By contrast with AAF, the ENDOR spectra of FAAF (figure ) depend strongly on field position. At the low-field side, the ortho/para protons give rise to a single peak centred at +5 MHz. The meta protons produce the peak The mirror image symmetry between the two panels originates in the same effect as discussed for the 1D ENDOR spectra. at -2.5 MHz. As the field position approaches the centre of the ESR spectrum, the ENDOR data becomes more symmetric relative to the Larmor frequency. At the centre fields (1212.9 mT -1214.1 mT), the FAAF spectra are effectively identical to AAF (apart from the Larmor peak). As the high-field side of the ESR spectrum is approached, the ENDOR spectra become almost perfect mirror images of the corresponding low-field spectra. As explained in the SI, the presence of the Larmor peak and the mirror image symmetry of the low/high-field ENDOR spectra are signatures of a triplet state. The only problem is the similarity of the FAAF and AAF data at the centre fields. In the absence of other evidence, the data at 1213.5 mT would be consistent both with a doublet/triplet admixture and with a pure triplet state. This is because, when the anisotropy of the dipolar coupling is taken into account, both triplet transitions 𝑇 0 → 𝑇 + and 𝑇 0 → 𝑇 -are excited at the centre field position (i.e. at the mid-point between 𝑋 𝑌 + and 𝑋 𝑌 --figure -corresponding to an angle of 54.7 • between 𝐷 𝑧 and the magnetic field). Hence, a pure triplet ENDOR spectrum recorded at the centre field would contain a Larmor peak and hyperfine peaks on either side of it, as would the ENDOR spectrum of a doublet/triplet admixture. Nonetheless, we already know from all the previously presented results that the admixture is truly the origin of the similarity between the ENDOR spectra of AAF and FAAF at the centre field position. However, in more complicated systems, such clear spectral simulations and nutation experiments are the exception, not the rule. In those circumstances, how could we test, experimentally, whether the 1213.5 mT ENDOR spectrum in figure is caused by a doublet/triplet admixture or a pure triplet? The answer lies in the hyperfine-correlated ENDOR experiment shown in figure . As explained in the SI, hyperfine-correlated ENDOR produces 2D spectra where the peaks in the usual 1D-ENDOR spectra are correlated with the hyperfine couplings from which they originate. A doublet electronic spin coupled to multiple protons will give rise to hyperfine correlation ridges along the 2|𝜈 rf -𝜈 H | line in the hyperfine dimension. This line is shown in dashed blue in figure . For AAF, the data in the top left panel is fully consistent with an electronic spin quantum number of 1/2. When the measurement is performed on the central field position of the FAAF spectrum, the result is effectively identical to AAF. A triplet electronic spin produces hyperfine correlation ridges along the |𝜈 rf -𝜈 H | line (dashed red in figure ). The bottom panels in figure show that the assignment of a triplet state to the shoulders in the pulse ESR spectra of FAAF is correct. Overall, the hyperfine-correlated ENDOR experiment demonstrates uniquely, without relying on subjective interpretation, overly parametrised simulations, and external references with known 𝑆, that solutions of the FAAF diradical contain a mixture of doublet and triplet species. It is worth mentioning other advantages of using hyper-fine-correlated ENDOR to assign the electronic spin quantum number. The technique achieves a frequency discrimination between two multiplets which is proportional to the ratio of the magnetic quantum numbers 𝑀 𝑆 1 and 𝑀 𝑆 2 involved in an ENDOR transition. This is because the hyperfine field experienced by a nucleus is proportional to the magnetic quantum numbers. For doublets vs. triplets, this simply implies a frequency discrimination factor of 2 (illustrated by the dashed red/blue lines in figure ). By contrast, the nutation experiment (SI) achieves, at best, a frequency discrimination factor of √ 2 between doublets and triplets. Moreover, unlike nutation experiments, hyperfine-correlated ENDOR is not subject to distortions caused by microwave field inhomogeneities or complications due to the limited excitation bandwidth of the microwave pulses.