Magnetic Resonance in Chemistry最新文献

Pyridines are a crucial class of heterocycles with widespread applications in natural products, pharmaceuticals, and fluorescent organic materials. In this manuscript, we report the results from a kinetic and mechanistic investigation of an inverse-electron-demand Diels-Alder (IEDDA) cycloaddition involving an oxazole-type diene synthesized via an Ugi-Zhu multicomponent reaction (UZ-3CR). This heterodiene reacts efficiently with various dienophiles such as E-4-oxopentenoic acid, fumaric acid, and monoethyl maleate, yielding highly substituted pyridines in good to excellent yields. Reaction conditions were optimized, and the influence of solvent polarity on regioselectivity was evaluated. The necessity of protonation for successful cycloadditions was probed using structurally diverse dienophiles, revealing the essential role of the carboxylic acid group in triggering the reactions. Mechanistic insights were supported by a comprehensive NMR study (¹H, ¹³C, and ¹⁵N), which provided indirect evidence of in situ protonation of the oxazole ring. Notably, ¹⁵N NMR revealed significant downfield shifts of the oxazole nitrogen, consistent with its protonation, and the emergence of new nitrogen signals corresponding to pyridine products. This study demonstrates the synthetic utility of Ugi-Zhu-derived 5-aminooxazoles in IEDDA cycloadditions and highlights the critical role of acid-promoted activation in enabling efficient pyridine synthesis. We report the results from a kinetic and mechanistic investigation of an IEDDA cycloaddition involving an oxazole-type diene synthesized via an UZ-3CR.

In our study, chemical investigation of the marine fungus Penicillium sp. led to the isolation of four isocoumarin derivatives, including two new compounds (1 and 2) and two known analogues (3 and 4). The structures of 1 and 2 were determined by comprehensive analysis of the 1D (¹H and ¹³C NMR spectra) and 2D NMR data (COSY, HSQC, HMBC, and NOESY spectra). The structure of 1 was confirmed by comparing the calculated and experimental ¹³C NMR data. Compounds 1 and 2 are isocoumarins that lack substitution at the C-5, C-6, and C-7 positions, a structural feature that is uncommon among this class of natural products. The current study enriched the chemical diversity of fungus-derived isocoumarin derivatives.

We report the nuclear magnetic resonance dispersion (NMRD) profile of superparamagnetic iron oxide nanoparticles (SPIONs) and compare the values obtained from its analysis with those from standard methods. Recorded at ¹H Larmor frequencies from 10 kHz to 600 MHz (0.23 mT to 14.1 T, static magnetic field), this profile covers water ¹H dynamics across a broad timescale encompassing all medically relevant magnetic field strengths in magnetic resonance imaging (MRI). This extensive coverage enables the extraction of accurate physicochemical parameters of the nanoparticles that dictate their efficiency (relaxivity). To support this approach, other conventional characterization methods, including TEM, DLS/zeta potential, and VSM, were employed and compared with the NMRD findings. To ensure robustness, the NMRD profiles were recorded at both 25°C and 37°C. Analysis of NMRD profiles, in conjunction with the appropriate theoretical model, successfully characterized SPIONs, yielding coherent parameters that compare favorably with those from conventional characterization methods.

The determination of the relative and absolute configuration of natural compounds is a very challenging task. Among other anisotropic NMR parameters, residual chemical shift anisotropy (RCSA) induced by anisotropic media is an invaluable tool to determine relative configurations of natural and synthetic organic molecules in solution. This review introduces various RCSA-based methodologies for the structural elucidation of natural products. The current availability of alignment media in organic solvents for RCSA measurements is also discussed as are applications of RCSAs for structural analysis of various natural products.

The article reviews the electron paramagnetic resonance research carried out in India over the last three decades. After providing a broad picture of the number and type of publications pertaining to the period, studies on condensed matter physics topics such as manganites, high-Tc superconductors and water are spotlighted.

The molecular dynamics of ionic liquids (ILs) can be probed using fast field cycling (FFC) NMR relaxometry. Conventionally, such studies focus on ILs where only one ionic species carries NMR-active nuclei or on systems combining ${}^1$ H nuclei on the cations with ${}^{19}$ F nuclei on the anions. This way, the dynamics of cations and anions can be resolved individually. However, the situation becomes considerably more complex in fully protonated systems where both ions contain protons, because the various relaxation pathways can no longer be disentangled. Here we report the first FFC NMR investigation of such a case, using the IL triethylammonium methanesulfonate ([TEA][OMs]). Our strategy exploits selective partial deuteration of the ionic species, which enables the separate evaluation of cation and anion dynamics. We demonstrate for the first time that, from the known partial relaxation rates together with the determined interionic distances and self-diffusion coefficients, the relaxation contribution arising from cation-anion interactions can be quantified. Remarkably, this approach even allows reconstruction of the total relaxation rate observed experimentally for the fully protonated IL. This methodology provides a fundamentally new route to overcoming the limited spectral resolution of FFC NMR relaxometry at low fields. More broadly, it establishes a framework for disentangling relaxation processes in complex multicomponent systems, thereby extending the applicability of FFC NMR to more challenging classes of ILs and related materials.

A rapid, reagent-free method for the quantification of chitosan in aqueous solutions was developed using ¹H nuclear magnetic resonance (NMR) relaxometry. Transverse relaxation times (T₂) of chitosan solutions (5-1000 mg/L) were measured with a Carr-Purcell-Meiboom-Gill (CPMG) sequence on a benchtop NMR relaxometer. The data were processed using three approaches: monoexponential fitting (Bruker software), inverse Laplace transformation (CONTIN), and one-component curve fitting (Excel Solver). Calibration curves derived from these models demonstrated high linearity and reproducibility, particularly when concentration intervals were segmented. Among the approaches, the most robust correlation was achieved by plotting the absolute area (AA) of the T₂ distribution obtained from CONTIN analysis versus chitosan concentration, yielding R² values of up to 0.9978 for 5-100 mg/L. Comparative analysis at 40°C and 21°C confirmed the method's temperature stability, with improved sensitivity at elevated temperature. Unlike conventional spectrophotometric or chromatographic methods, the proposed protocol requires no chemical derivatization or complex sample preparation. This technique provides a fast, accurate, and non-destructive alternative for chitosan quantification, especially suitable for material science applications where precise concentration monitoring is critical, such as surface modification of nanomaterials.

Spinocerebellar ataxia 12 (SCA12) is a progressive degenerative neurological disorder, primarily characterized by impaired coordination and balance. To investigate the correlation between proton (¹H) magnetic resonance spectroscopy (MRS) and structural imaging indices in patients with SCA12. T1-weighted MRI, DTI, and single voxel MRS point resolved spectroscopy (PRESS) in the left hemispheric cerebellum were acquired using a 3-T MR scanner in 40 SCA12 patients and 25 healthy controls. Correlations between metabolites, gray and white matter volume of lobules, fractional anisotropy (FA), and clinical, nonclinical, and genetic data were examined. Three machine learning algorithms (KNN, LDA, and SVM) were used to analyze the metabolic feature differences between SCA12 and HC groups. Significant decreases in choline (Cho [GPC (glycerophosphocholine) + PCh (phosphocholine)]) and N-acetyl aspartate (NAA) levels, along with increases in myo-inositol ratios to creatine, FA, and white matter volume values (p < 0.05), were observed in the cerebellum of the SCA12 group compared to healthy controls. Positive correlations were observed between NAA levels and cerebellar lobule volume, the SPM IQ score with the right crus II in the SCA12 group. The International Cooperative Ataxia Rating Scale (ICARS) score showed a negative correlation with white matter and specific cerebellar lobules. Disease duration and cytosine, adenine, and guanine (CAG) repeat length were negatively correlated with right lobule VIIIB, lobule IX, and left lobule X. Machine learning algorithms achieved an accuracy of over 95% in MRS data, and 88.89% in volumetric data. MRS, VBM, and DTI techniques reveal neuronal degeneration in SCA12 compared to healthy individuals.

The spin Hamiltonian parameters (SHPs) (g factors g_// and g_⊥ and hyperfine structure constants A_// and A_⊥) and their concentration (x) dependences for Cu²⁺ in xNaI·(30-x)·Na₂O·70B₂O₃ (5 ≤ x ≤ 25) glasses are theoretically investigated in a consistent way. The [CuO₆]^10- complexes are subject to the Jahn-Teller effect, leading to the relative elongations of about 7%. The concentration dependences of the SHPs are suitably attributed to the nonmonotonic piecewise relationships of cubic field parameter, covalency factor, relative tetragonal elongation ratio, and core polarization constant with NaI concentration x due to the modifications of local crystal fields and electron cloud distribution around impurity Cu²⁺. The calculated cubic field splittings E₁, g factors, and A_// at various concentrations x agree well with the measured results, and the values of corresponding A_⊥ are also predicted. The properties of optical and EPR spectra are discussed, and the microscopic mechanisms of the concentration dependences of the related quantities are also analyzed.

A chemical investigation of the 95% EtOH extract of Dendrobium nobile Lindl. led to the isolation of one new sesquiterpene component (1), together with one known compound (2). Its structure was completely and unambiguously assigned by 1D and 2D NMR spectra (¹H NMR, ¹³C NMR, HSQC, HMBC, COSY, and NOESY), and HR-ESI-MS spectroscopic analysis and the absolute configuration was confirmed by ECD calculation.