Solid state nuclear magnetic resonance最新文献

Under normal experimental conditions in an achiral environment, NMR spectra of enantiomers have chemical shifts and J couplings which are not differentiable. In this work, the reproducibility of spectral intensities for pairs of amino acid enantiomers, as well as factors influencing these intensities, is assessed using ¹³C and ¹⁵N cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. Prompted by a recent literature debate over a possible influence of the chirality-induced spin selectivity (CISS) effect on spectral intensities obtained in CP/MAS NMR experiments carried out on enantiomers, a number of control experiments were performed with recycle delays of at least 5T₁. These included the analysis of proton-decoupled Bloch decay solid-state NMR spectra as well as solution NMR spectra where the cross polarization process is absent. Bloch decay and CP/MAS NMR spectra yield the same relative intensities for pairs of enantiomers while solution NMR spectra provide relative intensities closest to unity. Differences of plus-or-minus a few percent in the D/L spectral intensity ratios observed in all solid-state NMR experiments are due to sample preparation (i.e., grinding, particle size, partial amorphization) and limitations on sample purity. As previously described in the literature, more drastic intensity differences on the order of 50% are easily created by ball milling the samples. Finally, apodization is shown to invert the apparent D/L ratio in low signal-to-noise ¹⁵N CP/MAS NMR spectra of aspartic acid enantiomers. In summary, no spectral intensity differences attributable to enantiomerism are identified.

¹⁴N NMR of magnetically oriented microcrystals is reported. With a home-built ¹H–¹³C–¹⁴N probe capable of modulating the rotation of the sample around the axis normal to the magnetic field, magnetically oriented microcrystal suspension (MOMS) of l-alanine is made. ¹⁴N NMR spectra acquired with various timings during intermittent rotation lead to a rotation pattern of the MOMS similar to that of a single crystal. The effect of orientational distribution of the microcrystals to broadening of the resonance line is discussed.

Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.

Deuterium rotating frame solid-state NMR relaxation measurements (²H $R_{1\rho }$) are important tools in quantitative studies of molecular dynamics. We demonstrate how ²H to ¹³C cross-polarization (CP) approaches under 10–40 kHz magic angle spinning rates can be combined with the ²H $R_{1\rho }$ blocks to allow for extension of deuterium rotating frame relaxation studies to methyl groups in biomolecules. This extension permits detection on the ¹³C nuclei and, hence, for the achievement of site-specific resolution. The measurements are demonstrated using a nine-residue low complexity peptide with the sequence GGKGMGFGL, in which a single selective −¹³CD₃ label is placed at the methionine residue. Carbon-detected measurements are compared with the deuterium direct-detection results, which allows for fine-tuning of experimental approaches. In particular, we show how the adiabatic respiration CP scheme and the double adiabatic sweep on the ²H and ¹³C channels can be combined with the ²H $R_{1\rho }$ relaxation rates measurement. Off-resonance ²H $R_{1\rho }$ measurements are investigated in addition to the on-resonance condition, as they extent the range of effective spin-locking field.

The development of NMR crystallography methods requires a reliable database of chemical shifts measured for systems with known crystal structure. We measured and assigned carbon and hydrogen chemical shifts of twenty solid natural amino acids of known polymorphic structure, meticulously determined using powder X-ray diffraction. We then correlated the experimental data with DFT-calculated isotropic shieldings. The small size of the unit cell of most amino acids allowed for advanced computations using various families of DFT functionals, including generalized gradient approximation (GGA), meta-GGA and hybrid DFT functionals. We tested several combinations of functionals for geometry optimizations and NMR calculations. For carbon shieldings, the widely used GGA functional PBE performed very well, although an improvement could be achieved by adding shielding corrections calculated for isolated molecules using a hybrid functional. For hydrogen nuclei, we observed the best performance for NMR calculations carried out with structures optimized at the hybrid DFT level. The high fidelity of the calculations made it possible to assign additional signals that could not be assigned based on experiments alone, for example signals of two non-equivalent molecules in the unit cell of some of the amino acids.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful technique for characterizing the local structure and dynamics of battery and other materials. It has been widely used to investigate bulk electrode compounds, electrolytes, and interfaces. Beside common ex situ investigations, in situ and operando techniques have gained considerable importance for understanding the reaction mechanisms and cell degradation of electrochemical cells.

Herein, we present the recent development of in situ magic angle spinning (MAS) NMR methodologies to study batteries with high spectral resolution, setting into context possible advances on this topic. A mini cylindrical cell type insert for 4 mm MAS rotors is introduced here, being demonstrated on a Li/VO₂F electrochemical system, allowing the acquisition of high-resolution ⁷Li MAS NMR spectra, spinning the electrochemical cell up to 15 kHz.

Field-stepwise-swept solid-state ¹²⁷I NMR experiments of 1,4-diiodobenzene, C₆H₄I₂, applied to a Zeeman-perturbed NQR region, have been presented. A series of QCPMG measurements is performed at T = 90 K with resonant frequencies of 271 MHz in the range of magnetic fields from 2.5 T to zero with the interval of 12 mT. The spectral simulation, in which a numerical calculation involves the diagonalization of the combined Zeeman-quadrupolar Hamiltonian, provides quadrupole coupling constant (C_Q) = 1863(5) MHz and the asymmetry parameter (η_Q) = 0.04(2). The ¹²⁷I NQR spectrum is observed at T = 90 K, which is consistent in the above experimental results.

The proton-phosphorus (H–P) cross-polarization (CP) is effective in Sn(HPO₄)₂·H₂O despite of the presence of paramagnetic ion impurities. Polarization constants T_H-P and ¹H T_1ρ times are measured in static Sn(HPO₄)₂·H₂O by the kinetic variable-temperature H–P CP experiments. The temperature dependence of the ¹H T_1ρ times is interpreted in terms of proton movements in the interlayer space occurring between the phosphate groups without participation of the water molecules. The process requires an activation energy of 8.7 ± 0.7 kcal/mol. The MAS effect on the ¹H T_1ρ times is shown and discussed.

High-resolution low-field nuclear magnetic resonance (NMR) spectroscopy has found wide application for characterization of liquid compounds because of the low maintenance cost of modern permanent magnets. Solid-state NMR so far is limited to low-resolution measurements of static powders, because of the limited space available in this type of magnet. Magic-angle sample spinning and low-magnetic fields are an attractive combination to achieve high spectral resolution especially for paramagnetic solids. Here we show that magic angle spinning modules can be miniaturized using 3D printing techniques so that high-resolution solid-state NMR in permanent magnets becomes possible. The suggested conical rotor design was developed using finite element calculations and provides sample spinning frequencies higher than 20 kHz. The setup was tested on various diamagnetic and paramagnetic compounds including paramagnetic battery materials. The only comparable experiments in low-cost magnets known so far, had been done in the early times of magic angle spinning using electromagnets at much lower sample spinning frequency. Our results demonstrate that high-resolution low-field magic-angle-spinning NMR does not require expensive superconducting magnets and that high-resolution solid-state NMR spectra of paramagnetic compounds are feasible. Generally, this could introduce low-field solid-state NMR for abundant nuclei standard as a routine analytical tool.