Advanced Nuclear Magnetic Resonance Spectroscopy Research Articles

One-dimensional alignment of defects in a flexible metal-organic framework.

Crystalline materials are often considered to have rigid periodic lattices, while soft materials are associated with flexibility and nonperiodicity. The continuous evolution of metal-organic frameworks (MOFs) has erased the boundaries between these two distinct conceptions. Flexibility, disorder, and defects have been found to be abundant in MOF materials with imperfect crystallinity, and their intricate interplay is poorly understood because of the limited strategies for characterizing disordered structures. Here, we apply advanced nuclear magnetic resonance spectroscopy to elucidate the mesoscale structures in a defective MOF with a semicrystalline lattice. We show that engineered defects can tune the degree of lattice flexibility by combining both ordered and disordered compartments. The one-dimensional alignment of correlated defects is the key for the reversible topological transition. The unique matrix is featured with both rigid framework of nanoporosity and flexible linkage of high swellability.

State-of-the-art accounts of hyperpolarized 15N-labeled molecular imaging probes for magnetic resonance spectroscopy and imaging.

Hyperpolarized isotope-labeled agents have significantly advanced nuclear magnetic resonance spectroscopy and imaging (MRS/MRI) of physicochemical activities at molecular levels. An emerging advance in this area is exciting developments of 15N-labeled hyperpolarized MR agents to enable acquisition of highly valuable information that was previously inaccessible and expand the applications of MRS/MRI beyond commonly studied 13C nuclei. This review will present recent developments of these hyperpolarized 15N-labeled molecular imaging probes, ranging from endogenous and drug molecules, and chemical sensors, to various 15N-tagged biomolecules. Through these examples, this review will provide insights into the target selection and probe design rationale and inherent challenges of HP imaging in hopes of facilitating future developments of 15N-based biomedical imaging agents and their applications.

Intrinsic anisotropy parameters of a series of lanthanoid complexes deliver new insights into the structure-magnetism relationship

Intrinsic anisotropy parameters of a series of lanthanoid complexes deliver new insights into the structure-magnetism relationship

Structural composition of immobilized fertilizer N associated with decomposed wheat straw residues using advanced nuclear magnetic resonance spectroscopy combined with 13C and 15N labeling

When crop residues are returned to soil, nitrogen (N) from applied chemical fertilizers can be immobilized by microbial growth and incorporated back into the residues themselves to form organic N compounds with poor bioavailability. However, the chemical composition of these compounds has remained poorly understood. Knowledge of the chemical composition of immobilized fertilizer N is crucial for understanding its fate in the field and its associated impacts on fertilizer applications and management. Here, we characterized immobilized fertilizer N by coupling isotopic labeling of wheat straw (13C) and fertilizer N (urea-15N) with two advanced nuclear magnetic resonance (NMR) techniques, (13C{15N} rotational-echo double resonance, REDOR) and two-dimensional (2D) 15N−13C heteronuclear single quantum coherence (HSQC) spectroscopy. We found that the immobilization of N under aerobic conditions was greater than that under anaerobic conditions (52% of fertilizer 15N vs 38% of fertilizer 15N). Under anaerobic conditions, most of the immobilized fertilizer 15N was in the form of protonated amide N. Compared to anaerobic conditions, 15N spectra of aerobic samples showed more diversity amongst N containing functional groups, including peptide-amides (54%), methylated amides (22%), amines (7%), anilides (5%) and heterocyclics (3%). Additionally, from the perspective of its structural composition, 55-80% of the immobilized fertilizer 15N was estimated to be a potentially labile N fraction. Collectively, these results indicate that the immobilization of fertiziler N during aerobic decomposition of straw residues produces organic N functional groups with a strong potential for remineralization available for use by crops when they are released into soil.

Structural characterization of gilsonite bitumen by advanced nuclear magnetic resonance spectroscopy and ultrahigh resolution mass spectrometry revealing pyrrolic and aromatic rings substituted with aliphatic chains

Gilsonite, a naturally occurring asphaltite bitumen, consists of a complex mixture of organic compounds. In the present study, advanced one and two dimensional solid state and solution 1H, 13C and 15N nuclear magnetic resonance (NMR) and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) were employed to investigate its composition and structure. 13C NMR yielded a carbon aromaticity of 27%. Aromatic moieties in gilsonite were primarily single rings or small clusters of fused rings. Half of the aromatic carbons of gilsonite can be accounted for by pyrroles. 15N and 13C cross polarization-magic angle spinning (CP-MAS) NMR showed that most nitrogen in gilsonite was pyrrolic. The aromatic rings were heavily substituted with alkyl chains, as evidenced by 1H13C correlation spectra. Advanced solid state NMR spectral editing techniques clearly identified specific functional groups such as CCH3, CCH2, and CCH2 (exomethylene). 1H13C wideline separation (WISE) NMR helped identify mobile and non-protonated alkyl carbons. FT-ICR-MS indicated that ∼64% of calculated formulae generated by ESI were aliphatic, while only about 0.8–2.5% of formulae contained possible aromatic rings. All of the assigned formulae contained at least one heteroatom (N, O or S), indicating that ionization by ESI was selective for the polar fraction of gilsonite and potentially less reflective of the overall chemical character of gilsonite than NMR spectroscopy. By combining the information obtained from advanced NMR and ultrahigh resolution MS we propose a structural model for gilsonite as a mixture of many pyrrolic and a few fused aromatic rings highly substituted with and connected by mobile aliphatic chains.