Rapid Acquisition of14N Solid-State NMR Spectra with Broadband Cross Polarization

Acquisition of 1H-Detected 103Rh and 99Ru Solid-State Nuclear Magnetic Resonance Spectra in Stationary Samples

The platinum group elements (PGEs) are among the most important in the periodic table due to their critical roles in a diverse array of applications. There is great interest in using solid-state nuclear magnetic resonance (SSNMR) for studying the structure and bonding in PGE complexes from the perspective of the metal nuclides, yet this has been limited to date. This is largely due to the inherently low Larmor frequencies of many of the PGE nuclides in addition to factors such as low natural abundances and/or large anisotropic interactions that reduce their receptivity to the NMR experiment. In this work, we demonstrate for the first time the ability to indirectly detect (with 1H, I = 1/2) wideline SSNMR powder patterns from stationary samples of compounds featuring 103Rh (S = 1/2) and 99Ru (S = 5/2) using the recently introduced progressive saturation of the proton reservoir (PROSPR) pulse sequence.

Optimal control-based nuclear spin cross-polarization in the presence of complicating anisotropic interactions

Cross-polarization is an indispensable part of solid state nuclear magnetic resonance spectroscopy to enhance sensitivity and extract structural information. However, the presence of certain anisotropic interactions, including chemical shift anisotropy and quadrupolar coupling, makes the inter-nuclear spin correlation experiments challenging. This impedes characterization of numerous materials and pharmaceutical compounds containing isotopes, such as 19F with large chemical shift anisotropy and 6/7Li, 23Na, 27Al, etc., with quadrupolar coupling. To address this problem, we introduce a new optimal control simulation-generated pulse sequence for Optimal Polarization Transfer In the presence of Anisotropic Nuclear Spin interactions (OPTIANS). Numerical simulations show high efficiency and robustness against experimental imperfections under a broad range of anisotropic interaction strengths for 19F-7Li, 19F-23Na, 19F-27Al, and 19F-13C polarization transfers. The polarization transfer curves show transient oscillations, which make the pulse sequence a quantitative method for dipolar coupling measurements. Experiments on a multi-metal fluoride system validate the predictions of the simulations by showing efficient PT in three spin pairs at varying experimental conditions. Remarkably, this method shows 50% better 19F-7Li PT efficiency at 14.1 T compared to the ramped cross-polarization experiment. The underlying polarization transfer mechanism is analyzed using the Fourier transform of the polarization transfer curves revealing that this optimal control method utilizes the chemical shift anisotropy and quadrupolar coupling to facilitate robust and efficient cross-polarization.

Spin-dynamics and efficiency of single 14N-1H cross-polarization at fast magic angle spinning in solids

The naturally abundant 14N isotope (>99 %) is sparingly employed for characterization in solid-state nuclear magnetic resonance (NMR) despite the importance of nitrogen atoms in shaping molecular structures and properties. This inhibition can be attributed to large quadrupolar couplings (∼several MHz), resulting in more involved spin methodologies for 14N nuclei. Experimentally, spin-½ nuclei are utilized for excitation and detection through two-way (1H→14N→1H) polarization transfer between spin-½ nuclei and 14N. Herein, we show direct 14N spin excitation followed by 14N→1H cross-polarization (CP) is an efficient method for polarization transfer even for 14N spins with a large quadrupolar coupling constant (3-4 MHz). This contrasts previous studies, which indicate that 1H-14N spectra can only be observed with a pair of at least a rotor period-long symmetric 14N pulses (J. Chem. Phys. 151 (2019) 154202). The 14N→1H CP spin dynamics have been experimentally established and can be explained in analogy to spin-½ Hartmann-Hahn CP if visualized in the quadrupolar jolting frame. The 14N→1H CP is ∼1.9-2.7 times more efficient in polarization transfer than other 14N edited experiments. Considering shorter 14N T1 relaxation times compared to protons, 14N edited spectra were recorded using 14N→1H CP, resulting in enhanced sensitivity per unit of time.

Significant 13C NMR signal enhancements in amino acids via adiabatic demagnetization and remagnetization cross polarization

Herein, we report an improvement over Hartmann-Hahn cross polarization for NMR signal enhancement: adiabatic demagnetization/remagnetization transfers that provide up to a 9-fold experimental speedup for 13C NMR signals in amino acids over conventional means. The experiment proved insensitive to site type, and we also demonstrate a means for making it compatible with high-resolution spectroscopy.

Sensitivity Enhancement of Ultra-Wideline NMR by Progressive Saturation of the Proton Reservoir Under Magic-Angle Spinning

Solid-state NMR of low-γ nuclides is often characterized by low sensitivity and by significant spectral broadenings induced by the quadrupolar and the chemical-shift anisotropy interactions. Herein, we introduce an indirect acquisition method, termed PROgressive Saturation of the Proton Reservoir Under Spinning (PROSPRUS), which could facilitate the acquisition of ultra-wideline NMR spectra under magic-angle spinning (MAS), in systems with a sufficiently long dipolar relaxation time, T1D. PROSPRUS NMR relies on the generation of so-called second-order dipolar order among abundant protons undergoing MAS, and on the subsequent depletion of this dipolar order by a series of looped cross-polarization events, transferring the proton order into polarization of the low-γ I-nuclei as a function of the latter's offsets. While the spin dynamics of the ensuing experiment is complex, particularly when dealing with narrow I spectral lines, it is shown that PROSPRUS can lead to faithful lineshapes for ultra-wideline spin-1/2 and spin-1 species, providing high sensitivity with extremely low RF power requirements. It is also shown that the ensuing 1H-detected PROSPRUS experiments can efficiently characterize I-spin lineshapes in excess of 1 MHz without having to retune electronics, while providing improvements in sensitivity per unit time over current broadband direct-detection methods by up to a factor of four.

Structure and bonding in rhodium coordination compounds: a 103Rh solid-state NMR and relativistic DFT study

This study demonstrates the application of 103Rh solid-state NMR (SSNMR) spectroscopy to inorganic and organometallic coordination compounds, in combination with relativistic density functional theory (DFT) calculations of 103Rh chemical shift tensors and their analysis with natural bond orbital (NBO) and natural localized molecular orbital (NLMO) protocols, to develop correlations between 103Rh chemical shift tensors, molecular structure, and Rh-ligand bonding. 103Rh is one of the least receptive NMR nuclides, and consequently, there are very few reports in the literature. We introduce robust 103Rh SSNMR protocols for stationary samples, which use the broadband adiabatic inversion-cross polarization (BRAIN-CP) pulse sequence and wideband uniform-rate smooth-truncation (WURST) pulses for excitation, refocusing, and polarization transfer, and demonstrate the acquisition of 103Rh SSNMR spectra of unprecedented signal-to-noise and uniformity. The 103Rh chemical shift tensors determined from these spectra are complemented by NBO/NLMO analyses of contributions of individual orbitals to the 103Rh magnetic shielding tensors to understand their relationship to structure and bonding. Finally, we discuss the potential for these experimental and theoretical protocols for investigating a wide range of materials containing the platinum group elements.

Broadband Cross-Polarization to Half-Integer Quadrupolar Nuclei: Wideline Static NMR Spectroscopy

Cross-polarization (CP) is a technique commonly used for the signal enhancement of NMR spectra; however, applications to quadrupolar nuclei have heretofore been limited due to a number of problems, including poor spin-locking efficiency, inconvenient relaxation times, and reduced CP efficiencies over broad spectral bandwidths─this is unfortunate, since they constitute 73% of NMR-active nuclei in the periodic table. The Broadband Adiabatic Inversion CP (BRAIN-CP) pulse sequence has proven useful for the signal enhancement of wideline and ultra-wideline (i.e., 250 kHz to several MHz in breadth) powder patterns arising from stationary samples; however, a comprehensive investigation of its application to half-integer quadrupolar nuclei (HIQN) is currently lacking. Herein, we present theoretical and experimental considerations for applying BRAIN-CP to acquire central-transition (CT, +1/2 ↔ -1/2) powder patterns of HIQN. Consideration is given to parameters crucial to the success of the experiment, such as the Hartmann-Hahn (HH) matching conditions and the phase modulation of the contact pulse. Modifications to the BRAIN-CP sequence such as flip-back (FB) pulses and ramped contact pulses applied to the 1H spins are used for the reduction of experimental times and increased CP bandwidth capabilities, respectively. Spectra for a series of quadrupolar nuclei with broad CT powder patterns, including 35Cl (S = 3/2), 55Mn (S = 5/2), 59Co (S = 7/2), and 93Nb (S = 9/2), are acquired via direct excitation (CPMG and WCPMG) and indirect excitation (CP/CPMG and BRAIN-CP) methods. We demonstrate that proper implementation of the sequence can enable 1H-S broadband CP over a bandwidth of 1 MHz, which to the best of our knowledge is the largest CP bandwidth reported to date. Finally, we establish the basic principles necessary for simplified optimization and execution of the BRAIN-CP pulse sequence for a wide range of HIQNs.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

3D Relaxation-Assisted Separation of Wideline Solid-State NMR Patterns for Achieving Site Resolution

There are currently no methods for the acquisition of ultra-wideline (UW) solid-state NMR spectra under static conditions that enable reliable separation and resolution of overlapping powder patterns arising from magnetically...

Nutraceuticals in Bulk and Dosage Forms: Analysis by 35Cl and 14N Solid-State NMR and DFT Calculations

This study uses ³⁵Cl and ¹⁴N solid-state NMR (SSNMR) spectroscopy and dispersion-corrected plane-wave density functional theory (DFT) calculations for the structural characterization of chloride salts of nutraceuticals in their bulk and dosage forms. For eight nutraceuticals, we measure the ³⁵Cl EFG tensor parameters of the chloride ions and use plane-wave DFT calculations to elucidate relationships between NMR parameters and molecular-level structure, which provide rapid NMR crystallographic assessments of structural features. We employ both ³⁵Cl direct excitation and ¹H→³⁵Cl cross-polarization methods to characterize a dosage form containing α-d-glucosamine HCl, observe possible impurity and/or adulterant phases, and quantify the weight percent of the active ingredient. To complement this, we also investigate ¹⁴N SSNMR spectroscopy and DFT calculations to characterize nitrogen atoms in the nutraceuticals. This includes a discussion of targeted acquisition experimental protocols (i.e., acquiring a select region of the overall pattern that features key discontinuities) that allow ultrawideline spectra to be acquired rapidly, even for unreceptive samples (i.e., those with long values of T₁(¹⁴N), short values of T₂^eff(¹⁴N), or very broad patterns). It is hoped that these experimental and computational protocols will be useful for the characterization of various solid forms of nutraceuticals (i.e., salts, polymorphs, hydrates, solvates, cocrystals, amorphous solid dispersions, etc.), help detect impurity and counterfeit solid phases in dosage forms, and serve as a foundation for future NMR crystallographic studies of nutraceutical solid forms, including studies using ab initio crystal structure prediction algorithms.

Broadband adiabatic inversion cross-polarization to integer-spin nuclei with application to deuterium NMR

Solid-state NMR (SSNMR) spectroscopy of integer-spin quadrupolar nuclei is important for the molecular-level characterization of a variety of materials and biological solids; of the integer spins, ² H (S = 1) is by far the most widely studied, due to its usefulness in probing dynamical motions. SSNMR spectra of integer-spin nuclei often feature very broad powder patterns that arise largely from the effects of the first-order quadrupolar interaction; as such, the acquisition of high-quality spectra continues to remain a challenge. The broadband adiabatic inversion cross-polarization (BRAIN-CP) pulse sequence, which is capable of cross-polarization (CP) enhancement over large bandwidths, has found success for the acquisition of SSNMR spectra of integer-spin nuclei, including ¹⁴ N (S = 1), especially when coupled with Carr-Purcell/Meiboom-Gill pulse sequences featuring frequency-swept WURST pulses (WURST-CPMG) for T₂ -based signal enhancement. However, to date, there has not been a systematic investigation of the spin dynamics underlying BRAIN-CP, nor any concrete theoretical models to aid in its parameterization for applications to integer-spin nuclei. In addition, the BRAIN-CP/WURST-CPMG scheme has not been demonstrated for generalized application to wideline or ultra-wideline (UW) ² H SSNMR. Herein, we provide a theoretical description of the BRAIN-CP pulse sequence for spin-1/2 → spin-1 CP under static conditions, featuring a set of analytical equations describing Hartmann-Hahn matching conditions and numerical simulations that elucidate a CP mechanism involving polarization transfer, coherence exchange, and adiabatic inversion. Several experimental examples are presented for comparison with theoretical models and previously developed integer-spin CP methods, demonstrating rapid acquisition of ² H NMR spectra from efficient broadband CP.

Broadband adiabatic inversion experiments for the measurement of longitudinal relaxation time constants

Accurate measurements of longitudinal relaxation time constants (T₁) in solid-state nuclear magnetic resonance (SSNMR) experiments are important for the study of molecular-level structure and dynamics. Such measurements are often made under magic-angle spinning conditions; however, there are numerous instances where they must be made on stationary samples, which often give rise to broad powder patterns arising from large anisotropic NMR interactions. In this work, we explore the use of wideband uniform-rate smooth-truncation pulses for the measurement of T₁ constants. Two experiments are introduced: (i) BRAIN-CPT1, a modification of the BRAIN-CP (BRoadband Adiabatic-INversion-Cross Polarization) sequence, for broadband CP-based T₁ measurements and (ii) WCPMG-IR, a modification of the WURST-CPMG sequence, for direct-excitation (DE) inversion-recovery experiments. A series of T₁ constants are measured for spin-1/2 and quadrupolar nuclei with broad powder patterns, such as ¹¹⁹Sn (I = 1/2), ³⁵Cl (I = 3/2), ²H (I = 1), and ¹⁹⁵Pt (I = 1/2). High signal-to-noise spectra with uniform patterns can be obtained due to signal enhancements from T₂ ^eff-weighted echo trains, and in favorable cases, BRAIN-CPT1 allows for the rapid measurement of T₁ in comparison to DE experiments. Protocols for spectral acquisition, processing, and analysis of relaxation data are discussed. In most cases, relaxation behavior can be modeled with either monoexponential or biexponential functions based upon measurements of integrated powder pattern intensity; however, it is also demonstrated that one must interpret such T₁ values with caution, as demonstrated by measurements of T₁ anisotropy in ¹¹⁹Sn, ²H, and ¹⁹⁵Pt NMR spectra.

Fast Acquisition of Proton-Detected HETCOR Solid-State NMR Spectra of Quadrupolar Nuclei and Rapid Measurement of NH Bond Lengths by Frequency Selective HMQC and RESPDOR Pulse Sequences

Fast magic-angle spinning (MAS), frequency selective (FS) heteronuclear multiple quantum coherence (HMQC) experiments which function in an analogous manner to solution SOFAST HMQC NMR experiments, are demonstrated. Fast MAS enables efficient FS excitation of ¹ H solid-state NMR signals. Selective excitation and observation preserves ¹ H magnetization, leading to a significant shortening of the optimal inter-scan delay. Dipolar and scalar ¹ H{¹⁴ N} FS HMQC solid-state NMR experiments routinely provide 4- to 9-fold reductions in experiment times as compared to conventional ¹ H{¹⁴ N} HMQC solid-state NMR experiments. ¹ H{¹⁴ N} FS resonance-echo saturation-pulse double-resonance (RESPDOR) allowed dipolar dephasing curves to be obtained in minutes, enabling the rapid determination of NH dipolar coupling constants and internuclear distances. ¹ H{¹⁴ N} FS RESPDOR was used to assign multicomponent active pharmaceutical ingredients (APIs) as salts or cocrystals. FS HMQC also provided enhanced sensitivity for ¹ H{¹⁷ O} and ¹ H{³⁵ Cl} HMQC experiments on ¹⁷ O-labeled Fmoc-alanine and histidine hydrochloride monohydrate, respectively. FS HMQC and FS RESPDOR experiments will provide access to valuable structural constraints from materials that are challenging to study due to unfavorable relaxation times or dilution of the nuclei of interest.

Efficient and sideband-free 1H-detected 14N magic-angle spinning NMR

Indirect detection via sensitive spin-1/2 nuclei like protons under magic-angle spinning (MAS) has been developed to overcome the low spectral sensitivity and resolution of ¹⁴N NMR. The ¹⁴N quadrupolar couplings cause inefficient encoding of the ¹⁴N frequency due to large frequency offsets and make the rotor-synchronization of the evolution time necessary. It is shown that ¹⁴N rf pulses longer than the rotor period can efficiently encode ¹⁴N frequencies and generate spinning sideband free spectra along the indirect dimension. Average Hamiltonian and Floquet theories in the quadrupolar jolting frame (QJF) are used to treat the spin dynamics of the spin-1 quadrupolar nucleus under long ¹⁴N rf pulses and MAS. The results show that the rf action can be described by a scaled and phase-shifted effective rf field. The large quadrupolar frequency offset becomes absent in the QJF and therefore leads to sideband-free spectra along the indirect dimension. More importantly, when a pair of long ¹⁴N rf pulses are used, the distribution of the phase shift of the effective rf field does not affect the ¹⁴N encoding for powder samples; thus, high efficiencies can be obtained. The efficient and sideband-free features are demonstrated for three ¹H/¹⁴N indirectly detected experiments using long ¹⁴N pulses under fast MAS.

Evaluation of excitation schemes for indirect detection of 14N via solid-state HMQC NMR experiments

It has previously been shown that ¹⁴N NMR spectra can be reliably obtained through indirect detection via HMQC experiments. This method exploits the transfer of coherence between single-(SQ) or double-quantum (DQ) ¹⁴N coherences, and SQ coherences of a suitable spin-1/2 'spy' nucleus, e.g., ¹H. It must be noted that SQ-SQ methods require a carefully optimized setup to minimize the broadening related to the first-order quadrupole interaction (i.e., an extremely well-adjusted magic angle and a highly stable spinning speed), whereas DQ-SQ ones do not. In this work, the efficiencies of four ¹⁴N excitation schemes (DANTE, XiX, Hard Pulse (HP), and Selective Long Pulse (SLP)) are compared using J-HMQC based numerical simulations and either SQ-SQ or DQ-SQ ¹H-{¹⁴N} D-HMQC experiments on l-histidine HCl and N-acetyl-l-valine at 18.8 T and 62.5 kHz MAS. The results demonstrate that both DANTE and SLP provide a more efficient ¹⁴N excitation profile than XiX and HP. Furthermore, it is shown that the SLP scheme: (i) is efficient over a large range of quadrupole interaction, (ii) is highly robust to offset and rf-pulse length and amplitude, and (iii) is very simple to set up. These factors make SLP ideally suited to widespread, non-specialist use in solid-state NMR analyses of nitrogen-containing materials.

Quantitative analysis of 14N quadrupolar coupling using 1H detected 14N solid-state NMR

Magic-angle spinning solid-state NMR is increasingly utilized to study the naturally abundant, spin-1 nucleus 14N, providing insights into the structure and dynamics of biological and organic molecules. In particular, the characterisation of 14N sites using indirect detection has proven useful for complex molecules, where the 'spy' nucleus provides enhanced sensitivity and resolution. Here we exploit the sensitivity of proton detection, to indirectly characterise 14N sites using a moderate rf field to generate coherence between the 1H and 14N at moderate and fast-magic-angle spinning frequencies. Efficient numerical simulations have been developed that have allowed us to quantitatively analyse the resulting 14N lineshapes to determine both the size and asymmetry of the quadrupolar interaction. Exploiting only naturally occurring abundant isotopes will aid the analysis of materials with the need to resort to isotope labelling, whilst providing additional insights into the structure and dynamics that the characterisation of the quadrupolar interaction affords.

Broadband adiabatic inversion cross-polarization phenomena in the NMR of rotating solids

We explore the use of cross-polarization magic-angle spinning (CPMAS) methods incorporating an adiabatic frequency sweep in a standard Hartman-Hahn CPMAS pulse scheme, to achieve signal enhancements in solid-state NMR spectra of rare spins under fast MAS spinning rates, including spin-1/2, integer spin, and half-integer spin nuclides. These experiments, dubbed Broadband Adiabatic INversion Cross-Polarization Magic-Angle Spinning (BRAIN-CPMAS) experiments, involve an adiabatic inversion pulse on the S-channel of a rare spin nuclide while simultaneously applying a conventional spin-locking pulse on the I-channel (¹H). The signal enhancement imparted by this CP scheme on the S-spin is broadbanded, while employing low RF field strengths on both I- and S-channels. A feature demanded by these BRAIN-CPMAS methods is to impose a selective adiabatic frequency sweep over a single MAS spinning centerband or sideband, to avoid interference between the MAS modulation and sweeps over multiple sidebands. Upon implementing this swept-CP method, a number of MAS-driven processes happen, including broadband zero- and double-quantum CP transfers, and MAS-driven rotary-resonance phenomena. When this CP method is applied to integer and half-integer quadrupolar nuclei at very fast MAS spinning rates, a favorable double-quantum CP condition is found that can be easily achieved, and avoids the level-crossings among various m_s energy levels that complicate quadrupolar CPMAS NMR experiments along lines first shown by Alex Vega. An additional CP mechanism was found in the ¹H-²H case, involving static-like zero-quantum CP modes driven by a quadrupole-modulated RF-dipolar zero-order recoupling under MAS. All these phenomena were examined using average Hamiltonian theory, numerical simulations, and experiments on model compounds. Sensitivity-enhanced, distortion-free CP over wide bandwidths were predicted and observed for S = 1/2 and for S = 1 (²H) under fast MAS rates. BRAIN-CPMAS also delivered undistorted central transition NMR spectra of half-integer quadrupolar nuclei, while utilizing low RF field strengths that avoid complex level-crossing effects under high MAS rates.

Indirect detection of broad spectra in solid-state NMR using interleaved DANTE trains

We analyze the performances and the optimization of ¹H-{I} HMQC experiments using basic and interleaved DANTE schemes for the indirect detection of nuclei I = 1/2 or 1 exhibiting wide lines dominated by chemical shift anisotropy (CSA) or quadrupole interaction, respectively. These sequences are first described using average Hamiltonian theory. Then, we analyze using numerical simulations (i) the optimal lengths of the DANTE train and the individual pulses, (ii) the robustness of these experiments to offset, and (iii) the optimal choice of the defocusing and refocusing times for both ¹H-{I} J- and D-HMQC sequences for ¹⁹⁵Pt (I = 1/2) and ¹⁴N (I = 1) nuclei subject to large CSA and quadrupole interaction, respectively. These simulations are compared to ¹H-{¹⁴N} D-HMQC experiments on γ-glycine and L-histidine.HCl at B₀ = 18.8 T and MAS frequency of 62.5 kHz. The present study shows that (i) the optimal defocusing and refocusing times do not depend on the chosen DANTE scheme, (ii) the DANTE trains must be applied with the highest rf-field compatible with the probe specifications and the stability of the sample, (iii) the excitation bandwidth along the indirect dimension of HMQC sequence using DANTE trains is inversely proportional to their length, (iv) interleaved DANTE trains increase the excitation bandwidth of these sequences, and (v) dephasing under residual ¹H-¹H and ¹H-I dipolar couplings, as well as ¹⁴N second-order quadrupole interaction, during the length of the DANTE scheme attenuate the transfer efficiency.

Practical considerations for the acquisition of ultra-wideline 14N NMR spectra

Several considerations for the acquisition, processing, and analysis of high quality ultra-wideline (UW) ¹⁴N solid-state NMR (SSNMR) powder patterns under static conditions are discussed. It is shown that the ¹⁴N quadrupolar parameters may be determined accurately using the frequencies of only two discontinuities in ¹⁴N NMR powder patterns that are dominated by the first-order quadrupolar interaction, thereby eliminating the need for the acquisition of the entire pattern and concomitantly reducing experimental time. A framework for utilizing the WURST-CPMG pulse sequence to improve the efficiency of UW ¹⁴N SSNMR experiments is explored in two parts: (i) a systematic investigation of the design and parameterization of the WURST pulse is presented, and (ii) the development of the practical aspects of CPMG refocusing for the acquisition of UW ¹⁴N SSNMR powder patterns is discussed, with a focus on maximizing both signal-to-noise and resolution, and minimizing spectral distortions. Finally, a strategy is demonstrated that allows for the measurement of the ¹⁴N quadrupolar parameters for any nitrogen moiety whose quadrupolar coupling constant falls within the range 0.8≤|C_Q|≤1.5MHz, by acquiring only two ¹⁴N NMR sub-spectra at strategically located transmitter frequencies; these results are compared to full powder patterns which are acquired using frequency-stepped methods. The methodologies and practical considerations outlined herein are not only useful for the rapid acquisition of UW ¹⁴N NMR spectra, but may also be modified and applied for UW NMR of a plethora of quadrupolar and spin-1/2 nuclides.

Adiabatic sweep cross-polarization magic-angle-spinning NMR of half-integer quadrupolar spins

The use of frequency-swept radiofrequency (rf) pulses for enhancing signals in the magic-angle spinning (MAS) spectra of half-integer quadrupolar nuclides was explored. The broadband adiabatic inversion cross-polarization magic-angle spinning (BRAIN-CPMAS) method, involving an adiabatic inversion pulse on the S-channel and a simultaneous rectangular spin-lock pulse on the I-channel (¹H), was applied to I(1/2)→S(3/2) systems. Optimal BRAIN-CPMAS matching conditions were found to involve low rf pulse strengths for both the I- and S-spin channels. At these low and easily attainable rf field strengths, level-crossing events among the energy levels |3/2〉,|1/2〉,|-1/2〉,|-3/2〉 that are known to complicate the CPMAS of quadrupolar nuclei, are mostly avoided. Zero- and double-quantum polarization transfer modes, akin to those we have observed for I(1/2)→S(1/2) polarization transfers, were evidenced by these analyses even in the presence of the quadrupolar interaction. ¹H-²³Na and ¹H-¹¹B BRAIN-CPMAS conditions were experimentally explored on model compounds by optimizing the width of the adiabatic sweep, as well as the rf pulse powers of the ¹H and ²³Na/¹¹B channels, for different MAS rates. The experimental data obtained on model compounds containing spin-3/2 nuclides, matched well predictions from numerical simulations and from an average Hamiltonian theory model. Extensions to half-integer spin nuclides with higher spins and potential applications of this BRAIN-CPMAS approach are discussed.

Measurement of 14N quadrupole couplings in biomolecular solids using indirect-detection 14N solid-state NMR with DNP

Insights into protein structure through the determination of ¹⁴N quadrupolar interactions using magic-angle spinning dynamic nuclear polarization NMR.

(14)N overtone NMR under MAS: signal enhancement using symmetry-based sequences and novel simulation strategies

Overtone ¹⁴N NMR spectroscopy is a promising route for the direct detection of ¹⁴N signals with good spectral resolution.

Overtone ¹⁴N NMR spectroscopy is a promising route for the direct detection of ¹⁴N signals with good spectral resolution. Its application is currently limited, however, by the absence of efficient polarization techniques for overtone signal enhancement and the lack of efficient numerical simulation techniques to aid in both the development of new methods and the analysis and interpretation of experimental data. In this paper we report a novel method for the transfer of polarization from ¹H to the ¹⁴N overtone using symmetry-based R-sequences that overcome many of the limitations of adiabatic approaches that have worked successfully on static samples. Refinement of these sequences and the analysis of the resulting spectra have been facilitated through the development of an efficient simulation strategy for ¹⁴N overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker–Planck equations.

Natural abundance 14N and 15N solid-state NMR of pharmaceuticals and their polymorphs

¹⁴N and ¹⁵N solid-state NMR at natural abundance are used in tandem for the investigation of pharmaceuticals and their polymorphs.

Solid-state NMR indirect detection of nuclei experiencing large anisotropic interactions using spinning sideband-selective pulses

Under Magic-Angle Spinning (MAS), a long radio-frequency (rf) pulse applied on resonance achieves the selective excitation of the center-band of a wide NMR spectrum. We show herein that these rf pulses can be applied on the indirect channel of Hetero-nuclear Multiple-Quantum Correlation (HMQC) sequences, which facilitate the indirect detection via spin-1/2 isotopes of nuclei exhibiting wide spectra. Numerical simulations show that this indirect excitation method is applicable to spin-1/2 nuclei experiencing a large chemical shift anisotropy, as well as to spin-1 isotopes subject to a large quadrupole interaction, such as (14)N. The performances of the long pulses are analyzed by the numerical simulations of scalar-mediated HMQC (J-HMQC) experiments indirectly detecting spin-1/2 or spin-1 nuclei, as well as by dipolar-mediated HMQC (D-HMQC) experiments achieving indirect detection of (14)N nuclei via (1)H in crystalline γ-glycine and N-acetyl-valine samples at a MAS frequency of 60kHz. We show on these solids that for the acquisition of D-HMQC spectra between (1)H and (14)N nuclei, the efficiency of selective moderate excitation with long-pulses at the (14)N Larmor frequency, ν0((14)N), is comparable to those with strong excitation pulses at ν0((14)N) or 2ν0((14)N) frequencies, given the rf field delivered by common solid-state NMR probes. Furthermore, the D-HMQC experiments also demonstrate that the use of long pulses does not produce significant spectral distortions along the (14)N dimension. In summary, the use of center-band selective weak pulses is advantageous for HMQC experiments achieving the indirect detection of wide spectra since it (i) requires a moderate rf field, (ii) can be easily optimized, (iii) displays a high robustness to CSAs, offsets, rf-field inhomogeneities, and fluctuations in MAS frequency, and (iv) is little dependent on the quadrupolar coupling constant.

Ultra-wideline 14N solid-state NMR as a method for differentiating polymorphs: glycine as a case study

¹⁴N solid-state NMR is useful for differentiating polymorphs and chemically distinct nitrogen-containing compounds. A case study of glycine is presented.

PRESTO polarization transfer to quadrupolar nuclei: implications for dynamic nuclear polarization

We show both experimentally and numerically that in experiments involving transfer of magnetization from ¹H to the quadrupolar nuclei under MAS, the PRESTO technique consistently outperforms the traditionally used CP method, affording more quantitative intensities, improved lineshapes, better sensitivity, and easier optimization.