## 2 Materials and Methods
### Experimental Setup  
Conventional (continuous wave--CW) and pulsed EPR measurements were performed, exploiting the abilities of Bruker Elexsys 580/680 spectrometer in X- (\(\
u_{\text{MW}}\) = 9-10 GHz), Q- (\(\
u_{\text{MW}}\) = 33-34 GHz), and W-band (\(\
u_{\text{MW}}\) = 94-95 GHz) ranges. At CW mode, experimental parameters (amplitude modulation, power of mw-source, and integration times) were chosen to avoid any distortion and saturation of absorption signals. Electron spin echo (ESE)-detected EPR spectra were recorded using a standard Hahn echo sequence with \(\pi\) = 32 ns and \(\tau\) = 300 ns for the X-band, and \(\pi\) = 64 ns and \(\tau\) = 240 ns for the W-band, unless otherwise specified in the text. Relaxation time measurements were taken using a 2-pulseHahn echo sequence and an inversion-recovery 3-pulse sequence for the spin-spin T\({}_{2}\) and spin-lattice T\({}_{1}\) relaxation times, respectively. Electron Spin Echo Envelope Modulation (ESEEM) was implemented using a well-known two-pulse Hahn sequence: \(\pi/2-\tau-\pi\), where the length of \(\pi\) was 32 ns, and \(\tau\) was equal to 200 ns. In this method, the integral intensity of the electron spin echo (ESE) was recorded, depending on the time interval \(\tau\) between two pulses at a fixed magnetic field. This parameter was increased to the required value (before the loss of information on nuclear modulations). To increase the sensitivity and spectroscopic resolution, the smallest possible step, t = 4 ns, was chosen and the ESE was integrated only at its peak with an integration time of t = 4 ns. Further spectral analysis of obtained results involved a Fourier transform using the program OriginPro. Electron nuclear double resonance (ENDOR) experiments were conducted using special (for nuclei and electron) cavities and a scheme of simultaneous electron-nuclear excitations called the Mims pulse sequence: \(\pi/2-\tau-\pi/2-\mathrm{T}-\pi/2\), with an additional radiofrequency (RF) \(\pi\)-pulse inserted between the second and third microwave \(\pi/2\) pulses, where the length of \(\pi/2\) was 64 ns, \(\tau\) = 250 ns, T = 20 \(\upmu\)s, and for the RF pulse, \(\pi\) = 18 \(\upmu\)s. Since the Larmor frequency of the nuclei is less than for an electron by about 2000 times, and is consequently in the radio frequency range, an additional RF-source with a wide frequency range (1-200 MHz) was used to stimulate the NMR transitions. To increase the spectroscopic resolution and separate close nuclear frequencies or overlapping splitting from each other in the RF-scale, experiments were performed in the high-field part of the spectrometer (W-band, \(\mathrm{v_{MW}=94\ GHz}\)). Due to method requirements for T\({}_{1}\) time measurements, the experiments were performed at a low temperature, T = 50 K.  
The ELDOR-detected NMR (EDNMR) method for the electron-nuclear study is based on the electron-electron double resonance (ELDOR) approach. The additional ELDOR module E580-400U, and a dielectric ring resonator ER 4118X-MD5, were used to investigate double electron-electron transitions. In this case, the initial (main) source of microwave radiation acts as the observation frequency (\(\mathrm{v_{mW1}}\) or \(\mathrm{v_{obs}}\)) at a fixed value (\(\mathrm{v_{MW}=9.59\ GHz}\)). At the same time, module E580-400U is necessary for generating and varying the second independent microwave frequency (\(\mathrm{v_{mW2}}\) or \(\mathrm{v_{pump}}\)) in the range from 9.3 to 10 GHz. Owing to its adjustable Q-factor, the ER 4118X-MD5 resonator allows for the optimization of the bandwidth of resonant frequencies in accordance with the specific requirements of the experiment. The duration of the selective pulse \(\mathrm{t_{sel}}\) was chosen to be 6 \(\upmu\)s, which corresponds to the excitation band in the EPR spectrum, with \(\Delta\)B = 7.2 \(\upmu\)T and the length of the detection pulse equal to 300 ns (\(\Delta\)B = 144 \(\upmu\)T). All of these tuning parameters allow for the recording of the spectra at a high resolution.