Characterization of the fullerene derivative [60]PCBM, by high-field carbon, and two-dimensional NMR spectroscopy, coupled with DFT simulations

High-resolution (600 MHz) ¹H and ¹³C chemical shift and 2D HETCOR NMR spectra of [60]PCBM were recorded. Resonances from every carbon atom of the ester, phenyl and cyclo-fullerenyl groups, were fully accounted. Assignments of the fullerene cyclopropa-ring, and all phenyl and ester carbons to their respective resonances were based on a HETCOR 2D NMR spectrum. Remaining fullerene assignments were made to a high level of confidence with the aid of an ωB97X hybrid HF/DFT simulation of the ¹³C NMR spectrum employing a triple zeta Dunning-type basis set. The best result was obtained with the range-separation parameter ω set effectively to zero. This indicates that the fraction of HF in the HF/DFT hybrid at very short range is the dominant factor in achieving good NMR results, that ωB97X with its 15.77% HF fraction at r_ij = 0 seems very well suited, and that allowing the HF fraction to increase with range is not particularly beneficial. The resulting spectrum had a remarkable qualitative agreement with experiment with a very low mean absolute error for fullerene carbons of 0.09 ppm, which was considerably lower than the 0.28 ppm of the more commonly used B3LYP/6-31G(d,p) method.