Rotational echo14N/13C/1H triple resonance solid‐state nuclear magnetic resonance: A probe of13C–14N internuclear distances

Accelerated acquisition of wideline solid-state NMR spectra of spin 3/2 nuclei by frequency-stepped indirect detection experiments

73% of all NMR-active nuclei are quadrupolar nuclei with a nuclear spin I > 1/2. The broadening of the solid-state NMR signals by the quadrupolar interaction often leads to poor sensitivity and low resolution. In this work we present experimental and theoretical investigations of magic angle spinning (MAS) 1H{X} double-echo resonance-echo saturation-pulse double-resonance (DE-RESPDOR) and Y{X} J-resolved solid-state NMR experiments for the indirect detection of spin 3/2 quadrupolar nuclei (X = spin 3/2 nuclei, Y = spin 1/2 nuclei). In these experiments, the spectrum of the quadrupolar nucleus is reconstructed by plotting the observed dephasing of the detected spin as a function of the transmitter offset of the indirectly detected spin. Numerical simulations were used to investigate the achievable levels of dephasing and to predict the lineshapes of indirectly detected NMR spectra of the quadrupolar nucleus. We demonstrate 1H, 31P and 207Pb detection of 35Cl, 81Br, and 63Cu (I = 3/2) nuclei in trans-Cl2Pt(NH3)2 (transplatin), (CH3NH3)PbCl3 (methylammonium lead chloride, MAPbCl3), (CH3NH3)PbBr3 (methylammonium lead bromide, MAPbBr3) and CH3C(CH2PPh2)3CuI (1,1,1-tris(diphenylphosphinomethyl)ethane copper(I) iodide, triphosCuI), respectively. In all of these experiments, we were able to detect megahertz wide central transition or satellite transition powder patterns. Significant time savings and gains in sensitivity were attained in several test cases. Additionally, the indirect detection experiments provide valuable structural information because they confirm the presence of dipolar or scalar couplings between the detected nucleus and the quadrupolar nucleus of interest. Finally, numerical simulations suggest these methods are also potentially applicable to abundant spin 5/2 and spin 7/2 quadrupolar nuclei.

Unravelling the mechanism of polarization transfer from spin-1/2 to spin-1 system in solids

An analytic theory based on the concept of "effective-fields" is proposed to explain the mechanism of polarization transfer from spin I = 1/2 to spin S = 1 in non-rotating (static) solids. Employing an isolated two-spin model system, the matching conditions responsible for polarization transfer in cross-polarization (CP) experiments are identified and described in terms of the single-transition operators. In contrast to other existing treatments, the polarization transfer among spins is quantified through analytic expressions highlighting the individual contributions emerging from all plausible CP matching conditions. The interplay between the CP matching conditions observed in experiments is outlined in both isotropic and anisotropic systems and verified through comparison between simulations based on analytic and exact numerical methods. The predictions emerging from the analytic theory are verified over a wide range of experimentally relevant parameters and could be beneficial in the optimization of the CP experiments.

Identification of phosphorus sites in amorphous LiPON thin film by observing internuclear proximities

Amorphous lithium phosphorus oxynitrides (LiPON), prepared by reactive magnetron sputtering, have become the electrolytes of choice for all-solid-state thin film microbatteries since its discovery in early 1990s. Nevertheless, there is still a lack of understanding of their atomic-level structure and its influence on ionic conductivity. Solid-state NMR spectroscopy represents a promising technique to determine the atomic-level structure of LiPON glasses but is challenging owing to its low sensitivity in the case of thin film materials. Recently, 31P solid-state NMR spectra of LiPON thin films were acquired under magic-angle spinning (MAS) conditions and assigned with the help of density functional theory (DFT) calculations of NMR parameters. However, the identification of the different P local environments in these materials is still a challenge owing to their amorphous structure and the lack of resolution of the 31P MAS NMR spectra. We show herein how the NMR observation of internuclear proximities helps to establish the nature of P sites in LiPON thin films. The 31P-14N proximities are probed by a transfer of population in double resonance (TRAPDOR) experiment, whereas 31P-31P proximities are observed using one-dimensional (1D) 31P double-quantum (DQ)-filtered and two-dimensional (2D) 31P homonuclear correlation spectra as well as dipolar dephasing experiments using DQ-DRENAR (DQ-based dipolar-recoupling effects nuclear alignment reduction) technique. The obtained NMR data further support the recently proposed assignment of 31P NMR signals of LiPON thin films. With the help of this assignment, the simulation of the quantitative 1D 31P NMR spectrum indicates that PO43- orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [O3PNPO3]5- dimeric anions. PO3N4- isolated tetrahedra and [O3POPO3]4- anions are also present but in smaller amounts.

Satellite-transition double cross-polarization HETCOR under fast MAS

Double-cross polarization to the satellite-transitions (STs) of half-integer quadrupolar nuclei is demonstrated using proton-detected heteronuclear correlation (HETCOR) under fast magic-angle spinning (MAS). By placing the rf frequency away from the central-transition (CT) and selective to the STs, average Hamiltonian theory shows a scaled effective rf field with a phase equal to the complex ST spinning sideband being irradiated. Such an effective rf field can excite and spinlock STs but the phase spread usually leads to signal cancellation in one-step excitation or cross polarization experiments. The cancellation does not occur for two-step double cross-polarization (DCP) HETCOR experiments, therefore high efficiencies can be obtained. With careful magic-angle calibration, ST and double-quantum ST (DQST) HETCOR experiments are demonstrated with the ³⁵Cl nuclei in histidine·HCl·H₂O. These experiments provide additional information over the commonly observed CT spectra and near isotropic resolution in the case of DQST of spin S = 3/2 quadrupolar nuclei.

Attached Nitrogen Test by 13C-14N Solid-State NMR Spectroscopy for the Structure Determination of Heterocyclic Isomers

Differentiation of heterocyclic isomers by solution ¹H, ¹³C, and ¹⁵N NMR spectroscopy is often challenging due to similarities in their spectroscopic signatures. Here, ¹³C{¹⁴N} solid-state NMR spectroscopy experiments are shown to operate as an "attached nitrogen test", where heterocyclic isomers are easy to distinguish based on one-dimensional nitrogen-filtered ¹³C solid-state NMR. We anticipate that these NMR experiments will facilitate the assignment of heterocyclic isomers during synthesis and natural product discovery.

Double echo symmetry-based REDOR and RESPDOR pulse sequences for proton detected measurements of heteronuclear dipolar coupling constants

¹H{X} symmetry-based rotational echo double resonance pulse sequences (S-REDOR) and symmetry-based rotational echo saturation pulse double resonance (S-RESPDOR) solid-state NMR experiments have found widespread application for ¹H detected measurements of difference NMR spectra, dipolar coupling constants, and internuclear distances under conditions of fast magic angle spinning (MAS). In these experiments the supercycled R4₁² (SR4₁²) symmetry-based recoupling pulse sequence is typically applied to the ¹H spins to reintroduce heteronuclear dipolar couplings. However, the timing of SR4₁² and other symmetry-based pulse sequences must be precisely synchronized with the rotation of the sample, otherwise, the evolution of ¹H CSA and other interactions will not be properly refocused. For this reason, significant distortions are often observed in experimental dipolar dephasing difference curves obtained with S-REDOR or S-RESPDOR pulse sequences. Here we introduce a family of double echo (DE) S-REDOR/S-RESPDOR pulse sequences that function in an analogous manner to the recently introduced t₁-noise eliminated (TONE) family of dipolar heteronuclear multiple quantum coherence (D-HMQC) pulse sequences. Through numerical simulations and experiments the DE S-REDOR/S-RESPDOR sequences are shown to provide dephasing difference curves similar to those obtained with S-REDOR/S-RESPDOR. However, the DE sequences are more robust to the deviations of the MAS frequency from the ideal value that occurs during typical solid-state NMR experiments. The DE sequences are shown to provide more reliable ¹H detected dipolar dephasing difference curves for nuclei such as ¹⁵N (with isotopic labelling), ¹⁸³W and ³⁵Cl. The double echo sequences are therefore recommended to be used in place of conventional S-REDOR/S-RESPDOR sequences for measurement of weak dipolar coupling constants and long-range distances.

Distance measurements to quadrupolar nuclei: Evolution of the rotational echo double resonance technique

Molecular structure determination is the basis for understanding chemical processes and the property of materials. The direct dependence of the magnetic dipolar interaction on the distance makes solid-state nuclear magnetic resonance (NMR) an excellent tool to study molecular structure when X-ray crystallography fails to provide atomic-resolution data. Although techniques to measure distances between pairs of isolated nuclear spin-1/2 pairs are routine and easy to implement using the rotational echo double resonance (REDOR) experiment (Gullion & Schaefer, 1989), the existence of a nucleus with a spin > 1/2, appearing in approximately 75% of the elements in the periodic table, poses a challenge due to difficulties stemming from the large nuclear quadrupolar coupling constant (QCC). This mini-review presents the existing solid-state magic-angle spinning NMR techniques aimed toward the efficient and accurate determination of internuclear distances between a spin-1/2 and a "quadrupolar" nucleus having a spin larger than one half. Analytical expressions are provided for the various recoupling curves stemming from different techniques, and a coherent nomenclature for these various techniques is suggested. Treatment of some special cases such as multiple spin effects and spins with close Larmor frequencies is also discussed. The most advanced methods can recouple spins with quadrupolar frequencies up to tens of megahertz and beyond, expanding the distance measurement capabilities of solid-state NMR to an increasingly growing number of applications and nuclear spin systems.

Deuteron Chemical Exchange Saturation Transfer for the Detection of Slow Motions in Rotating Solids

We utilized the ²H Chemical Exchange Saturation Transfer (CEST) technique under magic angle spinning (MAS) conditions to demonstrate the feasibility of the method for studies of slow motions in the solid state. For the quadrupolar anisotropic interaction, the essence of CEST is to scan the saturation pattern over a range of offsets corresponding to the entire spectral region(s) for all conformational states involved, which translates into a range of −60–+ 60 kHz for methyl groups. Rotary resonances occur when the offsets are at half-and full-integer of the MAS rates. The choice of the optimal MAS rate is governed by the condition to reduce the number of rotary resonances in the CEST profile patterns and retain a sufficiently large quadrupolar interaction active under MAS to maintain sensitivity to motions. As examples, we applied this technique to a well-known model compound dimethyl-sulfone (DMS) as well as amyloid-β fibrils selectively deuterated at a single methyl group of A2 belonging to the disordered domain. It is demonstrated that the obtained exchange rate between the two rotameric states of DMS at elevated temperatures fell within known ranges and the fitted model parameters for the fibrils agree well with the previously obtained value using static ²H NMR techniques. Additionally, for the fibrils we have observed characteristic broadening of rotary resonances in the presence of conformational exchange, which provides implications for model selection and refinement. This work sets the stage for future potential extensions of the ²H CEST under MAS technique to multiple-labeled samples in small molecules and proteins.

High-Pressure Synthesis and Gas-Sensing Tests of 1-D Polymer/Aluminophosphate Nanocomposites

Recently, filling zeolites with gaseous hydrocarbons at high pressures in diamond anvil cells has been carried out to synthesize novel polymer-guest/zeolite-host nanocomposites with potential, intriguing applications, although the small amount of materials, 10^-7 cm³, severely limited true technological exploitation. Here, liquid phenylacetylene, a much more practical reactant, was polymerized in the 12 Å channels of the aluminophosphate Virginia Polytechnic Institute-Five (VFI) at about 0.8 GPa and 140 °C, with large volumes in the order of 0.6 cm³. The resulting polymer/VFI composite was investigated by synchrotron X-ray diffraction and optical and ¹H, ¹³C, and ²⁷Al nuclear magnetic resonance spectroscopy. The materials, consisting of disordered π-conjugated polyphenylacetylene chains in the pores of VFI, were deposited on quartz crystal microbalances and tested as gas sensors. We obtained promising sensing performances to water and butanol vapors, attributed to the finely tuned nanostructure of the composites. High-pressure synthesis is used here to obtain an otherwise unattainable true technological material.

Recent developments in deuterium solid-state NMR for the detection of slow motions in proteins

Slow timescale dynamics in proteins are essential for a variety of biological functions spanning ligand binding, enzymatic catalysis, protein folding and misfolding regulations, as well as protein-protein and protein-nucleic acid interactions. In this review, we focus on the experimental and theoretical developments of 2H static NMR methods applicable for studies of microsecond to millisecond motional modes in proteins, particularly rotating frame relaxation dispersion (R1ρ), quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) relaxation dispersion, and quadrupolar chemical exchange saturation transfer NMR experiments (Q-CEST). With applications chosen from amyloid-β fibrils, we show the complementarity of these approaches for elucidating the complexities of conformational ensembles in disordered domains in the non-crystalline solid state, with the employment of selective deuterium labels. Combined with recent advances in relaxation dispersion backbone measurements for 15N/13C/1H nuclei, these techniques provide powerful tools for studies of biologically relevant timescale dynamics in disordered domains in the solid state.

Low-power synchronous helical pulse sequences for large anisotropic interactions in MAS NMR: Double-quantum excitation of 14N

We develop a theoretical framework for a class of pulse sequences in the nuclear magnetic resonance (NMR) of rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radiofrequency amplitude is smaller than or comparable to the spinning frequency. The treatment is based on average Hamiltonian theory and allows us to derive pulse sequences with well-defined relationships between the pulse parameters and spinning frequency for exciting specific coherences without the need for any detailed calculations. This framework is applied to the excitation of double-quantum spectra of ¹⁴N and is used both to evaluate the existing low-power pulse schemes and to predict the new ones, which we present here. It is shown that these sequences can be designed to be γ-encoded and therefore allow the acquisition of sideband-free spectra. It is also shown how these new double-quantum excitation sequences are incorporated into heteronuclear correlation NMR, such as ¹H-¹⁴N dipolar double-quantum heteronuclear multiple-quantum correlation spectroscopy. The new experiments are evaluated both with numerical simulations and experiments on glycine and N-acetylvaline, which represent cases with "moderate" and "large" quadrupolar interactions, respectively. The analyzed pulse sequences perform well for the case of a "moderate" quadrupolar interaction, however poorly with a "large" quadrupolar interaction, for which future work on pulse sequence development is necessary.

Effective Hamiltonian and 1H-14N cross polarization/double cross polarization at fast MAS

In this work we investigate in detail the underlying spin-dynamics associated with ¹H-¹⁴N double CP experiments under fast MAS, recently demonstrated by Carnevale et al. We employ matrix logarithm and Floquet theory to compute numerically the effective Hamiltonian associated to the time-dependent problem. Certain common features related to construction of effective Hamiltonians by both approaches are discussed. The main observations related to ¹H-¹⁴N CPMAS/double CP transfer are: (a) various spin terms of the effective Hamiltonian strongly depend on the crystallite orientation; (b) significant CP transfer occurs only when the magnitudes of the effective¹H and ¹⁴N RF strengths are comparable, and simultaneously all pure ¹⁴N terms in the effective Hamiltonian are small, except for the longitudinal and the RF terms; (c) the sign of ¹⁴N CPMAS signal follows the sign of ¹⁴N effective RF strength; (d) sign of the double CP signal is largely independent of crystallite orientation. We predict and verify matching conditions employing multiples of the spinning frequency or involving different ¹⁴N RF strengths. We provide an analytical proof for (d). The proof also provides an estimate for the ratio of ¹H-¹⁴N and ¹⁴N-¹H transfer amplitudes which is further substantiated through simulations. In addition, we find that double CP signals include contributions from several single-quantum coherences present after the first CP process. The uneven contribution from different coherences leads to a reversal of signal at very short contact times, a feature noted experimentally by Carnevale et al. The connection between CPMAS transfer and efficient spin-lock is discussed and illustrated. The factors affecting second-order quadrupolar lineshapes in double CP experiment are examined. With a linear ramp of ¹H RF amplitude we have observed that significant CP transfer occurs for more crystallite orientations resulting in improved sensitivity.

High-Resolution NMR of S = 3/2 Quadrupole Nuclei by Detection of Double-Quantum Satellite Transitions via Protons

Indirect NMR detection via protons under fast magic-angle spinning can help overcome the low sensitivity and resolution of low-γ quadrupole nuclei such as ³⁵Cl. A robust and efficient method is presented for indirectly acquiring the double-quantum satellite-transition (DQ-ST) spectra of quadrupole nuclei. For a spin S = 3/2, the DQ-STs have a much smaller second-order quadrupolar broadening, one-ninth compared to that of the central transition. Thus, they can provide a factor of up to 18 in resolution enhancement. The indirect detection of DQ-STs via protons is carried out using the heteronuclear multiple-quantum coherence (HMQC) experiment with the transfer of populations in double-resonance (TRAPDOR) recoupling mechanism. The resolution enhancement by detecting DQ-STs and the high efficiency of the TRAPDOR-HMQC experiment are demonstrated by ³⁵Cl NMR of several active pharmaceutical ingredients (APIs).

Deuteron Quadrupolar Chemical Exchange Saturation Transfer (Q-CEST) Solid-State NMR for Static Powder Samples: Approach and Applications to Amyloid-β Fibrils

We provide an experimental and computational framework for 2 H quadrupolar chemical exchange saturation transfer NMR experiments (Q-CEST) under static solid-state conditions for the quantification of dynamics on μs-ms timescales. Simulations using simple 2-site exchange models provide insights into the relation between spin dynamics and motions. Biological applications focus on two sites of amyloid-β fibrils in the 3-fold symmetric polymorph. The first site, the methyl group of A2 of the disordered N-terminal domain, undergoes diffusive motions and conformational exchange due to transient interactions. Earlier 2 H rotating frame relaxation and quadrupolar CPMG measurements are combined with the Q-CEST approach to characterize the multiple conformational states of the domain. The second site, the methyl group of M35, spans the water-accessible cavity inside the fibrils' core and undergoes extensive rotameric exchange. Q-CEST permits us to refine the rotameric exchange model for this site and allows the more precise determination of populations and rotameric exchange rate constants than line shape analysis.

Using the heteronuclear Bloch-Siegert shift of protons for B1 calibration of insensitive nuclei not present in the sample

Indirect rf field calibration using the heteronuclear Bloch-Siegert shift is presented. This method is useful for calibrating ω₁ = -γB₁ for the rf channels of small volume fast-spinning probes on which direct rf calibration is practically inconvenient or difficult for insensitive low-γ nuclei. Proton signals are observed for the rf calibration of the insensitive nuclei without requiring their presence in the sample. For a linearly modulated rf field, the heteronuclear Bloch-Siegert shift is given by, Δω_BS=ω₀ω₁²/ω₀²-ω_irr², where ω₀ and ω_irr are the Larmor and irradiation frequencies, respectively. A short protocol using full-echo acquisition of protons is described for measurement of the phase change induced by the Bloch-Siegert shift. The calibration procedure is validated by a comparison with direct ¹³C calibration and demonstrated for ¹⁴N rf field measurement of a 0.75 mm 100 kHz triple-resonance magic-angle spinning probe.

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.

Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement

We report a high-field in-situ solid-state NMR study of the hydration of CaAl₂O₄ (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state ²⁷Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS ²⁷Al NMR spectra, was used to monitor the ²⁷Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of ²⁷Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.

Deuterium Rotating Frame NMR Relaxation Measurements in the Solid State under Static Conditions for Quantification of Dynamics

The feasibility of static deuterium rotating frame NMR relaxation measurements for characterization of slow timescale motions in powder systems is demonstrated. Using a model compound dimethyl sulfone-d6 , we show that these measurements yield conformational exchange rates and activation energy values in accordance with results obtained with other techniques. Furthermore, we demonstrate that the full Liouvillian approach as opposed to the Redfield approximation is necessary to analyze the experimental data.

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.

Probing Interactions between Electron Donors and the Support in MgCl2-Supported Ziegler-Natta Catalysts

Olefin polymerization using Ziegler-Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst's MgCl2 support in binary adducts prepared by ball-milling. Besides the bare MgCl2 support and a MgCl2-TiCl4 adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different donor molecules, making it difficult to identify specific binding motives. The surface sites with large 35Cl NMR line widths can be observed using high radio frequency field strengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe additional surface sites with intermediately high quadrupolar couplings, suggesting a different surface structure for this particular adduct compared to the other systems. For 25Mg, pronounced effects of donor binding on the quadrupole interaction parameters are observed, both computationally and experimentally. Again the adduct with the 2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior of the surface sites compared to the other adducts, which display more asymmetric coordinations of the surface Mg sites. Identifying specific binding motives by comparing 25Mg NMR results to DFT calculations also proves to be difficult, however. This is attributed to the existence of many defect structures caused by the ball-milling process. The existence of such defect structures both at the surface and in the interior of the MgCl2 particles is corroborated by NMR relaxation studies. Finally, we performed heteronuclear correlation experiments, which reveal interactions between the support and Mg-OH surface groups, but do not provide indications for donor-surface interactions.

Pushing the limit of NMR-based distance measurements - retrieving dipolar couplings to spins with extensively large quadrupolar frequencies

Dipolar recoupling under magic-angle spinning allows to measure accurate inter-nuclear distances provided that the two interacting spins can be efficiently and uniformly excited. Alexander (Lex) Vega has shown that adiabatic transfers of populations in quadrupolar spins during the application of constant-wave (cw) radio-frequency pulses lead to efficient and quantifiable dipolar recoupling curves. Accurate distance determination within and beyond the adiabatic regime using cw pulses is limited by the size of the quadrupolar coupling constant. Here we show that using the approach of long-pulse phase modulation, dipolar recoupling and accurate distances can be obtained for nuclei having extensively large quadrupolar frequencies of 5-10 MHz. We demonstrate such results by obtaining a ³¹P-^79/81Br distance in a compound for which bromine-79 (spin-3/2) has a quadrupolar coupling constant of 11.3 MHz, and a ¹³C-²⁰⁹Bi distance where the bismuth (spin-9/2) has a quadrupolar coupling constant of 256 MHz, equaling a quadrupolar frequency of 10.7 MHz. For Bromine, we demonstrate that an analytical curve based on the assumption of complete spin saturation fits the data. In the case of bismuth acetate, a C-Bi₃ spin system must be used in order to match the correct saturation recoupling curve, and results are in agreement with the crystallographic structure.

Low-power broadband solid-state MAS NMR of 14N

We propose two broadband pulse schemes for ¹⁴N solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) that achieves (i) complete population inversion and (ii) efficient excitation of the double-quantum spectrum using low-power single-sideband-selective pulses. We give a comprehensive theoretical description of both schemes using a common framework that is based on the jolting-frame formalism of Caravatti et al. [J. Magn. Reson. 55, 88 (1983)]. This formalism is used to determine for the first time that we can obtain complete population inversion of ¹⁴N under low-power conditions, which we do here using single-sideband-selective adiabatic pulses. It is then used to predict that double-quantum coherences can be excited using low-power single-sideband-selective pulses. We then proceed to design a new experimental scheme for double-quantum excitation. The final double-quantum excitation pulse scheme is easily incorporated into other NMR experiments, as demonstrated here for double quantum-single quantum ¹⁴N correlation spectroscopy, and ¹H-¹⁴N dipolar heteronuclear multiple-quantum correlation experiments. These pulses and irradiation schemes are evaluated numerically using simulations on single crystals and full powders, as well as experimentally on ammonium oxalate ((NH₄)₂C₂O₄) at moderate MAS and glycine at ultra-fast MAS. The performance of these new NMR methods is found to be very high, with population inversion efficiencies of 100% and double-quantum excitation efficiencies of 30%-50%, which are hitherto unprecedented for the low radiofrequency field amplitudes, up to the spinning frequency, that are used here.

The coordinative state of aluminium alkyls in Ziegler–Natta catalysts

Multinuclear solid-state NMR provides insight in the interactions of aluminium alkyl based cocatalysts in MgCl₂-supported Ziegler–Natta catalysts.

Reprint of: Localization of Cl-35 Nuclei in Biological Solids using Rotational-Echo Double-Resonance Experiments

Chloride ions play important roles in many chemical and biological processes. This paper investigates the possibility of localizing ³⁵Cl nuclei using solid-state NMR. It demonstrates that distances shorter than 3.8Å, between ¹³C atoms and ³⁵Cl atoms in 10% uniformly labeled ¹³C L-tyrosine·HCl and natural abundance Glycine·HCl can be measured using rotational-echo (adiabatic passage) double-resonance (RE(AP)DOR). Furthermore the effect of quadrupolar interaction on the REDOR/REAPDOR experiment is quantified. The dephasing curve is plotted in a three dimensional chart as a function of the dephasing time and of the strength of quadrupolar interaction felt by each orientation. During spinning each orientation feels a quadrupolar interaction that varies in time, and therefore at each moment in time we reorder the crystallite orientations as a function of their contribution to the dephasing curve. In this way the effect of quadrupolar interaction on the dipolar dephasing curve can be fitted with a polynomial function. The numerical investigation performed allows us to generate REDOR/REAPDOR curves which are then used to simulate the experimental data.

Solvent signal suppression for high-resolution MAS-DNP

Dynamic nuclear polarization (DNP) has become a powerful tool to substantially increase the sensitivity of high-field magic angle spinning (MAS) solid-state NMR experiments. The addition of dissolved hyperpolarizing agents usually results in the presence of solvent signals that can overlap and obscure those of interest from the analyte. Here, two methods are proposed to suppress DNP solvent signals: a Forced Echo Dephasing experiment (FEDex) and TRAnsfer of Populations in DOuble Resonance Echo Dephasing (TRAPDORED) NMR. These methods reintroduce a heteronuclear dipolar interaction that is specific to the solvent, thereby forcing a dephasing of recoupled solvent spins and leaving acquired NMR spectra free of associated resonance overlap with the analyte. The potency of these methods is demonstrated on sample types common to MAS-DNP experiments, namely a frozen solution (of l-proline) and a powdered solid (progesterone), both containing deuterated glycerol as a DNP solvent. The proposed methods are efficient, simple to implement, compatible with other NMR experiments, and extendable past spectral editing for just DNP solvents. The sensitivity gains from MAS-DNP in conjunction with FEDex or TRAPDORED then permits rapid and uninterrupted sample analysis.

Advanced solid-state NMR spectroscopy of natural organic matter

Solid-state NMR is essential for the characterization of natural organic matter (NOM) and is gaining importance in geosciences and environmental sciences. This review is intended to highlight advanced solid-state NMR techniques, especially a systematic approach to NOM characterization, and their applications to the study of NOM. We discuss some basics of how to acquire high-quality and quantitative solid-state ¹³C NMR spectra, and address some common technical mistakes that lead to unreliable spectra of NOM. The identification of specific functional groups in NOM, primarily based on ¹³C spectral-editing techniques, is described and the theoretical background of some recently-developed spectral-editing techniques is provided. Applications of solid-state NMR to investigating nitrogen (N) in NOM are described, focusing on limitations of the widely used ¹⁵N CP/MAS experiment and the potential of improved advanced NMR techniques for characterizing N forms in NOM. Then techniques used for identifying proximities, heterogeneities and domains are reviewed, and some examples provided. In addition, NMR techniques for studying segmental dynamics in NOM are reviewed. We also briefly discuss applications of solid-state NMR to NOM from various sources, including soil organic matter, aquatic organic matter, organic matter in atmospheric particulate matter, carbonaceous meteoritic organic matter, and fossil fuels. Finally, examples of NMR-based structural models and an outlook are provided.

Localization of Cl-35 nuclei in biological solids using rotational-echo double-resonance experiments

Chloride ions play important roles in many chemical and biological processes. This paper investigates the possibility of localizing ³⁵Cl nuclei using solid-state NMR. It demonstrates that distances shorter than 3.8Å, between ¹³C atoms and ³⁵Cl atoms in 10% uniformly labeled ¹³C L-tyrosine·HCl and natural abundance Glycine·HCl can be measured using rotational-echo (adiabatic passage) double-resonance (RE(AP)DOR). Furthermore the effect of quadrupolar interaction on the REDOR/REAPDOR experiment is quantified. The dephasing curve is plotted in a three dimensional chart as a function of the dephasing time and of the strength of quadrupolar interaction felt by each orientation. During spinning each orientation feels a quadrupolar interaction that varies in time, and therefore at each moment in time we reorder the crystallite orientations as a function of their contribution to the dephasing curve. In this way the effect of quadrupolar interaction on the dipolar dephasing curve can be fitted with a polynomial function. The numerical investigation performed allows us to generate REDOR/REAPDOR curves which are then used to simulate the experimental data.

Theoretical study of a simple rotational-echo double-resonance NMR for homonuclear spin-1/2 pairs

We investigate theoretically intriguing aspects of a simple rotational-echo double-resonance (REDOR) NMR technique for homonuclear spin-1/2 pairs undergoing MAS. The simple technique sets Gaussian soft π pulses at every half MAS rotational period in the pulse sequence. The reduction in rotational echo amplitude (the REDOR echo reduction) is observed at the end of the evolution period te  = (n + 1)Tr, where Tr is a MAS rotational period. The exact average Hamiltonians for the homonuclear REDOR (hm-REDOR) technique are calculated by dividing the evolution period into four periods. We show theoretically and experimentally that the hm-REDOR technique produces the REDOR echo reductions for homonuclear spin-1/2 pairs. In addition, the theoretical results reveal that the REDOR echo reductions are independent of the chemical-shift difference, δ, under a simple condition of κ = δ/ωr  ≥ 6 and te  < 10 ⋅ (1/d'), where ωr is the sample spinning frequency and d' is the dipolar coupling constant expressed in Hz. We call this simple condition the master condition. This means that the REDOR echo reductions for a homonuclear spin-1/2 pair can be calculated under the master condition by considering only d' and ωr , which is the case for a heteronuclear spin pair. Finally, we demonstrate that four-phase cycling yields the multiple-quantum filtered hm-REDOR experiment, where the appearance of the REDOR echo reductions shows that the echo reductions are definitely attributable to the homonuclear dipolar interaction even if there is a slight unwanted effect from the recovered chemical-shift anisotropy in these reductions.

Determination of NH proton chemical shift anisotropy with (14)N-(1)H heteronuclear decoupling using ultrafast magic angle spinning solid-state NMR

The extraction of chemical shift anisotropy (CSA) tensors of protons either directly bonded to (14)N nuclei (I=1) or lying in their vicinity using rotor-synchronous recoupling pulse sequence is always fraught with difficulty due to simultaneous recoupling of (14)N-(1)H heteronuclear dipolar couplings and the lack of methods to efficiently decouple these interactions. This difficulty mainly arises from the presence of large (14)N quadrupolar interactions in comparison to the rf field that can practically be achieved. In the present work it is demonstrated that the application of on-resonance (14)N-(1)H decoupling with rf field strength ∼30 times weaker than the (14)N quadrupolar coupling during (1)H CSA recoupling under ultrafast MAS (90kHz) results in CSA lineshapes that are free from any distortions from recoupled (14)N-(1)H interactions. With the use of extensive numerical simulations we have shown the applicability of our proposed method on a naturally abundant l-Histidine HCl·H2O sample.

51V magic angle spinning NMR spectroscopy and quantum chemical calculations in vanadium bio-inorganic systems: current perspective

In recent years, 51V magic angle spinning (MAS) NMR spectroscopy has been widely used to characterize vanadium centers in biology, biomimetic complexes, and inorganic compounds of medicinal and industrial relevance. It has been demonstrated that 51V NMR parameters are sensitive probes of the coordination geometry and chemical environment of the metal center, beyond the first coordination sphere. To establish the relationships between NMR parameters and structure of the vanadium centers, over the past decade a large series of coordination complexes have been analyzed by MAS NMR spectroscopy. It has been demonstrated that the interpretation of the NMR parameters requires the use of theoretical methods, such as density functional (DFT) theory, whereby the experimental NMR observables are linked to the electronic and structural properties of a molecule. DFT calculations have been successfully employed to not only predict NMR parameters but to also yield valuable information regarding the structure and function of various vanadium compounds. In this report, we review the current state of the field, and present a survey of bioinorganic vanadium complexes as well as vanadium-dependent haloperoxidases analyzed using 51V MAS NMR spectroscopy and DFT calculations, to illustrate the rich information content available from such a combined approach.

Analysis of trivalent cation complexation to functionalized mesoporous silica using solid-state NMR spectroscopy

The first comprehensive study of Al(iii) and Sc(iii) interactions with a novel hybrid material, N-[5-(trimethoxysilyl)-2-aza-1-oxopentyl]caprolactam functionalized mesoporous silica, was conducted using solid-state NMR spectroscopy.

Acidity characterization of heterogeneous catalysts by solid-state NMR spectroscopy using probe molecules

Characterization of the surface acidic properties of solid acid catalysts is a key issue in heterogeneous catalysis. Important acid features of solid acids, such as their type (Brønsted vs. Lewis acid), distribution and accessibility (internal vs. external sites), concentration (amount), and strength of acid sites are crucial factors dictating their reactivity and selectivity. This short review provides information on different solid-state NMR techniques used for acidity characterization of solid acid catalysts. In particular, different approaches using probe molecules containing a specific nucleus of interest, such as pyridine-d5, 2-(13)C-acetone, trimethylphosphine, and trimethylphosphine oxide, are compared. Incorporation of valuable information (such as the adsorption structure, deprotonation energy, and NMR parameters) from density functional theory (DFT) calculations can yield explicit correlations between the chemical shift of adsorbed probe molecules and the intrinsic acid strength of solid acids. Methods that combine experimental NMR data with DFT calculations can therefore provide both qualitative and quantitative information on acid sites.

Solid-state NMR: a powerful tool for characterization of metal-organic frameworks

Metal-organic frameworks (MOFs) are a new type of porous materials with numerous current and potential applications in many areas including ion-exchange, catalysis, sensing, separation, molecular recognition, drug delivery and, in particular, gas storage. Solid-state NMR (SSNMR) has played a pivotal role in structural characterization and understanding of host-guest interactions in MOFs. This article provides an overview on application of SSNMR to MOF systems.

Insights into the spin dynamics of a large anisotropy spin subjected to long-pulse irradiation under a modified REDOR experiment

Distance measurements between a spin-1/2 and a second spin bearing a large anisotropy are performed using a modified rotational echo double resonance (REDOR) experiment. By applying pairs of rotor-synchronized π pulses on the detected spin and a single long pulse on the coupled spin the dipolar interaction is efficiently recoupled even at the sudden passage limit where both adiabaticity and the hard pulse approximation are not valid. In this manuscript we derive the theoretical basis for analyzing the behavior of single crystallites in order to gain insight into the mechanism of dipolar recoupling, and in order to find conditions for optimizing the experiment. The use of reduced time and frequency variables show that the signal depends on the ratios of the radio frequency strength ν(1) and the anisotropy, either the CSA (ν(σ)) or the quadrupolar interaction (ν(Q)), with respect to the spinning frequency ν(R). We derive expressions for the contribution of individual crystallites to the signal arising from the different frequencies mν(d) (m=0,1…2S) associated with the dipolar interaction and show that they result in a non-random distribution of intensities. For a spin-1/2 with a large CSA (up to 1MHz and more) we show using calculations and simulations that the result is a recoupling signal that takes maximal values ΔS/S(0) of ~0.6-0.7, beyond the saturation limit of 0.5, defined by equal contribution of all transitions. For a spin-3/2 we show that at certain conditions the non-random scrambling may result in an apparent saturation-like behavior. In all cases large RF amplitudes are not necessarily required for obtaining efficient recoupling. (13)C-(11)B LA-REDOR (Low-Alpha/Low-rf-Amplitude REDOR) dipolar recoupling experiments on 4-methoxyphenylboronic acid were performed following optimization of the spinning rates suitable for low amplitude radio-frequency power levels and show that efficient recoupling can be obtained for a spin-3/2, and that distance determination is not very strongly dependent on the actual value of the quadrupolar coupling constant.

Effects of the orientation of the 23Na-29Si dipolar vector on the dipolar mediated heteronuclear solid state NMR correlation spectrum of crystalline sodium silicates

Dipolar-Heteronuclear Multiple Quantum Correlation (D-HMQC) experiment based on SR4(2)(1) recoupling was shown as a very efficient probe of spatial proximities in ordered or disordered materials. As crystalline sodium silicates have been extensively studied using 1D and 2D MAS NMR experiments and DFT calculations, they have been used as candidate model systems to perform this D-HMQC experiment. In this work, we demonstrate that the combination of (29)Si and (23)Na NMR at high magnetic field and DFT calculations makes it possible to revisit the assignment of the NMR signature of the δ-Na(2)Si(2)O(5) polymorph. A D-HMQC experiment performed on this crystalline sample reveals lineshape distortions on the (23)Na powder patterns extracted from the 2D correlation. Numerical simulations showed that these distortions result from an effect of the relative orientation between the (23)Na quadrupolar tensor and the (23)Na-(29)Si dipolar vector at the origin of the magnetization transfer.

Suppression of probe background signals via B(1) field inhomogeneity

A new approach combining a long pulse with the DEPTH sequence (Cory and Ritchey, Journal of Magnetic Resonance, 1988) greatly improves the efficiency for suppressing probe background signals arising from spinning modules. By applying a long initial excitation pulse in the DEPTH sequence, instead of a π/2 pulse, the inhomogeneous B(1) fields outside the coil can dephase the background coherence in the nutation frame. The initial long pulse and the following two consecutive EXORCYCLE π pulses function complementarily and prove most effective in removing background signals from both strong and weak B₁ fields. Experimentally, the length of the long pulse can be optimized around odd multiples of the π/2 pulse, depending on the individual probe design, to preserve signals inside the coil while minimizing those from probe hardware. This method extends the applicability of the DEPTH sequence to probes with small differences in B₁ field strength between the inside and outside of the coil, and can readily combine with well-developed double resonance experiments for quantitative measurement. In general, spin systems with weak internal interactions are required to attain efficient and uniform excitation for powder samples, and the principles to determine the applicability are discussed qualitatively in terms of the relative strength of spin interactions, r.f. power and spinning rate.

Solid-state NMR detection of 14N-13C dipolar couplings between amino acid side groups provides constraints on amyloid fibril architecture

Solid-state nuclear magnetic resonance (SSNMR) is a powerful technique for the structural analysis of amyloid fibrils. With suitable isotope labelling patterns, SSNMR can provide constraints on the secondary structure, alignment and registration of β-strands within amyloid fibrils and identify the tertiary and quaternary contacts defining the packing of the β-sheet layers. Detection of (14)N-(13)C dipolar couplings may provide potentially useful additional structural constraints on β-sheet packing within amyloid fibrils but has not until now been exploited for this purpose. Here a frequency-selective, transfer of population in double resonance SSNMR experiment is used to detect a weak (14)N-(13)C dipolar coupling in amyloid-like fibrils of the peptide H(2)N-SNNFGAILSS-COOH, which was uniformly (13)C and (15)N labelled across the four C-terminal amino acids. The (14)N-(13)C interatomic distance between leucine and asparagine side groups is constrained between 2.4 and 3.8 Å, which allows current structural models of the β-spine arrangement within the fibrils to be refined. This procedure could be useful for the general structural analysis of other proteins in condensed phases and environments, such as biological membranes.

Efficient rotational echo double resonance recoupling of a spin-1/2 and a quadrupolar spin at high spinning rates and weak irradiation fields

A modification of the rotational echo (adiabatic passage) double resonance experiments, which allows recoupling of the dipolar interaction between a spin-1/2 and a half integer quadrupolar spin is proposed. We demonstrate efficient and uniform recoupling at high spinning rates (nu(r)), low radio-frequency (RF) irradiation fields (nu(1)), and high values of the quadrupolar interaction (nu(q)) that correspond to values of alpha=nu(1)(2)/nu(q)nu(r), the adiabaticity parameter, which are down to less than 10% of the traditional adiabaticity limit for a spin-5/2 (alpha=0.55). The low-alpha rotational echo double resonance curve is obtained when the pulse on the quadrupolar nucleus is extended to full two rotor periods and beyond. For protons (spin-1/2) and aluminum (spin-5/2) species in the zeolite SAPO-42, a dephasing curve, which is significantly better than the regular REAPDOR experiment (pulse length of one-third of the rotor period) is obtained for a spinning rate of 13 kHz and RF fields down to 10 and even 6 kHz. Under these conditions, alpha is estimated to be approximately 0.05 based on an average quadrupolar coupling in zeolites. Extensive simulations support our observations suggesting the method to be robust under a large range of experimental values.