2.2. NMR analysis
Solid-state 13C cross polarization (CP) NMR spectra were
acquired with magic angle spinning (MAS) at a 13C frequency of
50.3 MHz on a Varian Unity200 spectrometer. Samples were
packed in a 7 mm diameter cylindrical zirconia rotor with Kel-F
end-caps and spun at 5000 ± 100 Hz in a Doty Scientific MAS probe.
A 1-ms contact time and a 1-s recycle delay were used, and 4000
transients were collected for each spectrum. Free induction decays
were acquired with a sweep width of 40 kHz; 1216 data points
were collected over an acquisition time of 15 ms. All spectra were
zero-filled to 8192 data points and processed with a 50 Hz Lorentzian
line broadening and a 0.005 s Gaussian broadening. Chemical