Solid-state NMR experiments were conducted using a Varian (Agilent) DirectDrive 1 (VNMRs) NMR spectrometer operating at a 1 H frequency of 600 MHz and equipped with a 1.6mm HXY fastMAS probe filled with 2-6 mg of powdered sample (Agilent Technologies, Santa Clara, CA). The experimental protocol was adapted from previously published methodologies . 1D 13 C CPMAS spectra were recorded using typical 0.5-2 ms cross polarization times at 15-30 kHz spinning rates, and with a 3 s recycle delay and ~40000 transients. High-power heteronuclear 1 H decoupling (175-185 kHz) was achieved using the small phase incremental alternation (SPINAL) pulse sequence . To determine the carbon spin-lattice relaxation times, the recovery of 13 C signal intensity was monitored following cross polarization with a 1 ms crosspolarization time and inversion of the signal .