Magnetic Resonance in Chemistry最新文献

In NMR experiments, it is crucial to control the temperature of the sample, especially when measuring kinetic parameters. Usually, it takes 2 to 5 min for the temperature of the sample inside the NMR probe to stabilize at a fixed value set for the experiment. However, the NMR sample tubes are flame-sealed in some cases, such as when working with volatile solvents, atmosphere-sensitive samples, or calibration samples for long-term use. When these samples are placed inside the NMR probe, the spectrometer controls the lower portion (liquid phase) of the NMR sample tube with a gas flow at a fixed temperature, while the upper portion (vapor) is at ambient temperature. This probe design creates a unique temperature gradient across the sample, leading to vapor pressure build-up, particularly inside a sealed NMR tube. By analyzing the temperature-dependent spectral line shape changes of a chemical exchange process, we report that under standard experimental conditions, the sample temperature can take up to 2 to 3 h (instead of minutes) to stabilize. The time scale of the liquid–vapor equilibrium process is much slower, with a half-life exceeding 35 min, in contrast to the 2-min duration required to obtain each spectrum. This phenomenon is exclusively due to the liquid–vapor equilibrium process of the flame-sealed NMR tube and is not observable otherwise.

Current practices for structural analysis of extremely large-molecular-weight polysaccharides via solution-state nuclear magnetic resonance (NMR) spectroscopy incorporate partial depolymerization protocols that enable polysaccharide solubilization in suitable solvents. Non-specific depolymerization techniques utilized for glycosidic bond cleavage, such as chemical degradation or ultrasonication, potentially generate structural fragments that can complicate complete and accurate characterization of polysaccharide structures. Utilization of appropriate enzymes for polysaccharide degradation, on the other hand, requires prior structural knowledge and optimal enzyme activity conditions that are not available to an analyst working with novel or unknown compounds. Herein, we describe an application of a permethylation strategy that allows the complete dissolution of intact polysaccharides for NMR structural characterization. This approach is utilized for NMR analysis of Xylella fastidiosa extracellular polysaccharide (EPS), which is essential for the virulence of the plant pathogen that affects multiple commercial crops and is responsible for multibillion dollar losses each year.

The local structure and spin Hamiltonian parameters (SHPs) g factors (g_x, g_y, g_z) and the hyperfine structure constants (A_x, A_y, A_z) for Cu²⁺ doped in the LiTaO₃ crystal are theoretically investigated by the perturbation formulas for a 3d⁹ ion under rhombically elongated octahedral based on the cluster approach. The impurity Cu²⁺ was assumed to occupy the host trigonally-distorted octahedral Li⁺ site and experience the Jahn–Teller (JT) distortion from the host trigonal octahedral [TaO₆]¹⁰⁻ to the impurity rhombically elongated octahedral [CuO₆]¹⁰⁻. Based on the calculations, the impurity-ligand bond lengths parallel and perpendicular to the C₂-axis are found to be R_||(≈ 2.305 Å) and R_⊥ (≈ 2.112 Å) for the studied [CuO₆]¹⁰⁻ cluster, with the planar bond angle θ (≈ 78.2°). Meanwhile, the ground-state wave function for Cu²⁺ center in LiTaO₃ was also obtained. The calculated SHPs based on the above local lattice distortions agree well with the experimental data, and the results are discussed.

We report high-quality solid-state ^35/37Cl NMR spectra for chlorine atoms directly bonded to paramagnetic cobalt(II) ions (high spin S = 3/2) in powered samples of CoCl₂, CoCl₂·2H₂O, CoCl₂·6H₂O, and CoCl₂(terpy) (terpy = 2,2′:6′,2″-terpyridine). Because solid-state ^35/37Cl NMR spectra for paramagnetic cobalt(II) compounds often cover an extremely wide spectral range, they were recorded in this work in the form of variable-offset cumulative spectra. Solid-state ^35/37Cl NMR measurements were performed at three magnetic fields (11.7, 14.1, and 16.5 T) and analysis of data yielded information about ^35/37Cl quadrupole coupling and hyperfine coupling tensors in these paramagnetic cobalt(II) compounds. Experimental ^35/37Cl NMR tensors were found to be in reasonable agreement with quantum chemical calculations based on a periodic DFT method implemented in BAND.

Many-body expansion (MBE) fragment approaches have been applied to accurately compute nuclear magnetic resonance (NMR) parameters in crystalline systems. Recent examples demonstrate that electric field gradient (EFG) tensor parameters can be accurately calculated for ¹⁴N and ¹⁷O. A key additional development is the simple molecular correction (SMC) approach, which uses two one-body fragment (i.e., isolated molecule) calculations to adjust NMR parameter values established using ‘benchmark’ projector augmented-wave (PAW) density functional theory (DFT) values. Here, we apply a SMC using the hybrid PBE0 exchange-correlation (XC) functional to see if this can improve the accuracy of calculated ³⁵Cl EFG tensor parameters. We selected eight organic and two inorganic crystal structures and considered 15 chlorine sites. We find that this SMC improves the accuracy of computed values for both the ³⁵Cl quadrupolar coupling constant (C_Q) and the asymmetry parameter ($�{\eta }_Q$) by approximately 30% compared with benchmark PAW DFT values. We also assessed a SMC that offers local improvements not only in terms of the quality of the XC functional but simultaneously in the quality of the description of relativistic effects via the inclusion of spin–orbit effects. As the inorganic systems considered contain heavy atoms bonded to the chlorine atoms, we find further improvements in the accuracy of calculated ³⁵Cl EFG tensor parameters when both a hybrid functional and spin–orbit effects are included in the SMC. On the contrary, for chlorine-containing organics, the inclusion of spin–orbit relativistic effects using a SMC does not improve the accuracy of computed ³⁵Cl EFG tensor parameters.

The reaction of butyryl chloride with ethynylbenzene in the presence of AlCl₃ afforded a mixture of the Z/E-isomers of 1-chloro-2-phenylhex-1-en-3-one. 1,2-Diphenylethyne under these conditions gave a novel polycarbocycle core, 6aH-benzo[a]fluorene. The chemical structure of 11-chloro-5,6-diphenyl-6a-propyl-6aH-benzo[a]fluorene was established by means of IE-MS, ¹H, ¹³C NMR, COSY, HSQC, HMBC, and 2D INADEQUATE technique.

Externally calibrated quantitative nuclear magnetic resonance (NMR) approaches offer practical means to simultaneously evaluate chemical identity and content without the addition of calibrants to the test sample. Despite continuous advances in external calibration over the last few decades, adoption of these approaches has been slower than expected. Variations in NMR tube geometry are a commonly overlooked factor that can have a substantial effect on externally calibrated quantitation methods. In this report, we investigate the extent to which tube-to-tube volume variability can affect quantitative NMR outcomes. The results highlight the importance of considering tube quality during the development stages of externally calibrated quantitative methods. In addition, we propose a simple, yet effective volume correction strategy using the residual protonated solvent signal that, based on experiments with mixed NMR tubes of varying quality, alleviates the effect of tube-to-tube variability.

Biofilm formation protects bacteria from antibiotic treatment and host immune responses, making biofilm infections difficult to treat. Within biofilms, bacterial cells are entangled in a self-produced extracellular matrix that typically includes exopolysaccharides. Molecular-level descriptions of biofilm matrix components, especially exopolysaccharides, have been challenging to attain due to their complex nature and lack of solubility and crystallinity. Solid-state nuclear magnetic resonance (NMR) has emerged as a key tool to determine the structure of biofilm matrix exopolysaccharides without degradative sample preparation. In this review, we discuss challenges of studying biofilm matrix exopolysaccharides and opportunities to develop solid-state NMR approaches to study these generally intractable materials. We specifically highlight investigations of the exopolysaccharide called Pel made by the opportunistic pathogen, Pseudomonas aeruginosa. We provide a roadmap for determining exopolysaccharide structure and discuss future opportunities to study such systems using solid-state NMR. The strategies discussed for elucidating biofilm exopolysaccharide structure should be broadly applicable to studying the structures of other glycans.