonable resolution between C78 isomers 1 and 2, as well as the inefficiency of the commercially available octadecylsilyl phases commonly used for that purpose have justified the search for more efficient methodologies capable of delivering preparative amounts of these two fullerenes. Herein, an alternative approach in this direction is suggested. The isolation of the individual species is achieved through chemical modification (with ICl) of the initial fullerene mixture followed by the one-step baseline chromatographic separation of their derivatives. Through thermal dissociation of the respective fullerene halides, the parent species are obtained in isomerically pure form. The parent fullerenes are obtained in isomerically pure form through thermal dissociation of the respective fullerene halides. The second section of the present work deals with the single crystal X-ray studies of the crystal structures of the following fullerene halides: C76(1)Cl18, C78(2)Cl18, C78(3)Cl18, C78(5)Cl18 and C80(2)Cl12. Detailed analyses of the crystal, as well as the molecular structures of the reported compounds have shown some interesting features. On the basis of careful examinations of the addition patterns of chlorine atoms in all five fullerene halides, a proposal of a principle of higher fullerene reactivity has been made. The stability of the chlorinated fullerenes, as well as the selectivity of chlorine addition is explained by the relief in steric strain in the products compared to that in the parent species. The thorough analyses of the five crystal structures have resulted in the discovery of an interesting phenomenon apparently intrinsic to fullerene halides in general. The presence of numerous short intermolecular chlorine-chlorine contacts has been encountered in all structures (in C78(2)Cl18 . Br2 . TiCl4 also short chlorine-bromine contacts exist). The assumption that attractive interactions between the chlorine atoms belonging to neighboring fullerene molecules exist has been proven and a reasonable explanation for the contacts' nature given. Quantum chemical calculations aiming at further elucidation of the nature, as well as the magnitude of these attractive intermolecular forces have been initiated.