Synthesis and structural characterization of C70H38.

Abstract

Theoretical and experimental studies on addition reactions of fullerenes have been reported over the last few years as these reactions can lead to many different materials with various chemical and optical properties. The report of improved efficiency in photovoltaic cells through the use of fullerene-based materials has further increased the interest in this research field. We have shown that the high-pressure hydrogenation of C60 at 100 bar and 400 8C results in the direct synthesis of predominantly a C3v isomer of C60H18 in more than 95% purity without any further purification. We now report the successful synthesis and structural determination of the far more complex C70H38 structure. Theoretically, the number of isomers for a specific hydrogenated fullerene structure is enormous. For example, C60H18 has been calculated to have 6, 10 such isomers and C60H36 roughly 1 , 10, 5] but in practice stability constraints reduce that number significantly. For fullerene structures such as C60 and C70, the stability is mainly determined by the combination of p-electronic and steric contributions to the total energy, whereas most of the isomers of hydrogenated structures are thought to be stabilized by pair-wise addition of hydrogen atoms. According to semiempirical calculations, the initial pair-wise hydrogenation should occur at hex-hex edges. The aromaticity of the different isomers is also an important parameter, and it has been shown that particularly stable isomers exist for structures containing separated benzenoid rings. Hydrofullerenes that have isomers with separated benzenoid rings are C60H36 [5,9] and C70H36. [10] The importance of the separated benzenoid rings for stability has, however, been questioned for C70H36 by Fowler et al. , who conclude on the basis of MNDO calculations that the most stable structure of C70H36 does not contain benzenoid rings. [8] From these results, it is clear that further experimental studies are needed to fully explain the stability of hydrogenated fullerenes and related compounds. Hydrogenated fullerenes have been synthesized by several methods, with the resulting structures and compositions depending on the specific method used. In many cases the produced material needs significant purification, which is both costly and time-consuming. Our present study and earlier studies have shown that the hydrogenation of fullerenes at appropriate pressures and temperatures results in the selective preparation of materials with particular stoichiometries and consisting of only a small number of isomers. This result can, at least partly, be explained by a rearrangement of the hydrogen atoms on the fullerene surface as a result of the extreme experimental conditions, thereby leading to the formation of the most stable structure. However, prolonged hydrogenation of C60 results in fragmentation and partial collapse of the fullerene cage with formation of, for example, C59Hx and C58Hx. [12] Figure 1 shows the mass spectral analysis obtained by 9.4 T high-resolution atmospheric-pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS) of a C70 sample hydrogenated at 673 K and an H2 pressure of 100 bar for 72 h. Figure 1a shows a base signal that corresponds to the singly charged molecular ion of C70H37. As hydrocarbons have only an even number of hydrogen atoms in the solid state, the C70H37 ions must originate either from protonation of C70H36 during ionization or from loss of a hydrogen atom from C70H38 C. In contrast to NMR spectroscopic results, the high-resolution mass spectrum shows abundant non-hydrogenated C70 radical ions and oxidized C70H38 ions. Figure 1b indicates the presence of the C70Hx (x= 36, 38, 40, ...) series of hydrocarbons that undergo both protonation and radical-ion formation upon APPI. The splitting of the signals shown in Figure 1c confirms the presence of C70H38 in the mixture of hydrogenated fullerenes. Specifically, the monoisotopic [C70H38] C species is resolved from [C69 CH37] + (CH and C differ in mass by 4.4 mDa), and [*] Dr. M. Hedenstr m, Dr. I. Sethson, Prof. D. Johnels Ume% University Department of Chemistry 901 87 Ume% (Sweden) Fax: (+46)90-786-66-73 E-mail: Dan.Johnels@chem.umu.se

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