13 C Isotopjc Effect on the Raman Spectrum and Structure of C 60 Fullerene
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The carbon structures are known to be generally infinite or of undetermined size. Carbon 60 fullerene is the first carbone molecule found with a limited formula unit. The C60 molecules are often agglomerated in the solid state, but they can be isolated in very dilute solution in relatively inert solvent such as carbon disulfide. The truncated icosahedral structure of an isolated C60 fullerene molecule is a cage limited by 60 IV- V1I bonds, common to pentagons and hexagons, and 30 IVi - VII bonds joining by their intermediary hexagons to hexagons. These CC bonds are believed to be different and their bond lengths have been evaluated to be 1.47 A and 1.41 A repectively. These values are in agreement with the partial double bond characters of these chemical bonds [1] and explain the appearance of CC stretching vibration frequency around 1465 cm-1 in the Raman spectrum of C 60 . This C=C double bond vibrational frequency is lower than that observed around 1600 cm-1 in smaller molecules such as organic compounds, but higher than the CC stretching frequency 1331 cm-1 in diamond. It is well known that in small or non-spherical molecules, the C=C vibration is mostly localized on the CC bond itself, while in infinite system such as diamond, the observed frequency represents the movement in the whole solid network. It seems to be interesting to know the behaviour of the CC vibration in 463 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
the fullerene structure. One of the convenient methods to investigate this behaviour is to study the isotope effect. In this paper we present our results concerning the effect of 13C on the Raman spectrum of C60 fullerene. The analysis will be helped by comparison with the 13C isotopic effects on common organic molecules and on diamond. 2. EXPERIMENT 13C enriched C60 samples were prepared. Partial composition was found.
The Raman spectra of pure C60 and 13C enriched C60 were recorded on a DILOR Omar spectrometer, coupled with an optical microscope and a multichannel detector (both Silicon photodiodes and CCD). The ion Argon laser source used was a Spectra Physics model 2036 emitting at 514.5 nm. 3. RESULTS AND I)ISCUSSION The Raman spectrum of a C60 sample partially substituted by 13C is reproduced on figures 1 and 2, in comparison with that of pure C60 [2]. The absence of characteristic Raman band corresponding to C70 indicates that the sample is free of all higher fullerene than C60.
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Fig. 1 - Raman spectra (partial)of C60 and 13C enriched (8%) C60 . A frequency shift is observed by isotopic substitution especially for the curvilinear C =C stretching vibration around 1465 cm-1 ; smaller shifts are also recorded for other vibrations. Nevertheless, the effect is not similar to that usually observed for small and non-spherical molecules, such as organic compounds. One can remember that in a partial substitution of Carbon 12C by carbon 13C in these systems, three CC bonds can occur : 12C-
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