Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR

<p>¹³C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of ¹³C-enriched nanothreads enabled a detailed characterization of the reaction products by a full complement of advanced solid-state NMR techniques, with validation by <i>ab initio</i> calculation of chemical shifts. The ¹³C NMR spectrum was complex, with more than a dozen distinct features, but almost all (> 95%) represented CH moieties are as expected in nanothreads, with only 2–4% CH₂, 0.3% C=O, and 0.3% COO groups, according to spectral editing. Different components were quantified by integration of the fully equilibrated direct-polarization spectrum. Symmetric and asymmetric alkene-containing rings as well as trapped furan were identified by ¹³C-¹³C and ¹H-¹³C NMR. The most intriguing component observed was fully saturated perfect <i>anti</i> furan-derived nanothread segments, with two distinct, sharp peaks, accounting for ca. 10% of the material. The bonding patterns in these periodic structures deduced from DQ/SQ NMR was that of a [4+2] cycloaddition product. While the small number of chemically inequivalent carbon sites eliminated low-symmetry <i>syn/anti</i> threads, the large number of magnetically inequivalent ones (<i>i.e.,</i> distinct C-H orientations) in CODEX NMR was incompatible with the high-symmetry <i>syn</i> threads. <i>Anti</i> threads with two chemically and eight magnetically inequivalent sites provide the only consistent fit of the experimental data. These conclusions were convincingly corroborated by quantum-chemical simulations, which showed good agreement of isotropic chemical shifts only for the <i>anti</i> threads. This represents the first molecular-level identification of a specific type of nanothread. The typical length of the perfect, fully saturated thread segments was around 14 bonds and they accordingly constitute small clusters (according to ¹³C and ¹H spin diffusion analyses) which likely reside within an overall hexagonal thread packing along with other, less-perfect or less-saturated brethren. The relatively slow <i>T</i>_1C relaxation confirms the nanometer-scale length of the periodic perfect structure, indicates that the perfect threads are particularly rigid, and enables their selective observation in ¹³C NMR. </p>

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR

<p>¹³C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of ¹³C-enriched nanothreads enabled a detailed characterization of the reaction products by a full complement of advanced solid-state NMR techniques, with validation by <i>ab initio</i> calculation of chemical shifts. The ¹³C NMR spectrum was complex, with more than a dozen distinct features, but almost all (> 95%) represented CH moieties are as expected in nanothreads, with only 2–4% CH₂, 0.3% C=O, and 0.3% COO groups, according to spectral editing. Different components were quantified by integration of the fully equilibrated direct-polarization spectrum. Symmetric and asymmetric alkene-containing rings as well as trapped furan were identified by ¹³C-¹³C and ¹H-¹³C NMR. The most intriguing component observed was fully saturated perfect <i>anti</i> furan-derived nanothread segments, with two distinct, sharp peaks, accounting for ca. 10% of the material. The bonding patterns in these periodic structures deduced from DQ/SQ NMR was that of a [4+2] cycloaddition product. While the small number of chemically inequivalent carbon sites eliminated low-symmetry <i>syn/anti</i> threads, the large number of magnetically inequivalent ones (<i>i.e.,</i> distinct C-H orientations) in CODEX NMR was incompatible with the high-symmetry <i>syn</i> threads. <i>Anti</i> threads with two chemically and eight magnetically inequivalent sites provide the only consistent fit of the experimental data. These conclusions were convincingly corroborated by quantum-chemical simulations, which showed good agreement of isotropic chemical shifts only for the <i>anti</i> threads. This represents the first molecular-level identification of a specific type of nanothread. The typical length of the perfect, fully saturated thread segments was around 14 bonds and they accordingly constitute small clusters (according to ¹³C and ¹H spin diffusion analyses) which likely reside within an overall hexagonal thread packing along with other, less-perfect or less-saturated brethren. The relatively slow <i>T</i>_1C relaxation confirms the nanometer-scale length of the periodic perfect structure, indicates that the perfect threads are particularly rigid, and enables their selective observation in ¹³C NMR. </p>

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR

<p>¹³C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of ¹³C-enriched nanothreads enabled a detailed characterization of the reaction products by a full complement of advanced solid-state NMR techniques, with validation by <i>ab initio</i> calculation of chemical shifts. The ¹³C NMR spectrum was complex, with more than a dozen distinct features, but almost all (> 95%) represented CH moieties are as expected in nanothreads, with only 2–4% CH₂, 0.3% C=O, and 0.3% COO groups, according to spectral editing. Different components were quantified by integration of the fully equilibrated direct-polarization spectrum. Symmetric and asymmetric alkene-containing rings as well as trapped furan were identified by ¹³C-¹³C and ¹H-¹³C NMR. The most intriguing component observed was fully saturated perfect <i>anti</i> furan-derived nanothread segments, with two distinct, sharp peaks, accounting for ca. 10% of the material. The bonding patterns in these periodic structures deduced from DQ/SQ NMR was that of a [4+2] cycloaddition product. While the small number of chemically inequivalent carbon sites eliminated low-symmetry <i>syn/anti</i> threads, the large number of magnetically inequivalent ones (<i>i.e.,</i> distinct C-H orientations) in CODEX NMR was incompatible with the high-symmetry <i>syn</i> threads. <i>Anti</i> threads with two chemically and eight magnetically inequivalent sites provide the only consistent fit of the experimental data. These conclusions were convincingly corroborated by quantum-chemical simulations, which showed good agreement of isotropic chemical shifts only for the <i>anti</i> threads. This represents the first molecular-level identification of a specific type of nanothread. The typical length of the perfect, fully saturated thread segments was around 14 bonds and they accordingly constitute small clusters (according to ¹³C and ¹H spin diffusion analyses) which likely reside within an overall hexagonal thread packing along with other, less-perfect or less-saturated brethren. The relatively slow <i>T</i>_1C relaxation confirms the nanometer-scale length of the periodic perfect structure, indicates that the perfect threads are particularly rigid, and enables their selective observation in ¹³C NMR. </p>

Polymorphic Forms of Valinomycin Investigated by NMR Crystallography

A dodecadepsipeptide valinomycin (VLM) has been most recently reported to be a potential anti-coronavirus drug that could be efficiently produced on a large scale. It is thus of importance to study solid-phase forms of VLM in order to be able to ensure its polymorphic purity in drug formulations. The previously available solid-state NMR (SSNMR) data are combined with the plane-wave DFT computations in the NMR crystallography framework. Structural/spectroscopical predictions (the PBE functional/GIPAW method) are obtained to characterize four polymorphs of VLM. Interactions which confer a conformational stability to VLM molecules in these crystalline forms are described in detail. The way how various structural factors affect the values of SSNMR parameters is thoroughly analyzed, and several SSNMR markers of the respective VLM polymorphs are identified. The markers are connected to hydrogen bonding effects upon the corresponding (13C/15N/1H) isotropic chemical shifts of (CO, Namid, Hamid, Hα) VLM backbone nuclei. These results are expected to be crucial for polymorph control of VLM and in probing its interactions in dosage forms.

Исследование строения, ионной, молекулярной подвижности и термических свойств гидратов пентафторидоцирконата аммония

The effect of hydrate number on the structural changes, thermal properties, and ionic (molecular) mobility character in NH_4ZrF_5 ⋅ H_2O, NH_4ZrF_5 ⋅ 0.75H_2O crystal hydrates have been investigated by the methods of IR, Raman, nuclear magnetic resonance (NMR) (^1H, ^19F, including ^19F MAS), and TG-DTA spectroscopy. Differences in crystal hydrate structures—anion structure, molecular state of water, and O–H⋅⋅⋅F, N–H⋅⋅⋅F hydrogen bond strengths—have been corroborated by IR and Raman spectroscopy data. Isotropic chemical shifts of magnetic inequivalent positions have been determined and attributed to crystal structures of the studied compounds by the method of ^19F MAS NMR. It has been established that the removal of water molecules from NH_4ZrF_5 ⋅ H_2O and NH_4ZrF_5 ⋅ 0.75H_2O results in the transformation of chain or layered structures accompanied by the increase of the number of bridge bonds while retaining or increasing the dimensionality of the anion structural motif. According to the ^1H NMR data, the NH $$_{4}^{ + }$$ cation diffusion in NH_4ZrF_5 occurs only in the temperature range of 370–520 K.

A 13C solid-state NMR investigation of four cocrystals of caffeine and theophylline

We report an analysis of the 13C solid-state NMR chemical shift data in a series of four cocrystals involving two active pharmaceutical ingredient (API) mimics (caffeine and theophylline) and two diacid coformers (malonic acid and glutaric acid). Within this controlled set, we make comparisons of the isotropic chemical shifts and the principal values of the chemical shift tensor. The dispersion at 14.1 T (600 MHz 1H) shows crystallographic splittings in some of the resonances in the magic angle spinning spectra. By comparing the isotropic chemical shifts of individual C atoms across the four cocrystals, we are able to identify pronounced effects on the local electronic structure at some sites. We perform a similar analysis of the principal values of the chemical shift tensors for the anisotropic C atoms (most of the ring C atoms for the API mimics and the carbonyl C atoms of the diacid coformers) and link them to differences in the known crystal structures. We discuss the future prospects for extending this type of study to incorporate the full chemical shift tensor, including its orientation in the crystal frame of reference.