DNP-enhanced NMR of half-integer quadrupolar nuclei in solids

Quadrupolar nuclei with half-integer spin, which represent 66 % of the NMR-active isotopes, are present in a wide range of materials with applications in various fields, including heterogeneous catalysis, optoelectronics and energy. The solid-state NMR spectra of these isotopes are affected by quadrupolar interactions, which provide unique information on the local environment of these nuclei, in addition to their chemical shifts. These anisotropic interactions, which are generally larger than other internal spin interactions, split and broaden the NMR transitions, which reduce the sensitivity for the detection of these isotopes. In addition, the large dimensions of their density matrices and the numerous NMR transitions complicate the spin dynamics and can reduce the efficiency of coherence transfers, such as cross-polarization under magic-angle spinning (CPMAS), which is widely employed to boost the sensitivity for the detection of spin-1/2 isotopes. In the last decade, sensitivity gains provided by dynamic nuclear polarization (DNP) have been exploited to detect half-integer quadrupolar nuclei in solids. This review discusses the advantages and limitations of the different DNP-NMR techniques that have been proposed for the detection of these isotopes, including direct excitation and CPMAS, and two more recently introduced methods called PRESTO (Phase-shifted Recoupling Effects by Smooth Transfer of Order) and D-RINEPT (Dipolar-mediated Refocusing Insensitive Nuclei Enhanced by Polarization Transfer). We also show how these techniques can be applied to obtain new insights on the structure of materials, notably of their surfaces, and hence, contribute to extend the range of applications of the surface-enhanced NMR spectroscopy (DNP-SENS).