Figures and summarise the 1D-ENDOR results for AAF and FAAF. These ENDOR spectra were acquired across the ESR spectrum using microwave pulse lengths of 54 ns with a bandwidth of ca. 22 MHz ( 0.8 mT in field domain). For AAF, this pulse bandwidth is responsible for the small orientation selection effects observed in the ENDOR spectra acquired at different field positions. Apart from these small differences, the AAF ENDOR spectra are symmetric relative to the Larmor frequency (𝜈 rf -𝜈 H = 0), as expected for a doublet. The observed peaks are assigned to particular protons in AAF by performing a Gaussian deconvolution and comparing the results with the DFT predicted hyperfine tensors. As was the case for all data up to this point, the largest hyperfine coupling (blue line in figure ) corresponds to the ortho/para protons marked in the skeletal structures. The smaller hyperfine couplings to the meta protons and the anthryl protons are also clearly resolved in the data. ) corresponds to the ENDOR spectrum at 1213.5 mT. The dashed red line superimposed on the 1213.5 mT data represents the best-fitting of a sum of 5 symmetric bimodal Gaussian functions. By contrast, the dashed red line superimposed on the 1211.9 mT data represents the best-fitting of a sum of 5 ('single mode') Gaussian functions. Similarly to figure , the hyperfine couplings were assigned to the protons of FAAF by comparison with the DFT results. At the central field positions (1212.7-1214.3 mT), the ENDOR spectra of FAAF are very similar to the AAF (except for the appearance of the Larmor peak). However, the peripheral spectra, corresponding to the shoulders observed in the FAAF spectrum in figure , are strongly indicative of a triplet state (strong Larmor peak and pronounced asymmetry). As mentioned in the SI, the sign of the hyperfine coupling can be determined for triplet states if the sign of the 𝐷-value is known. The data shows that the ortho/para protons have a negative hyperfine coupling (i.e. an excess of 𝛽 spin density) and the meta protons have a positive hyperfine coupling (i.e. an excess of 𝛼 spin density). These relative signs are entirely reproduced by the phase of the DFT computed spin density.