Magnetic Resonance in Chemistry最新文献

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.

Making NMR spectroscopy accessible to a broad scientific community is an ongoing and exciting challenge as this analytical tool plays a central role in many fields of (bio)chemistry. With this in mind, benchtop NMR has enjoyed great success over the last 10 years, whether in academic research, industry, or even educational activities, offering the possibility of using this powerful spectroscopy in situations where high-field NMR is not feasible for economic or practical reasons. Furthermore, the lack of cryogenic fluids in benchtop NMR systems is becoming increasingly important in the current context as the scarcity and unstable pricing of liquid helium are significant concerns for many NMR platforms. This further requires no cryogenic maintenance, skipping routine, and safety concerns when working with cryogenic liquids. It is worth mentioning that the concept of “cryogen-free NMR” is not today limited to low-field permanent magnets, since a new type of cryogen-free power-driven high-temperature-superconducting (HTS) magnet has been recently proposed, which can operate until 9.4 T.

Current permanent magnets deliver fields of 1–2 T (or even 2.4 T for the most recent systems). While such magnetic fields are reminiscent of the early days of NMR in the mid-20th century, their impressive homogeneity results in narrow line widths (less than 0.5 Hz at half-height), making it possible to go beyond the analysis of highly concentrated small molecules. Furthermore, benchtop NMR has benefited from the most recent methodological developments originally proposed for high-field spectrometers, whether in the design of pulse sequences, signal processing, or data analysis based on algorithms. In particular, the implementation of gradient coils in recent benchtop devices has enabled the use of modern NMR experiments that rely on spatial encoding and diffusion contrast, as well as the application of solvent suppression schemes that are effective on both stationary and flowing samples. These improvements have paved the way for various reaction monitoring on-the-fly in standard reactors or within flow-chemistry platforms, along with quality control applications in different fields. As a result, benchtop NMR is becoming a valuable complement to high-field NMR, especially in environments where the latter is not accessible.

This special issue, entitled “Benchtop/cryofree NMR,” includes 19 research articles, one educational paper, and, finally, a mini-review exploring the analytical performance of an HTS magnet operating at moderate fields (9.4 T). This issue focuses mainly on NMR spectroscopy (i.e., FT-NMR), with method developments for the analysis of complex mixtures, solvent suppression, and diffusion measurement, while instrumental considerations for sample temperature control are also presented and discussed. The herein articles also present a wide range of applications in reaction monitoring and quality control in different matrices (pharmaceutical,

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.

Sesquiterpene lactones (SL) represent a class of secondary metabolites found in the Asteraceae family, notable for their unique structures. The SL α-santonin (1) and its derivatives are worthy of mention due to their diverse biological properties. Additionally, 4H-chromenes and 4H-chromones are appealing frameworks holding the capability to be used as structural motifs for new drugs. Furthermore, unambiguous structural elucidation is crucial for developing novel compounds for diverse applications. In this context, it is common to find in the literature molecules erroneously assigned. Therefore, the use of quantum mechanical calculations to simulate NMR chemical shifts has emerged as a valuable strategy. In this work, we conceived the synthesis of two halogenated 4H-chromenediones derived from photosantonic acid (2), a photoproduct arising from irradiation of α-santonin (1) in the ultraviolet region. The structure of the chlorinated and brominated products was determined by NMR analysis, with the aid of quantum mechanical calculations at the B3LYP/6-311 + G(2d,p)//M062x/6-31 + G(d,p) level of theory. All analyses were in agreement and led to the assignment of the brominated 4H-chromene-2,7-dione as (3S,3aS,5aR,9bS)-5a-(2-bromopropan-2-yl)-3-methyl-3,3a,5,5a,8,9b-hexahydro-4H-furo[2,3-f]chromene-2,7-dione (11b) and of the chlorinated 4H-chromene-2,7-dione as (3S,3aS,5aR,9bS)-5a-(2-chloropropan-2-yl)-3-methyl-3,3a,5,5a,8,9b-hexahydro-4H-furo[2,3-f]chromene-2,7-dione (12b). The diastereoselectivities of the reactions were explained based on products and intermediates formation energy calculated using B3LYP/6-31 + G(d,p) as the level of theory. Structures 11b and 12b were identified as the thermodynamic and kinetic products of the reaction among all candidates. Consequently, the strategy utilized in this study is robust and successfully illustrates the use of quantum mechanical calculations in the structural elucidation of new compounds with potential applications as novel drugs or products.

β-lactams are a chemically diverse group of molecules with a wide range of biological activities. Having recently observed curious trends in ²J_HH coupling values in studies on this structural class, we sought to obtain a more comprehensive understanding of these diagnostic NMR parameters, specifically interrogating ¹J_CH, ²J_CH, and ²J_HH, to differentiate 3- and 4-monosubstituted β-lactams. Further investigation using computational chemistry methods was employed to explore the geometric and electronic origins for the observed and calculated differences between the two substitution patterns.

T. Maschmeyer, B. Conklin, T. C. Malig, D. J. Russell, K. L. Kurita, J. E. Hein, J. G. Napolitano, Magn Reson Chem 2024, 62(3), 169. https://doi.org/10.1002/mrc.5421

This article, erroneously published in Magnetic Resonance in Chemistry, Volume 62, Issue 3, 2024, is an article from Special Issue: Benchtop/Cryofree NMR.

We apologize for this error.