Magnetic Resonance in Chemistry最新文献

This study presents a straightforward solution to the challenge of elucidating the structures of nitrogen containing compounds undergoing isomerization. When spectral line broadening occurs related to isomerization, be it prototropic tautomerism or bond rotations, this poses a significant obstacle to structural elucidation. By adding acids, we demonstrate a simple approach to overcome this issue and effectively sharpen NMR signals for acid stable prototropic tautomers as well as the conformational isomers containing a morpholine or piperazine ring.

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. ¹³C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO_2(aq) and another one for the species H₂CO₃, HCO₃⁻, and CO₃²⁻—herein collectively named H_xCO₃^x-2. The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO₃⁻ and CO₃²⁻. The detection of H₂CO₃ at low pH solutions is hindered, because of the concurrent liberation of CO₂ when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the H_xCO₃^x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H₂CO₃. The method employed the formation of frozen layers of NaH¹³CO₃ and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO₂ to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H₂CO₃ was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car–Parrinello molecular dynamics. The H₂CO₃ pK_a value determined by the present method was 3.41 ± 0.03, for 15% D₂O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.

Potentiometry is the primary pH measurement method, but alternatives are sought beyond glass electrodes operative limitations. In nuclear magnetic resonance (NMR) experiments, electrodeless pH sensing is important to track changes along titrations, during chemical reactions or inside compartmentalized environments inaccessible to electrodes, for instance. Although several interesting NMR pH indicators have been already presented, the potential of inorganic phosphate is overlooked, despite its common presence in NMR samples as the buffer main component. Its use for electrodeless pH determination can be expanded by exploiting all its three proton dissociations. This study was aimed at verifying the use of inorganic phosphate ³¹P chemical shift to sense pH variations, and at exploring the complementary use of pyrophosphate ions to cover a wide pH range. A simple set of equations is presented to utilize both phosphate and pyrophosphate ³¹P chemical shift in combination for accurate pH determination without a glass electrode over the 5–12 pH range, and without affecting the spectrum of other nuclei. The present study demonstrated an average deviation of 0.09 (maximum <0.2) pH unit from glass electrode measurements. The trimethylphosphate can be used as a suitable chemical shift reference for both ³¹P and ¹H (also ¹³C), with its hydrolysis being significant only at pH > 12. The method was also demonstrated by determining the pKa of three distinct molecules in a mixture and by comparing the results to those obtained when the glass electrode was used to measure the pH. The approach shown here can be easily tuned to different experimental conditions.

Benchtop diffusion nuclear magnetic resonance (NMR) spectroscopy was used to perform quantitative monitoring of enzymatic hydrolysis. The study aimed to test the feasibility of the technology to characterize enzymatic hydrolysis processes in real time. Diffusion ordered spectroscopy (DOSY) was used to measure the signal intensity and apparent self-diffusion constant of solubilized protein in hydrolysate. The NMR technique was tested on an enzymatic hydrolysis reaction of red cod, a lean white fish, by the endopeptidase alcalase at 50°C. Hydrolysate samples were manually transferred from the reaction vessel to the NMR equipment. Measurement time was approximately 3 min per time point. The signal intensity from the DOSY experiment was used to measure protein concentration and the apparent self-diffusion constant was converted into an average molecular weight and an estimated degree of hydrolysis. These values were plotted as a function of time and both the rate of solubilization and the rate of protein breakdown could be calculated. In addition to being rapid and noninvasive, DOSY using benchtop NMR spectroscopy has an advantage compared with other enzymatic hydrolysis characterization methods as it gives a direct measure of average protein size; many functional properties of proteins are strongly influenced by protein size. Therefore, a method to give protein concentration and average size in real time will allow operators to more tightly control production from enzymatic hydrolysis. Although only one type of material was tested, it is anticipated that the method should be applicable to a broad variety of enzymatic hydrolysis feedstocks.

Solution nuclear magnetic resonance (NMR) analysis of polysaccharides can provide valuable information not only on their primary structures but also on their conformation, dynamics, and interactions under physiological conditions. One of the main problems is that non-anomeric ¹H signals typically overlap, and this often hinders detailed NMR analysis. Isotope enrichment, such as with ¹³C and ¹⁵N, will add a new dimension to the NMR spectra of polysaccharides, and spectral analysis can be performed with enhanced sensitivity using isolated peaks. For this purpose, here we have prepared uniformly ¹³C- and/or ¹⁵N-labeled chondroitin polysaccharides –4)-β-D-glucuronopyranosyl-(1–3)-2-acetamido-2-deoxy-β-D-galactopyranosyl-(1– with molecular weights in the range from 310 to 460 k by bacterial fermentation. The enrichment ratios for ¹³C and ¹⁵N were 98.9 and 99.8%, respectively, based on the mass spectrometric analysis of the constituent chondroitin disaccharides. ¹H and ¹³C NMR signals were assigned mainly based on HSQC and ¹³C-detection experiments including INADEQUATE, HETCOR, and HETCOR-TOCSY. The carbonyl carbon signal of the N-acetyl-β-D-galactosamine residue was unambiguously distinguished from the C6 carbon of the β-D-glucuronic acid residue by the observation of ¹³C peak splitting due to ¹J_CN coupling in ¹³C- and ¹⁵N-labeled chondroitin. The T₂* and T₁ were measured and indicate that both rigid and mobile sites are present in the long sequence of chondroitin. The conformation, dynamics, and interactions of chondroitin and its derivatives will be further analyzed based on the results obtained in this study.

Since 1993, it has been known that ¹³C chemical shift tensor (i.e., δ₁₁, δ₂₂, and δ₃₃) provides information sufficient to distinguish between COOH and COO⁻ sites. Herein, four previously unreported metrics are proposed for differentiating COOH/COO⁻ moieties. A new relationship is also introduced that correlates the asymmetry (i.e., δ₁₁–δ₂₂) of COOH sites to the proximity of hydrogen bond donating partners within 2.6 Å with high accuracy (±0.05 Å). Conversely, a limitation to all proposed metrics is that they fail to distinguish between COO⁻ and hydrogen disordered COOH sites. To reconcile this omission, a new approach is proposed based on T₁ measurements of both ¹H and ¹³C. The ¹³C T₁ values are particularly sensitive with the T₁ for hydrogen disordered COOH moieties found to be nearly six times smaller than T₁'s from COO⁻ sites.

In recent years there has been a renewed interest in benchtop NMR. Given their lower cost of ownership, smaller footprint, and ease of use, they are especially suited as an educational tool. Here, a new experiment targeted at upper-year undergraduates and first-year graduate students follows the conversion of D-glucose into ethanol at low-field. First, high and low-field data on D-glucose are compared and students learn both the Hz and ppm scales and how J-coupling is field-independent. The students then acquire their own quantitative NMR datasets and perform the quantification using an Electronic Reference To Access In Vivo Concentration (ERETIC) technique. To our knowledge ERETIC is not currently taught at the undergraduate level, but has an advantage in that internal standards are not required; ideal for following processes or with future use in flow-based benchtop monitoring. Using this quantitative data, students can relate a simple chemical process (fermentation) back to more complex topics such as reaction kinetics, bridging the gaps between analytical and physical chemistry. When asked to reflect on the experiment, students had an overwhelmingly positive experience, citing agreement with learning objectives, ease of understanding the protocol, and enjoyment. Each of the respondents recommended this experiment as a learning tool for others. This experiment has been outlined for other instructors to utilize in their own courses across institutions, with the hope that a continued expansion of low-field NMR will increase accessibility and learning opportunities at the undergraduate level.

This paper reports the principal values of the ¹³C chemical shift tensors for five nitrogen-dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have ¹³C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from ¹H→¹³C cross-polarization magic-angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of ¹⁴N-¹³C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the ¹³C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double-hybrid functional PBE0-DH, along with the triple-zeta basis sets TZ2P or pc-3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate ¹³C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2-week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies.

The para spin isomer of hydrogen gas possesses high nuclear spin order that can enhance the NMR signals of a variety of molecular species. Hydrogen is routinely enriched in the para spin state by lowering the gas temperature while flowing through a catalyst. Although parahydrogen enrichments approaching 100% are achievable near the H₂ liquefaction temperature of 20 K, many experimentalists operate at liquid nitrogen temperatures (77 K) due to the lower associated costs and overall simplicity of the parahydrogen generator. Parahydrogen that is generated at 77 K provides an enrichment value of ~51% of the para spin isomer; while useful, there are many applications that can benefit from low-cost access to higher parahydrogen enrichments. Here, we introduce a method of improving parahydrogen enrichment values using a liquid nitrogen-cooled generator that operates at temperatures less than 77 K. The boiling temperature of liquid nitrogen is lowered through internal evaporation into helium gas bubbles that are injected into the liquid. Changes to liquid nitrogen temperatures and parahydrogen enrichment values were monitored as a function of helium gas flow rate. The injected helium bubbles lowered the liquid nitrogen temperature to ~65.5 K, and parahydrogen enrichments of up to ~59% were achieved; this represents an ~16% improvement compared with the expected parahydrogen fraction at 77 K. This technique is simple to implement in standard liquid nitrogen-cooled parahydrogen generators and may be of interest to a wide range of scientists that require a cost-effective approach to improving parahydrogen enrichment values.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical technique with the ability to acquire both quantitative and structurally insightful data for multiple components in a test sample. This makes NMR spectroscopy a desirable tool to understand, monitor, and optimize chemical transformations. While quantitative NMR (qNMR) approaches relying on internal standards are well-established, using an absolute external calibration scheme is beneficial for reaction monitoring as resonance overlap complications from an added reference material to the sample can be avoided. Particularly, this type of qNMR technique is of interest with benchtop NMR spectrometers as the likelihood of resonance overlap is only enhanced with the lower magnetic field strengths of the used permanent magnets. The included study describes a simple yet robust methodology to determine concentration conversion factors for NMR systems using single- and multi-analyte linear regression models. This approach is leveraged to investigate a pharmaceutically relevant amide coupling batch reaction. An on-line stopped-flow (i.e., interrupted-flow or paused-flow) benchtop NMR system was used to monitor both the 1,1′-carbonyldiimidazole (CDI) promoted acid activation and the amide coupling. The results highlight how quantitative measurements in benchtop NMR systems can provide valuable information and enable analysts to make decisions in real time.