Cross polarization from dipolar-order under magic angle spinning: The ADRF-CPMAS NMR experiment

Techniques for enhancing the signals arising from low-γ, insensitive (I) nuclei are central to solid-state nuclear magnetic resonance. One of the leading and best-established methods to sensitize these unreceptive species is Hartmann-Hahn cross polarization (HH-CP), a polarization transfer mechanism often executed under MAS. Herein, we explore the possibility of utilizing the 1H dipolar order created via adiabatic demagnetization in the rotating frame (ADRF), to enhance the unreceptive spins under MAS. It is found that an efficient polarization transfer via ADRF-CPMAS is not only possible but can exceed, at least in some instances involving plastic crystals, the efficiency of an optimized HH-CPMAS transfer. The experiment requires low radiofrequency nutation fields on both the 1H- and the I-spin channels, and displays unusual matching conditions that are reminiscent of the zero- and double-quantum matching conditions arising under CPMAS, albeit centered at zero frequency and demanding the simultaneous involvement of several spins. The origin of these multi-spin transfer processes is analytically derived and numerically simulated in predictions that compare well with experimental 13C and 15N results collected on model compounds at different spinning speeds. These derivations start from descriptions that depart from traditional thermodynamic arguments, and treat instead the ADRF processes in static and spinning solids on the basis of coherent evolutions. The predictions of these analytical derivations are corroborated by numerical simulations. The effects of additional factors, including chemical shift anisotropies, J-couplings, and radiofrequency inhomogeneities, are also theoretically and experimentally explored.