It is important to highlight the remarkable quality of the simulations of the AAF spectra in panels (a) and (b) of figure . The RMSD of the best fit is ∼ 0.5%. Such a good fit is not possible for the spectra of FAAF in a single component simulation. The reason why single-component simulations fail for FAAF can be most easily understood by visual inspection of the data in panel (d). A hyperfine coupling to two equivalent C nuclei ought to produce a spectrum which is the convolution of the unlabelled spectrum -panel (c) -with a 1 ∶ 2 ∶ 1 splitting pattern caused by the larger C couplings. Instead, the spectrum in panel (d) appears to be an overlap of the spectrum in panel (b) with another spectrum which has the expected 1 ∶ 2 ∶ 1 13 C splitting pattern. (d) correspond to the FAAF system with and without C labelling. The simulations of the FAAF spectra are comprised of a linear combination of a doublet spectrum (blue lines) and a triplet spectrum (orange lines). The corresponding simulation parameters are displayed in table .