Observation of proximities between spin-1/2 and quadrupolar nuclei in solids: Improved robustness to chemical shielding using adiabatic symmetry-based recoupling.

We introduce a novel heteronuclear dipolar recoupling based on the R21-1 symmetry, which uses the tanh/tan (tt) shaped pulse as a basic inversion element and is denoted R21-1(tt). Using first-order average Hamiltonian theory, we show that this sequence is non-γ-encoded and that it reintroduces the |m| = 1 spatial component of the Chemical Shift Anisotropy (CSA) of the irradiated isotope and its heteronuclear dipolar interactions. Using numerical simulations and one-dimensional (1D) 27Al-{31P} through-space D-HMQC (Dipolar Heteronuclear Multiple-Quantum Correlation) experiments on VPI-5, we compare the performances of this recoupling to those of other non-γ-encoded |m| = 1 heteronuclear recoupling schemes: REDOR (Rotational-Echo DOuble Resonance), SFAM (Simultaneous Frequency and Amplitude Modulation) and R42-1(tt). Such comparison indicates that the R21-1(tt) scheme is more robust to CSA, offset and radiofrequency field inhomogeneities than the other schemes. We take advantage of the high robustness of R21-1(tt) to CSA and offset to demonstrate the possibility to correlate the signals of 207Pb isotope with those of neighboring half-integer spin quadrupolar nuclei. Such approach is demonstrated experimentally by acquiring 11B-{207Pb} D-HMQC 2D spectra of Pb4O(BO3)2 crystalline powder.

Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR.

The field of dynamic nuclear polarization has undergone tremendous developments and diversification since its inception more than 6 decades ago. In this review we provide an in-depth overview of the relevant topics involved in DNP-enhanced MAS NMR spectroscopy. This includes the theoretical description of DNP mechanisms as well as of the polarization transfer pathways that can lead to a uniform or selective spreading of polarization between nuclear spins. Furthermore, we cover historical and state-of-the art aspects of dedicated instrumentation, polarizing agents, and optimization techniques for efficient MAS DNP. Finally, we present an extensive overview on applications in the fields of structural biology and materials science, which underlines that MAS DNP has moved far beyond the proof-of-concept stage and has become an important tool for research in these fields.

γ-Independent through-space hetero-nuclear correlation between spin-1/2 and quadrupolar nuclei in solids.

We introduce novel sequences using indirect detection to correlate quadrupolar nuclei and spin-1/2 isotopes, other than 1H and 19F. These sequences use γ-encoded symmetry-based RNnν schemes that reintroduce the space component |m| = 1 of the heteronuclear dipolar coupling. These schemes can be applied to the indirectly detected spin in Dipolar-mediated Heteronuclear Multiple-Quantum Correlation (D-HMQC) sequence or to the detected isotope in a novel sequence, named Dipolar-mediated Heteronuclear Universal-Quantum Correlation (D-HUQC). We show that the signal of these sequences using γ-encoded recoupling does not depend on the γ Euler angle relating the inter-nuclear vector between the coupled spins to the MAS rotor-fixed frame. Therefore, the transfer efficiency of these sequences is in principle higher than that of D-HMQC methods using non-γ-encoded recoupling. Furthermore, numerical simulations show that the heteronuclear correlation experiments employing γ-encoded recoupling are more robust to Chemical Shift Anisotropy (CSA) of the irradiated spin and MAS frequency fluctuations. These results are confirmed by 13C-{15N} heteronuclear correlation on glycine and 31P-27Al ones on VPI-5 and Na7(AlP2O7)4PO4. These experiments indicate that R1635 recoupling produces the highest signal-to-noise ratio in heteronuclear correlation 2D experiments when the detected spin-1/2 nuclei are subject to large CSA.

Insights into the catalytic activity of nitridated fibrous silica (KCC-1) nanocatalysts from (15) N and (29) Si†NMR spectroscopy enhanced by dynamic nuclear polarization.

Fibrous nanosilica (KCC-1) oxynitrides are promising solid-base catalysts. Paradoxically, when their nitrogen content increases, their catalytic activity decreases. This counterintuitive observation is explained here for the first time using (15) N-solid-state NMR spectroscopy enhanced by dynamic nuclear polarization.