13C NMR spectroscopy of C76, C78, C84 and mixtures of C86–C102; anomalous chromatographic behaviour of C82, and evidence for C70H12

Abstract

Using high pressure liquid chromatography, the fullerenes C₇₆, C₇₈. C₈₄, and C₈₆–C₁₀₂ have been separated (in some cases partially) from soot produced by the arc-discharge procedure. The ¹³C NMR spectrum for C₇₆ confirms that reported previously, whilst the spectrum for C₇₈ indicates the presence of C_2v, D₃, C_2v, isomers in the approximate relative yields (there is a small batch dependence) of 52 : 30 : 18%. The spectrum for the D₃ isomer (confirmed from two separate batches) differs from that given in the literature with respect to the location of one peak. The relative peak heights in the C₈₄¹³C NMR spectrum are similar to those reported for samples produced under different conditions. The stabilities of the component isomers must therefore be similar and so produced in a similar ratio irrespective of conditions; this indicates them to be the D₂and a O_2d isomers. The spectrum also contains over 70 minor peaks due to other isomers of C₈₄. Eleven of the peaks are more intense than the others, consistent with the presence of the D_3d and D_6h isomers, predicted to be of relatively high stability. Two fractions (giving yellow solutions in hexane) eluted after C₈₄: the first was separated into two components, shown by mass spectrometry to consist of minor amounts of C₈₆–C₉₂, and mainly C₉₀ and C₉₂ with lesser amounts of C₉₄–C,₁₀₂, respectively. The ¹³C NMR spectrum of the C₉₄–C₁₀₂ component indicated the presence of a fullerene isomer of high symmetry. The second yellow fraction consisted largely of C₈₂ together with traces of fullerenes up to C₁₀₈, but the quantity was insufficient for the ¹³C NMR spectrum to be obtained. The retention time of C₈₂ is inconsistent with values for the other fullerenes indicating that it consists of the C_3v isomer, predicted to be polar. C₉₀ eluted with two different retention times suggesting that the second fraction is also a polar isomer. One sample of C₈₄ contained a coeluent, identified by mass spectrometry as C₇₀H₁₂; proposals are made concerning its possible origin, structure, and stability relative to other hydrogenated derivatives.