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Detecting proximities between nuclei is crucial for atomic‐scale structure determination with nuclear magnetic resonance (NMR) spectroscopy. Different from spin‐1/2 nuclei, the methodology for quadrupolar nuclei is limited for solids due to the complex spin dynamics under simultaneous magic‐angle spinning (MAS) and radio‐frequency irradiation. Herein, the performances of several homonuclear rotary recoupling (HORROR)‐based homonuclear dipolar recoupling sequences are evaluated for²⁷Al (spin‐5/2). It is shown numerically and experimentally on mesoporous alumina thatoutperforms the supercycled S₃sequence and its pure double‐quantum (DQ) (bracketed) version, [S₃], both in terms of DQ transfer efficiency and bandwidth. This result is surprising since the S₃sequence is among the best low‐power recoupling schemes for spin‐1/2. The superiority ofis thoroughly explained, and the crucial role of radio‐frequency offsets during its spin dynamics is highlighted. The analytical approximation of, derived in an offset‐toggling frame, clarifies the interplay between offset and DQ efficiency, namely, the benefits of off‐resonance irradiation and the trough in DQ efficiency forwhen the irradiation is central between two resonances, both for spin‐1/2 and half‐integer‐spin quadrupolar nuclei. Additionally, density matrix propagations show that thesequence, applied to quadrupolar nuclei subject to quadrupolar interaction much larger than radio‐frequency frequency field, can create single‐ and multiple‐quantum coherences for near on‐resonance irradiation. This significantly perturbs the creation of DQ coherences between central transitions of neighboring quadrupolar nuclei. This effect explains the DQ efficiency trough for near on‐resonance irradiation, in the case of both cross‐correlation and autocorrelation peaks. Overall, this work aids experimental acquisition of homonuclear dipolar correlation spectra of half‐integer‐spin quadrupolar nuclei and provides theoretical insights towards improving recoupling schemes at high magnetic field and fast MAS.

Efficiently hyperpolarizing proton‐dense molecular solids through dynamic nuclear polarization (DNP) solid‐state NMR is still an unmet challenge. Polarizing agents (PAs) developed so far do not perform well on proton‐rich systems, such as organic microcrystals and biomolecular assemblies. Herein we introduce a new PA, cAsymPol‐POK, and report outstanding hyperpolarization efficiency on 12.76 kDa U‐¹³C,¹⁵N‐labeled LecA protein and pharmaceutical drugs at high magnetic fields (up to 18.8 T) and fast magic angle spinning (MAS) frequencies (up to 40 kHz). The performance of cAsymPol‐POK is rationalized by MAS‐DNP simulations combined with electron paramagnetic resonance (EPR), density functional theory (DFT) and molecular dynamics (MD). This work shows that this new biradical is compatible with challenging biomolecular applications and unlocks the rapid acquisition of¹³C–¹³C and¹⁵N–¹³C correlations of pharmaceutical drugs at natural isotopic abundance, which are key experiments for structure determination.

Efficiently hyperpolarizing proton‐dense molecular solids through dynamic nuclear polarization (DNP) solid‐state NMR is still an unmet challenge. Polarizing agents (PAs) developed so far do not perform well on proton‐rich systems, such as organic microcrystals and biomolecular assemblies. Herein we introduce a new PA, cAsymPol‐POK, and report outstanding hyperpolarization efficiency on 12.76 kDa U‐¹³C,¹⁵N‐labeled LecA protein and pharmaceutical drugs at high magnetic fields (up to 18.8 T) and fast magic angle spinning (MAS) frequencies (up to 40 kHz). The performance of cAsymPol‐POK is rationalized by MAS‐DNP simulations combined with electron paramagnetic resonance (EPR), density functional theory (DFT) and molecular dynamics (MD). This work shows that this new biradical is compatible with challenging biomolecular applications and unlocks the rapid acquisition of¹³C–¹³C and¹⁵N–¹³C correlations of pharmaceutical drugs at natural isotopic abundance, which are key experiments for structure determination.