2.5. 29Si magic angle spinning (MAS) nuclear magnetic resonance  

²⁹Si NMR spectra were recorded on a Bruker AVANCE 7.4 T operated at 59 MHz and equipped with a 4 mm double bearing MAS probe head spinning at 12 kHz. About 16 000 scans were accumulated after a 45° pulse, using a 10 s recycling delay. This delay was optimized to ensure a complete relaxation of the magnetization. ²⁹Si chemical shifts were reported relative to tetramethylsilane resonance. The spectra were simulated as a sum of individual Gaussian–Lorentzian functions, using the Dmfit program (Massiot et al., 2002 ▸). Their integrated intensities were used to estimate the amount of the differently coordinated species. The mean Si chain length (i.e. the mean number of Si atoms that are connected in a chain) was calculated following Richardson (2014 ▸).