*2.3. NMR Spectroscopy*

The NMR measurements were carried out using Bruker Avance III 600 and on a Bruker Avance III 400 spectrometers (Bruker GMbH, Karlsruhe, Germany). The Bruker Avance III 600 was equipped with 4 mm 1H/BB (31P–15N) CP/MAS probehead. The resonance frequencies for 1H, 13C, 15N were set at 600.13, 150.90, and 60.82 MHz, respectively. The 13C and 15N CP/MAS spectra were measured with a proton 90◦ pulse length of 4 μs, a 4–200 s repetition time and SPINAL64 decoupling (83 kHz amplitude). The 1H Very Fast MAS NMR spectra were recorded with a spinning speed of 60 kHz in a 1.3 mm zirconium rotor. Bruker Avance III 400 spectrometer experiments were performed for 1H, 15 N and 13C at 400.15, 40.53 and 100.63 MHz resonance frequencies, respectively. The cross polarization magic angle spinning (13C CP MAS) spectra were recorded with a proton 90◦ pulse length of 4 μs.

In typical CP/MAS experiments, samples were packed into a 4 mm ZrO2 rotor and spun at a rate of 8–12 kHz. The 1024 transients were acquired. FIDs were accumulated using time domain size of 3.6 K data points. Adamantane (resonances at 38.48 and 29.46 ppm) was used as a secondary 13 C chemical-shift reference from external tetramethylsilane (TMS). For 15N, a chemical shift calibration glycine signal of δ = 36.6 ppm was used as a secondary reference.
