Magnetic Resonance in Chemistry最新文献

Ethyl hydroxyethyl cellulose (EHEC) and methyl ethyl hydroxyethyl cellulose (MEHEC) are hydrophilic cellulose ethers commonly employed as rheology modifiers in diverse industrial applications. The performance of these polymers, and their resistance to degradation by various cellulase enzymes, depends on their intricate molecular structure. Distribution of the etherifying groups, within the anhydroglucose units and along the polymer chain, is the key property to control. However, characterizing such structural properties is challenging, necessitating the development of novel analysis methods. In this study, we demonstrate the application of solid-state nuclear magnetic resonance (NMR) spectroscopy, enhanced by dynamic nuclear polarization (DNP), for this purpose. We prove that the hydrophilic EHEC and MEHEC samples are homogenously swelled in D₂O/H₂O-based radical solutions, a necessity to ensure uniform DNP enhancement throughout the material. And we illustrate how the high sensitivity enhancements obtained can be used to perform selective, J-coupling-based C1 to C2 transfer experiments to measure the fraction of substituted C2 positions in these cellulose ethers. Moreover, with further refinement, the methodology outlined in this work holds promise for elucidating C3-specific substitution patterns.

We studied the solubilization of phenols with one and two hydroxyl groups (phenol, p-cresol, guaiacol, and pyrocatechol, resorcinol, hydroquinone) by micelles of the biological surfactant rhamnolipid using NMR diffusometry. We discuss the results within the framework of a model of two states of solubilizer molecules in solution: free in the aqueous phase and bound in surfactant micelles. The solubilization characteristics of rhamnolipid were calculated: the proportion of solubilized molecules р, micelle-water partition coefficient K_m, and molar solubilization ratio (MSR) depending on the concentration of rhamnolipid in solutions.

Non-uniform sampling (NUS) enables faster acquisition of NMR spectra. Concerns about spectral fidelity, particularly in high-dynamic-range experiments like NOESY, have limited its quantitative applications. In this study, we assessed whether optimised Poisson-gap sampling schemes can generate high-fidelity spectra suitable for quantitation and evaluated the effectiveness of NUS ranking tools, NUSscore and nus-tool, in identifying optimal sampling schemes. A total of 25,000 Poisson-gap sampling schemes were generated and ranked using NUSscore, with a subset of 11 of these spanning the score distribution, alongside 15 random-shuffle and the highest and lowest scoring Poisson-gap schemes determined using the signal apex-to-artefact ratio were used for comparison, all with 50% sampling coverage. Additionally, hybrid sampling schemes incorporating a long initial uniformly sampled section, termed US-NUS hybrid schemes, were evaluated. Spectral fidelity was evaluated on interproton distance accuracy, including the proportion of retained interproton distances and their deviation from uniformly sampled reference spectra. NUSscore showed a strong correlation with spectral fidelity. The peak-to-sidelobe ratio implemented in nus-tool showed no correlation, with the relative sensitivity metric showing a weak correlation. Signal-to-artefact apex ratio was also not predictive for identifying sampling schedules with maintained interproton distances. All Poisson-gap sampling schemes however outperformed random-shuffle. The US-NUS hybrids demonstrated improved interproton distance conservation than traditional Poisson-gap sampling schemes with a low seed dependence, making them a promising sampling schedule for quantitative NOESY analysis.

Calcium silicates react readily with CO₂ under aqueous conditions, forming CaCO₃ and silica gel. This is utilized to produce new cement binders and to sequester CO₂, thereby contributing to a lowering of the CO₂ footprint for the cement industry. The present work investigates aqueous carbonation of three hydraulic and three non-hydraulic calcium silicates with the aim of analyzing the impact of the Ca/Si ratio on the structure of the amorphous silica gel and on the extent and rate of carbonation. This information is obtained from ²⁹Si NMR experiments, whereas ¹³C NMR and FT-IR are used to characterize the polymorphic forms of CaCO₃ formed upon carbonation. The structure of the silica gel is not dependent on the type of carbonated calcium silicate or their Ca/Si ratio. In addition, the amounts of CaCO₃ from TGA analysis match well the theoretical maximum values. ²⁹Si and ²⁹Si{¹H} CP/MAS spectra of a commercial silica gel are very similar to those observed for the carbonated calcium silicates, which strongly suggests that a hydroxylated silica gel without incorporated Ca ions constitutes the silica gel in carbonated calcium silicates. From ¹³C NMR and FT-IR, it is found that calcite is the principal CaCO₃ polymorph for all samples carbonated for 6 h. However, aragonite and calcite do co-exist during the initial carbonation (20 min) of γ-Ca₂SiO₄. Comparison of the carbonation evolution for the hydraulic and non-hydraulic calcium silicates strongly suggests that an early hydration and formation of C-S-H is not a required initial step in the aqueous carbonation process.

NMR provides unprecedented molecular information, urgently needed by environmental researchers and policy makers. However, NMR is underutilized in environmental sciences due to the lack of available technologies, limited environmental-specific training opportunities, and easy-to-use workflows. NMR has considerable potential as a discovery tool for novel pollutants, and by-products, exemplified by the recent discovery of the degradation by-product of a rubber additive, 6PPD-quinone, now considered one of the most toxic compounds presently known. This work represents a proof-of-concept case study highlighting the use of NMR to profile effluents from 38 industries across Ontario, Canada. Wastewater effluents from various industrial sectors were analyzed using several 1D and 2D ¹H/¹³C NMR and ¹⁹F experiments and were screened both unconcentrated and after lyophilization. Common species could be identified using human metabolic NMR databases, but environmental-specific NMR databases desperately need further development. An example of manually identifying unusual NMR signatures is included; these resulted from phosphinic and phosphonic acids originating from the electroplating industry, for which the environmental impacts are not well understood. Basic ¹H NMR quantification is performed using ERETIC, while an optimized approach combining relaxation agents and steady-state-free-precession ¹⁹F NMR, to reduce detection limits (at 500 MHz) to sub-ppb (< 1 μg/L) in under 15 min, is demonstrated. The future potential of benchtop NMR (80 MHz) is also considered. This paper represents a guide to others interested in applying NMR spectroscopy to environmental media and demonstrates the potential of NMR as a complementary tool to assist MS in environmental pollutant and by-product discovery.