Magnetic Resonance in Chemistry最新文献

The present review focuses on the most recent advances in liquid-phase NMR of asphaltenes, leaving apart an overwhelming amount of publications dealing with solid-state NMR investigations in this field. Owing to the complexity of the coal-derived products, and in particular, asphaltenes, their ¹H and ¹³C NMR spectra consist of a number of overlapping signals belonging to different hydrocarbon types. Comprehensive studies of asphaltenes by means of NMR reveal the characteristic functional groups of their fractions together with the spectral regions in which they resonate. NMR studies of asphaltenes provide a straightforward guideline for their chemical composition and that of the related coal-derived products.

The distinction of enantiomers based on residual anisotropic parameters obtained by alignment in chiral poly-γ-benzyl-L-glutamate (PBLG) is among the strongest in high-resolution NMR spectroscopy. However, large variations in enantiodifferentiation among different solutes are frequently observed. One hypothesis is that the formation of hydrogen bonds between solute and PBLG is important for the distinction of enantiomers. With a small set of three almost spherical enantiomeric pairs, for which ¹D_CH residual dipolar couplings are measured, we address this issue in a systematic way: borneol contains a single functional group that can act as a hydrogen bond donor, camphor has a single group that may act as a hydrogen bond acceptor, and quinuclidinol can act as both hydrogen bond donor and acceptor. The results are unambiguous: although camphor shows low enantiodifferentiation with PBLG and alignment that can be predicted well by the purely steric TRAMITE approach, the distinction of enantiomers for the other enantiomeric pairs is significantly higher with alignment properties that must involve a specific interaction in addition to steric alignment.

The spatial magnetic properties (through-space NMR shieldings—TSNMRSs—actually the ring current effect in ¹H NMR spectroscopy) of the recently synthesized infinitene (the helically twisted [12]circulene) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. Both ¹H and ¹³C chemical shifts of infinitene and the aromaticity of this esthetically very appealing molecule have been studied subject to the ring current effect thus obtained. This spatial magnetic response property of TSNMRSs dominates the different magnitude of ¹H and ¹³C chemical shifts, especially in the cross-over section of infinitene, which is unequivocally classified as an aromatic molecule based on the deshielding belt of its ring current effect. Differences in aromaticity of infinitene compared with isolated benzene can also be qualified and quantified on the magnetic criterion.

Thiolate containing mercury(II) complexes of the general formula [Hg(SR)$�{}_{�n�}$]$�{}^{�2�-�n�}$ have been of great interest since the toxicity of mercury was recognized. ¹⁹⁹Hg nuclear magnetic resonance spectroscopy (NMR) is a powerful tool for characterization of mercury complexes. In this work, the Hg shielding constants in a series of [Hg(SR)$�{}_{�n�}$]$�{}^{�2�-�n�}$ complexes are therefore investigated computationally with particular emphasis on their geometry dependence. Geometry optimizations and NMR chemical shift calculations are performed at the density functional theory (DFT) level with both the zeroth-order regular approximation (ZORA) and four-component relativistic methods. The four exchange-correlation (XC) functionals PBE0, PBE, B3LYP, and BLYP are used in combination with either Dyall's Gaussian-type (GTO) or Slater-type orbitals (STOs) basis sets. Comparing ZORA and four-component calculations, one observes that the calculated shielding constants for a given molecular geometry have a constant difference of $�\sim$1070 ppm. This confirms that ZORA is an acceptable relativistic method to compute NMR chemical shifts. The combinations of four-component/PBE0/v3z and ZORA/PBE0/QZ4P are applied to explore the geometry dependence of the isotropic shielding. For a given coordination number, the distance between mercury and sulfur is the key factor affecting the shielding constant, while changes in bond and dihedral angles and even different side groups have relatively little impact.

Multilayered plastics are widely used in food packaging and other commercial applications due to their tailored functional properties. By layering different polymers, the multilayered composite material can have enhanced mechanical, thermal, and barrier properties compared to a single plastic. However, there is a significant need to recycle these multilayer plastics, but their complex structure offers significant challenges to their successful recycling. Ultimately, the use and recycling of these complex materials requires the ability to characterize the composition and purity as a means of quality control for both production and recycling processes. New advances and availability of low-field benchtop ¹H NMR spectrometers have led to increasing interest in its use for characterization of multicomponent polymers and polymer mixtures. Here, we demonstrate the capability of low-field benchtop ¹H NMR spectroscopy for characterization of three common polymers associated with multilayered packaging systems (low-density polyethylene [LDPE], ethylene vinyl alcohol [EVOH], and Nylon) as well as their blends. Calibration curves are obtained for determining the unknown composition of EVOH and Nylon in multilayered packaging plastics using both the EVOH hydroxyl peak area and an observed peak shift, both yielding results in good agreement with the prepared sample compositions. Additionally, comparison of results extracted for the same samples characterized by our benchtop spectrometer and a 500-MHz spectrometer found results to be consistent and within 2 wt% on average. Overall, low-field benchtop ¹H NMR spectroscopy is a reliable and accessible tool for characterization of these polymer systems.

The spin Hamiltonian parameters and defect structures are theoretically studied for the substitutional Mn²⁺ at the core of CdSe nanocrystals and in the bulk materials from the perturbation calculations of spin Hamiltonian parameters for trigonal tetrahedral 3d⁵ clusters. Both the crystal-field and charge transfer contributions are taken into account in the calculations from the cluster approach. The impurity-ligand bond angles are found to be about 1.84° larger and 0.10° smaller in the CdSe:Mn²⁺ nanocrystals and bulk materials, respectively, than those (≈109.37°) of the host Cd²⁺ sites. The quantitative criterion of occupation (at the core or surface) for Mn²⁺ in CdX (X = S, Se, Te) nanocrystals is presented for the first time based on the inequations of hyperfine structure constants (HSCs). This criterion is well supported by the experimental HSCs data of Mn²⁺ in CdX nanocrystals. The previous assignments of signals SI as Mn²⁺ at the core of CdS nanocrystals are renewed as Mn²⁺ at the surface based on the above criterion. The present studies would be helpful to achieve convenient determination of occupation for Mn²⁺ impurities in CdX semiconductor nanocrystals by means of spectral (e.g., HSCs) analysis.

Three new monacolin analogues, 3,6-dihydroxy-monacolin P (1), 6-methoxy monacolin S (2), and 6-methoxy dehydromonacolin S (3), were isolated from a fraction that strongly inhibited 3-hydroxy-3-methylglutaryl-CoA reductase from the ethyl acetate portion of red yeast rice ethanol extract. Their structures were determined through a combination of 1D and 2D NMR experiments, mass spectrometry analysis, and known literature reports.

Configurational and conformational analysis of the biologically relevant natural product artemisinin was conducted using carbon–carbon residual dipolar couplings (¹D_CC RDCs) at natural abundance. These RDCs were measured through the 2D-INADEQUATE NMR experiment using a sample aligned in a compressed poly (methyl methacrylate) (PMMA) gel swollen in CDCl₃. Singular value decomposition (SVD) fitting analysis of all carbon–carbon bonds, ¹D_CC RDCs, in relation to the full configuration/conformational space (32 diastereoisomers) of artemisinin, unambiguously identified the correct configuration of artemisinin.