Magnetic Resonance in Chemistry

Bakhmutov, Vladimir I.; Contreras‐Ramirez, Aida; Drake, Hannah; Zhou, Hong‐Cai

doi: 10.1002/mrc.5262pmid: 35229359

Solid‐state NMR experiments on 2H, 31P, 13C, and 1H nuclei, including 31P T1, 1H T1, and 1H T1ρ measurements, as well as on the kinetics of proton‐phosphorus cross‐polarization have been performed to characterize the crystalline and amorphous α‐zirconium phosphates, which were intercalated with D2O and/or CD3OD. The 13C{1H} CP MAS NMR experiment performed for compound 1‐CD3OD (Zr (HPO4)2.0.2CD3OD) with carbon cross‐polarization via protons of phosphate groups has provided a prove that the methanol was intercalated into the interlayer spaces of this compound. The variable‐temperature 2H solid‐echo MAS NMR spectra of intercalated compounds demonstrated that the methanol molecules, in contrast to the mobile water, were immobile, keeping, however, free CD3 rotations around the C3‐axis. It has been demonstrated that the intercalated species, D2O and CD3OD, do not affect the high‐frequency motions of the phosphate groups. By utilizing local structural models that satisfy the constraints of the experimental data, it has been suggested that the immobile methanol molecules are located in the cavity between two neighboring layers of the zirconium phosphates. Thus, the present work illustrates the reliable criteria in a comprehensive NMR approach to structural and dynamic studies of such systems.

Chen, Baojin; Zhang, Junlong; Li, Rong; Liang, Yajin; Feng, Cuidi; Zhang, Huaming

doi: 10.1002/mrc.5264pmid: 35246995

VO2+ (3d1) and Cu2+ (3d9) are the two complementary states that usually show opposite distortions when they are doped in crystals. In this work, the optical absorption spectra (OAS), electron paramagnetic resonance (EPR) parameters, and local structure (LS) for VO2+ (and Cu2+) in MgNH4PO4·6H2O (MPPH) are uniformly investigated on the basis of the high‐order perturbation formulas for a 3d1 (and 3d9) ion in tetragonally compressed (and elongated) octahedra, respectively. In the calculated formulas, the required crystal‐field parameters can be obtained from the superposition model and reasonably linked with the LS distortion for VO2+ and Cu2+ centers. Based on the calculations, the tetragonal compressed [VO(H2O)5]2+ cluster (and tetragonal elongated [Cu(H2O)6]2+ cluster) is found to suffer tetragonal compression ratio of 1.65% and tetragonal elongation ratio of 3.8% along C4‐axis, respectively, due to the Jahn–Teller (JT) effect. The theoretical EPR parameters based on the above lattice distortions agree well with the experimental data, and the LS of the VO2+ and Cu2+ centers in MPPH is discussed.

Knoll, Kevin; Herold, Dominik; Hirschmann, Max; Thiele, Christina M.

doi: 10.1002/mrc.5266pmid: 35266585

A supramolecular, lyotropic liquid crystalline alignment medium based on an azobenzene‐containing 1,3,5‐benzenetricarboxamide (BTA) building block is described and investigated. As we demonstrate, this water‐based system is suitable for the investigation of various water‐soluble analytes and allows for a scaling of alignment strength through variation of temperature. Additionally, alignment is shown to reversibly collapse above a certain temperature, yielding an isotropic solution. This collapse allows for isotropic reference measurements, which are typically needed in addition to those in an anisotropic environment, to be performed using the same sample just by varying the temperature. The medium described thus provides easy access to anisotropic NMR observables and simplifies structure elucidation techniques based thereon.

Scheidel, Sebastian; Östreicher, Laurina; Mark, Isabelle; Pöppler, Ann‐Christin

doi: 10.1002/mrc.5267pmid: 35277897

Although solid‐state nuclear magnetic resonance (NMR) is a versatile analytical tool to study polymorphs and phase transitions of pharmaceutical molecules and products, this work summarizes examples of spontaneous and unexpected (and unwanted) structural rearrangements and phase transitions (amorphous‐to‐crystalline and crystalline‐to‐crystalline) under magic angle spinning (MAS) conditions, some of them clearly being due to the pressure experienced by the samples. It is widely known that such changes can often be detected by X‐ray powder diffraction (XRPD); here, the capability of solid‐state NMR experiments with a special focus on 1H‐13C frequency‐switched Lee–Goldburg heteronuclear correlation (FSLG HETCOR)/MAS NMR experiments to detect even subtle changes on a molecular level not observable by conventional 1D NMR experiments or XRPD is presented. Furthermore, it is shown that a polymorphic impurity combined with MAS can induce a crystalline‐to‐crystalline phase transition. This showcases that solid‐state NMR is not always noninvasive and such changes upon MAS should be considered in particular when compounds are studied over longer time spans.